the impact of point source pollution on an urban river
TRANSCRIPT
1
The impact of point source pollution on
an urban river, the River Medlock,
Greater Manchester
A thesis submitted to the University of
Manchester for the degree of Doctor of
Philosophy in the
Faculty of Science and Engineering
2016
Cecilia Medupin
2
Table of Contents
The impact of point source pollution on an urban river, the River Medlock, Greater Manchester ...... 1
A THESIS SUBMITTED TO THE UNIVERSITY OF MANCHESTER FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
IN THE ............................................................................................................................................................... 1
FACULTY OF SCIENCE AND ENGINEERING ....................................................................................................... 1
2016 .................................................................................................................................................................. 1
Cecilia Medupin ................................................................................................................................................ 1
Abbreviations ............................................................................................................................................. 5
Words and meanings ................................................................................................................................. 5
Declaration ................................................................................................................................................ 6
Copyright Notice........................................................................................................................................ 6
The Author ................................................................................................................................................ 7
Acknowledgements .................................................................................................................................... 8
Abstract 9
Chapter 1 GENERAL INTRODUCTION ..................................................................................................... 10
1.1 COMBINED SEWER OVERFLOWS (CSOS) .................................................................................................... 12
1.1.1 CSOs and Environmental Regulation ............................................................................................. 14
1.2 PHYSICAL MODIFICATION ......................................................................................................................... 15
1.2.1 Channelisation ............................................................................................................................... 16
1.2.2 Examples of modification to the River Medlock ............................................................................. 17
1.3 RIVER ECOLOGY: IMPACTS OF PRECIPITATION .......................................................................................... 19
1.4 WATER POLLUTION .................................................................................................................................. 21
1.4.1 Physical and chemical characteristics of rivers................................................................................. 21
1.4.2 Biological characteristics of rivers: benthic macroinvertebrates .................................................. 22
1.5 WATER QUALITY IN THE RIVER MEDLOCK ....................................................................................... 25
1.5.1 Why study the impact of combined sewer overflows in the River Medlock? ..................................... 27
1.6 AIMS, OBJECTIVES AND HYPOTHESES ....................................................................................................... 28
1.6.1 Aims ............................................................................................................................................... 28
1.6.2 Objectives ................................................................................................................................... 28
1.6.3 Hypotheses ................................................................................................................................. 29
1.7 OVERVIEW AND STRUCTURE OF THE EXPERIMENTAL RESEARCH CHAPTERS ........................................... 29
Chapter 2 GENERAL METHODOLOGY AND APPROACH .................................................................. 33
2.1 SITE DESCRIPTION OF RIVER MEDLOCK .................................................................................................... 33
2.2 SAMPLING REGIME .................................................................................................................................... 34
2.3 FIELD AND LABORATORY ANALYSIS .......................................................................................................... 38
2.3.1 Benthic invertebrate sampling and analysis................................................................................... 43
2.3.1 Benthic invertebrate sampling ....................................................................................................... 43
2.3.2 Spatial and Statistical analysis ...................................................................................................... 44
2.4 WATER QUALITY STANDARDS .................................................................................................................. 46
2.4.1 WFD Standards .............................................................................................................................. 46
2.4.2 Classification of Invertebrates..................................................................................................... 47
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Chapter 3 LONG-TERM WATER QUALITY OF A HEAVILY URBANISED RIVER: A CASE
STUDY OF RIVER MEDLOCK, GREATER MANCHESTER, UK .......................................................... 51
ABSTRACT ....................................................................................................................................................... 51
3.1 INTRODUCTION ......................................................................................................................................... 52
3.2 METHODOLOGY AND APPROACH ............................................................................................................. 53
3.2.1 Study area ....................................................................................................................................... 53
3.2.2 Study sites and data collection......................................................................................................... 55
3.2.3 Water quality and ecological parameters ......................................................................................... 56
3.3 RESULTS .................................................................................................................................................... 58
3.3.1 Physical and chemical variables ....................................................................................................... 58
3.3.2 Benthic macroinvertebrates ............................................................................................................. 67
3.4 SUMMARY ................................................................................................................................................. 69
3.5 DISCUSSION ............................................................................................................................................... 70
3.6. CONCLUSION ........................................................................................................................................... 72
ACKNOWLEDGEMENTS ................................................................................................................................... 73
3.7 REFERENCES .............................................................................................................................................. 73
Chapter 4 SOURCES OF PO4-P IN AN URBAN RIVER: COMBINED SEWER OVERFLOWS VS
WASTEWATER TREATMENT WORKS .................................................................................................... 78
ABSTRACT ....................................................................................................................................................... 78
4.1. INTRODUCTION ........................................................................................................................................ 78
Study area: The River Medlock ................................................................................................................ 80
4.2. METHODS ................................................................................................................................................. 81
4.2.1 Low resolution long term EA data ................................................................................................... 81
4.2.2 Fortnightly spatial data .............................................................................................................. 84
4.2.3 Data collection ........................................................................................................................... 85
4.2.4 High resolution temporal dynamics ............................................................................................... 86
4.2.5 Data analysis................................................................................................................................... 86
4.3 RESULTS .................................................................................................................................................... 86
4.3.1 Low resolution long term data .................................................................................................... 87
4.3.2 Fortnightly spatial data .............................................................................................................. 92
4.3.3 Temporal dynamics .................................................................................................................... 96
4.3.4 Comparing PO4-P load and concentration ................................................................................. 98
4.4 DISCUSSION ............................................................................................................................................. 101
CSOs vs WwTW ................................................................................................................................... 103
Comparison of phosphorus load in Medlock with other rivers ................................................................ 104
4.5 CONCLUSION .......................................................................................................................................... 107
ACKNOWLEDGEMENTS ................................................................................................................................. 107
4.6 REFERENCES ............................................................................................................................................ 107
Chapter 5 CATEGORISING THE BENTHIC MACROINVERTEBRATE ASSEMBLAGES AND
WATER QUALITY IN A HIGHLY URBANISED RIVER ....................................................................... 112
ABSTRACT ..................................................................................................................................................... 112
5.1 INTRODUCTION ....................................................................................................................................... 113
5.2 METHODOLOGY AND APPROACH ........................................................................................................... 115
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5.2.1 Study area ..................................................................................................................................... 115
5.2.2 Sampling and data collection ......................................................................................................... 116
5.3 RESULTS .................................................................................................................................................. 127
5.3.1 Physical and chemical variables ..................................................................................................... 127
5.3.2 Benthic macroinvertebrates ........................................................................................................... 135
5.3.3 Relationship between physico-chemical, hydrogeomorphological variables and benthic
macroinvertebrate assemblages .............................................................................................................. 143
5.4 SUMMARY OF RESULTS ............................................................................................................................ 144
5.5. DISCUSSION ............................................................................................................................................ 145
5.6 CONCLUSION .......................................................................................................................................... 149
ACKNOWLEDGEMENTS ................................................................................................................................. 150
5.7 REFERENCES ............................................................................................................................................ 150
Chapter 6 SHORT TERM WATER QUALITY VARIABILITY IN AN URBAN RIVER SUBJECT TO
POINT AND DIFFUSE SOURCE POLLUTION ...................................................................................... 155
ABSTRACT..................................................................................................................................................... 155
6.1 INTRODUCTION ....................................................................................................................................... 155
6.1.1 Aims and objectives ....................................................................................................................... 157
6.1.2 Site Description ............................................................................................................................. 157
6.2 METHODOLOGY AND APPROACH ........................................................................................................... 159
6.2.1 Continuous sampling programme ................................................................................................. 159
6.2.2 Spot sampling ............................................................................................................................... 160
6.3 RESULTS ........................................................................................................................................... 161
6.3.1 Discharge and Precipitation ..................................................................................................... 163
6.3.2 Correlation of physico-chemical variables with discharge and intercorrelation between variables164
6.3.3 Temporal variability ................................................................................................................. 166
6.3.4 Chemical concentration vs discharge ........................................................................................ 168
6.3.5 PO4-P Load .............................................................................................................................. 177
6.4 DISCUSSION ............................................................................................................................................. 178
6.5 CONCLUSION .......................................................................................................................................... 180
ACKNOWLEDGEMENT: ................................................................................................................................. 181
6.6 REFERENCES ............................................................................................................................................ 181
Chapter 7 SUMMARY AND CONCLUSIONS ......................................................................................... 183
7.1 CONCLUSION .......................................................................................................................................... 187
7.2 GENERAL REFERENCES ........................................................................................................................... 187
Appendix ............................................................................................................................................... 198
Total word count: 47572
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Abbreviations
1. ASPT –Average Score Per Taxa 2. ANOVA- Analysis of Variance
3. BIOENV- Biota and Environmental
4. BMWP—Biological Monitoring Working Party
5. BOD—Biochemical Oxygen Demand
6. CSO—Combined Sewer Overflows
7. DO- Dissolved oxygen
8. EA – Environment Agency
9. EQR –Environmental Quality Ratio
10. FFD—Freshwater Fisheries Directive
11. LIFE – Lotic Invertebrate Index for Flow Evaluation
12. WFD-Water Framework Directive
13. SIMPER –Similarity Percentage
14. nMDS-Non-Metric Multidimensional Scaling
15. WwTW – Waste Water Treatment Works
16. WHPT ----Whalley Hawkes, Paisley and Trigg (WHPT)
Words and meanings
1. A combined sewer overflow (CSO) is a collection system of pipes and tunnels
designed to also collect surface runoff, domestic waste water and other waste water
especially during rainfall. It serves as a storage wastewater tank. It is usually available
in houses built before the mid-1960s
2. Hyetograph: Graphical representation of the distribution of rainfall over time.
3. Hydrograph: Graph showing the rate of flow (discharge) versus time
4. WwTW- is a waste water treatment plant where impurities in waste water are
removed with the aid of physical structures before the effluent is released to the
nearby water bodies.
5. Nutrient load: The quantity of nutrients entering the river in a given period of time
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Declaration
No portion of the work referred to in the thesis has been submitted in support of an
application for another degree or qualification of this or any other university or other
institute of learning.
Copyright Notice
i. The author of this thesis (including any appendices and/or schedules to this
thesis) owns any copyright or related rights in it (the “Copyright”) and s/he
has given The University of Manchester certain rights to use such Copyright,
including for administrative purposes.
ii. Copies of this thesis, either in full or in extracts and whether in hard or
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and/or Reproductions.
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http://www.manchester.ac.uk/library/aboutus/regulations) and in The
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The Author
The author has a degree in Biochemistry and a previous experience in industry which
led to enrolment for the MSc. Degree in Pollution and Environmental Control at
Manchester University. Further experience in industry, environmental regulation and
academia, led to an increased interest in integrated environmental
science/management and resulted in the author registering for a PhD, the result of
which is the current thesis.
8
Acknowledgements
I would like to thank my sponsors, The National Open University of Nigeria for
providing the funding for this study.
I would also like to thank my supervisors, Dr. Keith White and Dr. James Rothwell
for their continuous help during this research. Thanks to the Environment Agency
and to Andy Goodwin, Matthew Harris, Tracey Smith, and Gordon Hardman who
provided me with some information pertaining to this study.
I must also thank the people that have assisted me including Marc Attallah who
assisted me in the first field survey of high risk sample locations in order to identify
combined sewer overflows, walking miles through the Medlock and for all the hard
work post field sampling and laboratory analysis. To Dr. Rob Mansfield who assisted
me during the project planning, for my colleague Ismael Alkhamaisie, other
colleagues who at one time or the other assisted with field sampling- Daryl Teoh,
Emma Randle, Irene Okhade, Dr. Amit Bajhaiya, Dr. Merve Engin and a host of
others who came out with me on field work. Thanks to Deborah Ashworth, Jonathan
Yarwood, Mr. Karl Hennerman, Dr. Gail Challabi and Mr. Graham Bowden of the
Geography Department, University of Manchester and Mr. Gary Porteous who
analysed my nutrient and metal samples. All the people I have been with at Michael
Smith building whose presence provided some fun during my study and to Dr.
Andrew Dean, Professor Amanda Bamford for their support.
I acknowledge all the people who have supported me throughout my studies in no
small measures: Special gratitude to Dr. Eric Northey and Mrs. Julie Northey and the
entire Northey family, Dr. Thomas Keller, Mr. Mark Sullivan, The Manchester
Universities’ Catholic Chaplaincy and the Chaplains, Dr. Keith White who provided
me the free platform to engage with other aspects of the University in addition to my
studies, The Stopford and Michael Smith Building receptionists and security staff.
The Methodist International House, the wardens including Mr. Dimitri Brady and all
the students and researchers from all over the world who provided me a fun-filled
and comfortable house during my studies and writing up of this thesis. Many thanks
to Professor Olugbemiro Jegede, Professor Monioluwa Olaniyi, Dr. Felix Olakulehin,
and Dr. Seray Ozden. To my brothers and sisters, nieces and nephews and for my
parents who have been there for me and to God Almighty to whom I dedicate this
thesis.
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Abstract
The River Medlock is a small (22km) urbanised river, and is one of the five
main tributaries which forms part of the River Irwell Catchment in Greater
Manchester, UK. The river has a legacy of pollution from the 18th century and
continues to be affected by anthropogenic factors including point source pollution
from waste water treatment works (WwTWs) and combined sewer overflows
(CSOs). In order to investigate the impact of CSOs and the WwTWs on the river
hydrology, water quality and ecology of the lower largely urbanised reach, data sets
were obtained from the Environment Agency and from direct sampling of the river.
Load estimations from continuous discharge records from the river’s gauging
station plus estimates of sub-catchment area indicate the lower sites, classified as a
“highly modified water body” and downstream of treatment works had had a
higher load of discharge and PO4-P linked to point sources and episodic discharges.
Short term, continuous monitoring revealed that CSOs were active during high
velocity, but increased concentrations of nutrients post high velocity indicate
WwTW effects and possibly diffuse sources. This project reveals that the WwTW
are a major source of PO4-P and that the impact of CSOs on the river quality is
short-lived and depends on the degree of precipitation. Other parameters indicate
good water quality although the benthic macroinvertebrate community is degraded
as a result of episodic increases in the quantity of water destabilising the river bed.
Therefore pollution from the CSOs, the WwTW and rapid changes in discharge are
the reasons for the river’s failure to conform to WFD requirements.
University of Manchester
Cecilia Medupin
PhD Environmental Biology
Thesis title: The Impact of point source pollution on an urban river, the River
Medlock, Greater Manchester
Date: 15th December 2016
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Chapter 1 GENERAL INTRODUCTION
Urbanisation is one of the most significant human impacts on the biosphere
and exerts a major effect on water quality (Paul & Meyer, 2001) and water resources
(Semadeni-Davies et al. 2008). The impact of urbanisation on fluvial systems results
from changes in catchment characteristics due to increased land use for housing,
transport, industrial and commercial use (Gregory, 1976). Kulcsaar & White (2012)
suggest that urbanisation can proceed in two ways - either in the multiplication of the
concentration points (i.e. new conurbations) or in an increase in the size of individual
concentrations (i.e. expansion of existing urban areas). The increase in impervious
surface cover resulting from urbanisation alters the hydrology and geomorphology
of drainage systems and hence the amount of runoff into the receiving water. Rose &
Peters (2001) showed that peak flows were greatest in urban catchment areas,
increasing from 30% to more than 100%, compared to non-urbanised catchments.
When this happens, large numbers of properties are at risk of surface water flooding
(Houston et al. 2011). The high concentration of industrial and residential discharges
increase nutrient loads plus the amount of heavy metals and other contaminants
entering the receiving water course (Clark et al. 2007; Paul & Meyer 2001). Through
urbanisation, impervious surfaces such as roofs, roads and car parks replace the
natural ecosystems that adsorb pollutants and also degrade organic contaminants.
Consequently, urbanisation promotes the transport of more particulate material and
dissolved pollutants from runoff into river systems. Cost projections for supplying
water to urban areas are predicted to increase in coming years due to pollution from
such diffuse and point sources as well as from population growth (Serageldin 1995).
Income growth per capita will also spur increased agricultural production and this
prediction is projected to increase world-wide water pollution by more than 100%
(Serageldin 1995) resulting in a decline in the quality of water entering urban
waterbodies from surrounding agricultural areas.
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Mullis et al. (1997) found that precipitation intensity and total precipitation
volumes strongly influenced both the volume of storm discharges and the duration
of storm flows. Such increases in water quantity entering combined sewers systems
(CSS) result in releases of untreated sewage via combined sewer overflows (CSOs).
As a result CSOs have a significant impact on the quality of receiving waters,
resulting in the ecological degradation of the watercourse.
Increased rainfall affects surface urban runoff in three ways;
it increases runoff volume due to reduced rainwater infiltration and
evapotranspiration;
it increases in the speed of runoff due to hydraulic modification and;
it decreases the response time of the catchment area - that is the time
between the start of precipitation and the increase in discharge which is
reduced as a result of impervious urban surfaces
These urban rivers are subject to the ‘urban stream syndrome’ (USS), coined to
describe the ecological degradation of urbanised water courses (Walsh et al. 2005).
The symptoms of USS include increased impervious surfaces resulting in higher
surface runoff velocities and reduced lag time, increased peak discharges (Leopold,
1968), episodic (“flashy”) stream flow (Booth 2005; Roy et al. 2005), greater erosion
from altered channel morphology (Walsh et al. 2005; House et al. 1993) and increased
hydraulic conveyance efficiency (Goonetilleke et al. 2005). Effects of USS on river
quality include increased load of potentially toxic chemicals and organic matter,
sediment re-working (Meade 1982) and displacement (‘drift’) of benthic invertebrates
(Lenat & Crawford 1994). Increased water temperatures owing to loss of riparian
vegetation and warming of surface runoff on exposed impermeable surfaces are also
characteristic of USS (Sinclair Knight Merz 2013). The key sources/causes of urban
river pollution are discussed below.
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1.1 Combined sewer overflows (CSOs)
A combined sewer system (CSS) is a single system that connects the foul and
surface drains directly to the wastewater treatment plant. Combined sewer systems
can cause serious water pollution resulting in the operation of the CSOs when wet
weather flows exceed the sewage carrying capacity of the Wastewater Treatment
works (WwTW). An assessment of the impact of combined sewer overflows (CSOs;
Figure 1-1) on the River Medlock is a key aim of this study. Separate sewers include
surface water or storm water drains and foul drains respectively; CSOs release
dissolved contaminants, inorganic particulates, significant amounts of organic and
suspended solids (Even et al. 2004). The dissolved and particulate organic matter
has a significant effect on oxygenation due to their contribution to the biochemical
oxygen demand (BOD). Deoxygenation damages the biota and impairs the aesthetic
quality of receiving waters, including through the nutrients facilitating growth of
unsightly algae which upon death contribute to the BOD. Various studies have
related poor river quality to CSOs, including Barco et al. (2008), Welker (2008) and
Passerat et al. (2011). They all linked the ‘first flush’ pulse of effluent during intense
rainfall to pollution of the receiving waters.
Figure 1-1: Combined sewer overflow on the Medlock during dry weather (author’s own image)
13
Under wet weather conditions, CSSs impact the river quality spatially and
temporally (Even et al. 2004). Harwood & Saul (2001) describe three components of
flow: inflow, continuation flow, and spill flow in a CSO, as shown in Figure 1-1. The
impact of CSOs is greatest during wet weather as the CSSs are unable to transport all
the wastewater and urban runoff to the WwTW or the capacity of the works is
exceeded. Therefore excess flow is diverted to a watercourse (Harwood & Saul, 2001;
Passerat et al., 2011).
Figure 1-2: Schematic diagram of a combined sewer overflow (after Harwood & Saul, 2001).
In the UK, CSOs have long been recognised as one of the major causes of river
pollution (Butler & Davies, 2000; Myerscough & Digman, 2008). In 1970, 37% of
14,440 CSOs in England and Wales were reported as unsatisfactory by the Ministry of
Housing and Local Government Technical Committee on Storm Overflows and the
Disposal of Storm Sewage (1970). Some of the problems reported included aesthetic
pollution, leading to public complaints, or a marked deterioration in the chemical
and biological quality of the receiving watercourse. In 2011, the policy document of
the Marine Conservation Society reported over 30,000 CSOs polluted the water
courses in the United Kingdom with an uncertified number of discharges to UK
water courses (Marine Conservation Society 2011).
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1.1.1 CSOs and Environmental Regulation
Early work carried out by the then National Rivers Authority (NRA) and the
Foundation for Water Research to assess CSO performance focussed mainly on visual
inspection and archived data, as reviewed by Blanksby (2002) and Lau et al. (2005).
They reported that statutory sampling programmes were carried out randomly
during working hours and that river monitoring specifically at these times was
inadequate to highlight the effect of wet weather flow on the CSOs. Since 1989, CSOs
have been regulated by the NRA’s successor the Environment Agency (EA) and they
set standards and discharge consents for the commercial water companies who own
and manage CSOs (Ayton 1994) such as United Utilities in Northwest England. CSOs
are regulated under the European Union’s (EU) Urban Waste Water treatment
Directive (EUUWWTD) (Council of the European Union 1991) and the EU Water
Framework Directive (WFD). The conditions listed in the UK regulation imply that
the type, nature of discharge and location of CSOs are critical to their management.
These conditions can be summarised below (Discharge Licenses, personal
communication with the EA, 2013).
a. Discharge from CSOs will “only occur as a result of rainfall or snowmelt”
therefore there shall be no discharge in the absence of rainfall.
b. The size of solids in the CSOs should not be greater than 6mm. This is
achieved in the design of mechanically raked screens to prevent heavy floating
solids from entering the water courses.
c. The condition indicates that flows to the storage should only occur when the
flow being passed forward in the sewer is at least 831 Ls-1 (0.831m³sˉ¹). This
implies that the discharge to river shall only occur when the storage is full and
the flow being passed forward in the sewer continues to be at least 831 Ls-1 .
d. The concentration of the discharge should comply with the standard
requirements for effluent released in the river. The CSO discharge should
therefore not contain more than 15mgLˉ¹ of biochemical oxygen demand
(BOD), 6mgL ˉ¹ ammonia and 35 mgL ˉ¹ of suspended solids.
15
Various studies have reported on the pollution status of the River Medlock since
the industrial revolution (Douglas et al. 2002; Burton 2003; Williams et al. 2010; James
et al. 2012) arising from point sources. James et al., (2012) suggest that CSOs may be
key reasons for many of the rivers of the Irwell catchment (including the Medlock)
not achieving the legally required EU water quality standards.
A previous study of the impact of CSOs on the lower Irwell and the upper
Manchester Ship Canal by Rees & White (1993) showed how River Irwell catchment
was influenced by storm water overflows, which discharged directly into the river
carrying contaminants with a high BOD plus suspended solids, ammonia and PO4-P.
The River Medlock receives episodic discharges from more than fifty CSOs, including
29 within the area under investigation (United Utilities’ ersonal communication,
2013). On the basis of discharge from CSOs, WwTW and river canalisation, the
Environment Agency’s classification of the river using the General Quality
Assessment had been “poor” and has also not met the European Union (EU) Water
Framework Directive which required “good ecological status”.
This study is the first comprehensive assessment of the impact of CSOs on the
physico-chemical parameters and their interaction with the benthic
macroinvertebrates in the River Medlock.
1.2 Physical modification
A water body is regarded as highly modified when its physical characteristics
and hydromorphology have been substantially changed e.g. for flood protection and
this will have a significant adverse impact on the water use or on the water
environment. Globally, changes in land-use practices affect the integrity and
conservation of water resources. Changes to the catchment include vegetation
removal and deforestation for urban development including housing and industry.
Modification of rivers include culverting, channelisation (to improve transportation),
and the creation of dams for irrigation and potable water. These modifications have
been shown to alter flow regimes and are regarded as the most serious threat to the
16
ecological sustainability of rivers and flood plain wetlands (Bunn & Arthington,
2002).
Degradation of the biological components of urban rivers and streams,
including the riparian areas, has been documented in a number of studies including
Tong & Chen (2002) who revealed that there was a significant correlation between
land use and in-stream water quality on a regional scale in Ohio, USA. Steiger et al.
(2003) reported that the disturbance of riparian vegetation was a major cause of
increased sediment loads in rivers. Miserendino et al. (2011), demonstrated that
urban sites showed lower biodiversity resulting from domestic sewage inload to
rivers and reported large variations in water quality, especially in terms of
conductivity, nutrients and dissolved oxygen. High loadings of fine suspended
particulates in rivers accumulate in the sediment and affect fish and invertebrates
that depend on well-oxygenated habitats (Mainstone et al., 2008 and Heaney et al.
2001).
1.2.1 Channelisation
Rivers undergo channelisation either naturally or by human-induced
modification (Gregory 2006; Gregory et al. 1992) and the latter result in increased
surface runoff in urban catchments (Grimm et al. 2000). The process of re-routing
river channels for navigation, reducing erosion (Marshall et al. 1978; Duan et al. 2014)
and for flood protection, all contribute to continuous disturbance of the river.
River channelisation significantly impacts on the environment and is
regulated through the EU Environmental Impact Assessment Directive (85/337/EEC).
This recognises the potential destruction of the habitat specifically through the
disconnection of the river from the flood plain, loss of wetland habitat, silting up of
the river and damage to the aquatic ecology. In the UK, the directive is implemented
under the Town and Country Planning (Environmental Impact Assessment)
Regulations 2011 for major development works and their impact on natural
conservation.
17
In the British Isles, channel modification began in the 17th century for the
purposes of navigation, dredging of existing watercourses to improve drainage and,
in the 19th century, to provide gravel for the construction of railway embankments
(Sear and Archer, 1998 in Acreman, 2000). In England and Wales approximately
8504km of rivers have been channelised in response to urbanisation with a
channelised density of 0.06 km/km2 and a further 35,500 km of rivers which are
regulated (Brookes et al. 1983). Between 1939 and 1945, war-time demand for
increased agricultural output and, in later years, the EU’s Common Agricultural
Policy and funding for land drainage improvements, led to intensive and extensive
channel modification to ensure continued high rates of agricultural productivity
(Acreman, 2000).
As these physical modifications do not take place uniformly along the river
channel (Arnold et al. 1982) they have uneven and specific effects. For example, they
directly cause changes in water velocity (Laws & Roth 2004); reduce hydraulic
connectivity between river channel and the riparian zone (Laws & Roth 2004);
increase the load of suspended solids (Lane et al. 2007); accumulate sediment
especially in low-energy river systems (Wilby et al. 1997); and increase contaminant
metal fluxes (Longfield & Macklin 1999) associated with soil erosion and fine
sediment transport from land (Leemans & Kleidon, 2002). Such modifications have
all had their effects on the River Medlock.
1.2.2 Examples of modification to the River Medlock
Studies on urban rivers in the Irwell catchment have been described in James
et al. (2012). The EA has classified some parts of the River Medlock as a “highly
modified water body” based on the degree of modification, in particular the lower
10km reach immediately upstream of the confluence with the River Irwell. Gill’s
(2006) study showed that stretches of most urban rivers in Manchester, including the
Medlock, were culverted as a result of their historical use as sewers, and that
18
culverting reduces their ability to cope with the large fluctuations in flow rate from
precipitation events.
Figure 1-3 shows the River Medlock before and after restoration work in 2014
at Clayton Vale. This stretch of the river was lined with bricks to enhance the flow
after serious flooding in 1872 (National Rivers Authority North West 1994). Recent
improvements included the removal of brick lining and weirs (Figure 1-4) (James et
al. 2012; Manchester City Council 2014). These modifications were aimed at restoring
the river to its near-natural state to enhance the invertebrate and fish populations,
while at the same time improving the amenity value in areas of public open space
(National Rivers Authority North West 1994).
Figure 1-3: Restoration of the River Medlock. View before and after removal of brick lining at
Clayton Vale, Manchester. The wall was also removed to allow growth of marginal plants. (Image
by Manchester City Council, 2014) NGR: SJ 87359 99209
Before
After
19
Figure 1-4: Restoration of the River Medlock. View before and following removal of the weir at
Clayton Vale (Image by Manchester City Council, 2014) NGR: SJ 88461 99429
1.3 River ecology: Impacts of precipitation
River habitats are modified by precipitation-induced changes in flow
magnitude and timing which directly influence benthic invertebrate community
structure. In addition, changes to the flow regime rework the substrate that also
affects the biota (Konrad & Booth 2005). Although river biota has evolved to cope
with variations in water velocity (Nilsson & Renöfält 2008), the increased episodicity
of urban rivers amplifies the shift in community structure (Willemsen et al. 1990) to
Before
After
20
favour species capable of withstanding continuous habitat change (Pedersen &
Perkins 1986). Precipitation in urban catchments has been shown to affect water
quality (Nilsson & Renöfält 2008; Poff et al. 1997) through the operation of CSOs at
high discharge rates (Mulliss et al. 1996).
River hydrographs show increased flood peaks created during storm periods
(Leopold 1968). Such changes in flow pattern in urban rivers are likely to be
accentuated by global warming affecting weather patterns (Hulme et al. 2002). For
example, high winter flows or short-term increases in discharge from summer storms
lead to soil erosion, scouring out of occupied habitats and increased nutrient and
suspended solid load which also impact on the biota (Stanley et al. 1994). Low flows
significantly modify in-stream communities in lotic systems (Boulton 2003; Lytle &
Poff 2004) by the deposition of silt substrate which exacerbates the impact of other
stressors such as high organic pollution and toxins (Boulton 2003) and impact on the
river biota. Frequent changes in the sediments, arising from flow-induced deposition
and erosion impact on the biota (Whitehead et al. 2009) because changes in bed
sediment favour species adapted to unstable habitats such as Chironomidae and
Oligochaeta (Pedersen & Perkins 1986). Higher invertebrate diversity has been
observed in stable and coarser sediments due to the increased number of niches
(Collier 1995).
Urban rivers in Manchester including the Medlock catchment (Figure 1-5) are
subject to heavy rain and prolonged periods of wet weather conditions with over
1,000 mm of rain per year and short periods of high precipitation reaching 0.11 mm
hr-1 (National River Flow Archive). One of the objectives of this study was to monitor
the river during wet and dry weather conditions. The aim was to assess the impact of
flow on water quality and the benthic invertebrate biota.
21
1.4 Water Pollution
Point and diffuse source pollution include biodegradable organic material,
metals and nutrients. These result in changes in physico-chemical characteristics of
rivers, in particular temperature, inorganic and organic suspended solids, nutrients,
dissolved oxygen, biochemical oxygen demand (BOD), conductivity and pH. Levels
of trace metals can increase as a result of pollution. Changes in discharge and water
velocity can also be due to human impact as described above.
1.4.1 Physical and chemical characteristics of rivers
River temperature is a key physical parameter which influences river ecology
(Webb & Walsh 2004) and it is in turn influenced by altitude and source. In Britain,
lowland river temperature can become as warm as 25°C, while at high altitudes,
streams remain cool at round 11oC, which is the national average (Hynes 1960; Orr et
al. 2010). Temperature has an indirect influence on the mobilisation as well as the
toxicity of pollutants. For example (Li et al. 2013) and Doudoroff & Katz (2014) found
that increased temperatures enhanced the release of phosphorus from sediments.
Temperature also influences the distribution and abundance of benthic
macroinvertebrates due to interspecific differences in thermal tolerance(Quinn &
Hickey 1990; Leunda et al. 2009). Total Suspended Solids (TSS) represents the actual
measure of mineral and organic particles transported in the water column by mass.
TSS can originate from sewage pollution, soil erosion, agricultural activities or
industrial runoff and algal blooms. Resuspension of silt, sand, clay or gravel from the
interstices of the river bed will also contribute to the suspended solid load depending
on water velocity (Everest et al. 1987). Concentration of ammonia-N and 5-day
biochemical oxygen demand (BOD5) are used to determine the impact of organic
pollution on water quality and the animal community (Donald et al. 2002) Ammonia-
N is released by the largely microbial-mediated breakdown of organic material while
BOD is an indirect measure of the amount of biodegradable organic material (Drury
et al. 2013).
22
Trace metals, including cadmium, chromium, copper, mercury, nickel, lead
and zinc are ubiquitous in nature (Gasperi et al. 2012) and hence are found in rivers
due to erosion. As a result of the anthropogenic factors described above,
mobilisation and transport of trace metals are increased by storm events and
flooding (Rose et al., 2004). An example of trace metal pollution of an urban
catchment area is the River Irwell. Work by Eyres & Pugh-Thomas (1978), indicated
a reduced number of benthic invertebrates due to metal pollution, plus the
presence of pollution tolerant taxa such as Chironomidae and Oligochaeta.
1.4.2 Biological characteristics of rivers: benthic macroinvertebrates
Elliott et al. (1980) define macroinvertebrates as those organisms that are
retained by a net or sieve with an aperture of 0.6mm. However a larger aperture,
generally as 0.95mm, is more commonly used by the research community and
regulatory bodies such as the UK’s Environment Agency although such a size will
not retain the early stages of some aquatic insects.
The dissolved oxygen (DO) content of a river affects the types of invertebrates.
Many invertebrates such as Plecoptera and Ephemeroptera require well aerated
water while some such as Oligochaetaes- (commonly of the family Tubificidae) are
abundant and the diptera Chironomidae are very abundant in poorly oxygenated
water. Both feed on detritus, including anthropogenic sources (Butcher et al. 1927).
Tubificidae feed on detritus in the sediment and many Chironomidae feed on
bacteria and detritus on the sediment surface. A rise in temperature affects the
amount of oxygen saturation, especially in turbulent streams where there is ready
exchange with the atmosphere and hence the water is fully oxygenated.
The nature of a river bed, which can be eroding or depositional, also
determines the types of invertebrates to be found. Most invertebrates show structural
adaptations which enable them to live either in fast or slow flowing water. Mayfly
nymphs, of the family Ecdyoduridae (synonym Heptageniidae), are flattened with
clawed appendages and apply themselves close to stones to reduce resistance to flow.
23
Many case-bearing caddisflies make their cases of stones to increase their density.
The caseless caddis larvae use silken threads for anchoring themselves, and the larva
of the black fly Simulium, spins a small mat of silken thread and attaches itself to this
via a complex circlet of tiny hooks (Hynes 1970).
Feeding habits among benthic macroinvertebrates vary and include
carnivores, such as many large stoneflies, some caddisflies (e.g. Rhyacophilidae),
beetle larva (Coleoptera) and leeches (Hirudinea). Most of the nymphs of mayflies
and many caddisflies, scrape (graze) algae off stones (Moon, 1939). Detritus of
terrestrial (allochthonous) vegetable origin is fed on by many organisms of the
shredder and collector guilds such as some stonefly nymphs (Hynes 1960) and
caddisfly larvae (Percival & Whitehead 1929). Food is also carried away by currents,
including small particles of detritus, detached algae and small benthic invertebrates
that have lost their attachment. This is then available to animals downstream such as
the blackfly Simulium, which filters the water by making continuous grasping
movements with a pair of mouth brushes.
The assessment of water quality using benthic macroinvertebrates has several
advantages over physical and chemical analyses as it not only gives an indication of
the quality of the water for living organisms but is also able to detect and integrate
environmental change that could only be directly assessed through continuous
monitoring of physico-chemical parameters. Hellawell (1986) argues that benthic
macroinvertebrate analysis has the following benefits:
they are the most useful indicators for monitoring water quality because they
are sensitive to toxic pollutants and the general degradation of the river,
they are reasonably sedentary and are therefore representative of local
conditions (Cook 1976),
different taxa display different levels of sensitivity to organic and other
pollutants and their responses are well understood,
24
they have lifespans long enough to provide a record of environmental quality
(Pratt & Coler 1976),
they are relatively large and easily identified,
they are ubiquitous in freshwaters and their biogeography is similar
throughout the world.
There are various ways to assess the resident macroinvertebrate community in a
river. These include passive sampling, including the use of colonisation samplers
which are regarded as a passive method by Hellawell (1978); and active sampling,
using, for example, hand-held nets. Colonisation samples have the advantage of
eliminating differences arising from changes in substrate and hence facilitate inter-
site comparisons; however, they may not reflect the indigenous community at a given
site. Various samplers that are used for invertebrates include traps (drift and
emergence traps); colonisation samplers (multiplate); and immediate samplers (grab,
air lift, corer, dredge) (Elliott et al. 1980). The period of exposure of these traps and
colonisation samplers in freshwater varies from four to six weeks (Weber 1973). The
main advantage of active sampling is that the animals are collected at the same time
as the sampling is conducted and that it can be performed on a range of substrates.
Various biological indices have been used to indicate pollution and reflect the
differing sensitivity of freshwater organisms to organic pollution, including the
Biological Monitoring Working Party (BMWP) (Hawkes 1997) and its variant the
Whalley, Hawkes, Paisley & Trigg (WHPT) metric, (Paisley et al. 2014; WFD-UK
Technical Advisory Group (UKTAG) 2014). Other workers have assessed indices
based on relative abundance, including MacNeil et al., (2002) who proposed the
Gammarus:Asellus ratio. Ephemeroptera, Plecoptera and Trichoptera (EPT) are taxa
widely used in combination as bio-indicators as they are relatively intolerant of
organic pollution (Hoiland et al. 1994; Malmqvist 2002; Ode et al. 2005).
25
1.5 Water Quality in the River Medlock
Various studies on the Medlock showed it has been polluted since at least the
mid-nineteenth century due to its industrial legacy (Burton 2003; MacKillop 2012;
Williams et al. 2010) and urban catchment (Willey 2011) resulting in diffuse sources
of pollution, WwTWs and localised storm events which had led to frequent spill
events from combined sewer overflows (CSOs) (Rees & White 1993; James et al. 2012;
Douglas et al. 2002; Frost et al. 1976; Tyson & Foster 1996 & Environment Agency
2009c).
Previously, the Medlock in common with other UK rivers was classified
based on the General Quality Assessment (GQA) and General Quality Assessment
Headline Indicators (GQAHI). Data obtained from 1990 to 2009 from the source to
Lumb Brook 12km downstream showed the river was rated by the EA as “very
good”; from Lumb Brook for a further 6.5km to the confluence of the River Irwell
the river was rated from “poor” to “fairly good”. This latter section is classified by
the EA as a “heavily modified river” based on channel modification. A similar
spatial pattern was observed for nutrients between 1990 and 2009. While NO3-N
concentrations in the river were generally “very low” from the source to Lumb
Brook, the concentration recorded from Lumb Brook to the confluence with the
River Irwell 6.5km downstream were higher and therefore were classed
“moderate”. For the same period, PO4-P concentration was “high” at the upper sites
(with a range of 0.2mgP Lˉ¹) and the sites below the WwTW, 0.5km below Lumb
Brook had “very high” concentrations (1mgPLˉ¹). The EA therefore attributed the
greatest challenge to good water quality on the Medlock to the “very high”
concentration of PO4-P from the WwTWs plus CSOs (EA, personal communication,
2013).
The River Medlock is typical of many rivers in the UK in that it has a mixed
use catchment; in this case 40% urbanised in the lower reaches with agricultural
areas (26%) and woodland (18.8%) above. In common with many northern UK
catchments, the agriculture in the upper Medlock catchment (Figure 1-5) is largely
26
sheep and cattle pasture with little arable farming (Farming and Countryside
Education (FACE) 2007). The River Medlock, like other urban rivers, is subject to
multiple stressors (Heathwaite 2010) including poor water quality arising from
point sources, in particular WwTWs and CSOs. Diffuse pollution within the
urbanised reach includes road runoff, badly connected sewers, old landfills, and
agriculture (James et al. 2012). Re-engineering of the Medlock, although not as
extensive as in some other urban rivers such as the Irwell (Williams et al., 2010) will
also influence flow and hence pollutant behaviour and ecology, including the
benthic invertebrate community.
Figure 1-5: River Medlock catchment boundary with urban settlements. The sampling sites (S1 to
S6). (Source: ArcGIS)
27
Several studies have looked at the impact of point source pollution on urban
rivers from various single angles. But this study provides a holistic understanding on
the basis of water quality (physico-chemical parameters); biology (benthic
macroinvertebrates), nutrient dynamics and the impact of precipitation on water
quality. The catchment of the Medlock contributes to local understanding of the River
Catchment Development Project which feeds into the national, EU Water Framework
Directive and global water management plans (Sustainable Development Goals).
Thus, the solution following this study will be sensitive to local needs as it provides
information necessary to control pollution and its challenges at a local scale. Also, it
will inform effective decision-making processes and practical solutions, in this case,
the control of discharge entering the river.
1.5.1 Why study the impact of combined sewer overflows in the River
Medlock?
CSOs have been seen as major point sources of pollution of urban rivers after
the WwTWs (James et al. 2012). Although both sources are controlled through
legislative requirements, discharges from CSOs are less regulated and also storm
dependent. Thus, while the WFD requires good ecological status, many rivers have
yet to comply with these standards. Therefore, in establishing the magnitude of CSO
impact, river managers will be able to focus efforts on controlling high discharging
CSOs. In addition, the assessment of CSO impact on the river relative to WwTWs
would assist water managers and environmental regulators to implement cost-
effective and focussed mitigation strategies to facilitate compliance with the WFD.
One of the aims of this study is therefore to identify and quantify the main
source(s) of PO4-P in the Medlock by determining its concentration at different
seasons and to assess the points on the river with the highest concentration. In
combination with knowledge of discharge it will then be possible to quantify
amounts of PO4-P both from WwTWs and from CSOs.
28
1.6 Aims, Objectives and Hypotheses
1.6.1 Aims
The overall aim is to examine the impact of point-sources, specifically WwTWs and
CSOs, on the water quality and benthic invertebrate community of the River
Medlock. More specifically, this study aims to:
1. Characterise the chemical and biological status of the River Medlock based on
the requirements of the WFD. This includes long term (10 years) changes in
the physicochemical condition and the benthic macroinvertebrate community.
2. Assess the relative contribution of CSOs and WwTWs on the water quality of
River Medlock. This will include the relative importance of these point sources
on the load of selected pollutants, including PO4-P, and during episodic
rainfall events.
3. To characterise the benthic macroinvertebrate community in the river and to
assess the relative importance of the physical, chemical and
hydromorphological impacts on the community.
1.6.2 Objectives
The above aims give rise to the following objectives.
1. To assess past and current (2000-2013 and 2013-2014) water quality status by
measurements of key physico-chemical and hydromorphological parameters
indicative of anthropogenic-induced change, including re-engineering of the
river and the degree of urbanisation.
2. Estimate fluvial WwTW NO3-N, and PO4-P load and quantify the relative
importance of other sources, in particular CSOs.
3. To determine the role of short-term (15min- over 92 days plus spot sampling at
intervals corresponding to rainfall events) discharge dynamics on water quality.
4. To determine the diversity and abundance of the benthic macroinvertebrates
community and the relationship with the above environmental variables.
29
5. To use the data obtained over the sampling period to assess the extent of
compliance of the River Medlock with the EU WFD standards. This will include
an examination of the relative importance of chemical quality and
hydromorphological parameters in defining the status of the benthic
macroinvertebrate community. Such information will aid in the formulation of a
strategy for the future management of the Medlock and other urban rivers.
Long-term changes where examined from data provided by the EA and the
current (2013-2014) water quality and ecological status was assessed from
sampling and analysis at a number of points along the river.
1.6.3 Hypotheses
1. CSOs contribute to the poor water quality and reduction in the benthic
macroinvertebrate community in the River Medlock.
2. Control of CSOs will reduce the pollution of the River Medlock rather than
further improvements in the WwTW effluent.
3. “Good” water quality as defined by the WFD does not result in “Good
Ecological Status”.
Although it is appreciated that the recent decision to withdraw from the EU will have
implications for water management, including compliance with the WFD, EU
standards will still apply until withdrawal, and possibly thereafter (Miller 2016).
1.7 Overview and Structure of the Experimental Research Chapters
The experimental chapters 3 to 5 are written in the form of papers. Each
chapter therefore has an abstract, introduction, aims and objectives, methods, results,
discussion and conclusion.
Prior to the experimental chapters, chapter 2 provides a more detailed account
of the methodology and approach used by the EA, and in this study, to assess water
quality and benthic invertebrate biota. It includes a description of the study area, the
rationale for site selection and the parameters analysed during the period 2013-2014
seasonal survey and the short term examination of discharge and water quality
30
dynamics. Analytical methods and statistical analytical tools are described. The EA’s
General Quality Assessment and the WFD requirements for rivers are outlined.
Chapter 3 is an overview of the studied reach of the river based on the datasets
between 2000 and 2013 obtained from the EA prior to my study between 2013 and
2014. One study site was located upstream and two downstream of the main
Wastewater Treatment Works (WwTW). The major tributary to the study area, Lord’s
Brook was also analysed to assess its pollution impact to the river. The study
indicated that high PO4-P concentration was a major barrier to the river’s compliance
with the WFD. The benthic invertebrate community failed to achieve the “good
ecological status” required by the WFD as it was dominated by pollution-tolerant
taxa even though water chemistry, other than PO4-P, indicated good water quality.
High suspended solid concentrations greater than the EU Freshwater Fisheries
Directive requirement of 25mgL¯1 were recorded at certain periods.
Subsequent chapters 4 to 6 determined through further interrogation of the EA
datasets plus sampling and analysis by myself at six sites, between 2013 and 2014. In
chapter 4, the source, dynamics, load and relative contribution of PO4-P in relation to
river episodic conditions was determined: the long term EA data, annual PO4-P
concentrations from the fortnightly data at the six sites from 2013 to 2014 and the
high resolution dataset were obtained from August 2014 to 31st October. These
dataset provided a detailed account in time and space of the PO4-P dynamics in the
river. Although the PO4-P load estimated (Webb et al. 1997) was less than 3.5
kgha⁻¹yr⁻¹ during any of the study periods, it was within the range found in other
urban areas and agricultural sites. The treatment works contributed an average of
92% of PO4-P load which made it a major contributor, rather than the CSOs or other
diffuse sources.
In Chapter 5, the environmental variables which created the greatest impact
on the benthic macroinvertebrate community in the Medlock were identified using
the annual fortnightly datasets and a combination of biotic indices and multivariate
tools. The results indicate the river invertebrates were influenced by a number of
31
hydrogeomorphological and chemical factors, mainly discharge, altitude, slope and,
PO4-P. Altitude, slope, were linked to site location and hence reflects discharge and
flow. The apparent relationship between the invertebrate community and PO4-P
concentration was attributed to the relationship with discharge. It is also suggested
that the community was impacted by the river’s sandy substrate (<2mm) which are
not suitable for pollution-sensitive taxa. The catchment of the uppermost site was
only 33% urbanised compared to the other sites, which had >40% urban catchment.
Given that streamflow patterns are modified by urban development it is suggested
that hydrological changes impact on the biota. The new biotic indices introduced by
the WFD in 2015 i.e. Whalley Hawkes, Paisley Trigg (WHPT) scores and WHPT
Average score per taxon (WHPT ASPT) were used in the assessment alongside the
old BMWP score. Both indices gave a very similar indication of the degree of
environmental degradation at all sites. It is concluded that the key stressor that
degrades the invertebrate community of the Medlock is urban stormwater runoff
released to the river by the hydraulically efficient drainage system that responds
rapidly to changes in precipitation. Other stressors, such as pollution (rather than
discharge) from CSOs plus WwTWs are considered of less importance. Therefore the
Medlock can be considered to be suffering from the “urban stream syndrome”
(Walsh et al., 2005) which adversely affects the distribution and abundance of benthic
macroinvertebrates in urbanised rivers. Until the symptoms of the urban stream
syndrome are addressed the Medlock is unlikely to comply with the WFD.
Chapter 6 employed the higher resolution continuous datasets taken at the
EA Gauging station to examine short-term changes in water quantity and quality
arising from the hydraulically efficient and hence ‘flashy’ nature of the urbanised
lower reaches of the Medlock. This part of the study showed that the CSOs were
sometimes active during short-term high rainfall events although other sources,
probably agricultural and road runoff contributed PO4-P and suspended solids to
the river during such events. The study therefore confirmed that CSOs were not
the sole pollution sources as revealed by the EA for some highlighted peak
32
discharge periods (EA personal communication, 2015). The results of this chapter
supported the earlier studies as to the importance of discharge on river sediment
destabilisation, and hence on the benthic macroinvertebrate community. This
chapter is informative for policy makers and the water companies as they tend to
focus on PO4-P reduction, removal and possibly, recovery from the WwTW.
However, dealing with phosphorus and other pollutants will not fully address the
reasons for the degraded invertebrate community due to the urban stream
syndrome. Therefore suggestions for reduction of discharge into the river are
included. Chapter 7 provides a summary of the chapters and a general conclusion
from the study.
This comprehensive study provides a clear understanding of the issues
within the River Medlock catchment. It also serves as a pilot study that will help
to integrate projects which will address catchment restoration plans in a cost-
effective way. The study therefore contributes effectively to knowledge of river
catchment studies.
33
Chapter 2 GENERAL METHODOLOGY
AND APPROACH
2.1 Site Description of River Medlock
The River Medlock (Figure 2-1) is a third-order stream in one of the heavily
urban areas of Greater Manchester. The river forms part of the River Irwell
catchment which in turn is a component of the Mersey catchment and which is one
of the largest in the UK. The Medlock is sourced from the moorland to the north
east of Oldham (National Grid Reference: SD 95308 05431) where it flows for 22km
through Ashton-under-Lyne and continues in a south easterly direction to
discharge into the River Irwell immediately downstream of Manchester city centre
(SJ 85781 97858).
The Medlock has a catchment area of approximately 57km2 and about 40% of the
land cover is urbanised (National Rivers Flow Archive, 2016); the remainder is
recreational or agricultural land (Tyson & Foster 1996). The main tributaries of the
River Medlock are Thornley, Taunton, Glodwick, Lumb Clough and Lord’s Brooks.
For the last 10 km, the River Medlock flows through the Manchester city centre
mainly in underground culverts and artificial channels until it confluences with the
River Irwell.
The river has a continuously operational waste water treatment works (WwTW)
at Failsworth (NGR: SJ 89674 99800), fifty combined sewer overflows (CSOs)
(personal communication, United Utilities, 2013) and an unknown number of
surface water drains within the study area (
Figure 2-2). Failsworth WwTWs is situated 12.6km south of the river’s source.
The daily discharge from this WwTW was obtained from the water utility
company, United Utilities. Table 2-1 provides estimated values of the
frequency/volume of discharge from the CSOs in 2013 (Personal Communication,
34
United Utilities, 2013). Population served by WwTW is 21,624 (United Utilities’
personal communication, 2016).
Figure 2-1: The River Medlock showing the EA sample sites, EA gauging station, CSOs
(graduated values),the WwTWs and the urbanised areas of the catchment. Dashed line shows the
source to Lumb Brook & from Lumb Brook to the confluence with the River Irwell; see
introduction (Section 1.5). CSO Data from United Utilities (Source: ArcGIS.)
Table 2-1: Combined sewer overflows (CSOs) on the River Medlock classified on the basis of
number of spill events, duration and volume of discharge (United Utilities, 2013)
S1&S2 S3 S4 S5 S6
No of CSO events
(spills/year) 219.1 112.4 420.1 72.1 1
Duration (hours) 603.3 171.5 3547.9 79.9 0.6
Volume (m3) 1,029,584 52,945 585,608 29,902 1,005
2.2 Sampling regime
Long term datasets from 2000 to 2013 were obtained from the EA. Physico-
chemical parameters were available for three sites on the river: here designated S0
(Medlock Vale), S4 (Millstream Lane) and S6 (Pin Mill Brow) and, for the tributary
Lord’s Brook from 2000 to 2006. Complete physico-chemical datasets were available
for S6, while S0 and S4 were monitored by the EA between 2000-2004 and 2010-2012;
35
hence no data is available between 2005 and 2009. The variables analysed were pH,
dissolved oxygen, temperature, conductivity, suspended solids, biochemical oxygen
demand (BOD), ammonia-N, NO3-N and PO4-P. Benthic macroinvertebrates were
assessed by the EA for S6 in autumn and spring, and at the Lord’s Brook between the
period of 2000 and 2008.
Annual sampling was carried out by the author on the river from March 2013 to
April 2014 at six locations (S1to S6). These sites were selected upstream and
downstream of twenty-nine CSOs and the main WwTW. Two of the sites, S4 and S6
corresponded to the EA sites (long- term sampling) while the EA site S0 was not
sampled in this study due to poor access. Sampling frequency was fortnightly for
physico-chemical variables and monthly for benthic invertebrates. On each
sampling date, samples were taken from three sections of each site to obtain an
average for the measured parameters. S3 was not sampled for benthic
macroinvertebrates due to poor access and S6 was sampled less frequently due to
safety considerations at certain periods, in particular during high flows.
Table 2-2 describes the sample location, sub-catchment area, and distance from
the source, velocity, depth and width of the river.
Figure 2-2 shows the position of the sampling sites.
36
Table 2-2: Sample locations on the River Medlock, sub-catchment area, distance from source, and
geographical variables. Site 0 was only sampled by the EA
Site name Site No.
Catchme
nt area
(km²)
Catchment
area as % of
the total
catchment
Distance
(km)
from
source
Latitude Longitude Altitude
(m)
Slope
(%)
Mill Brow S1 15 26 6.60 53.5173 -2.0892 138.51 2.69
Park Bridge
Road S2 23.5 41 8.50 53.51282 -2.0997 117.87 2.34
Daisy Nook
Garden S3 29.7 52 10.30 53.50107 -2.12398 88.81 2.21
Millstream
Lane S4 43.9 76 13.00 53.49258 -2.16317 66.81 1.92
Purslow Close S5 53.7 93 16.10 53.48197 -2.21164 47.35 1.67
Pin Mill Brow S6 54.4 95 17.40 53.47726 -2.21571 42.39 1.57
WwTW n/a n/a n/a 12.60 n/a n/a n/a n/a
Lord's Brook
4.5 2.58 12.07 53.49422 -2.15519 68.16 -
Medlock vale S0 65 37.2 12.23 53.49311 -2.15135 74.82 -
Figure 2-2: Study sites (S1-S6), Lord’s Brook, Environment Agency’s Gauging station and the
WwTw. Dashed line shows the source to Lumb Brook & from Lumb Brook to the confluence with
the River Irwell; see introduction (Section 1.5). Blue circles represent estimates of volumes of
discharge from CSOs entering into the River Medlock. (Source: ArcGIS).
37
Figure 2-3: Photographs of sample sites S1 to S6 (shown on the map of
Figure 2-2). S6 shows the debris screen which is aimed to retain large objects and prevent flood
damage.
The study sites had riparian vegetation, largely erosional at S1-S3 and
depositional downstream of the river, hence the large silty substrate observed
downstream at S6.
S1 S2
S3 S4
S5 S6
NGR: SD 94183
02262
NGR: SD 93489
01798
NGR: SD 91874
00493
NGR: SJ 89272
99554
NGR: SJ 86052
98382
NGR: SJ 85781
97858
38
Invertebrate colonisation samplers were installed in the river at S2 and S6 for
four-months from September to December 2014.
Higher resolution sampling was carried out at the Environment Agency’s
gauging station site from 1st August 2014 to 31st October 2014. Monitoring of pH,
dissolved oxygen, temperature, conductivity and turbidity at 15-minute intervals was
carried out. Fifteen minute continuous discharge records from the gauging station
was obtained from the EA for the duration of sampling.
The summary of the sampling regime carried out on the river for the study is
provided on Table 2-3.
Table 2-3: Summary of the measurements carried out on the River Medlock. * Spot samples
collected during this period.
Data from the
Environment
Agency
This study:
seasonal
analysis
This study: high
resolution analysis
Duration 2000-2012 March 2013 -
April 2014
1st August 2014 -
31st October 2014
Sampling Regime Monthly Fortnightly 15-minute
continuous - high
resolution
Temperature √ √ √
Dissolved Oxygen √ √ √
pH √ √ √
Conductivity √ √ √
Nutrients NO3-N, PO4-P,
ammonia-N
NO3-N, PO4-P,
TP, ammonia-N
*NO3-N, *PO4-P, *
ammonia-N
BOD √ √ -
Suspended solids √ √ √
Turbidity - - √
Benthic
invertebrates
S6 Spring/Autumn
between 2000 &
2008
Monthly at S1-
S2, S4 -S6
4 months
2.3 Field and laboratory analysis
Sampling of physico-chemical parameters and benthic macroinvertebrates
were obtained at the river. Photographs of sites S1 to S6 are shown in Figure 2-3.
39
The historical data obtained directly from the EA used standard analytical
methods as described in the Standard Committee of Analysts Publications, (2011).
For the seasonal analysis between March 2013 and April 2014, measurements
of pH, dissolved oxygen (percentage saturation and mgL-1), temperature and
conductivity were obtained using a pre-calibrated hand-held multiparameter water
quality meter (YSi 556 Multi probe system YSI, Yellow Springs, Ohio, USA).
Fortnightly discharge was calculated for each sub-catchment areas on the basis
of their relationship to the total catchment area. Discharge records obtained at the
continuously monitored EA gauging station was considered a preferable option to in-
situ measurements because discharge based on estimates of velocity and cross-
sections area was not always possible or accurate due to limited safe access,
particularly at periods of high velocity and discharge. Therefore, the data presented
in Table 2-4 are approximations. Continuous discharge data was available only at the
EA’s Gauging station 0.5km below S6. The river discharge measured in cubic metre
per second (m³s-¹) for each study location was obtained by estimation using a simple
linear regression equation which correlated the catchment area (km2) with mean
discharge (Q) for twenty-eight rivers within Greater Manchester including the River
Medlock (National Rivers Flow Archive, 2014). The model y = 0.0218x - 0.0422, which
indicated a strong correlation between the catchment areas and river discharge (R² =
0.873) was used to estimate the discharge at the sites in relation to the sub-catchment
areas.
Water velocity was measured using the float method i.e. by recording the time
taken for the float to travel over a given distance (10m) along the river. The results
obtained from this exercise was compared with the EA velocity records for the same
period. River substrate class was obtained by estimating the percentage substrate
observed at each site and is shown on Table 2-5. Substrate composition of benthic
habitats was determined at each sample site by visual examination of the percentage
coverage of each particle class: silt, sand, gravel, pebble, stones, boulder, and
40
bedrock. Substrate composition was divided into the above classes based on the
modified Wentworth scale (Cummins 1962).
Table 2-4: Values for average (no. = 23) of Width (W) (m), minimum and maximum depth (D) (m),
Velocity (V) (msˉ¹) and discharge (Q) (msˉ¹) at sample sites.
W D V Q
Sites Ave
W
(m)
Ave
D
(m)
Min
D
(m)
Max
D
(m)
Ave V
(msˉ¹)
Min V
(msˉ¹)
Max V
(msˉ¹)
Ave Q
(m³sˉ¹)
Min Q
(m³sˉ¹)
Max Q
(m³sˉ¹)
S1 5.7 0.22 0.13 0.3 0.27 0.16 0.53 0.15 0.05 0.46
S2 8.2 0.23 0.12 0.59 0.26 0.11 0.59 0.23 0.08 0.72
S3 8.6 0.44 0.28 0.55 0.27 0.13 0.53 0.29 0.1 0.9
S4 8.8 0.27 0.14 0.64 0.6 0.14 1.25 0.43 0.15 1.34
S5 8.8 0.29 0.11 0.69 0.57 0.13 1 0.53 0.18 1.63
S6 8.5 0.29 0.15 0.58 0.57 0.13 1 0.53 0.18 1.66
One litre water sample was collected in acid-washed (10% hydrochloric acid)
polypropylene bottles. A 300ml aliquot was filtered through a dried (heated to 500°C
for three hours) pre-weighed 0.45 µm glass microfiber filter (Whatman GF/C filter,
VWR International, Leicestershire UK) to remove any organic particulates. The filter
paper was then oven dried at 105°C for 24 hours to remove moisture, weighed
andthe difference in weight taken to determine the total suspended solids (SS) in the
filtered sample.
Part of the aliquot containing inorganic constituents was filtered through a
0.45µm Millipore (Millipore Limited, UK) for measurement of ammonia (mgL-1 as
ammonia-N), NO3 (mgL-1 as nitrate-N), PO4 (mgL-1 as phosphate-P) and trace metals
(µgL-1) while 40ml unfiltered sample was preserved for the analysis of total
phosphorus by acid digestion. The trace metal samples were acidified with two drops
of ultra-pure reagent nitric acid to pH~2 to retain the metals in solution.
41
The nutrients NO3-N and PO4-P were analysed using a SEAL Auto Analyzer 3
High Resolution instrument (SEAL Analytical Ltd, Southampton, UK). This
equipment has a high level of precision and ultra-low detection limits (SEAL
Analytical, 2013). Detection limit for phosphate measured as P was 0.004 mgLˉ¹ and
nitrate measured as N was 0.05 mgLˉ¹. For further information on the methods
employed by the auto analyser see SEAL Analytical (2013). Phosphorus is measured
by the Environment Agency as soluble reactive phosphate, again as the element P in
mgL-1. Total phosphorus (a measure of the total inorganic and organic phosphorus)
was processed through a pressure digestion technique with sulphuric acid and
potassium persulphate (Mackereth et al. 1978) and analysed using ion
chromatography.
Ammonia was analysed using the Hanna low range reagents kit (HI-93700-01;
Hanna Instruments Ltd, Leighton Buzzard, UK) and the colour change quantified by
spectrophotometry at an absorbance of 500nm. The limit of detection for ammonia
measured as N was 0.01 mgLˉ¹.
For measurement of Biochemical oxygen demand (BOD), a brown glass bottle
was used for the collection of samples from each site to avoid autotrophic metabolism
and incubated at 20°C for five days. The five-day BOD was calculated as the
difference between dissolved oxygen at day zero and day five measured using a pre-
calibrated Hanna dissolved oxygen meter (Hanna Instruments Ltd, Leighton
Buzzard, UK).
The trace metals chromium, cadmium, copper, nickel, lead and zinc were
analysed by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) using an
Agilent 7500cx (Agilent Technologies, Santa Clara, USA) spectrometer. Calibration
was by matrix-matched standards.
Geographical information including sub-catchment altitude was obtained
from the internet map tools (www.freemaptools, www.daftlogic.com) for the study
sites using the sub-catchment latitude and longitude information. The sub-catchment
42
slopes were obtained by dividing each site’s elevation from the river’s source by the
distance of the sample site from the source (See Table 2-2).
To obtain the high resolution temporal dynamics discharge, a 15-minute
duration discharge record was obtained from stage records supplied by the EA’s
gauging station SJ849975 at London Road, Manchester. Precipitation data of 15-
minute duration was obtained from the Whitworth Meteorological Observatory,
Manchester.
Spot samples in parallel with the high resolution discharge data were collected
for measurement of PO4-P, NO3-N, ammonia-N, and suspended solids, from the
London Road gauging station. The samples were obtained over a range of low and
high flow conditions between August and October 2014 in order to establish
concentration-discharge relationships. Samples were obtained by lowering a bucket
into the river from the bridge located immediately upstream of the gauging station.
These samples were then decanted into a one-litre sample container for subsequent
analysis.
Table 2-5: Mean values for types of substrate found at sample sites examined at different times. The
text in bold shows the dominant substrates to be stones and sand, which constitutes 60% of the total
substrate composition recorded in the river.
Site Boulders
(%)
Stones
(%)
Pebbles
(%)
Gravel
(%) Sand (%) Silt (%) Mud (%)
S1 6 37 7 5 30 15 3.5
S2 10 39 7 7 27 12 3
S3
S4 4 27 1 23 32 12 3
S5 14 43 3 6 28 8 0
S6 6 18 1.4 3.8 42 16 2
% total
composition 8 33 4 9 32 13 2
43
2.3.1 Benthic invertebrate sampling and analysis
All macro invertebrate samples collected through kick sampling and from
artificial colonisation samplers were preserved in 70% ethanol and counted in the
laboratory (Pawley, Dobson, & Fletcher, 2014). All macroinvertebrate specimens
apart from Oligochaeta were identified to family level using the taxonomic groups
used in the biological monitoring working party (BMWP) score.
2.3.1 Benthic invertebrate sampling
2.3.1.1 Kick-net sampling
Samples were collected using a 1mm mesh hand net by the three-minute kick
net sampling technique outlined in the Water Framework Directive, UK policy report
(UK Technical Advisory Group 2008). A one-minute manual search was also carried
out to collect benthic invertebrates that could have been missed through kick
sampling.
2.3.1.2 Inter-site comparisons using colonisation samplers
Colonisation samplers allow a comparison between sites as colonisation is
independent of the natural substrate (Czerniawska-Kusza 2004). Colonisation
samplers were therefore applied in this study to facilitate examination of the impact
of stream hydrology and substrate on the invertebrate community.
Sampler Description
The polypropylene pall ring colonisation samplers were based on the S.Auf.U
colonisation samplers described by Watton & Hawkes (1984). The pall rings were
strapped together in one position using plastic ties to form the colonisation sampler.
The samplers were attached at the base to a one-mm mesh size white polyester
netting to trap the benthic macro-invertebrates which are defined as those that are
retained on a 1mm mesh (Tagliapietra & Sigovini 2010) (Figure 2-4). The samplers
were anchored to two house bricks joined together (21 cm x 20 cm). They served to
44
weigh down the sampler in the river which was attached to a rope. The surface area
available for colonisation was 0.042m².
Figure 2-4: Invertebrate colonisation sampler mounted on a brick
Sampling strategy
Two artificial colonisation samplers were positioned on the river bottom at
sites S2 and S6 (see Figure 2-4). Site S2 had lower concentration of suspended solids,
low discharge and nutrient concentration. Also, S2 was upstream of the main
operational WwTWs and the substrate was composed of a mixture of stones, gravel
and sand. Site S6 had a higher concentration of suspended solids, higher nutrient
concentration, had a higher discharge and is located below the WwTWs. Site S6 had a
sandier substrate compared to S2. The colonisation samplers were left in the river for
a four month period from September 2014 to December 2014 and removed at 30 day
intervals. The 30-day period is suitable for the development of a representative
community of organisms (Weber 1973; Meier et al. 1979).
2.3.2 Spatial and Statistical analysis
The sub-catchment areas were determined using the package Terrain Analysis
System, GIS (Lindsay 2005) and a 50m (horizontal) and 0.1m (vertical) digital terrain
model (Centre for Ecology and Hydrology DTM). Statistical analysis were carried out
45
using Microsoft Excel 2013, SPSS (IBM, 2013), GraphPad Prism version 6 and
PRIMER 6 (Clarke & Warwick 2001). Differences between the physico-chemical
variables and the biotic indices (BMWP, ASPT) at each site were analysed using One-
way Analysis of Variance (ANOVA) on the assumption of data homogeneity and
normal distribution for each category of independent variable Pearson correlation
analysis was used to investigate how the various metrics changed and how these
variables were related to pollution impact
Principal component analysis (PCA) based on a correlation matrix between
samples was used to analyse physico-chemical variables. The first few PCs allow an
accurate representation of the true relationship between the samples in the original
high dimensional space as summarised by the percentage variation explained (Eigen
values). All the datasets were standardised in order to obtain comparable scales
(Clarke & Warwick 2001).
Non-metric multidimensional scaling (nMDS) were used to test for similarities
in benthic invertebrate abundance. The computation of similarity was performed
using the triangular matrix generated through Bray-Curtis similarity test (Clarke &
Warwick 2001).
Differences in benthic macroinvertebrate composition among sample sites and
between the sample seasons were analysed using the similarity percentages
(SIMPER) routine. The abundance data was square root transformed in order to give
more weight to abundance in comparison species. This was followed by the Bray-
Curtis similarity test which was employed among all pairs of samples and seasons to
describe assemblage similarity (Clarke & Warwick 2001).
The BIOENV procedure (Clarke & Warwick 2001) was used to select
environmental variables (EVs) that best explained benthic invertebrate community
patterns by maximising a rank correlation between their respective resemblance
matrices. Data were square root transformed and normalised to allow comparison at
46
the same scale. The weighed Spearman rank correlation coefficient (ρ) between the
physico-chemical and benthic invertebrate community similarity matrices was the
basis for this procedure. The physico-chemical with the largest ρ was taken to
identify the best match with the benthic invertebrates. Multivariate analysis were
performed using the PRIMER-6 software package (Clarke & Warwick 2001),
Statistical package for social sciences (SPSS) for the analysis of variance (ANOVA).
Particle size- flow distribution: The particle size distribution and the flow
conditions under which sediment was eroded, transported or deposited, was
estimated using the Hjulström-Sundborg Diagram (Figure 2-5) described by Earle
(2015).
Figure 2-5: The Hjulström-Sundborg diagram showing the relationship between particle size and
flow velocity. It shows the tendency of the sediments to be eroded, transported or deposited.
(Source: Earle,2015).
2.4 Water Quality Standards
2.4.1 WFD Standards
Under the Water Framework Directive, river chemistry is classified as shown
in Table 2-6 and includes altitude of the river as a surrogate variable for river
gradient and hence one of the natural characteristics that might influence ecological
47
communities. In addition the biota of fast flowing and hence high altitude rivers are
more vulnerable to organic pollution and this is reflected in the different standards
for rivers above and below 80m (UKTAG Water Framework Directive 2013). The
WFD classifies PO4-P from “High” to “Poor” for altitude either less than or greater
than 80m (WFD-UK Technical Advisory Group, 2012). River Medlock has an altitude
between 31 and 376m AOD according to the National Rivers Flow Archive.
Table 2-6: Classification of river chemistry according to the WFD at 90%ile
Variables High Good Moderate Poor
BOD (mgLˉ¹) (Altitude < 80m) 4.0 5.0 6.5 9.0
BOD (mgLˉ¹) (Altitude > 80m) 3.0 4.0 6.0 7.5
Ammonia-N (mgLˉ¹)
(Altitude < 80m) 0.6 0.6 1.1 2.5
Ammonia-N (mgLˉ¹)
(Altitude > 80m) 0.2 0.3 0.75 1.1
DO (% saturation) >80 79 64 50
PO4-P (mgLˉ¹)
(Altitude </> 80m) 0.05 0.12 0.25 1.0
2.4.2 Classification of Invertebrates
The Biological Monitoring Working Party Score (BMWP) is a method used in
the assessment of rivers with reference to the freshwater aquatic families (except
oligochaetes which are identified to class) based on the taxonomy of Maitland (1977)
and BMWP (1978). Families that are very sensitive to sewage pollution receive scores
of 10 and the most tolerant families receive a score of 1. Thus, the sum of the total
scores from the samples collected will determine the category to which the river is
classified as shown in Table 2-7. The average score per taxon (ASPT) is the ratio of the
BMWP Score to the number of scoring taxa (N): BMWP Score/N and is less
dependent on sampling effort. In this study, the BMWP score and Average score per
taxa (ASPT) were calculated in each instance to allow comparison with earlier work.
48
Table 2-7: BMWP Score, ASPT and interpretation (Hawkes 1997)
BMWP Score ASPT Category Interpretation
0-10 ≤3.9 Very poor Heavily polluted
11-40 4.0 - 4.9 Poor Polluted or impacted
41-70 5.0 -5.9 Moderate Moderately impacted
71-100 6.0 - 6.9 Good Clean but slightly impacted
>100 > 9 Very Good Unpolluted/unimpacted
However, the BMWP which was formerly used for water quality status
classification is being replaced with a new measure called the Whalley, Hawkes,
Paisley & Trigg (WHPT) metric which aligns with the requirements (Article 8; Section
1.3 of Annex II and Annex V) of the WFD (2000/60/EC). The River Invertebrate
Classification Tool (RICT) (Paisley et al. 2014) is a model used to contextualize WHPT
scores by using a model to predict site-specific reference values and provide a WFD
compliant probabilistic classification. WHPT was designed to detect organic
enrichment as well as other stressors to the invertebrates Therefore WHPT relates the
response of invertebrate taxa to organic enrichment using a ‘pressure sensitivity
score’ (PSs). The PSs is the sum of the PSs assigned to each taxon present in a single
sample from a single season (WFD-UTAG, 2008).
There are two differences between the original BMWP and the new WHPT.
The first is that WHPT considers numerical abundance (Paisley et al. 2014) as shown
on Table 2-8. Therefore increasingly abundant intolerant taxa attract a higher score
(e.g. Perlidae: AB1, 12.6; AB4, 13.0) and the reverse is the case for low scoring tolerant
taxa (e.g. Glossiphonidae: AB1, 3.4; AB4, 0.8). Secondly, the BMWP is based on
analysis of 82 taxa whereas the WHPT is based on 106 taxa so its sensitivity is slightly
greater.
49
Table 2-8: WHPT (Whalley & Hawkes, Paisley and & Trigg) logarithmic abundance categories
Abundance category Numerical Abundance
AB1 1-9
AB2 10 – 99
AB3 100 – 999
AB4 >1000
The new metric comprises of WHPT NTAXA (sum of the number of different
taxa contributing to the assessment from the same sampling site) and WHPT ASPT
(average score per taxon) which is applied as an abundance weighted metric AB (AB
= abundance related pressure sensitivity score for each taxon contributing to the
assessment). Both metrics are assessed separately and then combined in a “worst of”
approach to provide the overall invertebrate classification.
The WHPT ASPT is applied as abundance weighted metric (Table 2-8)
calculated as WHPT Classification: Count/Abundance category/Score; WHPT ASPT =
Sum AB/WHPT NTAXA. Therefore, observed value of ASPT = PSs ÷ NTAXA. The
observed value is then converted to bias-corrected values. Bias correction is estimated
for the value of ASPT for taxa missed because of sample sorting and identification
errors by using the equation: Estimated ASPT of missed taxa = 4.29 + 0.077x observed
value of NTAXA where the observed value of NTAXA is the value prior to bias
correction.
Therefore, in order to determine the biological status of the river based on the
WFD criteria, the ASPT of the samples observed (Obs) is divided by the predicted
(Pred) pristine condition score using the RICT statistical model with the WFD setting.
The RICT model therefore provides a classification Ecological Quality Ratio (EQR)
and an estimate of the probability of the result belonging to any of the WFD classes as
shown in Table 2-9 for both metrics based on observed data (UKTAG), 2014). EQR =
Observed/Predicted; EQR values close to one therefore indicate invertebrate
communities close to the natural state, those near to zero indicate a high level of
pollution or disturbance.
50
Table 2-9: EQR for WHPT-ASPT and WHPT-NTAXA
WHPT
ASPT- EQR NTAXA EQR
High/Good 0.97 0.80
Good/Moderate 0.87 0.68
Moderate/Poor 0.72 0.56
Poor/Bad 0.59 0.47
51
Chapter 3 LONG-TERM WATER
QUALITY OF A HEAVILY URBANISED
RIVER: A CASE STUDY OF RIVER
MEDLOCK, GREATER MANCHESTER,
UK
Abstract
This paper examines the water quality and ecology of an urbanised 5km reach of
the River Medlock catchment for over a decade between 2000 and 2013. The aim
was to identify the main challenges to achieving good water quality, including
compliance with European Union Directives. Dataset were obtained from the
Environment Agency for physico-chemical parameters and benthic
macroinvertebrates. Three locations were examined: one upstream and one
immediately downstream of the single operational wastewater treatment works
(WwTW), plus a third 5km further downstream and 6.5km above the confluence
with the River Irwell. The tributary Lord’s Brook which brackets the first two sites
was assessed to determine its status and impact on the Medlock. The WwTW was
the major source of PO4-P and although concentrations reduced with time remains
much higher than the 0.1mgPLˉ¹ standard indicative of good water quality. Other
variables including ammonia and BOD were generally within standard
requirements for EU Rivers. Despite the generally good water quality, biotic indices
(BMWP, ASPT and EQR) indicate the river to be moderately polluted which
suggests the impact of non-sewage related pollution or some other stressors. On the
basis of the WFD standard for PO4-P and ecological quality, the river remains
polluted and has not markedly improved over the period 2000-2013.
Key words: River Medlock, urban water quality, waste water treatment works,
benthic macroinvertebrates
52
3.1 Introduction
The River Medlock in Greater Manchester, UK is a mixed use catchment with
two-thirds of the catchment being classified as “highly modified” by the UK
Environment Agency (EA). The modified section of the river starts from one of the
river’s tributaries, Lumb Brook until its confluence 6.5km downstream with the River
Irwell. The lower part of the Medlock drains a highly urbanised catchment and
receives effluent from a major waste water treatment work (WwTW).
For 200 years, the rivers of Greater Manchester, including the Medlock,
deteriorated in quality due to increases in industrialisation and urbanisation. Serious
flooding in the mid-Nineteenth Century led to canalisation, culverting, and
installation of weirs. These activities increase the flow of flood water and are likely to
damage the ecology of the river (Williams et al. 2010).
De-industrialisation and improvements to wastewater treatment resulted in
some recovery and according to a 2007 report by Manchester City Council (2007)
river quality had improved considerably, including reduced sewage contamination
and nutrient concentrations compared to the 1990s. Water quality improvements
resulted from improvements to the WwTWs plus a reduction in NO3-N following
improved farming and agricultural practices (Environment Agency, 2007; European
Union 2010). However, the continued high PO4-P concentration in the river is linked
to the discharge from the WwTW and CSOs as reported by The Irwell Catchment
Pilot Steering Group (James et al. 2012). High PO4-P and the effect of physical
modifications degrading the benthic invertebrate community are the major reasons
why the river had not met the requirements of the Water Framework Directive
(WFD)(Council of the European Union 2000; European Environment Agency 2015).
To manage a river with a history of pollution such as the Medlock and hence
to achieve compliance with the WFD requires knowledge of long term water quality
plus qualitative and quantitative information on point and diffuse sources of
53
pollution. Until now, no published study has been carried out to establish the long-
term changes in river quality of the River Medlock.
The WFD requires all waters to reach ‘good ecological status’ by 2027 so the
results from this study can contribute to identifying reasons for non-compliance by
the Medlock. The overall aim of this study was to assess the efficacy of the water
management measures on the river’s quality over time from EA’s long term water
quality dataset and suggest potential improvements.
Objectives
1. Using EA datasets, assess the long-term water quality dynamics and
determine the abundance and diversity of the benthic macroinvertebrates
community.
2. To determine the major source of pollution to the river.
3. To determine the status of the river with respect to the Water Framework
Directive (WFD).
3.2 Methodology and Approach
3.2.1 Study area
The River Medlock (Figure 3-1) rises in the Pennines that surround Strinesdale
to the north east of Oldham in Greater Manchester (National Grid Reference: SD
95308 05431). The river has a catchment area of 57.5km2 and the major tributary
within the study area is Lord’s Brook. The Medlock catchment is heavily urbanised
(40%), including light industry that extends from the south Pennine to Manchester
(CEH, 2014). The Medlock flows for 22km through steep sided woodlands and
continues in a south westerly direction to discharge into the River Irwell immediately
downstream of Manchester city centre (SJ 85781 97858). The river receives episodic
discharges from more than fifty CSOs (United Utilities, personal communication,
2014).
54
The river has two WwTWs: a major treatment works at Failsworth (SJ8982
9979) and a smaller treatment works at Park Bridge (SD 9394 0253). The Failsworth
WwTW operates primary, secondary (biological filters) and tertiary (Nitrifying
trickling filters) treatments while the treatment works at Park Bridge (SD 9394 0253),
operates solely by the secondary rotating biological contactors. While, the
Environment Agency permits the Failsworth WwTW to discharge effluent volume
limit of 16,000 cubic metres per day, with a maximum dry weather flow of 6180
m³d¯¹, the Park Bridge WwTW has a markedly lower discharge permit volume limit
of 20 cubic metres per day at a rate of 0.0007m³s¯¹ (0.7 Ld¯¹) (Environment Agency,
personal communication, 2014).
Figure 3-1: River Medlock, Greater Manchester showing catchment and urban settlements. Insert:
Map of the UK with location of Greater Manchester. (Source: ESRI, GIS)
55
3.2.2 Study sites and data collection
The EA river sites were selected based on their relative proximity to the main
WwTW, the location of CSOs on the river and the extent of datasets collected from
such locations. The study sites (as shown on Figure 2-1) are S0 - Medlock Vale
(SJ9032899673); S4-Millstream Lane (SJ8909799436) and S6- Pin Mill Brow
(SJ8567297756). The major tributary along these three study sites -- Lord’s Brook
(SJ8987999773) is 0.8km upstream of the main WwTW was also included in the
assessment. The EA collected samples from the river monthly, with varied sampling
frequencies at each site between the period 2000 and 2013.
Site 0 (S0) is 3km above the main WwTW, 8km downstream of a high
discharging CSO plus others discharging less frequently and the small WwTW at
Park Bridge (SD 9394 0253). It is located within the Medlock Vale Park.
Site 4 (S4) is located 0.6km downstream of the Failsworth WwTW and hence
receives effluent from the WwTW, and also from Lord’s Brook. The river also runs
through an urbanised area for 0.1km and then Clayton Vale local nature reserve. In
2014, the brick lining at Clayton vale, created after the flooding in 1872, was removed
together with weirs and other barriers as part of the EA project of restoring the river
back to its natural state (Manchester City Council 2014).
Site 6 (S6) is located 5km downstream of the Failsworth WwTW within a
highly urbanised area and, because of its shallow slope collects silt and debris. The
EA therefore installed a debris screen to collect debris and hence reduce impediments
to flow during flood conditions. The river flows underneath the A6010 in Manchester
city centre before the confluence with the River Irwell.
In order to identify CSO infrastructure discharging into the River Medlock,
effluent discharge licences were obtained from the EA in 2013. This information was
complemented by the water companies’ provision of specific points of discharge in
the study areas, simulation data on spill analysis, including frequency of spills per
56
year, duration and volume of CSO discharge per year. The information extracted for
the study showed that twenty-nine CSOs were located within the study areas.
The sub-catchment area for each sampled location, their sizes relative to the
EA’s gauging station and their distance from the river’s source at Strinesdale
reservoir is summarised on Table 3-1.
Table 3-1: River Medlock catchment information
Station Name Station
No.
Catchment
area (km²)
Catchment
area as % of
total
Distance
(km) from
source
Discharge
(m³s¯¹)
Altitude
(m)
Lord’s Brook 2.58 4.5 12.07 0.01 68.16
Medlock Vale S0 37.2 65 12.23 0.77 74.82
Millstream Lane S4 43.9 76 13.04 0.91 66.81
Pin Mill Brow S6 54.4 95 17.40 1.14 42.39
3.2.3 Water quality and ecological parameters
Monthly physico-chemical datasets of biochemical oxygen demand (BOD),
suspended solids, conductivity, temperature, dissolved oxygen, nitrate (as NO3-N)
and phosphate (as PO4-P) were obtained from the Environment Agency for the three
sites and tributary for the following periods: S0 and S4 from 2000 to 2004 and 2010 to
2013 (no data from 2005 to 2009) while S6 had a complete dataset from 2000 to 2013.
Datasets for Lord’s Brook was obtained from 2000 to 2006 and, had been included in
the study in order to assess its contribution to pollution of the Medlock and receives
discharge from CSOs. The water quality was analysed in accordance with the
Standard Committee of Analysts Publications (EA Standard Committee of Analysts
Publications, 2011) and APHA (1989). Benthic macroinvertebrates were sampled by
the EA twice yearly during spring and autumn. Benthic invertebrate data was only
available at S6, and only between 2000 and 2008 and for Lord’s Brook between 2000
and 2006. Benthic macroinvertebrates were identified to family level with the
exception of Oligochaeta which was identified to class in accordance with the
requirements of the UK’s Biological Monitoring Working Party (BMWP) score
57
(Wright, Moss, Armitage, & Furse, 1984). A summary of the EA sampling regime is
shown on Table 3-2.
The assessment of the macroinvertebrates based on the spring and autumn
seasons was carried out to apply the results to the River Invertebrate Classification
Tool (RICT) in order to assess the environmental quality of the river. This model
compared observed information to an expected “pristine” condition expected of the
river and classified on the basis of environmental quality bands from A = “Very
good” to F = ”Very bad”. The river is therefore classified with the Environmental
Quality Index on the basis of the BMWP score and the ASPT of the sample obtained
for the two seasons.
Table 3-2: Datasets obtained from the EA
Date/site S0 S4 S6 Lord's Brook
2000-2004 √ √ √ √
2005-2009 n/a n/a √ 2005-2006
2010-2013 √ √ √ n/a
Dissolved oxygen (% saturation) √ √ √ √
Suspended solids (mgLˉ¹) √ √ √ √
Conductivity (µScmˉ¹) √ √ √ √
NO3-N (mgLˉ¹) √ √ √ √
PO4-P (mgLˉ¹) √ √ √ √
Benthic macroinvertebrates n/a n/a √ (between 2000
and 2008) √
Continuous discharge data was available only at the EA’s Gauging station
0.5km below S6. Therefore river discharge was measured in cubic metre per second
(m³s-¹) for each study location and this was obtained by estimation using a simple
linear regression equation which correlated the catchment area (km2) with mean
discharge (Q) for twenty-eight rivers within Greater Manchester including the River
Medlock (National Rivers Flow Archive, 2014). The linear regression equation y =
0.0218x - 0.0422 which indicated a strong correlation between the catchment areas
and river discharge (R² = 0.873) was used to then estimate the discharge at the sites in
relation to the sub-catchment areas of S0, S4, S6 and Lord’s Brook. Discharge was
determined in order to estimate the nutrient load entering the river at each sub-
catchment area.
58
The ratio of mean NO3-N to mean PO4-P concentration was determined for all
data obtained from the EA in order to determine the impact of the Waste water
Treatment Works (WwTW).
3.3 Results
3.3.1 Physical and chemical variables
3.3.1.1 River Medlock
Figure 3-2A-H show the pattern of dissolved oxygen, conductivity, suspended
solids, BOD, ammonia-N, NO3-N and PO4-P at sites S0, S4 and S6. The mean and
standard error of the mean were calculated for the three sites on the basis of available
data i.e. from 2000 to 2004 and from 2010 to 2013 for S0 and S4 and from 2000 to 2013
for S6. pH had an overall mean of 7.9 + 0.3 (SD) and was within the normal range for
rivers (WFD-UK Technical Advisory Group (UKTAG) 2012) and hence not
suggestive of pollution. Temperature had an overall mean of 10.37+ 0.64. pH and
temperature were not analysed further since there was no difference between sites or
over time.
Over the 2000-2013 period of study, the average discharge of the Medlock was
0.73m³sˉ¹ (Figure 3-2 A). A one-way analysis of variance indicated there was no
significant (p >0.05) difference with year, but larger variations were found in 2000 and
2008 due to the higher than average rainfall in February, from September to
November in 2000 and, in January, and from July to November in 2008.
Between 2000 and 2004, & 2010 and 2013, higher dissolved oxygen (>90%) was
recorded in the river (Figure 3-2B), indicating very good level of oxygenation. While
there was no difference between the sites for the average conductivity, suspended
solids, BOD and ammonia-N (Figure 3-2C-F) in the river, a significant (p<0.05)
difference between the three sites was found for NO3-N and PO4-P concentration
(Figure 3-2G-H, Table 3-3). The peak concentration (mgLˉ¹) observed for BOD,
ammonia-N and suspended solids in 2001 (25/9/2001) at S6, suggested a pollution
59
incident. Communication with the EA revealed that a significant pollution incident
had been reported to the EA two days earlier on the 23/09/2001. The pollution was
caused by discharge of organic chemicals/products into the river about three miles
upstream of S4 (NGR: SD 90699 03183). Following this date, readings were found to
be within the normal range of concentration by the EA samplers (EA, personal
communication 2016). Elevated concentration of suspended solids recorded at the
three study sites in 2001 (18/10/2001) and 2012, indicates the influence of high
precipitation and therefore elevated discharge levels (EA, personal communication
2016). The highest total precipitation recorded for these elevated concentrations
occurred in October, 2001 and during January and April, 2012.
NO3-N and PO4-P were highest at S4 and S6. One-way analysis of variance
(ANOVA. post Hoc, LSD) showed a significantly (p<0.05) lower concentration at S0
for the periods assessed (Table 3-3). Between 2000 and 2013, S6 showed a decline in
the concentration of PO4-P from 0.77 mgLˉ¹ to 0.60mgLˉ¹, which indicated “poor”
quality (Table 3-4). Although NO3-N was lower than the recommended General
Quality Assessment, the values of PO4-P were higher than the recommended level
for good ecological status of 0.1PO4-PmgLˉ¹ (European Directives 91/676/EEC;
91/271/EEC; 96/61/EEC; 2000/60/EC). A peak in ammonia-N was observed in 2001
for S6 (Figure 3-2F) with mean concentration recorded at 1.26mgLˉ¹. This was
classified as “poor”. Although BOD values reached 5.23mgLˉ¹ (Figure 3-2E) for the
same period and location, it was within the WFD requirement.
All other measured variables complied with the WFD standards (UKTAG
Water Framework Directive 2013) during the period of measurement. However, the
concentration of suspended solids was higher than the recommended 25 mgLˉ¹ of
the EU Freshwater Fisheries Directive (78/659/EEC & 2004/44/EC).
60
Figure 3-2: Mean± SEM annual water quality at the three sites S0, S4 and S6 from monthly samples
on the River Medlock, 2000-2004, 2005-2009 and 2010-2013. A. Discharge; B. DO; C. Conductivity; D.
Suspended solids; E. BOD; F. Ammonia-N; G. NO3-N; H. PO4-P. The dotted lines B to H represent
WFD standard requirement for “good ecological status” for surface water quality, Freshwater
Altitude > 80m
Altitude <80m
Altitude > 80m
B
C D
E F
G H
Altitude >
80m
61
Fisheries Directive for Suspended solids (D). In some cases the WFD standards vary with altitude
</> 80 m AOD.
Table 3-3: One way ANOVA to compare S0, S4 and S6 for variables measured from 2000 to 2004 and
from 2010 to 2013. No. of samples at S1= 9, S2 = 9; S3=14
S/No Variables 2000 - 2004
(S0, S4 & S6)
2010 - 2013
(S0, S4 & S6) Comments
1 Dissolved
oxygen (%)
F2,12 =11.32,
p < .05
F2,9 = 0.77,
p > .05 High DO levels
2 Conductivity
(μScmˉ¹)
F2,12 = 0.721,
p > .05
F2,9 = 0.09,
p > .05
No difference
between sites
3
Suspended
solids
(mgL¯¹)
F2,12 = 0.50,
p > .05
F2,9 = 0.09,
p > .05
No difference
between sites
4 BOD (mgL¯¹) F2,12 = 1.85,
p > .05
F2,9 = 3.71,
p > .05
No difference
between sites
5 Ammonia-N
(mgL¯¹)
F2,12 = 1.56,
p > .05
F2,9 = 5.93,
p < .05
No difference
between sites
between 2000 and
2004. Higher at S6
between 2010 and
2013
6 NO3-N
(mgL¯¹)
F2,12 = 32.40,
p < .01
F2,9 = 22.69,
p < .01
Highly different at
the sites
7 PO4-P
(mgL¯¹)
F2,12 = 111.27, p
< .01
F2,9 = 28.97,
p < .01
Highly different at
the sites
Table 3-4: Pearson correlation of variables with time at S6 from 2000 to 2013. Number of samples at
S6 = 14
S/No Variables
1 Dissolved oxygen (%) p < .05 r = 0.57
2 Conductivity (μScmˉ¹) p < .05 r = 0.86
3 Suspended solids (mgL¯¹) p > .05 r = - 0.02
4 BOD (mgL¯¹) p > .05 r = - 0.30
5 Ammonia -N (mgL¯¹) p > .05 r = - 0.42
6 NO3-N (mgL¯¹) p > .05 r = 0.84
7 PO4-P (mgL¯¹) p < .05 r = - 0.72
62
The substrate (Figure 3-4) recorded during the spring and autumn seasons at S6,
between 2000 and 2008 showed that the river was largely composed of pebbles
(≤64mm, 33% to 48%), gravel (≥2mm; 30% to 32%) and sand (≤ 1mm, 19% to 35%).
Co
bb
les
Pe
bb
les
Gra
ve
l
Sa
nd
Sil
t
0
2 0
4 0
6 0
S u b s t r a te T y p e s
%S
ub
str
ate
at
S3
S p rin g A u tu m n
Figure 3-3: Types of substrate found at the River Medlock at S6 during spring and autumn seasons
for 2000 and 2008.
3.3.1.2 Lord’s Brook
Over the period 2000 and 2006, the dissolved oxygen levels recorded at the
Brook were high (>80%), as in the river. Similarly, Brook pH and temperature were
similar to the records on the river. Although conductivity declined from a maximum
value of 600µScm¯¹ in 2001 to 468µScm¯¹ in 2004, the levels recorded were similar to
that in the river (Figure 3-4A).
Average concentration of suspended solids complied with the standards of the
Freshwater Fisheries Directive of 25 mgLˉ¹. However, a peak value of 38 mgLˉ¹ was
recorded in year 2000 and linked to above average precipitation as found in the river
(Figure 3-4B). All other parameters analysed at Lord’s Brook were lower than the
WFD requirements, including BOD and ammonia-N (Figure 3-4 C & D); the
exception was PO4-P (Figure 3-4F) and which recorded the highest concentration in
63
2005 (0.79mgLˉ¹). There was no reported incident to the EA to account for the peak
concentrations of BOD and ammonia recorded in 2002. (EA, personal communication
2016).
Figure 3-4: Mean (± SEM) annual water chemistry parameters at Lord’s Brook between 2000 and
2006. (A) conductivity; (B) suspended solids; (C) BOD; (D) Ammonia-N; (E) NO3-N; (F) PO4-P. The
dotted horizontal line represents the WFD standards concentration on the basis of altitude, except
suspended solids which is based on the EU Freshwater Fisheries Directive.
3.3.1.3 Relationship between NO3-N and PO4-P concentrations
A correlation between of NO3-N and PO4-P often indicates the WwTWs as a
contamination source (Jarvie et al. 1998). A correlation analysis of NO3-N and PO4-P
data indicated a significant (p<0.05) positive correlation at S4 with r = 0.791, n =100
and S6 (r = 0.679, n =168) while there was no correlation at S0 (Figure 3-5). This
analysis suggests that the WwTW is the likely source of PO4-P discharge to the two
downstream sites.
64
0 1 2 3 4 5
0 .0
0 .2
0 .4
0 .6S 0
N O 3 -N (m g L- 1
)
PO
4-
P (
mg
L-
1)
0 2 4 6 8 1 0 1 2 1 4
0 .0
0 .5
1 .0
1 .5
2 .0
2 .5
3 .0
S 4
N O 3 -N (m g L- 1
)
PO
4-
P (
mg
L-
1)
0 2 4 6 8 1 0 1 2 1 4 1 6
0 .0
0 .5
1 .0
1 .5
2 .0
2 .5
3 .0
S 6
N O 3 -N (m g L- 1
)
PO
4- P
(m
gL
-1)
Figure 3-5: Relationship between average concentration of PO4-P (mgL¯¹) and average NO3-N
(mgL¯¹) at site S1 and S2 (from 2000 to 2004; 2010 to 2013) and from S3 (2000 to 2013).
The average NO3-N / PO4-P ratio at the three sites further indicated the
influence of the point sources on the river as shown on Table 3-5 and,
Table 3-6 for Lord’s Brook. The highest N/P ratio was found at S0 (>20) while
S4, S6 and Lord’s Brook had ratios which were less than 11 for the periods. N/P ratios
which are typically less than 11 confirm the influence of the WwTW and as found
elsewhere by Jarvie et al. (1998). It indicated periods when the nitrogen was limiting
resulting from greater availability of the PO4-P from sewage inputs, followed by the
influence of urban and industrial activities. The low ratio recorded at Lord’s Brook
suggested the influence of episodic pollution from CSOs since the brook was located
above the treatment works.
65
Table 3-5: Comparison of average concentration of NO3-N/PO4-P ratio from 2000-2004; 2010-2013 at
the Medlock (S0, S4 and S6).
Period S0 S4 S6
NO3-N
S0
PO4-P
S0
N/P
S0
NO3-N
S4
PO4-P
S4
N/P
S4
NO3-N
S6
PO4-P
S6
N/P
S6
2000 1.99 0.14 14.21 3.75 1.02 3.68 3.34 0.77 4.34
2001 1.71 0.09 19.00 4.29 0.94 4.56 3.75 0.89 4.21
2002 1.80 0.08 22.50 5.23 1.02 5.13 3.94 0.58 6.79
2003 1.50 0.11 13.64 6.07 1.06 5.73 5.00 0.92 5.43
2004 1.64 0.13 12.62 5.46 1.02 5.35 3.89 0.65 5.98
2010 1.07 0.04 26.75 7.36 1.00 7.36 4.77 0.56 8.52
2011 1.24 0.06 20.67 6.49 0.84 7.73 4.78 0.57 8.39
2012 1.20 0.06 20.00 4.03 0.50 8.06 3.07 0.36 8.53
2013 1.14 0.05 22.80 5.03 0.76 6.62 4.19 0.60 6.98
Table 3-6: Average NO3-N/PO4-P ratio at Lord’s Brook, 2000-2006.
Mean N/P ratio Lord’s Brook 2000-2006
NO3-N PO4-P N/P ratio
2000 2.18 0.55 3.99
2001 2.84 0.52 5.44
2002 2.54 0.61 4.19
2003 1.86 0.42 4.43
2004 1.50 0.41 3.66
2005 2.44 0.79 3.11
2006 2.43 0.42 5.76
3.3.1.4 Summer vs Winter water chemistry
Average summer PO4-P and NO3-N concentrations showed that the nutrients (
Figure 3-6A & B) were mostly elevated in the summer especially for S4 and S6 as few
winter concentrations were observed at S0. Higher summer nutrient concentration
points to the effectiveness of the treatment works during the low discharge at
summer months when dilution is reduced. Average concentrations of ammonia-N
and suspended solids were highest during winter months which suggests that a rise
in precipitation increases run-off plus releases from CSOs. There is no clear pattern
with BOD (mgL¯¹) which suggests the influence of both point and diffuse pollution
sources on the river.
66
Figure 3-6: Average winter vs summer water chemistry (A) PO4-P;(B) NO3-N; (C) Ammonia-N; (D)
BOD; (E) Suspended solids. Each icon, S0 (blue); S4 (red) and S6 (green) per
site represents a year
A B
C
D E
67
3.3.2 Benthic macroinvertebrates
3.3.2.1 Invertebrate abundance and BMWP scores
The abundance of benthic macroinvertebrates at S6 (Figure 3-7) between 2000
and 2008 showed the dominance of taxa in the following order: Oligochaeta >>
Baetidae > Chironomidae > Simulidae. The number of taxa identified at S6 and at
Lord’s Brook was 21 and 25 respectively. At Lord’s Brook (
Figure 3-8), a similar pattern of distribution to S6 was identified between 2000
and 2006. The dominance of tolerant benthic invertertebrates was indicated by the
low BMWP scores and ASPT. Overall the BMWP score and ASPT placed the river at
S6 in the “polluted” category while Lord’s Brook was “moderately polluted”. This
pattern agrees with the Environmental Quality Ratio (EQR) for the same period with
ASPT= 0.66, Number of taxa = 0.44 which placed the Medlock at the “moderate”
pollution boundary for benthic macroinvertebrates (EA Data, 2016).
68
Figure 3-7: Benthic macroinvertebrate abundance at S6 between 2000 and 2008 and at Lord’s Brook
between 2000 and 2006.
69
Figure 3-8: BMWP scores and ASPT at S6 and at Lord’s Brook
3.4 Summary
i. On the basis of the WFD classification, the physico-chemical variables except
PO4-P were “good”. High PO4-P concentration placed water chemistry in the
“poor” category.
ii. The major change in the river was observed between S0 and S4 which are
respectively upstream and downstream of the WwTw. The treatment works
significantly increase PO4-P concentration at S4.
S6
70
iii. Summer and winter results showed the effect of episodic pollution with a
deterioration in water quality during high winter precipitation.
iv. The benthic invertebrate community was indicative of moderate pollution as
indicated by the EQR and the BMWP scores.
3.5 Discussion
The overall aim of this study was to assess the efficacy of water management
measures on the river Medlock’s quality and ecology over time from Environment
Agency’s long term water quality dataset. This study encompassed more than a
decade between 2000 and 2013 and allowed an assessment of the impact of
regulatory policies specifically the European Water Framework Directive (WFD,
2000/60/EC).
Apart from NO3-N and PO4-P concentration which was higher at the
downstream sites (S4 and S6) of the WwTW, other physico-chemical analysed in
this study were similar at all sites. BOD was low throughout the study period
indicating that sewage was effectively treated at the WwTW and there was little
contribution from CSOs. Information obtained from the EA indicated significant
efforts to reduce BOD concentrations in order to comply with the GQA (EA,
personal communication 2016).and this has been achieved throughout the period of
this study. Also, low BOD was shown by the high DO which in combination with
the well-mixed water resulted in 80% saturation throughout the study period.
PO4-P concentration may have declined in the Medlock from 2000 to 2012, but
would need to be justified with further analysis to ascertain the level of reduction.
However, the concentration of PO4 is still often higher than the 0.1PO4-PmgLˉ¹ WFD
limit, especially in downstream urban areas of rivers such as the Medlock where this
study showed concentration to average 0.5 mgLˉ¹. In common with many other rivers
(Howell, 2010), a major source phosphorus in the Medlock is from the (single)
WwTWs due to the lack of PO4-P removal from the effluent (Neal et al. 2008). James
et al. (2012) showed that diffuse pollution is another reason for rivers in the Irwell
71
catchment failing to meet the legally required EU standards for phosphorus. As a
result of continued PO4-P pollution from agriculture and runoff, most UK Rivers
including the Medlock may not comply with the WFD until the next scheduled
deadline of 2027 (Priestley 2015).
There are no statutory targets for NO3-N concentrations in UK surface waters
under the WFD. However, the World Health Organization, (2007) states that NO3-N
concentration in surface water is normally low at between 0 –18 mgLˉ¹ but can reach
high levels as a result of agricultural runoff, refuse dump runoff or contamination
with human or animal wastes. Within the EU, NO3-N concentrations in rivers
declined by 0.8% each year over the period 1992 to 2012 following measures to
reduce NO3-N from agricultural land and improvements in wastewater treatment
(European Environment Agency 2015).
Although NO3-N mirrors PO4-P concentration, a higher summer concentration
as observed in the Medlock points to the effectiveness of the WwTW (Bowes et al.,
2015; Neal et al., 2005) which provides a constant effluent source and is less dilute in
the summer low-flow months. A similar pattern to the Medlock was reported in the
urban reach of the River Frome at Bristol (Bowes et al. 2009).
A summer-winter relationship showed that the concentration of ammonia-
and suspended solids increased during winter in the Medlock. Increased
concentration of ammonia could be linked to impact of CSOs (Mulliss et al. 1996;
Mullis et al. 1997) and high surface runoff (Martin, 1995) during the winter season
(Sigleo & Frick 2003) and Wang (2014) found high ammonia concentration during
rainstorms in the Harlem river, New York. However, the increased concentration of
ammonia is temporary as the River Medlock was highly oxygenated and hence
conversion to NO3-N is rapid. The elevated suspended solids concentration may be
also be linked to episodic storm events as was observed within the wider Irwell
catchment (APEM, 2007). The higher suspended solids concentrations recorded in the
Medlock in 2000 and at 2012, which are above the EU FFD standards, may be linked
72
to the increased rainfall during these years as indicated by the Meteorological Office
(Online archive of the UK Meterological Office).
Biotic indices analysed for BMWP scores and ASPT from 2000 to 2008 at S6
indicate the river to be “impacted” by pollution and “moderately impacted” at Lord’s
Brook between 2000 and 2006 according to Hawkes (1997). The possible reasons for
the low scores and degraded community could be linked to episodic discharge and
urban runoff which transports suspended solids, PO4-P and other materials into the
river (Paul & Meyer 2001; Walsh 2000). The benthic invertebrates were similar in the
river and tributary and were dominated by pollution tolerant taxa including
Oligochaeta, Baetidae, Chironomidae and Simulidae. Goodnight, (1973) describes
Oligochaete as normal members of the stream biota and therefore, the classification of
good and bad stream conditions will depend on their percentage contribution
relative to the total stream biota. If the contribution of Oligochaete falls between 60%
and 80%, this shows that the river was in bad condition and if < 60%, this would
indicate good condition. On the basis of a high contribution of Oligochaete which
constituted over 60% of the community in this study, the Medlock would fall under
the bad category. Therefore, a subsequent chapter will explore the relationship
between water quality and the degraded benthic invertebrate community.
3.6. Conclusion
There had been no change in water quality in the River Medlock with time
for the variables measured except for PO4-P. Although PO4-P declined, in particular
at the most downstream (and hence urbanised) site, the concentration directly
downstream of the treatment works exceeded WFD requirements. Thus, the
WwTWs was shown to be the major pollution source of PO4-P. As the
concentrations of other physico-chemical variables were conducive to a diverse
benthic invertebrate community, it is suggested that they were adversely affected
by high precipitation which causes increased discharge, runoff and episodic
73
pollution from CSOs. These changing conditions are likely to destabilise the benthic
fauna and hence cause their impoverishment.
It is therefore apparent that the River Medlock has not improved in water quality
and does not comply with both the requirements of the WFD for good chemical and
ecological status. The EA data provided an overview of the river quality and
revealed high concentrations of phosphorus, the next chapter will investigate the
PO4-P dynamics in the river in an attempt to identify and quantify the major
sources of this contaminant.
Acknowledgements
I am grateful for financial support from The National Open University of Nigeria.
Thanks to the Environment Agency, Warrington, UK for supplying the long-term
datasets used for this study.
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78
Chapter 4 SOURCES OF PO4-P IN AN
URBAN RIVER: COMBINED SEWER
OVERFLOWS VS WASTEWATER
TREATMENT WORKS
Abstract
This study examines concentrations and load of PO4-P in the urban River Medlock,
Greater Manchester UK. This river has a history of pollution from the industrial
revolution with significant contributions from wastewater treatment works (WwTW)
and combined sewer overflows (CSOs). Data was obtained from the Environment
Agency between 2000 and 2013, water samples were collected from the river every
fortnight from March 2013 to April 2014 and a short term high resolution data was
obtained at a single site over a period of four months. Concentrations of PO4-P in the
river significantly reduced over the last decade but still show high PO4 concentrations
>0.1mgLˉ¹ were observed, most commonly at all the sample sites below the WwTW
where amounts reached 0.57 mgLˉ¹. PO4-P load varied from 1.01kgha⁻¹yr⁻¹ to
3.17kgha⁻¹yr⁻¹ and, on the basis of load estimate, about 90% came from the single
WwTW discharge.
Keywords: PO4-P; River Medlock; PO4-P load; wastewater treatment works; Water
Framework Directive.
4.1. Introduction
The adverse effects of elevated PO4-P has been a major environmental problem
in water bodies for decades (Mainstone et al. 2000; Meybeck & Helmer 1989). These
problems have been linked to diffuse pollution from urban runoff, storm water
drains and combined sewer over flows (CSOs) in urban rivers by various authors
including Barco et al. (2008); Hatt et al. (2004) and from discharge from wastewater
treatment works (WwTWs) (Williams et al. 2010).
In the Northwest England, urban pollution affecting rivers was associated
with the industrial revolution of the 19th century (Manchester City Council 2007;
79
Burton 2003). In the 21st century, these pollution effects were associated with point
and diffuse sources in the river catchments (James et al. 2012). Following the
implementation of various European Union Directives (Urban Wastewater
Treatment Directive, Freshwater Fisheries Directive and Nitrate Directive) aimed at
reducing contaminants discharged to surface waterbodies, the concentration of
trace metals; NO3-N, ammonia-N and biochemical oxygen demand have reduced.
However, levels of PO4-P have remained elevated as reported by the European
Union Water Framework Directive (WFD) requirements (Council of the European
Union 2000).
Efforts to reduce the PO4-P concentration to less than the WFD standard of
0.1mgLˉ¹ have largely failed (Bowes et al. 2009; Neal et al. 2005;Jarvie et al. 2006)
due to contributions from WwTWs that lack tertiary treatment to remove this
element from the effluent stream. While WwTW has been identified as a pollution
source, the quantification of PO4-P from other sources including CSOs have yet to
be determined. CSOs are storage drains which receive both urban runoff and
wastewater and which are discharged into rivers during storms. Some of the effects
of CSO discharges are seen in increased chemical pollution including PO4-P plus
increased flow and discharge (Even et al. 2004; Wang 2014). In the UK,
approximately 31,000 CSOs have been reported to discharge into fresh running
watercourses (Marine Conservation Society 2011).
PO4-P concentrations from WwTWs (Bowes et al. 2005), other point sources
(Nyamangara et al. 2013; Hirsch 2012) and from agricultural land (Edwards &
Withers 2007; Edwards & Withers 2008) have been reported in the literature. There is
however a significant research gap in quantifying the contribution to PO4-P load of
CSOs and WwTW given that CSOs are only monitored in the case of localised
impacts on water quality and the biota (Marsalek et al. 2006). This study therefore
investigates the dynamics of PO4-P released to the largely urbanised lower River
Medlock by assessing long term load over different periods and discharge regimes
80
using past data from the Environment Agency and primary data collection. The
overall aim is to quantify the load of PO4 as P arising from the WwTW in order to
assess its contribution to the river. The questions to be addressed in the study are as
follows:
1. What is the relationship of PO4-P with discharge?
2. What is the spatial and temporal pattern of PO4-P concentration and flux?
3. What is the contribution to PO4-P load from, respectively, CSOs and the single
operational WwTW?
Study area: The River Medlock
The River Medlock catchment (Figure 1-5) is heavily urbanised, in particular
the lower reaches (CEH, 2014). The catchment area of the Medlock is 57.5 km2 and the
average rate of flow in the river is 0.8m³s¯¹ (CEH, 2014). The River Medlock rises in
the hills to the NE of Oldham in Greater Manchester (National Grid Reference
(NGR): SD 95308 05431). The Medlock flows for 22km through steep sided woodland
and continues in a south westerly direction to discharge into the River Irwell
immediately downstream of Manchester city centre (NGR: SJ 85781 97858). The study
area encompasses both less and more heavily urbanised areas and extends from Mill
Brow Bridge, 6.6km from the river’s source to the confluence with the River Irwell
17.4 kilometres downstream (Figure 2-2). This reach was also chosen as it
encompasses the single continuously operational wastewater treatment works at
Failsworth (NGR: SJ 89666 99802) plus twenty-nine CSOs (Personal Communication,
United Utilities, 2015). In the past, the river had a history of pollution especially from
industrial effluent, and inadequately treated sewage (Douglas et al. 2002; MacKillop
2012;Williams et al. 2010). Rees & White (1993) also attributed pollution to localised
storm events which led to frequent spill events from CSOs.
81
4.2. Methods
Phosphorus in the form of PO4-P was used throughout this study. This is the
bioavailable form of P utilised by plants including algae (Jarvie et al. 2006).
The long term Environment Agency data were selected for three river sites
within the study reach and the main tributary, Lord’s Brook. The EA data was
complemented by bi-monthly sampling at six sites from March 2013 to April 2014 to
encompass a full season. CSOs were identified during this study but water samples
were not obtained due to safety considerations and no past measurements were
available. High frequency spot sampling from a single site from August 2014 to
October 2014 in order to examine changes in PO4-P concentration during high and
low discharge was carried out to examine the effect of rainfall events on PO4-P
concentration and load.
4.2.1 Low resolution long term EA data
Three sites designated S0, S4 and S6 were selected from a number of EA
sampling locations on the Medlock as these were the only sites within the study area
that bracket the WwTW. Site S0 is 3km above the WwTW, S4 and S6 are 0.8km and
5km downstream of the WwTW respectively. Lord’s Brook is 0.5km upstream of the
WwTW and was also assessed to determine its effect on the river although data was
only available from 2000 to 2006. The sampling frequencies over the study period
between 2000 and 2013 varied with each site. While S0 and S4 had datasets from 2000
to 2004 and from 2010 to 2013, there was no data from 2005 and 2009. S6 had
complete data sets from 2000 to 2013.
Instantaneous continuous discharge records at 15 minute intervals was
obtained from the EA at the single gauging station (NGR: SJ 849 975) on the Medlock
to estimate PO4-P load at the three sites. Discharge readings for the three locations
were determined on the basis of each site’s sub-catchment area as shown on Table
4-1. Each sub-catchment area was divided by the total catchment area (57.5km²) and
82
the result multiplied by the instantaneous discharge records obtained from the
gauging station. Daily discharge records from the WwTW were obtained from the
water company, United Utilities.
Table 4-1: Environment Agency sampling sites S0, S4 and S6, Lord’s Brook at the River Medlock
Station Name No. Catchment
area (km²)
Catchment
area as % of
gauging
station
Distance
(km) from
source
Mean
Discharge
(m³s¯¹)
2000-2004
Mean
Discharge
(m³s¯¹)
2010-2013
Lord’s Brook
2.58 4.5 12.07 0.03 0.03
Medlock Vale S0 37.20 65 12.23 0.50 0.46
Millstream Lane S4 43.90 76 13.04 0.59 0.55
Pin Mill Brow S6 54.40 95 17.40 0.73 0.68
4.2.1.1 Estimating load
PO4-P load was estimated using two methods (extrapolation and interpolation;
Littlewood 1992) in order to verify the outcomes of each. Estimates of PO4-P load to
the river were derived using the regression/rating curve method (Walling & Webb
1985). The extrapolation method was applied to the EA datasets due to the limited
number of PO4-P concentration measurements to provide estimates of the PO4-P
load. A relationship between the continuous daily discharge (m³s¯¹) and the PO4-P
concentration (mgL¯¹) was employed to generate continuous PO4-P flux (kgha ˉ¹
yearˉ¹) between the sites. The load was estimated using the following equation:
Load = K. Δt. Σ(Ci . Qi)
where K is a constant, Δt is the data time interval; Ci is the concentration of
sample and Qi is the discharge at the time of sampling.
All concentration and discharge were transformed using the power law function
(extrapolation 1) and log-log regression (extrapolation 2) before the derivation of
rating curves. By using the rating curves, PO4-P concentrations were estimated for
every 15-minute interval which corresponded with the discharge records. In the
estimation of river loads, Walling & Webb (1985) assessed five interpolation methods
of nutrient load estimation (“Methods 1 to 5”) to determine their reliability. While
other methods were based on the estimates of time-weighted rather than on the flow-
83
weighted value, the “Method 5” interpolation technique was considered
representative of conditions occurring between sampling occasions. The resultant
load estimate will depend entirely upon the representativeness of the flow-weighted
mean concentration value derived from a small number of samples. “Method 5” is
also a preferred method recommended by the Paris Commission for assessing river
inputs of Red List and other substances to the North Sea (Littlewood 1992).
Interpolation “Method 5” was also applied to the samples collected during this
study- fortnightly and the high frequency samples using the following equation:
K = a conversion factor to account for (a) the period of load estimation and (b) units;
time interval (in seconds) over which the load was calculated (kgyearˉ¹): (86400 =
number of seconds in a day) x 1000 (correcting m³ to Litres))/106 (correcting mg to kg)
= sample concentration; = discharge at sample time;
= annual mean discharge for period of load estimate (record); n = number of
samples; (i=1,…n) are the times at which the samples were taken.
The resulting load estimates for both methods (extrapolation and interpolation)
were compared to each other in order to examine differences or similarity in output.
84
4.2.2 Fortnightly spatial data
Water samples were obtained fortnightly from the River Medlock from March
2013 to April 2014 at the six monitoring locations as shown on Table 4-2.
Table 4-2: Description of sampling sites on the Medlock including values for average (no =23) of
width, depth, velocity (V) and discharge (Q) of each site.
Station Name Station No. Catchment
area (km²)
Dist.
(km)
from
Source
Ave.
Width
(m)
Average
Depth
(m)
Average
V (ms-1)
Average
Q (m³s-
1)
Mill Brow S1 15.00 6.60 5.70 0.17 0.15 0.15
Park Bridge
Road S2 23.50 8.50 8.20 0.25 0.11 0.23
Daisy Nook
Garden S3 29.70 10.3 8.60 0.21 0.16 0.29
Millstream
Lane S4 43.90 13.00 8.80 0.27 0.18 0.43
Purslow Close S5 53.70 16.10 8.80 0.22 0.27 0.53
Pin Mill Brow S6 54.40 17.40 8.50 0.30 0.21 0.53
WwTW N.A N.A 12.60 N.A N.A N.A N.A
More sample sites were selected compared to the EA temporal data to obtain a
higher spatial resolution and hence identify changes in flow patterns and PO4-P
concentration from groups of CSOs and the WwTW. S1 to S3 are upstream and S4 to
S6 are below the WwTW. Two sites S4 and S6 correspond to the equivalent EA sites.
CSOs are monitored by the United Utilities’water Company and a simulated spill
analysis (frequency, duration and volume of discharge) from CSOs was obtained
from them. A summary of the results are presented in Table 4-3. The sub-catchment
areas were determined using the Terrain Analysis System, GIS (Lindsay 2005) and a
50m (horizontal) and 0.1m (vertical) digital terrain model (DTM) (Centre for Ecology
and Hydrology DTM 2015).
85
Table 4-3: Simulated prediction of frequency, duration and volume from CSOs (Source: United
Utilities)
Sample sites No. of CSOs
discharging into each
sample site
Frequency
(spills/yr)
Duration
(hours)
Volume
(m³)
S1/S2 7 46 176.8 735,617
S3 8 285.5 598 346,912
S4 7 420.1 3547.9 585,608
S5 5 72.7 79.9 29,902
S6 1 1 0.6 1,005
4.2.3 Data collection
For the fortnightly samples, one litre water sample was collected from the six
sample sites in acid-washed (10% HCl for 24 hours) polypropylene bottles. Part of the
water samples was filtered through 0.45µm Millipore cellulose acetate filter for the
measurement of PO4-P and 40ml unfiltered sample was preserved for the analysis of
total phosphorus (TP) by acid digestion (Mackereth et al. 1978). Total phosphorus
was analysed for the estimation of TP load in the river.
Water velocity was measured using the float method i.e. by recording the time
taken for the float to travel over a given distance (10m) along the river. Fortnightly
discharge was calculated for each sub-catchment area on the basis of their
relationship to the total catchment area. Discharge records used for the sub-
catchment area obtained at the continuously gauged Environment Agency site was
considered a viable option because the discharge calculated based on the river
measurement varied extensively due to poor access, and at periods of high flows.
PO4-P samples were processed within 24 hours of the sample collection and
analysed using a SEAL Auto Analyzer 3 High Resolution instrument (SEAL
Analytical Ltd, Southampton). Detection limits for PO4-P measured as P is 0.004
mgLˉ¹. For further information on the methods employed by the auto analyser see
SEAL Analytical (2013).
86
4.2.4 High resolution temporal dynamics
Hydrograph monitoring and precipitation: A 15-minute duration discharge record
was obtained from stage records supplied by the EA’s gauging station. Precipitation
data over the 15-minute periods was obtained from the Whitworth Meteorological
Observatory, Manchester.
Spot sampling: Water samples were collected from the EA gauging station over a
range of low and high flow conditions between August 2014 and October 2014. There
was no direct access into the river; therefore, the water sample was obtained by
lowering a bucket into the river immediately upstream of the gauging station. These
samples were then decanted into a one-litre sample container for subsequent
analysis. The sample was analysed as described above.
4.2.5 Data analysis
Statistical analyses were carried out using Microsoft Excel 2013, SPSS (IBM, 2013) or
GraphPad Prism version 6.
4.3 Results
The results are presented in four sections- low resolution long term EA data
for the river and the tributary (between 2000 and 2013), fortnightly sampling (2013-
2014), high frequency three month sampling (2014, August to October) and a
comparison of the results. The rationale is that the long-term changes in the river
(section 4.3.1), the seasonal pattern, concentration-discharge relationship, load at
different locations provides an overview of the river’s conditions with time; the
fortnightly data (section 4.3.2) provides changes over an annual cycle with more
spatial resolution; high frequency spot sampling was determined so as to understand
temporal changes in the river at the gauged site in relation to rainfall and discharge
(section 4.3.3). The last section 4.3.4 compares the PO4-P load on the Medlock for the
entire period of study between 2000 and 2014.
87
4.3.1 Low resolution long term data
The mean PO4-P concentration between 2000 and 2004, and from 2010 to 2013
was shown on Figure 3-2H (Chapter 3). For both periods, the concentration of PO4-P
increased with site S6>S4>>S0 and only S0 complied with the WFD requirements of
<0.1 mgLˉ¹. A one-way ANOVA to distinguish the concentration between the three
study sites revealed a highly significant (p < .01) difference in the period 2000 to
2004 (F2,12 = 111.27) and from 2010 to 2013 (F2,9 = 28.97). At site S6, where continuous
data was available from 2000 to 2013, Pearson correlation showed a significant (p <
.05, r = -0.72) reduction in the concentration of PO4-P between 2000 and 2013.
Average summer and average winter PO4-P concentration at the three sites (
Figure 3-6A (Chapter 3) shows that the concentration was elevated in the
summer at all sites, with some periods of elevated winter concentrations at S0.
Elevated concentrations suggest the impact of discharge from the WwTW during
low flow summer periods which is indicative of continuous sewage discharge.
4.3.1.1 PO4-P concentration-discharge relationship
The 14-year measurement of PO4-P concentration and discharge dataset was
divided into two-five-year and one-four-year period (from 2000 to 2004, 2005 to 2009
and 2010 to 2013). This duration was considered appropriate as the UK water
industry operates on five-yearly cycles called Asset Management Plans (AMP) as
directed by the Office of Water Services (OFWAT) (http://www.ofwat.gov.uk/). The
AMP is a plan which delivers specific objectives on water infrastructure by the
combination of multi-disciplinary management techniques over the water life cycle
by the water companies (dream report.net, 2014). In this study the AMP covers AMP
3 (2000-2004); AMP 4 (2005-2009) and AMP 5 (2010 -2014). Each of the five-year data
subsets was modelled as power law functions for the three sites S0, S4 and S6 (Figure
4-1, Figure 4-2, Figure 4-3) (Bowes et al., 2009). The line of best fit shown by the
coefficient of determination R² for the concentration-discharge plots are described.
88
A weak positive correlation occurred at S0 (Figure 4-1) from 2000-2004 (r =
0.46, n=56, p=0.0004) and from 2010 to 2013 (r = 0.59, n=44, p=0.0001) which indicate
diffuse source pollution such as agricultural runoff upstream. However, a negative
correlation between PO4-P concentration and discharge occurred at S4 (2000-2004,
r= -0.65, n=54, p=0.000 & from 2010 to 2013, r = -0.57, n=46, p = 0.000) (Figure 4-2)
and at S6 (2000-2004, r= -0.40, n=58, p=0.0017; from 2005-2009; r= - 0.47, n=60,
p=0.0001 & from 2010 to 2013 r= -0.52, n=49, p=0.0001) (Figure 4-3). The results
showed the dilution behaviour of the river as both downstream sites (S4 and S6)
showed a decrease in PO4-P concentration with increasing discharge and suggest
pollution from the WwTW situated above these sites.
89
Figure 4-1: PO4-P concentration versus discharge at S0, 2000-2004 and at 2010-2013 estimated by
extrapolation method using power law functions.
Figure 4-2: PO4-P concentration versus discharge at S4, 2000-2004 and at 2010 -2013 estimated by
extrapolation method using power law functions.
Figure 4-3: PO4-P concentration versus discharge at S6, 2000-2004, 2005-2009 and 2010-2013 estimated
by extrapolation method using power law functions.
90
4.3.1.2 PO4-P load for the low resolution dataset
Figure 4-4 shows the estimated PO4-P load into the River Medlock at the three
sample sites by using the methods of extrapolation and interpolation. The two
methods were found to be similar in output as no significant (p > 0.05) difference was
found. This suggests that either method was a suitable predictive tool for the
estimation of load.
Table 4-4 presents the mean and range of PO4-P load for the three EA study
sites, the WwTW, and the main tributary on the study location, Lord’s Brook, by
interpolation “Method 5”. The mean rate of PO4-P load input at each site was
computed using extrapolated method (power law function) at all the sites over the
duration of monitoring and by dividing each catchment’s export rate by the annual
discharge. The results for the low resolution data between 2000 and 2004, S0 to S6,
Lord’s Brook had an estimated load range of 0.60 to 3.13 kghaˉ¹yrˉ¹ and between 2010
and 2013; a range of 0.26 to 2.16 kghaˉ¹yrˉ¹. Between 2010 and 2013, the increased
load at S4 may be attributed to increased release of PO4-P from the WwTWs.
91
Figure 4-4: Comparison of Interpolation Method 5 and two extrapolation methods to estimate PO4-P
load kg haˉ¹ yrˉ¹ at sites S0, S4 and S6.
92
Table 4-4: Mean and range of PO4-P load estimated at sites S0, S4, S6, at the WwTW and Lord’s
Brook for 2000 to 2004 and 2010 to 2013.
PO4-P kghaˉ¹yrˉ¹
Sites
2000-2004 2010-2013
mean range mean range
Lord’s
Brook
1.91 0.86-2.80
WwTW (2012=1.21)
(2013=1.18)
1.18-1.21
S0 0.6 0.35-1.04 0.26 0.18-0.37
S4 3.13 2.63-3.83 2.16 1.55-3.18
S6 2.52 1.83-3.17 1.45 1.01-1.95
4.3.2 Fortnightly spatial data
The concentration of PO4-P (mgLˉ¹) at the six sample locations S1-S6 is
shown on Figure 4-5. Higher concentrations of PO4-P were obtained at sites S4, S5
and S6 which were located downstream of the WwTWs compared to sites S1, S2
and S3 which were upstream. The difference was confirmed in a one-way analysis
of variance (ANOVA) which showed a highly significant (p <0.001) difference
between the upstream and downstream sites sites (F5, 132 =13.2). Post hoc Tukey
multiple comparisons tests showed a high significant difference (p<0.0005) between
S1 and S4 (p=0.0004) and between S1 and S6 (p=0.0008). Differences were found
between S2, S3 and the downstream sites, p= 0.005).
93
S 1 S 2 S 3 S 4 S 5 S 6
0 .0
0 .2
0 .4
0 .6
0 .8
1 .0
1 .2
1 .4
1 .6
1 .8
S a m p le L o c a t io n s
PO
4-
P (
mg
L-1
)
Figure 4-5: PO4-P concentration at sample locations S1 to S6 in relation to distance from the source
of the River Medlock. Box and whiskers represent 25% and 75%, median, minimum and maximum
values.
The relationship between PO4-P concentration and discharge at sites S1 to S6 is
shown on Figure 4-6. While S1, S2 and S3 showed no (p > .05) correlation, S4, S5 and
S6 showed a significant (p < .05) negative correlation (S4 r= -0.57; S5 r= -0.48; S6 r = -
0.56) of PO4-P with discharge. This pattern is similar to data obtained from the EA
and shown on 0. This decrease in concentration of PO4-P with increased discharge is a
further suggestion of the influence of the discharge from the WwTW.
94
Figure 4-6: PO4-P concentration vs discharge at sites S1 to S6. Regression line showed a decline in
concentration with increased discharge.
95
The summer and winter relationship (Figure 4-7) showed higher PO4-P
concentration during the summer compared to the winter season. During the low
rainfall summer period there is limited dilution of PO4-P whereas during winter,
higher concentration suggests overflows from combined sewers especially during
storms and of runoff from urban surfaces.
Figure 4-7: Summer and winter PO4-P concentration (mgLˉ¹; note log scale) in the River Medlock
using the fortnightly datasets collected between March 2013 and April 2014.
4.3.2.1 PO4-P load for fortnightly spatial data at sample sites
Table 4-5 shows the PO4-P load (kgdayˉ¹) contributions from the upstream site
and WwTW. The concentration and discharge data used for the calculation of the
latter was obtained from the EA and United Utilities. From the table, the total PO4-P
load entering the river downstream of the treatment works is between 14.54 and
29.48 kgdayˉ¹ (average 22 kgdayˉ¹). Therefore percentage contribution from the
WwTW is calculated to be between 71 and 99% and shows the treatment works as the
major contributor of PO4-P to the Medlock at site S4
96
Table 4-5: Contribution of PO4-P load from the WwTW.
PO4-P Load kgdayˉ¹
Date Discharge
(m³sˉ¹) S3 WwTW
Total load
(S3+STW)
% WwTW
Contribution
09-04-13 0.13 8.66 20.82 29.48 70.62
24-04-13 0.12 0.25 19.96 20.21 98.77
26-06-13 0.12 0.02 14.52 14.54 99.85
13-08-13 0.25 1 15.34 16.35 93.88
25-11-13 0.17 0.68 17.73 18.41 96.29
11-12-13 0.26 0.61 25.57 26.18 97.68
14-01-14 0.24 5.7 22.12 27.82 79.51
18-02-14 0.9 1.14 17.87 19.01 94.00
Min 14.54 70.62
Max 29.48 99.85
Average 21.50 91.325
4.3.3 Temporal dynamics
PO4-P concentration vs discharge relationship (
Figure 4-8) showed a significant negative correlation (r= -0.269, p <0.05) which
usually indicates dilution from a point source particularly the WwTW (Bowes et al.
2015).
Figure 4-8: Concentration of PO4-P vs discharge measured from 1st August and 31st October 2014.
WFD standard 0.1mgL¯¹.
97
4.3.3.1 Hydrograph of chemical concentration vs discharge
In the hydrograph of Figure 4-9 based on the 15-minute continuous discharge
record was plotted against spot sample analysis of PO4-P collected over the same
period. The hydrograph/PO4-P plot revealed higher concentration of PO4-P especially
at low discharge levels which could indicate WwTW operation. However, higher
discharge records with corresponding higher PO4-P concentration was recorded on
the 10th August 2014, concentration of PO4-P (0.73mgL¯¹) with increased discharge
(6.82m³s¯¹) which suggests contributions from CSOs following increased volume
arising from rainfall events. The highest PO4-P concentration in the river during the
study period was recorded at 1.17mgL¯¹ with a discharge of 0.29m³s-¹ (5th August
2014). This value is 12-fold higher than the WFD standard of 0.1mgL⁻¹.
Figure 4-9: Hydrograph of 15-minute discharge at the gauging station and spot sample analysis of
PO4-P measured over the study period from 1st August 2014 to 31st October 2014. Sharp discharge
spikes and high concentration suggest diffuse pollution sources.
4.3.3.2 Rainfall and discharge
The total precipitation recorded from 1st August to 31st October was
209.75mm with a mean 0.02mm, with minimum and maximum rainfall ranging from
no precipitation to 7.37mm during the sample duration.
98
The hyetograph (Figure 4-10) is shown of precipitation over the sampling
period. A mean discharge of 0.49m3s¯¹ was recorded throughout the sampling period
with a minimum and maximum discharge of 0.16m3s¯¹ and 6.91m3s¯¹ respectively.
During these times, 69 peak discharges between 0.16m3s¯¹ and 6.91m3s¯¹ out of 8825
measurements were recorded manually. A significant correlation was found between
discharge and precipitation (Pearson correlation, n=8825, p < .01) which, as expected,
indicates that precipitation influenced discharge in the river.
Figure 4-10: Hyetograph of 15-minute precipitation and discharge over the study period from 1st
August to 31st October 2014.
4.3.4 Comparing PO4-P load and concentration
Table 4-6 compares the PO4-P load over the duration of study from EA
datasets, bimonthly sampling and the short-term (between 1st August and 31st
October 2014) spot sampling at the gauging station. S6 has been estimated based on
the value obtained from the gauged station. The mean PO4-P load estimated at the
river between 2000 and 2004 ranged from 3.13 to 0.6 kgPhaˉ¹yrˉ¹; between 2010 and
2013, load ranged from 2.16 to 0.26 kgPhaˉ¹yrˉ¹; and, in 2014, the range was from 0.92
to 0.26kgPhaˉ¹yrˉ¹.
99
Table 4-6: Summary of mean load/range of PO4-P for long term EA data (2000-2004, 2010-2013),
fortnightly data (2014 for sites S4 and S6) and 2014* (high resolution spot sampling expressed
kgPha⁻¹yr⁻¹ for comparison).
kgPha⁻¹yr⁻¹
Sample locations -
-EA & fortnightly
sampling
Catchment area
as % of EA
Gauging Station
2000-2004
2010-2013
2014
2014*
WwTW 1.20 0.94
S0 65 0.60 0.26 0.82
S4 76 3.13 2.16 0.92 0.96
S6 95 2.52 1.45 0.68 1.2
The results show that between 2000 and 2014, the load of PO4-P in the river
decreased with time across the sites. However, between sites, S4 was significantly
higher than other sites. Between S4 and S0 and from 2000 to 2004, the load at S4 was
five times higher (3.13 kgPha⁻¹yr⁻¹) than the load measured at S0, having a mean load
of 0.60 kgPha⁻¹yr⁻¹ and, eight times much more between 2010 and 2013. For S4 and S6
and between 2000 and 2004, S4 was 19% higher than S6 and by 33% between 2010
and 2013. In 2014, the difference between both sites was by 26%. These values further
emphasized the relative impact of the WwTW discharge on the river, especially at
site S4 which is directly below the WwTW. However, an estimated load value from
high frequency sampling in 2014 showed the load at S4 was lower than what was
obtained at S6, suggesting the influence of CSOs or other sources during episodic
(rainfall) events contributing more PO4-P between S4 and S6.
The change in concentration over two decades is presented in Table 4-7. The %
decrease in the concentration of PO4-P (mgLˉ¹) was calculated over a period of three
decades in the River Medlock based on mean PO4-P concentrations for EA sites and
the WwTW. Between two decades, i.e. 2002 & 2012, 2003 & 2013 (Figure 4-11), more
than a 60% reduction in PO4-P concentration was observed in the effluent discharged
from the WwTW. These changes were also reflected in the PO4-P concentration
recorded in the river with > 30% reduction at S4 and S6.
100
With reference to simulated CSO spill analysis obtained from the water
company (Table 4-3), spatial analysis of PO4-P concentration in this study showed
that CSO discharge volume and duration do not significantly increase the
concentration of PO4-P. Lower PO4-P concentrations was analysed for the upper S1,
S2 and S3 and higher concentrations were recorded downstream of the WwTW at S4,
S5 and S6. The impact of CSOs is greatest during short-duration episodic conditions.
During the high frequency sampling regime, high PO4-P was captured at one of the
periods.
Table 4-7: Summary of mean PO4-P concentration (mgLˉ¹) measured at the WwTW and other EA
sites between 2000 and 2012, 2003 and 2013, 2004 and 2014.
PO4-PmgLˉ¹/Period
Sites 2002 2012 2003 2013 2004 2014
WwTW 3.14 1.21 3.89 1.18
S4 1.02 0.5 1.16 0.76 1.02 0.57
S6 0.58 0.36 0.92 0.6 0.65 0.45
% change in PO4-PmgLˉ¹
(2002/2012) (2003/2013) (2004/2014)
WwTW -61.46 -69.67
S4 -50.98 -34.48 -44.12
S6 -37.93 -34.78 -30.77
101
Figure 4-11: Mean PO4-P concentration in the River Medlock between 2002 and 2012, 2003 and 2013
at S4, S6 and at the WwTW
4.4 Discussion
High PO4-P concentration at the Medlock has been a major problem with
regard to the river’s compliance with the WFD. The overall aim of this study was to
determine the load of PO4-P entering the river from CSOs and the single operational
WwTW, and to examine the effects of seasonality. A further aim was to confirm the
changes in PO4-P concentration and load in the river.
The long term and fortnightly measurements suggest that the sites below the
WwTWs had higher concentration of PO4-P (> 0.1mgL¯¹) and the main tributary
Lord’s Brook contributed a very small amount of PO4-P to the river.
Although long term datasets obtained from the EA were incomplete for some
years at S0 and S4, analysis at the lowermost site S6, indicated a decrease in the
concentration of PO4-P with time. Over two decades more than a 60% reduction in
PO4-P concentrations was shown to have occurred at S6 and this is highly likely to
have resulted from a decrease in releases from the WwTW as there are no other major
sources of PO4-P. Between these periods, more than 30% reduction were recorded at
the site S4 which is 0.5km below the treatment works. Information provided by the
EA indicated there was no limit for PO4-P prior to year 2000 and therefore no
102
improvement plan was initiated for its discharge to rivers. However, stringent permit
requirements were enforced under the UK River Ecosystem classification for the
reduction of ammonia from 6mgL¯¹ to 3 mgL¯¹, BOD, which reduced from 30mgL¯¹ to
15mgL¯¹ and suspended solids which reduced from 45mgL¯¹ to 35 mgL¯¹. The EA
suggests that enforcement of these standards could have resulted in a reduction in
PO4-P (EA, personal communication 2016).
Various suggestions to reduce PO4-P in UK rivers have been including the
United Utilities (DEFRA 2014; DEFRA 2012). The construction of underground
storage tanks which will hold excess water and improve water quality entering the
Manchester ship canal is proposed by United Utilities for the Salford area of the city.
New or improved technologies such as pile cloth media filtration; membrane
filtration; ballasted coagulation; nano-particle embedded ion exchange; immobilised
algal bioreactor; and absorption media reed beds aimed at reducing phosphorus from
WwTWs are currently being evaluated at some Universities in the UK (WWT, March
2016). Tertiary treatment has been shown to reduce concentration of PO4-P elsewhere
such as in the River Kennet and the River Thames (Jarvie et al., 2002) and could be
applied to the WwTWs on the Medlock. As at 2010, the River Medlock fell within
44% of the rivers in North West England that have average PO4-P concentrations
>0.1mgLˉ¹ (Rothwell et al. 2010) which indicates that PO4-P contamination is a
common problem in the region’s rivers, in part due to the lack of tertiary treatment of
effluent from the WwTWs.
The concentration-discharge relationship supports the fact that the WwTW
was the major point source. Howell (2010) and Halliday et al.(2014) pointed out that
monitoring sites which directly received discharge from WwTW contained the
highest PO4 concentration. Fortnightly sampling over a single season showed no
relationship between discharge and PO4-P concentration at the upstream sites.
However, the long term EA datasets indicate that PO4-P sometimes increased with
increasing discharge (Figure 4-1, 2010-2013). This suggests the influence of diffuse
source impacts (Bowes et al. 2009). While concentrations can be reduced from point
103
sources such as WwTWs, diffuse sources from agriculture and from urban areas at
the Medlock (Figure 1-5) runoff are more difficult to control as they would require
changes to the use of PO4-P, including the use of fertilizers by the agriculture
industry and other domestic applications of PO4-P.
The EA average summer and average winter PO4-P concentrations revealed
that the highest concentrations occurred during the summer months when the river
flow was lowest. The winter fortnightly datasets may suggest that during storm
events the concentration of PO4-P could increase in the river due to remobilisation
of the sediments (Bowes et al. 2015; Bowes et al. 2008).
PO4-P load measured during the high resolution sampling showed a higher
load compared to those monitored during the long term and fortnightly sampling
periods. This may be attributed to the remobilisation of particulate bound
phosphorus from the bed sediment during flushing events where, large amounts of
particulates were re-suspended (Brunet & Astin, 1998). Thus, ecological
degradation occurs from the interaction between urban surfaces and other instream
processes (Mulliss et al. 1996). The study period for the high resolution sampling
(August-October, 2014) was reported to be the wettest months in 2014 by the UK
Meteorological Station. Since August is the season of planting in the UK (Farming
and Countryside Education (FACE) 2007), fertiliser application prior to high rainfall
could increase nutrient load. Hence, timing of application is crucial to nutrient
export (Beaulac & Reckhow 1982). Such diffuse pollutants pose a particular
problem as they are generally widespread, hard to detect and to quantify (Beven et
al. 2005). This study may suggest the influence of diffuse pollution, probably from
headwaters, as well as from CSOs (based on data obtained from the United
Utilities’ company) during high flow conditions.
CSOs vs WwTW
Due to the risk of sampling directly from CSOs especially during discharge,
the contribution of PO4-P from the WwTW was determined. This method examined
the importance of PO4-P load from upstream locations and, export from the WwTW
104
into the river. The results indicate that the WwTW exported higher PO4-P (average
of 92%) load compared to other sources in particular CSOs, although storm drains
may make a small contribution (Houston et al. 2011; Walsh et al. 2001). In this
study, the shorter high frequency sampling between August and October 2014
indicate the influence of CSOs in providing very high concentration of PO4-P
especially during storm conditions when CSOs are operational. This further
suggests that, CSOs operated mainly during storm conditions. Therefore short
duration sampling regimes can be used to assess the impact of CSOs under
differing flows rather than a routine sample regime which may miss such events.
Comparison of phosphorus load in Medlock with other rivers
The load of total phosphorus in the River Medlock measured during the
fortnight sampling was compared with other studies of urban, semi-urban and
agricultural catchments (Table 4-8). Table 4-8 indicates that semi-urban sections of the
River Medlock (S1/S2) were comparatively low compared to some other rivers such
as Pevensey Levels which was ten times higher, and twice higher for River Ant.
Urban sections of the river Medlock (S3-S6) had lower TP load compared to other
urban rivers with a ratio of 0.65:3. TP load recorded from urban construction
activities and from agriculture were shown to be high while woodland had the
lowest export rate (Table 4-8) on the basis of low fertiliser application and high
infiltration rates (Line et al. 2015; Johnes 1996). Approximately 80% of total
phosphorus recorded at River Avon (Bowes, Hilton, Irons, & Hornby, 2005) and
Pevensey Levels were associated with WwTWs.
This study showed the relative importance of point source pollution sources to
the Medlock. However, several challenges were encountered especially when
information on CSOs (i.e. CSO location, concentration of discharge, spill volumes and
frequencies) were required from both the EA and the water companies. A similar
pattern faced during this study has been encountered by other workers and
presented as a policy document (Marine Conservation Society 2011) which calls for
105
the cooperation of the regulators, and the water company in order to work effectively
to facilitate improvements in water quality.
With the provision of available datasets, future work could include the use of
Geographical Information Systems (GIS). The concentration of PO4-P at the various
locations could be superimposed on a GIS map, together with the CSO locations and
other information obtained on the river infrastructure. This information would
represent high risk conditions within the shorted possible time. It is also important to
determine the relative contribution of PO4-P concentration present in runoff and the
river.
106
Table 4-8: Published phosphorus load based on different land uses.
Land use Catchment
(km²)
Range
(TP
kghaˉ¹yrˉ¹)
Mean
(TP kghaˉ¹yrˉ¹) Source
Urban
(Commercial) 0.56-3.36 (Donigian et al., 1994)
Urban (Residential) 1.30 (EPA, 1983)
Urban (Residential) 0.025 2.30 (Line et al., 2015)
Urban (Residential)
0.96 (Hartigan et al. 1983)
Urban
(Commercial) 3.40 (EPA, 1983)
Urban
(Golf courses &
construction sites) 0.083
3.00 (Line et al., 2015)
Semi-urban 0.69
46.90 (Bales et al. 1999)
Agriculture
(Cropping) 0.10-3.25 0.94 (Beaulac & Reckhow 1982)
Agriculture
(Cropping &
irrigated pasture )
0.14 (Ierodiaconou et al., 2005)
Agriculture
(Pasture) <0.01-4.90 0.82 (Beaulac & Reckhow, 1982)
Agriculture
(Pasture ) 0.062 4.30 (Line et al., 2015)
Woodland 0.02 (Johnes, 1996)
Woodland 0.033 1.0 (Line et al., 2015)
Slurry and farm
yard manure 31.90 (Tamminga, 1992)
Semi-urban
(Pevensey Levels) 56 6.10 (Mainstone et al., 2000)
Semi-urban (River
Ant) 49.3 1.00 (Johnes et al. 1994)
57.5
S1 (semi-urban) 0.46
This study
S2 (semi-urban) 0.40
S3 (urban) 0.23
S4 (urban) 2.20
S5 (urban) 1.92
S6 (urban) 1.55
107
4.5 Conclusion
This study has shown the concentration and load of PO4-P decreased with
time in River Medlock. However, the river is yet to comply with the WFD
requirements for PO4-P. The WwTW is the largest source of PO4-P load in the river
and its contribution is greatest during the summer due to reduced discharge.
Evidence was also presented of significant PO4 contribution from CSOs during
episodic storm conditions. High precipitation during fertiliser application period
could also increase diffuse pollution. The comparison of total PO4-P load from the
Medlock with other areas showed that the Medlock was influenced by a combination
of pollution sources enhanced by modified urban areas. The most cost-effective
reduction in PO4-P concentration in the River Medlock could be achieved by
investing in technologies which remove PO4-P from the treatment works effluent and
also of reducing discharge entering the river. One option could be the use of
wetlands to reduce the concentration of PO4-P in some parts of the river where land is
available such as Clayton Vale.
Acknowledgements
I am grateful for financial support from The National Open University of Nigeria.
Furthermore, thanks to the Environment Agency for the long term data sets,
Warrington and water company, United Utilities for supplying information on the
CSOs.
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Chapter 5 CATEGORISING THE
BENTHIC MACROINVERTEBRATE
ASSEMBLAGES AND WATER
QUALITY IN A HIGHLY URBANISED
RIVER
Abstract
Previous studies have shown the water quality of urban River Medlock to be good
except for PO4-P and a degraded benthic invertebrate community. This study aims to
investigate the variables that degrade the benthic macroinvertebrates and so prevent
compliance with the EU WFD. Water samples were collected fortnightly over a full
season (from March 2013 to April 2014) from five sites and the benthic invertebrate
community was sampled monthly. The sample sites were selected upstream and
downstream of the WwTW and CSOs over a distance of 17 km. Physico-chemical
variables consisting of dissolved oxygen, pH, conductivity, temperature, suspended
solids, nutrients, flow and discharge plus the benthic macroinvertebrate community
were analysed. The effect of substrate was investigated by the use of colonisation
samplers. In this assessment, the use of biotic indices and multivariate statistic tool is
presented as an objective tool in the classification of the river. Correlation between
the variables using PRIMER-6 BIOENV showed the assemblages of benthic
macroinvertebrates were strongly associated with natural variables (altitude, slope
and catchment area) plus anthropogenic influenced discharge, conductivity and
nutrients. The Medlock was characterised on the basis of benthic invertebrate
assemblage into upstream good and downstream poor sites. The poor section had
abundant but moderately pollution sensitive taxa dominated by Gammaridae.
Seasonal studies revealed a greater abundance of pollution tolerant taxa during
winter. This study suggests the Medlock is influenced by the urban stream syndrome
and questions the possibility of the Medlock in achieving a good ecological status due
to the extent of re-engineering undertaken in the previous two centuries.
Keywords: River Medlock, benthic macroinvertebrates, pollution, urbanisation,
PCA, BIOENV, SIMPER
113
5.1 Introduction
Benthic macroinvertebrate communities and biological indices derived from
them have been used in the assessment of running water ecosystems since the
Nineteenth Century (Tate & Heiny 1995; Hellawell 1986; Borja & Franco, 2000). A key
reason is that benthic macroinvertebrates are abundant and reasonably sedentary in
the absence of a marked change in the physico-chemical environment which allows
them to integrate with environmental stress (Paul & Meyer, 2001). Indices such as the
Biological Monitoring Working Party (BMWP) (Hawkes 1997) score and ASPT
(average score per taxon) are commonly used to assess environmental conditions
(Davy-Bowker et al. 2008). A new index based on the BMWP score called the Whalley
Hawkes, Paisley and Trigg (WHPT) metric (Paisley et al. 2014) includes an
abundance weighting and inclusion of further taxa, replaced the BMWP score in the
UK in 2014 (Paisley et al. 2014; Environment Agency 2015b). Benthic invertebrates
were classified by the UK Environment Agency on the basis of the biological
monitoring working party (BMWP) score under the WFD monitoring cycle 1 (2009 -
2015). The new WHPT index is being employed under WFD cycle 2.
The degree at which observed benthic macroinvertebrate community differs
from the expected can be predicted using key physico-chemical factors, including
hydromorphological variables (Wright et al., 1984). Among the key
hydromorphological variables is water discharge. Water discharge plays a key role in
invertebrate movement by drift associated with flood conditions and as a result of
physical disturbance of the substrate (Brittain & Eikeland 1988).
The Lotic-invertebrate Index for Flow Evaluation (LIFE) score was developed
to evaluate benthic invertebrate communities in a river based on the flow regime in a
water body (Extence et al. 1999). Discharge and flow will be markedly affected by the
increased and highly variable run-off characteristic of urbanised rivers (Lytle & Poff
2004) plus re-engineering of the stream-bed for flood control (Paul & Meyer 2001).
Urbanisation exerts a major effect on water quality due to release of industrial
and domestic wastes plus contaminated run-off (Paul & Meyer, 2001). Reduced
114
infiltration due to a large impermeable surface area results in a hydraulically efficient
drainage system (Walsh et al. 2005) characterised by high runoff velocities, episodic
“flashy” stream flows, increased peak discharge and greater erosion. The changes in
the urban river as a result of these conditions are termed, the ‘urban stream
syndrome’ (USS) (Walsh et al. 2005). Thus, the term USS has been coined to describe
the ecological degradation of urbanised water courses (Walsh et al. 2005). Willemsen
et al. (1990) showed that increased episodicity in urban rivers amplifies the change in
community structure as the biota has to cope with these variations in velocity
(Nilsson & Renöfält 2008). Therefore, the effects of USS on river quality would impact
on compliance with the European Union’s Water Framework Directive (WFD;
Council of the European Union, 2000).
The River Medlock is subject to urban pollution and flooding (Environment
Agency 2009a) and is classified as a “highly modified water body” (Environment
Agency 2009b) subject to pollution from urban runoff, combined sewer overflows
(CSOs) and from wastewater treatment works (WwTWs). As a result the river fails to
meet the requirements of the WFD (EA, personal communication 2014). While point
source pollution are continuously monitored and regulated, diffuse sources are less
controlled and, this has been perceived to have prevented the river from compliance
with the WFD (James et al., 2012).
Invertebrate colonisation was investigated on the Medlock in order to
determine the effect of substrate on the invertebrate community. Colonisation
samples have the advantage of eliminating differences arising from changes in
substrate and hence facilitate inter-site comparisons (Davies 2002). The period of
exposure of these samplers in freshwater varies from four to six weeks (Weber 1973).
Active sampling which uses the kick-sampling method was applied at all sites while
colonisation of invertebrates were employed at two sites upstream and downstream
of the WwTWs.
115
The aim of this chapter is to determine the relative importance of water
quality and quantity (discharge and flow) on the benthic macroinvertebrate
community of the River Medlock. A subsidiary aim was to compare the BMWP
scores used in the analysis of historic data with the new WHPT index in response to
organic pollution. The hypothesis is that physical rather than chemical pollution is
the major determinant of ecological status in the river. A second hypothesis is that the
new WHPT index would provide better representation of the river’s benthic
macroinvertebrates compared to the BMWP scores.
The specific objectives are therefore to:
1. Characterise the benthic macroinvertebrate community in the River Medlock
spatially and temporally;
2. Identify the water quality variables that influence the benthic
macroinvertebrate community;
3. Investigate the relationship between the invertebrate community and selected
physico-chemical factors, including flow rate and discharge;
4. Compare the ASPT and number of taxa from BMWP scores with the WHPT
ASPT and WHPT NTAXA.
5.2 Methodology and Approach
5.2.1 Study area
The Mersey catchment is one of the most urbanised catchments in the UK with
a catchment size of 4680 km2. The River Medlock (See Figure 1-5) rises in the hills to
the NE of Oldham in Greater Manchester (National Grid Reference (NGR): SD 95308
05431), and it flows for 22km, initially through a steep-sided wooded area for 10 km
before entering a largely urbanized area and, continues in a SW direction to
discharge into the River Irwell immediately downstream of Manchester city centre
(NGR: SJ 85781 97858). The catchment area of the Medlock is 57.5 km2 and the
average rate of flow as recorded in the National Rivers Flow Archive was 0.82m³s¯¹
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(CEH, 2016). The majority of the catchment (37%) is heavily urbanised and includes
light industries extending into parts of Manchester and the southerly Pennine hills
(CEH, 2016).
Eleven kilometres of the river were examined from Mill Brow Bridge (NGR:
SD 94183 02262) to Pin Mill Brow (NGR: SJ 85781 97858). The rationale is that the
catchment of the surveyed reach is largely urbanised. The sub-catchment area of each
sample location and other characteristics is shown on Table 5-1. The survey reach of
the river has a continuously operational waste water treatment works (WwTW) at
Failsworth (NGR: SJ 89674 99800), about 30 CSOs (EA, personal communication 2016)
and an unknown number of surface water drains.
Table 5-1: Catchment area of the sampling sites and WwTw on the Medlock and the distance from
the river’s source
Site name Site
No.
Catchment
area (km²)
Catchment
area as %
of total
Dist (km)
from
source
%
urban
cover
Latitude Longitude
Altitude
(m)
Slope
(%)
Mill Brow S1 15 26 6.60 33 53.5173 -2.0892 138.51 2.69
Park Bridge
Road
S2 23.5 41 8.50 45 53.51282 -2.0997
117.87 2.34
Daisy Nook
Garden
S3 29.7 52 10.30 42 53.50107 -2.12398
88.81 2.21
Millstream
Lane
S4 43.9 76 13.00 45 53.49258 -2.16317
66.81 1.92
Purslow
Close
S5 53.7 93 16.10 47 53.48197 -2.21164
47.35 1.67
Pin Mill
Brow
S6 54.4 95 17.40 48 53.47726 -2.21571
42.39 1.57
WwTW n/a n/a n/a 12.60 n/a n/a n/a
5.2.2 Sampling and data collection
Five sample sites (S1 –S2, S4 to S6) were selected on the river; upstream and
downstream of the major WwTW, Failsworth plus major CSOs (United Utilities,
personal communication, 2013) and sampled from March 2013 to April 2014 (Table
5-1, Figure 2-2). The presence of riparian vegetation stabilises the river bank and
helps to slow down flood water and acts to deposit sediments which serve to build
the banks. Failsworth WwTW is situated 12.6 km south of the river’s source and is
located between sites 2 and 4. The site numbers S1, S2, S4 to S6 had been retained to
concur with the sample sites in previous chapters. Physico-chemical variables were
117
recorded every two weeks but benthic invertebrates were collected monthly since no
major change would be expected within two weeks. The water quality data was
averaged over the previous two weeks in order to align to the benthic
macroinvertebrate samples.
The sub-catchment areas (Table 5-1) were determined using the Terrain
Analysis System, GIS (Lindsay 2005) and a 50m (horizontal) and 0.1m (vertical)
digital terrain model (DTM) (Centre for Ecology and Hydrology DTM).
118
Figure 5-1: Photographs of sample sites S1 to S6 and the riparian vegetation. S6 shows the debris
screen which is aimed to retain large objects and prevent flood damage.
The sample sites on the Medlock as shown on Figure 2-2 and photographs on
Figure 5-1 showed that the river is shallow, the upper sites S1 and S2 are largely
erosional while sites S4 to S6 are deeper and depositional with finer substrate. S6 has
been canalised for flood control and all sites showed extensive riparian vegetation.
S1 NGR: SD 94183 02262 S2: NGR: SD 93489
01798
S3 NGR: SD 91874
00493
S4 NGR: SJ 89272 99554
S5 NGR: SJ 86052 98382 S6 NGR: SJ 85781 97858
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5.2.2.1 Physical and chemical measurement
At each site, measurements of pH, dissolved oxygen, temperature and
conductivity were taken using a pre-calibrated hand-held multiparameter water
quality meter (YSi 556 Multi probe system YSI, Yellow Springs, Ohio, USA). Water
velocity was measured at intervals using the float method i.e. by recording the time
taken for the float to travel over a given distance (10m) along the river.
Fortnightly discharge was calculated for each sub-catchment areas on the basis
of their relationship to the total catchment area (Table 5-2).
Table 5-2: Average (no.= 23) width (m), depth (m), flow velocity (msˉ¹) and discharge (msˉ¹) at each
sample site
Site Width Depth (m) Velocity (msˉ¹) Discharge (m³sˉ¹)
Average Min Max Average Min Max Average Min Max
S1 5.7 0.22 0.13 0.3 0.27 0.16 0.53 0.15 0.05 0.46
S2 8.2 0.23 0.12 0.59 0.26 0.11 0.59 0.23 0.08 0.72
S4 8.8 0.27 0.14 0.64 0.6 0.14 1.25 0.43 0.15 1.34
S5 8.8 0.29 0.11 0.69 0.57 0.13 1 0.53 0.18 1.63
S6 8.5 0.29 0.15 0.58 0.57 0.13 1 0.53 0.18 1.66
Discharge records obtained at the continuously gauged Environment Agency
site was considered a preferable option because discharge could not be accurately
calculated due to poor access and high flows. The calculation of discharge data for
the sample sites have been described in Chapter 2. Velocity readings were obtained
using the float method and confirmed with EA velocity readings obtained from the
gauging station described in section of Chapter 2. Table 5-2 showed that average
velocity in the river ranged from 0.27ms¯¹ to 0.57ms¯¹ with higher velocity
downstream of the WwTW (S4 to S6). Similarly, average discharge from the river
showed a range of 0.15m3s-1 to 0.53m3s-1. The results showed that increased volume of
water comes from the treatment works flows downstream of the river. At each
sampling date a one-litre water sample was collected in an acid-washed
polypropylene bottle. A 300ml aliquot was filtered through a 0.45 µm glass fibre
paper (VWR International) and oven dried at 105°C for 24 hours to determine total
suspended solids. The remainder (700ml) of water sample was filtered through a
120
0.45µm Millipore (Millipore-UK, Limited) hydrophilic, 0.45 µm, 47 mm cellulose
acetate filter for measurement of ammonia (ammonia-N), NO3-N (nitrate-N), PO4-P
(phosphate-P) and trace metals. The trace metal sample was acidified with two drops
of ultra-pure reagent nitric acid to pH=2 to retain the metals in solution.
A brown glass bottle was used for the collection of samples for measurement
of BOD from each site to avoid autotrophic metabolism and incubated at 20°C for
five days. The difference between the dissolved oxygen levels was measured using a
calibrated Hanna meter (Hanna Instruments Ltd, Bedfordshire, UK) to determine the
BOD5 recorded on the first day and the fifth day was calculated to determine the
BOD5 (mgL-1).
The trace metals chromium (Cr), cadmium (Cd), copper (Cu), nickel (Ni), lead
(Pb) and zinc (Zn) were analysed by Inductively Coupled Plasma-Mass Spectrometry
(ICP-MS) using an Agilent 7500cx (Agilent Technologies, Santa Clara, USA)
spectrometer. Calibration was by matrix-matched standards.
5.2.2.2 River Substrate
River substrate was characterised by estimating the percentage substrate
contribution (Table 5-3) using the Wentworth scale. The highest contribution was
from sand which accounted for 36% of the total. The sites downstream have higher
finer substrates.
121
Table 5-3: Types of substrate found at sample sites. *The text indicated in bold showed the
dominant substrates to be stones and sand.
Site Boulders
(%)
Stones
(%)
Pebbles
(%)
Gravel
(%)
Sand
(%)
Silt
(%)
Mud
(%)
S1 6 37 7 5 30 15 3.5
S2 10 39 7 7 27 12 3
S4 4 27 1 23 32 12 3
S5 14 43 3 6 28 8 0
S6 6 18 1.4 3.8 42 16 2
% total
contribution
7.5 30.84 3.65 8.43 35.55 11.85 2.16
5.2.2.3 Nutrient analysis
NO3 and PO4 samples were processed within 24 hours of sample collection
and analysed using a SEAL Auto Analyzer 3 High Resolution instrument (SEAL
Analytical Ltd, Southampton) based on a segmented flow analysis. For further
information on the methods employed by the autoanalyser see SEAL Analytical
(2013). Throughout this study, concentrations of nutrients are presented as elemental
concentrations, i.e. mgLˉ¹ P, not PO4 and mgLˉ¹ N, not of NO3; ammonia as mgLˉ¹ of
N not NH3.
Analysis of PO4-P is based on the molybdenum blue method in which
orthophosphate reacts with molybdate and ascorbic acid to form an intensely blue
compound which is measured at 660nm. Detection limits for PO4-P measured as P
was 0.004 mgLˉ¹.
Nitrate (NO3-N), measured as N following the DIN 38405 and ISO/DIS 13359
standard methods and with a detection limit of 0.01 mgLˉ¹. The analysis was based
on the cadmium reaction method in which the sample is reduced from nitrate-N to
nitrite by hydrazine in alkaline solution, with a copper catalyst, after which it is,
reacted with sulfanilamide and coupled with α-napthyethlene diamine
dihydrochloride to form a pink compound measured at 520 nm. Phosphoric acid is
added at the final stage to reduce the pH and thus avoiding precipitation of calcium
122
and magnesium hydroxide. The addition of zinc to the reducing agent suppresses the
complexing of copper by organic material. These analyses comply with the Standard
Committee of Analysts Publications, (2011).
Ammonia-N concentration (mgL¯¹) was analysed using the Hanna (Hanna
Instruments Ltd, Leighton Buzzard, Bedfordshire, low range reagents (HI-93700-01)
kit by spectrophotometry. The limit of detection for ammonia-N measured as N was
0.01mgLˉ¹. The analysis of ammonia-N is based on the Nessler Method in which the
Nessler Reagent (K2HgI4) reacts with the ammonia-N present in the sample under
strongly alkaline conditions to produce a yellow-coloured species. The intensity of
the colour is in direct proportion to concentration of ammonia-N concentration. The
measurement wavelength is 425 nm.
Geographical information including sub-catchment altitude was obtained
from the internet map tools (www.freemaptools, www.daftlogic.com) for the study
sites using the sub-catchment latitude and longitude. The sub-catchment slopes were
obtained by dividing each site’s elevation from the river’s source by the distance of
the sample site from the source.
5.2.2.4 Benthic invertebrates sampling and collection
Samples were collected from five sites (S1, S2, S4 to S6) using a 1mm mesh
hand net by the three-minute kick net sampling technique outlined in the Water
Framework Directive, UK policy report (UK Technical Advisory Group 2008). An
additional one-minute manual search was carried out by collecting benthic
invertebrates that could have been missed through the kick sampling. The samples
were preserved in 70% ethanol, identified and counted in the laboratory (Pawley,
Dobson, & Fletcher, 2014). Biotic indices used in this study were the Biological
Monitoring Working Party (BMWP), Average Score Per Taxa (ASPT), Whalley,
Hawkes, Paisley & Trigg (WHPT), River Invertebrate Classification Tool (RICT) for
Environmental Quality Ratio and, Lotic Invertebrate-Index Flow Evaluation (LIFE).
123
The BMWP score and WHPT were used simultaneously in this study because
both indices fall under WFD reporting period encompassed by this study. While the
BMWP scores is still used for local EA operations as the scores can easily be
communicated to and understood by members of the public, this is not yet the case
with WHPT index. The WHPT index is currently rarely used for river classification
due to the quantification required and therefore, only applied in EA reports meant
for reporting for specialised audience. The EQR from the RICT is derived from both
indices and provides an overall WFD status for a site. This study falls within the two
reporting cycles of the WFD and therefore applied in this study to compare and
contrast output of the ASPT and NTAXA.
Under the BMWP score, all macroinvertebrates apart from Oligochaeta were
identified to family level using the taxonomic groups used in the biological
monitoring working party (BMWP)-score (Hawkes 1997). Invertebrate families that
are very sensitive to sewage pollution receive scores of 10 and the most tolerant
families receive a score of 1 and the sum of the total scores determine the category to
which the river is classified as shown on Table 5-4.
Table 5-4: BMWP Scores and interpretation (Hawkes 1997).
BMWP Score ASPT Category Interpretation
0-10 ≤3.9 Very poor Heavily polluted
11-40 4.0 - 4.9 Poor Polluted or impacted
41-70 5.0 -5.9 Moderate Moderately impacted
71-100 6.0 - 6.9 Good Clean but slightly impacted
>100 > 9 Very Good Unpolluted / unimpacted
The Whalley Hawkes, Paisley and Trigg (WHPT) Average Score Per taxa
(WHPT ASPT) and WHPT number of taxa (WHPT NTaxa) were assessed separately
and then combined in a “worst of” approach to provide the overall invertebrate
classification. The WHPT ASPT was applied as abundance weighted metric. Table
5-5 shows WHPT logarithmic abundance categories and Environmental Quality
124
Ratio (EQR). Thus, the sum of the total scores from the samples collected will
determine the category to which the river is classified.
Table 5-5: WHPT logarithmic abundance categories and Environmental Quality Ratio (EQR) for
WHPT-ASPT and WHPT-NTAXA.
Abundance
category
Numerical
Abundance
WHPT EQR
NTAXA ASPT
AB1 1-9 High/Good 0.80 0.97
AB2 10-99 Good/Moderate 0.68 0.87
AB3 100-999 Moderate/Poor 0.56 0.72
AB4 >1000 Poor/Bad 0.47 0.59
The UK River Invertebrate Classification Tool (RICT) is used to contextualise
WHPT scores by predicting site specific reference values and provides a WFD
compliant probabilistic classification (Davy-Bowker et al. 2008). The ecological
quality of the river is classified for spring and autumn seasons.
The ASPT of the samples observed (Obs) is divided by the predicted (Pred)
pristine condition score to provide a classification Ecological Quality Ratio (EQR)
belonging to any of the WFD classes as shown on Table 5-5. EQR values close to 1
indicate invertebrate communities close to the natural state, those near to zero
indicate a high level of pollution or disturbance.
LIFE scores are determined by using the family abundance and ecological
associations with flow as shown on Table 5-6. The sum of the individual invertebrate
families is divided by the number of scoring families to produce the overall LIFE
Score: LIFE =
LIFE scores less than 6.00 generally indicate sluggish or still water conditions.
As current velocity increases, so do LIFE scores. LIFE values greater than 7.5 indicate
very fast flows.
125
Table 5-6: Life Flow Groups.
ABUNDANCE CATEGORY
LIFE Flow Group
Flow association
Velocity (cmsˉ¹)
A (1-9)
B (10-99)
C (100-999)
D/E (1000 - > 10,000)
I Rapid flows Typically >100cmsˉ¹
9 10 11 12
II Moderate to fast flows
20 cmsˉ¹ to >100cmsˉ¹
8 9 10 11
III Slow to sluggish
< 20cmsˉ¹ 7 7 7 7
IV Slow and standing
N/A 6 5 4 3
V Standing water
N/A 5 4 3 2
VI Drying and drought impacted
N/A 4 3 2 1
5.2.2.5 Colonisation samplers
The colonisation method was used in addition to the kick sampling (active
method) in order to allow comparison between sites as colonisation is independent of
the natural substrate (Czerniawska-Kusza 2004). Colonisation will therefore facilitate
examination of the impact of stream substrate on the community. Two artificial
colonisation samplers (Figure 5-2) were positioned at two locations, upper S2 and
lower S6 for a thirty-day period over four month duration from September 2014 to
December 2014. The 30-day period was critical for the development of a
representative community of organisms (Weber 1973; Meier et al. 1979). The two sites
were selected in order to determine if the substrates impacted on the benthic
invertebrate community as S2 is largely erosive and S6 is partly depositional, having
a sandier substrate. Both sites are located upstream and downstream of the major
WwTW respectively. At the end of the 30 day period the colonisers were removed
from the river and washed in a bucket to be processed at the laboratory using the
same procedure as the kick samplings.
126
Figure 5-2: Invertebrate colonisation sampler before and after 30 days’ colonisation. Source, Author,
2014
5.2.2.6 Statistical analysis
Data were tabulated and analysed using Microsoft Excel 2013, GraphPad
Prism 6 and multivariate analysis (similarity percentages (SIMPER) routine; Principal
component analysis (PCA); Non-metric multidimensional scaling (nMDS); BIOENV
(Biota and Environmental) procedure were performed using the PRIMER-6 software
package (Clarke & Warwick 2001). Multivariate approaches (Cao et al. 1996) were
used to identify the water quality variables that affect the macroinvertebrate
community in the River Medlock. The study variables include in addition to benthic
macroinvertebrate assemblages, physico-chemical parameters collected at a number
of sites along the river over a full season between March 2013 and April 2014.
Differences between the physico-chemical variables and the biotic indices
(BMWP, ASPT, and WHPT) at each site were analysed using One-way Analysis of
Variance (ANOVA). Pearson correlation analysis was used to investigate how the
various metrics changed and how these variables impacted on the river.
127
5.3 Results
The following results are presented in three sections- physico-chemical
variables, benthic macroinvertebrates and the relationship between physico-chemical
parameters and benthic macroinvertebrates. The rational is because conventionally,
physico-chemical variables are presented first and the water quality would need to be
established in order to determine if the biota will correspond to the physico-chemical
parameters. The description of each parameter in the results is referred in terms of
sample location and distance from source.
5.3.1 Physical and chemical variables
The spatial differences and ordination of the physico-chemical variables were
determined. On the basis of the Gregorian calendar used in separating the seasons-
December, January and February (Winter); March, April and May (Spring); June, July
and August (Summer); September, October and November (Autumn), there was no
significant (p>0.05) difference between the water quality variables (including DO,
pH, temperature, conductivity, BOD, Ammonia, NO3-N and PO4-P) with season and
is therefore not presented.
5.3.3.1 Ordination of environmental variables
Ordination of variables was described in two scenarios in order to determine
the differences in outcomes between using all or fewer environmental variables in the
PCA. All data matrix were transformed and ordinations were executed on the basis
of a distance matrix.
Scenario 1: Twenty physico-chemical variables, specifically dissolved oxygen, pH,
temperature, conductivity, suspended solids, BOD, ammonia-N, NO3-N, PO4-P,
discharge, velocity, catchment area, altitude, slope, boulders, stones, pebbles, gravel,
sand and silt were combined in a PCA. (Figure 5-3, Table 5-7). The first PCA axis
accounted for 34% of the overall variance and was most heavily weighted to altitude,
slope, catchment area, PO4-P, NO3-N and velocity while the second PC axis
accounted for 17% of the variance and was dominated by substrate, specifically
128
boulders, stones, sand and silt. The PCA plot representing PC1 and PC2 showed a
difference between the sites especially with reference to the river’s physical attributes.
Table 5-7: All environmental variables (p<0.05) based on ordination with principal components.
Selected PC characters in bold indicated environmental variables that controlled the river.
Variable PC1 PC2 PC3 PC4 PC5
DO -0.072 -0.06 0.231 -0.009 -0.562
PH 0.141 -0.081 -0.295 -0.373 -0.282
TEMP 0.022 -0.001 -0.444 0.39 0.287
COND 0.115 0.02 0.333 -0.269 0.221
BOD 0.138 0.025 -0.024 -0.234 0.033
NH3-N 0.045 0.081 0.42 0.222 0.364
NO3-N 0.304 0.008 -0.259 0.068 0.133
PO4-P 0.318 -0.007 -0.208 0.154 0.001
SS 0.148 0.18 0.185 0.314 0.169
DISCHARGE 0.212 0.116 0.451 -0.004 -0.075
VELOCITY 0.239 0.068 -0.007 0.174 -0.263
CATCHMENT AREA 0.372 0.071 -0.001 -0.094 -0.008
BOULDERS -0.007 0.485 -0.074 -0.238 0.116
STONES -0.168 0.474 -0.071 0.032 -0.1
PEBBLES -0.356 0.124 -0.055 -0.117 0.107
GRAVEL 0.136 -0.098 0.077 0.475 -0.35
SAND 0.15 -0.445 0.05 -0.169 0.199
SILT -0.144 -0.487 0.059 -0.009 0.137
ALTITUDE -0.368 -0.051 0.008 0.132 -0.014
SLOPE -0.364 -0.061 0.002 0.151 -0.047
129
-6 -4 -2 0 2 4
PC1
-4
-2
0
2
4
PC
2
Site Number1
2
4
5
6
DO PH
TEMP
CONDBODNH3-N
NO3-N
PO4-P
SSDISCHARGEVELOCITYCATCHMENT AREA
BOULDERSSTONES
PEBBLES
GRAVEL
SANDSILT
ALTITUDESLOPE
Figure 5-3: Ordination diagram of 20 environmental variables at each of the sites on the River
Medlock. Variables are DO (Dissolved oxygen), pH, temperature (TEMP), conductivity (COND),
velocity, BOD, ammonia-N, NO3-N, PO4-P, SS (suspended solids), discharge, substrates, catchment
area, altitude and slope.
Scenario 2: In order to determine impact of water quality variables, altitude, slope,
catchment area and substrates were excluded in a further PCA combination. The first
PCA axis accounted for 26% of the overall variance and was most heavily weighted
on NO3-N, PO4-P, velocity and suspended solids. The second axis accounted for 21%
of the variance and was dominated by discharge, ammonia-N and conductivity
(COND) (Figure 5-4, Table 5-8). These groups indicated the importance of water
quality on river assessment.
The output for scenarios 1 and 2 were similar especially in the distinction
between the sample sites S1 and S2; and, S4 to S6. The exclusion of certain variables
in the second scenario showed that the nutrients and suspended solids concentration
(34%)
(17%)
130
was influenced by velocity and discharge which would be increased with decreasing
slope.
-4 -2 0 2 4
PC1
-4
-2
0
2
4P
C2
Site Number1
2
4
5
6
DO
PH
TEMP
COND
BOD
NH3-N
NO3-NPO4-P
SS
DISCHARGE
VELOCITY
Figure 5-4: Ordination diagram of 11 environmental variables at each of the sites on the River
Medlock. Variables are DO (Dissolved oxygen), pH, temperature (TEMP), conductivity (COND),
velocity, BOD, ammonia-N, NO3-N, PO4-P, SS (suspended solids) and discharge.
(26%)
(21%)
131
Table 5-8: Selected environmental variables (p<0.05) based on ordination with principal
components. PC characters in bold indicated the water quality variables including nutrients and
DO influenced the river.
Variable PC1 PC2 PC3 PC4 PC5
DO -0.208 0.163 0.165 0.531 0.095
pH 0.217 -0.284 0.549 0.076 -0.119
TEMP 0.142 -0.411 -0.477 -0.171 0.075
COND 0.148 0.365 0.300 -0.316 -0.466
BOD 0.246 0.02 0.411 -0.384 0.543
Ammonia-N 0.071 0.455 -0.267 -0.325 0.415
NO3-N 0.521 -0.172 -0.012 -0.031 -0.095
PO4-P 0.516 -0.132 -0.045 0.117 -0.017
SS 0.280 0.273 -0.316 -0.003 -0.433
DISCHARGE 0.246 0.508 0.041 0.233 0.069
VELOCITY 0.354 0.055 -0.100 0.512 0.292
5.3.3.2 Spatial differences at sample locations
Significant (p<0.05) differences between S1, S2 and from S4 to S6 were
observed for pH, conductivity, discharge, NO3-N and PO4-P (Figure 5-5,Table 5-10).
Low conductivity upstream indicated that the river’s conductivity is not influenced
by the river’s geology (and from the moors) which is made up mainly of mixed
permeability superficial deposits. As expected, discharge increased significantly
downstream of the river along with increased concentration of nutrients from sites S4
to S6 (Figure 5-5; D&E). The change occurred from S4 (which is 0.5km below the
WwTW) to S6 as all three sites are located downstream of the WwTW. No significant
(p>0.05) difference was recorded for temperature, BOD, ammonia-N, dissolved
oxygen and suspended solids. However, periods of high concentrations of suspended
solids were observed from S4 to S6 and this was linked to increased discharge from
runoff and CSOs especially during high rainfall events. Post Hoc, LSD for substrates
indicates that sand and stones were abundant in the river compared to other
substrates (p < 0.001, F6, 28= 24.963).
Within the context of the Water Framework Directive, all average chemical
variables fulfilled the WFD requirements except PO4-P (European Union, 2000). Mean
suspended solids concentration conformed to the requirement of the Freshwater
132
Fisheries Directive even though there were periods (probably during storm
conditions) when the concentration of suspended solids exceeded the standard of
25mgLˉ¹.
Trace metals were below environmental quality standards as shown on Table
5-9. They are therefore highly unlikely to influence benthic invertebrate community
structure.
Table 5-9: Mean and standard deviation of trace metals sampled at six sites on the River Medlock
between April 2013 and April 2014.
Trace
metal
standard
(ug/L)
Cr
standard
(50 ug/L)
Cu
standard
(50 ug/L)
Cr standard
(3000 ug/L)
Cr
standard
(5 ug/L)
Cr
standard
(1 ug/L)
Cr
standard
(50 ug/L)
Sites
Number
of
samples Cr (ug/L)
Cu
(ug/L) Zn (ug/L)
Cd
(ug/L)
Hg
(ug/L) Pb (ug/L) Ni (ug/L)
S1 12 0.24±0.12 6.59±4.01 24.26±12.48 0.03±0.01 0.07±0.0 0.91±1.53 2.45±0.69
S2 12 0.25±0.12 7.09±3.22 24.95±17.22 0.03±0.01 0.07±0.01 0.52±0.39 2.85±0.94
S3 12 0.30±0.17 7.49±4.42 34.43±45.41 0.03±0.01 0.08±0.01 0.55±0.27 2.62±0.52
S4 12 0.39±0.21 9.55±4.26 44.15±62.47 0.03±0.01 0.07±0.00 0.68±0.89 2.71±0.38
S5 12 0.33±0.13 9.40±3.32 22.92±13.89 0.03±0.01 0.07±0.00 0.38±0.20 2.56±0.40
S6 12 0.35±0.10 8.94±2.39 21.48±10.79 0.04±0.02 0.07±0.00 0.61±0.51 2.62±0.40
The influence of land use on the five sampling sites was examined by
comparing the percentage size of the sub-catchment areas. The results showed S1 had
significantly (p <0.05) low percentage land use compared to other sites along the
river.
133
S 1 S 2 S 4 S 5 S 6
0 .0
0 .5
1 .0
1 .5
2 .0
S a m p le L o c a t io n s
Dis
ch
ar
ge
(m
3s
-1)
A
S 1 S 2 S 4 S 5 S 6
0
2 5
5 0
7 5
1 0 0
1 2 5
1 5 0
S a m p le L o c a t io n s
Su
sp
en
de
d s
oli
ds
(m
gL
- ¹)
B
S 1 S 2 S 4 S 5 S 6
0
2 0 0
4 0 0
6 0 0
8 0 0
1 0 0 0
S a m p le L o c a t io n s
Co
nd
uc
tiv
ity
(µ
Sc
m- ¹)
C
S 1 S 2 S 4 S 5 S 6
7 .0
7 .5
8 .0
8 .5
9 .0
9 .5
1 0 .0
S a m p le L o c a t io n s
pH
(p
H u
nit
s)
D
S 1 S 2 S 4 S 5 S 6
0
2
4
6
8
1 0
1 2
1 4
S a m p le L o c a tio n s
NO
3-N
(m
gL
-1)
E
S 1 S 2 S 4 S 5 S 6
0 .0
0 .5
1 .0
1 .5
2 .0
S a m p le L o c a tio n s
PO
4-P
(m
gL
¯¹)
F
Figure 5-5: (A) Discharge, (B) Suspended solids, (C) Conductivity, (D) pH, (E) NO3-N and (F) PO4-P
at all sample locations on the River Medlock. Box and whiskers represent 25% and 75%, median,
minimum and maximum values of the variable measured. The dotted lines show the WFD
standards.
134
Table 5-10: One-way ANOVA to compare sites S1 and S2, S4 to S6 for mean environmental
variables, BMWP, WHPT scores and ASPT of benthic invertebrates and trace metals. The results are
compared with European Union’s standard requirements.
Variables ANOVA
P ( post hoc
test)
Average
conc.
Standards
(WFD, FFD)
Interpretation
Discharge
(m³s ˉ¹)
F4, 110 = 7.061 *** S1-S2 &
S4-S6
0.37
Conductivity
(µScmˉ¹)
F4, 103 = 8.081 *** S1& S3-
S6; ***S2 &
S3-S6
615.30 N/A
pH F4, 108 = 3.232 ***S2 & S4-
S6
8.03
Good
Temperature
(°C)
F4, 108 = 0.324 ns 10.13
DO (%sat) F4, 108 = 0.899 ns 101.58 >80% High
Suspended
solids
(mgL ˉ¹)
F4,108 = 1.633 ns 9.81 ≤25mgLˉ¹ Good (FFD)
NO3-N
(mgL ˉ¹)
F4, 110 =14.34 **S1-S2 & S4
–S6
2.89
Ammonia-N
(mgL ˉ¹)
F4, 108 = 0.379 ns 0.52 <0.6 mgL¯¹ Good
PO4-P
(mgL ˉ¹)
F4, 110 = 13.32 *** S1&S4 &
***S1 &S6
0.33 <1mgL¯¹ Poor
BOD5
(mgL ˉ¹)
F4, 108 = 0.879 ns 2.54 <5 mgL¯¹ High
Trace metals
(Cr, Cu, Zn, Cd,
Ni, Pb) (µgL ˉ¹)
ns Below
environmental
quality
standards/detec
tion limits
Very Good
Where p < .0001***; p < .05*; not significant (ns)
Correlation analysis showed a significant positive (p < .05) relationship
between most of the variables including NO3-N and PO4-P. Discharge correlated with
most variables suggesting its key influence on this system.
The maximum and minimum velocity (in cms¯¹) (Table 5-11) recorded in the
river fortnightly from March 2013 to April 2014 were estimated from instantaneous
velocity readings at the gauging station during the study period and related to each
site’s-catchment velocity. This aimed to establish the maximum and minimum
velocity that would influence the movement of substrate in the river. By using the
Hjulström-Sundborg diagram (Earle 2015), the particle size distribution was
135
estimated on the basis of the velocity conditions, under which sediment was either
eroded, transported or deposited. The results showed that at maximum velocity
(52.63 -100 cmsˉ¹), particles ≤ 0.1mm e.g. silt are transported in suspension while at
minimum velocity (11.36 -15.72cmsˉ¹; particles ≤ 1 mm e.g. sand are transported and
particles ≤ 2mm e.g. gravel are deposited as bed load. These results suggest an
unstable sediment regime which is likely to affect the community of benthic
macroinvertebrates.
Table 5-11: Maximum and minimum velocity recorded at the Medlock with for fortnightly data and
at the sample locations and instantaneous readings from the EA gauging station (*EA data indicates
that records were not obtained during the period).
Sites Maximum Velocity
(cmsˉ¹)
Minimum Velocity
(cmsˉ¹)
S1 52.63 15.72
S2 58.82 11.36
S4 100.00 0*
S5 100.00 13.33
S6 66.67 0*
5.3.2 Benthic macroinvertebrates
5.3.2.1 Community structure
A total of 32 benthic macroinvertebrate families were recorded in the River
Medlock at the five sites between March 2013 and April 2014 where benthic
invertebrates could be safely collected. The benthic macroinvertebrate families were
distributed across three phyla (Annelida, Arthropoda and Mollusca). The phylum
Arthropoda was important as they contributed 23 benthic invertebrate families which
made up 72% of the total identified at the river. The most common taxa which
occurred at all the sites were largely insects (Baetidae, 19%; Chironomidae, 17%) and
Annelids (Lumbriculidae, 14%, Tubificidae, 38%).
136
5.3.2.2 Spatial analysis of assemblage structure
Benthic macroinvertebrate assemblages in the river were analysed using the
Non-metric multidimensional scaling (nMDS) and Similarity Percentages (SIMPER)
multivariate tests. nMDS plot in Figure 5-6 indicated that sites S1 and S2 were similar
in composition and, sites S4 to S6 also grouped together.
Site Number1
2
4
5
6
2D Stress: 0.27
Figure 5-6: MDS ordination plots of benthic invertebrates at sample sites 1-2, 4-6 grouped together.
Similarity percentages (SIMPER) were used to distinguish invertebrate
families that made the greatest contribution to the differences identified by the
ordination plots in Table 5-12. The results from SIMPER analysis of the invertebrate
assemblages showed that both S1 and S2 were similar at 44% while between S4, S5
and S6 the percentage similarity was 53%. The highest average dissimilarity between
all sample sites was observed for S1 and S5 at 64.71% (Table 5-12). This difference
could be associated with the distance between the upper S1 and lower S5 and the
difference in urban extent and differences in altitude. While S1 had an urban area of
33%, 6.6km from source, S5 had an urban area of 47% and 16.1km from source. Also
S1 had more pollution sensitive taxa including Heptageniidae, Leuctridae,
Ephemerellidae and Perlodidae compared to S5.
137
Table 5-12: The top-ranked SIMPER contributors to % dissimilarity in benthic macro invertebrate
composition between S1 and S5. Figures in bold were highlighted based on the high ranking
pollution sensitive taxonomic groups.
Average dissimilarity = 64.71%
Site and distance
from source
Site 1
(6.60km)
Site 5
(16.10km)
No. Benthic
invertebrates
Average
Abundance
Average
Abundance Contribution
(%)
Cumulative
contribution
(%)
1 Gammaridae 0.19 1.65 12.19 12.19
2 Heptageniidae 1.38 0.47 8.78 20.97
3 Chironomidae 1.15 0.98 8.02 28.98
4 Simuliidae 0.81 0.19 5.99 34.97
5 Tubificidae 0.84 0.07 5.94 40.92
6 Paediciidae 0.79 0.11 5.66 46.58
7 Lumbriculidae 0.69 0.45 4.77 51.34
8 Leuctridae 0.41 0 4.69 56.03
9 Baetidae 1.41 1.73 4.65 60.69
10 Ephemerellidae 0.54 0.07 4.64 65.32
11 Hydropsychidae 0.35 0.42 4.29 69.61
12 Perlodidae 0.46 0 3.99 73.6
13 Lumbricidae 0.38 0.47 3.85 77.45
14 Tipulidae 0.61 0.47 3.6 81.06
15 Erpobdellidae 0.25 0.56 3.23 84.29
16 Asselidae 0.27 0.30 2.99 87.28
17 Rhyacophilidae 0.19 0.24 2.33 89.61
18 Limnephilidae 0.08 0.21 2.3 91.91
5.3.2.3 Temporal variation in benthic invertebrates and biotic scores
Temporal variation in the benthic macroinvertebrates was analysed using the
multidimensional scaling ordination (MDS) plot as shown on Figure 5-7. The MDS
plot showed slight differences between the seasons. The highest average dissimilarity
in the assemblage of benthic invertebrate families occurred between summer and
winter at 61.62%, based on similarity percentage (SIMPER) analysis (Table 5-13)
while autumn and spring were more alike suggesting a low degree of intra-seasonal
variability in invertebrate community. Tubificidae, Tipulidae, Lumbriculidae,
Heptageniidae, Simuliidae, Hydropsychidae and Paediciidae were more abundant in
138
the winter than in the summer. Baetidae and Gammaridae were abundant in the
summer.
SeasonSp
Su
A
W
2D Stress: 0.26
Figure 5-7: MDS ordination plot of benthic invertebrates based on seasonal patterns of spring (Sp),
summer (Su), autumn (Au) and winter (W).
139
Table 5-13: Assemblage of benthic macroinvertebrates between the summer and winter detected by
SIMPER. Figures in bold showed the taxa which dominated the river based on the average
abundance.
Average dissimilarity = 61.62%
Summer
(June –
August)
Winter
(December –
February)
No Benthic
invertebrates
Average
Abundance
Average
Abundance
% contribution Cum.%
1 Tubificidae 0.20 1.02 8.95 8.95
2 Tipulidae 0.18 0.89 7.97 16.92
3 Lumbriculidae 0.21 0.83 7.41 24.33
4 Heptageniidae 0.38 1.14 7.31 31.65
5 Baetidae 1.67 1.18 7.16 38.81
6 Chironomidae 1.11 1.57 7.10 45.90
7 Simuliidae 0.18 0.81 5.64 51.54
8 Hydropsychidae 0.34 0.60 5.53 57.07
9 Paediciidae 0.28 0.66 4.67 61.74
10 Erpobdellidae 0.12 0.49 4.61 66.35
11 Ephemerellidae 0.67 0.07 4.60 70.95
12 Gammaridae 0.80 0.58 3.93 74.88
13 Asselidae 0.17 0.38 3.57 78.46
14 Lumbricidae 0.15 0.28 3.52 81.97
15 Rhyacophilidae 0.18 0.34 3.43 85.40
16 Leuctridae 0.45 0.00 3.29 88.70
17 Limnephilidae 0.30 0.07 2.70 91.40
Classification of the river using the River Invertebrate Classification Tool
(RICT) during spring and autumn showed that there was a decline in the number of
taxa at the study sites (Table 5-14) even though no significant (p>0.05) difference was
found between both seasons. Based on the number of taxa in the river, the
Environmental Quality Ratio (EQR) classified S1 as “Good” with a value of 0.73,
“Moderate” at S2 (with a value of 0.63) and “Bad” from S4 to S6 (with a value <0.42).
In order for the river to achieve a high/good category, the number of taxa (NTAXA)
must be close to an EQR of 0.8. Classification which was based on the average score
per taxon (ASPT) weighed abundance metric indicated the river to be “moderately
polluted” at all the sites except S5 which was “Poor”.
140
Table 5-14: Biological classification results with environmental quality ratio (EQR)
Sites Indices EQR Status
Indices EQR Status
S1 NTAXA 0.73 Good ASPT 0.85 Moderate
S2 NTAXA 0.63 Moderate ASPT 0.86 Moderate
S4 NTAXA 0.41 Bad ASPT 0.78 Moderate
S5 NTAXA 0.35 Bad ASPT 0.75 Poor
S6 NTAXA 0.35 Bad ASPT 0.76 Moderate
5.3.2.4 BMWP, WHPT, ASPT and LIFE scores
BMWP scores and ASPT declined downstream of the river (Figure 5-8). A one-
way ANOVA showed a significant (p < .05) difference between the sites S1 and S6 for
the BMWP score (F4, 63= 3.889) and for ASPT (F4, 63 = 3.513). The average BMWP
scores and ASPT did not exceed 40 and 4.5 indicating the river to be polluted or
otherwise impacted (Hawkes 1998).
S 1 S 2 S 4 S 5 S 6
0
2 0
4 0
6 0
8 0
1 0 0
S a m p le L o c a t io n s
BM
WP
Sc
or
es
S 1 S 2 S 4 S 5 S 6
0
2
4
6
8
1 0
1 2
S a m p le L o c a t io n s
AS
PT
Figure 5-8: Box and whisker plot with 25% and 75%, median, minimum and maximum values of
BMWP score (A) and ASPT (B) with distance along River Medlock
While a significant (p<0.05) difference was found between the sample sites for WHPT
ASPT (F4, 62 =5.60), there was no difference for the number of taxa (Figure 5-9).
A B
141
S 1 S 2 S 4 S 5 S 6
0
2
4
6
8
1 0
S a m p le L o c a t io n s
WH
PT
AS
PT
S 1 S 2 S 4 S 5 S 6
0
2
4
6
8
1 0
1 2
1 4
S a m p le L o c a t io n s
WH
PT
NT
ax
a
Figure 5-9: Box and whisker plot with 25% and 75%, median, minimum and maximum values for
monthly samples obtained between March 2013 and April 2014 of (A) WHPT ASPT (B), (B) WHPT
NTaxa with distance along the river Medlock
Comparative analysis of BMWP ASPT, number of taxa with WHPT ASPT and
WHTP NTAXA (Figure 5-10) showed a significant (p<0.05) correlation. ASPT
(r=0.877, Pearson correlation) and the number of taxa showed an almost perfect
relationship. This result indicates that either BMWP or WHPT ASPT or NTAXA
could equally be used in the classification of invertebrates. The comparison of the two
indices for tributaries of River Swale’s metal impacted catchment showed some
similarities (Barber 2014).
0 2 4 6 8 1 0 1 2 1 4
0
2
4
6
8
1 0
1 2
1 4
W H P T N T a x a
BM
WP
NT
ax
a
3 4 5 6 7
2
3
4
5
6
7
W H P T A S P T
BM
WP
AS
PT
Figure 5-10: Comparison of WHPT and BMWP number of taxa and ASPT
The results of LIFE index (Table 5-15) suggested the river to be fast flowing,
having an overall LIFE score average of 7.5. This result might be expected given the
maximum and minimum velocities recorded during the survey and suggests
moderate to high flow conditions.
A B
142
Table 5-15: LIFE results summary
Sample
sites
n taxa LIFE
S1 147 18 8.17
S2 146 23 6.35
S4 137 17 8.06
S5 129 16 8.06
S6 90 11 8.18
5.3.2.5 Composition of the benthic macroinvertebrate community in colonisation
samplers
A total of 12 macroinvertebrate taxa were found in the colonisation samplers
deployed at S2 (4km above the WwTW) and S6 (5km below the WwTW). After each
30-day period over a four-month duration, the colonisation samplers were
dominated by the crustacean family Gammaridae (67%), followed by the insects
Chironomidae (14%) and Hydropsychidae (8%) plus a further crustacean, Asellidae
(5%) (Figure 5-11).
Gammaridae was more abundant at the lower site S6 compared to upstream
S2 which was expected to be in the cleaner part of the river. Kick sampling results
also showed that Gammaridae was abundant at the downstream sites. The results
suggest that a difference in substrate was not the reason for the abundance of
Gammaridae.
143
Figure 5-11: Benthic invertebrate community composition in the invertebrate colonisation samplers
at S2 (4km upstream of the WwTW) and S6 (5km downstream of the WwTW).
5.3.3 Relationship between physico-chemical, hydrogeomorphological
variables and benthic macroinvertebrate assemblages
All environmental variables combined in a PC matrix (Table 5-7) were
combined with benthic macroinvertebrate assemblages in the BIOENV analysis to
determine which variable(s) affected benthic invertebrate abundance and
distribution. BIOENV analysis revealed that the most important variables structuring
benthic macroinvertebrate communities were conductivity, PO4-P, discharge,
catchment area, altitude and slope based on correlation matrix ρ = 0.274 (Table 5-16).
Several correlation analyses were performed with fewer variables to test if there were
144
any changes in the BIOENV outcome. With selected variables from Table 5-8, the
most important variables were conductivity, NO3-N, PO4-P and discharge, (ρ = 0.225).
Table 5-16: Correlation between physico-chemical variables and benthic invertebrate assemblages
using the BIOENV procedure. The correlation was carried out using a series of different number of
variables.
No. of
variables
Weighted
Correlations
ρ Selections of variables
0.274 5
Conductivity, Discharge, Catchment area,
Altitude, Slope
0.273 4
Conductivity, Discharge, Catchment area,
Altitude
0.272 5
Conductivity, PO4-P,Discharge, Catchment
area, Altitude
0.272 2 Conductivity, Catchment area
0.271 5
Conductivity, NO3-N, Discharge, Catchment
area, Altitude
0.271 5
Temperature, Conductivity, Discharge,
Catchment area, Altitude
0.269 5
Conductivity, PO4-P, Discharge, Catchment
area, Slope
0.268 5
DO, Conductivity, Discharge, Catchment area,
Altitude
0.268 5
Conductivity, NO3-N, Discharge, Catchment
area, slope
0.267 4
Conductivity, Discharge, Catchment area,
Slope
5.4 Summary of results
• Dissolved oxygen, pH, BOD, ammonia-N, NO3-N showed that the water
quality is “Good”/ “Low pollution” except PO4-P which was >0.1mg/L and
classifies the river as “poor”. However the benthic invertebrate classification
indicates the river to be “moderately polluted”.
• Spatial variation for physico-chemical and benthic macroinvertebrates showed
the upstream sites (S1 and S2) of the WwTW were better than downstream
sites (S4 to S6). Gammaridae, a taxa found in moderately polluted rivers, was
145
found to be dominant at downstream sites especially S5 for (kick sampling)
and S6 (colonisation samplers) even though BMWP scores indicated pollution.
• There was a strong seasonal dissimilarity in benthic macroinvertebrate
abundance between winter and summer. While Oligochaeta, Chironomidae
and Simulidae were abundant during winter, Gammaridae was abundant
during summer.
• The relationship between physico-chemical variables and benthic
invertebrates using the BIOENV analysis showed conductivity, discharge,
catchment area, altitude, slope and nutrients as factors which described the
differences between the study sites. While altitude, slope and catchment area
could be related to the location of the sampling sites, the concentration of
conductivity and nutrients in the river is related to increase in discharge.
• The LIFE index indicates that the benthic macroinvertebrate assemblages were
affected by variation in flow and therefore probably affect the sediment
instability.
• The NTAXA and ASPT from BMWP were compared with WHPT NTAXA and
ASPT and the results revealed no (p>0.05) difference between the indices.
5.5. Discussion
The aim of this chapter was to determine the relative importance of water
quality and quantity (discharge and flow) on the benthic macroinvertebrate
community of the River Medlock. The results of macroinvertebrate sampling from
March 2013 to April 2014 and the colonisation sampling which took place from
September to December 2014 revealed that the Medlock was a moderately polluted
system on the basis of fewer numbers of pollution sensitive taxonomic groups.
Various studies of benthic invertebrates in urban rivers have revealed similar
patterns of fewer or absent pollution sensitive taxa including Beavan et al. (2001) in
the River Tame catchment, UK and worldwide from studies in Brazil, USA, Australia
and Canada (Grapentine et al. 2004; Guimaràes et al. 2009; Mikalsen 1989; Silveira et
al. 2006; Walsh et al. 2001; Wright et al. 2007; Whitehurst & Lindsey 1990). These
146
studies have all associated faunal impoverishment, degradation of the river and the
loss of sensitive taxa to urbanisation rather than water quality. Thus, the loss of
sensitive invertebrates, biological degradation in other urban catchments (Duda et al.
1982) plus the Medlock are part of the complex changes in urban rivers resulting
from a flashy hydrograph and altered channel morphology collectively termed the
“urban stream syndrome”(USS) (Walsh et al. 2005; Walsh et al. 2012). The most
common taxa recorded at all sites throughout the study were Baetidae,
Chironomidae, Lumbriculidae, and Tubificidae. While these taxonomic groups
tolerate organic pollution, some researchers have also attributed their dominance to
the deposition of silt and sand substrate arising from episodic discharge (Macan 1962;
Chutter 1969; Langford & Bray 1969).
There are a number of possible reasons for the less degraded invertebrate
community at the upper sites. S1 has 33% average sub-catchment urban area,
whereas the average at S4 to S6 was 45% and are classified as “heavily modified
waterbody” (HMWB) by the Environment Agency (Environment Agency, 2009).
Waterbodies such as the Medlock are identified as HMWBs when physical
modifications to the river negatively impact on its quality as a result of the USS. The
upstream and downstream pattern in river quality corroborated with the studies of
Guimaràes et al., (2009) who recorded a more diverse invertebrate community at an
upstream site which had a vegetation corridor and therefore less pollution from
diffuse source runoff. While S1/S2 reflect the lack of direct anthropogenic activity, S4
to S6 are located within the urban area and therefore subject to various impacts such
as erosion and modification of substrate, as well industrial and WwTw effluent. Some
pollution tolerant taxa including Tubificidae, Lumbricidae, Lumbriculidae,
Chironomidae and Simuliidae were found at the upstream sites. These taxa are most
likely to be exhibiting responses to episodic discharges from stormwater outfalls or
CSOs (Grapentine et al. 2004) which are of a sufficient magnitude to select for tolerant
benthic taxa. Gammaridae were abundant at the downstream sites. This taxa favours
a highly oxygenated river system for reproduction and are present in moderately
147
polluted rivers (Hynes 1970). However, their dominance in certain systems has been
linked to their feeding plasticity. Gammaridae fall within the herbivore/shredder
guild (Macneil et al. 1997), and hence the presence of organic detritus and other
foodstuff within the substrate, plus allochthonous leaf litter and the associated
microbial community contribute to an increase in population (Macneil et al. 1997) in
the lower Medlock.
SIMPER analysis of the invertebrate assemblage showed the importance of
temporal variation as average dissimilarity between winter and summer was 62%.
Oligochaeta, Chironomidae and Simuliidae were abundant in winter compared to
summer assemblages. High precipitation during winter increases flow and discharge,
increasing run-off and causing instability to the river bed, thereby providing more
food for pollution tolerant, fast-growing and fast colonising deposit/suspension-
feeding taxa such as Oligochaetes, Chironomidae and Simuliidae (Fonseca and Hart,
1996; Grapentine et al., 2004 and Silveira et al., 2006). During summer Gammaridae
increased in abundance, particularly at the downstream sites. Thus, Gammaridae
exploit seasonal changes in abundance of specific foods and also are able to rapidly
colonise new and variable habitats (Schwartz 1992). Furthermore, the ability of
Gammaridae to exploit a variety of foods is a selective advantage in rapidly changing
environments, predating on Asellus as well as exploiting feeding on allochthonous
material (Macneil et al. 1997). The Gammarus to Asellus index tend to be more
sensitive in organic enriched systems (Whitehurst 1991; Whitehurst & Lindsey 1990)
which the Medlock is not and provides further indication of other pollution sources.
The relationship between environmental variables and benthic
macroinvertebrate community suggests the impact of discharge, conductivity,
altitude, slope, catchment area and nutrients. Indicators of organic pollution,
specifically ammonia and BOD did not appear to influence the invertebrate
community, probably because both were low due to effective treatment of sewage
by the WwTWs and little contribution from CSOs (see Chapter 3). Altitude is a
surrogate for river gradient (Roesner & Bledsoe, 2003), slope and catchment area are
148
factors which describe the differences between the location and distance of study
sites. Increased conductivity at sites downstream between 483 and 693μScmˉ¹ could
be related to the catchment geology and the influence of urban streams (Walsh et al.
2005).
Although changes in flow during and after storm events have not been
assessed in this study, the LIFE (Lotic Invertebrate-Index Flow Evaluation) index
indicates that the river was fast-flowing and therefore could impact on pollution
sensitive assemblages (Extence et al. 1999). The influence of flow variability on the
benthic invertebrate community will be masked by pollution (e.g. Monk et al., 2006)
but the use of the LIFE index here is appropriate in the absence of marked pollution
impacts. Other studies have identified the impact of flow on benthic
macroinvertebrates including Monk et al. (2006) from a study of 83 catchments in
England and Wales. In this study, various factors including river flow is shown to be
a valuable predictor of the instream physical environment and provides a better
understanding of river ecosystems than water quality alone (Poff et al. 1997).
PCA which was used to reduce the large number of variables to few
parameters suggested that apart from water quality variables, substrate type was one
of the factors which structured macroinvertebrate assemblage in this study. This
result corroborate other studies that state the suitability of a substrate as a primary
factor governing colonisation by benthic macroinvertebrates (Hynes 1970; Silveira et
al. 2006). The instability of river substrates is unsuitable for the colonisation of
benthic macroinvertebrates as it reduces both diversity and density. Hynes, (1970)
and McCulloch, (1986) have reported reduced densities and diversity in sandy and
heavily silted streams such that occurs during periods of low flow in the Medlock.
Comparison of BMWP ASPT/NTAXA and the WFD’s WHPTASPT/NTAXA
index showed no significant difference in output. However, the similarity of both
systems provides an advantage as BMWP serves the basis for communication to non-
river ecologists and also for routine monitoring (Armitage et al. 1983; Hawkes 1997)
while the WHPT could be used for more technical communication and reports where
149
a greater degree of precision is required. In addition, the similarity allows
comparison with the historical data-sets. However, WHPT could provide a better
indicator of subtle changes in macroinvertebrate abundance or community structure
than the BMWP which may indicate environmental stress (Environment Agency
2015b).
This study suggests that a comparison of observed with reference, pristine
conditions determined by RICT for WFD standards is inadequate to classify an urban
river. This is because RICT does not take into account direct effects of flow and
discharge, plus an indirect effects on substrate arising from the ‘flashy’ nature of
urbanised catchments. This study has shown that the quality of urban rivers such as
the Medlock is strongly influenced by variability in the magnitude of discharge and
frequency of flow and requires an integrated classification tool that accounts for
change in flow and discharge. Reducing the magnitude of changes in discharge and
flow requires the cooperation of the various stakeholders to facilitate changes to land-
use and effluent management to moderate the flow regime, including the application
of both hard and soft engineering solutions.
5.6 Conclusion
The various multivariate tools and biotic metrics used in this study
contributed to show the physico-chemical variables which influenced the River
Medlock. This river is subject to variability in the magnitude of discharge and
frequency of flow. The quality of the Medlock, though of good water quality except
for PO4-P, is however classed as moderately polluted on the basis of biotic indices
and therefore cannot comply with the EU WFD which requires both water chemistry
and ecology to achieve “good ecological status”. These changes are due to direct and
indirect effects of discharge. Thus, the Medlock, like other urban rivers within the EU
is influenced by the integration of factors collectively termed the urban stream
syndrome as it is influenced by natural, semi-natural and anthropogenic factors.
150
Acknowledgements
I am grateful for financial support from The National Open University of Nigeria.
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Chapter 6 SHORT TERM WATER
QUALITY VARIABILITY IN AN
URBAN RIVER SUBJECT TO POINT
AND DIFFUSE SOURCE POLLUTION
Abstract
Fifteen minute in situ conductivity, turbidity, dissolved oxygen, pH, and
temperature measurements plus, continuous discharge data were obtained from 1st
August 2014 to 31st October 2014 at the Environment Agency gauging station on the
lower River Irwell, Manchester. This study aimed to determine the impact of
combined sewer overflows (CSOs) during short duration events on the basis of the
water quality variables. The concentration-discharge relationship showed some
variables to be lowered (pH, conductivity, PO4-P and NO3-N) while suspended solids
increased with discharge. However, during separation of hydrographs, the study
revealed increased PO4-P concentration at high discharge. Peaks of suspended solids
and PO4-P observed on the hydrographs suggest spills from CSOs while continuous
high concentration during the limb recession points to other pollution sources. All the
variables conformed to the requirements of the WFD standards at all discharges apart
from PO4-P and suspended solids.
Key words: Conductivity, suspended solids, discharge, gauging station, Water
Framework Directive
6.1 Introduction
Most river catchments in the United Kingdom are affected by urbanisation
(Lamb et al. 2003) and its forecast that urban areas will house 68% of the total global
population by 2050 (United Nations, 2014) and hence will continue to increase in
extent. In the UK, urban areas already contain 79% of the total population and this is
projected to increase to 86% by 2050.
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Rivers can be classified based on either topology or discharge and water
chemistry (Lindsay, et al. 2008). Two categories based on discharge and water
chemistry where identified in the River Medlock; from the river’s source to Lumb
Brook 12km downstream and from Lumb Brook, 5 km to the confluence of the River
Irwell. The current ecological status from the source to Lumb Brook is regarded as
“good” based on Environment Agency’s 2015 prediction while from Lumb Brook to
the confluence with the River Irwell the Medlock is designated a ‘heavily modified
water body’ (HMWB) and classified as “poor potential” for ecological status and
considered at risk (Environment Agency 2015a).
Chapters 3 and 4 examined the long term and medium term variability of the
physico-chemical parameters on the Medlock. These studies aimed to establish the
historic patterns, spatial and temporal conditions of the river and to identify which
variable affected the river’s overall quality. The results suggested that river discharge
was an important factor which influenced the physico-chemical variables, plus
abundance and distribution of benthic macroinvertebrates (Chapter 5). As the
physical variables and, with the exception of PO4-P, chemical indicators of water
quality indicated “good” chemical quality, it was expected that the Medlock would
also be of good ecological status under the WFD. The degraded invertebrate
community was ascribed to direct and indirect effects of changes in discharge and
flow.
In this chapter, the short term variability in the water quality of the River
Medlock, was examined at the Environment Agency’s gauging station. By this point
the river has received discharges from a variety of sources described above, including
WwTWs, CSOs, surface water drainage and agricultural runoff. This study aims to
understand the dynamics of physico-chemical variables in the lower Medlock by
examining high resolution datasets that allow an examination of extreme events
compared to monthly and or biweekly records (Jarvie et al. 1998). As part of this
study, the phenomenon of the “first flush”, described as the first part of runoff which
is the most polluted during storm events, is examined. The first flush is composed of
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runoff from rainwater, roofs, discharge from separate and combined sewer systems
(Deletic 1998; Lawler et al. 2006; Lee et al. 2002) and will be examined on the basis of
the changes in the physico-chemical variables during storm conditions.
A combination of 15-minute continuously monitored pH, conductivity,
turbidity, temperature plus spot samples collected for suspended solids, NO3-N and
ammonia-N and PO4-P were examined from the 1st of August to 31st October 2014 at
the EA’s London Road gauging station. In addition suspended solids concentration at
15 min intervals was estimated from the relationship between sampled suspended
solid concentration and the corresponding turbidity record. The gauging station is
approximately 2.9km upstream of the river’s confluence with the Irwell and 6km
downstream of the main WwTW on the Medlock.
6.1.1 Aims and objectives
The overall aim is to identify the key influences of water quality during storm events
by examining high resolution datasets over a period of three months and to
determine the changes in the concentrations following continuous precipitation and
discharge.
The objectives are:
1. to determine the relationship between pollutant concentration and discharge,
2. to examine the contribution of episodic pollution to the concentration and load
of key variables
3. to estimate the nutrient load exported from the river
The hypothesis is that short-term (less than 24 hr) changes in water quality are due to
discharges from combined sewer overflows (CSOs) rather than the WwTW.
6.1.2 Site Description
The River Medlock is one of five rivers which form part of the River Irwell
catchment. The River Medlock rises to the northeast of Manchester (Figure 6-1) and
158
drains a largely urbanised catchment of 57.5km². (National Rivers Flow Archive,
2015).
Water quality data were captured at the EA’s river discharge gauging station
(SJ 848975) 2.9 km from the confluence with the River Irwell in Manchester city
centre. At the gauging station, the river has a mean annual flow of 0.819m3s-1 (1974-
2013; National Rivers Flow Archive, 2015). Average annual rainfall over the Medlock
catchment between 1961 and 1990 was 1033mm (National Rivers Flow Archive,
2015). The gauged station consists of a non-standard short crested weir 8.5m wide
with a sloping downstream face. The weir is located in a rectangular concrete channel
with vertical walls upstream of a large culvert. The measurement of discharge is by
stilling well and float. The maximum gauged level is 0.48m, maximum gauged flow
is 6.75m³sˉ¹ and the bankfull stage i.e. the stage at which a river overflows its natural
banks and is likely to cause damage is 3.55m (49.92m³s¯¹).
159
Figure 6-1: Location of River Medlock showing catchment area and degree of urbanisation and
the gauging station (Source, EDINA).
6.2 Methodology and approach
6.2.1 Continuous sampling programme
A pre-calibrated YSI 6600 V2 multiparameter water quality sonde (Yellow
Springs Incorporated, Ohio-USA) was installed adjacent to the gauging station from
the 1st of August 2014 until the 31st October, 2014, a total of 92 days. pH, temperature,
dissolved oxygen, turbidity and conductivity were recorded at 15-minute intervals.
The sonde was calibrated using a turbidity (0NTU distilled water and Xylem
160
Analytics 126NTU suspension), pH (Xylem Analytics pH7 and pH10 buffer
solutions) and conductivity (Xylem Analytics 1413µScm¯¹) standards following the
manufacturer’s calibration procedures. Dissolved oxygen was calibrated using the
“open-cup” calibration method in which a container is filled with small amount of
water which is allowed to equilibrate with the surrounding atmospheric conditions,
as per the manufacturer’s procedures and calibrated monthly.
Continuous turbidity measurements allows direct access to the dynamics of
particulate pollution (Franklin et al. 2001; Hannouche et al. 2011) and can be used as
a surrogate variable for the measurement of suspended solids (Métadier & Bertrand-
Krajewski 2011). In this study, suspended solids measured at the laboratory and the
corresponding turbidity sample collected from continuous data were plotted to
produce a suspended solids-turbidity linear regression equation y = 0.4405x + 4.486; r
= 0.58, n=48 p<0.0001 (see Appendix). Therefore, on the basis of this relationship, the
concentration of suspended solids determined was used in the description of result
and analysis of this study.
A 15-minute discharge record was calculated from stage records using a rating
curve supplied for the gauging station. Precipitation data was obtained from the
Whitworth Meteorological Observatory (SJ 84681 96760) managed by the Centre for
Atmospheric Science, School of Earth and Environmental Sciences, University of
Manchester.
6.2.2 Spot sampling
Water samples were collected from the gauging station in order to establish
concentration-discharge relationships. Samples were obtained by lowering a bucket
into the river from the bridge located immediately upstream of the gauging station.
These samples were then decanted into a one-litre sample container for subsequent
analysis back at the laboratory. Water samples were filtered through a 0.45 μm
Whatman G/F membrane filter and stored at 4°C prior to analysis. Samples were
161
analysed for NO3-N, PO4-P, ammonia-N and suspended solids as described above in
Chapter 2.
Hydrograph separation were identified based on high correlation-discharge
coefficients (Caissie et al. 1996). This graphical method (Dingman, 2002) was also
used in this study as it is straightforward and used by many researchers to describe
river hydrodynamimcs (Blume et al. 2007).
Data and statistical analysis was carried out using MS Excel 2010 and
GraphPad Prism version 6.
6.3 Results
Table 6-1 provides a summary of the data collected from the river between
01/08/14 and 31/10/14. The relative standard deviation indicates a large temporal
variation for all variables except pH and dissolved oxygen. Discharge varied 40-fold
from 0.16m³s-1 to 6.91m³s-1; conductivity ranged from 188µScm-1 to 682µScm-1 with a
mean of 519µScm-1; turbidity ranged from 1NTU to 1448NTU with a mean of 34NTU
and suspended solids (SS, derived from the SS/turbidity relationship) ranged from 4
mgL-¹to 646 mgL-¹ with a mean of 19mgL-¹. PO4-P was present in very high
concentration that varied 7-fold from 0.17mgL-¹ to 1.20mgL-¹ and with a mean of
0.49mgL-¹. NO3-N and ammonia-N were present in very low concentrations in the
river with ammonia-N having a minimum concentration below the detection limit of
0.01 mgL-¹ and not exceeding 1.1 mgL-¹and NO3-N having a mean concentration of 3.9
mgL-¹. pH measurements showed the river to be near neutral, ranging from 7.3 to 8.3
and a mean of 7.9 while dissolved oxygen ranged from 59% to 106% with a mean of
84% and water temperature from 9°C to 18°C. Maximum precipitation during
recorded the period was 7.3mm.
162
Table 6-1: Mean and range of physico-chemical variables in the River Medlock at London Road
gauging station between August and October 2014. Data is either from 15-minute continuous
analysis or spot samples for ammonia-N, NO3-N and PO4-P. Precipitation is from the Whitworth
Meteorological Observatory. N = number of samples
Parameter N Minimum Maximum Mean Std. Dev. RSD
(%)
Precipitation
(mm) 8825 0.00 7.37 0.02 0.18 736.44
Flow velocity
(ms¯¹) 8825 2.70 120.00 8.5 8.3 98.32
Discharge
(m³s¯¹) 8825 0.16 6.91 0.49 0.48 98.30
Temperature
(°C) 8825 9.3 18.03 13.00 1.8 13.17
Conductivity
(µScm¯¹) 8825 188.00 682.00 519 86.00 16.67
pH 8825 7.30 8.30 7.90 0.16 1.98
Turbidity
(NTU) 8825 1.00 1448.00 34 103 304.14
Dissolved
oxygen (%
saturation)
8825 59.00 106.00 84.00 7.80 9.21
Suspended
solids (mgL¯¹) 8825 0 646.00 19.00 46 241.85
Ammonia-N
(mgL¯¹) 48 ≤0.01 1.1 0.34 0.24 72.00
NO3-N (mgL¯¹) 50 1.1 8.40 3.9 2.00 52.41
PO4-P (mgL¯¹) 50 0.17 1.20 0.49 0.25 51.54
163
6.3.1 Discharge and Precipitation
Table 6-2 presents discharge and corresponding precipitation records during the
study and the hyetograph of the relationship is shown on Figure 6-2. The mean
discharge at the gauging station measured for the sampling duration was 0.49m3s-1,
mean precipitation was 0.02mm, and total precipitation recorded was 209.69mm.
August 2014 was very wet, including numerous days with light rainfall; no rain fell
only on 4th and 23rd August 2014. The highest precipitation and peak discharge
(6.91m³s⁻¹) was recorded on the 11th August 2014. As shown on Table 6-2 other high
discharges were also recorded in August 2014 while the lowest records were
obtained in September 2014 (5th & 23rd September 2014) and from 11th to 13th October
2014. A significant correlation was found between discharge and precipitation
(Pearson correlation, n=8825, p < .01) which, as expected, showed that there was a
trend of increasing water volume with increased rainfall.
Table 6-2: Total precipitation and average discharge from 1st August to 31st October 2014.
Sample
months Number of days
Total precipitation
(mm)/month
Average discharge
(m3s-1)/month
Aug-14 31 128.12 0.68
Sep-14 30 17.7 0.28
Oct-14 31 63.87 0.5
Total 92 209.69
164
Figure 6-2: Hyetograph taken over 15-minute duration from 1st August 2014 to 31st October 2014.
6.3.2 Correlation of physico-chemical variables with discharge and
intercorrelation between variables
The relationship between, respectively, conductivity, pH, precipitation and
spot samples of NO3-N, PO4-P, ammonia-N and mean continuous discharge over 90
days is shown in Figure 6-3. 15-minute interval measurements are shown for pH,
conductivity and suspended solids (Figure 6-3A-C) while discharge readings
corresponding to daily spot sample were plotted for PO4-P (r = -0.26, n=50, p>0.05);
NO3-N (r=-0.42, n=50, p<0.05) and ammonia-N(r=0.22, n=48, p>0.05) (Figure 6-3D-F).
Discharge was negatively correlated with all the variables (conductivity r= -0.69,
p<0.0001, n=8825; pH r= -0.36, p<0.0001, n=8825) except suspended solids (r = 0.18,
p<0.001, n=8825) and ammonia-N which showed a positive relationship. Suspended
solids vs discharge plotted in Figure 6-3C was influenced by the river’s desiltation
during the sampling period (see hydrograph of Figure 6-6B). While an increase in
discharge indicated the dilution of nutrients and dissolved salts, a positive
correlation with suspended solids suggested other sources of pollution to the river
including road runoff and CSOs. No correlation was found between discharge and
ammonia-N which indicates that the concentration of ammonia-N in the river was
independent of river discharge.
165
0 2 4 6 8
7 .2
7 .4
7 .6
7 .8
8 .0
8 .2
8 .4
D is c h a r g e (m ³s- 1
)
pH
0 2 4 6 8
0
2 0 0
4 0 0
6 0 0
8 0 0
D is c h a r g e (m ³s- 1
)
Co
nd
uc
tiv
ity
(µ
Sc
m- ¹)
0 1 2 3 4 5
0
2 0 0
4 0 0
6 0 0
D is c h a r g e (m ³s- 1
)
Su
sp
en
de
d s
oli
ds
(m
gL
- ¹)
0 1 2 3 4
0 .0
0 .5
1 .0
1 .5
D is c h a r g e (m ³s- 1
)
PO
4-
P (
mg
L-1
)
0 .0 0 .5 1 .0 1 .5 2 .0 2 .5 3 .0
0
2
4
6
8
1 0
D is c h a r g e (m ³s- 1
)
NO
3-N
(m
gL
-1)
0 .0 0 .5 1 .0 1 .5 2 .0 2 .5 3 .0
0 .0
0 .5
1 .0
1 .5
D is c h a r g e (m ³s- 1
)
Am
mo
nia
-N (
mg
L-1
)
Figure 6-3: Correlation of chemical determinands with discharge. (A) pH (B) Conductivity (C)
Suspended solids (D) PO4-P (E) NO3-N and (F) ammonia-N. Figures A, B and C are based on 15-
minute discharge while D, E and F correspond to the dates when spot samples were collected from
the river.
E F
D
A B
C
166
Figure 6-4: Relationship between physico-chemical concentrations measured in the River Medlock
at the gauging station between 1st August 2014 to 31st October 2014. (A) Mean daily pH and mean
daily conductivity (B) NO3-N and PO4-P
Figure 6-4 shows the strongest correlation to be between NO3-N and PO4-P (r =
0.87) and conductivity and pH (r = 0.43).
6.3.3 Temporal variability
Monthly changes in the physico-chemical parameters in the river from 1st
August 2014 to 31st October 2014 are presented in Figure 6-5A-I. All the variables
showed a significant (p<0.05) difference with month except ammonia-N. As expected,
there was a gradual decline in temperature from August to October. There was
consistently high percentage saturation of dissolved oxygen in the river. Very high
concentration of suspended solids (>25mgL¯¹) were recorded when compared to the
standard annual mean of 25 mgL¯¹ under the EU Freshwater Fish Directive (DEFRA,
2010). PO4-P concentration classified the river as “poor” under the WFD over more
than 80% of the sampling period as the concentration exceeded the 0.12mgL¯¹
standard. Concentrations indicative of “moderate” pollution of between 0.12mgLˉ¹
and 0.25mgL¯¹ were recorded, mainly in October. Mean concentration of ammonia-N
complied with the WFD standard of 0.6 mgL¯¹; however a few samples in August
had moderately high ammonia-N concentrations with a range of 0.79 to 1.06mgL¯¹.
A B
167
A u g S e p t O c t
0
2
4
6
8
S a m p l e m o n t h s
Dis
ch
ar
ge
(m
³s
-¹)
A
A u g S e p t O c t
0
2 0 0
4 0 0
6 0 0
8 0 0
B
S a m p l e m o n t h s
Co
nd
uc
tiv
ity
(µ
Sc
m-¹)
A u g S e p t O c t
7 .2
7 .4
7 .6
7 .8
8 .0
8 .2
8 .4C
S a m p l e m o n t h s
pH
A u g S e p t O c t
8
1 0
1 2
1 4
1 6
1 8
2 0
D
S a m p l e m o n t h s
Te
mp
er
atu
re
(°
C)
A u g S e p t O c t
4 0
6 0
8 0
1 0 0
1 2 0
E
S a m p l e m o n t h s
Dis
so
lv
ed
ox
yg
en
(%
sa
tu
ra
tio
n)
A u g S e p t O c t
0
2 0 0
4 0 0
6 0 0
8 0 0 F
S a m p l e m o n t h s
Su
sp
en
de
d s
olid
s (
mg
L-¹)
A u g S e p t O c t
0 .0
0 .5
1 .0
1 .5G
S a m p l e m o n t h s
Am
mo
nia
-N
(m
gL
-1)
A u g S e p t O c t
0
2
4
6
8
1 0
H
S a m p l e m o n t h s
NO
3-N
(m
gL
-1)
A u g S e p t O c t
0
2
4
6
8I
S a m p l e m o n t h s
PO
4-P
(m
gL
¯¹)
A
Figure 6-5: Box and whisker plots showing the variability in physicochemical parameters based on
15-minute continuous data collection or spot sampling between 1st August 2014 and 31st October
2014. (A) Discharge, (B) Conductivity, (C) pH, (D) Temperature, (E) Dissolved oxygen, (F)
Suspended solids, (G) Ammonia-N, (H) NO3-N, (I) PO4-P.
168
6.3.4 Chemical concentration vs discharge
On the basis of the correlation found between the chemical parameters and
discharge, hydrographs were plotted for conductivity and suspended solids by using
the continuous measurement discharge records (Figure 6-6) and spot samples for
PO4-P and NO3-N for the sample period (Figure 6-7).
Conductivity declined following high discharge as highlighted by the black
boxes on 1st August, 29th August 2014, 6th and 7th October (Figure 6-6A) indicating
dilution by run-off that presumably contains large quantities of low conductivity
rainwater. Suspended solids concentration was shown to be very high on 14 August
2014, 22- 28 September and 25 -29 September 2014 (Figure 6-6B). While the record
suggests sediment influx from the surrounding catchment which has been washed
into the river by rainfall events prior to 14th August, some days after the high
precipitation, the river still recorded high concentration of suspended solids.
169
Figure 6-6: Hydrographs of (A) Conductivity; (B) Suspended solids over 15-minute intervals from
1st August and October 2014.
The time series for continuous discharge and nutrients (Figure 6-7) showed
high nutrient concentration during periods of low discharge; a pattern which
suggests the effect of the discharge from the WwTWs. Occasionally, such as 10th
August 2014, a very high PO4-P level was recorded (0.73mgL¯¹) with increased
discharge, 6.82m³s¯¹ which suggests contributions from episodic point source,
probably CSOs, following storm events. The 1st of August provided a good example
of the variability in nutrient concentration at high discharge. At 11:00 a discharge of
B
A
170
0.19m3s-1, had a corresponding increase concentration of PO4-P of 0.82mgL¯¹; and NO3-
N of 6.43mgL¯¹; at 15:45, an increase in discharge to 0.44m³s-¹ resulted in 1.12mgL-1 of
PO4-P and 8.38mgL¯¹ NO3-N at 20:00hr. A further increase in discharge to 1.10m³s-¹
however resulted in fall in the PO4-P concentration to 0.68mgL⁻¹ and NO3-N to
3.89mgL¯¹. This implied that at the initial periods, where high PO4-P concentration
was recorded, the impact of episodic discharges (“first flush”) triggered by increases
in discharge was followed by dilution with continuous discharge. These results
indicate that both WwTW and CSOs contribute to PO4-P concentration in the river
although the latter only during initial periods of high rainfall. Neither WwTWs nor
CSO are significant sources of NO3-N.
171
Figure 6-7: Continuous time series of discharge in the River Medlock from 1st August 2014 to 31st
October 2014 and concentrations of (A) PO4-P and (B) NO3-N from spot sampling during low and
high flows.
Peak discharge events were selected for further analysis using hydrograph
separation to study the impact of storm events on chemical concentrations as seen in
other urban systems (e.g. Pilgrim et al. 1979). On the basis of peak discharge
identified in earlier hydrographs, the 10th August 2014 which was one of the periods
172
with the highest river discharge at 6.82m³s-¹ was used for hydrograph separation in
order to determine the effect of rainfall on conductivity, pH and suspended solids
(Figure 6-8A-C). Conductivity decreased with increasing discharge indicating some
dilution. Continued precipitation led to a further increase in discharge from 1.53m³s-¹
at 2100hr to 6.82m³s-¹ at 2315hr followed by an initial slight increase in conductivity
and, with continuous precipitation followed a progressive decrease in conductivity
levels. Increased precipitation over two hours increased discharge retention leading
to decreased conductivity. pH followed a similar pattern to conductivity but the
change was small with a fall of around 0.4 for one hour. Suspended solids correlated
very strongly with discharge (Figure 6-8C), (r=0.928, p < .05, Two-tailed t-test) with
both increasing proportionately.
173
Figure 6-8:Hydrograph separation of high discharge on the 10th August 2014 using 15-minute
continuous data for (A) conductivity; (B) pH; (C) suspended solids.
174
Figure 6-9 and Figure 6-10 below show the response of the river during two
rainfall events; long-duration at 27mm for 11 hours 15 minutes on the 1st August 2014
(Figure 6-9) and short-duration 15mm rainfall for 1hour 30 minutes on the 8th of
August 2014 (Figure 6-10). Total precipitation recorded on the 1st August 2014 when
it rained continuously for 11 hours 15 minutes between 1045hr and 2200hr, was
27mm. There was no recorded change in discharge and conductivity (Figure 6-9A)
for five hours when rainfall rate increased from 0.1mm to 6.76mm by 1530hr.
Conductivity levels decreased after 30 minutes by 16% from 674µScm¯¹ to 562µScm¯¹.
A decrease in precipitation and increasing discharge led to lower conductivity, falling
to 364 µScm¯¹ (i.e. 46% less than the initial value) at 2015hr followed by an increase to
518µScm¯¹ at 2100hr and then a decrease to 362 µScm¯¹ at a discharge of 1.58m³sˉ¹
when the rain stopped four hours later. The converse was the case with suspended
solids (Figure 6-9B) which increased with increasing precipitation and discharge,
from 8mgL- ¹ to 46mgL¯¹ at 2145hr. As earlier observed, there is a significant (r²= 0.939,
p < .001) correlation between suspended solids and discharge.
175
Figure 6-9: Hydrograph separation for the long duration rainfall (27mm) event on 1st August 2014
using 15-minute continuous reading for (A) Conductivity (B) Suspended solids with discharge
data and total precipitation. The ellipse highlights the decrease in in conductivity levels with the
rising limb of the hydrograph and increase in suspended solids concentration.
A slightly different pattern was found for the shorter duration rainfall event at
15mm (Figure 6-10 A and B) on the 8th August 2014 when it rained from 1345hr to
1515hr. As with the long duration rainfall event, conductivity declined when
discharge started to rise and precipitation was at its peak at 7.4mm/day. However,
the recovery time was faster due to the short duration of the rainfall event and
decline in discharge. A higher suspended solids concentration was reached to
92mgL¯¹ but declined after six hours (2315hr) to 17mgL¯¹ after rainfall ceased. Both
conductivity and suspended solids concentration recovered to their pre-rainfall
concentrations faster than during the continuous rainfall event indicating duration of
rainfall has a strong influence on the concentration and that larger events are more
176
likely to have the “first flush” event. The peak concentration associated with the
rising limb of the hydrograph for suspended solids (Figure 6-9B and Figure 6-10B)
suggests mobilisation of sediment and/or first flush of particulates from combined
sewer systems (CSS) via CSOs into the river. This was followed by storm water
dilution and the consequent recovery to base flow conditions.
Figure 6-10: Hydrograph separation of 8th August 2014 total rainfall event at 15mm using 15min
continuous reading for (A) Conductivity (B) Suspended solids with discharge and total
precipitation *the ellipse indicates increase in concentration with the rising limb of the
hydrograph.
The ellipses on Figure 6-10 A and B highlight the effect of discharge on conductivity
and suspended solids.
177
6.3.5 PO4-P Load
The PO4-P load estimated at the gauging station showed that August 2014 had
the highest load record 18kgPhaˉ¹yearˉ¹ as shown on Table 6-3 due to increased
precipitation and hence discharge from episodic point sources such as CSOs.
However, daily load showed the values were less than 0.1kgPhaˉ¹dayˉ¹.
Table 6-3: Total precipitation and PO4-P load from the river Medlock from 1st August to 31st October
2014.
Sample months Total
Precipitation/month
(mm)
PO4-P load
(kgha ˉ¹yearˉ¹)
PO4-P load
(kgha ˉ¹ dayˉ¹)
Aug-14 128.12 18.25 0.05
Sep-14 17.70 1.95 0.01
Oct-14 63.87 2.21 0.01
In summary, all the variables analysed in this study showed dilution with
increased discharge apart from ammonia-N and suspended solids. The concentration
of suspended solids recorded during the period following increased precipitation
could be as high as 600mgLˉ¹ indicating the effect of “first flush”. This was followed
by dilution with continuous discharge. Hydrograph separation served to show the
trend of selected variables over a sample time and the changes that occurred during
the period of sampling. With reference to compliance, the mean PO4-P was higher
than WFD requirement of 0.12mgLˉ¹ and during the period of study, the standard
was not achieved even when there was no rainfall/high discharge which indicates the
continuous influence of the treatment works. Evidence of “first flush” episodic
phenomena was observed for PO4-P in August when a high precipitation/discharge
corresponded to a very high concentration.
During long duration rainfall, the recovery time taken for concentration of the
measured water quality parameters to return to their pre-storm levels was slow,
while the reverse is the case for short duration rainfall event. Apart from ammonia-N
178
which showed no significant change, all variables measured were significantly
different between the sample months. PO4-P load estimated during the sampling
period indicated that August 2014 was impacted by episodic pollution due to high
rainfall events. Other sampled months had <3kgPhaˉ¹yearˉ¹, which is similar to the
estimated load during the fortnightly sampling period (Chapter 4).
6.4 Discussion
The short-term temporal dynamics of the River Medlock was assessed from
continuous monitoring of key water quality parameters. This included hydrograph
separation for conductivity, pH and suspended solids following examination of the
concentration-discharge relationships which indicated the variables to be separated
by hydrographs. This pattern was used by Caissie et al., (1996) to determine the
relative contribution of groundwater (pre-event) flow to total flow (or event water)
during storm events of different magnitude.
The hydrographs were ‘flashy’, rising steeply with minor attenuation
especially during high flows. Peaks in discharge which correlated positively with
rainfall were observed for suspended solids and occasionally for PO4-P. Decreased
conductivity and pH indicate dilution with increased discharge. High conductivity
during autumn and winter could be linked to the runoff of de-icing salts into the
river. While NO3 and PO4-P responded to stream flow by dilution, ammonia-N and
suspended solids did not follow the same pattern. This implies that river
concentrations during the high precipitation “first flush” event could be linked to the
duration, frequency and magnitude of spills which could then impact on urban river
quality.
Turbidity was used as a surrogate variable for suspended solids concentration.
(Bilotta & Brazier 2008). The high concentration of suspended solids observed in the
Medlock in October 2014 (6th, 22nd and 27th) was not “first flush” event but was
attributed to the desilting of the river by the Environment Agency as part of flood
defence and control activity (EA personal communication, 2015) and exacerbated by
179
high rainfall as was the case in August. Also, high concentrations recorded in the
river some days after the rainfall could be linked to “late” arrival of suspended
materials in the storm (i.e. reverse first flush) indicating a departure from “first flush
models”(Lee et al. 2002). This delay indicates that CSOs were not the major
contributors to the changes observed in the river at short duration high flows.
Increase in rainfall increases the hydraulic gradient and also saturates the soil (Blume
et al. 2007). Thus, elevated suspended solids during high discharge could indicate
upstream erosion (Chebbo et al, 2001) as increased flow from the upper sub-
catchments resulted in the sediments being transported downstream to the gauging
station as shown by increased concentration of suspended solids from 22mgLˉ¹ at
2145hr to 167mgLˉ¹ at 2330hr. Such high suspended solid concentrations are likely to
adversely affect the benthic macroinvertebrates (Beck et al. 2004; Newcombe &
Macdonald 1991; Crabtree 1989) and hence contribute to the degraded community in
the River Medlock (Chapters 3 and 5).
Concentrations of both nutrients were highest in August reflecting the
seasonality in precipitation and fertiliser application. Agricultural fertilisers are
applied to the soils during August in the UK (Farming and Countryside Education
(FACE) 2007) and the high rainfall in August (total precipitation: 128mm) will
transport the fertilisers to the Medlock. This is exacerbated by crop harvesting and
planting of winter crops as land is left bare and ploughed; such activities also provide
a potential sediment source to the Medlock. Apart from August 2014, PO4-P load
estimated for the three month period had similar output (<3kgPhaˉ¹yearˉ¹) for the low
resolution and monthly spatial datasets (Chapters 4 and 5 respectively). The results
there also showed that a reduction of effluent PO4-P from the WwTWs would reduce
the concentration and load. The various datasets have revealed that the single
WwTWs is the largest source of PO4-P and suspected by the water company (United
Utilities, personal communication, , 2014).
Increased PO4-P and suspended solids concentration identified in this study
showed that the river is subject to episodic conditions which influence its quality.
180
Although maximum concentrations were recorded at the beginning of the storm
events, the recovery time of the river varied with the duration and volume of
rainfall/runoff. Thus, CSO effects are temporal while other pollution sources
including WwTW, diffuse runoff are continuous and these are integrated in the
quality of the Medlock.
High dissolved oxygen levels recorded during the sampling period showed
that the effect of organic pollution as a result of “first flush” is limited, in part due to
the high degree of re-aeration resulting from the turbulent flow. pH was near-neutral
and also correlated with conductivity and both variables were diluted with increased
rainfall. This further implies that the “first flush” activity was rare. This study has
shown that the urban River Medlock especially the part classified as a “highly
modified water body” is subject to urban stress which is exacerbated by increased
precipitation and discharge from point and diffuse sources. Such stormwater run-off
combined with hydraulically efficient drainage is a key reason why the Medlock does
not display “good ecological potential” as required by the WFD.
6.5 Conclusion
High resolution datasets provided information on the short term variability of
physico-chemical variables at the River Medlock. The potential for “first flush” events
markedly influencing water quality was shown by PO4-P and suspended solids. The
duration of rainfall events can have a strong influence on the river’s water quality
due to rapid dilution of first-flush pollutants. Concentrations increased during large
storm events and gradually returned back to pre-event concentrations.
Simultaneously, other variables were diluted with rainfall-induced increases in
discharge. The results show that the activities of CSOs were short-lived and that the
Medlock was subject to other point and diffuse sources which may affect its quality.
As a highly modified water body, the River Medlock is subject to urban stress due to
the hydraulically efficient drainage; therefore at high discharge it is influenced by
181
water quantity as well as quality. This study does not support the hypothesis that
short-term water quality is solely due to discharges from CSOs.
Acknowledgement:
I am grateful for financial support from The National Open University of Nigeria.
Thanks to Cascade Consulting (now Renovo), Manchester for the use of their YSi
meter and data collection at the Medlock, and Dr Gareth Martins of Cascade for help
and advice with the installation.
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183
Chapter 7 SUMMARY AND
CONCLUSIONS
The first research chapter (three) provided an overview of the state of the
Medlock over the previous decade with the aid of EA long-term datasets from three
sites on the lower reaches. Interrogation of such data provides an indication of trends
in water quality and allows the effectiveness of management strategies and policy
changes to be assessed. Although some data was not available for two sites, analysis
of the results suggests the pollution was largely non-sewage related as BOD and
ammonia-N were low. Although PO4-P declined with time the concentration was still
higher than the WFD standard. This data reflects the current policy of reducing BOD,
ammonia and suspended solids from WwTWs, but not always PO4-P due to the cost
of tertiary treatment to remove this compound (Mainstone et al., 2000; Mainstone &
Parr, 2002). Occasionally, high suspended solids recorded over the study period
indicate the impact of high precipitation events. This chapter identifies the WwTWs
as a key influence on river quality, specifically PO4-P.
In chapter four, the contribution of WwTW, CSOs and other sources to the
high PO4-P loading was investigated with the use of the long-term EA data plus
fortnightly data from a larger number of sites to improve the spatial resolution and
data from a high temporal resolution study at a single site. The results suggested that
the WwTW was a major contributor to the PO4-P load with an average of 92% from
this source. The remainder of the contribution was associated with CSOs and other
diffuse sources and which varied with discharge. Non-WwTWs sources of PO4-P
were greatest during storm events during initial high discharge. It also showed that
reduction of PO4-P from the WwTW would reduce the river load considerably. A
comparison of total phosphorus load per unit area of catchment (<3kgTPha⁻¹yr⁻¹)
estimated in this study was commonly around a third of total phosphorus load
obtained in the literature for other urbanised areas.
184
Chapter five demonstrates the usefulness of multivariate techniques and
interactions within and between samples/variables and, the potential of biotic indices
in determining the benthic status of urban rivers such as the Medlock. The study
revealed that the degraded benthic community in the Medlock was not primarily due
to poor water quality but as a direct and indirect result of water quantity. With the
exception of PO4-P, water quality indicators including DO, ammonia and BOD
conformed to the requirements of the WFD, including at high discharge and are
hence unlikely to adversely impact on the invertebrate community. Tolerance to
disturbance and suspended solids plus feeding habits and ability to rapidly colonise
a habitat were the main factors determining survival. Therefore, taxa such as
Oligochaeta and Chironomidae that are resistant to the instability caused by
changing flow conditions in the river dominate. Gammaridae were also common at
the polluted sites, particularly during the summer, as they are also able to take
advantage of the plentiful supply of allochthonous material. Both the BMWP and the
WHPT are equally effective at detecting change in the invertebrate community
despite the latter also including a measure of abundance that may have been
expected to have resulted in greater discrimination between sites.
In chapter six, high resolution sampling confirmed that Medlock water quality
was influenced not only by CSOs during high discharge events but was also subject
to other point and diffuse sources. Increased concentration of suspended solids and
PO4-P at peak discharge indicated the effect of water quantity on the river’s quality.
This part of the study has brought further clarity to an understanding of the
behaviour of the River Medlock especially as it pertains to the unstable conditions
that degrade the benthic macroinvertebrate community as shown in Chapter 5.
The individual chapters have highlighted the challenges to the River Medlock,
and hence other urban rivers in achieving the required WFD standards. The non-
achievement of this standard is linked to the contribution of both continuous,
episodic point and possibly diffuse sources that influence both the quantity and
185
quality of the receiving water. This study also showed that effective sampling in time
and space is necessary to describe patterns and processes. While the long term
datasets provided an overview, the fortnightly sampling provided a more focussed
and spatially coherent sampling of both water quality and ecology. This enabled the
characterisation of river quality and the benthic invertebrate community. The high
resolution sampling quantified the scale of the changes during peak discharge and re-
emphasized the impact of diffuse pollution and effects of the urban stream
syndrome.
Like the Medlock, PO4-P pollution has been a major problem in Europe and
the US, its management to date has focussed on controlling its removal from
municipal wastewater. Almost half of the rivers and three-quarters of lakes in
England suffer from eutrophication as a result of elevated phosphorus concentration
(Davey & VanLeirde 2015). Substrate instability is a further problem highlighted by
this study resulting in peaks of suspended solids during storms which exceeded the
EU Freshwater Fisheries Directive of ≤25mgLˉ¹ for 9% of the storm duration. Finally,
the Medlock is characterised by a ‘flashy’ hydrograph characteristic of urban rivers
(e.g. Paul & Meyer 2001). This study therefore does not support the hypotheses that
CSOs are usually the key contributor to the poor water quality and reduction in the
benthic macroinvertebrate community in the river.
The term ‘‘urban stream syndrome’’ as described in Chapter 5 was coined by
(Meyer et al. 2005) to describe the long-term ecological degradation of rivers and
streams draining urban catchments. Key symptoms of the urban stream syndrome
are a flashy hydrograph, high nutrients and other contaminants, altered channel
morphology due to re-engineering, and reduced biodiversity and increased
dominance. The mechanisms driving the urban stream syndrome are numerous and
interactive, but result from a few major sources, specifically storm water runoff but
also CSOs and WwTWs plus the legacy of pollution from previous activities within
the catchment (Meyer et al. 2005;Walsh et al. 2005). On the basis of the above
definition and criteria, there is no doubt that the Medlock suffers from the urban
186
stream syndrome. Because of the urban stream syndrome, “Good” water quality as
defined by the WFD will not result in “Good Ecological Status” for the River
Medlock.
In order to effectively manage and control pollution, it would be important to
reduce the frequency and intensity of river discharge through hard and soft
engineering which will promote biodiversity at the Medlock. Some measures to
reduce water quantity is the construction of sustainable urban drainage systems
(SuDS)(Maltby 2012). SuDS are systems which mimic natural systems such as
‘raingardens’ to drain surface water and release it slowly back into the environment.
Such approaches aim to address urban flooding and sewage overflow while
promoting urban greening. The sustainability of urban rivers would be enhanced by
installing permeable pavements which increase infiltration and, also to involve and
link various stakeholders who will ensure continuity of green infrastructural
improvements (Royal Geographical Society (with IBG) 2012). Furthermore, the use of
wetland vegetation could be explored to reduce pollutants such as phosphorus in the
less urban areas of the Medlock catchment where land is available such as Clayton
Vale. Such wetlands will also reduce the risk of flooding down-stream. Such a
scheme is being installed on the lower Irwell where a five-hectare wetland will also
increase biodiversity by containing marsh, reed-beds, gravel islands and ponds
(SalfordOnline, 2015). The water company, United Utilities is constructing an
underground storage tank which will hold excess storm water of around 17,800 m3sˉ¹
during storm conditions. This storm tank is expected to reduce the frequency of CSO
releases to the Irwell from Davyhulme WwTW (United Utilities
http://www.unitedutilities.com/2601.aspx). The Medlock would benefit from a
similar installation to reduce the frequency of CSO discharges and hence levels of
suspended solids. However, this would not reduce total phosphorus load in the
absence of subsequent treatment. PO4-P concentration in the River Medlock can be
reduced by stripping from the WwTW effluent as has been explored elsewhere (Neal
et al. 2005) and to improve agricultural practices upstream of the river.
187
Further work
To investigate the effect of sediment traps on the river ecology
To investigate the impact of extreme conditions in relation to climate change
(increased/ reduced rainfall) and the resilience of urban streams.
7.1 Conclusion
The overall aim of the thesis is to examine the impact of point-source pollution
from WwTWs and CSOs on the water quality. While the WwTW was shown to be a
major source of PO4-P pollution, the effect of other sources including CSOs, runoff
and high precipitation were revealed. The benthic invertebrate community are
influenced mainly by the direct and indirect effects water quantity, specifically the
highly variable (‘flashy’) hydrograph rather than river quality. The introduction of
storm water control measures would mitigate pollution entering the River Medlock.
This could include the construction of storm water purification tanks at the Medlock
for the control of extreme flood events, thereby reducing flow, suspended solids and
nutrients.
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Appendix
199
Table i. Concentration of suspended solids measured at the river at the EA’s gauging station and
corresponding turbidity levels
Date Time (hr:mm:ss) Turbidity (NTU) SS (mgL¯¹)
01-08-14 11:15:52 2.2 4.00
01-08-14 16:00:52 24.9 3.00
01-08-14 20:15:52 52.4 48.00
02-08-14 12:15:52 18.4 9.67
02-08-14 14:00:52 7.9 6.67
02-08-14 18:00:52 6.7 14.67
03-08-14 17:45:52 1.8 5.67
05-08-14 12:30:52 2.2 5.00
06-08-14 13:00:52 3.1 0.00
08-08-14 15:15:52 136.9 50.00
08-08-14 15:30:52 108.2 41.33
08-08-14 17:30:52 148.6 237.33
10-08-14 09:45:52 11.8 4.33
10-08-14 12:00:52 103.1 107.33
10-08-14 16:45:53 93.3 33.00
10-08-14 17:30:52 72.5 50.33
11-08-14 16:00:53 23 6.33
11-08-14 18:45:52 34.4 13.33
12-08-14 11:30:52 11.4 9.00
12-08-14 16:30:52 8.3 6.00
13-08-14 13:00:52 11.3 7.33
14-08-14 14:00:52 238.7 50.00
14-08-14 18:00:52 80.6 87.67
17-08-14 14:00:53 6.4 6.33
17-08-14 18:30:53 6.4 4.33
18-08-14 09:45:53 8 4.00
19-08-14 10:00:53 5.1 1.00
27-08-14 19:45:53 185.3 8.00
28-08-14 12:00:52 2.1 0.00
04-09-14 12:45:53 2.5 0.00
09-09-14 18:00:53 2.6 9.00
10-09-14 10:30:53 4.7 11.33
15-09-14 17:30:53 12.6 11.33
16-09-14 13:00:53 27.7 7.67
19-09-14 10:15:52 4.5 0.00
20-09-14 16:00:53 4.4
23-09-14 17:15:53 28 0.00
23-09-14 20:00:53 16.1 1.67
24-09-14 11:30:53 14.7 3.00
26-09-14 14:00:53 48.6 2.33
29-09-14 12:00:53 2.5 0.00
30-09-14 09:30:53 3 0.00
30-09-14 16:00:53 2.8
25-10-14 18:15:54 3 0.00
26-10-14 15:00:54 3.8 0.00
27-10-14 18:45:54 7.2 0.00
28-10-14 15:00:54 40.4 9.67
29-10-14 17:30:54 8.9 5.33
30-10-14 12:00:54 18.1 11.00
31-10-14 16:15:54 3.1 53.60