1

JAMES ROTHWELL & DR MARTIN EVANS

Upland Environments Research Unit,

School of Environment and Development,

The University of Manchester.

Flux of heavy metal pollution from

eroding southern Pennine peatlands

2

This report represents a distilled version of my PhD research. Some of the research is

still ongoing, whilst other areas of research have been completed. Therefore, the report

does not contain all the results of the project. I plan to submit my PhD in 6 months and

copies will be made available.

James Rothwell

3

CONTENTS

1 SUMMARY 5

2 IMPETUS FOR RESEARCH 6

2.1 Suspended sediment 9

2.2 Sediment-water interactions 9

2.3 Dissolved heavy metals 10

3 RESEARCH AIMS 10

4 METHODS 11

4.1 Study catchments 11

4.2 Alport Moor 12

4.2.1 Peat coring 12

4.3 Upper North Grain 13

4.3.1 Peat coring 13

4.3.2 Gully wall traps 13

4.3.3 Overbank sediments 14

4.3.4 Storm water 15

4.4 Torside Clough 17

4.5 Determination of Pb 17

4.5.1 Solid phase 17

4.5.2 Dissolved phase 18

5 RESULTS 18

5.1 Spatial variability of Pb contamination at Alport Moor 18

5.2 Pb Inventory 20

5.3 Modelling Pb peak from Pb inventory 21

5.4 Mapping heavy metal contamination of Peak District 22

blanket peats

4

5.5 Sediment-associated Pb concentrations 23

5.5.1 UNG Upper Weir 23

5.5.2 UNG Main Weir 24

5.5.3 Torside Clough 25

5.5.4 Site comparisons 26

5.6 Contaminated sediment 29

5.7 Sediment-water interactions 30

5.8 Stormflow dissolved Pb 31

5.8.1 Stormflow dynamics 31

5.8.2 Autumn flush 34

5.9 Pb in the sediment-water system 35

6 CONCLUSIONS 36

7 ACKNOWLEDGEMENTS 38

8 REFERENCES 38

5

1 SUMMARY

High concentrations of industrially-derived, atmospherically-transported heavy metals

are stored in the upper peat layer of the blanket peats of the Peak District, southern

Pennines. Peak concentrations of lead (Pb) in the upper peat layer are typically in excess

of 1000 mg kg−1

. However, peak Pb concentrations can vary significantly over a small

spatial area. Erosion of the contaminated upper peat layer is releasing Pb, associated

with eroded peat particles, into the fluvial systems of the region. Stormflow sediment-

associated Pb concentrations are variable over space and time, influenced by a

combination of geomorphological and hydrological conditions of the catchment. At

Upper North Grain and Torside Clough, stormflow Pb concentrations in suspended

sediments commonly exceed 100 mg kg−1

and peak values can exceed 350 mg kg−1

. A

strong positive correlation between dissolved Pb and DOC indicates that DOC is a

vector for dissolved Pb transport in the fluvial systems of the Peak District. Many

headwater steams of the Peak District recharge drinking water reservoirs. The legacy of

the 19th

century English Industrial Revolution is affecting the quality of sediment and

water entering reservoirs of the region.

6

2 IMPETUS FOR RESEARCH

Anthropogenic emissions of pollutants to the atmosphere have increased significantly in

the last 200 years, and large quantities of toxic heavy metals, which have long-term

ecological and human health impacts, are dispersed globally (Nriagu, 1996). Blanket

peats are largely ombrotrophic, that is, they are nourished almost entirely by

atmospheric deposition (Shotyk, 2002). Those blanket peats situated close to, and

downwind from major centres of industrial activity, have typically been repositories for

high concentrations of atmospherically-derived pollutants, such as heavy metals. These

pollutants are stored within the upper peat layer (Jones, 1987). Such pollutants are the

by-products of fossil fuel combustion, iron and steel manufacture, and vehicle emissions

(Petrovsky and Ellwood, 1999). The main focus of research over the last few decades

has centred upon the vertical distribution of pollutants within peat profiles, in relation to

historical changes in pollutant deposition (Livett et al., 1979; Weiss et al., 2002). In

recent years there has been growing interest in the mobilisation and delivery of toxic

heavy metals stored in the upper layers of catchment peats, to freshwater ecosystems.

Of the small body of research, work has focused upon heavy metal pollutants in

dissolved form, remobilised by acidic groundwater (e.g. Lucassen et al., 2002, Lawlor

and Tipping, 2003, Tipping et al., 2003). However, in a study at Lochnagar, Scotland,

Yang et al. (2001) proposed that eroded peat transported in the fluvial environment

could be a significant source of heavy metals to the lake system at Lochnagar. Eroded

peat particles, sourced from the contaminated upper peat layer, could potentially be an

important vector for sediment-associated heavy metal mobilisation and transport in

hydrological systems.

The blanket peats of the Peak District, southern Pennines, are situated in the heartland

of the 19th

Century English Industrial Revolution, between the cities of Sheffield and

Manchester. Consequently, very high concentrations of anthropogenic, industrially-

derived, atmospherically-transported toxic heavy metals are stored in the upper peat

layer in this region (Lee and Tallis, 1973; Livett et al., 1979). Globally, there is great

spatial variability in Pb concentrations in peatland environments (Table 1). Proximity to

pollutant source, severity of pollutant source, and a combination of accumulation and

7

preservation processes at a receiving peatland, are some of the major factors controlling

the degree of contamination recorded in a peatland environment. As peatlands generally

receive contaminant inputs from diffuse sources, they are not as heavily contaminated

as regions with point pollution sources, such as areas containing contemporary or

historical metalliferous mining and/or smelting activities. When the peatlands of the

Peak District are compared with other peatlands from around the globe that receive their

contaminant inputs from diffuse sources, they are amongst the most contaminated in the

world (Table 1).

Location

Pb (mg kg−1)

maximum Author(s)

Alport Moor, Derbyshire, England 1647 Rothwell & Evans (unpublished data)

Ringinglow Bog, Derbyshire, England 1230 Jones & Hao, 1993

Forest-steppe zone, southern Russian Plain 856 Zhulidov et al., 1997

Tinsley Park Bog, Lower Don Valley, Sheffield 827 Gilbertson et al., 1997

Grassington Moor, Wharfedale, Yorkshire 800 Livett et al., 1979

Ringinglow Bog, Derbyshire, England 700 Markert & Thornton, 1990

Snake Pass, Derbyshire, England 570 Lee & Tallis, 1973

Ringinglow Bog, Derbyshire, England 548 Jones, 1987

Kola Peninsula, Russian Artic 510 Zhulidov et al., 1997

Bozi Dar, Czech Republic 479 Vile et al., 2000

Lochnagar, Scotland 400 Yang et al., 2001

Tor Royal, central Dartmoor, England 400 West et al., 1997

Flanders Moss, central Scotland 388 Farmer et al., 1997

Northern Black Forest, southern Bavaria 339 Kempter et al., 1997

Langmoos Bog, Mondsee, Austria 230 Holynska et al., 2002

Hajavalta, southwest Finland 204 Nieminen et al., 2002

Ystwyth valley, Wales 200 Mighall et al., 2000

Canton Jura, Switzerland 120 Shotyk, 2002

Penido Vello, northwest Spain 84 Martinez Cortizas et al., 2002

Point Escuminac, eastern Canada 53 Wiess et al., 2002

Stokkanmyra Bog, Trondheim, Norway 50 Gorres & Frenzel, 1997

Table 1. Maximum Pb concentrations in a selection of peatland environments.

The UK Department for the Environment, Food and Rural Affairs (DEFRA) has

established a series of Soil Guideline Values (SGVs) for concentrations of contaminants

in soils. These SGVs are used in the assessment of contaminated land, and the

subsequent risks to human health from chronic exposure to it (DERFA, 2002). The

DEFRA SGV for the heavy metal Pb is 750 mg kg−1

for contaminated

commercial/industrial land, and 450 mg kg−1

for contaminated residential land. At

8

Alport Moor the peak Pb concentration of 1647 mg kg−1

is more than double the SGV

for Pb for contaminated commercial/industrial land. Pb concentrations in the upper peat

layer at Alport Moor also exceed all contaminated land guidelines in Sweden, the

Netherlands, Canada and Australia (Table 2).

Table 2. Contaminated land guidelines for selected countries. DEFRA, Department for Environment

Food and Rural Affairs, UK; SGV, Soil Guideline Value; SEPA, Swedish Environmental Protection

Agency; VROM, Netherlands Ministry of Housing, Spatial Planning and the Environment; NGSO,

National Guidelines and Standards Office, Canada; CSoQGs, Canadian Soil Quality Guidelines; NEPC,

National Environmental Protection Council, Australia; SIL, Soil Intervention Level.

The peat catchments of the Peak District are unquestionably the most eroded in Britain

(Tallis, 1997). The first systematic study of blanket peat erosion in the southern

Pennines was that of Bower (1960), which extensively mapped peat erosion in the

southern Pennines. The work concluded that the form of peat erosion falls into one of

two categories: dissected peat gullies or bare peat flats. Although more recent work has

focused upon process mechanisms driving peat erosion (e.g. Francis, 1990; Labadz et

al., 1991; Holden and Burt, 2002), Bower’s qualitative descriptions of the form of peat

erosion still withstand today. The Peak District is characterised by large expanses of

bare peat flats and hundreds of kilometres of dissected peat gullies. Uncontrolled

Country Organisation Contaminated land guideline

Pb

Concentration

(mg kg−1) Source

UK DEFRA SGV - Residential land 450 DEFRA (2002)

SGV - Commercial / Industrial land 750 DEFRA (2002)

Sweden SEPA Guideline Value - Polluted soils 80 SEPA (2002)

Netherlands VROM Intervention Value - Polluted soil 530 VROM (2000)

Canada NGSO CSoQGs - Agricultural land 70 CCME (2002)

CSoQGs - Residential / Parkland 140 CCME (2002)

CSoQGs - Commercial land 260 CCME (2002)

CSoQGs - Industrial land 600 CCME (2002)

Australia NEPC SIL - Residential land 300 NEPC (1999)

SIL - Parkland 600 NEPC (1999)

SIL - Commercial / Industrial land 1500 NEPC (1999)

9

accidental fires are a major contributor to the loss of plant cover and exposure of bare

peat flats on moorlands in the Peak District. Annual rates of loss in extensive gently-

sloping peat flats can be as high as 53 mm yr−1

(Anderson et al., 1997). Pollutants, such

as heavy metals, stored in the upper peat layer will be readily mobilised in particulate

form, associated with eroded peat particles, if the upper peat layer is eroded. Erosion of

bare peat flats by sheet erosion will therefore potentially mobilise greater quantities of

heavy metals than erosion of deep peat gullies, but the depth and rate of erosion on bare

peat flats will control sediment-associated heavy metal mobilisation into the fluvial

system.

2.1 Suspended sediment

The role of suspended sediment as a vector for contaminant transport in fluvial systems

is well established (e.g. Dawson and Macklin, 1998; Horowitz et al., 2001; Blake et al.,

2003). The transport of sediment, and its associated chemical constituents, in the fluvial

environment, is related to the geomorphological and hydrological conditions of the

system, in particular, the processes of erosion, transportation and deposition. All these

processes are highly variable in space and time, and because suspended sediment does

not behave conservatively, there are constant exchanges of sediment between the water

column, the river banks, the river bed, and the floodplain environment (Horowitz,

2000). Therefore, a thorough understanding of the dynamics of erosion, transportation

and deposition is essential for calculating heavy metal flux and elucidating the fate of

sediment-bound contaminants in hydrological systems.

2.2 Sediment-water interactions

Sediment-associated heavy metals in natural waters are subject to adsorption and

desorption processes, where heavy metals can move between solid and dissolved phases

(Foster and Charlesworth, 1996). Those heavy metals that are less strongly bound to

sediment, such as exchangeable metal ions, are the most susceptible to mobilisation into

the water column. Such interactions are controlled by chemical (Eh and pH), physical,

10

and biological conditions of stream water (Kersten, 2002). Streams draining peatland

catchments are often highly acidic as organic acids and industrially-derived acid species

are flushed out of the peats during storm flow. pH values in such fluvial environments

may be as low as pH 3, and under such conditions sediment-associated heavy metals

may be desorbed from sediment into the water column. This may therefore substantially

increase the levels of toxic heavy metals already present in peatland streams, which are

derived from in-situ heavy metal release (Lucassen et al., 2002, Lawlor and Tipping,

2003, Tipping et al., 2003;).

2.3 Dissolved heavy metals

Depositional fluxes of heavy metals to peat soils occur by wet and dry deposition

(Gelinas & Schmit, 1998). Humic substances, which are ubiquitous in peat soils, are

dominant complexing agents for heavy metals deposited in solution (Tipping et al.,

2003). Dissolved organic carbon (DOC) in stream waters draining peatland

environments is a good indication of humic substances (Tipping, 2002). Therefore,

DOC is important in the mobility and transport of heavy metals in streams draining

peatland catchments contaminated with heavy metals. Previous work on dissolved

heavy metals at moorland sites in the UK has highlighted the importance of DOC in

complexing heavy metals and the importance of leaching of previously deposited heavy

metals stored in the catchment peats (e.g. Lawlor and Tipping, 2003).

3 RESEARCH AIMS

The aims of this research project are to

• Assess and map heavy metal contamination in Peak District blanket peats.

• Calculate heavy metal flux (particulate and dissolved) from eroding peat

catchments in the Peak District, that exhibit contrasting types of peat erosion

(sheet erosion vs. gully erosion).

• Determine the significance of within-stream sediment-water interactions to

dissolved heavy metal fluxes in these fluvial systems.

11

• Assess the role of DOC in mobilising and transporting heavy metals in fluvial

systems of the Peak District

• Construct heavy metal budgets for the contrasting study catchments.

• Evaluate and model the conditions controlling these heavy metal fluxes.

Overall, this study will provide a detailed assessment of the problems associated with

contamination of blanket peat catchments of the Peak District.

4 METHODS

4.1 Study catchments

The first study catchment, Upper North Grain (UNG), is located east of Alport Moor.

UNG is a peatland catchment dominated by gully erosion, with very few exposed areas

of bare peat. At UNG there are two study sites, the ‘Upper Weir’ and the ‘Main Weir’.

The Upper Weir is a 0/1st order stream which has incised only partially into the

underlying peat. At this site, there is no exposure of the underlying geology of the

catchment. The Main Weir is located downstream of the Upper Weir and the stream at

this point in the catchment is 2nd

/3rd

order. This weir is characterised by a mineral floor

to the stream and the source area of this weir includes both organic and minerogenic

areas. Geologically, the Peak District is dominated by sedimentary rocks. The

sedimentary rocks of the catchments are interbedded sandstones and shales of the

Millstone Grit Series (MGS), of Namurian (Middle Carboniferous) age (Wolverson-

Cope, 1976). Some of the bedrock of the catchments is overlain by head deposits, the

product of solifluction processes active during the late Devensian and Loch Lomond

(Younger Dryas) stadials, during which times this area was subjected to periglacial

conditions (Burek, 1991). Because sandstones, shales and head deposits are all

susceptible to erosion in the UNG catchment, suspended sediment may therefore consist

of a variety of mineral components, as well as eroded organic matter. The second study

catchment is Torside Clough, which is located in the Longdendale Valley, situated

between Harrop Moss and Sykes Moor. Harrop Moss and Sykes Moor are characterised

12

by large expanses of exposed peat. The bare peat on Sykes Moor is derived from a 60 ha

fire that occurred in 1970. In 1980 a catastrophic burn on Torside, which covered 224

ha, extended the 1970 burn on Sykes Moor and exposed large are of peat on Harrop

Moss (Anderson et al., 1997). The stream at the sampling site at Torside Clough is 3/4th

order, passing through a steep valley with exposed geology.

4.2 Alport Moor

4.2.1 Peat coring

To investigate small scale spatial variability of Pb contamination of a peatland

environment, Alport Moor was chosen because of the intact nature of the peat dome, it’s

appropriate size and for logistical reasons (Figure 1).

Figure 1. Lidar (hill shade) image of Alport Moor.

Area of coring transects is marked on the image by the red rectangle.

13

During summer 2004, three parallel transects, measuring 500 m in length, 100 m apart,

were marked on the peat dome at Alport Moor using a series of ranging poles. At

approximately 63 m intervals along each of transect, two peat cores were extracted

using a 50 cm Russian corer (8 sites per transect, 24 sites in total). The two peat cores

from each site were taken within approximately 50 cm of each other. In total, 48 peat

cores were extracted. All cores were wrapped in cling film to avoid atmospheric

contamination during transit and subsequent storage. Each core was labelled and stored

at 4 °C until further analysis was carried out. Two cores were taken so as to enable

contiguous sampling to be carried out one core and storage of the second core for

further work at a later date. From the 24 coring locations, 10 cores were selected

randomly (by random number generation) for laboratory analysis. The 10 cores were

sliced at contiguous 1 cm intervals for the top 40 cm of the core. Each 1 cm slice was

oven dried at 105 °C for 24 hours, disaggregated, weighed, and prepared for Pb analysis

as described below.

4.3 Upper North Grain

4.3.1 Peat coring

Eight peat cores were collected from around the UNG catchment with good spatial

coverage. A 50 cm Russian corer was used to extract the peat cores and each was

wrapped in cling film and labelled. On return to the laboratory each core was sliced at 1

cm intervals for the top 10 cm. All peat core samples were then oven dried at 105 °C for

24 hours, disaggregated, weighed, and prepared for Pb analysis as described below.

4.3.2 Gully wall traps

To investigate Pb flux from a gully wall face, a half drainpipe section, 50 cm in length,

was secured into the face with aluminium pins. This trap was positioned to intercept

peat falling off the gully wall face (Figure 2). Approximately every three months, over a

14

two year period (December 2002 – December 2004), any peat that had collected in the

trap was collected, taken back to the laboratory and prepared for Pb analysis as

described below.

Figure 2. Trap positioned at the base of the gully wall.

Note the stream directly below the peat face.

4.3.3 Overbank sediments

On an overbank deposition area at the top of the catchment, Astroturf mats were secured

with tent pegs and positioned to collect any overbank sediments deposited during

stormflow events (Figure 3). These mats were emptied after storm events, and the

sediments taken back to the laboratory for Pb analysis as described below.

15

Figure 3. Zone of overbank deposition and an Astroturf mat in situ.

4.3.4 Storm water

At UNG two Sigma 900 automatic water samplers were positioned at two different

locations in the UNG catchment. These samplers were programmed to collect 24 water

samples during storm events. The samplers were triggered automatically by a rise in

stage. Stage was recorded at the two stream locations by Intelysis pressure transducers

and data loggers.

The first automatic water sampler was located on the 2nd

/3rd

order reach of UNG (Main

Weir – Figure 4). Storm water samples were collected from this site from August 2002

to November 2004. Pb analysis on storm water samples was carried as described below.

16

Figure 4. Main Weir. Automatic water sampler located out of shot to the right.

The second sampler was positioned at the top of the catchment, beside a 0/1st stream

(Upper Weir – Figure 5). A V-notch weir was constructed to allow discharge

measurements of the stream at this site to be calculated. Storm water samples were

collected at this site between October 2003 and November 2004. Preparation of storm

water samples for Pb analysis is described below.

Figure 5. Upper Weir. Automatic water sampler located out of shot to the left

17

4.4 Torside Clough

Another water sampler was set up at Torside Clough. This sampler was positioned well

above the high water level because of the very flashy nature of this site (Figure 6).

Storm water samples were collected from Torside Clough between August 2004 and

December 2004. Samples were prepared for Pb analysis as described below.

Figure 6. Torside Clough water sampler location.

4.5 Determination of Pb

4.5.1 Solid phase

Water samples from the all storm events were filtered through pre-weighed Whatman

GF/C glass microfibre filter paper circles, in order to retain the suspended sediment

fraction of the water sample. These filter papers, along with peat samples and overbank

sediments were oven dried at 105 °C for 24 hours, weighed and then digested on a hot-

plate at approximately 100 °C for 4 h using 5 ml of concentrated HNO3 (Analar, BDH).

After digestion, all samples were then made up to 25 ml with high purity distilled water,

filtered through Whatman GF/C glass microfibre filter paper, and stored at 4 °C in

sterile polythene tubes prior to analysis. Pb concentrations in all samples were

determined using Atomic Absorption Spectrophotometry (Thermo Unicam S11) using

18

an air–acetylene flame with hollow cathode. Lead standard solutions (Spectrosol, BDH)

were used to calibrate the AAS. Certified Reference Material LGC6139 River Sediment

(LGC Promochem) was digested and analysed along with the sediment samples, and a

recovery of 105 % was obtained for lead. Blank samples (digested, unused filter papers)

were also analysed and were below the detection limit of the AAS.

4.5.2 Dissolved phase

Storm water samples analysed for dissolved Pb were first filtered through Whatman

0.45µm Cellulose Nitrate filter papers. Samples were then acidified to 2% with

Ultrapure HNO3 and stored in sterile polythene vials. Dissolved Pb was measured on an

Inductively Coupled Plasma Mass Spectrometer (VG Elemental Plasmaquad 2 STE) at

the Williamson Research Centre for Molecular Environmental Science, School of Earth,

Atmospheric and Environmental Sciences, The University of Manchester.

5 RESULTS

5.1 Spatial variability of Pb contamination at Alport Moor

In order to produce a map of Pb contamination in the Peak District, the degree of spatial

variability of contamination over a small area needed to be addressed with respect to the

suitability of a taking a single peat core for assessment of contamination status at a

particular site. Traditionally, single peat cores have been taken from peatland

environments, and this single record used to determine contamination status (e.g. Wiess

et al., 2002). However, the utility of this approach needed to be fully tested before

embarking upon such an extensive investigation of mapping contamination in the Peak

District. The results of the Pb analysis of the Alport Moor peat cores (Figure 7) show

that over a relatively small spatial scale, there is significant spatial variability in Pb

contamination. Peak Pb concentrations at the 10 sites vary from 662.8 to 1647.2 mg

kg−1

(Core 8, and 2 respectively), and taking one core from this area and using the peak

concentration as representative would in fact have a 25 % error associated with it.

19

Figure 7. Pb concentrations in peat cores taken from Alport Moor

20

Some of the cores have distinct pollution peaks (Cores 2, 7 and 10), whilst others have

less well defined pollution peaks (Cores 4, 5 and 6). Some of the Pb profiles show more

significant pollution in lower depths than other cores (Cores 7 and 9). Ombrotrophic

peatlands receive contaminant inputs from diffuse sources and the accumulation of peat

over time records historical atmospheric pollutant inputs. However, the integrity of this

record is dependant on deposition, accumulation and preservation processes, which are

in turn influenced by differences in hydrological, biological and chemical status,

vegetational composition, topography and peatland dynamics. Differences in one or a

combination of these factors may explain the dramatic differences in the Pb records

from the Alport Moor peat cores. Due to the differences in Pb contamination over a

relatively small spatial area, a single core analysed for Pb concentration on a depth basis

is not suitable for mapping contamination status.

5.2 Pb Inventory

The total inventory of Pb contamination at a particular site is a more suitable assessment

of contamination status, rather than Pb concentration on a depth basis, and the use of

peak Pb concentrations. An inventory represents the total store of contaminants. For

each of the ten Alport Moor peat cores analysed for Pb content, a Pb inventory was

calculated for each centimetre depth in each core on a mass basis. A cumulative

inventory per centimetre depth was then determined for each core for the top 40 cm and

a mean was generated for each centimetre, based on the 10 cores. Based on this mean,

percentage inventory recorded per cumulative centimetre was calculated for the top 40

cm (Figure 8). The assumption of this percentage calculation was that 100 % of the Pb

contamination occurred in the top 40 cm. Based on the result from the percentage

inventory recorded per cumulative centimetre, 95 % of the total pollution occurs in the

top 30 cm. For Alport Moor, the inventory of Pb in the top 30 cm is 9.62 ± 1.44 g m−2

.

The error associated with this figure (15%), is due to the spatial variability of Pb

contamination at the site. However, this error is significantly less than that associated

with using peak Pb concentrations to assess pollution. Instead of taking multiple

downcore measurements of Pb pollution, a relatively rapid assessment of pollution

21

status can therefore be undertaken by bulking and thoroughly mixing the top 30 cm of a

peat core, weighing its mass and calculating the Pb concentration in a subsample.

Figure 8. Percentage cumulative inventory recorded per cm.

5.3 Modelling Pb peak from Pb inventory

Peak Pb concentrations in peat profiles can be useful for assessing pollution status of a

site if small scale variability can be quantified. Peaks also provide essential information

on timing of peak pollution with respect to industrial development, especially if such a

peak can be securely positioned with a well constrained chronological framework. To

determine if the Pb inventory of a peat core can be used to model peak Pb

concentration, the relationship between peak Pb concentration for the ten peat cores

from Alport Moor and the cumulative inventory for each centimetre depth, was

undertaken using linear regression. The strongest correlation between Pb peak and Pb

inventory was for an inventory based on the top 9 cm, giving an r2 value of 0.7513

(Figure 9).

22

y = 209.62x - 5.3373

R2 = 0.7513

0

400

800

1200

1600

2000

0 2 4 6 8 10

Pb inventory (mg)

Pb

peak (

mg

/kg

)

Figure 9. Linear regression of Pb inventory of the top 9cm and Pb peak for the Alport Moor peat cores

5.4 Mapping heavy metal contamination of Peak District blanket peats

The methodology for mapping heavy metal pollution of the blanket peats of the Peak

District will be based on the information from the inventory calculations for the Alport

Moor peat cores. It is planned that 30 cm peat cores will be taken from the selected

sites. The top 9 cm of the core will be bulked and inventory calculated. This will then be

used to model peak heavy metal concentration. The remainder of the core will be bulked

and an inventory calculated. The two inventories will be added together, ultimately

providing information on the contamination status of a particular site. However, there

are two main assumption of this work. Firstly, the methodology used for Pb will be

applicable to other heavy metals, and secondly the methodology used for the Alport

Moor location can be scaled up to an investigation of a larger spatial area. To test the

first assumption, heavy metal inventories for all heavy metals to be investigated would

have to be calculated for the Alport Moor peat cores. As the number of samples in the

Alport Moor dataset amounts to 400, validating this assumption would be extremely

time consuming. It is expected that the total inventory calculations will be suitable for

other heavy metals, but calculating peak concentrations, based on the inventory, will be

less accurate, but could be tested by analysing a few cores for the heavy metals on a

depth concentration basis. Due to the extensive work at Alport Moor, and an

understanding of the degree of variability at one site, it is expected the scaling up to a

larger spatial scale will be successful.

23

5.5 Sediment-associated Pb concentrations

5.5.1 UNG Upper Weir

Between October 2003 and November 2004, 9 storms events were sampled from the

Upper Weir at UNG. Figure 10 shows sediment-associated Pb concentrations for a

storm event collected on the 18/4/04. The sediment-associated Pb concentrations and

the within-storm Pb patterns in the 18/4/04 event are typical of the other 8 storm events

sampled. The mean sediment-associated Pb concentration for the 18/4/04 storm event is

138.20 mg kg−1

, but values range from 80.65 to 158.23 mg kg−1

.

50

100

150

200

01:55 03:07 04:19 05:31 06:43 07:55

Time

Pb

(m

g/k

g)

0

50

100

150

200

250

300

SS

C (

mg

/l)

Pb

SSC

Figure 10. Variation in suspended sediment concentration (SSC) and Pb content of suspended sediment

samples during a flood event at UNG Upper Weir on the 18/4/04

24

5.5.2 UNG Main Weir

At the Main Weir site at UNG, a total of 24 storm events were sampled between August

2002 and November 2004. In Figures 11 and 12, two storm events from the dataset are

displayed. Figure 11 shows a fairly large storm event that occurred on the 1/11/02. This

storm event is characterised by a flush of highly contaminated sediment at the beginning

of the storm event, prior to the peak in suspended sediment concentration (SSC). After

the initial Pb flush, sediment-associated Pb values drop sharply

0

100

200

300

400

500

15:22 16:22 17:22 18:22 19:22 20:22

Time

Pb

(m

g/k

g)

0

150

300

450

600

750

SS

C (

mg

/l)

Pb

SSC

Figure 11. Variation in suspended sediment concentration (SSC) and Pb content of suspended sediment

samples during a flood event at UNG Main Weir on the 1/11/02.

The sediment-associated Pb value of this initial Pb flush is 392.7 mg kg−1

. This high

sediment-associated Pb value only occurs only once in all the storm events sampled at

UNG Main Weir, and therefore does not represent the true pollution status of the

system. However, this high sediment-associated Pb value highlights that highly

25

contaminated sediment is transported on occasions in the fluvial system of UNG. Figure

12 shows sediment-associated Pb concentrations for a storm event on the 16/11/04, the

most recent of all the storm events sampled. Sediment-associated Pb values during this

storm event are more typical of the storms sampled over the two year period. During

this storm event the mean sediment-associated Pb value is 124.92 mg kg−1

, with a range

of Pb concentrations from 105.04 to 159.57 mg kg−1

.

50

100

150

200

19:12 20:24 21:36 22:48 00:00 01:12

Time

Pb

(m

g/k

g)

0

50

100

150

200

250

SS

C (

mg

/l)

Pb

SSC

Figure 12. Variation in suspended sediment concentration (SSC) and Pb content of suspended sediment

samples during a flood event at UNG Main Weir on the 16/11/04

5.5.3 Torside Clough

At Torside Clough, 7 storm events were sampled between August and December 2004.

Figure 13 shows a large storm event collected from Torside Clough on the 9/8/04. This

was the largest recorded at the site during the monitoring period, with a peak SSC of

2172 mg l−1

. Sediment-associated Pb values progressively increase as SSC fall. The

26

mean sediment-associated Pb concentration for this storm event is 104.55 mg kg−1

, with

values ranging from 69.26 to 152.84 mg kg−1

. Although, this was the largest storm event

from Torside Clough, sediment-associated Pb values are within the range of sediment-

associated Pb values recorded in the other storm events sampled.

50

100

150

200

16:48 18:00 19:12 20:24 21:36 22:48 00:00 01:12

Time

Pb

(m

g/k

g)

0

500

1000

1500

2000

2500

SS

C (

mg

/l)

Pb

SSC

Figure 13. Variation in suspended sediment concentration (SSC) and Pb content of suspended sediment

samples during a flood event at Torside Clough on the 9/8/04

5.5.4 Site comparisons

Sediment-associated Pb concentrations for all the storm events from each of the sites

have been summarised in Table 3. The sediment-associated Pb data for each of the sites

is normally distributed, and because of the large number of samples collected, the

statistics generated are meaningful and the differences between sites is statistically

different.

27

TORSIDE

CLOUGH

UNG

MAIN WEIR

UNG

UPPER WEIR

Mean 113.99 102.53 137.62

StDev 35.20 38.96 32.53

Minimum 22.94 4.43 77.43

Maximum 205.48 392.64 240.38

n 162 212 94

CL (95%) 5.46 5.28 6.66

Table 3. Descriptive statistics for sediment-associated Pb values for storm events

collected from the study catchments

All suspended sediments collected from UNG and Torside Clough during the

investigation period are contaminated with Pb. The degree of contamination is

determined by the ratio of contaminated particles to uncontaminated particles that make

up the suspended sediment. Particles sourced solely from the contaminated upper peat

layer will have the highest concentration of sediment-associated Pb. However,

suspended sediment which is composed of particles from this contaminant layer and

uncontaminated sediment sources in the catchment (deep catchment peat, millstone grit

sandstone), will have lower sediment-associated Pb values.

The Upper Weir is characterised by organic sources only and incomplete incision into

the deep peat layers (Figure 5). The Main Weir is characterised by a source area for

suspended sediment that is dominated by deep gullies incising into the minerogenic

bedrock (Figure 14). Suspended sediment at the Main Weir is therefore composed of

mineral sources, but also organic sediment derived from uncontaminated deep peat

exposed by gullies. Mean sediment-associated Pb values from the Main Weir (102.53 ±

5.28 mg kg−1

) are lower than the mean sediment-associated Pb value for the Upper Weir

(137.62 ± 6.66 mg kg−1

). This is because the source area for suspended sediment for the

Upper Weir is organic sources only, and because of the incomplete incision into deep

peat, there is a smaller source area for uncontaminated peat than the Main Weir.

28

Figure 14. Incising gully at UNG. This gully is approximately 3 m deep.

At Torside Clough the mean sediment-associated Pb value is 113.99 ± 5.46 mg kg−1

.

The catchment of Torside Clough is characterised by a high proportion of bare peat flats

but also some deeply incising gullies (Figure 15). The underlying bedrock at Torside

Clough is also exposed in many parts of the catchment. Because of the bare peat flats, it

was expected that the mean sediment-associated Pb concentrations from Torside Clough

would be much higher than UNG, because of potentially more exposure and subsequent

mobilisation of contaminated sediment from the bare peat flats. However, there may be

three reasons for the lower than expected mean sediment-associated Pb concentration at

Torside Clough. Firstly, and probably the most likely, is that erosion in the Torside

Clough catchment has removed a large portion of the contaminated peat layer at some

time in the past. Monitoring work over an 11 year period by Anderson et al. (1997)

revealed that the accidental burn on Harrop Moss, located next to Torside Clough, led to

a maximum loss of 24 cm over the 11 year study period. Therefore, it is likely that this

hypothesis is correct. Torside Reservoir, at the bottom of Torside Clough, would be the

final sink for contaminated sediment derived from the polluted catchment. Sediment in

Torside Reservoir is therefore expected to be contaminated. Work by Shotbolt et al.,

(2001) on the sediments of Howden reservoir reveal that they are contaminated with Pb.

29

Figure 15. Bare peat flat in the Torside Clough catchment

The second reason may due to lower Pb concentration in the catchment peats of Torside

Clough. Torside Clough is located approximately 6 miles from UNG and would

therefore expect to have similar Pb concentrations in the upper peat. The third reason

may be due to the nature of the Torside Clough catchment just upstream of the storm

water sampling point. Torside Clough passes through a very steep valley, approximately

1000m long, flanked by outcrops of Millstone Grit and shale. Sources of suspended

sediment will include such material, which may act to dilute sediment-associated Pb

values because of the low Pb content of such materials. However, suspended sediment

sampled in small storm events, and during events sampled when low flow is re-

established, is devoid of large mineral grains. Low sediment-associated Pb values in

such storm events nullify this hypothesis, but to rule it out completely, suspended

sediment samples should be taken from upstream of the valley.

5.6 Contaminated sediment

Sediment-associated Pb concentrations from UNG and Torside Clough indicate that

erosion of the upper peat layer in both catchments is releasing Pb into these fluvial

systems. Values for sediment-associated Pb in rivers draining urban/industrialised

catchments, or regions containing contemporary or past metalliferous mining activity,

are usually significantly higher than rivers draining agricultural land or natural areas.

30

Land use at UNG and Torside Clough is predominately rough grazing, yet the Pb values

are significantly higher than those of other agricultural or natural environments.

In the UK, there are at present no established guideline values for the quality of

sediment in fluvial systems. However, the National Guidelines and Standards Office in

Canada have established Canadian Sediment Quality Guidelines (CSeQGs) for the

Protection of Aquatic Life. For sediment-associated Pb in the fluvial environment, there

is a ‘Probable Effect Level’ (PEL) to aquatic life of 91.3 mg kg−1

(CCME, 2002). Mean

sediment-associated Pb values from UNG and Torside Clough exceed the Canadian

PEL during storm events, possibly impacting aquatic life.

5.7 Sediment-water interactions

Figures 16 and 17 show the results of mixing experiments, conducted to determine if

heavy metals desorb from contaminated peat into stream water. The experiments were

conducted on a ‘worst case-scenario’ basis, whereby the most contaminated peat (top

5cm) was added to low pH stream water (pH 3.5). The first experiment (Figure 16) was

conducted keeping suspended sediment concentration (SSC) constant, taking aliquots of

water at predetermined time intervals throughout the experiment.

y = 0.6802Ln(x) + 6.9754

R2 = 0.9524

0

2

4

6

8

10

12

14

0 200 400 600 800 1000 1200 1400 1600

Time (mins)

Pb

(µ

g/l

)

Figure 16. Dissolved Pb concentrations in stream water during the duration of the experiment

31

The second experiment (Figure 17) involved three different SSC over one time interval

(24 hrs). The results of the first experiment (Figure 16) indicate that rapid Pb desorption

from sediment to water occurs during the first hour of the experiment. Results for the

second experiment (Figure 17) indicate that desorption of Pb increases as SSC

increases. The laboratory mixing experiments reveal that under a ‘worst-case scenario’,

Pb desorption from sediment to water occurs, and therefore sediment-associated heavy

metals in the fluvial systems of eroding peat catchments may influence dissolved heavy

metal flux. However, because the mixing experiments were carried out under a ‘worst-

case scenario’, true environmental conditions of the system may not have been

accurately represented in the lab experiments.

9

10

11

12

13

14

0.5 1.6 3

SSC (g l�¹)

Pb

(µ

g/l

)

Figure 17. Dissolved Pb concentrations in stream water for different suspended sediment concentrations.

5.8 Stormflow dissolved Pb

5.8.1 Stormflow dynamics

Storm water samples from 18 storm events were collected from UNG and from Torside

Clough, and subsequently analysed for dissolved Pb. Figure 18, 19 and 20 show one of

the storm events sampled from Torside Clough. During this storm event pH fell by

nearly a whole unit and DOC increased from 4.7 to 29.7 mg l−1

(Figures 18 and 19

respectively). These trends can be attributed to flushing of humic acids from upper

layers of peat within the catchment.

32

25

30

35

40

45

19:1

2

20:2

4

21:3

6

22:4

8

00:0

0

01:1

2

02:2

4

03:3

6

Time

Sta

ge (

cm

)

3.50

3.70

3.90

4.10

4.30

4.50

4.70

pH

Stage

pH

Figure 18. Variation in pH during a flood event at Torside Clough on the 5/08/04

25

30

35

40

45

19:1

2

20:2

4

21:3

6

22:4

8

00:0

0

01:1

2

02:2

4

03:3

6

Time

Sta

ge (

cm

)

0

5

10

15

20

25

30

35

DO

C (

mg

/l)

Stage

DOC

Figure 19. Variation in DOC during a flood event at Torside Clough on the 5/08/04

Figure 20 shows dissolved Pb concentrations for the water samples collected during the

event. Pb concentrations increase from ~ 1 µg l−1

to ~ 9 µg l−1

. The stream at Torside

Clough recharges Torside Reservoir at the bottom of the catchment. The EU Drinking

Water Directive standard for Pb is 10 µg l−1

(EUROPA, 2003). Although dissolved Pb

levels in the later half of the storm on the 5/8/04 come close to this standard, the mean

Pb concentration is 5.66 µg l−1

for this event, and this water entering Torside Reservoir

would be mixed and diluted with water containing lower Pb concentrations.

33

25

30

35

40

45

19:1

2

20:2

4

21:3

6

22:4

8

00:0

0

01:1

2

02:2

4

03:3

6

Time

Sta

ge (

cm

)

0

1

2

3

4

5

6

7

8

9

10

Pb

(µ

g/l

)

Stage

Pb

Figure 20. Variation in dissolved Pb during a flood event at Torside Clough on the 5/08/04.

The strong positive correlation between Pb and DOC (r2 = 0.9751 – Figure 21) indicates

that DOC is effectively mobilising and transporting Pb in the fluvial system of Torside

Clough. In the other storm events sampled, there is also a strong positive correlation

between Pb and DOC. The work from UNG and Torside Clough supports the work by

Lawlor & Tipping (2003), and the importance of DOC in heavy metal transport, but

gives a new insight into the dynamics of dissolved Pb flux over an event level time

scale.

y = -0.2472x2 + 5.656x - 0.824

R2 = 0.9751

0

5

10

15

20

25

30

35

0 1 2 3 4 5 6 7 8 9 10

Pb (µg/l)

DO

C (

mg

/l)

Figure 21. Relationship between Pb and DOC for the storm event on the 5/8/04 from Torside Clough.

34

5.8.2 Autumn flush

During summer months, water table drawdown, reduced precipitation and higher

temperatures result in high DOC generation in peatland environments. During the early

autumn DOC is flushed out of the peat into the fluvial system with the onset of more

frequent precipitation and a rise in water table (Evans et al., 1999, Worrall et al., 2002).

Of the 18 storm events analysed for Pb and DOC, 4 storms were collected at UNG over

a one month period during autumn 2004 (13/10/04, 21/10/04, 4/11/04 and 16/11/04).

Figure 22 shows the relationship between Pb and DOC for the four storm events

sampled during the autumn of 2004.

Figure 22. Relationship between Pb and DOC for autumn 2004 storm events from UNG Main Weir.

35

The graph not only shows the good positive correlation between Pb and DOC (r2 =

0.8486), but also the flushing of Pb from the UNG system over the autumn period.

Highest Pb levels occur during the early autumn (~ 10 µg l−1

), progressively declining

as the autumn period progresses. Rising concentrations of DOC have been observed in

many UK rivers over the last 30 years (Charman, 2002). Freeman et al., (2001) and

Worrall et al., (2004) have associated the observed increase in DOC with rising

temperatures. Increases in DOC over the last 30 years may therefore have also been

accompanied with increases in dissolved Pb levels from stream draining peatland

catchments contaminated with Pb. The latest climate change scenarios suggest a higher

summer mean temperature (Hulme et al., 2002). This may result in significant

drawdown of peatland water tables, especially in large boreal peatland environments.

Warmer conditions and lower water tables might be expected to increase and accelerate

the supply of DOC through microbial decomposition, resulting an in increase in the

export of DOC (Charman, 2002). One deleterious effect of this increase in DOC fluxes

may be increased fluxes of dissolved Pb during autumn flush periods in catchments

contaminated with heavy metals.

The strong positive correlation between dissolved Pb and DOC seems to suggest that

instream interactions between contaminated sediments and the water column appear to

be negligible. However, it cannot be ruled out that contaminated sediments in the fluvial

environments of the Peak District have no influence on dissolved heavy metal fluxes.

More mobile elements like Zinc may significantly influence dissolved levels in peatland

streams.

5.9 Pb in the sediment-water system

Physical, chemical and biological processes operating at different scales in the study

catchments have mobilised Pb into many parts of the sediment-water system. Figure 23

details some of the Pb levels in the sediment-water system at UNG, including the lower

gully faces and overbank deposition areas.

36

Intact peatNear surface peak

concentrations ranging from

785.42 - 1613. 65

Lower gully facesMean = 60.83StDev = 32.18

Min = 31.66Max = 118.58

n = 6

CL (95%) = 33.73Overbank sediments

Mean = 57.12

StDev = 36.13

Min = 22.16Max = 123.01

n = 13

CL (95%) = 21.83

Suspended sedimentMean = 102.53

StDev = 38.96Min = 4.43

Max = 392.64n = 212

CL (95%) = 5.28

Stream waterMean = 5.75

StDev = 2.08Min = 1.99

Max = 10.71n = 230

CL (95%) = 0.27

Figure 23. Pb in the sediment-water system at UNG.

Pb concentration for sediments in mg kg−1

and dissolved concentrations in µg l−1

Suspended sediment and dissolved concentrations are based on data from the Main Weir.

6 CONCLUSIONS

The peat catchments of the Peak District are heavily contaminated with industrially-

derived, atmospherically-transported heavy metals. Heavy metals such as Pb are stored

in the upper peat layer, with the majority of contamination located in the top 30 cm.

However, peak Pb concentrations vary significantly over small spatial scales. Therefore,

heavy metal inventories, rather than concentrations on a depth basis, are more accurate

37

for assessing contamination status of peatland areas, and providing information on total

heavy metal stores. Erosion in some of the peat catchments of the peak District is

severe. Eroded peat particles, sourced from the contaminated upper peat layer, are a

vector for sediment-associated heavy metal mobilisation and transport in the fluvial

systems of the Peak District. Sediment-associated heavy metal flux from eroding peat

catchments is both spatially and temporally variable, influenced by a combination of

geomorphological and hydrological conditions. Laboratory experiments reveal that

under certain hydrological and physico-chemical conditions, dissolved concentrations of

heavy metals in these fluvial systems may increase as a result of interactions between

contaminated peat and stream water. However, from extensive analysis of storm waters

for dissolved Pb, this toxic heavy metal is mobilised and transport by DOC, and

dissolved concentrations do not appear to be influenced by instream processes of

desorption of Pb from contaminated sediments in the water column. The results

indicate that particulate and aqueous heavy metal flux from eroding peat catchments is

dynamic, because catchment-specific processes and conditions act to deliver and

transport heavy metals within the fluvial system. The results from this study highlight

the importance of high resolution water quality measurements.

There is historical evidence that previous phases of peat erosion may have been initiated

during prolonged periods of drought. Therefore, predicted trends in global warming

may result in an increase in peat erosion, with the possible mobilisation of increased

loads of contaminated sediment. Such environmental change may also increase fluxes of

DOC from peatland catchments, and those with significant stores of contaminants in the

surface peat, may experience increases in dissolved heavy metal fluxes.

38

7 ACKNOWLEDGEMENTS

We thank Moors for the Future for the provision of a Small Project Grant and the

School of Environment and Development, The University of Manchester, for fieldwork

funding. The authors are grateful to The National Trust for allowing work to be carried

out at Upper North Grain and to United Utilities for allowing work to be carried out at

Torside Clough.

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