sustainable coastal aquifer management in urban areas
TRANSCRIPT
Proceedings of the Resilient Cities 2013 congress
Session: B4 Adapting Urban Water Management
Presentation: Sustainable coastal aquifer management in urban areas:
The role of groundwater quality indices
El-Fadel, M.; Tomaszkiewicz, M.; Abou Najm, M.
Abstract:
Urbanized coastal areas are vulnerable to salt water intrusion into fresh water aquifers due to
increased water demand associated with population growth and exacerbated by potential
climate change and sea level rise resulting in serious socio-economic impacts related to the
deterioration of groundwater resources. Groundwater quality indices (GQIs) that constitute a
reliable management tool in defining coastal aquifer vulnerability to seawater intrusion (SWI)
are relatively limited. This study aims to develop GQIs using water quality from 60 groundwater
wells during vulnerable periods of early and late summer to ensure their representativeness
under worst-case conditions. Generalized and SWI-specific GQIs were developed from various
water quality indicators and spatially analyzed through GIS. The results indicated that
generalized GQI was successful in capturing pollution issues related to wastewater
contamination whereas the SWI-specific GQI was helpful in understanding the extent of saline
water intrusion. Such results contribute to filling a gap in GQI definition, particularly when
accounting for seasonal variability of SWI under urban stress. They form a basis for planning
effective water quality management towards sustainable exploitation of groundwater resources
in coastal urban areas particularly during summer periods when recharge is limited.
Keywords:
Groundwater quality index, sustainable aquifer management, vulnerability mapping.
Proceedings of the Resilient Cities 2013 Congress
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1. Introduction
Water resources are increasingly strained due to population growth, development, and
environmental degradation with policy makers and planners often facing serious challenges in
ensuring the long-term sustainability of existing resources and meeting future water demands
for various sectors. This is especially true along coastal zones where population centers tend
to develop resulting in high densities with increased water demands that are invariably
satisfied at the expense of overexploitation and exposure of coastal aquifers to various urban
pollution sources partly due to inadequate infrastructure (Howard, 2002; Appleyard, 2004)
and mostly due to saltwater intrusion (Chang et al. 2011; El-Moujabber et al. 2006; Konikow &
Kendy, 2005). The proposed research is driven by the need to understand the combined
effects of climate change and human impacts on saltwater intrusion. It is predicted that
climate change will aggravate salt water intrusion as 1) a direct consequence of sea level rise,
and 2) an indirect consequence of the changing climate, where a) decreases in precipitation
will affect recharge of the coastal aquifers, and b) increases in temperature will heighten
water demand in addition to affecting recharge of the coastal aquifers through increased
evapotranspiration. Hence, anthropogenic activities such as over-pumping and excess paving
in urbanized areas are currently the major causes of saltwater intrusion, and are expected to
interact with climate change to exacerbate the problem given predictions of population
growth and expected increases in groundwater overexploitation. Increased saltwater
intrusion will carry a significant socio-economic burden (i.e. damage to infrastructure such as
piping, distribution and storage systems, household appliances and supplies, and when
relevant, the cost of averting health threats; salinity damages to soils, impacts on crop yields,
impacts of saline runoff, and consequences for drainage management), which is likely to
touch upon national economies as well as vulnerable and fragile local livelihoods. In this
context, it is essential to quantify the intensity, as well as the spatial and temporal extent, of
saltwater intrusion into freshwater coastal aquifers in order to develop vulnerability maps
and mitigation measures / adaptation strategies that take into consideration socio-economic
impacts and area characteristics. For this purpose, vulnerability mapping is a valuable tool
for environmental planning and decision-making by using index methods coupled with GIS-
based spatial analysis to ascertain aquifer vulnerability (Machiwal et al. 2011). Vulnerability
can be categorized into either intrinsic, based on the sensitivity of aquifers to human and
Proceedings of the Resilient Cities 2013 Congress
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natural activities (Metni et al. 2004), or specific, based on one or more water contaminants
(Gogu & Dassargues, 2000). Intrinsic vulnerability mapping can be developed using one of
several overlay and index or statistical models, such as DRASTIC (Aller et al. 1987) but has
been widely criticized for its limited ability to predict the aquifer’s reaction to particular
pollutants (Kouli et al. 2008). Specific vulnerability mapping is argued to be a preferable
alternative due to its capability to derive an objective analysis based on the spatiotemporal
variability of monitored water quality parameters that can be translated into a GIS-based
groundwater quality index (GQI).
Given that water resources are continuously facing increasing pressures, research into their
vulnerability can provide useful information for those concerned with environmental policy
and planning. Populations living in vulnerable areas would benefit from better-informed
policy that alleviates the impact of climate change on groundwater quality. As such, in this
study, a groundwater sampling program was implemented and the results were used to map
the vulnerability to a set of indicators and determine corresponding GQIs. Different sources
of contamination were considered in the mapping process (individually and in combination)
and compared to international guidelines used in groundwater quality classification with the
ultimate objective to provide a basis for planning effective water quality management
towards sustainable exploitation of groundwater resources in coastal urban areas
2. Materials and Methods
2.1. Study area characteristics
The study area consisted of a coastal karstic aquifer underlying the city of Tripoli, Lebanon
located along the Eastern Mediterranean (Figure 1). The area is characterized by a semi-arid
climate with mild wet winters (average daily temperatures ranging from 11 to 280C with 636
mm of average annual precipitation from October through March) and moderately hot dry
summers (average daily temperatures ranging from 19 to 270C with 75 mm of precipitation).
With a predominantly residential and commercial land use, the area’s high population density
exceeding 35,000 capita per km2 (Awad & Darwich, 2009), exerts a pressure on existing
water and sanitation infrastructure. The City id one of two pilot areas considered because of
their highest density of urbanization, population growth, and development.
Proceedings of the Resilient Cities 2013 Congress
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2.2. Field sampling program
Two groundwater sampling campaigns were conducted in early (June) and late summer
(September) to capture conservative seasonal fluctuations when groundwater recharge is
minimal and most vulnerable thus ensuring a representative GQI. During each campaign,
samples were collected from 60 privately owned wells spread throughout the study area
(Figure 1). On site and laboratory analysis included pH; Temperature; Alkalinity; Total
Hardness; Calcium; Magnesium; Sodium; Potassium; Chloride; Fluoride; Nitrate; Nitrite;
Bicarbonate; Sulfate; Phosphate; Fecal Coliform (FC); Total Coliform; and Total Dissolved
Solids (TDS) in accordance with Standard Methods for the Examination of Water and
Wastewater (APHA/AWWA/WEF, 2005).
2.3. Development of GQIs
Decision and policy makers are continuously challenged with varied and complex water
quality problems that require the understanding and monitoring of the spatial variability of
critical water quality indicators. Depending on the complexity of the problem, water quality can
be represented with single-pollutant concentration maps or require the use of sophisticated
generalized or problem-specific GQIs. The use of single-pollutant concentration maps can be
Figure 1. Pilot study area with distribution of groundwater wells and population densities
MEDITERRANEAN SEA
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ineffective and misleading for researchers as well as decision and policy makers since water
quality indicators have different and at times conflicting impacts. In such cases, multi-
parameter GQIs become more suitable to translate water quality concentrations into a single
indicator that resolves a pre-defined environmental problem. In the context of saltwater
intrusion, two GQI methods were considered in this study, a generalized GQI (after Babiker et
al. 2007) and a modified, saline intrusion-specific GQI The generalized GQI normalized
results from different water quality concentrations using World Health Organaization (WHO)
threshold-standards and aggregated the results into a single value. Water quality indicators
with potential health impacts were weighted heavier than other indicators in the GQI
calculations. The generalized GQI can be adopted to address a specific environmental
problem through the selection of a range of contributing pollutants. While the generalized GQI
has been successfully used in addressing some environmental problems, it is limited in scope
to applications where related processes follow linear increasing or decreasing trends.
However, environmental problems like saltwater intrusion are more complex and involve
processes that causes non-linear trends. For this reason, a saltwater intrusion-specific GQI
was established. This GQISWI accounts for hydrogeochemical processes associated with
saltwater intrusion that are currently explained in common graphical methods like the Piper
diagram (Figure 2). The problem with the latter approach is the inability to georeference them
for use by decision and policy makers. Equations 1 to 4 represent the generalized GQI and 5
to 8 those used to develop the GQISWI of the Piper diagram (Singhal & Gupta, 2010). Thus,
the advantage of the GQISWI lies in its ability to simplify multiple non-linear processes
involving several water quality pollutants into an indicator that can be quantified and spatially
referenced. On the other hand, and in a Piper diagram,water analysis results are presented
on a trilinear plot consisting of cation and anion triangles, which extend to a two-coordinate
diamond diagram. Generally, seawater composition has a uniform chemistry where Cl- and
Na+
make up approximately 84% of the total ionic composition. On the other hand, freshwater
composition varies widely, although Ca2+
and HCO3- commonly dominate (Richter & Kreitler,
1993). As a result, seawater and freshwater appear in distinct areas of the Piper diagram.
Likewise, mixed groundwaters appear in characteristic areas on the diagram representative of
various hydrogeochemical processes associated with saltwater intrusion (Singhal & Gupta,
2010). Points on the two Piper diagram triangles were translated into a gridded value and EC
was normalized against a reference range in groundwater. The three values were then
aggregated into the GQISWI thus allowing the mapping and the visual interpretation of the
spatiotemporal variability of SWI.
CI = (X – Y) / (X + Y) (1)
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R = 0.5xCI2 + 4.5xCI + 5 (2)
GQI = 100 – (R1w1 + R2w2 + … + Rnwn) / N (3)
w = mean R for Ca2+
, Mg2+
, Na+, Cl
-, SO4
2-, TDS or mean R + 2 for Fl
-, NO2
-, NO3
-, FC, TC (4)
Where X = Derived water quality parameter concentration at each pixeled grid location (from Kriging analysis); Y = WHO threshold value; CI: Normalized concentration index (ranging from -1 to +1); R = Ranked value (ranging from +1 to +10 , from lowest to highest water quality based on individual parameters); w = weighted rank values for impacting WQ parameters with potential health impacts; and N = number of parameters to develop index.
{( ( ))
( ( )) }
5
{( (
))
( )
} 6
{
}
7
2
GQIGQIGQIGQI ECanioncation
SWI
8
Figure 2. Description of the Piper Diagram (adapter after Piper, 1944)
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3. Results and Discussion
A comparison of the results of groundwater quality analyses with WHO threshold standards
revealed generally higher concentrations in late summer, which is typically expected after a
long period of pumping and no recharge. Coupled with subsequent geostatistical analysis,
the primary groundwater pollution sources consist of sewage and saltwater intrusion.
3.1. Sewage contamination indicators
While total coliforms tested positive for most sampled wells in the study area during both late
and early summer, fecal coliform contamination (Figures 3a and 3b) is significantly higher in
the late summer after a prolonged dry period. This can be attributed to sewer network
leakage and the absence of recharge and dilution coupled with increased groundwater
extraction to meet water demand shortages during the summer as well as slightly higher
groundwater temperature, which induces higher biological activity rates (Paul et al. 2004).
Karstic aquifers are highly vulnerable to bacterial pollution, partly due to their large pore
spaces which transmit organisms easily through the aquifer with minimal soil filtration that
usually occurs in non-karstic media (Appleyard, 2004; Kacaroglu, 1999). Total coliforms which
covers all bacteria sources including soil and plant sources, exhibited a smilar trend during
the late summer (Figures 3c and 3d).
Similarly, nitrate levels (Figures 3e and 3f) exceeded the WHO guideline of 50 mg/l in part of
the study area, but with spatiotemporal patterns that are different from bacterial contamination,
thus indicating contribution of different sources. While nitrate pollution often results from
domestic wastewater infiltrating into the groundwater and can be linked with bacterial
contamination particularly in densely populated areas in developing countries, it is equally
linked to agro-chemical and fertilizer usage at nearby agricultural fields (Zhang et al. 2004).
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Figure 3. Distribution of sewage contamination pollutant indicators
Greens represent concentrations within desirable limits; reds represent concentrations beyond desirable limits
3.2. Saltwater intrusion indicators
Signs of salinization are manifested through elevated chloride concentrations, ion exchanges,
and characteristic hydro-geochemical ratios. Seawater has a generally uniform chemistry
with an excess of Cl- over alkali ions (Na
+ and K
+) and with Mg
2+ dominating Ca
2+, whereas
fresh groundwater composition can vary greatly although primary cations and anions are Ca2+
,
Mg2+
, Cl-, HCO3
-, and SO4
2- (El Moujabber et al. 2006). In many cases, Ca
2+ is in excess of
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Mg2+
in fresh groundwater, particularly in karstic limestone aquifers rich in carbonate rocks,
and typical in the study area.
Elevated chloride concentrations are a primary indicator of salinity, whereby the range of 300-
600 mg/l is considered typical for initial mixing of freshwater and seawater (Oude Essink 2001;
Petalas & Diamantis 1999). The results of groundwater sample analyses indicated that
chloride exhibited similar trends between late and early summer (Figures 4a and 4b), with
increased concentrations along the western coastline. As the dry season progresses,
chloride concentrations are expected to increase due to groundwater pumping with no
recharge. In the absence of chloride monitoring data, total dissolved solids (TDS) can also
serve as an indicator of imminent salinity intrusion. High TDS concentrations (Figures 4c and
4d) in groundwater typically correlate well with elevated chloride concentrations (Park et al.
2005). Initial saltwater intrusion can be identified by TDS concentrations exceeding 640 mg/l,
corresponding to an electrical conductivity (EC) of 1,000 μS/cm (Petalas & Diamantis, 1999;
Pulido-Leboeuf et al. 2003),
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3.3. Generalized groundwater quality index, GQI
The GQI developed by Babiker et al (2007) was adopted where a comparative assessment
with reported values in the literature suggests that the GQI can realistically range from 60 to
100, where lower and higher values indicate poor and better water quality values,
respectively. Moreover, the GQI is dependent on the specific parameters selected for
analysis and is best suited for relative rather than absolute assessments (Babiker et al. 2007).
Recent studies characterized groundwater in their respective study areas and categorized the
Figure 4. Distribution of saltwater intrusion pollutant indicators
Greens represent concentrations within desirable limits;
reds represent concentrations beyond desirable limits;
yellows represent initial freshwater-seawater mixing
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classification of groundwater as poor, moderate, and good quality based on 10% intervals
between the minimum and maximum GQI although the calculated GQI differed significantly
between studies (Babiker et al. 2007; Machiwal et al. 2011). Therefore, finding a specific
range of GQI to define the acceptability of groundwater is challenging at best. Three
scenarios were considered:
First, a GQI using nine parameters (Ca2+
, Mg2+
, Na+, Cl
-, NO3
-, SO4
-, TDS, TC, FC) was
developed. Classification criteria of the general water quality was based on the assumption
that exceeding maximum WHO guidelines is considered poor while being below half the
maximum threshold is good water quality. For the nine parameters considered, the water
quality is classified according to the GQI score as follows:
GQI < 76: poor groundwater quality,
76<GQI<83: moderate water quality,
GQI > 83: good water quality.
With this definition, the corresponding GQI distribution for the study area is depicted in
Figures 5a and 5b which indicates poor groundwater quality in late summer (mean GQI = 72.5
with the entire area below the 76 threshold) and moderate groundwater quality in early
summer (mean GQI = 78.9 with a few zones exhibiting a GQI > 83 while an appreciable zone
remaining below the 76 threshold).
Second, although bacterial contamination adversely affects water quality due to its potential
health impacts, its contribution to seawater intrusion is debatable. Thus, a comparative
assessment was conducted to define the GQI by eliminating the impact of bacterial
contamination (Figures 5c and 5d). Using the same approach (maximum or half the
maximum threshold imposed by the WHO guidelines), the water quality is classified according
to the GQI score as follows:
GQI < 83: poor water quality,
83< GQI < 88: moderate water quality,
GQI > 88: good water quality.
Under these conditions, both late summer (mean GQI = 87.5) and early summer (mean GQI
= 87.7) can be considered to have nearly moderate-to-good water quality with the exception
of the western coastline.
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Third, nitrate has a greater impact on the overall GQI but its contribution to seawater intrusion
is also debatable. Thus, a comparative assessment was conducted on the GQI by eliminating
nitrate, effectively leaving only the 6 parameters relevant for saline intrusion (Ca2+
, Mg2+
, Na+,
Cl-, SO4
2-, TDS). A comparison of the resultant GQI vulnerability map (Figures 5e and 5f)
concurs with other saline intrusion indicators, because chloride has the greatest impact on the
spatial pattern of the GQI due to its high mean rank value (w) and high standard deviation. In
this scenario, poor water quality is defined with a GQI < 85, moderate water quality has a GQI
ranging from 85 to 89, and good water quality has a GQI > 89. Although late summer (mean
GQI = 89.3) and early summer (mean GQI = 89.6) are considered to have generally good
water quality in this case, poor water quality remains confined to the western shoreline due to
saline intrusion.
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Figure 5. Groundwater quality index (GQI) vulnerability maps
Reds represent poor water quality and blues represent good water quality
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3.4. Saline intrusion groundwater quality index, GQISWI
The GQISWI aggregates data from the Piper diagram and EC into a single value ranging from
0, representing seawater, to 100, representing freshwater, which can be used as an index for
spatiotemporal mapping (Figure 6). Nearly 80% of the pilot study area is considered
freshwater in both late (mean GQISWI = 83.5) and early summer (mean GQISWI = 86.5).
However, seawater intrusion is evident along the western coastline, where the GQISWI is
below the threshold value of 75, indicative of mixed groundwater. Moreover, increasing
salinity is more problematic in late summer when groundwater recharge is at a minimum,
including along the ephemeral river bisecting the study area, which potentially allows
seawater to flow inland during the dry season.
Figure 6. GQISWI for (a) late summer and (b) early summer Reds indicate more saline and blues indicate more fresh aquifers
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4. Concluding remarks
Climate change is expected to have diverse influences on the availability of water resources
with most predictions indicating that acting alone, climate change will reduce the availability
of water in many areas of the globe and threaten to increase the salinity of water in coastal
aquifers through saltwater intrusion along coastal zones in particular. The vulnerability of
groundwater reserves already strained through high water demand in densely populated
coastal regions, resulting in over-extraction of aquifer water, is known to be an important
driver in increasing aquifer salinity. While it is not clear how much climate change impacts
will augment the existing over-extraction effects on groundwater salinity, or how climate
change, local geology and human activity will interact in different areas under different
conditions to further strain aquifer resources, it is widely believed that this change will only
exacerbate already stressed conditions. In general, those coastal areas occurring in less
developed countries or in localities that are home to underprivileged communities will be
the most vulnerable to changes in water availability and/or quality from the combination of
climate change and increased water demand. Thus, increased saltwater intrusion will carry a
significant socio-economic burden, which is likely to touch upon national economies, and
most importantly, vulnerable and fragile local livelihoods. In this context, using a GIS-based
water quality index is an effective means to aggregate chemical data into a quantifiable value
that can be spatially mapped providing helpful robust visual tool for researchers and policy
makers for defining corrective or adaptive measures and city planning that target areas most
affected/vulnerable and direct potential growth towards areas that have less pressure on
groundwater. Moreover, a GQI can help in performing an objective analysis, but care must
be exercised to standardize which indicators should be considered with a clear definition of
what constitutes poor, moderate, and good water quality to enable policy relevance. Targeted
mitigation efforts can then be based on a specific set of indicators and corresponding ratios
commonly linked to a certain source. While the development of GQIs delineates vulnerable
zones, it constitutes only a step in defining a groundwater management plan for aquifer
exploitation under the stress of population growth exacerbated by climate change impacts.
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In cooperation with the City of Bonn and the World Mayors Council on Climate Change
ICLEI does not accept any kind of liability for the current accuracy, correctness,
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Proceedings of the Resilient Cities 2013 Congress
Conference organisers: ICLEI – Local Governments for Sustainability
In cooperation with the City of Bonn and the World Mayors Council on Climate Change
ICLEI does not accept any kind of liability for the current accuracy, correctness,
completeness or quality of the information made available in this paper.
http://www.iclei.org/resilient-cities/
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Acknowledgements:
This study is part of a program on climate change and saltwater intrusion along the Eastern
Mediterranean funded by the International Development Research Center (IDRC) of Canada
at the American University of Beirut. Special thanks are extended to Mr. Mark Redwood and
Dr. Carrie Mitchel at IDRC for their support in implementing this program.
Proceedings of the Resilient Cities 2013 Congress
Conference organisers: ICLEI – Local Governments for Sustainability
In cooperation with the City of Bonn and the World Mayors Council on Climate Change
ICLEI does not accept any kind of liability for the current accuracy, correctness,
completeness or quality of the information made available in this paper.
http://www.iclei.org/resilient-cities/
The author(s):
Mutasem El-Fadel
Professor and Chairperson
Department of Civil & Enviromental Eng.
American University of Beirut
Email: [email protected]
Marlene Tomaszkiewicz
PhD Student
Department of Civil & Enviromental Eng.
American University of Beirut
Email: [email protected]
Majdi Abou Najm
Assistant Professor
Department of Civil & Enviromental Eng.
American University of Beirut
Email: [email protected]
Bios:
Mutasem El-Fadel got his BE in Civil Engineering from the American University of Beirut,
Lebanon, MS in Environmental Engineering, MS in Water Resources Engineering, and PhD
in Environmental Engineering, all from Stanford University. He is currently Professor and
Chairperson of the Department of Civil and Environmental Engineering, Faculty of
Engineering and Architecture, American University of Beirut, Lebanon where he also holds
the Dar Al-Handsah (Shair & Partners) Chair in Engineering.
Marlene Tomaszkiewicz got her BS in Civil Engineering from Illinois Institute of Technology,
and her MS in Civil Engineering from Louisiana State University. She is currently a PhD
student at the Department of Civil and Environmental Engineering, Faculty of Engineering
and Architecture, American University of Beirut, Lebanon.
Majdi Abou Najm got his BE in Civil Engineering and ME in Environmental and Water
Resources Engineering from the American University of Beirut, and his PhD from Purdue
University. He is currently an Assistant Professor at the Department of Civil and
Environmental Engineering, Faculty of Engineering and Architecture, American University of
Beirut, Lebanon.