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.
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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.
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
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/
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)
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/
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)
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/
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).
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/
Figure 3. Distribution of sewage contamination pollutant indicators