Arab J Geosci (2011) 4:483–494 DOI 10.1007/s12517-010-0135-0
ORIGINAL PAPER
Mapping groundwater contamination risk using GIS and groundwater modelling. A case study from the Gaza Strip, Palestine Husam Musa Baalousha
Received: 26 October 2009 / Accepted: 8 March 2010 / Published online: 9 April 2010 # Saudi Society for Geosciences 2010
Abstract Increasing pressure on water resources worldwide has resulted in groundwater contamination, and thus the deterioration of the groundwater resources and a threat to the public health. Risk mapping of groundwater contamination is an important tool for groundwater protection, land use management, and public health. This study presents a new approach for groundwater contamination risk mapping, based on hydrogeological setting, land use, contamination load, and groundwater modelling. The risk map is a product of probability of contamination and impact. This approach was applied on the Gaza Strip area in Palestine as a case study. A spatial analyst tool within Geographical Information System (GIS) was used to interpolate and manipulate data to develop GIS maps of vulnerability, land use, and contamination impact. A groundwater flow model for the area of study was also used to track the flow and to delineate the capture zones of public wells. The results show that areas of highest contamination risk occur in the southern cities of Khan Yunis and Rafah. The majority of public wells are located in an intermediate risk zone and four wells are in a high risk zone. Keywords Groundwater contamination . Risk mapping . GIS . MODFLOW . Capture zones . Vulnerability . Gaza Strip . Drastic
H. M. Baalousha (*) Hawke’s Bay Regional Council, 159 Dalton Street, Napier 4110, New Zealand e-mail:
[email protected]
Introduction Groundwater is an invaluable source of drinking water in many areas around the world. Due to extensive pumping, agricultural, and industrial activities, aquifers are at risk of being contaminated. Intensive application of pesticides and fertilisers, discharge of wastewater, and industrial effluent and excessive groundwater abstraction are just a few examples of activities that lead to groundwater contamination. These activities have resulted in the deterioration of water resources in various regions around the world (Pandey et al. 1999). Aquifers are valuable sources for water. Therefore, a quick action should be taken to prevent aquifers from contamination and to reduce the risk of contamination impact. Groundwater contamination risk mapping can help planners and decision-makers on proper land use and water resources management. This will enable incorporation of groundwater protection and health impact assessment in the analysis. Risk mapping is not only a preventative measure but it also assist with mitigation processes of groundwater contamination. Risk, by definition, is the probability of an event multiplied by its impact. In environment context, risk is the probability that a hazard will turn into a disaster. In groundwater context, risk can be defined as the probability that groundwater at a drinking well becomes contaminated to an unacceptable level by activities on the land surface (Morris and Foster 1998). Risk can be reduced by implementing a mitigation strategy with best management practice. Best practice avoids high-risk areas when locating a site of possible pollution potential.
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Geographical Information System (GIS) has been widely used in risk mapping (e.g., Bartels and Beurden 1998; Ducci 1999; Al-Adamat et al. 2003; Mimi and Assi 2009). It is very common to use intrinsic vulnerability either alone or coupled with other factors to assess groundwater contamination risk. The most widely used method for intrinsic vulnerability assessment is the DRASTIC approach (Aller et al. 1985). DRASTIC is based on seven hydrogeological parameters: Depth to water table, Recharge, Aquifer media, Soil media, Topography, Influence of vadose zone, and hydraulic Conductivity to assess the intrinsic aquifer vulnerability. Each map is classified and rated, then weighted based on standard DRASTIC weigh system (Aller et al. 1985). Vulnerability index is the sum of each rated map multiplied by its respective weight as shown in the equation below. The final DRASTIC product is a map showing vulnerability index. DRASTIC has been used in different studies to assess aquifer vulnerability (e.g., Lasserrea et al. 1999; Baalousha 2006; Nobre et al. 2007; Assaf and Saadeh 2009).
drawback of geostatistical approach is that it does not consider hydrogeological settings that have a significant effect on contamination risk. In this study, a new approach is proposed for contamination risk mapping. This approach depends on the idea that groundwater contamination risk is a product of probability of contamination occurring and contamination impact. A contamination risk map is a function of probability overlaid with a map of potential impact of groundwater contamination. The resultant convergence of probability of contamination and contamination impact is and assessed geospatially on map as a cross-product of the probability map and the contamination impact map. In the case study presented in this paper, the contamination probability map was created based on a previous work by the author using the DRASTIC approach (Baalousha 2006). In this paper, the vulnerability map was coupled with land use to represent the probability of contamination. The impact map was based on two factors: health impact of major contaminants in the area of study and the public water supply capture zone.
DRASTIC index ¼ Dr Dw þ Rr Rw þ Ar Aw Methodology
þ Sr Sw þ Tr Tw þ Ir Iw þ Cr Cw
ð1Þ
Where r and w denote DRASTIC rating and weight, respectively. In many studies, vulnerability map was coupled with hazard map or land use map to produce risk maps (Ducci 1999). For example, Al-Adamat et al. (2003) have coupled a DRASTIC vulnerability map with a land use map to produce a risk map. Intrinsic vulnerability is a good measure of weaknesses of an aquifer, as it considers the hydrogeological characteristics of the area under consideration. Intrinsic vulnerability alone, however, is not a measure of risk. Using vulnerability alone, or with land use to represent the risk, lacks the contamination impact, which is an essential factor for risk assessment. In addition, the movement of contaminants in the groundwater, which affects the capture zone around wells, is not considered in this approach. A highly vulnerable area, for instance, is not under contamination risk unless it is susceptible to a contamination source and the contamination impact is high. Other approaches of groundwater contamination risk mapping use a probability map of contaminants distribution (Zhu et al. 2001; Wackernagel et al. 2004; Amini et al. 2005). The probability map approach uses geostatistics (i.e., kriging) to interpolate the actual concentration of a certain contaminant in groundwater and to create a groundwater contamination probability map. The main
The assessment of groundwater contamination risk requires two main factors: probability of contamination and the contamination impact, as depicted in Fig. 1. For example, when a site has a high contamination probability but has a low impact of contamination, then the risk is low. But when both contamination impact and probability of contamina-
Fig. 1 The concept of groundwater contamination risk mapping
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tion are high, then the risk is high. Thus, the contamination risk (R) can be written as: R ¼ pðcontÞ \ pðimpactÞ
R 2 ½0; 1
ð2Þ
where, p(cont) is the probability of contamination; p(con) ∈ [0,1] and p(impact) is the impact of contamination; p (impact) ∈ [0,1. Using Eq. 2, a map of contamination probability can be classified in the probability space; that is p ∈ [0,1], and the same for contamination impact (Fig. 1). The cross-product of both maps is the risk map. A flowchart of stepwise methodology is shown in Fig. 2. The first step is to prepare the contamination probability map, which comprises intrinsic vulnerability and land use. In this study, DRASTIC intrinsic vulnerability map was used (Baalousha 2006). The vulnerability map was then coupled with land use to create a probability map. It was important to combine the land use map with the vulnerability map as both affect the probability of contamination. The vulnerability map shows intrinsic weaknesses of the hydrogeological system and the land use map represents potential sources of contamination (point source and non-point source) such as wastewater treatment plants. Aquifers in high vulnerability areas are more likely to be contaminated than other areas. The impact map was based on a combination of two factors. These factors are the contamination health-impact of major contaminants in groundwater and the capture zones of drinking water supply. The classification of the impacts of different contaminants in groundwater is important as different contaminants in groundwater have differing impacts on public health. Nitrate contamination health impact, for example, is worse than chloride contamination (Baalousha 2008). Because the impact of contamination is in the vicinity of the drinking water wells, it
Fig. 2 Groundwater contamination risk mapping
necessary to consider the capture zones of public water supply. For the study area, maps of vulnerability and land use were combined using the Spatial Analyst tool of ArcMap from ESRI® to produce a contamination probability map. The map of well capture zones was combined with the map of contamination health impact to produce a contamination impact map. The final risk map was produced by multiplying the contamination probability and the contamination impact maps. The study area The Gaza Strip area is located at longitudes 31° 25′ North and latitude 34° 20′ East. It extends 40 km along the south-eastern shore of the Mediterranean. The Strip is situated on a wide Palestinian coastal plain. The total area of Gaza Strip is about 365 km2 and more than 1.4 million inhabitants are living in this small area (Palestinian Central Bureau of Statistics (PCBS) 2009). The population density in Gaza Strip is one of the highest in the world, especially in the eight refugee camps. Because of its location, the Gaza Strip forms a transitional zone between the semi-humid coastal area in the north and the semi-arid Sinai desert in the south (EUROCONSULT and IWACO 1994). The area is characterised by a Mediterranean climate with 4 months of hot dry summer and a short winter with rain from November to March. The average summer and winter temperatures in Gaza Strip are 25°C and 7°C, respectively. Hydrogeology and water resources The aquifer system in Gaza Strip is part of the larger Palestinian coastal plain hydrogeological system, which extends from Haifa City in the north to Sinai desert in the south and over an area of about 2,000 km2 (Metcalf and Eddy 2000). The Palestinian coastal plain is characterised by flat relief, and is bounded to the east by the foothills of the West Bank mountain belt. This plain is narrow in the north and gets wider in the south. It has an average width of about 13 km. The main aquifer formation is composed of calcareous sandstone and gravel from the Pleistocene age and recent Holocene sand dunes. Some silts, clay, and conglomerate exist in the aquifer formation. Three main clay layers intercalate the aquifer and divide it into three main sub-aquifers in the west (Fig. 3). These clay layers extend from the shore in the west to about 3-5 km inland. Thus, the aquifer is mainly unconfined in the eastern part and confined/unconfined in the western part. Aquifer thickness varies from a few metres in the east of Gaza Strip to about 170 m near the shoreline. The aquifer overlies thick impermeable marine clay of the Tertiary age called the Saqaya Formation (EUROCONSULT and IWACO 1994).
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Fig. 3 Conceptual hydrogeological west-east cross-section
Fig. 4 Vulnerability map for the Gaza Strip based on DRASTIC approach
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Fig. 5 Land use map for the Gaza Strip and locations of wastewater treatment plants (WWTP)
Groundwater is the only source of water supply in the area as there is almost no surface water. The natural groundwater flow pattern is from east to west. However, the natural flow pattern was disturbed due to intensive pumping at different locations, especially in densely populated areas like Gaza City. Wadi Gaza (ephemeral stream) runs across the Gaza Strip from the Naqab desert and the Hebron Mountains in the east and drains into the Mediterranean. The catchment area of Wadi Gaza is about 3,500 km2 (United Nations 2003). Before 1967, flash floods in the Wadi closed the main motorway in the strip for few days each year (Al-Agha 1995). However, no water flows in this Wadi anymore since Israel has built many dams just behind the border of the Gaza Strip preventing most of the natural water flow from reaching Gaza (Al-Agha 1995; EUROCONSULT and IWACO 1994; United Nations 2003). Currently, the Wadi is used as an effluent discharge channel for the raw sewage from refugee camps adjacent to the watercourse, estimated at 6,0008,000 m3/day (United Nations 2003). The coastal aquifer beneath the Gaza Strip is recharged by rainfall, at an average annual rate of 300 mm (data
obtained from Palestinian Water Authority). Only part of this precipitation percolates into the aquifer and contributes to aquifer recharge and the rest is lost evapotranspiration. DRASTIC vulnerability The intrinsic vulnerability map for the Gaza Strip was created using the DRASTIC approach (Fig. 4), which was done by author (Baalousha 2006), and briefly discussed hereafter. Seven maps were prepared using ArcGIS. Hydrogeological parameters for DRASTIC mapping such as hydraulic conductivity and aquifer properties were based on literature data and data obtained from the Palestinian Water Authority (PWA). Groundwater recharge data was based on a previous study (Baalousha 2005). Topography data was obtained from PWA in digital format, and groundwater level records were obtained from monitoring data of PWA. Figure 4 shows that areas close to the coast have the highest vulnerability. This is because groundwater is shallow in that area and the area is covered by sand dune formations. In addition, this area receives the highest recharge. On the contrary, the area east of Khan Yunis has the lowest
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Table 1 Different land uses and their potential pollution impact level (Baalousha 1998) Land use
Impact
Wastewater treatment plants and unsewered areas Green houses Citrus Grapes Olives Dates and almonds Open fields (no irrigation)
6 (high) 5 4 3 2 1 0 (low)
vulnerability because the vadose zone is thick and the recharge rates are low.
agricultural practices, including the heavy application of pesticides and fertiliser and leakage from three sewage treatment plants. In built-up areas such as Khan Yunis City, the use of cesspits is the only means for domestic wastewater discharge. High levels of nitrate have been detected in groundwater in that area. The detected high nitrate concentrations are directly related to wastewater leakage (Baalousha 2008). Some types of agriculture such as citrus traditionally receive higher fertiliser loadings, and thus, their environmental impact is high. Other types of agriculture like dates and olives may have less potential impact on the environment. These have been classified from high to low in Table 1. Contamination probability map
Land use The land use map of the Gaza Strip is shown in Fig. 5. The main potential source of contamination in the area is from
Fig. 6 Probability of groundwater contamination in the Gaza Strip
The groundwater contamination probability map (Fig. 6) is a combination of the DRASTIC index vulnerability map (Fig. 4) and the land use map (Fig. 5). Both DRASTIC
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index and land use were given equal weights as they equally affect the groundwater contamination and reclassified within the probability space. Well capture zone An existing finite difference groundwater flow model (Baalousha 2003) was used as a basis to delineate capture zones of public drinking water supply wells in the area at different times (1, 2, 5 and 20 years). The advective transport code PMPATH of Processing MODFLOW package was used for this purpose (McDonald and Harbaugh 1988). The aquifer system consists of calcareous sandstone and conglomerate with intermittent clay layers of different thicknesses. The basement of the aquifer is a thick impermeable clay layer called the “Saqya Formation” (Fig. 3). Groundwater flows from east to west, perpendicular to the shoreline. The western boundary of the model (the Mediterranean) was considered a constant head boundary. Fig. 7 Modeled capture zones of public water supply wells in the Gaza Strip
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The recharge boundary was based on rainfall-recharge analysis. Annual rainfall varies between 200 mm in the south to 450 mm in the north. There are about 110 municipal water wells for public water supply in the Gaza Strip. Drinking water well abstraction data was obtained from PWA. The hydraulic properties were obtained from pumping test data, PWA, and from literature (i.e., Metcalf and Eddy 2000). Particle tracking (Pollock 1989) within the Processing Modflow Package was used to delineate the capture zones of public water wells at different time intervals. The backward tracking (Fig. 7) shows capture zones for periods of 1, 5, 10, and 20 years. Potential pollution in areas within capture zones of wells will have a higher impact on public health. All known public water supply wells have been considered in this study. Potential sources of pollutants and their possible impact In the Gaza Strip, nitrate is considered a major groundwater contaminant (Shomar et al. 2008) and is believed to be anthropogenic (Al-Agha 1997; Baalousha 2008).
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Fig. 8 Nitrate, chloride, and fluoride impact map for the Gaza Strip
Infants of 6 months or less are the most vulnerable to nitrate impact in drinking water, as it can cause methaemoglobinaemia (also known as blue baby syndrome). Several cases of methemoglobin have been reported in Gaza Strip in the last few years (Abu Maila et al. 2004). Pathogens and viruses can also arise from wastewater discharge and agricultural activities. While pathogens and viruses may be naturally eliminated through attenuation processes in the unsaturated zone, nitrate is more difficult to mitigate. The second widely spread contaminant in groundwater in the area is chloride, resulting in high salinity. There is no known health impact of high chloride in groundwater. The 250 mg/l maximum concentration limit assigned by World Health Organisation (WHO 2004) is aesthetic based. However, high chloride makes water non-potable. Less than 10% of groundwater in the area meets the WHO standards for chloride. There are two sources of high chloride concentrations in the area. The first is through seawater intrusion, especially in Khan Yunis area (Yakirevich et al. 1998) and the Gaza City area. The other source is up-
coning of brackish water and brines as a result of heavy pumping (e.g., Gaza City, Khan Yunis; Qahman and Larabi 2003). Fluoride in groundwater originates from phosphates derived by natural dissolution of phosphate minerals and long-term weathering of phosphates (Al-Agha 1995). High fluoride concentrations, above the WHO limit (1.5 mg/l) have been detected in the southern areas of Gaza Strip (i.e., Khan Yunis and Rafah). Excess amount of fluoride in groundwater can cause fluorosis, which affects the teeth and bones (WHO 2009). Long-term ingestion of fluoride can lead to potentially severe skeletal problems (WHO 2004). There is a high dental fluorosis index in Gaza Strip (WHO 1999). Shomar et al. (2004) have found a correlation between high fluoride concentration in drinking water and dental fluorosis among school children in the Gaza Strip. Other pollutants such as heavy metals were found to have concentrations below the maximum permissible limits assigned by the WHO. In summary, the major identified contaminants in the area are nitrate, fluoride, and chloride. Areas where these parameters exceed the maximum
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permissible limits for drinking water have been identified based on up to date monitoring data from PWA (WHO 2004). Figure 8 shows the resulting impact from a combination of exceedance of the three parameters. Nitrate and chloride data from 1990 to date was obtained from PWA. Fluoride data was obtained from the Ministry of Health. Nitrate and fluoride were given double weights as chloride in the pollutants impact map, as they pose higher threat to health than chloride.
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respectively). The resultant map of impact is shown in Fig. 9. Risk map The final risk map was obtained as a cross-product of the probability map (Fig. 6) and the impact map (Fig. 9). The global risk map is shown in Fig. 10. Risk varies between 0.0244, which is minimal to 0.786, which is high. The risk range was divided into equal intervals, as shown in Fig. 10.
Impact map The impact map was prepared as a combination of the capture zone of the drinking water wells and the health impact of different contaminant in the area of study. The following sections outline the procedure of each map preparation and the combination of the capture zone map and health impact map to produce the impact map. The final impact map was created by combining the capture zone map and pollutants impact map (Figs. 7 and 8,
Fig. 9 Impact map for the Gaza Strip based on well capture zones and potential contaminations in the groundwater
Discussion Contamination risk mapping of the Gaza Strip shows that approximately 34% of the area (124 km2) is located in the very low-risk zone and just less than half of its area 46.5% or 170 km2 falls within the low-risk zone (Fig. 10 and Table 2). Intermediate and high-risk areas constitute 19.45% of the entire area or 71 km2.
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Fig. 10 Risk map of groundwater contamination for the Gaza Strip
Analysis of results revealed that four municipal wells out of 110 municipal wells are located in a high contaminationrisk zone (risk more than 0.6). The four wells are L/87, L/ 127, L/43, P/124 and all are located in the Khan Yunis and Rafah areas. Sampling results of these wells show that they have nitrate concentration of more than 200 mg/l on average, which is four times higher than WHO drinking water standards. The majority of wells (88 wells) are located in the intermediate-risk zone, with the remainder in the low-risk zone. It is important to take action to mitigate the potential health impact of drinking water from wells in
Table 2 Results of the Gaza Strip groundwater contamination risk mapping
Risk class Very low Low Intermediate High
the high-risk zones. Such mitigation measure can be achieved using water from these wells for non-drinking purposes or treating the water before drinking. Figure 10 shows that there are areas where groundwater contamination risk is low or very low. These areas are the south-eastern and northern areas of Gaza Strip. However, the aquifer in the south-eastern area is non-productive as the vadose zone there is thick providing high contamination attenuation capacity. Some narrow coastal areas at Khan Yunis and Rafah in the south have low risk too, because these areas receive high recharge (covered by sand dunes)
Risk index
Area (km2)
Percentage of total area (%)
0.0-0.1 0.1-0.3 0.3-0.6 0.6-0.8
124.0 170.0 69.7 1.3
33.97 46.57 19.09 0.36
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and pumping is not intensive. Other areas of low risk are at the eastern boundaries of Gaza Strip, where the pumping and vulnerability are both low.
Conclusions This study proposes a new approach for groundwater contamination risk assessment accounting for hydrogeological factors, land use, public wells capture zones, and contamination potential impact. It is easy to implement and can be modified to suit local conditions. The use of GIS facilitates the preparation, classification, and mathematical calculation that are required to produce the intermediary and final maps. Results of the case study presented in this paper show that the majority of the area falls within the low-risk zones, with a small area within the high-risk zone. The majority of public wells are in the intermediate-risk zone with few in the high-risk zone in the southern part of the area. Wells in that area shows very high concentrations of nitrate and chloride (Baalousha 2008). Though the majority of public wells are located in the intermediate-risk zone, the risk will probably increase with time as a result of widening the well capture zone, and the ever increasing in groundwater abstraction. Intensive pumping results in upconning of lower brackish groundwater, and thus, deteriorating the groundwater quality and posing a threat to public health. It is recommended that a groundwater abstraction strategy be adopted in light of the findings of this study. The risk map created in this study can be used in the development of land management policy and practice, locating high pollution potential sources (i.e., treatment plants, landfills, etc.), and implementation of a mitigation strategy. One potential mitigation measure is to reduce groundwater pumping and gradually secure water from alternative source such as desalination, which is already implemented. An alternative measure is to use water from high-risk zones for nondrinking purposes.
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