ORIGINAL PAPER

Mapping groundwater contamination risk using GISand groundwater modelling. A case study from the GazaStrip, Palestine

Husam Musa Baalousha

Received: 26 October 2009 /Accepted: 8 March 2010# Saudi Society for Geosciences 2010

Abstract Increasing pressure on water resources worldwidehas resulted in groundwater contamination, and thus thedeterioration of the groundwater resources and a threat to thepublic health. Risk mapping of groundwater contamination isan important tool for groundwater protection, land usemanagement, and public health. This study presents a newapproach for groundwater contamination risk mapping, basedon hydrogeological setting, land use, contamination load, andgroundwater modelling. The risk map is a product ofprobability of contamination and impact. This approach wasapplied on the Gaza Strip area in Palestine as a case study. Aspatial analyst tool within Geographical Information System(GIS) was used to interpolate and manipulate data to developGIS maps of vulnerability, land use, and contaminationimpact. A groundwater flow model for the area of study wasalso used to track the flow and to delineate the capture zonesof public wells. The results show that areas of highestcontamination risk occur in the southern cities of Khan Yunisand Rafah. The majority of public wells are located in anintermediate risk zone and four wells are in a high risk zone.

Keywords Groundwater contamination . Risk mapping .

GIS .MODFLOW.Capture zones . Vulnerability .

Gaza Strip . Drastic

Introduction

Groundwater is an invaluable source of drinking water inmany areas around the world. Due to extensive pumping,agricultural, and industrial activities, aquifers are at risk ofbeing contaminated. Intensive application of pesticides andfertilisers, discharge of wastewater, and industrial effluent andexcessive groundwater abstraction are just a few examples ofactivities that lead to groundwater contamination. Theseactivities have resulted in the deterioration of water resourcesin various regions around the world (Pandey et al. 1999).Aquifers are valuable sources for water. Therefore, a quickaction should be taken to prevent aquifers from contamina-tion and to reduce the risk of contamination impact.

Groundwater contamination risk mapping can helpplanners and decision-makers on proper land use and waterresources management. This will enable incorporation ofgroundwater protection and health impact assessment in theanalysis. Risk mapping is not only a preventative measurebut it also assist with mitigation processes of groundwatercontamination.

Risk, by definition, is the probability of an eventmultiplied by its impact. In environment context, risk isthe probability that a hazard will turn into a disaster. Ingroundwater context, risk can be defined as the probabilitythat groundwater at a drinking well becomes contaminatedto an unacceptable level by activities on the land surface(Morris and Foster 1998).

Risk can be reduced by implementing a mitigationstrategy with best management practice. Best practiceavoids high-risk areas when locating a site of possiblepollution potential.

H. M. Baalousha (*)Hawke’s Bay Regional Council,159 Dalton Street,Napier 4110, New Zealande-mail: Baalousha@web.de

Arab J GeosciDOI 10.1007/s12517-010-0135-0

Geographical Information System (GIS) has been widelyused 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 aloneor coupled with other factors to assess groundwater con-tamination risk. The most widely used method for intrinsicvulnerability assessment is the DRASTIC approach (Alleret al. 1985). DRASTIC is based on seven hydrogeologicalparameters: Depth to water table, Recharge, Aquifer media,Soil media, Topography, Influence of vadose zone, andhydraulic Conductivity to assess the intrinsic aquifervulnerability. Each map is classified and rated, thenweighted based on standard DRASTIC weigh system (Alleret al. 1985). Vulnerability index is the sum of each ratedmap multiplied by its respective weight as shown in theequation below. The final DRASTIC product is a mapshowing vulnerability index. DRASTIC has been used indifferent studies to assess aquifer vulnerability (e.g.,Lasserrea et al. 1999; Baalousha 2006; Nobre et al. 2007;Assaf and Saadeh 2009).

DRASTIC index ¼ Dr � Dw þ Rr � Rw þ Ar � Aw

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 withhazard map or land use map to produce risk maps (Ducci1999). For example, Al-Adamat et al. (2003) have coupleda DRASTIC vulnerability map with a land use map toproduce a risk map.

Intrinsic vulnerability is a good measure of weaknesses ofan aquifer, as it considers the hydrogeological characteristicsof 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 contaminationimpact, which is an essential factor for risk assessment. Inaddition, the movement of contaminants in the groundwater,which affects the capture zone around wells, is not consideredin this approach. A highly vulnerable area, for instance, is notunder contamination risk unless it is susceptible to acontamination source and the contamination impact is high.

Other approaches of groundwater contamination riskmapping 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 acertain contaminant in groundwater and to create agroundwater contamination probability map. The main

drawback of geostatistical approach is that it does notconsider hydrogeological settings that have a significanteffect on contamination risk.

In this study, a new approach is proposed for contaminationrisk mapping. This approach depends on the idea thatgroundwater contamination risk is a product of probabilityof contamination occurring and contamination impact. Acontamination risk map is a function of probability overlaidwith a map of potential impact of groundwater contamination.The resultant convergence of probability of contamination andcontamination impact is and assessed geospatially onmap as across-product of the probability map and the contaminationimpact map. In the case study presented in this paper, thecontamination probability map was created based on aprevious work by the author using the DRASTIC approach(Baalousha 2006). In this paper, the vulnerability map wascoupled with land use to represent the probability ofcontamination. The impact map was based on two factors:health impact of major contaminants in the area of study andthe public water supply capture zone.

Methodology

The assessment of groundwater contamination risk requirestwo main factors: probability of contamination and thecontamination impact, as depicted in Fig. 1. For example,when a site has a high contamination probability but has alow impact of contamination, then the risk is low. But whenboth 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 contaminationrisk (R) can be written as:

R ¼ p contð Þ \ p impactð Þ R 2 0; 1½ � ð2Þwhere, 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 beclassified in the probability space; that is p ∈ [0,1], and thesame for contamination impact (Fig. 1). The cross-productof both maps is the risk map.

A flowchart of stepwise methodology is shown in Fig. 2.The first step is to prepare the contamination probabilitymap, which comprises intrinsic vulnerability and land use. Inthis study, DRASTIC intrinsic vulnerability map was used(Baalousha 2006). The vulnerability map was then coupledwith land use to create a probability map. It was important tocombine the land use map with the vulnerability map as bothaffect the probability of contamination. The vulnerabilitymap shows intrinsic weaknesses of the hydrogeologicalsystem and the land use map represents potential sourcesof contamination (point source and non-point source) such aswastewater treatment plants. Aquifers in high vulnerabilityareas are more likely to be contaminated than other areas.

The impact map was based on a combination of twofactors. These factors are the contamination health-impactof major contaminants in groundwater and the capturezones of drinking water supply. The classification of theimpacts of different contaminants in groundwater isimportant as different contaminants in groundwater havediffering impacts on public health. Nitrate contaminationhealth impact, for example, is worse than chloride contam-ination (Baalousha 2008). Because the impact of contam-ination is in the vicinity of the drinking water wells, it

necessary to consider the capture zones of public watersupply.

For the study area, maps of vulnerability and land use werecombined using the Spatial Analyst tool of ArcMap fromESRI® to produce a contamination probability map. The mapof well capture zones was combined with the map ofcontamination health impact to produce a contamination impactmap. The final risk map was produced by multiplying thecontamination probability and the contamination impact maps.

The study area

The Gaza Strip area is located at longitudes 31° 25′ North andlatitude 34° 20′ East. It extends 40 km along the south-easternshore of the Mediterranean. The Strip is situated on a widePalestinian coastal plain. The total area of Gaza Strip is about365 km2 and more than 1.4 million inhabitants are living inthis small area (Palestinian Central Bureau of Statistics(PCBS) 2009). The population density in Gaza Strip is one ofthe highest in the world, especially in the eight refugee camps.

Because of its location, the Gaza Strip forms a transitionalzone between the semi-humid coastal area in the north and thesemi-arid Sinai desert in the south (EUROCONSULT andIWACO 1994). The area is characterised by a Mediterraneanclimate with 4months of hot dry summer and a short winter withrain from November to March. The average summer and wintertemperatures in Gaza Strip are 25°C and 7°C, respectively.

Hydrogeology and water resources

The aquifer system in Gaza Strip is part of the largerPalestinian coastal plain hydrogeological system, whichextends from Haifa City in the north to Sinai desert in thesouth and over an area of about 2,000 km2 (Metcalf andEddy 2000). The Palestinian coastal plain is characterisedby flat relief, and is bounded to the east by the foothills ofthe West Bank mountain belt. This plain is narrow in thenorth and gets wider in the south. It has an average width ofabout 13 km. The main aquifer formation is composed ofcalcareous sandstone and gravel from the Pleistocene ageand recent Holocene sand dunes. Some silts, clay, andconglomerate exist in the aquifer formation. Three mainclay layers intercalate the aquifer and divide it into threemain sub-aquifers in the west (Fig. 3). These clay layersextend from the shore in the west to about 3-5 km inland.Thus, the aquifer is mainly unconfined in the eastern part andconfined/unconfined in the western part. Aquifer thicknessvaries from a few metres in the east of Gaza Strip to about170 m near the shoreline. The aquifer overlies thickimpermeable marine clay of the Tertiary age called theSaqaya Formation (EUROCONSULT and IWACO 1994).Fig. 2 Groundwater contamination risk mapping

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Fig. 3 Conceptual hydrogeological west-east cross-section

Fig. 4 Vulnerability map forthe Gaza Strip based onDRASTIC approach

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Groundwater is the only source of water supply in thearea as there is almost no surface water. The naturalgroundwater flow pattern is from east to west. However, thenatural flow pattern was disturbed due to intensive pumpingat different locations, especially in densely populated areaslike Gaza City.

Wadi Gaza (ephemeral stream) runs across the GazaStrip from the Naqab desert and the Hebron Mountains in theeast and drains into the Mediterranean. The catchment area ofWadi Gaza is about 3,500 km2 (United Nations 2003). Before1967, flash floods in the Wadi closed the main motorway inthe strip for few days each year (Al-Agha 1995). However,no water flows in this Wadi anymore since Israel has builtmany dams just behind the border of the Gaza Strippreventing most of the natural water flow from reachingGaza (Al-Agha 1995; EUROCONSULT and IWACO 1994;United Nations 2003). Currently, the Wadi is used as aneffluent discharge channel for the raw sewage from refugeecamps adjacent to the watercourse, estimated at 6,000-8,000 m3/day (United Nations 2003).

The coastal aquifer beneath the Gaza Strip is rechargedby rainfall, at an average annual rate of 300 mm (data

obtained from Palestinian Water Authority). Only part ofthis precipitation percolates into the aquifer and contributesto aquifer recharge and the rest is lost evapotranspiration.

DRASTIC vulnerability

The intrinsic vulnerability map for the Gaza Strip wascreated using the DRASTIC approach (Fig. 4), which wasdone by author (Baalousha 2006), and briefly discussedhereafter. Seven maps were prepared using ArcGIS. Hydro-geological parameters for DRASTIC mapping such ashydraulic conductivity and aquifer properties were based onliterature data and data obtained from the Palestinian WaterAuthority (PWA). Groundwater recharge data was based on aprevious study (Baalousha 2005). Topography data wasobtained from PWA in digital format, and groundwater levelrecords were obtained from monitoring data of PWA.

Figure 4 shows that areas close to the coast have thehighest vulnerability. This is because groundwater is shallowin that area and the area is covered by sand dune formations.In addition, this area receives the highest recharge. On thecontrary, the area east of Khan Yunis has the lowest

Fig. 5 Land use map forthe Gaza Strip and locationsof wastewater treatmentplants (WWTP)

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vulnerability because the vadose zone is thick and therecharge rates are low.

Land use

The land use map of the Gaza Strip is shown in Fig. 5. Themain potential source of contamination in the area is from

agricultural practices, including the heavy application ofpesticides and fertiliser and leakage from three sewagetreatment plants. In built-up areas such as Khan Yunis City,the use of cesspits is the only means for domesticwastewater discharge. High levels of nitrate have beendetected in groundwater in that area. The detected highnitrate concentrations are directly related to wastewaterleakage (Baalousha 2008).

Some types of agriculture such as citrus traditionallyreceive higher fertiliser loadings, and thus, their environ-mental impact is high. Other types of agriculture like datesand olives may have less potential impact on the environ-ment. These have been classified from high to low inTable 1.

Contamination probability map

The groundwater contamination probability map (Fig. 6) isa combination of the DRASTIC index vulnerability map(Fig. 4) and the land use map (Fig. 5). Both DRASTIC

Fig. 6 Probability of ground-water contamination in theGaza Strip

Table 1 Different land uses and their potential pollution impact level(Baalousha 1998)

Land use Impact

Wastewater treatment plants and unsewered areas 6 (high)

Green houses 5

Citrus 4

Grapes 3

Olives 2

Dates and almonds 1

Open fields (no irrigation) 0 (low)

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index and land use were given equal weights as theyequally affect the groundwater contamination and reclassi-fied within the probability space.

Well capture zone

An existing finite difference groundwater flow model(Baalousha 2003) was used as a basis to delineate capturezones of public drinking water supply wells in the area atdifferent times (1, 2, 5 and 20 years). The advectivetransport code PMPATH of Processing MODFLOW pack-age was used for this purpose (McDonald and Harbaugh1988).

The aquifer system consists of calcareous sandstone andconglomerate with intermittent clay layers of differentthicknesses. The basement of the aquifer is a thickimpermeable clay layer called the “Saqya Formation”(Fig. 3). Groundwater flows from east to west, perpendic-ular to the shoreline. The western boundary of the model(the Mediterranean) was considered a constant headboundary.

The recharge boundary was based on rainfall-rechargeanalysis. Annual rainfall varies between 200 mm in thesouth to 450 mm in the north.

There are about 110 municipal water wells for publicwater supply in the Gaza Strip. Drinking water wellabstraction data was obtained from PWA. The hydraulicproperties were obtained from pumping test data, PWA, andfrom literature (i.e., Metcalf and Eddy 2000).

Particle tracking (Pollock 1989) within the ProcessingModflow Package was used to delineate the capture zones ofpublic water wells at different time intervals. The backwardtracking (Fig. 7) shows capture zones for periods of 1, 5, 10,and 20 years. Potential pollution in areas within capture zonesof wells will have a higher impact on public health. All knownpublic 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 groundwatercontaminant (Shomar et al. 2008) and is believed to beanthropogenic (Al-Agha 1997; Baalousha 2008).

Fig. 7 Modeled capture zonesof public water supply wells inthe Gaza Strip

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Infants of 6 months or less are the most vulnerable tonitrate impact in drinking water, as it can cause methae-moglobinaemia (also known as blue baby syndrome).Several cases of methemoglobin have been reported inGaza Strip in the last few years (Abu Maila et al. 2004).

Pathogens and viruses can also arise from wastewaterdischarge and agricultural activities. While pathogens andviruses may be naturally eliminated through attenuationprocesses in the unsaturated zone, nitrate is more difficult tomitigate.

The second widely spread contaminant in groundwaterin the area is chloride, resulting in high salinity. There is noknown health impact of high chloride in groundwater. The250 mg/l maximum concentration limit assigned by WorldHealth Organisation (WHO 2004) is aesthetic based.However, high chloride makes water non-potable. Lessthan 10% of groundwater in the area meets the WHOstandards for chloride. There are two sources of highchloride concentrations in the area. The first is throughseawater intrusion, especially in Khan Yunis area (Yakirevichet al. 1998) and the Gaza City area. The other source is up-

coning of brackish water and brines as a result of heavypumping (e.g., Gaza City, Khan Yunis; Qahman and Larabi2003).

Fluoride in groundwater originates from phosphatesderived by natural dissolution of phosphate minerals andlong-term weathering of phosphates (Al-Agha 1995). Highfluoride concentrations, above the WHO limit (1.5 mg/l) havebeen detected in the southern areas of Gaza Strip (i.e., KhanYunis and Rafah). Excess amount of fluoride in groundwatercan cause fluorosis, which affects the teeth and bones (WHO2009). Long-term ingestion of fluoride can lead to potentiallysevere skeletal problems (WHO 2004). There is a high dentalfluorosis index in Gaza Strip (WHO 1999). Shomar et al.(2004) have found a correlation between high fluorideconcentration in drinking water and dental fluorosis amongschool children in the Gaza Strip.

Other pollutants such as heavy metals were found tohave concentrations below the maximum permissible limitsassigned by the WHO. In summary, the major identifiedcontaminants in the area are nitrate, fluoride, and chloride.Areas where these parameters exceed the maximum

Fig. 8 Nitrate, chloride, andfluoride impact map for theGaza Strip

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permissible limits for drinking water have been identifiedbased on up to date monitoring data from PWA (WHO2004). Figure 8 shows the resulting impact from acombination of exceedance of the three parameters. Nitrateand chloride data from 1990 to date was obtained fromPWA. Fluoride data was obtained from the Ministry ofHealth. Nitrate and fluoride were given double weights aschloride in the pollutants impact map, as they pose higherthreat to health than chloride.

Impact map

The impact map was prepared as a combination of thecapture zone of the drinking water wells and the healthimpact of different contaminant in the area of study. Thefollowing sections outline the procedure of each mappreparation and the combination of the capture zone mapand health impact map to produce the impact map.

The final impact map was created by combining thecapture zone map and pollutants impact map (Figs. 7 and 8,

respectively). The resultant map of impact is shown inFig. 9.

Risk map

The final risk map was obtained as a cross-product of theprobability map (Fig. 6) and the impact map (Fig. 9). Theglobal risk map is shown in Fig. 10. Risk varies between0.0244, which is minimal to 0.786, which is high. The riskrange was divided into equal intervals, as shown in Fig. 10.

Discussion

Contamination risk mapping of the Gaza Strip shows thatapproximately 34% of the area (124 km2) is located in thevery low-risk zone and just less than half of its area 46.5%or 170 km2 falls within the low-risk zone (Fig. 10 andTable 2). Intermediate and high-risk areas constitute19.45% of the entire area or 71 km2.

Fig. 9 Impact map for the GazaStrip based on well capturezones and potential contamina-tions in the groundwater

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Analysis of results revealed that four municipal wells outof 110 municipal wells are located in a high contamination-risk 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 andRafah areas. Sampling results of these wells show that theyhave nitrate concentration of more than 200 mg/l onaverage, which is four times higher than WHO drinkingwater standards. The majority of wells (88 wells) arelocated in the intermediate-risk zone, with the remainder inthe low-risk zone. It is important to take action to mitigatethe potential health impact of drinking water from wells in

the high-risk zones. Such mitigation measure can beachieved using water from these wells for non-drinkingpurposes or treating the water before drinking.

Figure 10 shows that there are areas where groundwatercontamination risk is low or very low. These areas are thesouth-eastern and northern areas of Gaza Strip. However,the aquifer in the south-eastern area is non-productive asthe vadose zone there is thick providing high contaminationattenuation capacity. Some narrow coastal areas at KhanYunis and Rafah in the south have low risk too, becausethese areas receive high recharge (covered by sand dunes)

Fig. 10 Risk map of ground-water contamination for theGaza Strip

Risk class Risk index Area (km2) Percentage of total area (%)

Very low 0.0-0.1 124.0 33.97

Low 0.1-0.3 170.0 46.57

Intermediate 0.3-0.6 69.7 19.09

High 0.6-0.8 1.3 0.36

Table 2 Results of the GazaStrip groundwater contamina-tion risk mapping

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and pumping is not intensive. Other areas of low risk are atthe eastern boundaries of Gaza Strip, where the pumpingand vulnerability are both low.

Conclusions

This study proposes a new approach for groundwatercontamination risk assessment accounting for hydrogeolog-ical factors, land use, public wells capture zones, andcontamination potential impact. It is easy to implement andcan be modified to suit local conditions. The use of GISfacilitates the preparation, classification, and mathematicalcalculation that are required to produce the intermediaryand final maps.

Results of the case study presented in this paper showthat the majority of the area falls within the low-risk zones,with a small area within the high-risk zone. The majority ofpublic wells are in the intermediate-risk zone with few inthe high-risk zone in the southern part of the area. Wells inthat area shows very high concentrations of nitrate andchloride (Baalousha 2008).

Though the majority of public wells are located in theintermediate-risk zone, the risk will probably increase withtime as a result of widening the well capture zone, and theever increasing in groundwater abstraction. Intensivepumping results in upconning of lower brackish ground-water, and thus, deteriorating the groundwater quality andposing a threat to public health.

It is recommended that a groundwater abstraction strategybe adopted in light of the findings of this study. The risk mapcreated in this study can be used in the development of landmanagement policy and practice, locating high pollutionpotential sources (i.e., treatment plants, landfills, etc.), andimplementation of a mitigation strategy. One potentialmitigation measure is to reduce groundwater pumping andgradually secure water from alternative source such asdesalination, which is already implemented. An alternativemeasure is to use water from high-risk zones for non-drinking purposes.

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