Assessment of Groundwater Quality in the Gaza Strip, Palestine Using GIS Mapping ()
1. Introduction
A Geographic Information System (GIS) is an important tool for integrating spatial data with other information. It allows one to analyze the integrated data and to represent the information spatially facilitating planning of resource development, environmental protection and scientific research [1–5]. This capability makes GIS a powerful tool for groundwater assessments. GIS not only provides tools for interpolating measured values of water quality parameters from specific locations, but also enables one to link water quality with land use, soil characteristics, and other relevant information. In addition, GIS provides sophisticated map-generation capabilities, useful in communicating results of data analysis [6–8]. Although GIS has been used to describe associations between water quality and both natural and anthropogenic activities in the world, in Palestine it has been applied in a limited fashion, but it has not been well implemented for planning, resource management and environmental protection at a national or regional level.
The Gaza Strip region is a fragile ecosystem suffering from increasing environmental assaults due to escalating population growth and limited availability of natural resources to support development. Groundwater is, perhaps, the most precious natural resource in the Gaza Strip as it is the only natural source of fresh water. Therefore, groundwater contamination can pose serious health and economic threats to the population that relies on this water for drinking, agriculture, and industry uses. The aquifer of Gaza is extremely susceptible to surface-derived contamination because of the high permeability of sands and gravels that compose the soil profile of Gaza [9,10]. It has already deteriorated in terms of quantity and quality as a result of over-exploitation and direct and indirect contamination [11–15].
The main objective of the current study is to use GIS to compare water quality data and related information collected during an eight-year monitoring program for groundwater quality in the Gaza Strip. A secondary objective is to portray the contaminant distribution in the groundwater of the Gaza Strip in easily viewed maps for use by the public and decision makers.
2. Study Area, Materials and Methods
2.1. Study Area
The Gaza Strip is one of the most densely populated areas in the world (4138 people per km2; [16]). For administrative purposes, the area has been divided into five regions: North, Gaza, Middle, Khan Younis and Rafah
(Figure 1). Approximately 85% of the population of the Gaza Strip drink from municipal groundwater wells and 15%, mostly in agricultural areas, use private wells to supply their drinking water [15].
The study area is part of the coastal zone in a transitional area between a temperate Mediterranean climate to the east and north and an arid climate of the Negev and Sinai deserts to the east and south. As a result, the Gaza Strip has a characteristic semi-arid climate. The hydrogeological features of the Gaza aquifer are well known. The coastal aquifer consists primarily of Pleistocene age
Kurkar Group deposits, including calcareous and silty sandstones, silts, clays, unconsolidated sands, and conglomerates. Near the coast, coastal clays extend about 2-5 km in land, and divide the aquifer sequence into three or four sub-aquifers, depending upon the location. Towards the east, the clays pinch out and the aquifer is largely unconfined [17]. Within the Gaza Strip, the total thickness of the Kurkar Group is about 100 m at the shore in the south, and about 200 m near Gaza city. At the eastern Gaza border, the saturated thickness is about 60-70 m in the north, and only a few meters in the south near Rafah. Local perched water conditions exist throughout the Gaza Strip due to the presence of shallow clays [18].
2.2. GIS Mapping
The GIS application was conducted according to Asadi et al. [6] and Carrera-Hernández and Gaskin [19] with modifications and changes to suit the study area of the Gaza Strip.
2.2.1. Thematic Maps
The base map data used for the study included digitized data sets originally developed by the Ministry of Planning, the Water Authority, and the Environmental Quality Authority in Palestine. The data sets were originally digitized from 1:20,000 scale topographic maps. The original digitized maps are registered to the National Grids-Palestinian Grid 1923. The study area matches these maps which cover the entire Gaza Strip area. Thematic maps including the base map (Figure 1) and network maps (Figures 3-20) were prepared from 1:20,000 scale topographic paper maps using AutoCAD and Arc/Info GIS software. All the maps were scanned and digitized to generate a digital output.
2.2.2. Water Quality Maps
The data used for the mapping water quality and other supporting maps were developed from the results of previously published studies [10,15,20–31]. Data for these studies were based on periodic fieldwork conducted by the Palestinian Water Authority for groundwater samples collected from predetermined locations of existing water wells in collaboration with Dr. Shomar who assisted with study design and was responsible for chemical analysis.
The sampling locations were integrated with the water data for the generation of spatial distribution maps of selected water quality parameters including electrical conductivity (EC), total dissolved solids (TDS), Cl-, F-, NO3-, SO42-, total hardness, Ca2+, Mg2+, Na+, K+, total Fe, total Cr, and total Zn. The water data were linked to the sampling locations using the basic geodatabase creation function of ArcGIS 9.2 software.
The present study used the Inverse Distance Weighted (IDW) method for spatial interpolation of water pollutants or other parameters. This method uses a defined or selected set of sample points for estimating the output grid cell value. It determines the cell values using a linearly weighted combination of a set of sample points and controls the significance of known points upon the interpolated values based upon their distance from the output point thereby generating a surface grid as well as thematic isolines. Important water quality parameters and their distribution patterns were studied in Gaza metropolis also with the help of cartographic techniques [19]. Thus, GIS enables us to present the cause and affect relationship visually.
The depth to water table (Figure 6) is based on the monitoring results of 500 groundwater wells. Data were obtained from both field surveys and databases of the Palestinian Water Authority.
3. Results
Owing to the large data set obtained from the analysis of 170 water samples for eight years, each having 27 parameters, this section focused on elements exceeding the World Health Organization (WHO) standards. Additionally, total Cr, Zn and Fe were presented to establish baseline values. Figures 7-20 showed the distribution of each parameter and the WHO standard. Maps showing land use, soil types, environmental hot-spots and socio-economic were included because these factors have direct and indirect impacts on groundwater quality.
3.1. Land Use
The area of the Gaza Strip is 363 km2 of which about
Table 1. Land use distribution in the Gaza Strip.
Figure 2. Major land use sectors in the Gaza Strip.
25% is urbanized. Table 1 and Figure 2 show the distribution of land use in the Gaza Strip based on estimated figures for 2008 and the available literature. About 40% of the land is being used for agriculture, most of which is in the eastern half of Gaza where population densities are low (Figure 3). The land use data were obtained from the analysis of aerial photographs taken in 2008. The industrial sector was discussed in detail in Shomar [31]. With an average population density of 4,091 people/km2, Gaza is one of the most densely populated areas in the world. About 80% live in the built-up areas shown on the map.
3.2. Environmental Hot Spots
In addition to the major pollution point sources of overloaded wastewater treatment plants, unprotected solid waste dumping sites and Wadi Gaza (Figure 4), hot spots appear sporadically in many locations due to Gaza’s inability to maintain adequate infrastructure. For example, frequent electricity outage or blackout causes paralysis of wastewater pumping stations and results in untreated wastewater infiltrating homes and streets. The lack of gasoline and diesel causes solid waste to accumulate in the streets without transportation to the dumping sites [31].
3.3. Soil Types
The Gaza Strip has several major soil types (Figure 5) including Arenosolic, Calcaric, Rhegosolic, and Calcaric Fluvisolic soils. Arenosolic (sandy) soils of dune accumulations are Regosols without a marked profile. The soils are moderately calcareous (5-8% CaCO3), with low organic matter, and are physically suitable for intensive horticulture. Calcaric Arenosols (loessy sandy soils) can be found some 5 km inland in the central and southern part of the Strip, in a zone along Khan Younis toward Rafah, parallel to the coast. This belt forms a transitional zone between the Arenosolic soils and the Calcaric (loess) soils. Typical Calcaric soils are found in the area between the city of Gaza and the Wadi Gaza and contain 8-12% CaCO3. Arenosolic Calcaric (sandy loess) soils are transitional soils, characterized by a lighter texture.
These soils can be found in the depression between the Calcareous (Kurkar) ridges of Deir El Balah. Apparently, windblown sands have been mixed with Calcareous deposits. Deposition of these two types of windblown materials originating from different sources has occurred over time and more or less simultaneously. These soils have a rather uniform texture. Another transitional form is the Arenosols over Calcaric soils. These are loess or loessial soils (sandy clay loam) that have been covered by a layer (0.20-0.50 m) of dune sand. These soils can be found east of Rafah and Khan Younis. Fluvisols (alluvial) and Vertisols (grumosolic), which are dominated by loamy clay textures, are found on the slopes of the northern depressions between Beit Hanoun and Wadi Gaza. Drilling east of El Montar ridge revealed that alluvial deposits of about 25 m in thickness occur. At some depth, calcareous concentrations are present. The CaCO3 content can be approximately 15-20%. Some of the soils have been strongly eroded, and the reddish-brown subsoils may be exposed on the tops of ridges and along slopes. The alluvial sediments are underlain by a calcareous layer.
3.4. Water Table
The depth to water table (Figure 6) varies between few meters in the west (very closed to the sea) to about 120m at some locations in the east.
Figure 6. Groundwater depth in the Gaza Strip 2008.
Figure 7. Electric conductivity in the groundwater of the Gaza Strip 2008.
Figure 8. Total dissolved solids in the groundwater of the Gaza Strip 2008.
3.5. Electric Conductivity
With relatively small, localized exceptions, the EC of municipal wells increases from north to south (Figure 7). The lowest EC value was 1,198 µS/cm and the highest was about 3,800 µS/cm. The most deteriorated and salty water was in the eastern regions of Khan Younis and Rafah with an average EC in the private groundwater wells of 5,000 µS/cm.
3.6. Total Dissolved Solids
Figure 8 illustrates that groundwater in most of the Gaza Strip exceeds the WHO TDS standard, which is 1000 mg/L. The TDS and EC maps show similar patterns as both parameters indicate the concentration of dissolved solids in water. The high TDS value in the eastern parts of Khan Younis (3000-4000 mg/L) makes water in the area undrinkable. More than 50% of the sampled groundwater showed TDS of more than 2000 mg/L.