Hydrochemical facies of groundwater in the Gaza Strip, Palestine

Hydrological Sciences Journal des Sciences Hydrologiques, 49(3) June 2004 359 Hydrochemical facies of groundwater in the Gaza Strip, Palestine MOHAMMAD R. AL-AGHA & HAMED A. EL-NAKHAL Department of Environment and Earth Science, The Islamic University of Gaza, PO Box 108, Gaza Strip, Palestine malagha@mail.iugaza.edu; hnakhal@mail.iugaza.edu Abstract Groundwater in the Gaza Strip is the only source of water for domestic, agricultural and industrial uses. Extensive pumping has caused serious quantitative and qualitative problems in the aquifer. The hydrochemical facies are evaluated using the trilinear diagram for 200 water samples. Groundwater in the north and west is mostly characterized by Ca-Mg-HCO 3 facies (alkaline water), and in the south and east by Na-Cl-SO 4 facies (saline water). Sand dunes and rainfall are the major factors controlling the distribution of hydrochemical facies. The eastern edge of the sand dune belt is considered the barrier that separates the two major facies. Brackish water flowing from the east is mixed with rainwater which infiltrates through the sand dunes to the aquifer. Other factors, e.g. seawater intrusion and extensive pumping, play a minor role in the distribution of the hydrochemical facies. Key words hydrochemical facies; groundwater; coastal aquifer; Gaza Strip; Palestine Faciès hydrochimiques de l eau souterraine dans la Bande de Gaza, Palestine Résumé L eau souterraine est la seule ressource en eau de la Bande de Gaza pour les usages domestiques, agricoles et industriels. Le pompage intensif a causé de sérieux problèmes quantitatifs et qualitatifs au niveau de l aquifère. Les faciès hydrochimiques sont évalués grâce au diagramme trilinéaire, pour 200 échantillons d eau. L eau souterraine est essentiellement caractérisée par des faciès Ca-Mg-HCO 3 (eau alcaline) dans le nord et l ouest, et par des faciès Na-Cl-SO 4 (eau saline) dans le sud et l est. Les dunes de sable et la pluie sont les facteurs principaux de contrôle de la distribution des faciès hydrochimiques. La limite est de la ceinture de dunes de sable apparaît être la barrière qui sépare les deux faciès principaux. L eau saumâtre s écoulant de l est est mélangée à l eau de pluie qui s infiltre à travers les dunes de sable jusqu à l aquifère. Les autres facteurs, comme par exemple l intrusion d eau marine et le pompage intensif, jouent un rôle mineur dans la distribution des faciès hydrochimiques. Mots clefs faciès hydrochimiques; eau souterraine; aquifère côtier; Bande de Gaza; Palestine INTRODUCTION The Gaza Strip is a narrow coastal region extending southeast along the Mediterranean Sea (Fig. 1). It is 45 km in length, and ranges from 5 to 12 km in width, with a surface area of 360 km 2. Groundwater is the only source for regular water supply. Running water is limited and rain is irregular, falling mainly during winter (October March) The area is subject to a semiarid Mediterranean climate. Annual precipitation ranges from 200 mm in the south to 400 mm in the north. The Gaza Strip is heavily populated: there are about 1 200 000 Palestinians. The population density is among the highest in the world. About 3500 Israeli settlers live in about 40% of the total area of the Gaza Strip. These settlers live in 18 settlements in the strip, most of which are Open for discussion until 1 December 2004

360 Mohammad R. Al-Agha & Hamed A. El-Nakhal (a) (b) Egypt Bar = 10 km Fig. 1 (a) Location map of the Gaza Strip; and (b) NW SE hydrogeological crosssection. along the Mediterranean Sea coast, and they have their own groundwater wells installed within the settlements. Geologically, the surface of the Gaza Strip is simple and exposures are limited. It is formed of a series of alternating elongated ridges and depressions which run parallel to the Mediterranean coast. Ridges consist of clayey sandstone and silt which are referred to as Continental Kurkar or Jarwal (El-Nakhal, 1968). Most of the surface is covered by Quaternary soil and wadi alluvium. The coastal 1 4 km wide belt along the Mediterranean Sea is covered with calcareous sand dunes (Fig. 2), which are important in increasing the natural recharge. More than 2500 water wells have been dug in the strip. Wells lying within the sand dunes area (in the west) tap unconfined water from a calcareous silty sandstone bed which constitutes the Continental Kurkar. Most of the remaining wells that lie outside the sand dunes, and usually have depth exceeding 60 m, tap confined to semiconfined water from the Marine Kurkar which underlies the

Hydrochemical facies of groundwater in the Gaza Strip, Palestine 361 Fig. 2 Plots of representative wells on Stiff diagrams showing the hydrochemical evolution of the groundwater along its regional flow path from southeast to northwest; the boundary of the sand dunes belt (after El-Nakhal, 1968). Continental Kurkar (El-Nakhal, 1968). The Marine Kurkar is highly porous and permeable due to the abundance of large voids. It consists of Pleistocene marine, medium to coarse quartz sandstone and calcareous shell fragments cemented by calcite

362 Mohammad R. Al-Agha & Hamed A. El-Nakhal (El-Nakhal, 1968; Al-Agha, 1995, 1999). Continental and Marine Kurkar are the main water-bearing beds in the area. They constitute the southern part of the Palestinian coastal aquifer which extends from the foothills of Mt Carmel in the mid-north of the Palestinian coast, to the Gaza Strip and northern Sinai in the south, running parallel to the Mediterranean seashore for about 120 km, and between 5 and 15 km inland (El- Nakhal, 1968). This aquifer is intercalated with several silty-clayey lenses. Within about 4 km from the sea, these lenses separate the aquifer into sub-aquifers (Fig. 1(b)). Eastwards, the lenses wedge out and disappear gradually with the formation of a single aquifer. The total thickness of the coastal aquifer ranges from 80 m in the east to 180 m in the west. The Marine Kurkar is underlain by the Saqiya Formation which is an aquiclude consisting of about 100 m of black shale of Pliocene age. Recharge of this aquifer comes from rainfall, and from inflow from the east (El-Nakhal, 1968; Yakirevich et al., 1998; Jayyousi, 2002). Some other sources recharging the groundwater reservoirs in the Gaza Strip produce a cocktail of contaminated water. These sources are cesspools ( black and grey water) providing a contaminated source; reflux of irrigation, industrial and domestic water also providing a contaminated source; and seawater intrusion providing a saline source (Al-Agha, 1995). The aquifer potentiometric surface indicates that the general groundwater flow is from the southeast to the northwest, i.e. seawards (El-Nakhal, 1968), (Figs 1 3). In some parts, where a high pumping rate is encountered, the flow direction is from northwest to Fig. 3 Potentiometric surface in metres, and flow direction of the groundwater in the Gaza Strip (after Palestinian Water Authority, 1995)

Hydrochemical facies of groundwater in the Gaza Strip, Palestine 363 southeast, which encourages the occurrence of seawater intrusion. The groundwater flow is believed to have the same direction in the aquifer system because the subaquifers are interconnected due to the presence of intermittent clay lenses. During the last few decades, the Gaza Strip has experienced intensive population, agricultural and industrial growth, which have increased the demand for groundwater supplies. This has led to a severe decline in both the quality and quantity of groundwater. As a consequence, 40% of the studied samples contain unacceptable concentrations of nitrates and 16% have unacceptable limits of chloride according to the standards of the World Health Organization (1996). Water (mainly groundwater) use for different purposes in 1997 was estimated as 132 10 6 m 3, while the total groundwater replenishment for the Gaza Strip aquifer (from different sources) in that year was 112 10 6 m 3 (Jayyousi, 2002). These values indicate that the water use is not sustainable as the withdrawal exceeds the renewable resources. There is an annual water deficit of at least 20 10 6 m 3. So, the water situation in the Gaza Strip is critical due to the very high pumping rate from groundwater reservoirs. Additionally, the strip is located in an arid to semiarid area, where the amount of rainwater is relatively low and cannot compensate for the water use/demand associated with the population increase. The aim of the present paper is to investigate the hydrochemical facies of groundwater and their relationships with the local hydro-environmental factors such as rainfall, sand dunes, seawater and the (regional) flow. The authors undertook this study to classify the chemical characteristics of water to be used as guidelines for future policy shaping and/or management scenarios. METHODOLOGY Water samples were collected from 200 wells in the Gaza Strip during April May 1995. These wells are used either for domestic and/or agricultural purposes. The wells were chosen to represent all the Gaza Strip, and on the basis of their geographical location (Fig. 4), they were grouped into four zones (Tables 1 and 2): (a) northern wells (90), (b) southern wells (58), (c) eastern wells (27), and (d) western wells (25). The depths of the representative wells plotted on Stiff diagram (Fig. 2) are shown in Table 3. The aim of such classification is to examine the relationship of the hydrochemical facies with the local hydro-environmental factors in each zone. Wells in each of these zones were defined in a single group. All water samples were analysed for the major ions: Ca 2+, Mg 2+, Na +, K +, SO 4 2-, HCO 3 -, Cl -, and NO 3 -. The main geochemical characteristics of the zones are summarized in Table 2. Analyses were performed in the laboratories of the Ministry of Agriculture of the Palestinian Authority. RESULTS AND INTERPRETATIONS Hydrochemistry and hydrochemical facies The chemical constituents of groundwater tend to increase with the length of its flow paths. Due to ion exchange and precipitation of least soluble salts first, the chemistry of water at an outcrop is modified as it flows through the ground. Groundwater evolves

364 Mohammad R. Al-Agha & Hamed A. El-Nakhal Fig. 4 A simplified map showing the distribution of the geographical zones of wells, and the hydrochemical facies with the sand dunes as a separation boundary. chemically towards the composition of seawater (Chebotarev, 1955; Freeze & Cherry, 1979). The main geological and hydrogeological factors which generally affect the groundwater geochemistry include rainfall, recharge, climate, soil, air, aquifer lithology, saline water and flow pattern. However, the present study is limited to investigating the effects of rainfall, seawater and sand dunes. The diagnostic chemical properties of water are presented by various methods, the most common of which are the hydrochemical facies, e.g. the Piper (1944) trilinear diagram. This diagram is useful in screening and sorting large numbers of chemical data, which makes interpretation easier. Furthermore, a Piper diagram can define the patterns of spatial change in the water chemistry among geological units, along a line of section or along a path line (Raji & Alagbe, 1997; Domenico & Schwartz, 1998). In the present study, the results of the chemical analyses of the total wells, as well as those of each of the defined geographical zones, are plotted on Piper diagrams (Figs 5 7). Additionally, representative water samples are plotted on Stiff diagrams (Fig. 2). Stiff diagrams show an interesting result that the four sections (except the

Hydrochemical facies of groundwater in the Gaza Strip, Palestine 365 Table 1 Geographical distribution and percentage of the identified hydrochemical facies in the Gaza Strip. Facies North: South: East: West: No. of wells % No. of wells % No. of wells % No. of wells Ca+Mg-SO 4 +Cl 2 2.2 0 0 0 0 0 0 Na+K-HCO 3 2 2.2 0 0 2 7.4 0 0 Na+K-Cl+SO 4 5 4.4 35 60.3 16 59.2 7 28 Ca+Mg-HCO 3 65 72.5 7 12 4 14.8 10 40 No pair up 5 5.5 8 13.8 0 0 7 28 No pair down 12 13.2 8 13.8 5 18.5 1 4 Total 90 100 58 100 27 100 25 100 % Table 2 Summary of the chemical characteristics of representative wells in Gaza Strip. Maximum (mg l -1 ) East (27 wells): Mg 91 Ca 314 Na 930 K 10 SO 4 710 NO 3 500 HCO 3 469 Cl 1778 West (25 wells): Mg 160 Ca 218 Na 700 K 37 SO 4 625 NO 3 450 HCO 3 406 Cl 1470 North (90 wells): Mg 328 Ca 178 Na 600 K 33 SO 4 1676 NO 3 550 HCO 3 510 Cl 889 South (58 wells): Mg 160 Ca 314 Na 960 K 37 SO 4 710 NO 3 550 HCO 3 448 Cl 1778 * SD: standard deviation. Minimum (mg l -1 ) 16 25 80 1 15 45 122 175 22 38 19 0 10 10 83 42 12 11 4 1 6 25 30 35 17 23 30 1 7 10 83 84 Mean (mg l -1 ) 55.22 83.78 508.70 4.59 261.96 164.92 279.22 749.30 71.83 116.67 224.21 6.54 184.79 105.42 202.92 438.46 50.85 77.12 123.45 2.90 74.93 133.07 242.47 218.51 50.48 82.64 413.26 5.95 227.47 173.26 213.22 590.72 SD* 19.90 57.68 281.68 2.65 215.92 128.26 94.63 469.64 41.75 52.14 202.85 9.65 172.91 94.07 63.37 384.05 35.79 32.97 132.58 4.20 178.17 98.49 89.49 192.56 26.68 53.11 273.44 5.47 192.81 146.63 70.74 417.75

366 Mohammad R. Al-Agha & Hamed A. El-Nakhal Table 3 Depths of representative wells in Gaza Strip. Well ID Depth (m) Well ID Depth (m) C22 (East) 102 M7 (East) 94 C125 64 L41 105 C128 115 L43 89 A63 32 L121 90 A83 (West) 28 L109 (West) 30 DB11 (East) 160 P94 (East) 70 S19 38 P42 76 G14 25 P121 55 G22 (West) 43 P123 (West) 45 Fig. 5 Piper diagram for the investigated wells of the Gaza Strip. second from the north) have low salinity in the west and high salinity in the east. Figure 5 demonstrates that the water samples occupy a wide area of the Piper diagram ranging from fresh to saline waters. An examination of Figs 5 7 demonstrates that two major hydrogeochemical facies are dominant in the area, as presented in Table 1; these are: (Ca+Mg-CO 3 +HCO 3 ) facies (Alkaline water Type I) and (Na+K-Cl+SO 4 ) facies (Saline water Type II).

Hydrochemical facies of groundwater in the Gaza Strip, Palestine 367 Type I (Ca+Mg-CO 3 +HCO 3 ) facies This facies represents 72.5% of the northern wells and 40% of the western wells (Fig. 4, Table 1). This may indicate that the hydrochemical properties as well as the factors affecting the hydrochemistry of these two locations are similar. In the Gaza Strip, the northern and western areas are classified as having the best water quality for domestic and agricultural uses. The important climatic factor is that the amount of the annual rainfall in the north is about 400 mm. This will contribute to improving the groundwater quality as a result of natural recharge. Another important factor is the existence of sand dunes in the western parts of the northern area (Fig. 2), which enhance the infiltration of rainwater to the aquifer during the natural recharge process. Stiff diagrams (Fig. 2) show that there is a gradual decrease in salinity in the representative wells from east to west in the sections except in G22-DB11 section. This is interpreted as being due to the narrowness of the sand dune belt in G22-DB11 section, where the local recharge is lower than that in other sections. The depths to the tapped aquifer are almost the same from north to south, ranging between 40 and 80 m from west to east respectively, as shown in the cross-section (Fig. 1, Table 3). The interconnection of the sub-aquifers explains the similarity of the western groundwater facies on the one hand, and the similarity of the eastern facies on the other. Sand dunes catch a large amount of water, either by direct precipitation, or from the small wadis which are dominant in the area. The downstream area of these small wadis is normally located in the sand dunes and the water never reaches the sea. As a consequence of the amount of water carried by these wadis and the large infiltration capacity of the sand dunes, the local natural recharge in the west is better than in the east and south. It is noticed that the depth of the wells and water salinity increase to the east (Fig. 2, Table 3). Type II (Na+K-Cl+SO 4 ) facies This facies represents 60.3% of the southern, 59.2% of the eastern, and 28% of the western wells (Fig. 4 and Table 1). Its source could be from saline water intrusion from the east, and/or seawater intrusion from the west. The occurrence of the saline water facies in the eastern parts (Fig. 2 Stiff diagram, Figs 6 and 7) is attributed to the effects of brackish groundwater flowing from the east. Salinization of this brackish water is due to hydraulic contact of the coastal aquifer at its eastern margin with underlying Eocene chalk and marl of Hashepela Group (Rosenthal et al., 1992; Vengosh & Rosenthal, 1994). In the eastern parts of Gaza Strip, groundwater has relatively high TDS (total dissolved solids) concentrations (Fig. 2 and Table 2). Along its pathway westwards, the groundwater receives gradually increasing amounts of rainfall. This leads to a general decrease in the concentration of chemical constituents, and groundwater gradually evolves to Ca- Mg bicarbonate facies. In the sand dune area, due to the effects of permeable sands coupled with increase in rainfall, and abrupt increase in infiltration, rainwater mixes with, dilutes and refreshes the brackish groundwater (coming from the east) giving rise to a new mixed water type (Ca-Mg bicarbonate), which characterizes the western regions (Figs 6 and 7). Consequently, the eastern margin of the sand dunes represents

368 Mohammad R. Al-Agha & Hamed A. El-Nakhal (a) North South (b) Fig. 6 Piper diagrams: (a) for the northern and the southern wells, and (b) for the easternmost and the westernmost wells. the boundary between the two major hydrochemical facies. Further to the west, salinity increases and the effects of seawater intrusion are encountered. In some areas, seawater intrusion is not detected. This may be attributed to the fact that the aquifer in such a location is protected from seawater by the interbedded clay lenses. SOCIO-ECONOMIC IMPACTS Water salinity in the south and east is very high when compared with that in the north and west. The mean chloride content in the eastern and western wells is 749 and 438 mg l -1, respectively, while it is 590 and 218 mg l -1 in the south and north, respectively (Table 1). This affects the taste and acceptability of water for use by the local population. Also it has negative impacts on the agriculture, especially citrus trees. About half of the citrus trees in the Gaza Strip were uprooted as a result of irrigation by brackish water, especially in the southern and eastern parts. As a result, the citrus

Hydrochemical facies of groundwater in the Gaza Strip, Palestine 369 Ca+Mg -SO 4+Cl Northern & western hydrochemical facies Eastern & southern hydrochemical facies Intrusion (Ca+Mg - CO 3 +HCO 3 ) Fresh Mixing Saline Na+K - Cl+SO 4 Freshening F resh Saline Fresh Saline Cations Anions Fig. 7 Piper plots showing the location of the identified hydrochemical facies in the Gaza Strip in relation to freshwater and seawater (the plots of freshwater and seawater after Appelo & Postma, 1993). growing areas have become less productive and consequently uneconomical. Citrus trees form a very important source of income for several thousand families. Thus socio-economic problems arise in the area, where the land use has been changed from agricultural purposes to urban and/or housing projects. DISCUSSION From a geographical point of view, there is a clear distinction between the chemical properties of water in the north and west on the one hand and in the east and south on the other. The occurrence of saline (brackish) water in both the south and east is attributed to the fact that the annual rainfall in the south is lower than that in the north (200 and 400 mm, respectively). Also, the unconfined nature of parts of the aquifer in the Gaza Strip suggests an open system (unconfined) for the natural recharge,

370 Mohammad R. Al-Agha & Hamed A. El-Nakhal especially in the dunes area along the Mediterranean coast. The other parts of the aquifer are of a confined to semiconfined nature. The structural geology, which may play a significant role in this regard, is neither well documented, nor well understood. Consequently, hydrogeological barriers could be assumed to be present, especially in the middle of the Gaza Strip, which separates the two chemical facies in the north and south. There is a clear trend shown in the Piper diagrams that the northern wells are concentrated in the calcium carbonate facies and low chloride content. Similarly, the southern wells show a trend to be concentrated in the saline water in the sodium chloride facies. Basically, such a distinction between the northern and southern waters is attributed to lithological and/or climatic factors in addition to the regional flow. It is now very observable that the regional flow mainly recharges the aquifer with saline water. This source of water contributes to improving the water quantity; however, it worsens the quality as discussed above. QUALITATIVE MANAGEMENT OF THE RESOURCE The results obtained indicate a very clear and distinctive relationship between water quality/hydrochemical facies and the geographical locations of the examined wells. Such a phenomenon may be used in formulating and shaping new policies in water resources management in the Gaza Strip. These should be related to the demographic and agricultural parameters, e.g. population density, agricultural cropping, types of crops, the infrastructure in these areas, the possibility of establishing a national water carrier in the area, environmental health issues, etc. Establishing such policies will significantly contribute to improving the water situation in this area. CONCLUSIONS Groundwater in the Gaza Strip is characterized by two main types of hydrochemical facies: Na+K-Cl+SO 4 and Ca+Mg-CO 3 +HCO 3 which have the tendency to dominate the eastern (outside sand dunes) and the western (inside sand dunes) parts, respectively. Rainfall and proximity to the sand dunes are the main hydroenvironmental factors controlling the hydrochemistry and spatial hydrochemical evolution of groundwater in the Gaza Strip. The eastern margin of the sand dunes belt is arbitrarily taken as the boundary between the Cl and HCO 3 hydrochemical facies. Moreover, the regional flow from the southeast to northwest has a significant impact on the spatial distribution of the hydrochemical facies. The geographical distribution of the hydrochemical facies is an important parameter in shaping new policies and management options for the resource. Such a result is of considerable value in introducing solutions in the qualitative and quantitative management of the aquifer. REFERENCES Al-Agha, M. R. (1995) Environmental contamination of groundwater in the Gaza Strip. Environ. Geol. 25, 109 113. Al-Agha, M. R. (1999) Impact of wastewater management on groundwater quality in the Gaza Strip, Palestine. In: Groundwater in Urban Environment (ed. by J. Chilton), 77 84. Balkema, Rotterdam, The Netherlands.

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