Trends and Sustainability of Groundwater in Highly Stressed Aquifers (Proc. of Symposium JS.2 at the Joint IAHS & IAH Convention, Hyderabad, India, September 2009). IAHS Publ. 329, 2009.

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Assessing the effect of over-exploitation on the Abdan-Dayer coastal aquifer, Iran MOHSEN REZAEI Geology Department, Tarbiat Moallem University, Tehran, Iran m_rezaei@tmu.ac.ir Abstract The impact of increasing abstraction on groundwater quality may be more critical than water table drawdown. The objective of this paper is to assess the effects of intensive exploitation on groundwater quality in the Abdan-Dayer Coastal Aquifer, Iran. In order to assess groundwater quality and quantity and to locate the most appropriate place for groundwater exploitation, groundwater level fluctuation, water budget and major element hydrochemistry have been studied. The result of water balance calculation indicates that outputs from the aquifer have been more than inputs for recent years, and so water level is falling. Groundwater samples were collected and analysed for major constituents (Na+, K+, Ca2+, Mg2+, Cl-, SO4

2-, and HCO3

-). Different methods, including composite diagrams, saturation indices and multivariate statistical methods, were employed in assessing groundwater quality. The results show that the Na-Cl water type as the main hydrochemical facies represents 60%, while the CaSO4 and MgSO4 types represent 40% of the total sampled water. The results of factor analysis indicate that the variables underlying the first and the most important factor are mainly controlled by salt water intrusion. The second and third proposed sources for salinity are dissolution of fine grain alluvium and gypsum in Aghajari formation in surrounding elevations, and evaporation from groundwater, respectively. Over-exploitation is the cause of intensification of seawater intrusion in coastal parts of aquifer. Key words hydrochemistry; multivariate analysis; saltwater; over-exploitation; Iran INTRODUCTION

Groundwaters in coastal arid and semi-arid areas in southern Iran receive increasing stress such as that contributed by anthropogenic factors. Since it is the most important source of water for the rural populations, pumping from the upper phreatic zone is widely practiced. Over-exploitation may cause an imbalance in the hydrological system. In such areas, groundwater is commonly the only water resource, and so the quality of groundwater is the most critical issue. Groundwater quality deterioration could be due to seawater intrusion as a result of over-exploitation. The characterization, interpretation and understanding of groundwater geochemistry are essential not only to identify groundwater quality, but to understand and characterize the factors controlling the basic hydrochemistry. These types of investigations provide useful techniques for studying chemical composition of groundwater in order to understand the main factors contributing to groundwater salinity (Wischusena et al., 2004; Hammer et al., 2005). Composite diagrams have been used as a tool to interpret groundwater chemistry data in various parts of the world. Back (1966), Henry & Schwartz (1990), Howard & Mullings (1996), Stober & Bucher (1999), Marie & Vengush (2001), Gosselin et al. (2001) and Cloutier (2004) used composite diagrams to identify salinity sources in their investigations. Hydrogeochemical modelling of the waters draining an aquifer not only characterize their spatial and temporal variations, but can also be useful in understanding the interaction between groundwater and the aquifer matrix. Thus, the saturation indices (SI) for minerals most representative of the rock can serve as a useful tool in understanding the interactions between groundwater and host rock. The calculation of saturation indices has been performed by a number of researchers (Nordstorm et al., 1989; Jeong, 2001). Statistical approaches such as multivariate analysis are very useful in the identification of relationships between variables (Johnson & Wichen, 1988). Clustering and factor analysis are used in the present groundwater geochemistry investigations. Factor analysis can be performed to identify the most important factors contributing to the data structure and similarities between the factors. When factor analysis is applied to chemical data of groundwater, the dominant processes can be identified as common factors that are sets of variables having strong associations with one

Copyright © 2009 IAHS Press

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another (Usunoff & Guzman-Guzman, 1989; Ritzi et al., 1993; Suk & Lee, 1999). In cluster analysis, water samples are classified based on their chemical similarities (Beatriz et al., 1998). Dawdy & Feth (1967), Hitchen et al., (1971), Dalton & Upchurch (1978), Usunoff & Guzman (1989), Subbarao et al., (1996), Raghunath et al., (2002), Liu et al., (2003) and Kalantary et al., (2007) used multivariate analysis for interpretation of groundwater chemistry. A case study was done on Abdan-Dayer coastal aquifer. This aquifer is under stress with the increasing number of shallow wells on the aquifer. The aquifer has deteriorated to a high degree during the last two to three decades; in quantity (groundwater level is about 10 m below sea level in some parts) and quality (the electrical conductance of groundwater is about 13 000 μs in some parts of the Abdan-Dayer Plain). This paper deals with characterization of the chemical composition of groundwater from shallow wells in the Abdan-Dayer Plain aquifer. The factor analysis and thermodynamic equilibrium analysis of hydrochemical data were applied to illustrate the dominant processes contributing to the chemical evolution and high salinity of the groundwater. Finally it is tried to illustrate the effects of over-exploitation on groundwater quality and some of the initiatives addressed to minimize potentially negative effects. STUDY AREA

Physical setting

The Abdan-Dayer Plain lies in the southeast of Bushehr City, south of Iran, extending between 27°37′–28°14′N and 51°19′–52°33′E. The main population centres of the study area are Kangan, Abdan and Dayer cities. A simplified geological map of the study area is presented in Fig. 1. Geologically the Plain area is sited over the Zagros tectonic zone, forming between two parallel anticlines directed northwest to the southeast. Kangan Anticline is located in the northeast and the Derang anticline is in the southwest of the Abdan-Dayer plain. Derang anticline is completely an outcrop of Aghajari formation (marl, sandstone) with some outcrops of Bakhtiary Conglomerate in margins of the anticline. Kangan anticline mainly consists of older formations such as, Bangestan Group (massive limestone, dolomite and shale), Pabdeh-Gurpi formation (shale and marl), Asmari-Jahrom formation karstic carbonate, Gachsaran formation (anhydrite, gypsum) and Mishan formation (marl with layers of limestone). The minimum elevation in the study area is zero on the coast of Persian Gulf and the maximum elevation is 1485 m above sea level in the Kangan anticline. The climate is arid with an average annual precipitation of 228 mm. The monthly temperature ranges from 15°C to 37.5°C, while the mean annual temperature recorded as 27.5°C. Hydrogeology

The Abdan-Dayer Plain aquifer and its vadose zone are hosted by Quaternary sediments. These sediments are coarse grained in the hillsides, containing gravel and sands, and the grain size reduces towards the central parts of the plain, changing to very fine sand, silt and clay with salt, gypsum and marls in the centre and the main parts of the aquifer. Alluvium thickness varies from 100 m in the northwest parts of the plain to 145 m in the central parts. According to the pumping tests, the transmissivity is in a range of 1000 m2/day to 4500 m2/day and specific yield of aquifer is about 4%. The bedrock of alluvial aquifer is Bakhtiary formation in the southwest mid and Aghajari and Mishan Formations in the northeast mid of the plain. Depth to water table varies from 2.1 m in the observation well no. 32 (utmx = 586269, utmy = 3098971) to 52.8 m in the observation well no. 22 (utmx = 5977001, utmy = 3088300) with an average of 22 m (Fig. 2). Evaporation from groundwater is taking place in an area of about 3.3 km2 around observation well no. 32. Groundwater flows from the mountains and recharges area (in the northwest) toward the sea coast (southwest). The maximum and minimum groundwater level is 18.2 m above sea level in the northwest part of aquifer (observation well no. 37, utmx = 575518 and utmy = 3095763) and 11.5 m below the sea level in the discharge area of aquifer (observation well no. 21 with utmx =

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Fig. 1 Map of the study area showing geology, water-well fields and location of city and villages.

Fig. 2 Location of observation wells and groundwater level contours.

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598890 and utmy = 3087060, 6 km from the sea coast) (Fig. 2). The average groundwater level is 10 m above sea level. Groundwater is abstracted via two well fields shown in Fig. 1. Well field 1 is in the vicinity of Abdan city and well field 2 is close to Kangan city. The bulk of abstraction is used for irrigation. MATERIAL AND METHODS

Groundwater quality is monitored in 26 production wells. Groundwater samples were collected from these wells. The sampling sites are shown in Fig. 3 as S1 to S26. Field measurements for temperature, pH and conductivity were done during sampling. Samples were filtered and bottled in polyethylene bottles and kept fridged. The major elements (Na+, Ca2+, Mg2+, K+, Cl-, SO4

2- and HCO3

-) were analysed in the laboratory using standard methods. A Piper diagram was used to display the major ion compositions of the groundwater samples (Piper, 1944). The extent of mineral saturation and PCO2 of all sampled water was determined using PHREEQC software package (Parkhurst & Appelo, 1999) and saturation indices were expressed as SI = logIAP/KT. The statistical technique of multivariate analysis was used to characterize hydrochemical processes through data reduction and classification. The factor analysis derived principal components from a correlation matrix and rotated axes with a quartimin rotation.

Fig. 3 Sampling locations and iso-electrical conductivity.

RESULTS

Chemistry

The range, arithmetic mean and standard deviation for cations, anions and physical parameters have been presented in Table 1. This table also evaluates groundwater quality for drinking

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consumption by comparing with maximum permissible concentrations in Iranian standards and standards of WHO (1971). Comparison of the water quality parameters with Iranian standards indicated that the quality of groundwater is not safe for drinking consumption. Most of the groundwater samples are highly saline with electrical conductance ranging from 2971 to 13 200 μs. The TDS ranges from 2075 to 9860 mg L-1. According to salinity classification by Rabinove et al. (1958), groundwater is slightly (TDS between 1000 and 3000 mg L-1) and moderately saline (TDS between 3000 and 10 000 mg L-1). The distribution of Electrical Conductivity in the aquifer is shown in Fig. 3. There are two areas of high electrical conductivity. One area is around sampling site no. 14 (Fig. 3) or observation no. 32 (Fig. 2) that evaporation from groundwater is taking place. The other high concentration area is in conformity with a well field and an area of high drawdown in groundwater level. The water table is about 10 m below sea level in that area and upconing of seawater can affect this well field. On average, for the major anions, the order of concentration was Cl->SO4

2->HCO3-, but this

sequence is not valid for all samples. There is a trend from sulphate dominant to chloride dominant water type from recharge area to discharge area, and bicarbonate dominant type was not seen in the study area (Fig. 4). According to hydrochemical modelling using PQREEQC, groundwater samples fall into three main hydrochemical groups: 60% in sodium-chloride group, 25% in calcium-sulphate group, and 15% in magnesium-sulphate group. This range of graphically classified water type can then be taken to indicate a water–rock reaction history. The degree of linear association between any two of the water quality parameters has been studied. It is observed that the correlation between EC and other chemical parameters is significantly positive, except bicarbonate (HCO3

-) (Table 2). The correlation between EC and Cl- (R2 = 0.94) and Na and EC (R2 = 0.87) was significantly positive, an indication of seawater influence on groundwater quality. Table 1 Some physico-chemical properties for groundwater samples and their statistical features.

Ca2+ Mg2+ Na+ K+ HCO3- Cl- SO4

2- EC TDS mg/L

pH μs/Cm mg/L

383.5 250.5 1040 15.97 181.2 1669 1526.4 7.84 6682.2 4960.7 Mean 217.48 116.3 777.5 8.03 72.58 1249 737.24 0.27 3445.3 2578.7 Std Deviation 900 450 2597 29.25 427 3905 3072 8.19 13200 9860 Maximum 80 72 224.3 6.63 85.4 402.9 417.6 7.37 2971 2075 Minimum 200 150 200 – – 600 400 6–9 – 2000 Iranian Standard 75 150 200 – – 500 450 5.6–5.8 – 500 WHO Standard

Table 2 Correlation between selected groundwater quality parameters. Correlation equation R2 Value Correlation equation R2 Value TDS = 0.7603 EC – 75.17 0.99 HCO3 = –8E–07 EC + 2.97 0.00 Ca = 0.0024 EC + 3.68 0.52 Cl = 0.0101 EC – 20.12 0.94 Mg = 0.0022 EC + 6.6318 0.56 SO4 = 0.003 EC + 11.74 0.42 Na = 0.0091 EC – 15.739 0.87 Saturation indices

It seems that the dissolution, precipitation, and cation exchange processes occur dominantly within the groundwater system of Abdan-Dayer Plain. The saturation of groundwater with respect to calcite, dolomite and gypsum are summarized in Table 3. The mineral phase gypsum is undersaturated in most of the groundwater samples. Calcite and dolomite are oversaturated in all samples. It is postulated that undersaturated (SI ≤ –0.1) mineral phases will tend to dissolve and oversaturated (SI ≥ 0.1) mineral phases will precipitate.

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Fig. 4 Piper diagram of the major ion composition of water samples in the Abdan-Dayer aquifer.

Table 3 Summary of saturation indices.

SIg SId SIc SIg SId SIc 0.65 0.06 0.31 S11 1.84 –0.37 0.95 S1 0.79 –0.13 0.39 S12 1.81 –0.52 0.95 S2 1.79 –0.75 0.78 S13 0.46 –0.36 0.22 S3 2.18 –0.65 1.02 S14 1.4 –0.54 0.71 S4 2.21 –0.27 0.98 S15 2.32 –0.07 1.12 S5 1.77 –0.98 0.73 S16 1.92 –0.36 0.79 S6 1.9 –1.17 0.65 S17 2.27 –0.41 0.95 S7 0.93 –0.56 0.25 S18 0.7 –0.26 0.27 S8 0.84 –1.27 0.28 S19 1.22 –0.3 0.62 S9 1.82 –0.34 0.85 S20 1.03 0.0 0.49 S10

Figure 5 shows the saturation states of water samples with respect to calcite, dolomite and gypsum vs chloride concentration. The chemical composition of groundwater has evolved from dissolution conditions to equilibrium with respect to gypsum (Fig. 5(c)). There is an intangible negative trend between dolomite and calcite saturation index and chloride concentration (Fig. 5(a),(b)). This may be due to the common ion effect or cation exchange processes removing Mg2+ from the water. The influx of calcium ions from gypsum weathering can drive dolomite deposition and as a result magnesium concentrations diminish with increasing salinity (Fig. 5(d)). Clustering analysis

Cluster analysis comprises of a series of multivariate methods which are used to find true groups of data. In clustering, the objects are grouped such that similar objects fall into the same class (Danielsson et al., 1999). The hierarchical method of cluster analysis, which is used in this study, has the advantage of not demanding any prior knowledge of the number of clusters, which the non-hierarchical method does. Clustering analysis was used for combining cases (water samples) into clusters. This clustering routine resulted in three groups of water samples on the basis of variables (pH, EC and major elements) (Fig. 6). Table 4 presents ionic concentration of three clusters.

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-1.2

0

1.2

0 5000 10000 15000

-1.4

0

1.4

0 5000 10000 15000

-2.5

0

2.5

0 5000 10000 15000

0

0.5

1

1.5

2

2.5

0 0.2 0.4 0.6 0.8 1 1.2 SIC

SIg

Fig. 5 Saturation indices of calcite, dolomite and gypsum in water samples vs chloride concentration.

SId

SId

SIC

Cl (mg/L)

Cl (mg/L)

(a)

(c)

(b)

(d)

Table 4 Ionic composition of each cluster

Cluster Variable 1 2 3

EC 2500–4500 4500–7000 8500–13500 TDS 2000–3000 3500–5500 7000–10000 SO4 500–1350 1500–2350 1500–3100 TH 500–1500 1250–3000 1600–4200 Water type CaSO4 NaCl NaCl Class. on the basis of TDS Rabinove et al. (1958)

Slightly saline

Moderately saline Moderately saline

1

3

2

Level of Similarity

Gro

undw

ater

Sam

ples

Fig. 6 Dendrogram of the Q-mode cluster analysis.

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Group 1: Slightly saline waters (EC ≤ 4500 μs and TDS between 2000–3000 mg/L) (samples 1, 2, 3, 4, 5, 6 and 11). These samples are located in the northwest part of study area (around Abdan city) in the recharge zone. Group 2: Moderately saline waters (4500 ≤ EC ≤ 7000 μs and TDS between 3500–5500 mg/L) (samples 7, 8, 9, 16, 17, 18 and 20). These samples lie at the central parts of the aquifer. Group 3: Moderately to very high saline waters (8500 ≤ EC ≤ 13500 μs and TDS between 7000 and 10 000 mg/L) (samples 10, 12, 13, 14, 15 and 19). These samples indicate more water–rock interaction and located in the southeast part of the plain. Factor analysis

In Abdan-Dayer aquifer, the groundwater salinization may be due to a lot of different possible factors such as: seawater intrusion, dissolution of evaporate minerals (from Aghajari, Gachsaran or Mishan formations), evaporation from groundwater (depth to water table is less than 3 m in some areas), or ion exchange (presence of clay minerals in alluvial aquifer). Factor analysis was applied to identify the dominant processes controlling major chemical components of groundwater. The variables for factor analysis were Na+, K+, Mg2+, Ca2+, HCO3

-, Cl- , SO42- and pH. The virmax

orthogonal rotation method was applied. Three factors are extracted to represent the contributions influencing chemical composition of groundwater. The results of factor analysis are summarized in Table 5. Table 5 Varimax factor matrix of chemical constituents and factor scores of groundwater samples. Factors Variables 1 2 3 Ca 0.26 0.83 –0.32 Mg 0.37 0.85 –0.00 Na 0.93 0.21 –0.21 K 0.87 0.41 –0.22 HCO3 0.52 –0.56 0.40 Cl 0.90 0.31 –0.22 SO4 0.25 0.90 –0.17 pH –0.38 –0.20 0.85 Percentage of variance explain with factor 38.9 36.0 14.5 Cumulative percentage of variance 38.9 74.9 89.4 The most significant factors indicate that three factors explain about 89.4% of total sample variance. The variables of Na, K and Cl, have high positive loading of factor 1. The variables Mg, Ca, and SO4 have high positive loading of factor 2. The factor loading of pH and bicarbonate show high positive value on factor 3. The groundwater of sodium-chloride type can be explained by factor 1. This main factor that explains about 38.9% of variance is in relation with chloride and sodium. This factor can be considered as influence of seawater mixing, halite dissolution or evaporation from the groundwater in the Abdan-Dayer aquifer. If halite dissolution is the sole source of salinity, the Na/Cl ratio must be close to unity. Richter & Kreitler (1986) have reported Na/Cl molar ratio for halite solution brine of more than 0.95, for deep basin brine less than 0.95. When salinity is due to influence of oil field brines Na/Cl ratio is less than 0.6 and if evapotranspiration from groundwater takes place in the aquifer the Na/Cl ratio is much less than unity due to cyclic wetting and drying effects (Richter et al., 1993). Analysed samples from the Abdan-Dayer Plain have Na/Cl molar ratios of 0.81 to 1.14, indicating different contribution of these different salinity sources in water samples. 36.0% of groundwater chemistry variation is controlled by the second factor. The sulphate water type may be explained by factor 2. Dissolution of gypsum and anhydrite minerals can also control concentrations of magnesium, calcium and sulphate. HCO3 and pH have very high positive loading of factor 3 (explaining about 14.5% of variance) and can be due to the influence of rainwater recharge (Lawrence & Upchurch, 1982).

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To investigate the importance of ion-exchange processes in groundwater chemistry, this study examined the relation between reactants and product of ion exchange reaction in x–y diagrams (Fig. 7). The driving process for salinity could be halite dissolution or seawater intrusion, but Na/Cl ratio is increased due to influence of ion-exchange processes. Figure 7(a) indicates that ion exchange is not the dominant process in the Abdan-Dayer aquifer. A weak correlation between alkaline earths (Ca + Mg) and HCO3 + SO4, is also evidenced for reverse ion exchange processes in the Abdan-Dayer Plain aquifer (Fig. 7(b)). When HCO3 + SO4 is low (less than 5 meq/L) and the samples plot on 1:1 lines in x–y diagram, this indicates that the dissolution of calcite and dolomite is the major process influencing water chemistry (Kalantary et al., 2007). But almost all samples from the Abdan-Dayer Plain aquifer lie under the 1:1 lines in Fig. 7(b) and HCO3 + SO4 is >10 meq/L, indicating simultaneous reverse ion exchange and gypsum dissolution processes.

0.1

1

10

0 5000 10000 150000

20

40

60

80

100

0 20 40 60 80

-30

0

30

-30 0 300

4000

8000

12000

0 0.2 0.4 0.6 0.8 1

Fig. 7 x–y diagrams illustrating relationship between major ions and reactant and products of ion-exchange processes.

Figure 7(c) shows Ca + Mg-HCO3-SO4 vs NaCl. There is a tendency toward the development of a clear –1 slope line in some groundwater samples, indicating the influence of ion exchange processes (Jankowski et al., 1998), but a lot of samples indicate reverse ion exchange processes. When ion exchange does not take place, the samples must be close to the zero value and plot close to horizontal line in Fig. 7(c). Finally there is a tendency toward a value of 1 for (Na-K)/(Na-K+Ca), with increasing TDS indicating ion exchange processes in the high saline samples of Abdan-Dayer aquifer. The effect of over-exploitation on groundwater quality

Groundwater is a critical resource on the Abdan-Dayer Plain as it is the main source for agricultural water. More than 30 MCM annual water discharge from the aquifer is taking place by about 625 wells via two well fields. The result of water balance calculation indicates that outputs from the aquifer have been more than inputs by 3 Mm3/year for recent years. As a result, groundwater storage is severely reduced and water table levels dropped by more than 2.5 m within

(c)

Ion Exchange

(a) (b)

(d)

Na/

Cl (

epm

)

TDS

(mg/

l)

Ca+

Mg-

SO4-

HC

O3 (

epm

)

Na-Cl (epm)

EC (μs)

(Na-K)/(Na-K+Ca) (epm)

Ca+

Mg

(epm

)

SO4+HCO3 (epm)

Ion Exchange

Reverse Ion Exchange Reverse Ion Exchange

Ion

Exch

ange

Reverse Ion Exchange

1: 1 Line

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a one year period from January 2007 to January 2008 (Fig. 8). Water levels are declining locally in areas of intensive pumping. On the other hand, three main sources of groundwater salinity, including seawater intrusion, evaporation from groundwater, and gypsum dissolution, were identified in Abdan-Dayer aquifer. As discussed previously, gypsum dissolution and evaporation from groundwater are the main sources of salinity in the northwestern part of the aquifer, while salt water intrusion is the main

Fig. 8 Distribution of drawdown in Abdan-Dayer aquifer.

0

1

2

3

4

5

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10

S11

S12

S13

S14

S15

S16

S17

S18

S19

S20

Sampling Site

Cl/S

O4 (

Mol

ar R

atio

)

Fig. 9 Molar ratio of Cl/SO4 in groundwater samples.

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source of salinity in coastal parts of the aquifer. Figure 8 indicates high drawdown in well field 2 and Fig. 3 shows low groundwater quality in this area. Over-exploitation from this well field can cause upconing and intrusion of seawater, leading to salinization and loss of groundwater quality. Figure 9 shows increasing of Cl/SO4 molar ratio in sampling site, from northwest to southeast (coastal area). This can be regarded as evidence of seawater intrusion in coastal part of aquifer. Reverse hydraulic gradient from the sea to the aquifer makes seawater influx into the aquifer reducing groundwater quality. It is proposed to reduce the abstraction rate from well field 2. Instead, the pumping rate from well field 1 could be increased to abstract groundwater before entering the groundwater evaporation area. CONCLUSION

The hydrogeochemical parameters of Abdan-Dayer aquifer indicate that the groundwater is low to moderately saline. Groundwater quality is not safe for drinking consumption. Three chemical types of groundwater, i.e. NaCl, CaSO4 and MgSO4 type are recognized in the Abdan-Dayer Plain aquifer. Groundwater samples were classified into three distinctive groups using clustering analysis on the basis of physiochemical variables (pH, EC and major elements). Samples of Group 1 (EC ≤ 4500 μs), lie in the aquifer recharge area. Groups 2 and 3 are more saline and lie in the Abdan-Dayer aquifer discharge area. The governing processes in groundwater composition, as inferred from the factor analysis are mainly determined: seawater mixing, evaporation from groundwater, and weathering of evaporites such as gypsum are the main factors controlling chemistry of groundwater in the Abdan-Dayer Plain aquifer. Finally, reverse ion exchange processes influence the concentration of cations such as Ca, Mg and Na. The effect of over-exploitation is totally different in well field 1 and well field 2. In the latter, the over-exploitation will intensify the sea water intrusion and will contribute to reducing the groundwater quality. On the other hand, increasing pumping from well field 1 will abstract groundwater with better quality before it reaches the evaporation zone. REFERENCES Back W. (1966) Hydrochemical facies and ground-water flow patterns in northern part of Atlantic Coastal Plain. US Geol.

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