hydrogeochemical processes operating within the main aquifers of kuwait

Journal of Arid Environments (1999) 42: 195–209Article No. jare.1999.0511Available online at http://www.idealibrary.com on

Hydrogeochemical processes operating withinthe main aquifers of Kuwait

A. Saleh, F. Al-Ruwaih & M. Shehata

Geology Dept., P.O. Box 5969, Kuwait University, Safat-13060, Kuwait

(Received 27 August 1998, accepted 2 March 1999)

The study area, Al-Wafra, is located in the extreme south of Kuwait andoccupies an area of 325 km2. The objective of this study was to develop aninterpretation and understanding of the geochemical processes and character-istics of the Kuwait Group and Dammam limestone aquifers. Therefore,hydrochemical data comprising the mean values and standard deviations of themajor ion concentrations, pH, PCO

2and saturation indices for calcite, dol-

omite, gypsum and halite have been determined in the ground-waters. Inaddition, the suitability of the brackish ground-water of the two aquifers forirrigation has been studied through the determination of exchangeable sodiumpercentage (ESP), boron, adjusted sodium adsorption ratio (adj. SAR), nitrateand several trace metals.

Chemical analyses of the samples have been interpreted with regard to theirsaturation levels of calcite, gypsum, halite and dolomite, and by simple dissolu-tion models involving carbonate, sulphate and chloride minerals. The cationconcentrations have probably been modified by exchange processes involvingclay minerals. The ground-water was found to be oversaturated with respect tocalcite, aragonite and dolomite, and undersaturated with respect to gypsum,anhydrite and halite. The calculated mean value of PCO

2is 5 ) 10~3 atm.,

which suggests a deep, closed environment system. The hierarchical clusteranalysis shows that the ground-water is separated into geochemical provincesbased on the similarity of total dissolved solids. Trace element concentrationsof Pb, Ni, Cd and Zn occur at concentrations below 1)0 mg l~1, whilst highnitrate values in the Kuwait Group ground-waters (mean"123 mg l~1) indi-cates the impact of agricultural activity on the study area. Ground-waterproved to be of rather poor quality, though it is used for irrigation due to thelimited natural water resources in arid Kuwait.

( 1999 Academic Press

Keywords: Kuwait aquifers; saturation indices; carbonate dissolution; clusteranalysis; exchangeable sodium percentage

Introduction

The State of Kuwait is located on the north-western side of the Arabian Gulf andoccupies an area of about 18,000 km2 of sand and gravel desert. Kuwait is a typicallyarid country, where rainfall is scarce and variable. The annual rainfall averaged some115 mm at the Kuwait International Airport station for the years 1962}1997. Potentialevaporation is very high and ranges from 2)5 mm day~1 to 16 mm day~1 (Anon., Met-eorological Department, Climatological Division, Ministry of Communications, Kuwait).

0140}1963/99/070195#15 $30.00/0 ( 1999 Academic Press

Figure 1. Geographical location and topographic map of the study area and the water well-fieldsin Kuwait.

196 A. SALEH ET AL.

Kuwait has very limited natural water resources with most of the natural water existingas ground-water, which is generally brackish to saline. The three brackish ground-waterwell-fields (Fig. 1) are Al-Sulaibiya, Al-Shagaya and Umm-Gudair, in which theground-water is produced from the Dammam limestone aquifer, with total dissolvedsolids (TDS) ranging from 2500}10,000 mg l~1 (Al-Ruwaih, 1995). Brackish ground-water from the Kuwait Group aquifer is obtained from Al-Wafra and Al-Abdaliagricultural farms, where the ground-water TDS ranges from 4000}10,000 mg l~1.Fresh ground-water resources with TDS ranging from 400}1000 mg l~1 are extremelylimited in Kuwait and receive very little annual replenishment. They are represented byAl-Rawdhatain and Umm Al-Aish in the north of Kuwait. Kuwait is anxious to exploreand use all available ground-water for irrigation of private and public gardens. About10}12% of the brackish ground-water is currently blended with the Municipal distilledwater supply to make it more suitable for drinking (Al-Ruwaih, 1994).

The area under investigation is the Al-Wafra agricultural farm located in the extremesouth of Kuwait (Fig. 1). The approximate total area is 325 km2 and the land surfaceslopes gently from an elevation of 180 m a.s.l. in the west to around 120 m a.s.l. in thevicinity of the eastern boundary.

Stratigraphy

The upper part of the stratigraphic sequence of Kuwait is divided into clastic sedimentsknown as the Kuwait Group and carbonate sediments called the Hasa Group. The

Figure 2. The stratigraphic sequence of the subsurface of Kuwait (modified after Owen & Nasr,1958).

HYDROGEOCHEMICAL PROCESSES IN KUWAIT AQUIFIERS 197

present study is concerned with both the Kuwait Group and the underlying DammamFormation of the Hasa Group of the study area, as shown in Fig. 2 (Owen and Nasr, 1958).

The Kuwait Group consists of sand, gravels, sandstone, clay, silts and limestone ormarls covering the entire surface of Kuwait and extending down to the top of theunderlying Dammam Formation. The Kuwait Group is subdivided from top to bottominto Dibdibba, Lower Fars and Ghar Formations. The thickness of the Kuwait Groupincreases from 150 m in the south-west to about 400 m in the north-east. The KuwaitGroup is of Miocene}Pleistocene age.

The Dammam Formation, the uppermost part of the Hasa Group, underlies theKuwait Group. It consists of over 200 m of whitish grey, porous, dolomitized limestone,nummulitic limestone and soft chalky limestone. The top of the Dammam Formation is

198 A. SALEH ET AL.

generally marked by the presence of a hard, cherty layer with a persistent grey}greenwaxy shale horizon providing a distinctive base of the Kuwait Group.

Aquifer system

There are two main aquifers in the study area, the Kuwait Group and the DammamFormation. Most of the private wells are drilled into the Kuwait Group aquifer to a depthof 9 to 46 m and each well pumps +518 to 1123 m3 day~1, with a TDS rangingbetween 4400}10,300 mg l~1 and this salinity increases from the SW to NE direction.

The Dammam limestone consists of various beds of shaly, sandy, chalky siliceous,dolomitic limestone. The aquifer salinity increases from the SW to NE direction andranges from 4680 to 8650 mg l~1. Although the ground-water for irrigation is obtainedmainly from Kuwait Group aquifer, study of the water quality of the underlyingDammam Formation is also important.

Hydrochemical interpretation

Laboratory analyses of Ca2`, Mg2`, Na`, K`, Cl~, SO2~4

and HCO~3

, along with fieldanalyses of temperature, pH and electrical conductivity have been carried out for 36ground-water samples collected from the Kuwait Group and nine samples from theDammam aquifer. The calculated mean and standard deviations of the major ionconcentrations, the pH, PCO

2and saturation indices, for anhydrite, aragonite, calcite,

dolomite, gypsum and halite for ground-water of both the Kuwait Group and theDammam limestone aquifers in the study area are listed in Table 1.

Table 1. Summary of the mean chemical data from the Kuwait Group and theDammam limestone aquifers at Al-Wafra area

Kuwait Group aquifer Dammam limestone aquifer

Parameter X~K

p X~D p

TDS (mg l~1) 7429 1496 5980 1123pH 7)5 0)32 7)4 0)2K` (mg l~1) 77 17)96 56 9)7Na` (mg l~1) 1345 341)16 1180 305)2Ca2` (mg l~1) 659 135)63 540 80)7Mg2` (mg l~1) 177 35)98 164 22)4HCO~

3(mg l~1) 150 30)63 173 7)1

Cl~ (mg l~1) 2578 653)87 2365 614)2SO2~

4(mg l~1) 1724 392)47 1032 65)6

PCO2

(atm.) 5)0 )10~3 6)0 ) 10~3SI calcite #0)49 #0)47SI dolomite #0)76 #0)76SI gypsum !0)17 !0)38SI aragonite #0)35 #0)32SI halite !4)22 !4)24SI anhydrite !0)37 !0)60

X~K"mean of 36 ground-water samples from the Kuwait Group aquifer; X~D"mean of nine ground-water

samples from the Dammam limestone aquifer.p"standard deviation.SI"Saturation index expressed in logarithmic form, log (IAP/K

41).


The chemical analyses indicate both important similarities and differences betweenthe ground-waters from the two aquifers. In each aquifer Cl~ is the dominant anionand SO2~

4is the second most abundant, while the concentration of HCO~

3is generally

low. In both aquifers the cation concentration order is (Na`#K`)'Ca2`'Mg2`.The concentration of total dissolved solids (TDS) of ground-water in the Kuwait Groupvaries from 4400 mg l~1 along the Saudi Arabian border to about 10,300 mg l~1 alongthe north-eastern corner (Fig. 3), while the TDS of the Dammam aquifer ranges fromaround 4680 to 8650 mg l~1.

The overall chemical properties of the ground-water in the two aquifers can be seen tobe dominated by alkalis, as shown in the Piper trilinear diagram (Fig. 4). In theinterpretation of the hydrochemical information, it was noted that the calculated meanPCO

2values for the ground-water from the Kuwait Group and the Dammam limestone

aquifers are 5 )10~3 atm. and 6 ) 10~3 atm., respectively, which are significantly abovethe PCO

2of the Earth’s atmosphere (10~3>5 atm.). This indicates that the ground-water

in the aquifers became charged with CO2

during infiltration through soil zones, and theground-water probably now represents a deep closed environment system since, accord-ing to Appello & Postma, (1994), Tellam (1994) and Stigter et al. (1998) PCO

2ranges

between 10~2>5 and 10~6>4 atm. in a closed system.Significantly, the pH of ground-water in the two aquifers ranges from 7)0 to 8)0. This

indicates that the dissolved inorganic carbon exists almost entirely as the HCO~3

ion.The concentration of HCO~

3ranges from 92 to 256 mg l~1 in the Kuwait Group

aquifer, and from 163 to 181 mg l~1 in the Dammam aquifer, which indicates that morecalcite and dolomite have dissolved in the water of the Kuwait Group than in theDammam aquifer.

Saturation indices

The natural composition of water is derived from many different sources of solutesincluding both gases and aerosols from the atmosphere, and the weathering and erosionof rocks and soils. Solution and/or precipitation reactions can occur below the landsurface where the concentration of many minerals is influenced by many environmentalfactors, especially the position and solubility of rock strata (water}rock interaction) (Lin& Clemency, 1980; Ronge & Claesson, 1982).

Saturation indices (SI) for 45 ground-water samples from both the Kuwait Group andthe Dammam limestone aquifers were calculated using the speciation code WATEQ4(Ball & Nordstrom, 1992). Results of the mean saturation indices of various differ-ent minerals are shown in Table 1. It was found that each aquifer is undersaturated(negative SI) with respect to gypsum, anhydrite and halite, and oversaturated (positiveSI) with respect to calcite, aragonite and dolomite. Saturation index is a measure of thethermodynamic state of a solution relative to equilibrium with a specified solid-phasemineral. In some parts of the aquifers, which are undersaturated with respect to calcite ordolomite, ground-water flow is capable of dissolving the aquifer rock, thereby increasingboth its porosity and permeability, whereas in other parts, where the SI is positive,mineral precipitation will have the opposite effect on the aquifer properties.

Influence of dolomite and calcite dissolution

Calcite and dolomite are common constituents of the Dammam limestone aquifersystem in the study area. It is of interest, therefore, to determine the effect ofdissolution of these minerals on ground-water chemistry. The carbonate aquifer in thestudy area is probably the least complex geochemical environment. The Ca2`, Mg2`,

Fig

ure

3.T

heis

osal

inity

map

(exp

ress

edin

mg

l~1)

ofth

eK

uwai

tG

roup

aqui

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atA

l-W

afra

area

.

200 A. SALEH ET AL.

Figure 4. Chemical analyses of the ground-water of the Kuwait Group and the Dammamaquifers, represented as percentage of total equivalents per liter on the Piper diagram.


HCO~3

and pH values from the carbonate aquifer of the study area are compared tovalues that would result from dissolution at 253C of pure dolomite in water charged withvarious CO

2pressures. The dissolution data were obtained using the equilibrium

constants of Langmuir (1971) and Wigley (1971). It is believed that Langmuir’s datarelating to dolomite solubility are the best available at present.

The Ca2`, Mg2` and HCO~3

field data are shown in Fig. 5 and generally plot abovethe equilibrium concentration lines computed from Langmuir’s dolomite solubilityvalues. The pH vs. Mg2` graph indicates that there is a considerable excess of Mg2`when related to simple dolomite dissolution. This condition is reflected in theCa2`/Mg2` molar ratios, which are nearly all less than 1)0. The Ca2`, Mg2`, HCO~

3and H` concentrations do not vary symmetrically with depth or lateral position in theflow system.

The hydrochemical data from the carbonate aquifer indicate that the dissolution ofdolomite could occur relatively rapidly as fresh water infiltrates into the aquifer in therecharge area. It is probable that the widespread condition of supersaturation occurs

Figure 5. Plot of pH, HCO~3

, Ca2` and Mg2` of the Dammam limestone aquifer, compared tothe dolomite dissolution model (Langmuir, 1971).

202 A. SALEH ET AL.

because of degassing of CO2

during deeper infiltration. As the water passes through thesoil horizons in the recharge areas, it is charged with CO

2produced by decaying organic

matter and root respiration. The water would then be supersaturated with respect to


dolomite and calcite. It may be expected that calcite or possibly aragonite wouldgradually precipitate as the solution adjusts to a new equilibrium. Precipitation ofCaCO

3would produce Ca2`/Mg2` molar ratios less than 1)0, as observed in the

chemical data from the aquifer. The aquifer is covered throughout most of the rechargearea by thin deposits of Recent sediments with low porosity and well-developed soilhorizons.

As indicated above, most of the ground-water in the study area contains moderate tohigh concentration of Na`, Cl~ and SO2~

4. It is possible that dissolution of dolomite

produces small quantities of K`, Na`, Cl~ and SO2~4

in solution as a result of impuritiesin the carbonate crystals, but, although these concentrations are believed to be signifi-cant, they would not be capable, by simple dissolution of the carbonate rocks, ofproducing significant concentrations of K`, Na` and SO2~

4in the ground-water relative

to the carbonate mineral ions Ca2`, Mg2` and HCO~3

. Other minerals, therefore,appear to be involved in the geochemical evolution of the ground-water. It is reasonableto expect that the concentration of Na`, Cl~ and SO2~

4and the excess Ca2` and Mg2`

are at least in part derived from highly soluble sulphate and chloride minerals. Dissolu-tion of silicate minerals can only contribute small concentrations of Na`, Ca2`, Mg2`and Cl~ under the pH and temperature conditions of the study area (Oetting et al.,1996). Ionic increases due to evaporation in the water-table zone would not explainthe progressive concentration increases of these anions in the interior of the flowsystems.

Mineral dissolution models, involving highly soluble sulphate and chloride mineralscombined with cation exchange on clay minerals, have therefore been used to interpretthe chemistry of water in the carbonate rock aquifers.

Statistical data treatment

Hierarchical cluster analysis was used to classify the data from the Kuwait Group andthe Dammam limestone aquifers into geochemical provinces based on their numericalsimilarity. In order to determine the correlation between groups, a series of measure-ments is conducted on n objects with m characteristics with the data set then forming ann]m matrix. A measure of resemblance or similarity between each pair of objects is thencomputed by measuring coefficients of resemblance such as the correlation coef-ficients. On each object, m variables are measured and the distance between each objectis computed. Thus a low distance indicates similarity between the two objects (Davis,1986).

These distances are then projected in two dimensions in the form of a dendrogram.The samples with the highest similarity, or the lowest within the group error sum ofsquares, form the nucleus of the clusters. The end product is a two-dimensionalhierachical dendrogram which recognizes geochemical relationships that exist amongthe samples and geochemical provinces (Anderberg, 1973).

Figure 6 represents a dendrogram which shows the classification of the data based ontheir similarity coefficients. Based on these groupings it was found that clusters 1, 2,4 and a portion of cluster 3 contain all the ground-water samples of Kuwait Group withTDS ranging between 4400}10,300 mg l~1. Cluster 3 includes all the ground-watersamples from the Dammam limestone aquifer, where TDS ranges from4680}8650 mg l~1, with a few samples from the Kuwait Group of similar salinity.Thus, it is clear that the ground-waters at Al-Wafra form a separate geochemicalenvironment of upper and lower aquifers; Kuwait Group and Dammam aquifers,respectively.

The correlation matrix of non-transformed data for the variables TDS, pH, K`, Na`,Mg2`, Ca2`, Cl~, SO2~

4and HCO~

3, and also the mean and standard deviations of the

two aquifers are listed in Tables 2 and 3. The tables show that the TDS is in good

Figure 6. Dendrogram showing hierarchical clustering of the Kuwait Group and the Dammamlimestone aquifers.

204 A. SALEH ET AL.

correlation with most of the variables except the ionic concentration of HCO~3

inthe Kuwait Group aquifer. Although the relation is positive (#0)31) the correlationcoefficient (r) value lies below the threshold value (r"0)6). This indicates thatthe recharge process is not local and the bicarbonates were precipitated as carbonatesalong the recharge route. The positive correlation between the ionic concentration ofCl~ and SO2~

4may be related to the mixing processes between different water

genesis.

Table 2. Correlation matrix of the non-transformed data input for the hydro-chemical variables of the Kuwait Group aquifer

Standardmode TDS pH HCO

3SO

4Cl Ca Mg Na K

TDS 1pH 0)03 1HCO

30)31 !0)06 1

SO4

0)7 0)21 0)13 1Cl 0)94 !0)07 0)29 0)52 1Ca 0)85 0)3 0)2 0)74 0)72 1Mg 0)15 0)13 !0)03 0)07 0)11 0)25 1Na 0)94 !0)02 0)3 0)61 0)95 0)72 0)13 1K 0)85 !0)21 0)2 0)64 0)85 0)61 0)14 0)85 1


The chemical suitability for irrigation

The chemical suitability of the ground-water of the Kuwait Group and the Dammamlimestone aquifers for irrigation is judged mainly by its electrical conductivity, sodiumadsorption ratio (SAR), adjusted SAR, boron concentration, exchangeable sodiumpercentage (ESP), nitrate and trace metals.

Electrical conductivity (EC) is a good measure of total dissolved solids (TDS) andhence a measure of the salinity hazard to crops. Excessive salinity reduces the osmoticacitivity of plants and thus interferes with the absorption of both water and nutrientsfrom the soil. Sodium adsorption ratio (SAR) is a measure of the alkali or sodium hazardto crops. When the sodium concentration is high, sodium ions tend to become adsorbedonto the clay soil particles, displacing Mg2` and Ca2` ions. Exchange of Na` for Ca2`and Mg2` results in a soil with a poor internal drainage and restricted circulation of airand water when wet. Such soils usually form hard and unmanageable clods when dry(Collins & Jenkins, 1996). SAR is defined as:

SAR"

Na`

J(Ca2`#Mg2`)/2(Eqn 1)

where Na`, Ca2` and Mg2` are the concentrations of the cations in meq l~1.

Table 3. Correlation matrix of the non-transformed data input for the hydro-chemical variables of the Dammam limestone aquifer

Standardmode TDS pH HCO

3SO

4Cl Ca Mg Na K

TDS 1pH 0)31 1HCO

30)64 0)23 1

SO4

0)41 !0)27 0)07 1Cl 0)97 0)36 0)52 0)49 1Ca 0)94 0)1 0)49 0)51 0)94 1Mg 0)23 0)14 0)16 0)54 0)34 0)25 1Na 0)96 0)4 0)53 0)39 0)99 0)94 0)32 1K 0)82 !0)02 0)26 0)58 0)88 0)94 0)39 0)88 1

206 A. SALEH ET AL.

With SAR values greater than 6 to 9, the irrigation water could be expected to causea permeability problem on shrinking and swelling types of clayey soils.

The older SAR procedure has been modified to include changes in soil waterconsumption that are expected to result from certain combinations of salts, which willeither dissolve lime from the soil or will result in deposition of lime from the soil water,giving an adjusted SAR (Calvache & Pulido-Bosch, 1994).

Adjusted SAR is calculated using the following equation:

Adjusted SAR"

Na`

J(Ca2`#Mg2)/2M1#(8)4!pHc)N (Eqn 2)

where pHc"P(Ca#Mg#Na)#P(Ca#Mg)#P(CO3#HCO~

3), and all cations

and anions are expressed in meq l~1.This formula predicts the soil’s permeability potential. Values of pHc below 8)4

indicate a tendency to precipitate lime (CaCO3) from the applied water.

The calculated mean values of the Adjusted SAR for the Kuwait Group and theDammam aquifers are 32)26 and 29)9, respectively, and the pHc value in the twoaquifers is about 6)7 which indicates that lime (CaCO

3) would be precipitated from the

irrigation water (Table 4).

Boron

Boron is toxic to plants at high concentration. McCarthy & Ellery (1994) proposed thelimits of boron concentration in irrigation waters as shown in Table 5.

However, the Kuwait Group water has a boron concentration range from1)3 to 5)7 mg l~1, and the Dammam limestone aquifer has a range from 1)5 to2)6 mg l~1. This makes the ground-water in the two aquifers generally unsuitablefor semi-sensitive crops, but it can be used carefully with semi-tolerant and tolerantcrops.

The exchangeable sodium percentage (ESP) is calculated as follows:

ESP"

100(!0)0126#0)01475 SAR)1#(!0)0126#0)01475 SAR)

(Eqn 3)

It can be observed from the results of chemical analyses that pH values are less than8)5 and ESP values are less than 15 for both the aquifers. This indicates that theground-water is saline. Soil salinity is the controlling factor for plant growth, while theESP indicates, the potential decline in soil structure.

Nitrate

Most of the nitrogen in the ground-water is probably derived from the biosphere.The nitrogen, originally fixed from the atmosphere, is mineralized by soil bacteriainto ammonium which, under aerobic conditions, is converted into nitrate bynitrifying bacteria (Tindall et al., 1995). This nitrate in the Kuwait Groupground-waters ranges from 94}202 mg l~1 with an average value of 123 mg l~1.This contrasts with the very low values of NO~

3in the lower aquifer (the Dammam

Formation) which has an average NO~3

value of 1)78 mg l~1 (Table 4). This makesnitrate a very good indicator for differentiating ground-waters from the twoaquifers.

Table 4. Summary of the mean values of the water quality criteria for irrigation ofthe Kuwait Group and the Dammam limestone aquifers at Al-Wafra area

Parameters Kuwait Group Dammam limestone

Electrical conductivity (EC)(ks cm~1 at 253C)

8996 8560

TDS (mg l~1) 7429 5980ESP 14)06 13)34SAR 11)96 11)34Adj) SAR 32)26 29)9pHc 6)71 6)79Boron (mg l~1) 2)57 1)86NO~

3(mg l~1) 123 1)78

Pb 0)14 0)017Zn 0)022 0)044Ni 0)071 }

Cd 0)019 }


Trace metals

Trace metals, which include Cd, Pd, Zn, Fe and Ni, rarely occur in ground-water atconcentrations large enough to comprise a significant component of the total dissolvedsolids. Trace metals in natural or even contaminated ground-water almost invariablyoccur at concentrations below 1)0 mg l~1. The concentrations are low because ofconstraints imposed by the solubility of minerals or amorphus substances and adsorp-tion on to clay minerals. A characteristic feature of most trace metals in water is theirtendency to form both hydrolysed and complexed species by combining with inorganicanions such as HCO~

3, CO2~

3, SO2~

4, Cl~, F~ and NO~

3. Using the speciation code

WATEQ4, the hydrolysed species and inorganic complexes of cadmium that couldform include CdOH`, Cd(OH)0

2, Cd(OH)2~

4, CdCl~, CdSO0

4and Cd(CO

3)02. Trace

elements which can cause growth reduction in crops include copper, zinc, cadmium,lead and nickel, amongst others. These metals were determined for the Kuwait Groupand the Dammam Formation ground-waters, and are presented in Table 4. However,the concentration of these trace elements in both aquifers was found to be lower than therecommended maximum concentration for irrigation water (Calvache & Pulido-Bosch,1994), and so these low levels will not normally have any adverse effects on plantsor soils.

Table 5. The permissible limits of boron for several classes of irrigation water

Boron class Semi-sensitive Semi-tolerant Tolerant cropscrops (mg l~1) crops (mg l~1) (mg l~1)

Excellent (0)33 (0)67 (1)00Good 0)33}0)67 0)67}1)33 1)00}2)00Permissible 0)67}1)00 1)33}2)00 2)00}3)00Doubtful 1)00}1)25 2)00}2)5 3)00}3)75Unsuitable '1)25 '2)50 '3)75

208 A. SALEH ET AL.

Conclusion

The study area, Al-Wafra, is one of the largest agricultural farms in Kuwait. There aretwo aquifers in the study area, the Kuwait Group and the underlying DammamFormation. Ground-water for irrigation is produced mainly from the Kuwait Groupaquifer. The ground-water salinity of the Kuwait Group is higher than the salinity of theDammam Formation. The ground-water in the two aquifers is dominated by alkali salts.The sequence of anions is Cl~'SO2~

4'HCO~

3, and the order of cations is

(Na`#K`)'Ca2`'Mg2`.The ground-water of the two aquifers is oversaturated with respect to calcite, aragon-

ite and dolomite, and undersaturated with respect to gypsum, anhydrite and halite. Thecalculated mean PCO

2for the two aquifers is 5 ) 10~3 atm., which is significantly above

the PCO2

of the Earth’s atmosphere (10~3>5 atm.), and probably represents a deep,closed environment. The hierarchical cluster analysis indicated that the ground-watersat Al-Wafra form a separate geochemical environment of upper aquifer and loweraquifer mainly based on the total dissolved solids.

Analyses of the field geochemical data indicates that the dissolution of dolomite andcalcite and the dissolution of highly soluble sulphate and chloride minerals coulddetermine the major ion chemistry of the ground-water in the study area. The dissolu-tion processes may be combined with cation exchange. The presence of highly solublesulphate and chloride minerals is inferred from the water chemistry data. Minerals suchas gypsum and halite could have been derived during the erosion of the aquifer materialsby mineral waters.

The suitability of the ground-water for irrigation has been judged by the determina-tion of the specific conductance, ESP, boron, sodium adsorption ratio, Adjusted SAR,trace metals and nitrate. Ground-water proved to be of rather poor quality, though it isused for irrigation due to the limited natural water resources. Trace metal concentrationsof the two aquifers are below 1)0 mg l~1.

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hydrogeochemical processes operating within the main aquifers of kuwait

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