Palaeohydrogeology and Water Quality Control of the Tertiary Aquifer, Kuwait

Transcription

1 International Water Resources Association Water International, Volume 32, Number 2, Pg , June International Water Resources Association Palaeohydrogeology and Water Quality Control of the Tertiary Aquifer, Kuwait F.M. Al-Ruwaih, L.A. Talebi, Department of Earth & Environmental Sciences, Kuwait University, Kuwait, and J. Almedeij, Civil Engineering Department, Kuwait University, Kuwait Abstract: The State of Kuwait is located at the northwestern corner of the Arabian Gulf and covers an area of about 18,000 km2. The aquifer under investigation is the Eocene Dammam limestone aquifer. This paper discusses the hydrochemistry of the aquifer through the period in order to identify the chemical and genetic types of groundwater, and to reveal the prevailing geochemical processes in the aquifer. Moreover, the spatial distribution of the brackish-brine groundwater is encountered to provide an overview of the regional scale hydrochemistry and to assess the possible factors controlling the aquifer quality. Accordingly, WATEQ4F program is used to compute the saturation indices of the minerals with respect to a state water composition. The Z- MAP and the STRATWORKS programs are used to construct the structural contour map and the subsurface geophysical logs of the Dammam Formation, respectively. The hydrochemical and geological investigations revealed that the Dammam limestone aquifer is occupied by a brackish-brine groundwater, with TDS values ranging from 2500 to 150,000 mg/l, which gradually increases towards the N NE in the flow direction. The main recognized groundwater chemical types are: Na2SO4, CaSO4 and NaCl. Three groundwater genetic types Na-SO4, Mg-Cl, and Ca-Cl are dominant and indicate a continentalmarine stagnant environment. Also, the study showed that the groundwater is oversaturated with respect to calcite and dolomite, and under-saturated with respect to gypsum. The calculated mean values of PCO2 range between 8.79 x 10-4 and 1.38 x 10-2 atm., which are above that of PCO2 of the earth s atmosphere. These high values of PCO2 suggest that the Dammam limestone aquifer represents a closed system with respect to CO2. The ion-exchange, reverse ionexchange and dedolomitization are the main geochemical processes prevailing in the aquifer. Furthermore, the carbonate weathering is most likely to operate in the aquifer. Controls on the distribution of water qualities reflect the ancient marine depositional environment, water-rock interaction, aquifer flushing including ion exchange effect, and the structural feature related to the Persian Gulf synclinorium. Key words: Limestone aquifer, saturation indices, brine water, dedolomitization, quality control, synclinorium. Introduction Saudi Arabia, Bahrain, United Arab Emirates, Qatar and Oman. It lies within an arid semi arid zone lacking Geography and Location renewable surface water; the only surface water are those The Arabian Peninsula, located in southwest Asia of the Tigris-Euphrates river system which become saline and with a population of 49 million, occupies approximately upon entering the Arabian Gulf. The State of Kuwait is 3,000,000 km 2. It includes the political units of Kuwait, located at the northwestern corner of the Arabian Gulf, 325

2 326 Palaeohydrogeology and Water Quality Control of the Tertiary Aquifer, Kuwait between latitudes 28 o o 30 N and longitudes 46 o o 30 E, and covers an area of about 18,000 km 2. Kuwait is bordered on the north by Iraq and on the south by Saudi Arabia. In general, dry lands are those regions which receive an average of less than 500 mm of rainfall each year (Williams, 2000). The maximum rainfall recorded in Kuwait was 216 mm, whereas the minimum value was 109 mm, with an annual average of 115 mm. Evaporation occurs all over the year, with an annual average of 265 mm. Therefore, precipitation is not considered a source of groundwater recharge. The areas under investigation are Al-Shagaya fields (Field-B, Field-C and Field-D), Al-Sulaibiya field and Umm-Gudair field penetrating the Eocene Dammam limestone aquifer, which is part of the most extensive limestone aquifer in the Gulf States. Moreover, a few groundwater samples from Burgan, Al- Rawdhatain and Sabiriyah oil fields were provided by Kuwait Oil Company (KOC) (Fig. 1). The objectives of this research are to study the hydrochemistry of the groundwater of the Dammam limestone aquifer throughout a thirty year period, , in order to identify the groundwater chemical and genetic types and to reveal the geochemical processes that are operating within the aquifer. Determination of mineral saturation indices (SI) for calcite, dolomite and gypsum is carried out in order to assess the degree of saturation of groundwater with respect to these minerals along the path flow. Eventually, the geological factors that influence the groundwater quality of the Dammam limestone aquifer from its recharge to discharge area are identified and interpreted. In the scope of this study, the geological and geochemical investigations of the Dammam limestone aquifer are mainly based on the collected data and reports from different sources. These sources include the Groundwater Administration of the Ministry of Energy (MOE) and the Kuwait Oil Company (KOC). The collected data of the studied fields were subjected to compilation, computation, analyses and interpretation. Measured field parameters included ph, temperature, electrical conductivity (E.C.) and O 2. Major element determinations were made by the MOE analytical laboratories: alkalinity (methyl orange), Cl (mercuric thiocyanate), NO 3 (hydrazine reduction to NO 2 ) by Figure 1. Location map of the study areas Figure 2. Structural elements map of Kuwait (Modified after Carman, 1996).

3 F. M. Al-Ruwaih, L.A. Talebi,.and J. Almedeij 327 autoanalyser colourimetry; total hardness (mg/l CaCO 3 ) and Ca 2+ by titrimetry (Mg 2+ was determined by difference); SO 4 by gravimetry (Fresenins et al., 1988). The chemical analyses of the major ions in the groundwater such as Ca 2+, Mg 2+, Na +, K +, HCO 3-, SO 4 and Cl -, expressed in mg/l, were converted to equivalent per million (e.p.m.) and to % e.p.m. by multiplying an appropriate conversion factor according to mathematical formulas (Todd, 2005). The accuracy of the chemical analyses for recording errors was checked carefully by calculating the cation anion balance percentage (Saether and de-caritat, 1997). The results of the chemical analyses of the studied fields have been plotted on a Piper trilinear diagram (1944) and the expanded Durov diagram (Lloyd and Heathcote, 1985) in order to determine the groundwater chemical types and to reveal the major geochemical processes prevailing in the Dammam limestone aquifer. Geotectonic Setting and Structure The Arabian Peninsula has two main geological components: the Precambrian Arabian Shield to the west, and the sequence of the overlying continental and shallowmarine sedimentary rocks of the Arabian Platform to the east. This sedimentary sequence consists mainly of sandstones and limestones, which outcrop in a great curved belt along the eastern margin of the shield, with the less resistant strata eroded into a series of lowland strips. Tectonically, Kuwait occupies the eastern extremity of the Arabian Shelf, and may be considered as marginal to the fore-deep zone of the unstable shelf bounding the Iranian orogenic belt (Beydoun, 1988). The eastern margin of the Arabian Gulf region has experienced complex structural evolution starting from Late Triassic to Palaeogene, followed by movements associated with Neogene Arabian plate motion. Pre-Neogene structures in Kuwait comprise north-south trending gentle domal features, faults and flexures, associated with a major structural zone characterized by repeated basement activation known as the Kuwait Arch (Fig. 2). This arch is considered the main structural feature of Kuwait and it trends through Kuwait from Saudi Arabia to southern Iraq. North-northwestern trending anticlinal and flexural structures also occur in the western part of Kuwait as offshoots of the Kuwait Arch, such as Umm-Gudair and Managish structures (Al-Sulaimi and Mukhopadhyay, 2000). Basins are located on both sides of the Kuwait Arch. The Dibdibba basin, which occurs to the west and the deeper northern part of the Arabian Gulf, mainly lies in the offshore area of Kuwait. Kuwait Arch structures existed as positive features during the Cretaceous and Tertiary periods, and were apparently subjected to notable uplift during the Late Cretaceous and Paleogene eras. Stratigraphy of the Dammam Aquifer The Tertiary - Quaternary sediments can be divided into two groups: the Kuwait Group and the Hasa Group (Owen and Nasr, 1958). The stratigraphy of the Dammam Formation had been studied in outcrops in Ahmadi Quarries in Kuwait, Saudi Arabia, Bahrain and Iraq (Al-Sharhan and Nairn, 1997). In Kuwait, the Dammam Formation is generally composed of Upper Eocene limestone and dolomite, with shale intercalation near the base and chart at the top. O Brien (1952) divided the Dammam Formation into three members from oldest to youngest. The lowest member ranges in thickness from 50 to 70 m and is composed of fossiliferous limestone with shale interlayers at the base and bituminous, banded, algal limestone interlayers towards the top. The middle member ranges in thickness from 30 to 40 m and consists of dark brown laminar, cherty and dolomitic limestone with lignite interlayers and vuggy, beige, fossiliferous and dolomitic limestone. The upper member is a friable, porous dolomite with occasional fossiliferous dolomite and chert intercalations, and the thickness of this member ranges between 60 to 90 m. Anhydrites of the Rus Formation together with the basal shale of the Dammam Formation act as an aquitard, separating the underlying Radhuma aquifer from the Dammam aquifer. Based on lithology, hydraulic properties and karstification, the middle part of the Dammam Formation represents the main aquifer in Kuwait (Al- Sulaimi and Al-Ruwaih, 2004). The lithological cross-section AA of some water wells located in the Dammam limestone aquifer in the direction of groundwater flow shows an increase in aquifer thickness toward N NE as shown in Figure 3.

4 328 Palaeohydrogeology and Water Quality Control of the Tertiary Aquifer, Kuwait Field/ Umm-Gudair Field Al-Shagaya Field-B Al-Shagaya Field-C Al-Shagaya Field-D Al-Sulaibiya Field Parameter Mean CV% Mean CV% Mean CV% Mean CV% Mean CV% TDS ph Ca Mg Na K Cl SO HCO NO log PCO SIcalcite SIdolomite SIGypsum Note: CV = Coefficient of variation, CV% = St. Dev. X 100 / Mean. Table 1. Summary statistics of characterisitc chemical analyses of the Dammam aquifer, Kuwait. Figure 3. Lithological cross-section (AA ) of water wells in the Dammam limestone aquifer in SSW - NE direction

5 F. M. Al-Ruwaih, L.A. Talebi,.and J. Almedeij 329 Hydrogeology The groundwater in the Arabian Peninsula is found in thick, high yield aquifers within the large sedimentary basins to the north, east and south of the Arabian Shield. Geological structures in the Arabian Plate control the geometry of the sedimentary basins and the hydraulic properties of major aquifers in the Arabian Gulf region. These basins contain groundwater of different ages, at variable depths, and with a wide range of salinity, hydraulic properties, and recharge mechanisms (Al-Sharhan et al., 2001). The water-bearing units in the Arabian Gulf can be divided into four aquifer groups or systems in the sedimentary rocks that gently slope towards the northeast (Burdon, 1973). These systems are: (1) The Palaeozoic System, (2) Triassic-Jurassic System, (3) Cretaceous system and (4) Tertiary-Quaternary System. Figure 4 shows the order of these systems in the region where the hydrogeological groundwater flow direction is from the southwest to the northeast. Tertiary-Quaternary system can be subdivided into two sub-systems: 1. The Eocene aquifer system. 2. The Neogene-Quaternary aquifer system. The Eocene aquifer system extends from the Dahna area in Saudi Arabia and consists of two limestone formations, the Radhuma and Dammam, which are separated by the anhydrite Rus Formation. This aquifer system is regionally confined except in its recharge outcrop area in Saudi Arabia and in the discharge areas of the Arabian Gulf coast. The recharge of the Dammam limestone aquifer occurs in the southern Iraqi desert, west of Kuwait and in Saudi Arabia (400 km south of Kuwait), where the exposures of the Dammam aquifer cover 1200 km 2 (Tleel, 1973). However, this aquifer system was recharged mostly during ancient periods, whereas the current recharge is weak. It is estimated that 80% of the lateral flow into the Dammam and the Kuwait Group aquifers comes from Saudi Arabia, and 20% from Iraq. The lateral outflow is directed eastwards to the Arabian Gulf and northward into Iraq (Mukhopadhuay et al., 1994). Discharge is indicated by the terrestrial springs upward leakage into the overlying aquifer and evaporation through coastal sabkhas. The Dammam aquifer has a higher Piezometric surface than that of the overlying Kuwait Group, and water moves upward from the Dammam aquifer, into the Kuwait Group aquifer. The transmissivity and storativity values of the Dammam limestone aquifer range from 22.4 to m 2 /day and 1 x 10-4 to 8 x 10-4, respectively (Qabazard, 2001). Figure 4. Schematic hydrogeological cross-section of the Easten Arabian Peninsula (after Al-Sharhan et. al., 2001).

6 330 Palaeohydrogeology and Water Quality Control of the Tertiary Aquifer, Kuwait Hydrochemistry of the Dammam aquifer The interpretations of the groundwater chemistry presented are based on a large archival data collected from the records of the Ministry of Energy (MOE) for 200 production wells penetrating the Dammam limestone aquifer of Al-Sulaibiya field, Al-Shagaya fields (field-b, field-c and field-d) and Umm-Gudair field, in addition to few groundwater samples provided by Kuwait Oil Company (KOC) for Al-Rawdhatain, Burgan and Sabiriyah oil fields (Fig. 1). A summary of the mean chemical analyses, including descriptive statistical calculations for each of the water well fields, is presented in Table 1. The Dammam groundwater has been classified by Omar et al. (1981) into three categories as in Table 2, reflecting differing degrees of salinity, as presented in Figure 5. and northeastern where salinity is generally > 8,000 mg/l. The sequence of cation dominance is Na + > Ca 2+ > Mg 2+ >K +. The water type is sulphate + chloride and changes to chloride +sulphate to the east and northeast. The salty water bounds the brackish from the north, northeast and east. The sequence of anion and cation dominance is Cl - > SO 4 > HCO 3 - and Na + > Ca 2+ > Mg 2+ >K +, respectively. The water type is chloride + sulphate. Brine water extends to the northeast of salty water, where the salinity reaches 190,000 mg/l as shown in Table 3. The brine water wells are indicated in Fig. 1. The studied groundwater samples exhibited three genetic water types: Na-SO 4, Mg-Cl and Ca-Cl. It suggests that the continental conditions promote the formation The water quality of the Dammam aquifer varies in salinity from brackish in the southwest of Kuwait to brine in the northeast. The brackish groundwater covers a large area of Kuwait and includes most groundwater well fields in Kuwait. The sequence of anion dominance is SO 4 > Cl - > HCO 3-, but changes to Cl - > SO 4 > HCO 3 - to the east Figure 6. Piper diagram representing the groundwater samples of Al-Shagaya Fields (Field-B, Field-C and Field-D), Al-Sulaibiya and some samples from Burgan, Al-Rawdhatain and Sabiriyah during the period Figure 5. Relative abundance of water quality and iso-salinity (mg/l) of the Dammam limestone aquifer (modified after Omer et al., 1981). Water type TDS range (mg/l) Cl - range (mg/l) Brackish Salty Brine ,000 10,000 50,000 50, , ,000 Table 2. Classification of the Dammam groundwater in Kuwait.

7 F. M. Al-Ruwaih, L.A. Talebi,.and J. Almedeij 331 of sodium-sulphate water by supplying soluble sulphate constituents to the water. The Mg-Cl type is generally indicative of marine environments and an evaporite sequence, while the Ca Cl genetic water type is associated with deep stagnant subsurface water. The brackish paleogroundwater of the Dammam aquifer has low values of δ 8 O (-4.5 per mil) and δd (-35 per mil), which reveal that these waters are related to the paleo-water from the cool wet period in eastern Arabia 11,000 to 60,000 years B.P. (Al-Ruwaih & Shehata, 2004). The plot of the chemical analyses of the Dammam aquifer during the last 30-year period, on Piper diagram, revealed that Ca 2+ and Mg 2+ exceed Na + and K +, and the strong acids Cl - and SO 4, exceed weak acids where the noncarbonate hardness exceeds 50%, whereas the groundwater samples of Al-Rawdhatain and Burgan located near its right vertex, the area of seawater in which the TDS >190,000 mg/l, indicate that the aquifer is occupied by brackish brine water (Fig. 6). At the same time, the groundwater samples on Durov diagram (Lloyd and Heathcote, 1985) show no dominant anions or cations and exhibit a simple dissolution or mixing process. Other groundwater samples represent the dominance of Cl -, and the groundwater may be related to reverse ion-exchange between sodium ions of the meteoric water and the magnesium and calcium ions of the groundwater, which give rise to Na + - Cl - waters. However, when the Cl - and Na + are dominant, this most probably indicates an end-point water (Fig. 7). Evolution of the carbonate system Saturation indices for various well fields were calculated using the speciation code WATEQ4F (Ball and Nordstrom, 1992). Calcite and dolomite mineral saturation (i.e. S.I. > 0) and undersaturation of gypsum predominates the carbonate system in all fields, as reflected in low values for the coefficients of variation (C.V. %) for ph, Ca 2+, HCO 3 - and log PCO 2 in comparison with other solutes (Table 1). The recharge area has a high total CO 2 and relatively low total dissolved solids. As the groundwater moves down gradient through the aquifer, it begins to dissolve calcite, dolomite and gypsum from the mineralogic framework. This results in an increase in the total CO 2 and increases in many of the other chemical constituents, particularly SO 4, Ca 2+ and Mg 2+. The Figure 7. Expanded Durov diagram representing the groundwater samples of Al-Shagaya Fields (Field-B, Field-C and Field-D), Al-Sulaibiya and some samples from Burgan, Al- Rawdhatain and Sabiriyah during the period Well No. / Burgan Sabiriyah Sabiriya h-wsw Parameter PH TDS Na K Ca Mg Cl SO4 HCO3 SIcalcite SIdolomite SIgypsum Al- Rawdhat ain Table 3. Chemical analysis of the brine water of the Dammam aquifer.

8 332 Palaeohydrogeology and Water Quality Control of the Tertiary Aquifer, Kuwait evolution of the carbonate system for the more saline-brine waters toward N-NE shows a general increase in Ca 2+ and Mg 2+. In addition to the fact that the concentrations of major cations and anions of the brine water in the direction of flow are above the seawater components (Table 3), this suggests a relatively long residence time of groundwater in the Dammam aquifer. The increase in SO 4 with Cl - also suggests mixing between brackish and saline source. Al- Ruwiah et al. (2005) indicated that the ion-exchange and reverse ion-exchange represented by Na + > Cl - and Cl - > Na + respectively, are the prevailing geochemical processes in the Dammam aquifer. In addition, the dedolomitization, as indicated by the gypsum dissolution and the calcite precipitation, and the carbonate weathering processes are most likely to control the water-rock interactions of the Dammam aquifer. Groundwater Quality Control The noticeable, steady increase in the groundwater salinity of the Dammam aquifer from southwest to the central part of the State of Kuwait, and then a sharp change in the salinity from brackish to salty, i.e. from 10,000 to 50,000 mg/l in a short distance, then followed by > 150,000 Country Age Lithology Environment Kuwait * Middle Eocene Shale, siliceous limestone and microporous limestone S a u d imiddle Limestone, dolomitic Arabia * Eocene Limestone, marl and shale Qatar * Middle Limestone, dolomitic Eocene Limestone, shale and chalk Bahrain* Middle Limestone (dolomitic and Eocene marl) marl and shale United ArabM-L Eocene Marly lime mudstone, Emirates * calcareous shale, nummulitic limestone Iraq * Upper Shale and dolomitized, Eocene microporous limestone Iran * Paleocene-Dolomitic limestone with L. Eocene bedded dolomite Subsurface formation Shallow marine Shallow water carbonates with subordinate fine clastics Shallow marine subtidal Subtidal, intertidal and slightly open marine Shallow marine shelf * Outcrop formation Shallow marine neritic Shallow marine restricted Table 4: The Correlation of the Dammam Formation in the Arabian Gulf Countries and Iran (compiled with modification from Al-Sharhan and Nairn, 1997). Countries Kuwait Saudi Arabia Qatar Bahrain United Arab Emirates Age (years) 22,000 16,000-20,000 10,000-17,000 13,000 Modern - 15,000 Oman Table 5: Groundwater age correlation of the Dammam limestone aquifer in the Arabian Gulf Countries according to stable and radioisotopes measurements (compiled from Yurtsever, 1999 and Al-Sharhan et al., 2001). < 10

9 F. M. Al-Ruwaih, L.A. Talebi,.and J. Almedeij 333 mg/l toward the north and northeast, will be discussed in order to explain the probable factors that might induce and cause these changes of groundwater quality. These factors are clearly related to the regional stratigraphy, structural geometry and formation characteristics. Al-Ruwaih (2001) and Al-Ruwaih et al. (2004), the Dammam aquifer exhibited three main genetic water types: Na-SO4, Mg-Cl and Ca-Cl represent a continental marine environment, and the Dammam Formation in Kuwait is related to the paleo-water from the cool wet period in the eastern Arabia 11,000 to 60,000 years B.P. Based on the regional stratigraphy of the Gulf Therefore, groundwater age correlation of the Dammam Cooperation Council (GCC), the Dammam Formation aquifers in Kuwait, Saudi Arabia, Qatar, Bahrain, United shows a similarity in age, lithology and depositional Arab Emirates and Oman, as presented in Table 5, shows environment, as shown in Table 4. According to Al-Sharhan that the Kuwait s Dammam groundwater is the oldest in et al. (2001), the lithostratigraphy of the Tertiary sediments the Gulf region. as shown in Fig. 8, indicate a major lithofacies variation, both laterally and vertically. In the Arabian Gulf region, the Based on the structural geometry -- the Dammam Tertiary sediments are made up of limestones, dolomites Formation is of Pre-Neogene age -- it should be affected and evaporites. Although the Oligocene sediments were by the Pre-Neogene structures. The Pre-Neogene removed by erosion due to a worldwide drop in sea level, unconformity, separating the underlying Dammam some areas remained submerged and Oligocene sediments Formation from the overlying Neogene sediments, is the crop out in Abu Dhabi and Oman and in the subsurface in most important erosional surface in the Tertiary sequence offshore United Arab Emirates. of the Arabian Gulf. The configuration of the top of the Dammam Formation (Fig. 9), shaped by this unconformity, According to the groundwater genesis study is considered to be the most influential single factor in carried out by Yurtsever (1999), Al-Sharhan et al. (2001), the determination of the distribution and thickness of Figure 8. Lithostratigraphic chart of the Paleogene-Neogene Formations in the Arabian Gulf and adjoining areas (after AlSharhan and Nairn, 1995).

10 334 Palaeohydrogeology and Water Quality Control of the Tertiary Aquifer, Kuwait the overlying younger clastics and, to some extent, their lithofacies. This erosion causes a direct contact between the Dammam Formation and the anhydrite Rus Formation, Figure 9. Structrual contour map of the top of the Dammam Foundation (derived from the KOC seismic data, 2003). which could be a source of mixing the Radhuma Formation water with the Dammam Formation water by upward leakage through the Rus Formation This increases the salinity of the Dammam limestone aquifer. Moreover, Kuwait is considered as marginal to the fore-deep zone of the unstable shelf bounding the Iranian orogenic belt of Late Cretaceous and Tertiary period Alpine belt (Al-Sulaimi and El-Rabaa, 1994 and Mina et al., 1967). Maurice (1970) described the whole region as a Persian Gulf Synclinorium, which can be defined by the outcrops of the corners. Kuwait and Saudi Arabia are sited at the western limb of the Persian Gulf Synclinorium. The Dammam Formation is outcropped in northeast of Saudi Arabia and in southwest of Iran. According to Al-Sharhan et al. (2001), the outcrop of the Dammam Formation in Saudi Arabia has a salinity level of less than 1000 mg/l, and according to the Iranian Report No. W/55436, 1995, the quality of the Jahrum Formation, which is equivalent to the Dammam Formation in Kuwait, also showed a salinity level less than 1000 mg/l. Figure 10. Cross-section of the Persian Gulf synclinorium (Modified after Maurice, 1970).

11 F. M. Al-Ruwaih, L.A. Talebi,.and J. Almedeij 335 The structure of the synclinorium is presented schematically in Figure 10. The Tertiary fore-deep facing the frontal fold tilted eastward of the sediments on the platform. So, the groundwater in the Dammam aquifer is trapped causing a sluggishness of water movement, which increases the possibility of water rock interaction and thus hinders any recharge to the aquifer from the Iranian side. Therefore, one can suggest that the changes in groundwater salinity in Kuwait along the path flow (SW-NE) are affected by the pre-neogene unconformity, as well as the Persian Gulf synclinorium structure. Furthermore, the effect of the formation characteristics of the Dammam aquifer on its water quality, have been investigated through the wells K1, K2, K3, K4 and K5 along the path flow from SW to NE direction. The geophysical logs of Gamma Ray (GR), Resistivity, Compressional Sonic travel time (DT) and Neutron logs of these wells were studied (Fig. 11). It is found that the resistivity values vary from 10 to 40 ohm-m in the southwest wells (K1 and K2), indicating a brackish water in which salinity ranges from 2,000 to 8,000 mg/l as NaCl equivalent. And the resistivity decreases gradually to the northeast direction to reach ohm-m in K5 area, suggesting a salinity of water greater than 170,000 mg/l as NaCl equivalent. Two highly porous sections designated as porous zone-i and porous zone-ii and are correlated in the cross-section were identified as shown in Table 6. The porosity has been determined by Wyillie equation (Helander, 1983): φ = t (log) - t (matrix) / t (fluid) - t (matrix) (1) where φ = fractional porosity t (log) = compressional wave transit time measured from the log (µsec/m). Well K1 K2 K3 K4 K5 Dammam top (m) Dammam Bottom (m) Porous Zone I (pu) Porous Zone II (pu) From To From To Table 6: Tops and bottoms of the Dammam Formation well logs with the porosity zones Figure 11. Sub-surface geophysical logs of the Dammam aquifer

12 336 Palaeohydrogeology and Water Quality Control of the Tertiary Aquifer, Kuwait t (matrix) = matrix transit time (limestone transit time = 14.5 (µsec/m). t (fluid) = transit time of the fluid saturating the porous media (water transit time = 57.9 (µsec/m). It is clear that the formation porosity decreases along the path flow from SW to NE, indicated from the remarkable decrease in the calculated values from K1 (55-58 pu) to K5 (30-45 pu). Moreover; the permeability has been calculated by the following equation (Serra, 1984): K (MD) = Exp [3 + (0.15 x porosity x 1000)] (2) where, K is the permeability applied in Equation (2) when the porosity is expressed in fraction form. Table 7 shows that the permeability decreases from SW to NE. However, besides the decrease of porosity and permeability, the saturated thickness of the Dammam Formation increases along the trend line, from K2 to K4 areas and dipping toward the northeast. Thus, the decrease in porosity and permeability and the increase of the saturated thickness of the Dammam Formation along the flow direction may minimize the ability of the groundwater flow in the aquifer, thus creating a stagnant zone of high salinity. Well Zone Resistivity* Porosity Permeability (ohm-m) (pu) (Darcy) K1 I II K2 I II K3 I II K4 I II K5 I II Resistivity = Directly from the log Table 7. The resistivity, porosity and permeability values of Zones I and II in the studied wells. Conclusions This study reveals that the Dammam aquifer exhibited a distinct zonation of water qualities of brackish, salty and brine, where the salinity ranges from 2,500 to 190,000 mg/l. This creates further limitations on the water resources development of the aquifer. The dominant characters of the groundwater chemistry of the Dammam limestone aquifer during the last 30 years are the prevalence of groundwater chemical types Na 2 SO 4, CaSO 4 and NaCl. Most of the studied groundwater samples show that the groundwater undergoes a simple mixing process or dissolution where Na + and SO 4 are the major cations and anions, respectively. The presence of genetic water types Na SO 4 of continental origin and Ca Cl of marine deep stagnant conditions suggests that the native ancient marine groundwater of the aquifer has been flushed and mixed with meteoric water through the processes of infiltration during the Pleistocene age. The groundwater is related to the paleo-water from the cool wet period in the eastern Arabia 11,000 to 60,000 years B.P. It is found that dolomitization, ion-exchange and carbonate weathering are the prevailing geochemical processes. The presence of gypsum disseminated throughout the limestone permits the process of dedolomitization to be a controlling factor of the chemical character of present-day groundwater. Calcite mineral saturation predominates the carbonate system in all fields, with incongruent dissolution of the low-mg calcite. However, change of the groundwater quality of the Dammam limestone aquifer, from SW toward N-NE from brackish brine, is explained by its genesis, as the aquifer is deposited in a shallow marine environment. The combined effect of the pre-neogene unconformity and the regional Persian Gulf Synclinorium structure is to prevent the groundwater from being discharged or moved. Finally, it is suggested that this decrease in the porosity and permeability and the increase of the saturated thickness of the aquifer downgradient are the influential factors in quality control. Acknowledgements We are deeply thankful to the Ministry of Energy

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