Hydrochemical evaluation of the Umm-er-Radhuma aquifer system, Saudi Arabia

Transcription

1 Hydrochemistry (Proceedings of the Rabat Symposium, April 1997) IAHSPubl.no. 244, Hydrochemical evaluation of the Umm-er-Radhuma aquifer system, Saudi Arabia A.M. AL BASSAM KingSaoud University, Riyadh 11495, Saudi Arabia MX HUSSEIN International University of Africa, P.O. Box 2469, Khartoum, Sudan M.A. SHARAF King Abdul Aziz University, Jeddah 21441, Saudi Arabia Abstract The Umm-er-Radhuma (UER) aquifer system is one of the most important reservoirs in the Arabian Peninsula. Several Arab Gulf states depend on the UER aquifer both for domestic and agricultural water supplies. The hydrogeological and hydrochemical characteristics of the UER aquifer in Saudi Arabia were evaluated. Recharge during this study was estimated to range from 10 to 15 mm year" 1. A complete hydrochemical study was conducted including the discussion of aerial distribution of the major ionic constituents, the regional evaluation of hydrochemical facies showed seven different water types, along with the investigation of the equilibrium situation, and the interpretation of isotopic data. BALANCE and PHREEQE computer programs were used to study the hydrochemical reactions responsible for observed water-quality variations. The simulation suggested that water quality is mainly controlled by hydraulic properties and sulphate minerals. INTRODUCTION The Umm-er-Radhuma (UER) aquifer system has been the subject of several geological and hydrogeological investigations since the early 1940s. Some of the previous research is summarized by Powers et al. (1966), Sogreah (1968), Watuki (1968, 1971) Italoconsult (1969), Dinçer et al. (1974), BRGM (1976, 1979), GDC (1979), Shampine et al. (1979), and Al Bassam (1983, 1987). In this paper, the hydrogeological and hydrochemical variations and facies in UER aquifer are evaluated. The purpose of this paper is to assess the hydrochemical variations of the UER aquifer system using reaction-path and mass-balance techniques. Specifically, PHREEQE and BALANCE computer programs were used for hydrochemical simulations. METHODS Site Characteristics The Umm-Er-Radhuma (UER) aquifer is located in the eastern part of Saudi Arabia, an area explored by ARAMCO oil drilling program (Fig. 1). It is an important aquifer system used for public water supply, irrigation and industry in the Arabian Peninsula.

2 14 AM. Al Bassam et al. Generally, the area is characterized by an arid to hyper-arid climate. Rainfall is scant and irregular and averages less than 100 mm year. Summer temperatures frequently exceed 48 C. Wind gusts of up to 100 km h" 1 have been recorded in the study area. Air masses move into the area primarily from the west and north. Fig. 1 Location map of the study area. Hydrochemical data for 160 boreholes distributed throughout the study were used in the simulations (Fig. 2). Climatological data were obtained from five meteorological stations including those at Riyadh, Hofuf, Harad and Jubayl (Fig. 2). For groundwater, the chemical parameters and constituents analyzed include salinity, alkalinity, ph, electrical conductivity (EC), total dissolved solids (TDS), and concentrations of major ions (Ca 2+, Mg 2+, Na +, K +, CI", S , C0 3 2 ', HC0 3 ") and minor ions (B, F). Standard chemical methods were used. Rainfall and runoff water quality also were evaluated to assess recharge quality. General Geology The UER aquifer is a hydraulically connected multi-layer groundwater system with groundwater flow occurring across layers; confining layers are either absent or significantly fractured to allow vertical flow. The hydraulic system consists of four different geological formations (Aruma, Umm-Er-Radhurna, Rus and Dammam) which range in age from Upper Cretaceous to Eocene. The lower part of the UER Formation rests conformably on the Aruma Formation, and the Rus Formation overlies conformably the UER. The base member of the Dammam Formation rests confromably on the chalky limestone of the Rus. The Aruma limestone crops out to the west of the study area in a belt trending northsouth. Its lithology is laterally uniform and consistent and is often locally fractured and highly permeable. The UER Formation crops out as a belt extending from the south (latitude 'N) to the Iraqi-Saudi Arabian borders, a distance of about 1220 km (Fig. 2) and consists of a repetitive series of light colored foraminiferal micritic and calcarenitic limestone, dolomitic limestone and dolomite. Limestone is progressively replaced by dolomite from south to north (GDC, 1979). The formation shows

3 Hydrochemistry of the Umm-er-Radhuma aquifer, Saudi Arabia 15

4 16 AM. Al Bassam et al. significant faciès changes from the north to the south. In the north, the deposits are of relatively deep-sea type consisting of fine-grained limestone often argillaceous, with gypsum and anhydrite beds near the top. In the centre, it contains calcarenitic beds, and in the south, porous detrital limestone and calcarenite with uniform thickness predominate. The fracture porosity of the formation in the south is higher than in the north, and intergranular permeability is low in all areas. Water is mainly transmitted through secondary porosity which is thought to be associated with dolomitization. The Rus Formation consists of light colored, soft chalky limestone and marl. In the subsurface, it is highly variable lithologically and in thickness. Generally, it is white, compact, fine crystalline calcite interbedded with shale and limestone. The variation in thickness is due to changes in the anhydrite content. The Dammam Formation crops out in widely scattered patches in the study area. It was divided into five members, generally consisting of light-colored limestone, marl and shale. The upper two members (Alat and Khober) consist of limestone, dolomitic limestone, marly limestone and marl. The middle (Alveodina) member consists of a thin bed of soft porous limestone. The two lower members (Saila and Midra) consist of shale and marl. Hvdrogeological Properties The piezometric surface for the UER aquifer follows the trend of the outcrop (Fig. 3) with a regional easterly hydraulic gradient. There are quite large variations in the gradient. In the west near the outcrop, the gradient is steep (1:200) and more gradual in the south (1:2000). The regional gradient is distorted by several groundwater mounds or ridges. These changes in the hydraulic gradient can be explained by variations in the horizontal permeability and transmissivity and by multilayering and associated vertical flow. These latter observation indicates the difficulty in attempting to describe a three-dimensional phenomenon by two-dimensional diagrams. The UER aquifer permeability ranges from 4. Ox 10" 5 to 1.1 x 10" 2 m s" l, and the large variation reflects local and regional variations in lithology and weathering. Vertical permeablities range from 4.4xl0" 8 to 3.2xl0" 2 m s" 1. Previously reported transmissivity varies markedly ranging from 4.4xl0" 3 to 1.5xl0 _1 m 2 s" 1 (BRGM,1979), from 7xl0" 5 to 6.2X10" 1 m 2 s" 1 (GDC, 1979), from 10~ 5 to 10" 3 (BRGM, 1976) and from 1.5xl0" 2 to 1.3xl0 _1 m 2 s" 1 (Italoconsult, 1969). Generally, hydrogeologic properties of the UER aquifer are controlled by the lithology of the formation and development of secondary features, such asfissuresand solution voids. RECHARGE ESTIMATION Recharge to the UER aquifer system has been estimation in other studies (Sogreah,1968; Italoconsult, 1969; Dinçer et al, 1974; BRGM, 1979; GDC, 1979). Each of the previous estimates were not comprehensive enough to account for all flows in the study area. In our view, previous recharge estimates were exaggerated. Using the water budget technique, recharge was estimated by considering average daily pan evaporation, number of rainy days, actual evaporation and runoff. Recharge was estimated to be mm year" 1, very little of which was would actually reach the saturated zone. Most of the groundwater recharge occurs from infiltration of runoff through concentrated wadi flow (GDC, 1979) or from rapid infiltration through the sand dunes (Dinçer et al, 1974)

5 Hydrochemistry of the Umm-er-Radhuma aquifer, Saudi Arabia 17 Fig. 3 Piezometric contour map of UER aquifer system. GROUNDWATER CHEMISTRY The regional UER groundwater chemistry is rather straight forward. Groundwater in the outcrop area has low salinity ( mg l" 1 ) having a high proportion of Ca 2+ and HC0 3 ". In the southern part of the outcrop area, salinity is higher and dominated by Na + and CI". At the downgradient middle zone, between the outcrops and the coastal area, salinity ranges from 1000 to 4000 mg l" 1. Several tongues of less saline water extend towards the east and northeast, these tongues represent major flow zones within the aquifer. The flow zones are controlled by lithological and structural factors that influence both the movement and salinity of the groundwater. The dominant anions are SO4 2 " and CI". In the eastern part, salinity is high ranging from 4000 to mg l" 1 and the dominant ions are Na + and CI". The high salinity within this zone is believed to be a mixture of old meteoric water and an older connate water rather than seawater intrusion (GDC, 1979).

6 18 A.M. Al Bassam et al. AREAL VARIATIONS The areal variations of the groundwater quality and the major ionic constituents of the UER aquifer system are illustrated on Fig. (2). In general, Ca 2+ concentrations increase from the outcrop area eastward to the coast. Tongues of low Ca 2+ concentrations coincide with low salinity zones. In the north, Ca 2+ concentrations increase markedly, and the increase might have resulted from the vertical movement of water through fractured anhydrite and gypsum of the Rus Formation. Ca 2+ concentrations increase markedly as well near the coastal area and in the northeast. Areal distribution of Na + concentrations is shown in Fig. 2. Near the outcrop area, Na + concentrations increase markedly to 200 mg l" 1 within a few kilometers. To the east and northeast, low Na + concentrations coincides with low salinity and collapse zone. In the coastal area and towards the northeast, Na + concentrations increase markedly, and the increase may have been caused by seawater intrusion due to extensive pumping in the area. Chloride concentrations range from 18 mg l" 1 in the outcrop area to > mg l" 1 near the coast. Chloride concentrations coincide with those of Na +. Recent recharge can be delineated in the central outcrop area by very low CI" concentrations. The absence of recharge, as occurs in the south, is shown by high CI" concentrations, e.g., near the outcrop area. For SO4 2 " concentrations, the study area can be divided into halves. Groundwater in the northern half is enriched with SO4 2 " and saturated with respect to gypsum and anhydrite. Whereas, groundwater in the southern half is marked by relatively low SO4 2 " concentrations near the outcrop area, which increase further south to more than 1800 mg l" 1. The area where S0 4 2 " concentrations increase coincides with the eastern limit of the collapse zone (Fig. 2). Bicarbonate concentrations show no pattern despite large variability in the aquifer. Carbonate is seldom found in the water because ph is too low (<7.5). Boron concentrations generally range from 0.3 to 1.0 mg l" 1, average less than 0.5 mg l" 1, and increase towards the coast to about 2.0 mg 1". Fluoride concentrations generally range from 1 to 3 mg l" 1 and increase near the coast to about 5 mg l" 1. VERTICAL VARIATIONS In addition to the spatial sampling, the UER aquifer was sampled at different depths (GDC, 1979). Hydrochemical results indicate that the groundwater is not vertically homogeneous. Salinity increases with depth from less than 2400 to about 3100 mg l" 1 (Fig. 4a). The salinity increase may be caused by an increase in temperature and a lithologie and associated permeability change with depth. Figure (4b) shows an opposite pattern where the salinity decrease from about 3600 to 3400 mg l" 1 with mainly a decrease in Ca 2+, S0 4 2 " and Mg 2+. Sodium and CI" concentrations increase with depth. A logical explanation is that the source of salinity for the top of the aquifer results from the vertical transfer of water from the overlying aquifers through the Rus anhydrite. This will result in increasing the concentration of Ca 2+ and S0 4. Another explanation is the increase in permeability with depth which results in a rapid movement of groundwater through lithologies containing less S0 4 2 " bearing minerals. The decline in Mg 2+ concentrations may indicate mixing with a sodium chloride water flowing through the aquifer.

7 Hydrochemistry of the Umm-er-Radhuma aquifer, Saudi Arabia 19 INCREASE IN! DEPTH ] ^ m 3 c a cc _ o o o en, "o epih Q en a Q. E a if) ^Vii <$r i./^ ' '5 S l cr ' I a I cr 1 UJ i 3! * i e 1 J oèi^^î; r o j -C a J Dl i S I "Q. e 1 a I to - Total dissolved solids lmg/1), BICARBONATE X 1 i! I i,,_!, Total dissolved solids! mg/l) Fig. 4 Vertical variations in groundwater salinity (a) borehole PN06-U, and (b) borehole PM01-U. WATER TYPES Durov's diagram was used to classify water types in the UER aquifer system (Fig. 5). Accordingly, groundwater was classified into seven major types among which calcium sulphate and sodium chloride are the most dominant faciès. The areal extent and distribution of these facies are shown on Fig. 6. The distribution of water types is explained as follows:

8 20 A.M. Al Bassam et al. Fig. 5 Burov's diagram. Type A: is dominated by Ca 2+ and bicarbonate concentration. It is believed to be of a recent recharge origin as supported by the radiocarbon data. Type B: is dominated by Na + and CI" with Ca 2+ becoming increasingly important. It is believed to be old recharge water that recharged during periods of high precipitation. The near absence of recharge recently is the reason for not flushing the water through the system. Type D: is dominated by Ca 2+ and S0 2 4 " which might be the result of gypsum and anhydrite dissolution from the Rus Formation, or from the UER Formation as it becomes increasingly gypsiferous. Type E: is dominated by Ca 2+ and S0 2 4 " with Na + and CI" concentrations becoming increasingly important. This water may be a mixture of sodium chloride water and calcium sulphate water. Type F: is dominated by Na + and S0 2 4 ". It may have been derived from water type E by precipitation of calcium carbonate or dolomite, or by Ca 2+ exchange as the formation becomes dominated by argillaceous limestone. Type G: is dominated by Ca 2+ and CI" which might result from ion-exchange as the CI" exceeds Na + concentrations by as much as 5 meq l" 1. Type H: is dominated by Na + and CI"; its presence near the coast is attributed to mixing of old recharge water and older connate water.

9 Hydrochemistry of the Umm-er-Radhuma aquifer, Saudi Arabia 21 This water classification does not reflect the vertical variations in water quality which might be very large as noted above. ISOTOPES Isotope data were acquired from the work by the GDC (1979) and includes 14 C, B C, 18 0, H (D), and 3 H. Some spatial variations are noteworthy (Fig. 7). For example, 14 C decreases from the outcrop towards the coast indicating that the relative age of groundwater increases towards the discharge area. However, several anomalies exist where old groundwater occurs at or near the outcrop area and relatively modern groundwater occurs near the coast. Also, 18 0 and D become more negative away from the outcrop. Tritium concentrations in groundwater in the outcrop area are generally less than 3 TU, but concentrations ranging from 3 to 10 TU have been reported in Hofuf and Jabrin areas suggesting downward leakage of recent recharge through the overlying layers. MASS-BALANCE AND REACTION PATH SIMULATION Seven hydrochemical cross-sections were simulated using mass balance and reaction path techniques (Fig. 2) The cross sections were chosen to represent changes in groundwater chemistry along the direction of flow. The calculations were performed in the following sequence: (a) BALANCE (Parkhurst et al., 1982) was used to calculate the amount of phases entering or leaving solution and the end members contributions along the flow path. (b) PHREEQE (Parkhurst et al., 1980) was used to check the thermodynamic viability of these reactions. A combination of four phases/reactions were used to calculate the amount of the phase that must enter or leave the solution to produce the observed areal variations. The phases or reactions include calcite, dolomite, gypsum, ion-exchange, de-dolomitization, kieserite and redox reactions. Kieserite (Mg SO4.H2O) has been used along with de-dolomitization as the most probable source of Mg, because Mg 2+ concentrations fluctuate in the profile whereas equilibrium calculations suggest that groundwater is saturated with respect to dolomite. Kieserite is probably not the actual mineral controlling the reaction due to the quantity required, but in the absence of halite, it is suggested because the model is unable to accurately model mixing of different end member solutions. De-dolomitization occurs as Ca 2+ -rich water moves downgradient through dolomitic rocks and Mg 2+ from the rock matrix replaces Ca 2+ in solution. Consequently, the Mg to Ca ratio of groundwater increases. Kieserite and de-dolomitization have been used alternatively in the simulation, depending on the Ca 2+ and SO4 2 " variations, to produce the Mg 2+ source. Because CI" concentrations exceed Na + concentrations in most samples, ion-exchange was used to control Na + losses or gains. Calcite dolomite and gypsum are the primary mineral phases of UER aquifer lithology. Linear mixing was used wherever results indicate mixing due to horizontal movement of groundwater, otherwise results were based totally on mass-balance calculations and chemical reactions regardless of the hydrodynamic situation. Results of the simulations for each of the seven sections (Fig. 2) and are consistent with interpretation discussed previously. Sections 2, 3 and 7 are from the southern part

10 22 A.M. Al Bassam et al.

11 Hydrochemistry of the Umm-er-Radhuma aquifer, Saudi Arabia 23 ' -<>-

12 24 A.M. Al Bassam et al. of the study area, Sections 1 and 4 are from the central part and sections 5 and 6 are from the northern part. In the south, the Ghawar anticline exerts lithologie and structural controls on the hydrochemical variations in groundwater. The high salinity zone near the outcrop could have resulted from low permeability, and/or lack or infrequent recharge. Salinity decreases downgradient due to mixing with more dilute freshwater moving downward from the overlying aquifers. The major chemical reactions in the southern part are precipitation of carbonates, dissolution of SO4 2 " minerals and de-dolomitization. Ion exchange may occur in thin shale horizons. Towards the east, dissolution evapprites dominate and mark the eastern limit of the collapse zone and beginning of a reducing environment. In the central and northern areas, groundwater begins as a calcium bicarbonate type and is undersaturated with respect to calcite, dolomite and gypsum. Evaporite and argillaceous faciès become more available for dissolution downgradient, gypsum continues to dissolve resulting in oversaturation with carbonate minerals, and calcite and dolomite begin to precipitation causing Ca 2+ and Mg 2 " 1 " to exchange with Na +. These processes cause the water type to change from calcium bicarbonate to sodium sulphate. As the water moves further downgradient, salinity decreases due to the decrease in Ca 2+, SO4 and Mg concentrations. The salinity decrease may have been caused by three processes: (a) precipitation of S0 2 4 " minerals, (b) reduction of SO4 2 " by bacteria, and (c) mixing with more dilute freshwater. Further to the east, groundwater begins to mix with highly saline water probably of connate origin. CONCLUSIONS The data analysis herein suggests that present-day recharge to the UER aquifer is restricted to the central part of the outcrop area. Furthermore, hydrochemical results suggest recharge does not occur appreciably in the southern part of the study area. Groundwater in the UER aquifer shows significant areal and vertical variations which can be subset into seven different water types. Calcium sulfate and sodium chloride types are dominate. Groundwater is saturated with respect to carbonate minerals, except in the central outcrop area, and is saturated with respect to SO4 2 " minerals only in the north-central study area. The simulation of the hydrochemical reactions suggests that the water quality is mainly controlled by hydraulic properties of the system and by the presence of S0 4 2 " minerals within the UER or in the overlying Rus Formation. Several other minor reactions occur within the aquifer, but are either restricted to a specific area or to a specific horizon of the aquifer. These reactions include ion-exchange, dedolomitization and redox. Hydrogeologcial and hydrochemical evidence suggest the occurrence of vertical flow between the UER and the adjacent aquifers. Thus the present attempt to simulate the hydrochemical variations within the UER aquifer can only be considered a crude approximation. However, with additional hydrogeological understanding and chemical data, these methods could prove extremely useful in identifying the more comprehensive suite of hydrochemical processes controlling water quality of throughout the UER aquifer system. Acknowledgments The authors would like to thank King Saoud University for funding this research, and Birmingham University for providing all facilities. Specials thanks are due to professor Lloyd, for general help and advice; his efforts made this project possible.

13 Hydrochemistry of the Umm-er-Radhuma aquifer, Saudi Arabia 25 REFERENCES Al Bassam, A.M. (1983) A quantitative study of Harad Wellfield, Umm-Er-Radhuma Aquifer.M.Sc. thesis, Ohio University, U.S.A Al Bassam, A.M. (1987) Hydrochemical computer modelling in groundwater related problems.ph.d thesis, University of Birmingham, U.S.A Backiewicz, W., Milne,D.M. & Noori, M. (1982) Hydrogeology of the Umm-Er-Radhuma Aquifer, Saudi Arabia with reference to fossil gradients. Quart. Jour. Eng. Geol. 15(1), BRGM (1976) Hydrogeological investigations of the Al Wasia Aquifer in the eastern province of Saudi Arabia. Unpub.report, Ministry of Agriculture & Water, Kingdom of Saudi Arabia. BRGM (1979) New data on groundwater resources of Al Kharj area.unpub. report. Ministry of Agriculture & Water, Kingdom of Saudi Arabia. Burdon, D.J. (1982) Hydrogeological conditions in the middle cast. Quart-Jour-Eng. Geol. 15(1), Dinçer, T.A.L., Mugrin, A. & Zimmerman, U. (1974) Study of the infiltration and recharge through the sand dunes and arid zones with special reference to the stable isotopes and thermonuclear tritium. J. Hydrol. 23, GDC (1979) Final draft of Umm-Er-Radhuma study. Unpub.report. Ministry of Agriculture & Water, Kingdom of Saudi Arabia. Iltaloconsult (1969) Water and agricultural development studies for area TV.Kingdom of Saudi Arabia. Unpub. report. Ministry of Agriculture & Water, Kingdom of Saudi Arabia. Parkhurst, D.L., Plummer, L.N.& Thorstenson, D.C. (1982) BALANCE: A computer program for calculation of chemical mass-balance. U.S. Geological Survey Water-Resources Invest Parkhurst, D.L., Thorstenson, D.C. & Plummer,L.N. (1980) PHREEQE: A computer program for geochemical calculations. U.S. Geological Survey Water-Resources Invest Powers, R.W., Ramirez, L.F. Redmond, CD. & EL Berg, E.L.(1966) Geology of the Arabia Peninsula. Sedimentary geology of Saudi Arabia. U.S. Geological Survey Prof. Paper 560D Shampine, W.J., Dinçer, T. & Noory, M. (1979) An evaluation of isotope concentrations in the groundwater of Saudi Arabia, In Isotope Hydrology, IAEA, Vienna, Austria Sogreah, (1968) Water and agricultural development studies, area V. Final report, Ministry of Agriculture & Water, Kingdom of Saudi Arabia. Watuki, G.M.G.H. (1968) Preliminary study of Umm-Er-Radhuma Aquifer Unpub. report Ministry of Agriculture & Water, Kingdom of Saudi Arabia. Watuki, G.M.G.H. (1971) Final report of water and wells, Faisal settlement project, Haradh. Unpub. report, Ministry of Agriculture & Water, Kingdom of Saudi Arabia.