Seventh Conference of Nuclear Sciences & Applications 6-10 February 2000. Cairo, Egypt En-10 Geochemical Characteristics and Environmental Isotopes of Groundwater Resources in some Oases in the Western Desert, Egypt EG0100098 M.S.Hamza, M.A.Awad, S.A.El-Gamal and M.A.Sadek Siting and Environmental Department, National Center for Nuclear Safety and Radiation Control, Atomic Energy Authority, 3 Ahmed El-Zomor St., Nasr City-11762, B.O.BOX 7551, Cairo-Egypt. ABSTRACT A study has been conducted using hydrochemistry and environmental isotopes (deuterium, oxygen-18 & carbon-14) on the Nubian Sandstone aquifer which underlies the Western desert oases. The concerned three oases (El-Farafra, El-Dakhla and El-Kharga) covers an area about 8000 km2 from the total area of the Western Desert. Seventy one water samples were collected from these three oases and subjected to both chemical and isotopic analysis to evaluate their groundwater resources. The hydrochemical data of these water samples reveals that thier salinity doesn't exceed 500 mg/I as well as the presence of marine and meteoric water types with different percentage. The mineralization of the investigated groundwater may be evoluated under flusing the original marine water entrapped between the pores of the aquifer matrix by meteoric water which is furthely modified through leaching, dissolution, cation exchange and oxidation-reduction processes. The investigated groundwater indicates some sort of quality hazards for drinking and domestic purposes due to the high concentration of both iron (agverage 6 mg/1) and hydrogen sulphide (average 2.5 nig/I ) relative to WHO standard. This water can be used safety for all kinds of livestocks. About 95% of the these groundwater have low to medium salinity hazards and low sodium content and can be used for irrigation. The mean geothermal gradient with depth is measured in two wells in El-Kharga and found to be slightly higher than the natural one, it is equal to about 1.4° C/l 00 m. The base tempetature was calculated by two cation geothermometers (K-Mg) and (Na-K-Ca) that indicate a generally higher temperature due to continuous rock-water interaction and/or mixing with old connate water which having assembly acquiring higher cationic temperatures. The corrosivity which noticed in some wells may be related to combined effect of high temperature of water, high concentration.of II2S and CO2 and low pH. The environmental isotopes data of the investigated groundwater samples reveals that theniain recharge source for the Nubian Sandstone aquifer is in situ precipitation in much cooler and humid climate thousands of years ago, since the carbon-14 age for the shallow and deep zones of six wells collected from these oases is > 20,000 years. Key Words : Environmental Isotopes/Geochemistry / The Western Desert Oases/ Nubian Sandstone Aquifer INTRODUCTION The River Nile is considered the main source of fresh water that covers the water demands for Egypt's population. Due to rapidly growth of population groundwater became an important integral part of the national policy of Egypt. 981 The Western desert oases have been inhabited since pharonic time (2000 B.C.)- The development of Nubian Sandstone aquifer in these oases has made to cultivate intensive large low land areas depending on the natural flowing water of this aquifer. This flowing water has ceased in many wells in the area due to continuous exploitation over the past years and thus pumpage and water lifting started in some of these oases as a result of declines of the piozometric pressure with time. So, it is necessary to identify the replenishment, flow dynamics, recharge mechnisms and the mean residence time of the groundwater in this vital aquifer using hydrochemical and environmental isotopes (I8O, D,1^) techniques. LOCATION The investigated area extends for about 8000 Kin2 between latitudes 24° 15 and 27° 30 N and between longitudes 27° 31 and 30° 27 E. It comprises three of the main oases in the Western Desert ( El-Kharga, El-Dakhla and El-Farafra ),as shown in Fig. (1). The climate of the study area is generally described as torrid and practically rainless. GEOLOGICAL AND HYDROLOGICAL ASPECTS The surface exposures throughout the whole area of study are entirely composed of sedimentary rocks belonging to Upper Cretaceous to Quaternary. The Basement rocks which only crops out at the southern part of El-Kharga oasis is unconformably overlained by the Jurassic to Upper Cretaceous Nubian Sandstone followed br Pre-Maestrichtian variegated shale, Maestrichtian Duwi Phosphate Formation, Maestrichtian to Danian Dakhla shale Formation, Landanion Tarawan chalky limestone Formation, Upper Paleocene-Lower Eocene Esna shale, Lower Eocene Thebes Formation and the Pleistocene and Recent sediments. The major homoclinal structure in the study area is generally interrupt by a number of dominantly oriented NE-SW and NW-SE upfols and downfolds. A great number of E-W faults are recognized mainly in the southern portion together with N-S and NW-SE trends/0 The Nubian Sandstone aquifer in Egypt is a part of a regional hydrogeological system with extension into Libya, Sudan and Chad. It contains a vast amount of water for agricultural and domestic uses ( 3x 10M million cubic meters ). Nubian Sandstone is the main groundwater bearing horizon outside the Nile Valley and Delta, it underlies the whole Western Desert and extends to the Eastern Desert and Sinai Peninsula. It consist mainly of alterning beds of sandstone and clay, the clay beds are laterally discontinuous and separate sandstone into muti-Iayer aquifer system bounded by impervious basement rocks. The thickness of the sediments varies between a few hundred meters in the south to 4,000 meters west of Abu Mongar. In the southern portion, the Nubian Sandstone aquifer is unconfined and depth of groundwater varies from 25-40 meters. Further north where the aquifer is overlain by Tertian.- shales groundwater is naturally confined, although springs are present originating from fissures in the confioning layers. In the erosional depressions where the shales have been removed groundwater is discharged at the surface forming the inhabited oases *\ The piezometric pressure ranges from 350 m (a.s.l., above sea level) at the southern corner of Egypt ( Uweinat) to about 100 m (a.s.l.) at the Kharga and Dakhla oases with a general flow direction from S.W to N.E and N as shown in Fig.(l). The huge depressions in the Western Desert below sea level constitute the natural discharge areas of groundwatcr contained in the Nubian Sandstone aquifer, thus considerable amounts of water reserves normalh- under pressure is expected to be lost by evaporation* '. METHODOLOGY AND TECHNIQUES A total number of seventy one groundwater samples were collected from the different bearing horizons of the Nubian Sandstone aquifer system in the concerned oases (El-Dakhla, El-Kharga 982 B 2000- I00O cr, Cr, 00 -WOO Pcm -2000 PRE-EOCENE (Duwi-Fm-Dakhla Fm-Kurkur Fm-Trawan Fm) JURASSIC-EARLY CRETCEOUS (Abu Simbil Fm-Abu Ballas Fm-El Borg Fm-Bahariya Fm) PRE-JURASSIC (Gilf Fm-and undiffcrentiatcd Paleozoic rocks) PRECAMBRIAN oo (Crystalline Basement Rocks) PLIOCENE-QUATERNARY (Terraces-Unconsolidated-Surficial deposits) PRE-PLIOCENE (Qattara Fm-Basalls-Moghra Fm-Marmarica Fm) EOCENE (Garra Fm-Dungul Fm-Thebes Fm-Minya Fm-Samalut Fm) Fig. (1) : Location map, sampling sites and generalized geological north-south cross section of the western desert and El-Farafra) in 1995. Sample identification, locality, pH, E.C., water's temp., H2S concentration were performed in situ. Chemical analysis were carried out according to standard methods <4\ The water samples were also analysed for oxygen-18 and deuterium using isotopic ratio mass spectrometer. The (3) ISQ/I&Q ratjQ wag measurecj after equlibrium between 5 ml of water sample and standard CO2 gas , while the 2HJ 'H ratio was determined after reduction reaction between 0.8 u ml from water sample and 25 mg Zn under vaccum at a temperature 450°C <6). The isotopic composition of a water samples is usually reporated in delta per mille (%o) deviation from the Vienna Standard Mean Ocean Water (V.SMOW) with uncertainty level of ± 0.1 %o for 5!SO and ± 1 %0 5 D. Benzene synthesis and liquid scintillation counting have been used for 14C measurements. The sampling was done in the field using about 100 to 120 liter containers to precipitate BaCO3 for laboratory analysis based on a modified IAEA technique<7). RESULTS AND DISCUSSION A- General chemical characteristics The results of the chemical analysis could be outlined as follows :- A. 1 - Salinity, major ion concentrations and water type The groundwater salinity and major ions concentration in the investigated oases vary within a narrow rang indicatig a high homogeneity in chemical composition due to the dominance of clastic facies. The values of total dissolved salts (TDS) does not exceed 500 mg/1 and increase generally with the decrease in depth of well due to the following factors : 1- The continuous rock-water interaction during water flow upward as a result of the high piezometric pressure and due to the long period of exploitation of shallow zone of the aquifer, which characterized by shally materials. 2- The mixing with irrigation water return and/or evaporation. The correlation coefficients between TDS and each of the major ions, Table (1), show that the most significant ions which control the TDS are Cl and Na followed by Ca and Mg. The low correlation coefficient between TDS and K could be attributed to the adsorpance of K on the shale content dominating in the highly saline water in shallow7 zone of the aquifer. The reduction of SO4 is indicated by the high H:S content, may explain the low correction coefficient between SO4 and TDS.