Hydro-geochemical Characterisation of Two Aquifers Overlying a Shale Basin: A construction of a water quality baseline in NW Saudi Arabia

Abstract

Hydro-geochemical characterization of groundwater systems should be considered as a verification tool to aid static and dynamic aquifer models. The general chemistry and isotopic signatures were analysed for two major aquifers in Northwest Saudi Arabia, and this is the first attempt to draw a geochemical baseline for these hydrogeological units. This attempt will not only help in verifying the flowpaths of the groundwater obtained by the dynamic models, but will also identify the natural conditions of these aquifers ahead of excessive shale gas activities, and investigate the connectivity of the different lithological units. A total of 79 water samples from Tawil and Jilh aquifers were analysed for general chemistry and strontium isotopes. A broad idea of the main types of the groundwater was developed by studying the chemistry of the water samples and linking their chemical composition to that of the host rocks. The dominant chemical reactions in the water were identified to be the added SO42- by dissolution and the ion-exchange between Ca2+ and Na+. The ion-exchange reactions urged the conduct of sequential extraction of different metals in the host rock. The chemical composition of the groundwater was distinct from that of the lithologies of their hosting rocks in terms of metal abundance i.e. Na+ was the dominant species in the water and K and Ca were the main constituents of the host rock. The most evolved water is thought to be undergoing an ion exchange due to the replacement of gypsiferous waters by Na+ rich waters as illustrated by the enrichment in Na+ and leaching out of Ca2+ that is directly proportional to SO42-. Sequential extraction of rock samples obtained from the underlying shale bed was also carried to uncover its metallic composition. Na was the dominant species in the shale samples, suggesting a potential interaction between the shale bed and the overlying aquifers, given the different chemical composition of the aquifers’ lithologies. The hypothesis of the interaction between the shale and the aquifer was predicated by the fingerprint of Sr isotope of the water samples, the aquifers’ rock samples, and the shale samples, coupled with attempts to age date the groundwater, which were compared to different stable isotope and radiocarbon data in the literature. The groundwater age was estimated to be at least 4577 yr BP, with noble gases feedback suggesting older residence times. The age of the groundwater alongside with radioactivity increase with depth, further supporting the interaction of the shale bed with the lower section of the aquifer.