Water-rock interaction of the Jilh and Tawil aquifers in the Wadi Sirhan Basin, NW Saudi Arabia

摘要

A total of 79 groundwater samples from the Upper Silurian-Lower Devonian Tawil and Triassic Jilh aquifers in NW Saudi Arabia were analysed for hydrochemical and strontium isotopic composition. A sequential extraction and ~(87)Sr/~(86)Sr analysis were conducted on 32 cutting samples from the groundwater host rocks and the underlying shale bed. The chemical composition of the groundwater was distinct from that of the host rock lithologies in terms of elemental abundance (i.e., Na~+ was dominant in the groundwater and K and Ca were the main constituents of the aquifer units). Principal component analysis (PCA) of water samples showed a positive loading of Na~+ and negative loadings of Ca~(2+) and SO_4~(2-). The replacement of gypsiferous waters by Na~+-rich waters is suggested by this ion exchange. The shale rock samples showed Na to be the dominant chemical constituent. ~(87)Sr/~(86)Sr ratios of the groundwater (0.707673-0.711577) and host rock samples (0.707930-0.712477) suggest that groundwater inherits the Sr signature of the rock from the exchangeable, carbonate, and the oxides phase, reflecting reducing conditions in Tawil aquifer. The elevated groundwater ~(87)Sr/~(86)Sr ratios were found in deeper sections of the aquifers, which coincided with the radiogenic ~(87)Sr/~(86)Sr of the deeper aquifer lithologies and the underlying shale bed.