Abstract
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 87Sr/86Sr 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 Ca2+ and SO42-. 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. 87Sr/86Sr 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 87Sr/86Sr ratios were found in deeper sections of the aquifers, which coincided with the radiogenic 87Sr/86Sr of the deeper aquifer lithologies and the underlying shale bed.
Highlights
The study area is located in the northwestern part of Saudi Arabia close to the border with Jordan and Iraq, with a total area of 71,225 km2 (Fig. 1)
At the areas where Tawil aquifer is truncated, Jilh is used as the alternative source of water, since Tawil groundwaters are believed to leak into the Jilh aquifer sediments [1]
The rock samples of both aquifers and the underlying shale bed were collected as cuttings from different depths of the wells during drilling
Summary
The study area is located in the northwestern part of Saudi Arabia close to the border with Jordan and Iraq, with a total area of 71,225 km (Fig. 1). The first aquifer forms part of the Tawil Formation of Upper Silurian-Lower Devonian age. The second aquifer is Jilh, which is composed of Triassic carbonates. Both aquifers overlie the Ordovician-Silurian Qusaiba Member, and are used to supply the region with water. Both aquifers are at similar depth, as the western part of the basin is structurally uplifted, causing the absence of the Tawil aquifer toward the west. At the areas where Tawil aquifer is truncated, Jilh is used as the alternative source of water, since Tawil groundwaters are believed to leak into the Jilh aquifer sediments [1]
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