Conceptual Hydrodynamic Model with Hydrochemical and Isotopic Signatures of the Triassic and Silurian-Devonian Groundwater Systems in Northwestern Saudi Arabia

Abstract

Groundwaters from an Upper Silurian-Lower Devonian sandstone-siltstone aquifer and an Upper Triassic calcareous unit in the northwestern part of Saudi Arabia were geochemically assessed to define their provenances, lateral migration pathways, secondary alteration processes, and potential connectivity between both aquifers. The water samples were analysed for their hydrochemical properties (n = 74), 87Sr/86Sr ratios (n = 14), 14C activities and noble gases (n = 5). The Devonian aquifer is of NaCl and Na-Cl-HCO3 water type, while the Triassic aquifer is of NaCl and Ca-Cl-SO4 type. A general increase in water salinity from 189 mg/L toward maximum concentrations of 5546 mg/L in the NW confirms potential recharge from Devonian outcrop areas with a lateral fluid migration from SE to NW. Principal component analysis (PCA) with nine variables (Na+, Ca2+, Mg2+, K+, Cl−, and SO42−, alkalinity, TDS, pH) revealed variations in TDS reflecting the observed lateral groundwater migration. A positive correlation between SO42− and Ca2+ loadings on PC2 illustrates the presence of gypsiferous water. An inverse correlation between Ca2+ and Na+ loadings on PC2 indicates cation exchange processes for both aquifers. The sandstone/siltstone-composed Silurian-Devonian aquifer groundwater was found to be dominantly undersaturated in terms of aragonite, calcite, and dolomite. Overlapping Ca/HCO3 and Na/Cl ratios for some Devonian and Triassic samples indicates mixing between both aquifers along the Hercynian Unconformity in the northwestern part of the study area. A depth-related increase in 87Sr/86Sr ratios (from 0.708611 to 0.711577) suggests the presence of water-rock exchange processes between the infiltrating groundwater and 87Sr/86Sr-enriched underlying units, likely by vertical segregating fluids. The general lack of measured 14C activities suggest the occurrence of recharge prior to Late Pleistocene Pluvial period or, more likely, a complete 14C decay during groundwater flow from recharge to the present discharge area. Elevated 4He/20Ne and depleted R/Ra ratios (0.01 to 0.05) of the groundwater indicate crustal radiogenic influence from the interaction with crustal fluids. 4He is produced in-situ with negligible contribution from the mantle. Evidence of noble gas isotopic fractionation was reflected in one water sample, indicating a preferential loss of some of the lighter isotopes of Ne and Ar.