Water-rock interaction processes in the Devonian aquifer system and Qusaiba Silurian shale of northwestern Saudi Arabia

Water-rock interaction (WRI) processes in Devonian sandstones and underlying Qusaiba Silurian shales in northwestern Saudi Arabia were investigated using the elemental abundance in core samples, groundwater hydrochemistry, and strontium isotopic ratios (⁸⁷Sr/⁸⁶Sr) of rock and groundwater samples. The partitioning of Na, Ca, Mg, K and trace metals (Sr, Ba, Zn, Fe, Mn) from Devonian and Qusaiba rock samples was assessed along five fractions: exchangeable, bound to carbonates, Fe–Mn oxides, organic matter, and residual fraction. Ca comprised the main constituent of the Devonian aquifer host rock, while Na was revealed to be the dominant constituent of the Silurian shales, especially in the exchangeable and the bound-to-carbonate phase. By integrating host rock elemental abundances (n = 17) with groundwater chemistry (n = 74), it was identified that the replacement of inherited Ca by Na represents the dominating ion-exchange process between host rock and groundwater. ⁸⁷Sr/⁸⁶Sr ratios for Devonian groundwater (0.707673–0.711577) and rock cuttings (0.707930–0.712477) are within a similar range, indicating enhanced WRI processes in the Devonian aquifer. The increase in ⁸⁷Sr/⁸⁶Sr with depth is temperature-triggered and attributed to a change in facies from a marine dominated environment of calcareous Devonian sandstones to a terrestrial depositional environment with K-rich terrane. Considerable quantities of Ca, Mg, and K in the Fe–Mn oxide leachates alongside with the inheritance of groundwater to the oxide phase ⁸⁷Sr/⁸⁶Sr ratios suggest the presence of a reducing environment in the Devonian aquifer.