Investigating groundwater recharge by means of stable isotopes in the Al-Khazir Gomal Basin, northern Iraq

Stable isotopes (δ¹⁸O and δ²H), ¹⁴C and groundwater chemistry were used in this study for gaining insight into the recharge mechanisms and groundwater geochemical evolution in a semi-arid region of northern Iraq. The 3,185 km² large catchment of the Al-Khazir Gomal Basin is divided into three regions (south, central, and north) depending on structural and geological aspects. In terms of hydrogeology, the vast syncline in the central part of the watershed is considered to be the most important region regarding groundwater quality and quantity. Groundwater samples of both confined and unconfined aquifers plot on or slightly above the local meteoric water line and the slope of the δ¹⁸O and δ²H regression line for the confined and unconfined aquifers is 7.26 and 7.01, respectively. This is interpreted as an indicator of considerable recharge with short residence time for the shallow groundwater, excluding the effect of evaporation before infiltration. This assumption was supported by low chloride concentrations, groundwater Ca-HCO₃ type and the high values of ¹⁴C activities. The variations in stable isotopes and ¹⁴C in the confined aquifer support the assumption of existence of different recharge resources and mechanisms. Hydrogeochemically, there are three main mechanisms contributing to the chemical evolution of the groundwater; these are the preferential chemical weathering of the carbonate cement of the sandstones, geochemical weathering of silicate minerals and cation exchange processes. Meteoric NaCl is considered to be the main source of dissolved Na and Cl ions in freshwater samples. Additional Na might stem from cation exchange with dissolved Ca and Mg.