Hydrogeochemical investigations of recharge and subsequent salinization processes at Al-Raudhatain depression in Kuwait

Fresh groundwater is uniquely present in the northern parts of Kuwait at Al-Raudhatain and Umm Al-Aish depressions, where it occurs in the form of lenses floating on top of Kuwait Group's brackish water. Earlier studies have provided a wealth of data on these depressions. Nonetheless, information on the recharge process is both scarce and scattered among different studies. Based on existing information, chemical and isotopic investigations, and geochemical modelling, a coherent description of the recharge and salinity evolution processes at Al-Raudhatain freshwater lenses is presented. Radioactive isotope (14C, 13C and 3H) results indicate that the groundwater sampled at the depression and its vicinity contain significant portions of recharge from recent (less than 500 years) rainfall events. Furthermore, the results of stable isotopes (18O and 2H) specify that recharging rainfall events took place during the last three to four decades. Examining the relationship of 18O/2H along with the Emirates meteoric water line (i.e. the nearest available meteoric water line) reveals that samples are showing sings of insignificant evaporation, which indicates rapid infiltration. Given the established low infiltration rates at the depression and the lack of surface runoff at the wadis, it was concluded that most of the recharge comes from infiltration outside the depression, followed by subsurface runoff to the depression where recharge takes place. Reverse geochemical modelling showed that the main processes controlling the geochemistry of the water in the depression are halite, albite, illite dissolution, as well as K-feldspar weathering and calcite precipitation. These processes take place during infiltration, subsurface runoff and percolation to the groundwater. The contribution of the recharge water in the makeup of the body of the lens versus the regional brackish groundwater was estimated according to their contribution in total dissolved solids concentrations using the salinity stratification and the results of 18O.