Contribution of Stables Isotopes to the Study of the Groundwater Recharge in the Southern Foothills of the Saharan Atlas: The case of Tolga Region, North Eastern Algeria

Abstract Tolga groundwater is one of Biskra's most important water supplies for both drinking and agriculture. The purpose of this study is to assess the hydrochemical and isotope (2H and 18O) signature of water in the Tolga area. To evaluate the hydrogeology of groundwater in the Tolga region, a chemical, piezometric, and isotopic research was employed. The results of the Piper diagram revealed that there is just one major hydrochemical facies in the research region, which corresponds to Ca–Mg–Cl–SO4. The piezometry of limestone sheets shows that as one goes westward, the turns become less substantial, with turns about 20 meters and a piezometric level of 140 m. For the first time, the local meteoric water line (LMWL) was drawn for Complexe Teminal (CT) and Continental Intercalaire (CI) on Tolga groundwater. The Tolga area LMWL refers to the equation δ2H= 7,18 δ18O + 4.17 with R2 about 0.85. CI and CT measurement locations indicate deuterium excess values ranging from 8.35 to 12.57. Indeed, the majority of values were more than 10‰ (mean of 10.36 ± 2.01‰). Furthermore, waters from Eocene have an isotopic composition of -7.57 ± 0.68‰ in δ18O and − 49.04 ± 4.69‰ In δ2H. The results of the sample characterisation analysis show that the majority of the groundwater samples analysed are of the Ca-Mg-SO4 water type. In addition, the intense exploitation aquifer water and the region's arid climatic conditions resulted in a large decrease in the piezometric limestone level. Finely, the signature of 2H and 18O data indicate that water comes from local precipitation and recent water, and groundwater recharge is influenced by quick infiltration.