Modeling groundwater recharge mechanisms in semi-arid regions: integration of hydrochemical and isotopic data

Abstract

Groundwater resources management is of strategic importance, particularly in arid and semi-arid regions, mainly associated to climatic environment and human pressures. Southeastern Tunisia, particularly Gabès region, is considered as one of the most promising areas for groundwater resources. Groundwater is the main water source used on agriculture and human consumption associated to the Continental Intercalary (CI) and Djeffara aquifers but to the interlayers from Ceno-Turonian and Miocene-Pliocene aquifer systems. The Cretaceous and Quaternary aquifer systems are the most exploited on the region. The present study aims to assess the hydrochemistry of the different aquifer systems as a contribution to the recharge mechanisms. 44 groundwater samples were analyzed for major ions, oxygen-18, and deuterium isotopes to assess the hydrochemistry and isotopic characteristics. Results highlighted that Total Dissolved Solids concentration ranges from 300 to 7200 mg/L jointed with a temperature variation from normal (19° C) to extremely high (>70° C). However, the cluster analysis reveals two main groups (A and B) with different chemical and isotopic characteristics. Isotope ratios of oxygen (δ18O) and hydrogen (δ2H) were applied to identify the recharge mechanism, which showed a relatively depleted 18O and 2H isotopic ratios and a mixing of groundwater from the Djeffara aquifer with deep thermal groundwater from CI. However, groundwater depth and local geologic structures also contribute to the mixing pattern. The obtained results suggest a more detailed structural characterization in the future to detect the spatial location and dominant properties of fractures, taking into account their relevance and contribution to regional groundwater recharge. Integration of hydrogeochemical data and isotopic groundwater composition will provide a better understanding of the geochemical processes that control the groundwater hydrochemistry and recharge of the different aquifer systems in the Gabès region and in areas with similar hydrogeological and structural conditions.