Origin of the groundwater mineralization in the Chougafiya basin
Intensive pumping of groundwater resources in the Chougafiya basin in Tunisia has largely influenced the hydrodynamic function of the shallow aquifer. It has resulted in a low hydraulic head and salt water intrusion from Sabkhas el Batten. In order to highlight the influence of salinization on the groundwater quality of the shallow aquifer, a multidisciplinary study based on hydrogeology and geochemistry methods was carried out. The hydrochemical study showed two chemical facies. The Cl-SO4-Ca water type characterizes groundwaters located in the western part of the study area whereas the Cl-Na water type is found in other parts. The principal geochemical processes contributing to groundwater mineralization are related to the water-rock interaction (dissolution of halite and sulfate minerals) and cation exchange reactions. However, the chemical composition of the brackish groundwaters located in the central and southern parts of the basin is clearly influenced by mixing process with Sabkhas salt water. Stable isotope studies (18O, 2H) indicated a large contribution from recent precipitation to the recharge of the shallow aquifer, whereas enriched values measured in groundwater from the southern and central parts are related to the mixing process with the Sabkhas salt intrusion. Data on the radiogenic (3H, 14C) isotopes provided insight into the presence of significant contemporaneous recharge in the western and northern areas, whereas low activities were recorded in the central, southern and eastern parts of the basin, reflecting the presence of low transmissivity in these zones. The study of the carbon isotopes made it possible to estimate the water residence time. The mean ages confirmed a modern recharge from upstream with ages of less than 2 ka BP, and indicated the presence of relatively old waters in the other parts of the aquifer (up to 5 ka BP). The carbon-13 enrichment revealed isotopic exchange processes with the carbonate matrix of the aquifer.