Relationship between Senonian and deep aquifers in Southern Tunisia

Steady increase in the salinity of water resources and the decline in the groundwater level are the most widespread processes that degrade water-quality and endanger future water exploitation in Southern Tunisia. Hydrological, geochemical and environmental isotope data were evaluated to characterize the recharge conditions of the Senonian aquifer and the flow pattern of groundwater mixing. The results show that groundwaters can be classified into two major water facies: Ca-SO4-to mixed and Na-Cl. The main hydrogeochemical processes that typically control the groundwater mineralization are dissolution of evaporites (halite, gypsum and anhydrite), evaporation, cation exchange and mixing process related to the up-flow leakage of deep groundwaters. The stable isotope signatures reveal that the main part of groundwater is significantly affected by evaporation process. Wide range (−4.5 to −8.4‰) and spatial heterogeneity of δ18O in groundwater indicate that the aquifer is characterized by a slow process of natural groundwater in its lateral extent. The 14C activity varies between 43 and 0.33 pmc. The 13C contents of the total dissolved inorganic C (TDIC) range between −10.8 and −3.6‰. The calculated 13C contents of the CO2 in equilibrium with the TDIC, varying between −15.1‰ to −11.4‰ indicate two origins of C in solution: the carbonate matrix (δ13C = −2‰) and the soil CO2 (δ13C from −26‰ and −22‰ for the cultivated areas). Mean residence times have been determined after correction of the initial activities for dead C from the rock matrix. The mean residence times confirm a modern recharge of the groundwater from Dahar upland and indicate the presence of palaeowaters in the northern and southern parts of the basin. Detailed investigations on identifying the potential recharge zones, flow-paths of mixing and recharge conditions will be useful to protect the groundwater resources from depletion and salinization.