Geochemistry and residence time estimation of groundwater from Miocene–Pliocene and Upper Cretaceous aquifers of Southern Tunisia

Miocene–Pliocene and Upper Cretaceous formations in Tunisia contain one of the most productive artesian aquifers in the country. They consist of Mio-Pliocene sands and Senonian/Turonian carbonates. Intensive pumping in Kébili and Nefzaoua regions over the past decades resulted in gradual lowering of water table at the rate of approximately 0.85 m y⁻¹. Forty-two groundwater samples have been collected from different sectors of the studied aquifers for chemical and isotopic analyses. The observed large spatial variability of chemical composition of groundwater in the study area is most probably linked to two processes: (i) dissolution of dolomite and gypsum, combined with calcite precipitation (dedolomitisation), and (ii) partial evaporation of water. The first process plays an important role in the study area due to abundance of evaporites. Partial evaporation occurs in the upper part of the unsaturated zone during infiltration, especially for groundwater sampled in the Kebili and Djerid regions. Apart from these processes, there are others which influence the salinity of the aquifers. In the Mio-Pliocene aquifer, which behaves as an open system to gases and which receives inputs of CO2 gas derived from intensive tectonic activity in the area, the interaction of carbon dioxide with carbonate matrix of the aquifer produces an increase in the alkalinity of water. In the Senonian and Turonian aquifers, the process of dedolomitisation evolves in a closed system with respect to CO2 gas. Ca²⁺/Na⁺ cation exchange and halite dissolution processes are also important. Stable isotope composition of water (δ¹⁸O, δ²H) indicates that the recharge occurs from the Dahar upland. The ¹⁴C activity varies between 89.5 (±1.5) and 3.7 (±2.1) pmc. The ¹³C content in the total dissolved inorganic carbon (TDIC) range between −13.9 and −3.6‰. The calculated concentrations of ¹³C in the CO2 gas in equilibrium with the TDIC vary between −22 and −11‰, indicating two sources of carbon in the solution: carbonate matrix (δ¹³C = −2‰) and soil CO2 (δ¹³C from −25 to −21‰ for the cultivated areas). Mean residence times of water have been determined after correction of the initial ¹⁴C activities for ¹⁴C-dilution processes including carbonate dissolution, calcite precipitation and cation-exchange. The dilution processes were quantified on the basis of geochemical and ¹³C mass balance equations. The calculated mean residence times of water confirm modern recharge from Dahar upland and the mountains surrounding depressions, and indicate the presence of paleowaters in the east and south-west region, and in the discharge zone.