Chemical and isotopic evaluation of groundwater salinization processes in the Djebeniana coastal aquifer, Tunisia

This study aims at identifying the sources and fate of salinity in the Djebeniana basin (Tunisia) aquifers. Groundwater samples from the shallow Plio-Quaternary (PQ) phreatic aquifer and deep Miocene aquifer were analyzed for major/minor ions, trace elements, and stable isotopes of the water molecule (delta 18OH2O - delta 2HH2O), chlorine (delta 37Cl) and bromide (delta 81Br). Two clusters of samples with distinct chemical compositions were identified. Cluster I samples correspond to Na-Cl mineralized waters with delta 37Cl from -0.3%o to 0.16%o and delta 81Br from 0.21%o to 0.39%o. Cluster II samples correspond to Ca-SO4 waters presenting a lower salinity than Cluster I, with delta 37Cl from -0.3%o to 0.09%o and delta 81Br from 0.28%o to 0.54%o. When plotted in a delta 2HH2O vs delta 18OH2O graph, most of Cluster I groundwater samples tend to align on a mixing line with seawater whereas Cluster II samples appear to define a local evaporation line. The spatial distribution of samples associated with these clusters allows for evaluating the hydrogeochemical processes associated with groundwater salinization: 1) recent recharge from rainwater dissolving salts and entailing the downward migration of saltwater in the higher reaches of the groundwater flow system, 2) upward leakage of mineralized waters from the deep Miocene aquifer, 3) seawater intrusion in the lower reaches of the coastal aquifer, and 4) ion-exchange reactions prompted by seawater intrusion. A conceptual model of groundwater salinization is proposed as a tool for better managing groundwater resources in the Djebeniana basin.