An appraisal of Chott El Jerid brine encroachment in the Tozeur-south shallow aquifer: Geoelectrical and hydrochemical approach

Abstract

Geoelectrical data were used in conjunction with hydrogeological information and hydrochemical measurements to investigate and conceptualize the Chott El Jerid saltwater encroachment into the Tozeur-south shallow aquifer, southern Tunisia. The hydrogeological investigation shows that the over-pumping of the unconfined aquifer groundwater has resulted in a localized water level decline, which enhanced the encroachment of the brines of the Chott El Jerid discharge area into the shallow water table. This transitional hydrodynamic state is further complicated by the upward vertical flow of relatively fresh-water from the underlying confined aquifer. The geoelectrical study has confirmed the brines encroachment through the predominance of relatively low resistivity values of the shallow aquifer groundwater close to the discharge area, which become progressively high as we move away towards the recharge zones. In fact, the toes shaped resistivity zones in the isoresistivity map bring insight to the mixing of shallow groundwater with Chott brines. However, the presence of a positive resistivity leakage plume confirmed the mixing with upward fresh-water leakages. Hydrochemically, the Chott brines encroachment is manifested by an evolution of hydrochemical facies from Na-Cl end-member towards a Ca-HCO3/SO4 end-member. The variation in groundwater hydrofacies between these two end-members, indicates a progressive evolution as we move along way from the Chott, by mix/Na-mix/SO4 and mix/Na-SO4. These findings are well illustrated in the conceptual model, which shows the impact of the stressed hydrodynamic regime on the behaviors of the electrical and the hydrochemical facies of the PQ aquifer and its groundwater, respectively.