Assessment of groundwater quality of El Ouara aquifer (southeastern Tunisia), geochemical and isotopic approaches

Abstract

Water scarcity is a major problem all over the world due to changes in precipitation patterns and intensity. Water quality is also so far affected. Despite the fact that water covers about 70% of the Earth’s surface, safe and clean drinking water is not available only to 1 in 9 people worldwide. And more than one third of Africa’s population lacks access to safe drinking water. Therefore, experts must target this concern and analyze the different complications related to the precious natural resource in order to save it from wastage. As a matter of fact, Tunisia is not an exception. It is struggling from the same water shortages. From here emerges the goal of this present study which is to determine the main factors and the geochemical processes that control water chemistry of the Mio-Plio-Quaternary (MPQ) aquifer for El Ouara plain, located in southeastern of Tunisia. And in order to monitor the water quality and assess the main physicochemical processes in this aquifer, a total of 30 samples were collected in Mio-Plio-Quaternary aquifer from wells located in Tataouine and Medenine regions and analyzed for various physical and chemical parameters, such as electrical conductivity, pH, dissolved solids (TDS), Na+, Mg2+, K+, Ca2+, Cl−, HCO3−, SO42−, NO3−, and Br−. The major and minor elements were analyzed by the liquid chromatography at the Laboratory of Water Geochemical Analysis at the Higher Institute of Water Sciences and Techniques of Gabes. Stable isotopes of δ18O and δ2H were measured by isotopic water liquid using laser absorption spectrometry and were expressed in δ‰ with respect to the Vienna Standard Mean Ocean Water (VSMOW). The hydrochemical data from a total of 30 groundwater samples indicate that the groundwater is characterized by the dominance of Na-Cl and Ca-Mg-SO4 water types. The plotting of all samples in Gibbs diagram illustrated that the groundwater chemistry is controlled by rock-water interaction and evaporation processes. Nitrate concentrations are significantly higher than WHO guideline drinking water values and are probably linked to the overfertilization, septic system, uncontrolled sewage, and animal waste. Isotope data reveal enriched groundwater in δ2H and δ18O suggesting an evaporation effect during recharge processes and depleted groundwater recharged during paleoclimate periods. This difference attests to the heterogeneity of recharge modes and probably longer residence time in the MPQ aquifer.