Geochemical characterization and water quality evaluation of springs emerging along with the Metlaoui range (Southwestern Tunisia)

In the beginning, irrigation water in Metlaoui oasis is largely derived from springs discharging mainly from sandy Miocene aquifer in the southern flank of Metlaoui range in southwestern Tunisia. It was only from the considerable drops in the flow rates of the springs that recourse was had to the digging of boreholes. Hydro-geochemical studies were carried out in this area with the objective of identifying the geochemical processes influencing spring water chemical composition. Water samples were selected from 12 springs and 11 wells to analyze δ 18 O and δ 2 H, major and trace element concentrations. Physical (pH, electrical conductivity, total dissolved solids) and hydro-chemical characteristics (Na, K, Ca, Mg, Sr, Li, HCO 3 , Cl, SO 4 , NO 3 , Br, B, F) of groundwater samples were determined. Water quality parameters show that the majority of studied springs are suitable for irrigational activities. However, the Water Quality Index (WQI) reveals that all springs have poor to very poor water quality. Springs have significantly different geochemistry and reflect interaction with different lithologies. Dissolution of evaporates and cation exchange represent the major mechanisms responsible for groundwater salinization. The multivariate technique of principal component analysis shows that loading distributions of the first factor pair Na and Cl, commonly attributed to the evolution of groundwater due to evaporation and dissolution of halite and evaporated minerals, and SO 4 , Mg, and Ca ions associated with gypsum and/or anhydrite dissolution and cation exchange processes. Stable isotopes represent the first group of spring plotting along the evaporation line. While the second spring group connected to the shallow unsaturated zone exhibits a mixing trend of evaporation-dissolution processes. Hamda Spring (sample N°4) was subjected to higher evaporation and is considered as a closed system not connected to the deep Miocene aquifer.