Multi-tracer investigation of groundwater in El Eulma Basin (northwestern Algeria), North Africa

The hydrogeochemical and isotopic compositions of groundwaters of the Mio-Plio-Quaternary (MPQ) aquifer of the El Eulma area, Northeast Algeria were examined to determine the main factors controlling groundwater chemistry and salinity as well as its hydrogeochemical evolution. Groundwater occurs in different water-bearing formations belonging to Quaternary, Neogene, Upper Cretaceous and Jurassic age. Different geochemical interpretation methods were used to identify the geochemical characteristics. Groundwater of the MPQ aquifer has the highest salinity values (564.5 ≤ total dissolved solids ≤ 2,333 mg/L) in the study area due to the impact of saltwater of Chotts and/or Sebkha “Bazar” and agricultural activities. Plotting data in a Piper diagram showed that Cl and SO4 are the dominant anions, whereas Na is the most dominant cation, although sometimes replaced by Ca and/or Mg in the groundwaters. Dissolution of carbonate and sulphate minerals in the aquifer matrices and recharge areas as well as cation exchange is shown to modify the concentration of ions in groundwater. Groundwater-mineral equilibria showed the prevailing dissolution-precipitation reactions in the groundwater. The groundwaters are depleted in 2H and 18O and display an isotopic signature close to that of meteoric water with d-excess values indicating present-day precipitation over the region and reflect the contribution of vapour masses from Mediterranean Sea and Atlantic origin. The isotopic features suggest that most of the groundwaters at the study area result from mixing between recent recharge and an older component recharged under climatic conditions cooler than at present.