Using geochemical and isotopic tracers to characterize groundwater dynamics and salinity sources in the Wadi Guenniche coastal plain in northern Tunisia

Northeastern Tunisia is a sub-humid region with annual rainfall ranging from 600 to 650 mm. Unconfined groundwater in this area is easily accessible and represents an important perennial water source for agricultural activities. The present study deals with a multi-parameter investigation of one of the most important basins in northeastern Tunisia: the Wadi Guenniche plain (130 km2). Hydrochemical (major elements and Br−) and isotopic (18O, 2H and 3H) investigations were carried out on 32 shallow wells to gain insights into groundwater recharge and salinity. The dominant water types were Ca–Mg/Cl–SO4 and Na/Cl with electric conductivity values varying between 0.86 and 6.6 mS/cm. Based on ion ratios and saturation indexes, groundwater salinity originated from weathering of carbonate and evaporite minerals and cation-exchange with clays. Additional influence of anthropogenic activities shown by high NO3− concentrations exceeding 100 mg/L was identified in different parts of the plain resulting from irrigation return flow. A six-year record of 18O and 2H signatures of daily rainwater (n = 293) enabled to define a Local Meteoric Water Line for the Bizerte area (δ2H = 7.02 δ18O + 8.27). The isotopic signature of groundwater was very similar to that of the mean annual rainfall in Bizerte suggesting direct recharge of the aquifer. Tritium levels in sampled wells were relatively high and indicated recent recharge and short residence time. Annual recharge of 100 mm was estimated using a mixing model based on 3H contents in precipitation and groundwater.