Groundwater chemistry and radon-222 distribution in Jerba Island, Tunisia

摘要

The present study integrates hydrogeological, hydrochemical and radiogenic data of groundwater samples taken from the Plio-Quaternary unconfined aquifer of Jerba Island, southeastern Tunisia, in order to interpret the spatial variations of the groundwater quality and identify the main hydrogeochemical factors responsible for the high ion concentrations and radon-222 content in the groundwater analysed. Thirty-nine groundwater samples were collected from open wells widespread on the island. Physical parameters (EC, pH, TDS and T degrees) were measured, major ions (Ca2+, Mg2+, Na+, K+, Cl-, SO42-, NO3- and HCO3-) were analysed and Rn-222 concentrations were determined using a RAD7-H2O. Hydrogeochemical characterisation revealed that groundwater from the Jerba aquifer has several origins. Basically, two water types exist in the island. The first one, characterized by a low to moderate salinity with a chemical fades Ca-Mg-Cl-SO4, characterizes the central part of Jerba (a recharge area) due to carbonate and gypsum dissolution. The second water type with high salinities, dominated by Na-K-Cl type, was observed in coastal areas and some parts having low topographic and piezometric levels. These areas seem to be affected by the seawater intrusion process. The Rn-222 concentrations in groundwater samples in Jerba varied from 0 Bq.L-1 to 2860 Bq.L-1 with an average of 867 Bq.L-1.