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

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 °) were measured, major ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl^-, SO₄^2-, NO₃^- and HCO₃^-) were analysed and ²²²Rn 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 facies CaMgClSO₄, characterizes the central part of Jerba (a recharge area) due to carbonate and gypsum dissolution. The second water type with high salinities, dominated by NaKCl 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 ²²²Rn 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. The highest values were registered in the western coastal wells and near the fault of Guellala. However, the central and eastern wells showed low radon levels. Compared to ²²²Rn activity in some countries with the same lithology, radon concentrations in the Jerba unconfined aquifer have higher values influenced by the structure of the aquifer and by seawater inflow enriched with ²²²Rn resulting from the decay of uranium derived from phosphogypsum deposits in the gulf of Gabes. The EC and ²²²Rn spatial variability in the study area were mapped using ARC Map 10.3 software. Hydrochemical results in addition to geological data and radon activities confirm the existence of vertical communication between the Miocene aquifer and the unconfined Plio-Quaternary aquifer through fault system and a lateral communication with the sea via seawater intrusion.