Identifying the origin of groundwater salinisation in the Sidi El Hani basin (central-eastern Tunisia)

In the Sidi El Hani basin, located in central-eastern Tunisia, a shallow aquifer is the main water source for agricultural practice. However, in the last few decades, it has undergone saline pollution. In this context, this study is carried out to identify the origin of the dissolved species and the processes involved in the increase of groundwater salinisation using hydrochemical tools and geochemical modelling with PHREEQC. To achieve this objective, water analysis was performed on 49 samples collected from 46 shallow wells and 3 observation wells during March and April 2015. The results indicate that for the samples located near the Sabkha Sidi El Hani, the dominant facies of the groundwater is sodium chloride (Na-Cl). The water samples are characterized by high salinity exceeding 6 g.L-1. However, in the centre of the basin, the water samples are a mixed type (Cl-Na-SO4-Ca-Mg). Our results show that salinisation of the groundwater was due to the dissolution of halite, cation exchange, and the precipitation of carbonate minerals such as calcite and dolomite coupled with the dissolution of gypsum, and evaporation. Intensive irrigation in the area leading leaching of salts from the surface soils to deep soil layers are additional factors. This soil leaching is the major process that accounts for salinisation of water and soil; it leads to the accumulation of a bitter brine solution after the precipitation of evaporite minerals.