Coupled hydrogeochemical modelling using KIRMAT to assess water-rock interaction in a saline aquifer in central-eastern Tunisia

To quantify water-rock interactions and forecast their spatiotemporal evolution in the shallow aquifer of Sidi El Hani located in central-eastern Tunisia, the numerical code KIRMAT was used. Based on available geological, mineralogical, thermodynamic, kinetic and hydrodynamic data, the model was built and calibrated on two transects of the aquifer oriented along the flow lines of groundwater. The numerical results show that the interactions between water and rock are responsible for groundwater salinity, which is strongly dominated by chlorides, sodium and sulphates. The variations of dissolved species in space and time are the result of halite dissolution, anhydrite and gypsum dissolution and precipitation, calcite dissolution and dolomite precipitation. The precipitation of dolomite can locally induce a reduction in permeability and further enhance an increase in leaching because the residence time between rock and water will be amplified. Three numerical sensitivity studies have been conducted to evaluate the influence of water-rock interactions on groundwater salinization. The study highlights the finding that the modelled fate of dissolved species in the unconfined aquifer is rather sensitive to the Darcy velocity but not very sensitive to the initial volume fractions of minerals in the two transects and the composition of groundwater injected at the upstream boundary.