Use of hydrochemistry and environmental isotopes for assessment of groundwater resources in the intermediate aquifer of the Sfax basin (Southern Tunisia)

摘要

Groundwaters from the intermediate aquifer of Sfax basin have been investigated using chemical tracers and environmental isotopes to reveal the hydrogeological features of this system and characterize the dynamics of groundwater salinization in this sector. The origin of salinity in the studied aquifer was investigated based on the chemical analyses of 30 groundwater samples. Groundwater is characterized by Na-Cl and Na-SO_4 water types. The saturation indices for calcite and gypsum, and binary diagrams of different ions showed that the main hydrogeo-chemical processes were the dissolution of carbonates (mainly calcite scattered through the reservoir rocks), the dissolution of evaporites (halite, gypsum and anhydrite) and the cation exchange processes. The isotopic composition investigation allowed the definition of two groups. The first group is represented by groundwater with the highest oxy-gen-18 content (δ~(18)O ranges between -5.36 and -4.22‰, and δ~2H ranges between -39.2 and -36 %o). The high δ~(18)O values in this group can be attributed to the evaporation effect. The second group includes the most depleted waters (δ~(18)O varies between -6.84 and -5.79 ‰, and δ~2H varies between -42.4 and -36.5 ‰). The combined analyses of stable isotopes and major ions (Cl~-) showed that mixing between old and recent water and evaporation were the main processes explaining the variation of salinity of the intermediate aquifer of the Sfax basin.