Hydrogeological investigation of karst aquifers using an integrated geomorphological, geochemical, GIS, and remote sensing techniques (Southern Mediterranean Basin—Tunisia)

Carbonate aquifers constitute a water reserve of essential importance for human supply. This study identified areas of extensive karstification, and groundwater resource potential in carbonate rocks in Tunisia, North Africa. Three karst systems of Jurassic, Upper Cretaceous, and Eocene ages are studied. A GIS-based predictive model is built using slope gradient, slope aspect, elevation, distance from the stream, fault kernel density, lithologic facies, and precipitation. Our model helps distinguish specific autogenic and allogenic recharge mechanisms and characterizes the respective contribution of the saturated and unsaturated zones. Geochemical monitoring was performed at the karst (springs) and in the unsaturated zone (caves and epikarstic springs). The dominant groundwater type in this area is calcium-bicarbonate-magnesium and sulfate. Total dissolved solids (TDS) of these waters vary from the minima observed in the springs located in the northwest of the study area (480 mg/l) to the peak TDS observed in the southern part of Tunisia (600–3000 mg/l). pH values are typically in the 6.1–8.3 range. The environment is locally rich in CO2; consequently, Ca2+, Mg2+, $${\text{HCO}}_{3}^{ - }$$ HCO 3 - and $${\text{SO}}_{4}^{2 - }$$ SO 4 2 - content in water increases along the water flow. Temperature is variable depending on the season, the region, and the age of the karst aquifer. In winter, it increases slightly with depth from 18 to 55 °C, and in summer, the maximum values (22–57 °C) are found just below the water level. The isotopic composition of δ18O and δ2H of all groundwaters of the study area ranged from −8.29 to −6.33‰ VSMOW and from −55.8 to −35.30‰ VSMOW, respectively. The 14C activities vary between 28.6 and 63%. The rechargeability period of these carbonate aquifers could be related to a paleoclimatic effect under colder climatic conditions than at present (Pleistocene and Holocene periods: Early Würm, Middle Würm and Late Würm humid periods).