Improving the Hydrogeological Conceptual Model of the Sidi Merzoug-Sbiba Aquifer System (North-West of Tunisia) Using Hydrochemistry and Isotopic Tools

The aim of the study is to improve the knowledge of the hydrogeological systems in the Sidi Merzoug Sbiba Basin (Northwestern Tunisia), using chemical and isotopic tools. Three major aquifers have been identified in this semi-arid region by previous hydrogeological studies: the Cretaceous, the Miocene and the Plio-Quaternary aquifers. Its hydrodynamic regime is largely influenced by tectonics, lithology and recharge conditions.

Given the heterogeneity of the multilayer aquifer system in a complex fractured zone, an hydrochemical and environmental isotope (2H, 18O,3H and14C) data were used to caracterised the groundwater flow and provide valuable information about the geochemical processes controlling groundwater quality and the circulation patterns of the different groundwater bodies. Three major processes control the chemical composition: i) dissolution of carbonate minerals, ii) cation exchange reactions and iii) evaporation process.

Stable isotopes indicate that most groundwater samples originate from infiltration of modern precipitation. A significant infiltration before evaporation takes place, indicating a major recharge directly from Cretaceous and Miocene formations of surrounding mountains and infiltration of surface water in the El Breck and Sbiba rivers. Downstream, the isotopic signature of evaporated water clearly indicates a recharge from rivers, irrigation fields or Sbiba dam. Tritium and 14C contents confirm the existence of modern groundwaters in the South-Western border and in the North -Eastern part of the basin and confirm the stratification of the system.