Origin of the groundwater salinity and geochemical processes in detrital and carbonate aquifers: Case of Chougafiya basin (Central Tunisia)

Abstract

Summary Comprehensive investigations of groundwaters were performed in the detrital and carbonate aquifers of the Chougafiya basin, central Tunisia. In the present review, hydrochemistry and isotopic tools were combined to get an insight into the processes controlling mineralization, recharge conditions, flow pattern of groundwater and C chemistry in the investigated hydrological system. Analysis of the dissolved constituents revealed that several processes controlled the observed chemical composition: (i) the dissolution of evaporitic minerals, (ii) cation exchange reactions, (iii) sulfate reduction under anaerobic conditions, (iv) incongruent dissolution of carbonate minerals (calcite, dolomite) coupled with gypsum dissolution and calcite precipitation, and (v) silicates weathering. Data inferred from 18O and deuterium isotopes in groundwater samples indicated recharge with modern rainfall. Water characterized by lower δ18O and δ2H values is interpreted as recharged by non-evaporated rainfall originating from Mediterranean and Atlantic air masses. However, water with relatively enriched δ18O and δ2H contents is thought to reflect the occurrence of an evaporation process related to the long term practice of flood irrigation. The radiogenic (3H) isotope data provided insight into the presence of two recharge periods in the investigated groundwaters. Waters with 3H contents of 1 TU clearly suggested the occurrence of a contemporaneous recharge probably during the last two decades. Carbon isotopes provided some insights into the timescales of groundwater flow, but mainly revealed that main sources of C are active in the system. These are likely: dissolved biogenic CO2, carbonate dissolution and incongruent reaction of the carbonate matrix. Mean residence times were determined after correction of the initial activities for dead C from the rock matrix and suggest ages ranging from the present day to the Holocene in both Upper Cretaceous and Mio-pliocene groundwaters.