Recharge of the plio-quaternary water table aquifer in Tunisian chotts region estimated from stable isotopes

Abstract

The hydrodynamic groundwater data and stable isotopes of water have been used jointly for better understanding of upward leakage and mixing processes in the Djerid aquifer system (southwestern Tunisia). The aquifer system is composed of the upper unconfined Plio-Quaternary (PQ) aquifer, the intermediate (semi-)confined Complex Terminal (CT) aquifer and the deeper confined Continental Intercalaire (CI) aquifer. A total of 41 groundwater samples from the CT and PQ aquifers were collected during June 2001. The stable isotope composition of waters establishes that the CT deep groundwater (depleted as compared to present Nefta local rainfall) is ancient water recharged during late Quaternary time. The relatively recent water in the shallow PQ aquifer is composed of mixed water resulting from upward leakage and sporadic meteoric recharge. In order to characterize the meteoric input signal for PQ in the study area, rainfall water samples were collected during 4 years (2000–2003) at the Nefta meteorological station. Weighted mean values of isotopic contents with respect to rainfall amounts have been computed. Despite the short collection period in the study area, results agree with those found in Beni Abbes (southwestern Algerian Sahara) by Fontes on 9 years of rainfall surveillance. Stable isotopic relationships provide clear evidence of shallow PQ aquifer replenishment by deep CT groundwater. The ¹⁸O/upward leakage rate allowed the identification of distinctive PQ waters related to CT aquifer configuration (confined in the western part of the study area, semi-permeable in the eastern part). These trends were confirmed by the relation ¹⁸O/TDS. The isotope balance model indicated a contribution of up to 75% of the deep CT groundwater to the upper PQ aquifer in the western study area, between Nefta and Hazoua.