Environmental isotopes as the indicators of the groundwater recharge in Tunisian Chotts region

Abstract

Hydrogeological investigations in arid zones tend to be logistically difficult and costly. However, isotopic techniques can be used to gain valuable reconnaissance information, for example, in advance of new drilling programmes and also during exploitation. Thus, the most cost-effective exploration in arid zones will include both good hydrogeological data combined with isotopic and supporting geochemical information. The question of recharge is one of the most critical for groundwater management in dry areas. Is the recharge currently taking place and at what rate? If recharge is occurring, further important problems arise dealing with possible mechanisms of recharge, which may occur through direct infiltration, upward leakage, lateral recharge from areas of rainfall, and lateral seepage of river (Fontes and Edmunds, Technical documents in hydrology 77, 1989). In this study, which is of interest for an important aquifer system in southern Tunisia, conjunction of environmental isotopes, i.e.: 18O, 2H, 3H, 14C and 13C and chloride contents was used to understand the mode of recharge of the shallow Plio-Quaternary (PQ) and the intermediate Complexe Terminal (CT) aquifers. Both are unconfined in the vicinity of the Tozeur uplift and the Dahar upland and show relatively modern water that originated from present-day rainfall. It has been demonstrated that the Continental Intercalaire deep groundwater contributes to the CT aquifer recharge, the latter to the upper PQ shallow aquifer. The shallow PQ is mainly recharged by return flow irrigation and upward leakage in oases areas. Away from oases zones, sporadic recharge may occur at the inter-dunes especially between the Chott Djerid and Chott El Gharsa, in the Bir Roumi region and in the Kebili sand fields.