Flow exchange between the deep and shallow groundwaters in the Sbe?tla synclinal basin (Tunisia): an isotopic approach

Abstract

Stable (18O, 2 H) and radiogenic (3H, 14C) isotopes of water have been used to constrain the source, origin, age, migration pathway and mixing processes in the Sbeitla (Tunisia) system. The system is composed of an upper unconfined “Middle Miocene” aquifer with a variable thickness from 10–300 m, an intermediate confined/unconfined “Lower Miocene” aquifer about 100 m thick and a deeper confined “Lower Cretaceous” aquifer about 150 m thick separated by a thin clay layer. A total of 53 groundwater samples from the three aquifers and spring samples were collected during February and March 2000 and isotopically analysed using conventional methods. The stable isotopes composition of waters establishes that the deep groundwater (depleted as compared to present corresponding local rainfall) is ancient water recharged probably during the late Pleistocene and the early Holocene periods. The relatively recent water in the superficial aquifer is composed of mixed waters resulting presumably from upward leakage from the deeper groundwater. The radiogenic (3H, 14C) isotopes data confirm that the recent water, with a tritium content between 6.5 and 19 TU, represents post-nuclear recharge and the ancient groundwater with low carbon-14 contents between 7 and 26 pmC infiltrated between 8,000 to 20,000 years ago. When used in conjunction with the stable isotopes data, the mixing process can be clearly identified, especially in the Sbeitla sill area. Groundwater of the upper aquifer exhibits isotopic signatures of both the old and recent waters. By using isotopic mass balance, the computed contribution of the deep groundwater in recharging the upper aquifer is up to 94%.