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

2005, Environmental Geology

AI-generated Abstract

Stable (18O, 2H) and radiogenic (3H, 14C) isotopes of water have been used to constrain the source, origin, age, migration pathway and mixing processes in the Sbeı¨tla (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 Sbeı¨tla 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%. Associated with extensive groundwater use are several deleterious consequences which have severely intensified over time. They include water level decline in both Miocene aquifers and their corollaries of drying springs, loss of artesian condition and water quality degradation presumably due to mixing with deep mineralized (up to 2.5 g/L) Lower Cretaceous groundwater. These consequences interfere with the prudent use of these invaluable resources on a 'sustainable' basis. An understanding of the origin, sources, ages, migration pathway and mixing rates is therefore necessary for viable long-term development. Major and minor ion chemistry is of limited value when dealing with these issues. They are however worthwhile in identifying the geochemical reactions occurring in the aquifer system.