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Hydro-(major and trace elements: Cd, F and Sr), isotope (O-18, H-2, H-3 and C-13) geochemistry and radiogenic carbon (C-14) of dissolved inorganic carbon (DIC) were used to investigate the sources of groundwater contamination and the hydrodynamic functioning of the multilayer aquifer system in the mining Gafsa basin (Southwestern Tunisia). The groundwater of the study area is subject to intense exploitation to accommodate all the water demands of this arid area. The Gafsa basin contains a multi-layered aquifer with four principal levels: Upper Zebbag (Cenomanian-Turonian), Abiod (Campanien-Maastrichian), Beglia (Miocene) and Segui (Plio-Quaternary) Formations. The hydrogeology of this system is largely affected by tectonics (Gafsa-Tebessa, Sehib, Negrine-Tozeur, Tabeddit and Metlaoui faults...). The groundwater of these aquifers undergoes a significant decline in water level (approximate to 0.5 m y(-1)), increasing salinity (TDS increase from 400 to 800-6000 mg l(-1): generally, TDS increases from the mountainous regions towards the discharge area) due to a long time of aridity, irregular rainfall and overexploitation (irrigation and industrial activities). Groundwater pumped from the semi-confined Complex Terminal (C.T) aquifers (Cretaceous and Mio-Plio-Quaternary: MPQ) and from the confined Continental Intercalaire (C.I) aquifers is an important production factor in irrigated oases agriculture and phosphate washing in Southwestern Tunisia. A rise in the groundwater salinity has been observed as a consequence of increasing abstraction from the aquifer during the last few decades. The salinization phenomena in the region are complex. Several possible causes for salinization exist: (1) the upwelling of saline and "fossil" water from the underlying, confined "C.I" aquifer; (2) as well as the backflow of agricultural drainage water; (3) phosphate and domestic wastewater; (4) brine intrusion from the salt lake (Sebkha/Garaat); (5) evaporate meteoric water dams (El Khangua and El Oudei); (6) reduced rainfall and (7) land and air alterations. The istopic study of waters establishes that the deep groundwater is "fossil" water (6000-37,000 years) recharged probably during the late Pleistocene and the early Holocene periods. The relatively recent water in the MPQ aquifer is composed of mixed waters resulting presumably from upward leakage from the deeper groundwater. (C) 2014 Elsevier Ltd. All rights reserved.