Use of Geochemical, Isotopic, and Age-Tracer Data to Develop Models of Groundwater Flow - A Case Study of Gafsa Mining Basin-Southern Tunisia - 2014

Geochemistry & Hydrochemistry

Hamed and 8 co-authors reported in 2014 that hydro-(major and trace elements: Cd, F and Sr), isotope (¹⁸O, ²H, ³H and ¹³C) geochemistry and radiogenic
carbon (¹⁴C) 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 development to accommodate all the water demands of this arid area.

The Gafsa basin contains a multi-layered aquifer with four principal levels:

1. Upper Zebbag (Cenomanian–Turonian),
2. Abiod (Campanien–Maastrichian),
3. Beglia (Miocene) and
4. 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 (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 production 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)
4.  brine intrusion from the salt lake (Sebkha/Garaat);
5.  evaporate meteoric water dams (El Khangua and El Oudeï); (6)
6.  reduced rainfall and
7.  land and air alterations.

The isotopic 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.