Use of geochemical, isotopic, and age tracer data to develop models of groundwater flow: A case study of Gafsa mining basin-Southern Tunisia

Highlights

• To investigate the sources of groundwater contamination in the semi-arid south-western Tunisia.

• To identify the hydrodynamic functioning of the multilayer aquifer system.

• To identify inter-aquifer mixing.

Abstract

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 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 (≈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 Oudeï); (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.

Introduction

Geochemical indicators constitute effective tools for solving various problems in hydrology, in particular in the arid and semi-arid regions (Fontes, 1980, Clark and Fritz, 1997, Cook and Herczeg, 2000, Etcheverry, 2002). About 30% of land area on the Earth is arid or semi-arid where potential evapotranspiration exceeds rainfall (McKnight and Hess, 2000). These areas, collectively called “the arid and/or semi-arid zone”, constitute much of the Earth’s land between latitudes 18° and 40° north and south of the equator and include most of northern and southern Africa, the Middle East, western USA and the southern areas of South America, most of Australia, and large parts of central Asia and even parts of Europe (NOAA, 2010). In arid/semi-arid countries, issues related to water resources are of growing concern due to different environmental, economic and social factors. Continuously increasing abstraction of groundwater resources to meet rising industrial, agricultural, domestic and touristic needs, coupled with severe drought periods during the past decades leads to growing deficit of water. The drawdown of piezometric levels, progressing degradation of water quality, extinction of the artesianism (drying of spring and disappear of the Foggara “Mkayel, traditional system of irrigation”) are the main consequences of such intensive exploitation of MPQ aquifer (2500 l s⁻¹) (Hamed et al., 2010b, Hamed et al., 2013a, Hamed et al., 2013b, Hamed et al., 2013c). But after 2011 the number of illegal wells has been increased; the quantity of water discharge by exploitation is about 3200 l s⁻¹. Due to extensive pumping, agricultural and industrial activities, aquifers are at risk of being contaminated (Choura, 2010, Hamed et al., 2012a, Hamed et al., 2013a). Intensive application of pesticides and fertilisers, discharge of wastewater, and industrial effluent and excessive groundwater abstraction are just a few examples of activities that lead to groundwater contamination. These activities have resulted in the deterioration of water resources in various regions around the world (Pandey et al., 1999, Al-Gamal, 2011, Hamed et al., 2012a, Hamed et al., 2012b). Aquifers are valuable sources for water in the mining basin of Gafsa. Therefore, a quick action should be taken to prevent aquifers from contamination and to reduce the risk of contamination impact. Groundwater contamination risk mapping can help planners and decision-makers on proper land use and water resources management. This will enable incorporation of groundwater protection and health impact assessment in the analysis. Risk mapping is not only a preventative measure but it also assist with mitigation processes of groundwater contamination. In groundwater context, risk can be defined as the probability that groundwater at a drinking well becomes contaminated to an unacceptable level by activities on the land surface (Morris and Foster, 1998). Risk can be reduced by implementing a mitigation strategy with best management practice. Best practice avoids high-risk areas when locating a site of possible pollution potential. In this study, hydrogeologic, hydrochemical and isotopic data from the aquifer system will be integrated and used to determine the main factors and mechanisms controlling the chemistry of groundwaters in the study area (southwestern Tunisia) and to identify the origins of water bodies and their migration pathways. The main issues that will be addressed by this study include: (1) the relative importance of geologic and hydrogeologic factors in controlling the groundwater circulation within the aquifer system; (2) the geochemical effects and age of leakage influencing groundwaters and (3) the appraisal of the value of chemical and isotopic (²H, ¹⁸O, ³H, ¹³C and ¹⁴C) approaches to delineate flowpaths and evaluate evolutionary processes.