Groundwater origins and mixing pattern in the multilayer aquifer system of the Gafsa-south mining district: a chemical and isotopic approach

Abstract

Major ion geochemistry and environmental isotopes were used to identify the origins and the mineralisation processes of groundwater flowing within the three aquifer levels of the multilayer system of the Gafsa-south mining district (Southwestern Tunisia). It has been demonstrated that groundwaters are characterised by a Ca-Mg-SO 4 water type. Geochemical pattern is mainly controlled by the dissolution of halite, gypsum and/or anhydrite as well as by the incongruent dissolution of dolomite. d 18 O and d 2 H values are much lower than the isotopic signature of regional precipitation and fall close to the meteoric water lines, indicating that groundwaters have not been significantly affected by evaporation or mineral-water reactions. The distribution of stable and radiogenic isotopes (d 18 O, d 2 H, d 13 C and 14 C) within the aquifer levels suggests that the deep confined aquifer receives a significant modern recharge at higher altitudes, while, the shallow unconfined aquifer has been mainly recharged under cooler paleoclimatic condition, likely during Late Pleistocene and Early Holocene humid periods. However, waters from the intermediate confined/unconfined aquifer have composite isotopic signatures, highlighting that they are derived from a mixture of the two first end-members.

Isotopic tracing for conceptual models of groundwater hydrodynamics in multilayer aquifer systems of central and southern Tunisia

“…Radiocarbon ( 14 C) results are reported as a percentage of the modern standard (pmC) and as the apparent radiocarbon age (BP), and 13 C results are expressed as δ 13 C, which is the deviation in per mil (parts per thousand; ‰) of the 13 C/ 12 C ratio of the sample, relative to the Pee Dee Belemnite (PDB) international standard (Craig 1957). The analytical error (2σ) was 0.1‰ for δ 18 O, 1‰ for δ 2 H, 0.7-1.4 TU for 3 H and 0.1‰ for δ 13 C. The new data of this study were combined with previously published isotopic data (Abidi 2007, Kamel 2007, Zammouri et al 2007, Dassi 2011, Hamed et al 2011, Tarki et al 2011 to mixing of waters from different sources (Sacks and Tihansky 1996). Their signatures in groundwater are generally discussed with respect to the Global Meteoric Water Line (GMWL) (Craig 1961) and the Regional Meteoric Water Line (RMWL), which correspond to the stable isotope signatures of modern rainfall at continental and regional scales, respectively.…”

“…These vary from 0 to 7 pmC in the Djerid-Nefzaoua basin, from 7 to 36 pmC in the Sbeïtla (Dassi 2011) and from 3 to 44 pmC in the Gafsa-south basin (Hamed et al 2011). This relatively low 14 C activity, which confirms the palaeoclimatic origin of these waters, is comparable to that of Late Pleistocene and Early Holocene palaeogroundwaters identified at a local scale in southern and central Tunisia (Fontes et al 1983, Mamou 1990, Ouda 2000, Yermani 2002, Edmunds et al 2003, Zouari et al 2003, Jéribi 2004, Dassi et al 2005, Kamel et al 2005, Kamel et al 2006, Hamed et al 2008, and at a global scale in North Africa, the Middle East and the Arabian Peninsula (Conrad and Fontes 1970, Gonfiantini et al 1974, Pearson and Swarzenski 1974, Edmunds and Wright 1979, Fontes et al 1983, Fontes 1985, Stute and Schlosser 1993, Daoud 1995, Clark and Fritz 1997, Edmunds et al 1997, Abd El Samie and Sadek 2001, Edmunds et al 2003, Guendouz et al 2003, Edmunds 2005).…”

“…They present relatively high radiocarbon activity, in the ranges 61-85 pmC in the Gafsa-south basin (Hamed et al 2011), 65-100 pmC in the Sbeïtla basin (Dassi 2011) and 77-103 pmC in the Djerid-Nefzaoua basin. These data support the stable isotope results and indicate that recent recharge has taken place.…”

“…The geology of these systems has been largely described by Dassi (2011), Hamed et al (2011 and Tarki et al (2011). Hydrogeologically, these systems are subdivided, based on lithology and position, into three superimposed hydrostratigraphic units .…”

“…Mixed groundwaters, which exist in the intermediate aquifer level in the Sbeïtla, Gafsa-south and Djerid-Nefzaoua basins, show wide ranges of 14 C activity, of 9.7-100 pmC (Dassi 2011), 3-75 pmC (Hamed et al 2011) and 0-28.5 pmC, respectively. These wide ranges of radiocarbon data bring insight to the mixing between a Pleistocene-Holocene original recharge component and modern groundwaters originating either from natural infiltration in the pediments of the mountains or from return flow in the irrigated areas.…”