Hydrochemical and isotopic study of groundwater in Wadi El Hechim–Garaa Hamra basin, Central Tunisia

Abstract

Hydrochemical and isotopic study of Miocene and Mio-Plio-Quaternary (M-P-Q) aquifers in Wadi El Hechim–Garaa Hamra basin, Central Tunisia was undertaken in order to investigate recharge mode and processes leading to mineralization of groundwater as well as interaction between both systems. The results revealed striking differences between the two aquifer systems. While the Miocene aquifer contains recently recharged waters with generally low mineralization (around 0.5 g L−1), stemming mainly from dissolution of carbonate minerals, the M-P-Q aquifer reveals TDS values reaching 3 g L−1, controlled mainly by dissolution of evaporitic minerals. Isotopic data indicate that the Miocene aquifer contains water recharged in past several decades (bomb tritium and bomb radiocarbon detected). The M-P-Q system appears to be much slower, with time scales of groundwater flow possibly reaching some thousands of years. Sharp discontinuity of hydrochemical and isotope characteristic of groundwater observed across the major tectonic fault separating the Miocene and M-P-Q aquifers supports the idea of very limited (if any) hydraulic interconnection between both studied systems. This in turn calls for revision of existing conceptual models of groundwater flow in the region postulating significant groundwater fluxes crossing the fault in the direction of M-P-Q aquifer and adjacent aquifers in the Wadi al Fakka plain.