Determination of the origins and recharge rates of the Sfax aquifer system (southeastern Tunisia) using isotope tracers

The origin of groundwater in the Sfax aquifer system was studied using environmental isotopic tracers (δ¹⁸O, δ²H, and ³H). In total, 164 water samples were analyzed for stable isotopes: 73 collected from the Sfax shallow aquifer, 63 from the Sfax middle aquifer, and 28 from the Sfax deep aquifer. Recent recharge of the groundwater in the Sfax aquifer was identified using tritium concentrations in 82 groundwater samples from different depths. The isotopic ratios of the shallow aquifer range from −5.55 to −1.59 ‰ for δ¹⁸O and from −38.38 to −14.19 ‰ for δ²H, and the isotopic ratios in the middle aquifer range from −6.86 to −2.97 ‰ for δ¹⁸O and from −44.18 to −22.38 ‰ for δ²H. The deep aquifer exhibited markedly lower isotopic values, ranging from −6.70 to −5.70 ‰ for δ¹⁸O and from −42.40 to −38.89 ‰ for δ²H. The mixing proportions inferred from stable isotopic mass balance calculations suggest that the deep aquifer contributes significantly to the middle aquifer through geologic structures and may reach 100 % in the Menzel Chaker region. The isotopic mass balance model also indicates that the middle groundwater aquifer may contribute up to 100 % of the shallow Plio-Quaternary aquifer, particularly in the western and northeastern parts of the study area, between Bir Ali ben Khalifa and Djebeniana. The tritium data support the existence of recent recharge. The tritium and stable isotope data clearly indicate the presence of mixing processes, especially in the northwestern and coastal portions of the study area. A conceptual model is established, explaining the pressure differences that generate vertical leakage, which is a reasonable mechanism for flow between the aquifers.