Integrated hydrochemical and multi-index assessment of groundwater quality in phreatic and deep aquifers of the Bou Omrane–Sabkhet Ennaouel area (Southern Tunisia)

Salinization and nitrate contamination are major global threats to groundwater sustainability, particularly in semi-arid regions. In southern Tunisia, these issues are intensified by the combine effect of surface-water scarcity, low average annual precipitation, and excessive groundwater abstraction, emphasizing the need for continuous integrated hydrochemical monitoring and further exploration labors. The Bou Omrane–Sabkhet Ennaouel is ~1668 Km2 country extending between southeastern Gafsa and southern Sidi Bouzid regions in south Central Tunisia and relies exclusively on its own groundwater resources for both domestic supply and agricultural irrigation. Groundwater abstraction therein targets both shallow (phreatic) and deep aquifers, both constitute together the primary freshwater sources of the region. In this work, a physicochemical characterization including analyses/ measures of pH, electrical conductivity (EC), Total dissolved Solids (TDS), major ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻, HCO₃⁻, CO₃²⁻), and nutrient-related pollution indicators (NO₃⁻, NO₂⁻, NH₄⁺, PO₄³⁻) was done at the Laboratory for Applied Geology and Hydrogeology, Department of Geology, Ghent University (Belgium), for 19 and 20 samples from shallow and deep aquifers, respectively. In addition, hydrochemical relationships were examined using correlation analyses, while water quality was evaluated through a multi-index framework integrating the Water Quality Index (WQI) for drinking purposes and the irrigation criteria index.

It was found that the groundwater chemistry is dominated by Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ among cations and SO₄²⁻ > Cl⁻ > HCO₃⁻ > NO₃⁻ among anions. Strong correlations between Na⁺–Cl⁻ and Ca²⁺–SO₄²⁻ suggest common geochemical controls associated with evaporite dissolution and salinization processes. The WQI results indicates the absence of excellent-quality water in both aquifers. In the phreatic aquifer, 12% of samples are classified as good, 21% as fair, 12% as poor, and 55% as extremely poor, whereas the deep aquifer exhibits more severe degradation, with 67% of samples falling into extremely poor class. For irrigation use, salinity constitutes the primary limiting factor. Electrical conductivity, as it is common measure of salinity, classifies 68% of samples as unsuitable for irrigation, while sodicity-related indicators are generally favorable, with most samples presenting SAR values below 10 and acceptable magnesium ratios. The permeability index (PI) and Kelley ratio (Kr) indicate suitable to moderate irrigation water quality, although some samples exhibit PI values below 25. Wilcox and USSL diagrams confirm the predominance of doubtful to unsuitable classes (C3–S1 to C4–S2), indicating significant agronomic risks related to soil structure degradation and crop productivity. Overall, the phreatic aquifer appears more vulnerable due to the limited thickness of the vadose zone, which facilitates contaminant infiltration. These findings highlight the urgent need to strengthen groundwater governance in the Bou Omrane–Sabkhet Ennaouel region through systematic monitoring of groundwater quality and abstraction, integration of field data into administrative databases, and the implementation of adaptive management strategies such as controlled drainage, water blending, and selection of salt-tolerant crops to ensure long-term groundwater sustainability.