Spatio-temporal variability of hydrochemical parameters and heavy metals in shallow groundwater of the area of Cebala–Borj–Touil, irrigated with treated wastewater (Tunisia)

The area of Cebala–Borj–Touil, located in northeastern Tunisia, is irrigated with treated wastewater (TWW) for over 26 years. The objectives of this study were to investigate the state of the hydrochemical characteristics and the heavy metals content and their spatial and seasonal variability. This area is affected by a shallow saline groundwater. Depth of this groundwater has been measured and water samples from 16 sites have been collected during two periods, September 2014 and March 2015, which correspond, respectively, to the post-irrigation period and before new cycle of irrigation, after the rainy season. Water samples were analyzed for pH, electrical conductivity (EC), major ions as well as the trace elements Zn, Fe, Cu, Cd, Co, Cr, Ni, Pb, and Mn. Concerning the hydrological condition, the depth average was 1.31 m in the summer and 0.70 m in the winter and a maximum 2.01 m at the end of the summer and a coefficient of variability (CV) about 35% in the summer and 78% in the winter indicating more high variability in the winter than in the summer. These values highlight bad drainage condition of the area. For chemical parameters, the water from both sampling periods is characterized by abundance of the major ions in the following order: Na⁺ >> Mg²⁺ > Ca²⁺ > K⁺ and Cl⁻ > SO₄²⁻ > HCO₃⁻. The mineral content of the groundwater is controlled by different factors: the water source, dilution in the wet season (March), and evaporation in the dry season (September). Spatially, the northeast and the southwest of the sector are more affected due to their relative low altitude. The content of heavy metals exceeds in some sites the values of the Tunisian standards of reuse. During both campaigns, the content of Cobalt (Co), Chromium (Cr), Manganese (Mn), and Iron (Fe) exceeds the limits in the groundwater. For lead (Pb), the exceeding varies between September 2014 and March 2015. Cadmium (Cd), nickel (Ni), zinc (Zn), and copper (Cu) did not exceed Tunisian standards.