Application of geochemical and isotopic tracers for the evaluation of groundwater quality in the irrigated area of the Sbiba plain (Central West Tunisia)

•Groundwater in the western, southern and central parts of the Sbiba plain is suitable for drinking, domestic and irrigation uses.•The deterioration of groundwater quality is linked to natural processes: water-clay interactions, dissolution and evaporation effects.•Anthropogenic pollution resulted from agricultural activities has a slight effect on WQI.•Tritium helps to define three recharge periods: from actual to 18 years, from 30 to 56 years, and older than 56 years. Hydrochemical and isotopic tracers of groundwater of the Mio-Plio-Quaternary aquifer in the Sbiba plain, central west Tunisia, are used to assess the groundwater quality for drinking and irrigation purposes as well as identifying sources of salinity variation. In this plain, the evaporation and water-rock interactions including dissolution of evaporite and carbonate minerals and cation exchange are the principal hydrochemical processes controlling the groundwater salinization and water types. Moreover, a secondary effect of salinity increase is due to the return flow of irrigation water and mixing between old and recent water within the aquifer. These phenomena are proved by the environmental isotope study. Several water quality parameters for drinking and irrigation purposes were applied. Water Quality Index (WQI) values indicate that 50 % of groundwater samples are suitable for drinking usage with “excellent” and “good” categories. According to Percent Sodium (%Na), Sodium Adsorption Ratio (SAR) and Residual Sodium Carbonate (RSC), all groundwater samples are suitable for agricultural irrigation activities. Based on the electrical conductivity (EC) and Wilcox classification, it appears that only 57 % of water samples are suitable for irrigation use; more than 50 % of these samples are “good” and “permissible”. In general, groundwater in the western and central parts of the Sbiba plain is suitable for drinking and irrigation purposes. However, an attention should be accorded to groundwater salinization processes.