Hydrogeochemical assessment and modeling of groundwater processes and pollution: a case study of the Grombalia aquifer in Northeast Tunisia

Groundwater in the Grombalia region often represents the main source of water, which is intensively exploited to meet the needs of human consumption and irrigation, threatening its quality. This study aims to assess the quality of groundwater in the Grombalia shallow aquifer, and to identify the hydrogeochemical processes controlling its mineralization, based on chemical analysis results of thirteen (13) groundwater samples taken in October 2020. The results show that the chemical element content of most groundwater samples exceeds WHO standards. Based on the abundance of cations and anions, the groundwater is characterized by a single hydrochemical facies (Cl–Ca–Mg), revealing the combination of natural and anthropogenic processes governing water chemistry in the study area. Multivariate statistical analysis, including principal component analysis, Pearson correlation and hierarchical cluster analysis, revealed the main processes controlling groundwater chemistry are agricultural activities and dissolution of evaporitic formations. Bivariate diagrams show that cation exchange reactions leading to the adsorption of Na+ into clay minerals and the simultaneous release of Ca2+ ions, as well as the dissolution of silicates, are also processes that influence groundwater chemistry. Finally, calculation of the nitrate pollution index (NPI) reveals that most groundwater samples are very heavily polluted (NPI > 3), which may be linked to anthropogenic activities, such as the return of irrigation water, as well as untreated urban waste. The suitability of groundwater for drinking was determined by using WHO guidelines and Water Quality Index (WQI) revealed the Grombalia aquifer have poor to very poorly quality water for drinking purposes.