In arid and semi-arid areas groundwater quality management is crucial for water sustainably. This study was conducted to determine the variation in hydrochemical facies, understand the evolution of hydrochemical processes and to assess the suitability of groundwater in shallow aquifer of Gafsa South El-Guettar (Southwestern of Tunisia) for both agricultural and drinking purposes. Groundwater samples were collected from shallow wells and analyzed for various physicochemical parameters. Piper's plot revealed that groundwater belongs to Ca2+-Mg2+-SO42- type, illustrating the presence of permanent hardness and the dominance of alkaline earths over alkali, with a nondominant cation type. The results indicate that groundwater has high concentrations of dissolved ions, primarily dominated by SO(4)2-, Mg2+ and Ca2+.The key processes driving the chemical evolution of Gafsa South El Guettar shallow aquifer include the dissolution of gypsum, anhydrite, and halite, along with direct and reverse ion exchange. quality. Concentrations of major anions, TDS, and EC exceed the admissible limits set by the World Health Organization (2011) and the National Water Exploitation and Distribution Company (SO.N.E.D.E, NT.09.14) guidelines for drinking water in most cases. Drinking Water quality index (DWQI) classed the water samples as unsuitable for drinking. Sodium adsorption ratio (SAR), sodium percentage (Na%), MH (Magnesium Hazard), KR Kelly's ratio and Sodium Soluble Percentage (SSP) were used to assess the water quality for agricultural propose (IWQI). One well has an 'excellent' water type, 68.75% of wells are classed as 'good' water type and 25% of wells are in permissible class based in Na% classification. Statistics analysis of the effective weight values of each water quality parameter in all studied locations shows that among the selected water quality parameters, EC, HCO3-, total Hardness and SO42- exhibit the largest mean effective weight values compared to the other parameters. Thus, these parameters are considered as the most effective parameters in the IWQI values. Results indicate that water quality is poor and should be monitored from a health perspective.Graphical AbstractThe graphical abstract visually summarizes our study on the identification of hydrochemical facies and the assessment of groundwater quality for drinking and agricultural use in the Gafsa South-El-Guettar shallow aquifer, Tunisia. It illustrates analytical techniques applied to water samples and the various methods used for data interpretation, including Piper and Gibbs plots, correlation matrix, and principal component analysis, to assess hydrochemical evolution and identify contamination sources. Mathematical models were used to calculate water quality indices, and GIS techniques were applied to visualize the spatial distribution of water parameters and quality indices for both drinking and agricultural proposes (DWOI, SSP, %MH, SAR, TDS, EC, %Na, KR, etc.). A key finding of this study is the first application of Water Quality Index (WQI) method in the studied region. Results indicate that groundwater samples predominantly fall within the Ca-2(+)-Mg-2(+)-SO42- water type, with water chemistry primarily influenced by dedolomitization which is a process involving dissolution reactions with carbonate minerals and gypsum, as well as reverse ion exchange. The study also reveals that agricultural activities and phosphate mining have significantly contributed to groundwater quality degradation. By illustrating these cause-and-effect relationships, graphical abstract enhances the understanding of the aquifer's hydrochemical facies and highlights the critical state of groundwater quality, emphasizing the urgent need for mitigation strategies.