This study presents a hydrochemical and microbiological assessment of groundwater quality in Al Marj City, northeastern Libya, to evaluate the risk of contamination of the karst aquifer caused by the city’s main sewage wastewater pond (216.58 ha) and uncontrolled cesspits. Groundwater samples were collected from five wells on a single day (2 June 2026) and analyzed for standard physicochemical and microbiological parameters. Pearson correlation analysis was also applied as a statistical tool to explore relationships among the measured variables. The results revealed that several water quality parameters exceeded the permissible limits established by Libyan drinking water standards and the World Health Organization (WHO). All groundwater samples were classified as very hard water, with total hardness exceeding 500 mg/L as CaCO₃. Microbiological analysis confirmed fecal contamination in all wells, with Well No. 4 exhibiting the highest Escherichia coli (E. coli) concentration of 387.3 MPN/100 mL. Statistically, a very strong positive correlation was observed between E. coli and Enterococci (r = 0.999), while a moderate positive correlation was found between total dissolved solids (TDS) and nitrate (r = 0.606). These correlations should be considered preliminary and exploratory because of the limited sample size. The findings indicate that groundwater from all five wells is unsuitable for drinking and provide strong evidence of contaminant migration through karst groundwater flow pathways. The study highlights the urgent need for groundwater treatment and comprehensive improvement of the city’s wastewater management system to protect public health................... Keywords:..................Wastewater infiltration, Faecal indicators, Hydrochemistry, Water hardness, Carbonate rocks.