This research article published by IWA publishing, 2021This study used the hydrochemical properties of water to reveal the causes of water quality degradation. The results showed that most samples located downstream of the mine tailings dam were slightly acidic with pH as low as 4.6. Samples with high levels of Na+, Cl−, and could not be isotopically linked to the local geochemistry, but the anthropogenic activities and evaporation were probably responsible for the observed water chemistry. The Piper diagram indicated cations were dominated by Ca and Mg, while anions were dominated by bicarbonates and sulphates. Pb and Hg levels (mean 70.29 and 17.95 μg/L, respectively) were all higher than the World Health Organization (WHO) and Tanzanian drinking water guidelines. Mining activities probably contributed to the observed low pH values and elevated concentration of cyanides, heavy metals, and metalloids. Stable isotope results indicated a vulnerability of the water to recent contamination that could be attributed to anthropogenic activities. Moreover, isotopic studies indicated a flow pattern from the eastern to the western side of the mine study site. Lowland samples were more 3H-enriched than highland ones. The present study concludes that groundwater recharge from recent local precipitation may have an impact on the sources studied.