The evolution of groundwater quality at the dry zone in Sri Lanka was investigated through chemical and isotopic analyses of field samples. The results indicated that the occurrence of high concentrations of major ions in the groundwater, and high electric conductivity were synonymous with large seasonal changes. Chloride and sodium were the major ions, which contributed to the increase in electric conductivity in both the rainy and dry seasons. In the groundwater, sodium-chloride was present in the final stage of the Chebotarev series. Evaporation and enrichment by air borne salts contributed to very high concentrations (E.C, chloride et al.) in some areas. Conversely, water seepage by water from a tank caused dilution, and the concentrations of major ions in the nearby groundwater were low. The isotopically lighter groundwater was found at the ridge of the valley in the rainy season. Under very heavy precipitation conditions, the slope of the regression line relating δD and δ18O to deuterium excess for groundwater were close to 8 and 10, respectively. In other cases, these slopes were much less, suggesting that the groundwater was strongly affected by the evaporation process.