Two industrial and highly populated cities namely Ibadan and Lagos, both located in Southwestern Nigeria have urban migration problems and resource limitations. As a result, the development of residential areas near waste sites and the indiscriminate dumping of municipal waste are common in both cities. Orita-Aperin and Oworonsoki neighborhoods in Ibadan and Lagos, respectively, both located near a waste site were studied. The two areas relied on the wells as sources of drinking water hence poor drinking water quality may have health consequences. A total of 51 groundwater samples (30 wells in Ibadan and 21 wells in Lagos) were monitored seasonally for two years in Ibadan and a year in Lagos. Results from this study revealed that some of the groundwater quality constituents determined exceeded the World Health Organization (WHO) standards for drinking water irrespective of source of pollution. Some of the groundwater samples were poor in quality in terms of pH, conductivity, total dissolved solids, chloride, nitrate, ammonia, COD, Al, Cd, Cr, Fe, Pb, Ni and total coliforms recorded. Thus, groundwater from some of these private wells requires further purification to ensure its fitness for human consumption. F-test, one-way parametric analysis of variance (ANOVA), Mann-Whitney 'T' test and Kruskal-Wallis H-test applied to upgradient and downgradient concentrations suggest impact of the waste sites on groundwater quality. The Mann-Whitney test only suggests that the downgradient values of Fe (Ibadan: dry season (1), sulfate (Ibadan: rainy season (2) and pH (Lagos: dry season (2) were significant at 5% level of significance. For lbadan wells, the Kruskal-Wallis test showed that variances of specific conductivity, dissolved solids, and chloride for lbadan upgradient and downgradient values were unequal. Also variances of specific conductivity and dissolved solids for upgradient and downgradient values in Lagos were unequal at 5% level of significance. Three downgradient wells in Lagos and four downgradient wells in Ibadan had significant impact due to leachate migration into drinking wells.
