Abstract: The biochemical effect of leachate (using Odo Iya Alaro landfill located in Ojota, Kosofe local government of Lagos State as a case study), on the quality of surrounding groundwater was assessed. Water samples were collected from two hand dug wells and two boreholes at about 2 km radius to the landfill. The physicochemical and microbial characteristics of the leachate-contaminated groundwater samples were determined. Compared to the tapwater, the leachate-contaminated groundwater samples and the simulated leachate showed significantly (p<0.05) higher concentrations of heavy metals such as Pb, Cd and Cr. Coliform bacteria such as E. coli, Shigella sp. and Salmonella sp. were present in the leachate-contaminated groundwater samples and simulated leachate. Overall, the data revealed that consumption of leachate-contaminated groundwater may lead to, among other things, heavy metal toxicity such as impaired renal function and possibly cancer. The evidences from this study, therefore, suggest that consumption of leachate-contaminated groundwater is hazardous and therefore should be discouraged.