When solid waste is disposed of at landfills, it undergoes a combination of physical, chemical and microbial processes. These processes help to transform the waste into various water-soluble compounds termed (leachate) and transfer the pollutants from the refuse to the percolating water, leading to contamination of surrounding groundwater and surface water. Characterisations of the leachate originating from the Isale-Ijagba and Oke-Diya dumpsites (situated in Sagamu, Ogun State, Nigeria) as well as evaluation of the pollution potential on the quality of the local aquifer were carried out.

The composite sampling method was employed and samples collected over a period of five weeks, parameters such as, Electrical conductivity (EC), pH, Total Dissolved Solid (TDS), Chemical Oxygen Demand (COD), Biological Oxygen Demand (BOD), NH3–N, PO4–P, SO42−, Cl−, K+, Cd, Mn, Zn, Ni, Fe and Pb were determined using standard method for water and waste water analysis. And the data gotten analysed using Microsoft excel spreadsheet.

Mean concentration values for TDS, BOD, COD, NH4+, PO4+, SO4+ and Cl- are 493 and 534 mg/L; 1.50 and 1.08mg/L; 0.28 and 0.60 mg/L; 0.46 and 0.28 mg/L; 0.46 and 0.72 mg/L; 18.46 and 25.13 mg/L for the two monitoring wells respectively. The highest observed BOD5 and COD values were 319 and 846 mg/L; 525mg/L and 1105mg/L for Isale-ijagba and Oke-Diya leachate samples respectively. The mean concentration values for Fe, Mn, Zn, Pb, Ni and Cr- in groundwater samples are 2.81 and 1.39mg/L; 0.02 and 0.08mg/L; 0.22 and 0.21mg/L ; 0.61 and 0.61mg/L; 0.17 and 0.14mg/L and 0.11 and 0.09mg/L for Isale-Ijagba and Oke-Diya monitoring wells respectively.

The mean concentrations of all measured parameters except, Pb, Fe and Cr conform to the stipulated World Health Organization potable water standard and the Nigerian Standard for Drinking Water Quality. It was however observed that in the absence of a properly engineered landfill system, percolation of leachates into waterbodies could be deleterious in the very near future. The study recommends an upgrade of both landfill so as to guarantee adequate protection of both the surface and the groundwater resources in the locality.