Time-lapse electrical resistivity tomography (ERT) and seasonal variation studies of the physicochemical properties of groundwater were carried out on the Olushosun dumpsite in Lagos South-western Nigeria, to monitor, track the depth of leachate contamination, and to investigate the impact of seasonal variation on groundwater quality around the dumpsite. Six 2-D resistivity imaging lines were investigated. The independent inversion of the dipole dipole and pole-dipole resistivity data indicated that contaminated zones are characterised by resistivity values ranging from 0.63 to 12.5 Omega m, and a maximum depth of 141 m was investigated. The pole-dipole models show clear evidence of vertical migration of contaminant with time, as depth of contamination increased from 106 m in May 2014 to about 120 m in December 2015 around the investigated portion of the dumpsite. Analysis of the seasonal variation of the examined physicochemical properties of the water samples taken from wells and boreholes within and around the site showed that there is increase in concentration of TDS and EC in the dry season study and a corresponding increase in the mean concentration of pH, Ca, Mg, hardness, Cu, Cr, NO3, SO4 and Na from the dry season results. Also, there is reduction in the mean concentration of Fe, Zn, Mn, PO4, Cl, and Ni in the dry season when compared with the wet season analysis. Generally, there is a strong correlation between the ERT results and the physicochemical parameters of ground water quality viz-a-viz the contamination status of the Olushosun dumpsite. This increased trend in the dry season period could be attributed to the increase in concentration of the dissolved metals due to evaporation, and on the other hand, dilution effect of the rainfall during the wet season. South East direction of contaminant flow was established from the water table contour lines produced for the area. The research has clearly shown that the groundwater within the study area has been impacted by the leachate from the decomposed refuse at the dumpsite and may constitute danger to the life of residents living around the dumpsite.
