Abstract
The study was aimed at assessing the vertical movement of leachate from 2001 through 2015, and groundwater physicochemical parameters analysis around the Olusosun dumpsite, through a time-lapse study.Tracking the leachate vertical migration profile was achieved through analysis of VES time-lapse data obtained from previous investigations on the dumpsite between 2001 and 2006, and 2D resistivity data acquired from this study in 2014 and 2015. Seventeen borehole and hand-dug well water samples were obtained randomly from 17 sampling locations around the dumpsite, first, in August 2014 (wet season) and then a repeat in December 2015 (dry season). The results of the VES data and 2D resistivity analysis showed that there is a constant and progressive increase in the depth of migration of the contaminants generated on this site into the subsurface environment over the years. It also showed that the contaminated zones are characterised by resistivity values ranging from 0.63 to 12.5 Ωm, and a maximum depth of 120 m has been impacted by the contaminants. The results of the water sample analysis showed clear variations for the various elemental constituents analysed for. Most of the physicochemical parameters analysed fell within the World Health Organisation (WHO) and the Nigerian Standard for Drinking Water Quality (NSDWQ) standard limit. Comparing the wet and dry season results, there was generally an increase in the mean concentrations of TDS, EC, temperature, hardness, Ca2+, Mg2+, K+, Na+, {text{Cl}}^{ - } , {text{NO}}_{3}^{ - } , Zn, Cu, Pb and Cr from the dry season study, particularly the samples with close proximity to the dumpsite. On the other hand, there was reduction in the mean concentrations of {text{SO}}_{4}^{2 - } , {text{PO}}_{4}^{3 - } , Fe, pH, and Ni from the dry season study when compared with the wet season. This trend could be attributed to the fact that in the absence of rainfall during the dry season period, contaminant accumulation in groundwater continued undiluted due to continuous breakdown of the biodegradables units with time, and this led to the generally high concentration of the physicochemical properties during the dry season. The implication is that the water around the study area has lower quality in the drier season when compared to the quality of water during the wet season, and so water treatment should be given more priority during the dry season before consumption and usage for other domestic purposes to avoid waterborne diseases. Generally, the information obtained from the geophysical study will facilitate decisions on improving protection for groundwater resources around the study area.
Highlights
An integrated geophysical survey consisting of 2D electrical resistivity imaging, time-lapse vertical electrical sounding (VES) and seasonal physicochemical measurement of groundwater parameters was conducted at the dumpsite with the aim of assessing and investigating the vertical migration profile of leachate contamination over time
The results show a general increase in the electrical conductivity (EC) concentration in the dry season when compared with the wet season
Analysis of VES data from previous investigations and 2D resistivity models obtained from this study show that there is a progressive increase in the depth of migration of the contaminants generated on this dumpsite into the subsurface environment over the years
Summary
An integrated geophysical survey consisting of 2D electrical resistivity imaging, time-lapse vertical electrical sounding (VES) and seasonal physicochemical measurement of groundwater parameters was conducted at the dumpsite with the aim of assessing and investigating the vertical migration profile of leachate contamination over time. Applied Water Science (2018) 8:142 that of pollution to the environment, and more challenging is how to determine the extent of environmental pollution to the surface and subsurface environment When it rains on a landfill, water percolates through the cells and soils in the landfill. There might be migration of different organic and inorganic chemical compounds to the unsaturated zone of the soil which may reach the saturated zone (Mondelli 2004). These contaminated liquids may leave the disposal site, enter the underlying groundwater system and make the groundwater unpotable. The methods are not used to detect contamination directly, but rather, they reveal contamination through sharp variation in subsurface resistivities as a result of the presence of these contaminants
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