Abstract
Geophysical and hydrochemical investigations have been undertaken in Ekehuan Dumpsite in Benin City, Southern Nigeria with a view of assessing the impact of effluent from the dumpsite on the soil and groundwater system. The geophysical investigation involved electrical resistivity methods using dipole-dipole profiling and Schlumberger Vertical Electrical Sounding (VES). Limited physicochemical analysis of soil sediments and water sample within the active dumpsite were carried out. A total of three(3) dipole-dipole profiling, seven (7) Schlumberger electrical soundings were carried out. Fifteen (15) soil/sediments were collected from two auger holes around the dumpsite at depth ranges of 0-3.5m and 0-6m, to gain insight into the near surface geology of the site. In addition, one water sample from the dumpsite precinct was collected. Both water and soil/sediments were analysed using the atomic absorption spectrometer(AAS) model-solaar 969 unicam series.The results from electrical surveys show that the study area is underlain by resolvable five to eight geoelectric layers namely, the topsoil, lateritic sand ,clayey sand, silty sand, clay, saturated sand and dry sand with resistivity value ranging from 14.97-250.87 Ohmm,90.38-3049.3 Ohm-m,6.98-1364.7 Ohm-m,24.95-3050.8 Ohm-m,54.09-999.44 Ohm-m,37.34-818.48 Ohm-m,140.45-3243.1 Ohm-m and 187.96-3511.7 Ohm-m respectively . Their thicknesses range from 0.15-0.68m,0.44-1.59m,1.15-5.40m,0.92-12.94m,3.31-46.06m and 8.29-53.81m. The results from the dipole-dipole profiling showed areas of low resistivity between 160m-180m interpreted as possibly indicating the presence of leachate. The analytical results showed that both the groundwater and soil/ sediments in the study area were acidic with pH values of 6.2 for the groundwater and a range of 4.6-5.0 for the soil sediments. The analytical results of groundwater provides the following heavy metals concentrations: Iron (0.542mg/l),Manganese(0.074mg/l),Copper (0.031mg/l), Zinc (0.122mg/l), Lead (0.021mg/l), Cadmium (0.015mg/l), Chromium (0.025mg/l), Nickel (0.010mg/l) and Vanadium (0.004mg/l). These results were compared to the World Health Organisation (WHO) and Nigerian Standard for Drinking Water Quality (NSDWQ) with the conclusion that groundwater has not been impacted. However, the near surface low resistivity values from the dipole-dipole results and a few above normal soil parameters results indicate the presence of leachate below the surface. It is therefore recommended that subsequent work should be carried out in the study area to monitor the transport of leachate into the groundwater
