Assessment of soil contamination at the central sewage treatment plant, University of Nigeria, Nsukka

Abstract

This paper integrated geophysical and geochemical methods to study the pollution of soil at the sewage plant at the University of Nigeria, Nsukka. The plant has two large stabilization ponds. Vertical electrical sounding (VES) was carried out to determine the depth of plume, and thus evaluate the safety of the underlying water table aquifer. Tracking of plume direction was by the application of horizontal profiling (HP) and induced polarization (IP). Geochemical analysis of 12 soil samples collected at 1 m and 5 m depths from 6 augured holes around the plant were carried out to evaluate the levels of concentration of the pollutants via their pollution indices (geoaccumulation index (Igeo), enrichment factor (EF), contamination factor (CF), pollution load index (PLI)). Results of the 22 VES shots showed the depth of plume to be about 40 m. Together with VES and aquifer vulnerability mapping techniques a plume migration rate of 0.76 m/yr was evaluated and thus, it would take 144 years to reach the aquifer at average water table depth of 110 m. The HP and IP showed concentrations of plume face towards the northeast of the plant. The total dissolved solids (TDS) and pollution indices were observed to be higher along the direction of migration of the contaminant plume than the other areas. The Igeo and CF show decreasing soil pollution down the profile. The EF showed no variation in enrichment pattern with depth. The pollution load index (PLI) was found to be 1.44 and 1.22 at the depths of 1m and 5m respectively, which indicates that the soil is polluted at both depths, yet less polluted down the profile. However, it may be safe to conclude that the aquifer is safe, considering the travel time and also that the pollution of soil at the site decreases with depth.