Environmental assessment of sewage disposal systems using 2D electrical-resistivity imaging and geochemical analysis: A case study from Ibadan, southwestern Nigeria

A case study is presented of integrated use of 2D electrical-resistivity imaging and geochernical analysis in environmental impact assessment of sewage disposal systems. This involved geoenvironmental studies around a septic tank in Ibadan, southwestern Nigeria. The area is underlain by quartzites and quartz schists, whereas the weathered profile consists of sandy and silty clay with some gravel. Twenty-two multi-electrode resistivity profiling lines were taken using the Wenner array. The electrode spacing ranged from 1 to 8 m with a station interval of 1 m. Resistivity data analysis involved inversion using a rapid least-square technique and construction of iso-apparent resistivity contour maps. Four pits were dug, with two of the pits located on points of resistivity lows and the other two pits on points of high resistivity values. The pits were dug to a total depth of 2 m and soil samples collected from each pit at 0.5-, 1.0-, 1.5-, and 2.0-m depths. The samples were analyzed for conductivity and for Pb, Fe, Cu, Cr, and nitrate concentrations. The resistivity survey yielded low resistivity values at locations close to the septic tank and higher values farther from it. This agreed with the electrical conductivities measured for soil samples. Statistical analyses of the conductivity variation with distance from the septic tank yielded a negative correlation coefficient of between -0.92 and -0.52, indicating an inverse relationship between the two quantities. Similar relationships were established for analyzed chemical constituents. The environment is impacted by the septic system as shown by soil sampling and geophysical results. The results demonstrate that pollution mapping in a basement complex environment can be carried out using the approach described in this paper.