Characterization of site contaminated with petroleum-effluents and evaluation of neighbouring groundwater quality using combined electrical resistivity tomography and geochemical methods

A geoelectrical characterization of a petroleum effluents-contaminated site and geochemical analysis of nearby water sources for polycyclic aromatic hydrocarbons (PAHs) were used to ascertain the extent of pollution of the subsurface around an oil depot site. Six traverses of 2-D electrical resistivity tomography (ERT) was laid using Wenner array with uniform electrode spacing ranging from 5 to 25 m at 5 m incremental step. The field resistivity data were processed and inverted using RES2DINV to obtain 2D model sections of the traverses. Water samples were collected from 15 sampling points and analyzed for polycyclic aromatic hydrocarbons (PAHs) using Agilent 7890A Gas Chromatography (GC) coupled with 5975C mass spectrometry. The 2D ERT sections showed low resistivity anomalies with values < 100 Ωm as mature/biodegraded LNAPLs plume up to average depth of 6—9 m. The concentrations of total PAHs in surface and groundwater ranged from 0.01 to 0.09 mg/L and 0.01 to 0.25 mg/L, respectively. The results show a good concurrence between the low resistivity anomalies and the summation of total PAHs greater than permissible limit in more than 70% of analysed water samples. The non-carcinogenic risk is normally below 1.0, whereas the total cancer risk (TCR) values for adults and children via oral ingestion and dermal contact are above the permissible limit of 1.00E-04, with children being the most susceptible. This study highlights the importance of integrated methods for better understanding of the extent of petroleum effluent contamination and recommends outright stoppage of effluent release into the adjoining stream.