International Journal of Science, Engineering and TechnologyAuthors: Ernest Ikotiko, E, Osayande, A.D
Abstract: Accurate understanding of the spatial and vertical distribution of petroleum hydrocarbons in post-spill soils is very important to develop effective intervention strategies, assess the processes of natural attenuation of spill impacts, and conduct risk assessment according to regulatory requirements. This study examined the spatial variability and vertical distribution of total hydrocarbon content (THC) and associated physicochemical properties in soils impacted by a sabotage incident that ruptured a pipeline at Obele-Ibaa, Emohua Local Government Area, Rivers State, Nigeria, on April 14, 2015. Soils were sampled approximately 1 year post-spill from three paired topsoil and subsoil locations at depths of 0-15 cm and 15-30 cm, respectively, and one uncontaminated control site. The total hydrocarbon content was determined using n-hexane as a solvent and ultraviolet spectrophotometry as the analytical method. Spatial variability was characterized using contamination factors (CF), coefficient of variation (CV), and vertical distribution ratios (VDR = subsoil THC/topsoil THC). The THC values ranged from 1,302.2 to 1,860.7 mg/kg with a mean of 1,495.6 mg/kg and a coefficient of variation of 17.3 % in the topsoil samples, indicating moderate uniformity of surface contamination. In contrast, the THC values ranged from 769.6 to 4,443.6 mg/kg with a mean of 2,021.2 mg/kg and a coefficient of variation of 84.8 % in the subsoil samples, indicating high spatial variability with vertical distribution ratios ranging from 0.59 to 2.39. This study has shown that there is a two-tier contamination architecture with a uniform surface layer formed by initial spill impacts and a spatially heterogeneous subsoil layer that is governed by preferential flow through macropores and structural discontinuities in the soils. Position P3 was identified as a sub-surface hotspot with high THC values of 4,443.6 mg/kg and a high vertical distribution ratio of 2.39, indicating preferential vertical flow through a high permeability path. All the contamination factors were above the Department of Petroleum Resources (DPR) threshold of 50 mg/kg by 15 to 2,020-fold. This study has shown that spatially uniform intervention strategies are inappropriate for spill impacts of this class and that sub-surface hotspots need to be targeted during cleanup operations. A spatial remediation prioritization framework based on Tier I, Tier II, and Tier III is proposed based on contamination factor values and vertical distribution ratio values.
