Spatial and Vertical Distribution of Hydrocarbon Contamination in Coastal Plain Sands of the Niger Delta, Nigeria

Abstract: The geologic Coastal Plain Sands is host to large numbers of onshore hydrocarbon exploration and processing activities and receives a greater part of the adverse environmental impact, including soil and groundwater contamination. This study explores the spatial and vertical distribution of hydrocarbon contamination in coastal plain sands typified by the Ogoni area (using indicator parameters of TPH and PAH) to assist with identification and delineation of vulnerable horizons as well as target specific contaminated areas as a remediation strategy. The investigations utilized a combination of Electrical Resistivity Tomography (ERT) and geotechnical boring to delineate hydrocarbon contaminated areas and laboratory testing for determination of concentrations/degrees of contamination. The 2-D ERT tomograph for the different survey lines were concatenated to provide a 3-D view which enabled horizontal slices corresponding to depths of interest to be assessed for possible contamination. The lithologs obtained from boring were further used in lithological modelling to precisely define the 3-D distribution of the contaminated zone. The results indicate that the ultimate distribution of hydrocarbons in the subsoil is influenced by topography, the physical and chemical characteristics of the compounds as well as those of the affected soil such as permeability and water table variation. In addition, climatic factors such as rainfall and temperature have a great influence on the reactions and the movement of the hydrocarbons plume in the subsoil. Because of their low densities, light hydrocarbons tend to spread laterally and are therefore commonly found at the interface of unsaturated-saturated zones while denser hydrocarbons tend to move vertically down through the subsoil into the saturated zone and migrate downward until they are restrained by an impermeable layer. This implies that the top soil in aged spill sites within the coastal plain sand become renovated by natural attenuation over time and tend to exhibit low levels of hydrocarbon contaminations.