Effects of groundwater flow on the distribution of bitumen contaminants in a shallow coastal plain sand aquifer of the Nigerian Dahomey basin

Abstract

In situ bitumen seeps within the shallow subsurface of a section of the coastal Dahomey basin of southwestern Nigeria act as a geogenic source of polycyclic aromatic hydrocarbons (PAHs), which contaminates soil and groundwater resources in the area. This study focuses on a hydrogeochemical characterization of PAHs contaminants from the bitumen seeps and assesses their distribution using a groundwater flow and transport model. To do this, we used geophysical results from a previous study to determine the location of three water wells drilled to depths of 15 m each. Soil and groundwater samples were retrieved from different depths during drilling and used for sedimentological and geochemical analysis. We also performed pumping tests on all 3 wells to determine the hydraulic properties of the aquifer in the area and, lastly, developed a MODFLOW-based groundwater flow and transport model to simulate the distribution of the contaminants. Bitumen-bearing sediments retrieved during the groundwater well installation confirmed earlier interpretations of geophysical results, which mapped subsurface lithologies that host the bitumen seeps. These lithologies, which act as PAH contaminant sources, occur at depths below 11 m towards the southern part of the study area. Hydraulic conductivity and porosity of the sandy aquifer were estimated to be 0.25 m/day and 46%, respectively. The United States Environmental Protection Agency's priority PAHs, including Naphthalene, Acenaphthylene, Fluorene, and Pyrene of geogenic sources, were identified with concentrations ranging from Below Detection Limit (BDL) to 5 ppm in both sediment and groundwater samples. We simulated the distribution of Naphthalene in groundwater in the area using the contaminant transport model, considering its high dissolution tendency. Hydrodynamic dispersion was observed as the dominant transport mechanism of the contaminants in groundwater. A distribution rate at the concentration of significant risk to human exposure was predicted to be 0.022 m/day and 0.015 m/day in the horizontal and vertical directions, respectively. This study confirmed groundwater contamination by PAHs from in situ bitumen seeps and provided a framework for future groundwater remediation within the area.