Development and application of human health risk-based screening levels for crude oil affected soils and sediments in the Niger Delta

Management of oil-affected sites in Nigeria is regulated through the Environmental Guidelines and Standards for the Petroleum Industry in Nigeria (EGASPIN). This includes an intervention value for total petroleum hydrocarbons (TPHs) of 5,000 mg/kg as a trigger for remedial action. This single intervention value does not differentiate between varied land uses or the specific chemical composition of spilled oil. The EGASPIN does allow development of refined screening levels based on human health risk assessment methods. Following these requirements, we applied the ASTM risk-based corrective action process to define Tier 1 risk-based screening levels (RBSLs) and Tier 2 site-specific target levels (SSTLs). Both are derived using USEPA guidance supplemented by equations correcting for the properties of chemical mixtures, presence of a residual oil phase, and preferential depletion of volatile oil components. The RBSLs were calculated for five Nigerian crude oils and six exposure scenarios specific to the Niger Delta, and SSTLs were calculated using sediment sampling results and four exposure scenarios from the Bodo oil spill area located in the eastern Niger Delta mangrove swamp area. Derived TPH RBSLs range between a factor of 2.5 lower (more stringent) for residential land use to >10 times higher for "commercial and industrial workers," when compared with the Nigerian EGASPIN intervention value. Derived SSTLs for TPH ranged between a value just above the intervention value for shoreline areas near Bodo Town to values approximately six times higher than the intervention value for most uninhabited mangrove areas. The derived RBSLs and SSTLs are protective of human health, but other criteria (aesthetic, ecological) may be more stringent. Additional criteria were introduced, including oil mobility (potential spreading of existing impacts) and criteria for ensuring the viability of the mangrove environment.