Application of Tier 2 Site-Specific Target Levels (SSTLs) for Certification and Close-Out of Hydrocarbon-Contaminated Soil at MOGHO, Bodo Community, Niger Delta

Abstract Certification and close-out of petroleum-contaminated sites in Nigeria often rely on fixed, generic regulatory thresholds that may not adequately reflect site-specific exposure conditions, contaminant behaviour, land use or receptor sensitivity. This creates uncertainty in residential or subsistence settings, where numerical compliance alone may not demonstrate protectiveness. This study addresses this gap by using a petroleum-contaminated site at MOGHO, Bodo Community, as a demonstration case for applying Tier 2 Site-Specific Target Levels (SSTLs) to support risk-based certification and close-out of petroleum-contaminated soil in Nigeria. The study links measured residual petroleum contamination with receptor-specific exposure pathways, petroleum hydrocarbon fractionation, cumulative human health risk and residential close-out decision-making within a Nigerian contaminated-land context. Measured concentrations of total petroleum hydrocarbons (TPH), polycyclic aromatic hydrocarbons (PAHs) and volatile organic compounds (VOCs) in surface soil were assessed using a tiered human health risk assessment framework. Tier 2 modelling was conducted for child and adult upper-percentile residential receptors, considering soil ingestion, dermal contact with soil, vegetable ingestion and inhalation of outdoor air. The results showed that carcinogenic risk exceeded the adopted target level of 1.0 × 10⁻⁶, with estimated risks of 3.4 × 10⁻⁶ for the child resident, 1.2 × 10⁻⁵ for the adult resident, and 1.5 × 10⁻⁵ for the combined residential receptor. Non-carcinogenic hazard indices also exceeded the acceptable threshold of 1.0, with values of approximately 33 for the child resident and 15 for the adult resident. Vegetable ingestion was the dominant exposure pathway for both risk endpoints, while PAHs mainly drove carcinogenic risk and heavier aromatic TPH fractions dominated non-carcinogenic hazard. The original TPH concentration of approximately 3,900 mg/kg exceeded the calculated TPH concentration at SSTL of approximately 110 mg/kg, requiring a reduction of about 97% to achieve the modelled site-specific target condition. Overall, the findings indicate that the current soil-source condition does not support unrestricted residential certification under the modelled exposure assumptions without further remediation, exposure-pathway control, land-use restrictions, or defensible refinement of the conceptual site model.