Distribution, sources, and ecological risk assessment of polycyclic aromatic hydrocarbons in agricultural and dumpsite soils in Sierra Leone

Abstract

This study investigates the concentration and distribution of polycyclic aromatic hydrocarbons (PAHs) in soils, potential sources, risk assessment, and soil physicochemical properties influencing PAH distribution in developed and remote cities in Sierra Leone. Seventeen topsoil samples (0–20 cm) were collected and analyzed for 16 PAHs. The average concentrations of Σ₁₆PAH in soils in the surveyed areas were 1142 ng g⁻¹ dw, 265 ng g⁻¹ dw, 79.7 ng g⁻¹ dw, 54.3 ng g⁻¹ dw, 54.2 ng g⁻¹ dw, 52.3 ng g⁻¹ dw, and 36.6 ng g⁻¹ dw in Kingtom, Waterloo, Magburaka, Bonganema, Kabala, Sinikoro, and Makeni, respectively. Based on the European soil quality guidelines, Kingtom and Waterloo soils were categorized as heavily and weakly contaminated soil PAHs respectively. The main PAH compounds of this study were 2-ring, 4-ring, and 5-ring PAHs. High molecular weight PAHs (4–6 rings) made up 62.5% of the total PAHs, while low molecular weight PAHs (2–3 rings) was 37.5%. In general, HMWPAHs were predominant in Kingtom, followed by Waterloo. The appointment of PAH sources using different methods revealed mixed sources, but predominantly pyrogenic sources (petroleum, biomass, coal, and fossil fuel contributions). Soil pH has a significant impact on PAH distribution. The toxicity equivalent quantity (TEQ_BaP) levels in soils pose a potential health risk to residents in developed cities but pose a negligible health risk to residents in remote cities. This study is significant as its findings reveal the status of PAH soil contamination in Sierra Leone. The results have important implications for policymakers and stakeholders to identify high-risk zones and establish proper environmental monitoring programs, pollution control measures, and remediation strategies to prevent future risks.