Ecological and human health risk of PB and AS In roadside soils of Benin City, Nigeria: a geoaccumulation and Monte Carlo simulation approach
Traffic-derived heavy metal contamination of roadside soils poses significant public health risks in rapidly urbanising sub-Saharan African cities, yet probabilistic risk assessment remains rare in this region. This study characterised lead (Pb) and arsenic (As) concentrations in surface soils (0–20 cm) at five traffic-corridor sites along an urban-to-peri-urban gradient in Benin City, Nigeria, and evaluated ecological and human health risks using geochemical pollution indices, the Håkanson ecological risk framework, and a USEPA multi-pathway human health risk assessment model augmented by Monte Carlo (MC) simulation. Soil Pb ranged from 4.38 to 16.80 mg kg⁻¹ and As from 0.81 to 2.13 mg kg⁻¹, exceeding the local geochemical control by 52–119-fold and 63–95-fold, respectively. Pollution indices referenced to global shale background classified all sites as uncontaminated (Igeo < 0; CF < 1), yet locally-normalised contamination factors indicated very high anthropogenic enrichment, exposing a critical limitation of global background standards for the inherently metal-poor soils of the Benin Formation. Ecological risk was low at all sites (RI: 3.24–5.73); however, As contributed disproportionately to composite risk due to its higher toxic response factor. Deterministic non-carcinogenic hazard indices remained below 1.0 for both children and adults. Total carcinogenic risk (TCR) from As ingestion exceeded the acceptable threshold of 1 × 10⁻⁶ at all contaminated sites (children: 1.52–2.33 × 10⁻⁵; adults: 1.31–1.99 × 10⁻⁵). Critically, MC simulation demonstrated that As carcinogenic risk exceeded 10⁻⁶ at probabilities of 61–96% for children and 43–89% for adults — identifying As ingestion as the modal, not worst-case, risk outcome. Soil As concentration (ρ = 0.71–0.83) and ingestion rate (ρ = 0.55–0.68) were the dominant sources of exposure variance. These findings provide the first probabilistic HHRA of roadside soils in Benin City and demonstrate that deterministic methods alone are insufficient to characterise As carcinogenic risk in low-concentration, high-slope-factor contexts.