Polycyclic Aromatic Hydrocarbons in Urban Motor-Park Soils of Asaba, Nigeria: Environmental Contamination, Source Characterization, and Human Health Risk.
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This study involved the characterisation of polycyclic aromatic hydrocarbon (PAH) concentration in soil at motor parks in Asaba, Nigeria. It explores the likely sources of PAHs and their potential health risks associated with multiple exposure pathways for adult and child populations. Topsoil composite samples (0-10 cm deep) were taken from 10 different motor parks and tested for 16 USEPA priority PAH pollutants using ultrasonic-assisted extraction and gas chromatography equipped with a flame ionisation detector (GC-FID). The Σ16 PAHs range between 1.18 and 2.53 mg/kg (mean 1.79 mg/kg), with approximately 69% of the total concentration by mass being "low-molecular-weight" PAHs. Estimated daily intake (EDI), benzo[a]pyrene toxic equivalency (TEQ), and incremental lifetime cancer risk (ILCR) were used for the ingestion, dermal contact, and inhalation risk assessments. Total EDI ranged from 1.76 × 10-6 to 3.78 × 10-6 mg/kg/day for children and 1.05 × 10-6 to 2.26 × 10-6 mg/kg/day for adult. TEQ ranged from 0.050 to 0.195 mg/kg BaP-eq, and four sites (AI, SM, KK, and IJ) exceeded the USEPA residential screening level of 0.11 mg/kg. ΣILCR ranged from 1.76 × 10-6 to 3.78 × 10-6 for children and 1.05 × 10-6 to 2.26 × 10-6 for adult; all sites fell below the 10-4 upper bound. The results from exploratory source assessment based on hierarchical cluster analysis and diagnostic ratios suggested potential mixed combustion and petroleum-related source inputs, along with some biomass or coal signatures. Sensitivity analysis suggested that ΣILCR is directly proportional to concentration, exposure frequency, and exposure duration and inversely proportional to body weight. Monte Carlo simulation (10,000 iterations; lognormal site-level ΣILCR fitted to ten samples; ±25% exposure-parameter uncertainty) gave a child median ΣILCR of 2.6 × 10-6 (5th-95th percentile 1.5-4.3 × 10-6) and an adult median of 1.5 × 10-6 (0.9-2.6 × 10-6). In 99.8% of child iterations and 91% of adult iterations, the risk was greater than 10-6 and less than 10-5.