Variance decomposition of probabilistic health risk of potentially toxic metals at two Nigerian artisanal mining fields.
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Artisanal and small-scale mining supports an estimated 44.75 million people directly worldwide, and about 260,000 artisanal gold miners in Nigeria with a further 1.3 million livelihoods dependent on the sector, yet exposure in host communities is rarely quantified probabilistically. We assessed potentially toxic element contamination and health risk on the working faces and tailings aprons of a gold mining field (Moro) and a beryllium mining field (Ifelodun) in Kwara State, North-Central Nigeria. Seventy-two surface soils were analysed by flame atomic absorption spectrophotometry after aqua regia digestion. Cadmium exceeded world-soil background 113-fold at Moro and 73-fold at Ifelodun; lead averaged 448.75 mg kg-1 at Moro, above the 2024 USEPA residential screening level in every sample. Monte Carlo simulation (100,000 iterations) gave adult hazard indices below unity (0.38 Moro; 0.20 Ifelodun) but child indices of 3.49 and 1.77, exceeding unity with probabilities of 99.0% (95% CI 98.3-99.5) and 82.4% (75.9-88.0) under a two-dimensional simulation separating variability from uncertainty. Cancer risk lay between 10-6 and 10-4 for all receptors. The child result survived every assumption tested: setting hexavalent chromium to 1% of total chromium lowered the Moro child index only to 1.75. Soil ingestion rate ranked first in every hazard-index scenario, explaining 19-57% of output variance, while the seven soil concentrations combined explained less. Neither correlation structure (KMO 0.47, 0.50) nor spatial analysis supported source apportionment. Children in mining households are the population of concern, and characterising childhood soil ingestion, rather than further analytical precision, would most reduce uncertainty.