Heavy Metal Speciation and Bioavailability in Agricultural Soils of the Jos Plateau Tin Mining Belt, North Central Nigeria

Tin mining and smelting activities on the Jos Plateau have deposited legacy heavy metal contamination in surrounding agricultural soils, yet the chemical speciation and plant-available fractions of metals such as lead, cadmium, arsenic, and zinc have not been rigorously characterised, limiting the accuracy of ecological and human health risk assessments for farming communities. This study applies the modified Tessier sequential extraction procedure and the single-step DTPA extraction method to characterise heavy metal speciation and bioavailability in contaminated agricultural soils from six sites across the Jos Plateau tin mining belt, Plateau State, North Central Nigeria. Six fractions are targeted: exchangeable, carbonate-bound, iron and manganese oxide-bound, organic matter-bound, sulphide-bound, and residual. Pseudo-total metal concentrations are determined by aqua regia digestion and measured by inductively coupled plasma optical emission spectrometry. Grain samples of maize and sorghum grown on sampled soils are analysed for plant uptake of each metal to calculate bioconcentration factors and evaluate health risks via dietary intake calculations using Monte Carlo simulation. Geochemical indices including geo-accumulation index, enrichment factor, and contamination factor are calculated to distinguish anthropogenic from lithogenic metal sources. Multivariate statistical analysis, including principal component analysis and cluster analysis, elucidates contamination source apportionment. Findings bridge a critical gap between total metal concentration data and the fraction available for crop uptake and ecological harm, providing more accurate inputs for remediation priority-setting in Jos Plateau mine-affected farmlands. Keywords: heavy metal speciation, sequential extraction, Jos Plateau, bioavailability, tin mining contamination