Geochemical Fractionation, Mobility and Environmental Risk of Lead, Cadmium and Zinc in Agricultural Soils from an Abandoned Mining Area in Jos-South, Nigeria

Abstract

The total amount of a potentially toxic element in soil, on its own, provides limited information about its mobility or bioavailability. This paper re-examines the geochemical partitioning, extractability and environmental status of lead (Pb), cadmium (Cd) and zinc (Zn) in agricultural soils surrounding abandoned mining excavations at Dahwol-Vwana village, Jos-South Local Government Area, Plateau State, Nigeria. Soil collected around three disused mining holes was subjected to a modified Tessier sequential extraction scheme, which splits Pb, Cd and Zn into an exchangeable-plus-carbonate fraction, a reducible (Fe/Mn oxide-bound) fraction, an oxidisable (organic-matter/sulphide-bound) fraction, and a residual fraction, with each fraction quantified by atomic absorption spectrometry. The results show that Pb, Cd and Zn were predominantly associated with the less mobile fractions: roughly 71.5% of Pb and 81.8% of Cd occurred in the residual fraction, while 89.8% of Zn was bound to organic matter. The exchangeable-plus-carbonate pool accounted for only 7.8% of Pb and 7.5% of Cd, and the oxide-bound (reducible) pool for 12.7% of Pb and 6.5% of Cd; for Zn, the reducible pool accounted for about 3.5% and the residual fraction just 2.9%. Calcium-chloride extractability was correspondingly low, at 3.95–7.30 mg kg⁻¹ for Zn, 3.85–9.59 mg kg⁻¹ for Pb and 0.09–0.44 mg kg⁻¹ for Cd equivalent to extractable proportions of only 0.03–0.05% (Zn), 0.05–0.12% (Pb) and 0.03–0.14% (Cd) of the total inventory. Enrichment factors were 5.6, 15.4 and 2.1 for Pb, Cd and Zn respectively, with corresponding geoaccumulation indices of 1.9, 3.4 and 0.5. Cd therefore combines strong apparent enrichment and a high geoaccumulation classification with a fractionation profile dominated by the residual phase and very low extractability, implying limited immediate mobility despite that enrichment. Overall, the fractionation pattern points to a substantial lithogenic contribution to the metal inventory, though shifts in soil pH, redox status, organic-matter turnover or physical disturbance could still alter mobility over time. The results demonstrate why total-metal concentration should be interpreted alongside chemical fractionation and extractability when environmental risk is being assessed in agricultural soils affected by historical mining.