Geochemical Fractionation, Stability and Potential Mobility of Heavy Metals in Soils from Oil-field Communities of Delta State, Nigeria

Abstract

The total concentration of a heavy metal in soil provides limited information about its environmental behaviour, since mobility and potential bioavailability depend strongly on the geochemical phase with which the metal is associated. This study investigated the fractionation, stability and potential mobility of Pb, Zn, Cd, Cu, Ni and Fe in soils from the Apkai and Umusadege oil-field communities of Delta State, Nigeria. A modified Tessier sequential extraction scheme partitioned each metal into exchangeable + carbonate (F1), reducible (F2), oxidizable (F3) and residual (F4) fractions, and three quantitative indicators were derived from these fractions: residual fraction (RF), non-residual fraction (NRF) and mobility factor (MF). The residual fraction dominated for every metal investigated, ranging from 68.2–75.8% in Apkai and 52.5–84.2% in Umusadege. Ni showed the strongest residual association, reaching 84.2% in Umusadege, while Fe showed the weakest, at 52.5% in the same location. The corresponding non-residual fraction ranged from 24.2–31.8% in Apkai and 15.8–47.5% in Umusadege, with Fe in Umusadege recording the largest non-residual pool (47.5%), followed by Cu in Umusadege (33.1%) and Apkai (31.8%). The mobility factor ranged from 6.9–13.8% in Apkai and 7.4–19.0% in Umusadege; Pb consistently showed the lowest mobility factor of the six metals, while Cu (Apkai) and Fe (Umusadege) showed the highest. Mean RF, NRF and MF across the six metals were 72.98%, 27.02% and 10.33% in Apkai, compared with 69.55%, 30.45% and 11.68% in Umusadege. Across all twelve metal–location combinations, NRF and MF were strongly and significantly correlated (Pearson r = 0.841, p < 0.001), confirming that these two derived indicators move together as expected from their shared basis in the non-residual fractions. These findings indicate that the investigated heavy metals were predominantly associated with relatively stable mineral phases, consistent with an important lithogenic contribution, while the appreciable non-residual and exchangeable pools identified, particularly for Fe and Cu, remain potentially responsive to future changes in soil geochemical conditions. The results demonstrate that total metal concentrations and pollution indices should be complemented by chemical fractionation and quantitative mobility indicators when evaluating the environmental significance of heavy-metal contamination in oil-field soils.