Effect of a Six-month Attenuation Study on the Concentration of Heavy Metals in Crude-oil-impacted Soil from Ubeji, Nigeria

Abstract

Crude oil contamination alters the soil chemistry and can enhance the mobility and bioavailability of heavy metals. At toxic concentrations, these trace metals pose significant risks to ecological integrity, soil functionality, plant physiology, and human health. This study evaluated the capacity of indigenous soil microorganisms to modulate the bioavailability of selected heavy metals (Cd, Cr, Fe, Mn, Ni, and Pb) in crude oil impacted soils from Ubeji, Nigeria. Contaminated soil samples were incubated under ambient conditions for six months, after which heavy metal concentrations and selected physicochemical parameters were assessed. The study integrated measurements of salinity, water holding capacity, particle size distribution, pH, total nitrogen, organic carbon, available phosphorus, nitrate content, and dehydrogenase activity, alongside flame atomic absorption spectrometric determination of selected heavy metals, to provide a comprehensive assessment of soil quality changes across impacted and control sites. Statistical analysis was performed using R software (version 2025.09.1+401), and results were considered statistically significant at p < 0.05. Overall, the results demonstrated a reduction in the concentration of the investigated metals following the attenuation period, with Cr, Fe, and Mn exhibiting statistically significant declines after incubation. The observed decrease in heavy metal concentration suggests active microbial mediation by autochthonous soil microorganisms during natural attenuation. These findings provide important insights into heavy metal dynamics in petroleum contaminated soils and highlight the potential of monitored natural attenuation as a sustainable and cost-effective remediation strategy for crude oil polluted environments in the region.