Assessment of Heavy-metal Pollution and Ecological Risk in Soils and Water of Oil-field Communities in Delta State, Nigeria, Using Integrated Pollution Indices

Abstract

Petroleum exploration and production can alter the geochemical quality of surrounding soils and water through the introduction and redistribution of potentially toxic metals. This study assessed the degree of heavy-metal pollution and associated ecological risk in soils and water from the Apkai and Umusedege oil-field communities of Ndokwa East and Ndokwa West Local Government Areas, Delta State, Nigeria. Ten heavy metals, namely Ni, Cr, Cu, Co, Pb, Cd, Fe, Mn, As and Zn, were investigated in soil, while the detected subset was assessed similarly in water. Single-element pollution was evaluated using the contamination factor (CF), geo-accumulation index (I-geo), enrichment factor (EF) and potential ecological risk factor (Er); overall environmental quality was evaluated using the pollution load index (PLI), summed pollution index (PIsum), risk index (RI), average pollution index (PIave) and Nemerow pollution index. In soil, Cr, Co, Pb, Cd, As, Zn, Cu and Ni showed moderate contamination, while Fe and Mn showed considerable contamination; Ni, Cr, Cu, Pb and Zn were classified as low ecological risk, and Cd and As as moderate risk; most metals fell within the depletion-to-minimal-enrichment category, with As as the exception at moderate enrichment; and Cr, Fe, Mn and As showed light-to-moderate geo-accumulation, while the remaining metals were non-contaminated. In water, Cr, Cd and Ni showed moderate contamination, Fe and Mn showed considerable contamination, and Pb, Zn and Cu showed very high contamination; Pb showed considerable ecological risk, while Zn, Cu and Cd showed low-to-moderate risk. Mean PLI was 1.3 for soil and 1.7 for water, indicating pollution in both media under the adopted criterion; mean PIsum was 13.3 and 36.6, mean RI was 67.3 and 142.9, and mean PIave was 1.3 and 4.5 for soil and water, respectively, while the Nemerow index was almost identical between the media at 171.8 and 171.9. Collectively, the integrated indices point to measurable degradation of environmental quality in both compartments, with water showing consistently greater pollution pressure than soil, particularly through Pb, Zn and Cu. The results indicate that heavy-metal pollution in this oil-field environment warrants continued environmental monitoring and management, while also establishing an index-based baseline for the geochemical fractionation and human-health risk assessments that build on this dataset.