Heavy-metal Contamination of Soils and Water in the Apkai and Umusedege Oil-field Communities, Ndokwa East and Ndokwa West, Delta State, Nigeria

Abstract

Petroleum exploration and production can introduce potentially toxic metals into the soils and water bodies of host communities, with consequences for both ecosystem integrity and human exposure. This study assessed the concentrations and distribution of ten heavy metals: Ni, Cr, Cu, Co, Pb, Cd, Fe, Mn, As and Zn in soil and water from two oil-field communities, Apkai (Ndokwa East Local Government Area) and Umusedege (Ndokwa West Local Government Area), Delta State, Nigeria. A total of 63 soil samples and 11 water samples were collected across the two communities and a nearby control site (Obetim). Soil metals were determined by atomic absorption spectrophotometry (AAS) and, for Zn, Fe and Cu, cross-checked by X-ray fluorescence (XRF); water metals were determined by AAS alone. Overall mean soil concentrations were 148.58, 254.41, 53.15, 0.16, 229.98, 21.85, 2905.61, 1228.55, 0.58 and 175.87 mg kg⁻¹ for Ni, Cr, Cu, Co, Pb, Cd, Fe, Mn, As and Zn, respectively, with Fe as the dominant element throughout. Relative to the control, Cr, Cu, Co, Pb, Cd, Fe, Mn, As and Zn were all elevated in soil, while Ni was slightly lower; the largest soil enrichment ratios were recorded for Mn (2.10×), As (1.81×) and Cr (1.76×). In water, mean concentrations were 0.14, 0.04, 0.13, 0.35, 0.03, 6.91, 1.25 and 0.30 mg L⁻¹ for Ni, Cr, Cu, Pb, Cd, Fe, Mn and Zn, respectively (Co and As were not detected), with Pb (10.77×) and Zn (10.17×) showing by far the largest enrichment relative to the control. The AAS–XRF comparison showed statistically significant differences for Zn, Fe and Cu (p < 0.05 in all cases); XRF consistently returned higher values than AAS. Measured against the soil guideline values reproduced in the underlying study, Ni, Cr, Cu, Pb, Cd, Fe and Mn all exceeded their respective thresholds, most markedly Mn, Fe, Cd, Cr and Pb; in water, Fe, Pb, Cd, Ni and Mn were flagged as being of particular concern against U.S. EPA, WHO and SON reference values. Taken together, the results indicate substantial, spatially uneven alteration of soil and water metal concentrations in this oil-field environment relative to the surrounding background, consistent with a combination of lithogenic and anthropogenic contributions, and establish a concentration baseline for the pollution-index, fractionation and health-risk assessments that build on this dataset.