Assessment of Heavy Metal Assemblages in Coastal Sediments of the Cross River Estuary, Southeast Nigeria

Abstract

Surface sediment contamination by heavy metals poses significant ecological and human health risks in estuarine environments due to the persistence and bioaccumulative nature of these pollutants. This study assessed the concentrations, spatial distribution, contamination status, and ecological risks of heavy metals in bottom sediments of the Cross River estuary, Southeast Nigeria. Fifty-five surface sediment samples were collected along a 65 km stretch of the estuary and analyzed for Cu, Zn, Cd, Ni, Pb, and Fe using Atomic Absorption Spectrometry (AAS). Mean metal concentrations (mg/kg) were 0.241 for Cu, 0.356 for Zn, 0.027 for Cd, 0.439 for Ni, 0.003 for Pb, and 0.840 for Fe, following the order Fe > Ni > Zn > Cu > Cd > Pb. All measured concentrations were substantially lower than the world average shale values and the Department of Petroleum Resources (DPR) sediment quality guideline values. Geo-accumulation index (Igeo) values were ≤ 0 for all investigated metals, indicating unpolluted sediments, while contamination factor (CF) values were consistently < 1, reflecting low contamination. Degree of contamination (DC) values ranged from 0.007 to 0.254, and pollution load index (PLI) values ranged from 0.000 to 0.003, confirming the absence of overall sediment pollution. Ecological risk factor (ER) values varied between 0.000 and 7.000, whereas ecological risk index (RI) values ranged from 0.030 to 7.083, both indicating low ecological risk throughout the estuary. Hierarchical cluster analysis classified the investigated metals into two distinct assemblages, comprising Cu–Zn–Ni–Pb–Cd and Fe, suggesting mixed anthropogenic and lithogenic sources, with iron predominantly controlled by natural geological processes. Although the present contamination levels are low, continuous anthropogenic activities, including maritime transportation, urban runoff, dredging, and indiscriminate waste disposal, may progressively increase heavy metal accumulation. Continuous environmental monitoring is therefore recommended to ensure the long-term ecological sustainability of the Cross River estuary.