Integrated geochemical and multivariate assessment of heavy metal contamination and ecological risk in Ikpoba and Ogba Rivers Benin city Edo State, Nigeria

Abstract

Urbanization-driven contamination of freshwater systems has become an increasing environmental concern due to the persistence, bioaccumulation potential, and ecological toxicity of heavy metals in aquatic environments. This study presents an integrated geochemical and multivariate assessment of heavy metal contamination and ecological risk in the Ikpoba and Ogba Rivers located in Benin City, Edo State, Nigeria. Surface water and sediment samples were collected and analyzed for Pb, Cd, Mn, Fe, Cu, Zn, and Ni using atomic absorption spectrophotometry. To determine contamination levels and ecological implications, several geochemical indices including contamination factor (CF), geo-accumulation index (Igeo), enrichment factor (EF), and potential ecological risk index (PERI) were applied. In addition, multivariate statistical approaches such as Pearson correlation analysis, principal component analysis (PCA), and hierarchical cluster analysis were employed to identify relationships among metals and distinguish between lithogenic and anthropogenic sources. The results showed substantially higher concentrations of heavy metals in sediments compared with water, indicating strong sediment–water partitioning influenced by adsorption processes involving Fe–Mn oxides and particulate matter. The dominance of iron and manganese reflects natural lithogenic contributions associated with tropical lateritic geology, while strong associations among Cd, Zn, Ni, Pb, and Cu indicate common anthropogenic inputs from urban runoff, industrial discharge, and municipal waste pathways. Although contamination factors and geo-accumulation indices suggest generally low to moderate contamination, enrichment factors revealed notable anthropogenic enrichment of cadmium and copper. Cadmium was identified as the major ecological risk driver due to its high toxic-response coefficient. Overall, the study demonstrates that the Ikpoba and Ogba Rivers are experiencing early-stage anthropogenic metal enrichment moderated by sediment retention processes but remain vulnerable to future ecological risk under changing environmental conditions.