Ecological Risk and Mobility Assessment of Heavy Metals in Soils from the Ikole–Itapaji Area, Southwestern Nigeria

This study evaluates the ecological risk and mobility behavior of heavy metals in soils from the Ikole–Itapaji area, Southwestern Nigeria, using the Ecological Risk Index (ERI) and Mobility Factor (MF) within a multi-fraction geochemical framework. The study area lies within the Precambrian Basement Complex of the West African Craton, characterized by migmatite–gneiss, granitic, and schistose lithologies that influence the natural geochemical background of soils. Surface soil samples collected from seventeen locations were subjected to a seven-step sequential extraction procedure to determine metal partitioning across water-soluble, exchangeable, carbonate-bound, Fe–Mn oxide-bound, organic-bound, sulfide/strongly bound, and residual phases. The Ecological Risk Index results indicate spatial variability in contamination levels, with SQ4 and SQ15 exhibiting very high ecological risk (ERI = 629.72 and 655.88, respectively), while most other locations fall within moderate to considerable risk categories. The Mobility Factor results reveal generally low mobility across the study area (2.7%–4.0%), indicating that most heavy metals are predominantly associated with stable geochemical phases. However, slightly elevated mobility at SQ4 and SQ15 suggests localized enhancement of metal reactivity and potential bioavailability. Integrated interpretation shows a strong coupling between ecological risk and metal mobility, with cadmium identified as the primary contaminant driving both enrichment and ecological hazard. The combined ERI and MF approach confirms that while the Basement Complex geology exerts a dominant control on metal stabilization, localized anthropogenic inputs contribute to elevated contamination and environmental risk in specific hotspot areas. The study highlights the need for targeted environmental monitoring and sustainable land-use management in the identified high-risk zones