Geochemistry and Mobility of Trace Elements in Soils Developed on Different Geological Substrates Across the Niger Delta, South South Nigeria: An Original Risk Framework

This dissertation provides an original geochemical characterisation of trace element distribution and mobility in soils developed on different geological substrates in the Niger Delta, South South Nigeria, and proposes an original risk framework for trace element contamination assessment in the region. The Niger Delta's complex alluvial, coastal plain sand, swamp forest, and mangrove soil mosaic was formed from diverse geological materials with varying natural trace element loadings, creating a baseline geochemical heterogeneity that is poorly constrained, making it difficult to distinguish natural geogenic enrichment from anthropogenic petroleum industry contamination. This study analyses 200 soil profiles across Bayelsa, Delta, and Rivers States for a suite of 30 major and trace elements by ICP-MS following total digestion, sequential chemical extraction (Tessier five-step), and XANES speciation analysis for arsenic, chromium, and selenium. Geochemical baseline maps are constructed for the full trace element suite using robust statistical procedures. A receptor-specific bioavailability model is developed for agricultural and residential land use scenarios. The dissertation proposes the Niger Delta Trace Element Risk Framework (NDTERF) as an original contribution, which integrates geogenic background uncertainty into contamination assessment through probability-based enrichment factor analysis, replacing the current practice of applying temperate soil guideline values to geochemically distinct Nigerian deltaic soils. The NDTERF is validated against human biomonitoring data from Niger Delta communities. Original contributions include the geochemical baseline database, the XANES speciation dataset, and the NDTERF assessment protocol. Keywords: trace element geochemistry, Niger Delta, mobility, risk framework, geogenic baseline.