Long-Term Geochemical Evolution of Contaminated Floodplain Soils in the Industrial Corridor of Apapa-Ijora, Lagos State: Speciation Dynamics, Mineralogical Transformation, and Ecological Risk Trajectory

Floodplain soils in heavily industrialised urban corridors accumulate contaminants over decades through complex geochemical pathways, yet understanding of long-term speciation dynamics and mineralogical transformation processes affecting metal bioavailability in tropical industrial settings is lacking. This dissertation investigates the geochemical evolution of contaminated floodplain soils in the Apapa-Ijora industrial corridor, Lagos State, South South West Nigeria, using a chronosequence approach spanning sediment cores with archived contamination histories spanning approximately 50 years. Sediment cores were collected and dated by lead-210 and caesium-137 geochronology. Vertical profiles of metal speciation were analysed by BCR sequential extraction and micro X-ray absorption spectroscopy at synchrotron facilities. Mineralogical characterisation was conducted by synchrotron powder diffraction and electron microprobe analysis. Sequential extraction speciation data were integrated with thermodynamic stability calculations and geochemical modelling using PHREEQC to develop a quantitative model of metal speciation evolution over time. Biological endpoint measurements in core sections representing distinct historical contamination periods included earthworm bioassays and enzyme activity profiles as indicators of evolving ecological risk. Results reveal a progressive transformation of originally mobile lead and zinc fractions toward more crystalline iron oxide-associated forms over decades, reducing mobility but increasing long-term persistence. Cadmium showed distinctly different behaviour, maintaining elevated exchangeable fractions despite ageing. A predictive geochemical trajectory model is developed as an original contribution. Original ecological risk trajectory functions derived from biological endpoint and chemical speciation co-data represent a novel framework for long-term contaminated land risk forecasting. Keywords: geochemical evolution, floodplain soils, metal speciation, Lagos, contaminated land