Source-pathway-sink controls on heavy-metal contamination in urban soils: Insights from environmental magnetism and machine learning

Urban roadside soils are important repositories of traffic-derived contaminants; however, the coupled influence of anthropogenic emissions, contaminant transport, and soil-controlled retention processes remains insufficiently understood, particularly in rapidly urbanizing environments. This study developed and validated an integrated Source-Pathway-Sink framework by combining environmental magnetism, heavy-metal geochemistry, geotechnical characterization, spatial modelling, multivariate statistical analysis, and machine-learning techniques to investigate roadside contamination in Minna, North-Central Nigeria. Low- and high-frequency magnetic susceptibility, heavy-metal concentrations, contamination indices, and geotechnical parameters were integrated to evaluate contaminant distribution, transport, and retention behaviour. Spatial analyses revealed pronounced contamination gradients, with increase low-frequency magnetic susceptibility (χlf ) and heavy-metal concentrations occurring adjacent to major road corridors and decreasing systematically with increasing distance from traffic sources, confirming the dominant influence of vehicular emissions. Strong magnetic-geochemical relationships, particularly between χlf and Lead (Pb), demonstrated that magnetic susceptibility is a reliable proxy for traffic-derived contamination. Five-fold cross-validation of Simple Linear Regression and Random Forest models further confirmed the predictive capability of χlf for estimating the Heavy Metal Pollution Index (HPI). Vertical retention analyses showed maximum contaminant confinement within the 10–20 cm soil horizon, where low permeability, high plasticity, and enhanced adsorption capacity promoted contaminant accumulation. The proposed Vertical Retention Capacity Index (VRCI), supported by multivariate statistical analyses, successfully differentiated anthropogenic contaminant inputs from soil-controlled retention processes. The integrated SPWS framework provides a robust, rapid, and cost-effective approach for urban roadside contamination assessment, offering a transferable methodology for environmental monitoring, pollution mitigation, and sustainable urban planning in rapidly urbanizing regions.