Hydrogeomorphic connectivity governs heavy-metal bioavailability, source apportionment, and human-health risk in a tropical agro-riverine watershed.

Anthropogenic landscape alteration increasingly modifies contaminant transport and geochemical processes in agro-riverine systems. Still, the mechanisms linking landscape transformation, hydrogeomorphic connectivity, contaminant bioavailability, and human exposure remain poorly understood, especially in tropical watersheds. Hydrogeomorphological connectivity can relate landscape alteration and exposure to contamination by controlling the transfer and deposition of sediment-attached PTEs. Still, the relationship among connectivity, landscape alterations, bioavailability, and contaminant exposure remains unclear. Here, we investigated this connectivity-mediated pathway in the River Hawul catchment, northeastern Nigeria, using an integrated source-transport-transformation-exposure (STTE) framework combining land-use and land-cover (LULC) analysis, geomorphometric indices, Diffusive Gradients in Thin Films (DGT), Pb isotopic fingerprinting, Positive Matrix Factorisation (PMF), Structural Equation Modelling (SEM), and probabilistic health-risk assessment. Surface soils and major food crops were analysed for Cd, Pb, Cr, Ni, Cu, Zn, As, and Hg. Although total PTE concentrations indicated generally low-to-moderate contamination, elevated DGT-labile fractions in hydrologically connected floodplains indicated that exposure-relevant contamination was associated with hydrogeomorphic and biogeochemical conditions rather than contaminant loading alone. Agricultural expansion and vegetation loss were associated with Cd, Pb, and As accumulation (r = 0.86-0.90) and with human health risk (r = 0.91). In comparison, landscape transformation explained 83% of the spatial variability in health risk (R² = 0.83, p < 0.001). PMF and Pb isotopic fingerprinting identified agricultural activities, atmospheric deposition, and geogenic weathering as dominant PTE sources, while SEM linked anthropogenic pressure, hydrogeomorphic connectivity, PTE redistribution, bioavailability, and exposure. This implies that such interconnected floodplains provide important deposition zones where contaminants are more prone to transformation and retention, thereby increasing their exposure. The STTE framework therefore advances beyond concentration-based assessment by explicitly linking landscape transformation with contaminant redistribution, bioavailability, and exposure, providing a transferable process-based approach for identifying contamination hotspots and supporting sustainable management of tropical agro-riverine ecosystems.