Seasonal heavy metal mobilization in a tropical artisanal mining system: a source-to-receptor assessment

Artisanal mining poses a widespread threat to tropical water-food systems, yet the coupled influence of seasonal hydrology and subsurface structural architecture on contaminant dispersal remains poorly resolved. Here, we integrate multi-season hydrogeochemical monitoring (n = 83) with audio-magnetotelluric (AMT) geophysical imaging across the Ifewara schist belt to establish a process-based source-pathway-receptor model for heavy metal contamination. Groundwater and soil were sampled during dry and wet seasons, alongside three AMT traverses delineating fracture-controlled flow paths. Results reveal that seasonal mobilization is metal-specific and mechanistically distinct. Dry-season evaporative concentration drives extreme Hg and Cd enrichment (up to 93- and 11.8-fold above WHO guidelines), while wet season acidification (pH 6.05 → 5.60) enhances Pb runoff and leaching (2.8-fold increase). AMT resistivity models expose a regional low-resistivity conductive horizon (20-30 m and 40-60 m depth; 0.5-2Ω·m in log₁₀), correlated with the Ifewara Shear Zone, which functions as a contaminant pathway. This interpretation is validated by significant inverse correlations with metal loads (ρ = -0.43 to -0.65, p < 0.05) and by 12-29% improved prediction via co-kriging. Agricultural soils exhibit a stark metal gradient from source to receptor (Pb 119-fold higher at mine sites), with a potential ecological risk index of 52.04 (low risk) but disproportionate Hg contribution (72%), signaling chronic bioaccumulation concern. Importantly, this work demonstrates that contaminant dispersal in crystalline basement terrains is structurally controlled, not merely distance dependent. Our integrated hydrogeophysical framework provides a replicable template for seasonally adaptive remediation, targeting source stabilization, fracture zone pathway interruption, and receptor protection. This approach offers direct pathways toward SDG 6 (Clean Water) and SDG 15 (Life on Land) in data-scarce mining regions worldwide.