Landscape-scale controls on trace metals partitioning and mobility in tropical soils affected by legacy lead smelting

The long-term legacy of a decommissioned lead smelter in Santo Amaro, Bahia (Brazil), has produced one of the most metal-contaminated urban areas worldwide. This study investigates the spatial distribution, geochemical partitioning, and potential mobility of trace metals across contrasting landscape units, namely hillslope and floodplain soils. A total of 120 soil samples were analyzed using portable X-ray fluorescence (XRF), scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), and the BCR sequential extraction procedure. XRF results revealed extremely high concentrations of Pb (up to 24,962 mg kg^-1), Zn (up to 8572 mg kg^-1), and Cd (up to 454 mg kg^-1), with strong spatial heterogeneity related to distance from the former smelter and landscape position. Cadmium and Pb were predominantly associated with labile and reducible fractions, indicating high chemical lability (mobility factor) and sensitivity to environmental conditions. SEM-EDS observations revealed contrasting stabilization mechanisms of Pb, associated with carbonate phases in hillslope soils and co-precipitated with Fe oxyhydroxides in floodplain soils. In contrast, Cr, Ni, and Cu were mainly associated with the residual fraction, indicating dominant lithogenic control and limited mobility. Geostatistical modeling still showed strong spatial dependence for Pb, Zn, and Cd with decreasing concentrations away from the smelter. The results demonstrate that floodplain soils do not act as permanent sinks but as transitional environments, where metals mobilized from hillslopes are temporarily retained and subsequently transferred toward downstream compartments.