Received: Nov 10, 2025 / Accepted: Mar 10, 2026 / Published: Jun 30, 2026
Trace metal (TM) contamination in post-mining soils represents a long-term environmental risk due to potential leaching into groundwater. This study applied an integrated risk assessment approach—combining sequential extraction, in situ soil solution sampling, and HYDRUS-1D modelling—to evaluate TM behaviour along the soil–groundwater pathway at a recently abandoned bauxite mining site in the Central Highlands of Vietnam, currently used for cassava cultivation. Our results revealed that less than 1% of the total TM concentrations (As, Cu, Zn, Pb) are mobilizable, indicating a negligible risk of groundwater contamination under the current conditions. This low mobility is primarily attributed to strong sorption by Fe- and Al-(hydr)oxides, despite the moderately acidic soil pH (5.3 ± 0.2). With the exception of Pb, the in situ soil solution samples showed consistently higher TM concentrations than predicted by sequential extraction (1.1–4.9-fold higher). This trend suggests that natural complexation with dissolved organic matter and minor redox fluctuations may enhance TM mobility more than estimated by lab experiments under controlled conditions. Simulation of the soil water balance indicated an annual infiltration of 3,654mm, evapotranspiration of 845mm, and seepage of 2,809mm, with TM loads far below admissible regulatory limits. However, continuous monitoring is recommended, as potential changes in pH, organic matter, or precipitation patterns could alter TM solubility and increase leaching risks in the future. These findings demonstrate that total TM concentrations may serve as valid first-order approximation for risk assessments in similar tropical post-mining soils, though site-specific conditions should always be considered.
