Heavy metal transfer and health risk assessment in an abandoned zinc mining-affected soil-rice system in western Thailand

Abstract

Mining activities are a major source of heavy metal contamination in agricultural systems, posing risks to food safety and human health. This study investigated the soil-rice system in a zinc (Zn) mining-affected area of western Thailand to: (1) quantify heavy metal contamination across soil and rice compartments; (2) evaluate compartment-specific transfer factors (soil-root, root-stem, and stem-grain) to elucidate metal mobility and accumulation pathways; and (3) identify soil properties influencing metal retention and mobility. Soil, root, stem, and grain samples were collected from 20 paddy fields and analyzed for As, Cd, Cr, Cu, Mn, Pb, Zn, Hg, and Fe. The results showed that although bulk soil concentrations were within national standards, the geo-accumulation index (Igeo) indicated extremely severe Cd and Hg contamination at all sites, with Zn also reaching extremely severe levels at several locations. In addition, consistently elevated pollution load index (PLI) values (> 1) indicated widespread multi-metal pollution across the entire study area. Soil organic matter, cation exchange capacity, and texture were strongly correlated with heavy metal retention and transfer (p < 0.05). Transfer factor analysis revealed high root accumulation of As and Hg, restricted Pb and Cd mobility, and substantial grain translocation of Cu, Zn, and Mn. Despite limited mobility, grain concentrations of As (1.16 mg kg-1), Cd (0.98 mg kg-1), Pb (4.61 mg kg-1), and Hg (0.08 mg kg-1) exceeded national food safety standards. Health risk assessment showed that both children and adults faced extremely high non-carcinogenic and carcinogenic risks from rice consumption, with hazard index and cancer risk values exceeding safe thresholds by several orders of magnitude. These findings demonstrate that the legacy of mining activities continues to threaten food safety and public health through rice-based exposure, highlighting the urgent need for targeted remediation, and routine monitoring to mitigate risks and support sustainable agriculture in mining-impacted regions.