Leachability and Health Risk Assessment of Cadmium and Other Heavy Metals in Agricultural Soils from the Mae Tao Watershed, Northern Thailand

Decades of unregulated zinc mining activities in the Mae Tao watershed, located in Mae Sot District, Tak Province, northern Thailand, have resulted in the pervasive contamination of agricultural soils with heavy metals, particularly cadmium (Cd), zinc (Zn), lead (Pb), and manganese (Mn). This legacy pollution has significantly impacted multiple environmental compartments-including surface water, groundwater, and sediments-and poses chronic health risks to local populations. This study investigates the key geochemical and physicochemical factors governing the leachability and mobility of these metals from contaminated soils and evaluates the associated human health risks. Controlled leaching experiments demonstrated that ionic strength exerts a more pronounced influence on metal mobilization than pH or other tested variables, suggesting that the electrolyte composition of pore water plays a dominant role in heavy metal transport. Despite elevated total concentrations of Cd, Zn, Pb, and Mn in the soils, hazard quotient (HQ) calculations indicated no significant non-carcinogenic risk under typical exposure scenarios. However, Cd exhibited a carcinogenic risk above the acceptable threshold at both average and peak soil concentrations, underscoring its potential to adversely affect human health. These findings enhance the understanding of heavy metal behavior in contaminated agroecosystems and provide a scientific basis for targeted risk management and long-term monitoring strategies in the Mae Sot region.