Assessing Potential Health Impacts of Cyanide-Contaminated Seepage in Paddy Field Near a Gold Mine in Thailand: Cyanide Speciation and Vapor Intrusion Modeling

Gold mining in Asia utilizes cyanide to extract gold from ores, and the resulting cyanide-contaminated mine tailings are disposed of in a tailing storage facility (TSF). Nevertheless, several incidents of leakage of cyanide-contaminated water from TSFs have been reported. This study examined the source and assessed health risk of mysterious cyanide-elevated (2.13 mg/L) black seepage in a paddy field near a TSF of the largest gold mine in Thailand. Using cyanide speciation—including free, weak metal complex (WMC), and strong acid dissociable (SAD) forms—as a forensics tool, we revealed that TSF1 of the mine is a potential source of the seepage in the paddy field because the majority of cyanide in the affected paddy was WMC and free cyanide while the SAD cyanide was minor, similar to the underdrain of a TSF1 and opposite to the natural cyanide in unaffected paddies. Furthermore, because free hydrogen cyanide (HCN) in the seepage may cause vapor intrusion problems for nearby villagers, the potential HCN exposure was determined by coupling HCN volatilization from the black water via the two-film theory with atmospheric HCN dispersion using AERMOD. We found that villagers dwelling in 3 to 23 villages may be at risk of acute cyanide toxicity when free cyanide concentration in the black seepage is greater than 0.65 mg/L, emphasizing the importance of prevention and remediation of leakage of TSFs. This is the first systematic assessment of vapor intrusion risk from recurring gold mine leakage in Asia.