Metals and arsenic concentrations of Ultisols adjacent to mine sites on limestone in Western Thailand

• Concentrations of As, Cd, Cu, Pb and Zn in soils in a mined area, Western Thailand were analyzed.
• High levels of As and Pb occur in soils including some soils distant from mine sites.
• Concentrations of Cd, Pb and Zn are highest in surface soil.
• SEM/EDS results confirm that Pb is associated with clay minerals and Fe/Mn oxides.
• The abandoned Pb-Zn mining sites are “hot spots” of soil contamination.

Soil samples collected from Ultisols at various distances from mine sites were analyzed to map metals and arsenic (As) concentrations in a Pb/Zn mineralized limestone area, Western Thailand. Total metals and As in soil samples (< 2 mm) were determined by acid digestion (aqua regia). Chemical forms of metals and As were investigated using sequential extraction and SEM/EDS. High median concentrations in soils at mine sites were 76 mg/kg As and 835 mg/kg Pb, which are higher than the limits for agricultural and residential uses. The high Cd, Pb and Zn concentrations in the surface soil horizon at sites throughout the area indicate that contamination of soils by dust from mining activities has occurred. Arsenic and Cu are present at uniform concentrations to one meter depth in soils indicating that these elements have been derived by weathering of underlying mineralized limestone. Some As and Pb are associated with the NH4-oxalate fraction being bound to poorly crystalline Fe/Mn oxides. SEM/EDS results confirm that Pb is associated with clay minerals and Fe/Mn oxides. The results suggest that the abandoned PbZn mining sites are “hot spots” of soil contamination and that contamination also affects soils distant from mines so metals and As contamination of soils poses a pervasive environmental problem for local land uses.