Seasonal effects of natural attenuation on drainage contamination from artisanal gold mining, Cambodia: Implication for passive treatment.

In Mondulkiri province, Cambodia, artisanal gold miners dump tailings and wastewater from gold processing into a tributary of the Prek Te River. In the rainy season, heavy metal concentrations in the tributary decrease below the WHO drinking water standard levels through natural attenuation; however, this does not occur in the dry season. To further understand the natural attenuation mechanism, detailed analyses of the wastewater from tailing and tributary water, tributary sediments, waste rock, and ore minerals were undertaken in both seasons. The high concentration of dissolved Fe in the contaminated tributary plays a significant role in As removal during the rainy season, whereas other elements such as Ni, Se, and Cu concentration decrease due to dilution. Schwertmannite formation, controlled by iron-oxidizing bacteria, was only found at the bottom of the tributary during the rainy season. In the dry season, As, Ni, Se, and Cu concentrations remained at their original levels because there was no formation of schwertmannite or dilution by rainwater. The existing schwertmannite also starts to dissolve as the pH decreases. Seasonal dynamics cause the failure of natural attenuation; thus, methods for maintaining its effectiveness in the dry season are needed. In addition, geochemical modeling was conducted to determine the significant roles of schwertmannite formation and dilution of rainwater in the tributary. Schwertmannite is a potential adsorbent for As removal from drainage. However, dilution provided indirect and direct impacts on the tributary, such as increasing the pH and diluting the concentration of toxic elements.