Assessment of seasonal changes in groundwater quality of waste rock dump in temperate continental climate, northern Japan.

Understanding seasonal groundwater quality changes in temperate continental climate waste rock dumps (WRDs) is necessary for sustainable environmental risk prevention and legacy mine contamination management. Therefore, we conducted a field investigation of a WRD to determine the mechanisms controlling its groundwater quality dynamics. The research aimed to understand the impact of seasonal changes on heavy metals released from the WRD. Three monitoring wells were installed in the WRD to investigate the pH, electrical conductivity (EC), and groundwater level (GL). The mineral composition of the waste rock was determined. Groundwater and river water samples from the monitoring wells and rivers surrounding the WRD were collected for chemical analysis. The sphalerite and galena concentrated in the WRD were assumed to be the main sources of Zn, Pb, and Cd contamination. Summer rainfall was the dominant recharge source of river water, which rapidly infiltrated to the WRD, altering the pH, EC, and GL of the groundwater. The pH, EC, and GL were stable in winter because snowpack covering the surface soil prevented groundwater recharge to the WRD. However, snow melting affected the pH, EC, and GL in the WRD. The sources of groundwater recharge (rainfall, river water, and snowmelt) altered the behaviour of the heavy metals in the WRD through two main mechanisms: the dissolution of sulphide minerals and efflorescent salts upon contact with the recharge water, and the dilution effect of the recharge water, which mixes with the groundwater in the WRD, reducing the heavy metal concentration. Sulphide mineral and efflorescent salt dissolution were significant in the deepest monitoring well and rainfall was the dominant recharge source which increased sulphide mineral and efflorescent salt dissolution in the WRD.

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.