Porewater Monitoring Under Different Layer Systems on a Sloping Surface at a Closed Mine Site

Abstract

This paper addresses the geochemical evolution, volumetric water content, and temperature of porewater when constructing different soil layers to improve the surface acidic conditions on a slope at a closed mine. Three cases were set under different layer systems. Case 1 was solely composed of surface-weathered rocks. A vegetation layer was constructed on the surface rocks in case 2, whereas a top vegetation and bottom low-permeable layers were constructed on the rocks in case 3. In both cases, a soil–cement layer was constructed to prevent landslides. Porewater sampling systems and soil sensors were set at different depths to collect porewater and measure the volumetric water content and temperature. The results showed that, when no layers were applied (case 1), high concentrations of heavy metals and low pH values were observed regardless of the depth and season. When a vegetation layer (case 2) was applied, a dramatic decrease in heavy metal concentrations was observed, similar to the results in case 3. In both cases, pH values were circumneutral. Moreover, the addition of the low-permeable layer reduced the infiltration of rainfall through the layers by considering the changes in volumetric water content. Also, the results of case 1 were compared with those obtained at a flat surface under similar conditions. On the slope, the pH was more acidic, and heavy metal concentrations were higher. These suggest that the dissolution of heavy metals from the weathered rocks into the porewater is enhanced on the sloping surface due to a longer solid–liquid interaction time.