Evaluating Impacts of Legacy Mining and Nature-Based Interventions on a Watershed Scale

Abstract

Comprehensive environmental assessment and monitoring are vital in the efficient planning and evaluation of remediation and restoration efforts. Detailed assessment and monitoring are especially important at large, complex legacy mining sites, where contaminant sources such as mine drainage and waste rock have been allowed to interact with and degrade the quality of surrounding water, soil, and habitat. This study examined the effects of point sources, such as mine drainage seeps, and nonpoint sources, such as stormwater runoff interacting with surface-stockpiled mine waste, on water quality in a mining-impacted watershed. The first section of this study compared point and nonpoint source loading in a mining-affected watershed and identified stream reaches where nonpoint source loading was more likely to occur. The second section examined the hyporheic zone of a mining-impacted stream to determine whether diffuse groundwater upwelling was a source of metals of concern. The third section focused on water quality and quantity in mine waste and tailings stockpiles acting as perched aquifers and their potential effects on metals loads in an adjacent stream. Nature-based interventions to decrease diffuse metals loading, including partial remediation of a mine waste pile, construction of a vegetated detention pond in a flood plain that was recently buried under mine waste, and the use of manure-based biochar as a sorbent for metals introduced to surface water by interaction with mine waste were also examined. Diffuse loading of metals was principally noted in the area where partial removal of mine waste occurred, as well as other areas where mine waste interacted with surface water. However, these diffuse loads usually comprised a smaller fraction of the loads of most metals of concern than those contributed by mine drainage discharges. Similarly, diffuse groundwater discharge was not found to be a substantial source of metals loading, and porewater interaction with organic material in a volunteer wetland sequestered metals within the hyporheic zone. A volunteer riparian area growing on one of the studied mine waste piles acted as a buffer between the surface water and the perched groundwater inside the mine waste, where much greater concentrations of metals of concern were observed. The vegetated detention pond constructed in the area uncovered by the partial removal of mine waste did not decrease metals concentrations in the runoff of the site due to the lack of metals in the particulate fraction that could be removed via settling and a lack of wetland functions to decrease metals in the dissolved fraction. Finally, manure-based biochar introduced undesirable concentrations of potassium and phosphate in addition to not acting as an effective sorbent of metals of concern. This study has found that mine waste, secondarily contaminated soils and sediments, and unabated mine drainage discharges all negatively impacted water quality in the watershed. Future remediation at the site should thus take a more holistic approach to effectively improve water quality.