Acid Mine Drainage at Ore Knob Tailings Pile -- Hydrologic and Geochemical Characterization and Bioremediation

Acid Mine Drainage (AMD) is produced in the vadose zone of the Ore Knob Mine tailings pile (Ashe County, NC) by oxidation of sulfide minerals and subsequent transport of the AMD to the water table within the pile. Surface water from the upstream watershed enters the pile through a series of pools or wetlands on the pile perimeter and rapidly transports the AMD through the pile to the downstream embankment face. Water discharges the embankment face as a series of seeps or springs with high levels of dissolved Fe, SO4, acidity, Cu, and Zn and impairs water quality in 1.5 km of Ore Knob Branch and 4.7 km of Peak Creek feeding the New River. In this work, a detailed hydrological and chemical characterization was performed to better understand the physical and chemical processes controlling AMD generation within the tailings pile. This information was then used to develop a remediation process that could be used to treat AMD within the tailings pile without the high capital and operating costs associated with traditional treatment approaches.;Spatial variations in the physical and hydraulic characteristics of the Ore Knob tailings cause large variations in water and oxygen transport, with associated changes in the amount and concentration of AMD produced. Tailings in the upstream areas are finer grained, with lower air filled porosity and oxygen diffusivity, which reduces the rate of sulfide mineral oxidation. In downstream areas near the embankment face, the original tailings were coarse grained, with lower water retention and high oxygen diffusivity, increasing the oxidation rate. However, weathering processes have increased the fine grained fraction in the oxidized zone and hardpan layer, increasing water retention and lowering oxygen diffusivity. The greater thickness of the downstream oxidized zone combined with increased water retention due to weathering may have significantly reduced acid generation in these areas.;The water budget model DRAINMOD was calibrated using soil physical properties and meteorological data and used to estimate recharge rates on the tailings pile surface and upstream watersheds that drain into the pile. This information was then used to calibrate the groundwater flow model, MODFLOW, to help understand water movement within the pile and the impact of different remediation alternatives on water distribution and movement. Recharge rates were higher in the coarse grained areas and lower in fine grained areas. Infiltrating water transports AMD constituents from the vadose zone where they are discharged to the water table within the pile. Surface water that enters the pile from upstream areas then transports the AMD constituents to the downstream embankment face where these constituents are discharged to surface water.;A pollutant load analysis using the recharge analysis results and average long term monitoring results showed that over 89% of the Fe, SO4, acidity, and Al released to the Ore Knob Branch are produced in the vadose zone of the pile. The total annual load of acidity discharged from the Ore Knob tailings pile is estimated to be over 220 tons/yr. Given the very high levels of dissolved Fe and acidity produced within the pile, traditional end-of-pipe treatment approaches will have very high capital and operating costs. An affordable passive treatment method is needed to treat AMD within the pile before it is discharged to the surface.;A 20 month long in situ pilot test was conducted to evaluate the surface application of waste glycerol (WG) to reduce release of acid mine drainage (AMD) constituents from mine tailings. Beneficial characteristics of the WG include high aqueous solubility, high organic content, and high alkalinity. Four columns were packed with fine grained sulfide rich tailings and incubated in the field under ambient temperature and precipitation conditions. Columns were periodically pumped to maintain unsaturated condition. In the two replicate untreated control columns, diffusion of oxygen into the tailings resulted in large increases in dissolved Fe, SO4, Mn, Mg, Al, Zn, and hydrogen peroxide acidity with an associated drop in pH. In the two replicate treated columns, WG was blended into the top 0.18 m of tailings seven months after the columns were established, resulting in large reductions in Fe, SO 4, hydrogen peroxide acidity, Al, Cu, and Mn. Observed pollutant reductions resulted from a combination of processes including: (a) neutralization of acidity by the KOH present in the WG (b) reduction of SO4 to H 2S with subsequent precipitation of dissolved metals, and potentially (c) consumption of oxygen, slowing oxidation of the tailings.