Evolution of Acid Mine Drainage from a Coal Waste Rock Pile Reclaimed with a Simple Soil Cover

Abstract

Waste rock piles (WRPs) are commonly remediated with cover systems to limit water and oxygen influx and mitigate the impacts of acid mine drainage (AMD) on the environment. While numerous types of cover systems exist, simple, single-layer soil covers remain an attractive option due to their low cost and simplicity of installation. Since knowledge of their long-term performance in humid climates is limited, this study was undertaken to assess and predict a single-layer cover system at a WRP in Nova Scotia, Canada. A two-dimensional finite element model was developed to simulate variably saturated flow and solute transport at the WRP and surrounding area. Key parameters collected during five years of field monitoring, including moisture contents, groundwater levels and dissolved metal concentrations, were used to produce a well-calibrated and verified model. Early results confirm that the cover system has already decreased AMD into both groundwater (reduced water infiltration/seepage in the WRP) and surface water (eliminated contaminated surface water runoff). Long-term acidity depletion rates indicate that all sulphidic minerals within the pile will be oxidized within 34 years, but due to the slow leaching rates into water, it will take over 9000 years to deplete all acidity. Numerical simulations predict the evolution of groundwater and surface water quality over time until full acidity depletion. Current work involves kinetic tests on waste rock samples to more accurately access the annual generation and release of AMD.

A novel empirical approach to measuring pore gas compositional change in mine waste storage facilities: A case study from northern Europe

“…The application of cover/capping systems (ranging from dry, wet and organic systems) at waste storage facilities is a common closure engineering strategy that aims to prevent O2 ingress into wastes (Skousen et al 2017). Measuring of pore-gas concentration within wastes has previously been used as a parameter assess the effectiveness of cover systems, although O2 has been the focus of intrusive on-site investigations (Harries & Ritchie 1985;Ramasamy & Power 2019). Site-specific variability (e.g., mineralogy and climate) and the inter-play of O2 and carbon dioxide (CO2) during relevant reactions (sulfide oxidation, silicate and carbonate weathering) mean the development of a standardised empirical measurement method of pore-gas change would be of benefit to the mining industry.…”

“…As all samples had similar water contents, this CO2 decrease could have been due to the diffusion of CO2 into the interstitial water of mineral grains. Though this process can be the first step in aqueous carbonation (Rackley 2017), diffusion of CO2 into water is a fundamental physical process that occurs irrespective of mineralogy (Power et al 2014). Since CO2 is highly soluble at the temperature range at which the barrels were kept (Stokreef et al 2022), the CO2 uptake in all barrels may have been largely due to CO2 diffusion into water and not necessarily potential subsequent carbonation reactions.…”

Mine Waste Rock: Insights for Sustainable Hydrogeochemical Management

“…Surface runoff from waste-rock piles may be deliberately minimized to avoid erosion on the pile batters, in which case it is typically insignificant for the water balance. Yet, certain reclamation strategies rely on maximizing runoff from compacted (subsurface) cover layers to limit percolation into underlying reactive material [236,247]. Material compaction (e.g., in traffic surfaces) can facilitate surface runoff and ponding, especially under flashy precipitation patterns.…”