Thermal-Hydrological-Chemical Modeling of a Covered Waste Rock Pile in a Permafrost Region

In order to reduce contaminant mass loadings, thermal cover systems may be incorporated in the design of waste rock piles located in regions of continuous permafrost. In this study, reactive transport modeling was used to improve the understanding of coupled thermo-hydrological and chemical processes controlling the evolution of a covered waste rock pile located in Northern Canada. Material properties from previous field and laboratory tests were incorporated into the model to constrain the simulations. Good agreement between simulated and observational temperature data indicates that the model is capable of capturing the coupled thermo-hydrological processes occurring within the pile. Simulations were also useful for forecasting the pile’s long-term evolution with an emphasis on water flow and heat transport mechanisms, but also including geochemical weathering processes and sulfate mass loadings as an indicator for the release of contaminated drainage. An uncertainty analysis was carried out to address different scenarios of the cover’s performance as a function of the applied infiltration rate, accounting for the impacts of evaporation, runoff, and snow ablation. The model results indicate that the cover performance is insensitive to the magnitude of recharge rates, except for limited changes of the flow regime in the shallow active layer. The model was expanded by performing an additional sensitivity analysis to assess the role of cover thicknesses. The simulated results reveal that a cover design with an appropriate thickness can effectively minimize mass loadings in drainage by maintaining the active layer completely within the cover.

Modeling of Thermal-Hydrological-Chemical (THC) Processes During Waste Rock Weathering Under Permafrost Conditions

The oxidation of sulfide minerals such as pyrite present in waste rock results in elevated sulfate, enhanced metal loadings and in many cases low pH conditions. Recently, many mines have opened in remote areas, including regions subject to permafrost conditions. In these regions, freeze-thaw cycles and the possible development of permafrost in mine waste add to the complexity of weathering processes, drainage volumes and mass loadings. To assess weathering in these waste rock piles, the reactive transport code MIN3P-HPC has been enhanced by implementing constitutive relationships related to freeze-thaw cycles that control flow patterns, solute transport, generation and transport of heat, as well as geochemical reactions and their rates. Simulations of a hypothetical pyrite-rich waste rock pile placed onto natural permafrost were conducted under reference climate conditions. Additionally, the effect of a warming climate was also studied through a sensitivity analysis. The simulation results indicate a potentially strong coupled effect of sulfide mineral weathering rates and a warming climate on the evolution and persistence of permafrost within waste rock piles and the release of acidic drainage. For relatively low sulfide mineral oxidation rates, the simulations indicate that permafrost can develop within waste rock piles, even under warming climate conditions. However, the results for low reactivity also show that mass loadings can increase by >50% in response to a slight warming of climate (3°C), relative to reference climate conditions. For the chosen reference reaction rates, permafrost develops under reference climate conditions in the simulated waste rock pile; however, permafrost cannot be maintained for a marginally warmer climate, leading to internal heating of the pile and substantially increased production of acidic drainage (>550%). For high reaction rates, the simulations suggest that internal heating takes place irrespective of climate conditions. Evaluation of thermal covers indicates that significant reductions of mass loadings can be achieved for piles with low and reference reactivity (91–99% in comparison to uncovered piles), but also suggest that thermal covers can be ineffective for piles with high sulfide content and reactivity. Together, these simulations provide insights into the complex interactions controlling waste rock weathering in cold-region climates.

Mapping Potentially Acid Generating Material on Abandoned Mine Lands Using Remotely Piloted Aerial Systems

Weathering and transport of potentially acid generating material (PAGM) at abandoned mines can degrade downstream environments and contaminate water resources. Monitoring the thousands of abandoned mine lands (AMLs) for exposed PAGM using field surveys is time intensive. Here, we explore the use of Remotely Piloted Aerial Systems (RPASs) as a complementary remote sensing platform to map the spatial and temporal changes of PAGM across a mine waste rock pile on an AML. We focus on testing the ability of established supervised and unsupervised classification algorithms to map PAGM on imagery with very high spatial resolution, but low spectral sampling. At the Perry Canyon, NV, USA AML, we carried out six flights over a 29-month period, using a RPAS equipped with a 5-band multispectral sensor measuring in the visible to near infrared (400–1000 nm). We built six different 3 cm resolution orthorectified reflectance maps, and our tests using supervised and unsupervised classifications revealed benefits to each approach. Supervised classification schemes allowed accurate mapping of classes that lacked published spectral libraries, such as acid mine drainage (AMD) and efflorescent mineral salts (EMS). The unsupervised method produced similar maps of PAGM, as compared to supervised schemes, but with little user input. Our classified multi-temporal maps, validated with multiple field and lab-based methods, revealed persistent and slowly growing ‘hotspots’ of jarosite on the mine waste rock pile, whereas EMS exhibit more rapid fluctuations in extent. The mapping methods we detail for a RPAS carrying a broadband multispectral sensor can be applied extensively to AMLs. Our methods show promise to increase the spatial and temporal coverage of accurate maps critical for environmental monitoring and reclamation efforts over AMLs.

Nitrogen removal from waste rock drainage in northern Sweden with denitrifying woodchip bioreactor

<p>Although nitrogen is not a traditional contaminant when considering the detrimental impacts of mine waste leachate on aquatic ecosystems, it is a common pollutant of concern in underground iron ore mining where waste rock leachate has a neutral pH and a low metal content. This is the case in northern Sweden, where environmental authorities, supported by the EU Water Framework Directive, have imposed strict regulations on nitrogen discharges to oligotrophic surface water systems. Requirements for lower nitrogen releases has driven the development and application of a bioreactor technology for nitrate removal at LKAB’s Kiruna iron ore mine.</p><p>A full-scale woodchip denitrifying bioreactor has been in operation since September 2018 in Kiruna for the removal of nitrate (NO<sub>3</sub><sup>-</sup>) from waste rock leachate. Drainage from the waste rock pile is intercepted in a subsurface groundwater collection reservoir at the toe of the waste rock pile and pumped at an average rate of 22 m<sup>3</sup>/d into the bioreactor. Leachate from the low-sulfur waste rock is characterized by neutral pH (average pH 7), moderate alkalinity (108 mg/L HCO<sub>3</sub><sup>-</sup>), and elevated concentrations of sulfate, NO<sub>3</sub><sup>-</sup> and chloride (average concentrations 670, 61 and 102 mg L<sup>-1</sup> respectively).</p><p>During 2019, and average nitrogen removal efficiency was 77%: during the 165 day sampling period, 189 kg NO<sub>3</sub>-N were removed in the bioreactor, which is primarily attributed to denitrification. A net production of 26 kg of NO<sub>2</sub>-N was measured. Nitrous oxide (N<sub>2</sub>O) emissions in gas and aqueous phase are low from the bioreactor and the primary product of denitrification is assumed to molecular nitrogen (N<sub>2</sub>). Dissolved N<sub>2</sub>O concentrations were on average greater at the bioreactor inlet (277 µg L<sup>-1</sup>) than at the outlet (179 µg L<sup>-1</sup>), although variations were substantial during the summer months with a net dissolved N<sub>2</sub>O export from the bioreactor on one occasion in late summer. The flux as N<sub>2</sub>O from the bioreactor surface varied from 1 – 28 mg N<sub>2</sub>O m<sup>-2</sup> d<sup>-1</sup>. In addition to nitrate removal, zinc concentrations were reduced by, on average, 88%. </p>