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Abstract: Characterizing infiltration through acid generating waste rock is essential in addressing critical environmental issues attributed to mining activities. At it’s peak operation, Faro Mine in Yukon, Canada was one of the world’s largest lead and zinc mines, where 320 million tonnes of waste rock was accumulated during operation and has been leaching contaminants over time. In efforts to remediate the existing waste rock, a reactive transport model is planned to characterize geochemical reactions within the waste rock pile. Prevalence of acid rock drainage is a function of available moisture, which is driven by the infiltration of precipitation events. The purpose of this research is to utilize a soil – atmosphere model to determine the boundary condition for a reactive transport model in the form of net percolation. Field and laboratory investigations were paired with numerical modeling to compute net percolation. The field and laboratory studies aimed to characterize in-situ physical and hydrological properties of the waste rock materials which govern saturated / unsaturated flow. The overall nature of the surface materials were found such that unsaturated flow of water and water vapour contributing to geochemical reactions is predominantly through fine fraction of the waste rock matrix materials. An existing finite element soil – atmosphere model (SoilCover) was used to compute net percolation for three meteorological scenarios: wet year, average year, and an extreme event. Sensitivity analyses were conducted to understand the effect of saturated hydraulic conductivity and initial matric suction on the predicted net percolation. For average and wet year simulations, the baseline net infiltration was computed to be 12% and 15 % of the total yearly precipitation respectively. When saturated hydraulic conductivity was altered during sensitivity analyses, a range of net infiltration computed was found to be 2.1% and 20% of total precipitation across all simulations for average and wet year. Net percolation was also computed for an extreme event found within the representative wet year analysis. During the extreme event, baseline infiltration was found to be 47% of the total rainfall event, where minimum and maximum values were computed to be between 25% and 90%. The saturated hydraulic conductivity had a large effect on the infiltration for all scenarios. Increasing the hydraulic conductivity by one order of magnitude doubled the amount of predicted infiltration for the extreme event. Effects were calculated on water balance parameters such as runoff and actual evaporation. Initial matric suction had a larger effect on extreme events than the overall yearly water balance simulations. Results of net percolation analyses were in good agreement with results of previous in-situ investigations performed on the surface of the Main Dump. Matric suction profiles computed by the soil – atmosphere model showed that the surface soils become saturated to a depth of approximately 1.0 m as a result of large infiltration events, particularly the freshet season. In-situ measurements show that large infiltration events and freshet can reach deeper elevations within the waste rock pile than computed by the water balance model. It is thought that differences between field behaviours and numerical simulations is a result of spatial variability of fines contents, as well as the existence of preferential flow paths beneath the surface of the waste rock storage facility.

Abstract: This research aims to develop a model using a systems dynamics approach that can be coupled with TMSim to account for the deposition of waste rock at hard rock mines at the feasibility stage of mine planning. The model includes waste rock placement techniques to minimize the environmental impacts of acid rock drainage at many hard rock mines. The GoldSim software was used to develop the model, with two aspects, the deposition of waste rock based on mine processes to create the “pile”, and the environmental conditions and associated variably saturated flow of water within the waste rock pile.The conceptualization of the waste rock in the pile was constructed in four lifts. These four lifts were divided into 15 rows and 16 columns to create a pseudo 3-D structure of 240 1-D columns. The model includes waste rock deposition, precipitation, snowmelt, evaporation, runoff, infiltration, and seepage. The variably saturated system was modelled using a modified version of the variably saturated flow sub-model for a tailings cover. Modifications of the model included potential evaporation calculation, snow accumulation and meltwater calculation, and simplification of runoff. The model was run using a case study for a mine site at the feasibility stage to demonstrate the application of the model. Overall, the model behaved as expected with general pile water balance behaviour observed over the course of construction and long-term behaviour. The models clearly show the benefits of paddock dumping versus end dumping with reduced water storage volumes within the pile.

Abstract: The extraction of sulfide-bearing waste rock can contribute to the generation of acid rock drainage (ARD). ARD is the effluent generated due to the oxidation of sulfide minerals which can be toxic to the receiving environment. Waste rock material which contains potentially acid-generating (PAG) material is often covered with a soil cover to mitigate the production of ARD. When a soil cover system is implemented in a permafrost environment such as northern Canada, it may realize additional PAG management capabilities as a result of annual freezing temperatures. ARD is a known issue at the Diavik Diamond Mine (the Site). It is located on an island in Lac de Gras roughly 300 km northeast of Yellowknife, Northwest Territories. The Site is located on the boundary of continuous and discontinuous permafrost with an active layer ranging between 1.5 and 5 m in depth. The mean annual average temperature of the Site was determined to be -8.9 degrees Celsius. The waste material at this Site is separated according to sulfide content and therefore its acid-generating potential. The key material types included in this research include the Type I (T1) material which is classified as non acid generating (NAG), Type III (T3) material (PAG), and till material (NAG). The bulk of the material on Site is the PAG T3 material, which is stored in a large, 80 m tall waste rock storage area called the North Country Stock Pile (WRSA-NCRP). A proposed cover system for the WRSA-NCRA included 3.0 m of T1 waste rock overlying 1.5 m of a finer layer of till. The till material properties allow for the formation of a latent heat layer due to a low permeability and high moisture content. An adjustment to this design included the removal of fine fractions of the T1 material to form a higher permeability upper layer of the cover system with the intent to induce natural convective cooling. This material was denoted T1 (coarse), and the system was called an Air Convective Cover (ACC). Previous research included the development of a simulation model to analyze this effectiveness of this ACC design on a structure like the WRSA-NCRP in a program called COMSOL Multiphysics® (Comsol). The field conditions were approximated assuming heat transfer and fluid flow coupling. This simulation considered an annual surface temperature increase as a result of climate change. Results demonstrated the success of the soil cover system in maintaining the underlying PAG material frozen after 100 years. This thesis builds on these findings in order to optimize the soil cover design prior to moving towards test cell recommendations for the WRSA-NCRP. The optimization process was designed to minimize volume requirements while maintaining the material frozen for the same long-term timeline of 100 years. Similar to previous modelling, this research will also account for an annual surface temperature increase due to climate change. The key objectives in this research were to demonstrate a successful model benchmark analysis, conduct a sensitivity analysis, optimize soil cover thicknesses, and provide suggestions for future test cell constructions. A model benchmarking analysis was established and the rebuilt model in Comsol was deemed an acceptable equivalence to the original model simulation. A sensitivity analysis explored the parameter uncertainties of the till volumetric moisture content, the T1 (coarse) permeability, and the annual temperature increase as a result of climate change. Each parameter had a functional range that would produce accurate and expected results. The model optimization of the soil cover included different combinations of materials including T1 (coarse), T1 (all fraction), and till. The model optimization step provided options that account for material availability at the site. The accepted optimization combinations were further classified according to the definition of success for each simulation which provided four levels of added contingency. The optimized results from the lowest contingency buffer category were either 1.00 m T1 (Coarse) + 1.25 m Till or 2.00 m T1 (coarse) + 0.25 m Till. The highest contingency buffer resulted in an optimized prescription of 2.25 m T1 (coarse) + 1.25 m Till. Proposed field test plot prescriptions were generated according to these optimization results.

Abstract: Soil cover systems are engineered barriers designed to isolate hazardous mine waste from climatic water and oxygen. Assessment of water flow at the interface between the atmosphere and the ground surface is paramount to successful cover design. For decades, cover systems were designed based on infiltration and evaporation models often overlooking surface runoff. Runoff is a fundamental part of the rainwater cycle intertwined with both infiltration and evaporation but rarely adequately examined in the context of cover system design. This thesis provides a laboratory and numerical modelling program for comprehensive physical evaluation of rainfall runoff responses in soil cover systems. In the laboratory, rainfall runoff tests were conducted using a specially designed rainfall simulator apparatus. A series of controlled rainfall experiments targeted at low permeability and capillary barrier profiles were completed. The experiments focused on observing the runoff phenomenon and quantifying the volume, rate, and time until runoff occurred in response to variation of applied rainfall intensities. Changes in matric suction and volumetric water content were monitored as wetting fronts propagated through the soil profiles. Soil profiles were investigated under different saturation states. The study focused on one central question: is it possible to predict rainfall runoff based on measurable soil properties? Laboratory observations suggest that yes, rainfall runoff rates and volumes are primarily governed by the applied rainfall intensity and saturated hydraulic conductivity in the case of saturated soil surfaces, whereas rainfall runoff rates are governed by the applied rainfall intensity and infiltration capacity for unsaturated soil profiles. The laboratory experiments were numerically replicated for each profile using the SVFlux model. One- and three-dimensional models were assessed for saturated and unsaturated initial states. One-dimensional models produced runoff cumulative volume results within 6% accuracy for most low permeability profiles. Less rigorous results were observed in the capillary barrier profiles, where accuracy varied between 1% and 32%. Three-dimensional models marginally improved the results for capillary barrier profiles. Overall, the results of numerical predictions testify to a reasonably good capability to predict runoff fluxes for controlled laboratory conditions. Lastly, a case study of the Savage River mine in Australia was evaluated to ascertain temporal and spatial variability effects on numerical predictions in field settings. Comparisons of field measured rainfall, and runoff volumes with predictions made by SVFlux were discussed. Both non-vegetated water-shedding cover system and uncovered tailings dam were examined. The study encompassed detailed sensitivity analyses of surface runoff predictions in response to the changing input of rainfall intensity resolution and saturated hydraulic conductivity. The results showed that runoff predictions were highly sensitive to both the resolution of precipitation rate and change in saturated hydraulic conductivity input. Close attention to site conditions is vital when choosing soil parameters to attain meaningful results.

Abstract: Mine waste rock is one of the largest waste streams produced from precious metal mining that must be managed over the long-term. Of particular concern is the management of chemical oxidation of sulphide minerals termed acid rock drainage (ARD). This thesis presents a field and laboratory investigation of two historic waste rock stockpiles at Detour Lake Mine in Ontario, Canada. 100 physical samples and in situ measurements of unsaturated conditions were collected. Laboratory analyses determined particle size distribution and unsaturated and hydraulic characteristics. Digital image processing techniques evaluated large scale grain size, porosity, and water storage capacity. The stockpiles were unsaturated, clast supported structures with features typical of end-tipped deposition. On average, 17% of the material was

Road construction through sulfidic materials in Virginia has resulted in localized acid rock drainage (ARD) that threatens water quality, fill stability, integrity of building materials, and vegetation management. The objectives of this study were: i) to develop a state-wide sulfide hazard rating map based on characterization of the geologic formations associated with acid roadcuts, ii) to estimate depth to sulfidic sediments in the Coastal Plain based on landscape relationships, and iii) to evaluate potential acidity testing procedures on diverse materials. Geologic formations associated with acid roadcuts were characterized by potential peroxide acidity (PPA) and S content, and grouped into four categories. Listed in order of increasing severity, these formations included: the Tabb Formation (Coastal Plain), the Lynchburg Group of the Ashe Formation (Blue Ridge), the Chesapeake Group and Lower Tertiary deposits (Coastal Plain), the Millboro shale, Marcellus shale, Chatanooga shale and Needmore Formation (Valley and Ridge), and the Quantico Formation (Piedmont). Evaluation of landscape parameters near Richmond, Virginia, indicated that the likelihood of encountering sulfidic materials within a given depth at a specific location was related to elevation and mapped soil types. Elevation and soil map units were assigned to risk classes to indicate the likelihood of encountering sulfides within a depth of 9 m. Comparison of PPA and S content for 296 diverse samples indicated that S may serve as a screening tool to evaluate materials without carbonates. Comparison of PPA and conventional Acid-Base Accounting (ABA) for 14 diverse samples indicated that PPA and ABA were highly correlated, with PPA yielding 0.60 to 0.95X the amount of acidity as ABA. Potential acidity by Soxhlet extraction and PPA were equivalent for 3 of 4 diverse samples. Average acidity and metal contents of leachate from Soxhlet extractors were correlated with acidity and metals of road drainage. Sulfide hazard analysis should be an essential step in the pre-design phase of highway construction and other earth-disturbing activities.