Analysis of Surface Infiltration Through Acid Generating Waste Rock at Faro Mine
Creators: Wahl, Gabriella
Publisher: name: University of Alberta Library
Year: 2023

<p>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. </p>

<p>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. </p>

<p>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. <br>
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.</p>

<p>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. </p>