Research Highlight - Is the Water Balance of Your Waste Rock Pile Reliable? A framework for Improving Assessment of Water Inputs and Outputs for a Typical Storage Facility
“The simulations facilitated estimation of the partitioning of rain and snowmelt into infiltration, evapotranspiration, and overland flow at the land surface, changes in moisture storage within the subsurface, and drainage from the base of the waste rock using a physically based approach. ”
Thompson, C., Thompson, R., Zawadzki, W. (2025). Is the Water Balance of Your Waste Rock Pile Reliable? A framework for Improving Assessment of Water Inputs and Outputs for a Typical Storage Facility. International Mine Water Association (IMWA) 2025 conference.
We’re pleased to highlight this research by Craig Thompson, Randi Thompson, and Willy Zawadzki of BGC Engineering, which presents a practical framework using HydroGeoSphere modelling to improve water balance assessments in mining waste rock piles. This research was presented at the International Mine Water Association (IMWA) 2025 conference. With increasing pile heights, evolving geochemical considerations, and growing scrutiny around drainage and treatment, the study highlights the importance of simulating how waste rock emplacements evolve over time and how that evolution impacts water movement and seepage predictions.
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The researchers conducted one-dimensional numerical simulations to assess the influence of pile growth over the mine life on water input, storage, and basal drainage under a range of Canadian climate conditions. Their approach compared two scenarios: a conventional static waste rock pile at full buildout and a more realistic transient pile that increased in height by 10 m increments every two years during a 20-year mining period. An additional 100-year closure period was also simulated. The study accounted for both “soil-like” and “rock-like” material types, incorporating matrix and dual-domain flow properties. In some simulations, seasonal near-surface freezing was introduced to capture the impact of ice formation on percolation dynamics.
The results reveal that neglecting the temporal development of the pile can significantly delay predicted drainage onset (wet-up time) by more than a decade. For example, at a temperate site (Site A), the transient configuration showed drainage beginning within 2 years, compared to 15–19 years in the static scenario. Variations in basal drainage, moisture distribution, and saturation profiles all highlight the importance of modelling how water flow conditions shift as new lifts of waste rock are added. These findings have practical implications for water treatment infrastructure planning and for evaluating long-term contaminant transport pathways.
To perform these simulations, the researchers used HydroGeoSphere (HGS), a fully integrated surface-subsurface modelling platform. HGS enabled the team to simulate the coupled movement of rain and snowmelt through the waste rock system, including infiltration, overland flow, evaporation, and drainage under varying climate regimes. The physically-based framework provided by HGS was crucial for capturing transient storage dynamics and drainage timing, offering a more nuanced understanding of how pile growth influences seepage. The authors note that, while their current approach used one-dimensional modelling for efficiency, the HGS simulations can be extended to two- and three-dimensional configurations depending on project scale and data availability— making this methodology scalable and adaptable for real-world mine planning.
Abstract:
Piles of waste rock placed during mining operations commonly increase from initial heights of several metres to more than 100 m during the life of a mine. Depending on geochemical properties of the waste rock, seepage emanating from the base of piles may require collection and treatment which necessitates adequate planning through development of reliable estimations of the waste rock pile water balance. Using a suite of numerical simulations of a synthetic waste rock pile, consideration of the pile’s temporal development is shown to be an important factor on the distribution of water within the waste rock and the timing and magnitude of basal drainage from the pile for a range of climate regimes.