Case Study 1 - Open Pit Mine in North Central British Columbia
Case Study 1 is situated in north central B.C. on the plateau between the Coast Mountain and Rocky Mountain ranges, just north of the town of Fort St. James. The open pit project will recover upwards of 60,000 tonnes per day from the copper-gold deposit with a projected mine life of approximately 20 years.
The mine site ("Site") occupies a valued ecosystem comprising fish-bearing creeks and small lakes hosted in geologic materials indicative of a glacial valley. The main project components will include a two-stage open pit development, a prominent tailings storage facility (TSF), on-site crushing and concentrator plants, as well as a concentrate load-out facility (Figure 1).
Figure C1-1: Mine site layout for Case Study 1.
Development of the open pits will induce inflow to the pit and the operation of the TSF will likely result in seepage of mine-affected water to the environment. A numerical groundwater modelling effort was undertaken to assess the impacts of these activities on the receiving environment.
During the application process for an environmental assessment certificate, the Proponent and stakeholders identified groundwater and surface waters as valued ecosystem components (VECs). Subsequently, numerical modelling was undertaken to address the following areas of concern:
- Baseline hydrogeology for the project area, including:
- Verification of the Site water balance model
- Interpretation of integrated groundwater and surface water systems
- Indication of the groundwater flow pattern
- Inflows to the proposed open pit development
- Seepage from the proposed TSF
The regional baseline groundwater flow model would be created initially, and later modified to address the issues of open pit inflow and TSF seepage.
Data used to develop the Site conceptual model included information from borehole drilling logs and geologic mapping, sourced primarily from previous field investigations.
The data used to develop the baseline regional groundwater model comprised groundwater elevations, aquifer hydraulic test results, and observed flows. Additional flow estimates were obtained from the site-wide baseline water balance that was completed as part of this modelling exercise.
Limited climate data from the Site was calibrated to local municipal datasets to generate a synthetic series for the Site. This data was used for water balance modelling and for assigning recharge to the numerical groundwater model.
The Proponent proposed that the objectives could be met by both analytical and numerical modelling approaches. An analytical solution for fault zone contributions to pit inflow, for example, could be used to support a more rigorous numerical solution. Furthermore, both 2D and 3D methods could be employed for modelling, depending on the objective and the complexity of the problem.
The three-dimensional finite difference code MODFLOW-SURFACT was selected for baseline modelling for two primary reasons:
- The code allows for verification that the groundwater recharge, storage, and discharge as defined by the site-wide water balance satisfies groundwater flow theory, and
- The code employs an updated wetting/drying function which minimizes convergence problems typically associated with MODFLOW simulations in steep terrain with steep groundwater gradients.
Auxiliary software tools were also used to facilitate the model set-up and interpretation of results, including:
- Geographic Information Systems (GIS) packages,
- Contour plotting software, and
- Database management programs
For the determination of pit inflow, both analytical and numerical solutions were used.
TSF seepage was modeled first with a 2D cross-sectional model, before using a modified version of the 3D baseline numerical model to confirm the results.
Baseline Groundwater Flow Model
The boundaries of the baseline groundwater flow model are depicted in Figure 2.
Figure C1-2: Boundaries of baseline groundwater flow model (red).
The domain is bounded primarily by the natural groundwater divides formed by creeks to the north, south, and east, and by bedrock topography to the west. The extent of the domain is such that the boundaries are not likely to bias the flow solution.
Creeks, lakes, and ponds were represented as constant head boundaries in the model while ephemeral streams and seeps were simulated with drains (Figure 3). The drains were assigned a constant conductance for all model runs. Aerial (meteoric) recharge was applied to the model according to earlier water balance modelling for the Site. The distribution of recharge zones across the model domain is shown in Figure 4.
Figure C1-3 - Baseline boundary conditions.
Figure C1-4: Zones of recharge for the baseline model.
The finite difference grid was uniformly distributed across the domain. The domain comprises approximately 180 rows and 200 columns, encompassing an area of 40 km2 . Vertically, the model was discretized into five hydrostratigraphic layers representing glacial outwash, till, and bedrock (Figure 5). All layers were modeled as confined layers to avoid convergence issues due to low recharge values and steep topography.
Figure C1-5: Vertical discretization of model domain.
The model was first run with steady state conditions to evaluate regional flow and then run with transient conditions to assess groundwater contribution to streamflow under low recharge conditions.
Open Pit Inflow Model
The baseline numerical model was used to estimate pit inflows according to the modelling objectives. The setup of this model is identical to that of the baseline model in terms of model domain, discretization, and initial conditions; however, boundary conditions in the area of the open pit (surface water features) were removed from the model. All aquifer properties were identical to the baseline model.
Dewatering of the open pit was simulated using MODFLOW by assigning pumping wells to the pit area which caused the simulated water level to be lowered according to the mine plan. The model was run at steady state at three distinct times during the mine life. Figure 6 shows the drawdown contours after 7 years of mining.
Figure C1-6: Drawdown contours (red) in the vicinity of the open pit after 7 years of mining.
The influence of the TSF on pit flows was not explicitly modeled at this stage, but covered later in the TSF-specific modelling phase. The results from the TSF modelling were subsequently used to verify the assumptions regarding TSF contribution to the pit inflow model.
As fault zones were intersected during drilling in the open pit area, a Darcy based analytical approach was used to conservatively estimate the discharge from an intersecting fault zone. This value was summed with the numerical inflow calculation and the estimate of TSF seepage contribution to determine a conservative pit inflow estimate.
Tailings Storage Facility (TSF) Seepage Model - 2D Analysis
Prior to setting up the three dimensional TSF model, a series of two dimensional VADOSE/W models were created to meet the following objectives:
- Estimate seepage using a range of hydraulic conductivity values for the glacial foundation materials (sensitivity analysis)
- Identify key flow pathways
- Estimate groundwater recharge under unsaturated conditions
VADOSE/W was selected for its applicability to unsaturated flow problems. The 2D model incorporates hydrostratigraphic units consistent with the baseline model and materials representing the gradation of tailings and TSF structural elements (i.e. the core, filter, and shell). The three sections chosen for modelling are presented in Figure 7.
Figure C1-7: Section alignments for 2D VADOSE/W modelling (red lines).
The sections represent the most direct pathways to sensitive receptors downstream of the TSF. Section B is shown in Figure 8 as an example of the TSF elements and hydrostratigraphy.
Figure C1-8: Model Section B in VADOSE/W.
Boundary conditions for the VADOSE/W sections comprise recharge to the upper surface and constant head boundaries representing the tailings pond level and the surface of downstream water bodies. All models were run in transient state for at least two years.
Tailings Storage Facility (TSF) Seepage Model - 3D Analysis
Following the 2D seepage modelling, the 3D baseline groundwater model was modified to explicitly include the tailings storage facility. This model included most of the elements of the baseline model but also included layers to represent the tailings materials and refinement of the hydrostratigraphy beneath the TSF based on the VADOSE/W modelling. The model was first run with steady state conditions over a range of recharge and hydrogeologic scenarios at a time period equal to 15 years of operation. Subsequent steady state runs at 18 and 35 years of operation were completed with only the most reasonable hydrogeologic conditions determined from the initial runs. A summary of the runs is provided in Table 1.
Table C1-1: Simulation runs for 3D numerical model of TSF.
The model domain was decreased in extent to exclude the steep topography west of the mine. This was done to facilitate model convergence. Furthermore, surface water features in the footprint of the TSF were removed and constant head boundaries were used to simulate pumping wells (proposed to be installed in the more transmissive materials beneath the facility).
To account for the open pit, while not explicitly including the excavation in the model, a general head boundary was assigned to the eastern pit boundary in the most transmissive model layer.
The tailings pond was simulated with the MODFLOW-96 river package which permits flow between surface water features and groundwater based on a conductance assigned to the bottom of the surface water body. As two distinct gradations of tailings are to be stored in the TSF, two values of conductance were used for the respective areas of the facility.
Meteoric recharge was simulated at several different rates based on climatic expectations and the degree of exposure of the tailings beach (i.e. wetted beach length, vegetation coverage, and supernatant pond coverage). The type of tailings exposed was also considered as finer grained tailings may inhibit recharge to the subsurface.
The hydrogeologic components of the TSF were assigned typical values of hydraulic conductivity supported by independent laboratory testing. The values of hydraulic conductivity assigned to the TSF components are presented in Figure 9.
Figure C1-9: Distribution of hydraulic conductivity in the TSF model.
Baseline Groundwater Model Calibration
The baseline numerical groundwater model was calibrated to hydraulic head targets and groundwater estimates of flux between sub-catchments from the site-wide water balance. During dry periods (late winter), baseflows were measured and reproduced with the water balance, improving the uniqueness of the solution. In addition to these calibration targets, the following factors were also reviewed at the calibration stage:
- Visual assessment of flow direction, especially in the vicinity of assumed hydrologic divides and imposed boundaries
- Comparison of calculated groundwater head contours to those interpreted from field water level surveys (Figure 10 and Figure 11)
- Quantitative comparison of model recharge values to those derived from the water balance
- Qualitative comparison of as-modeled hydraulic conductivity value to those inferred from aquifer hydraulic testing
- Comparison of the simulated groundwater flow system to the conceptual groundwater flow model
Figure C1-10: Groundwater level contours (dashed blue) interpreted from field survey. Red dashed line indicates a groundwater divide between two creeks.
The values of recharge and hydraulic conductivity were varied until the calibration targets were met. A comparison of sub-catchment discharge from the water balance model and the numerical groundwater model is presented in Table 2. The comparison of measured and simulated groundwater heads is presented in Figure 11.
Table C1-2: Comparison of water balance discharge to calibrated numerical model discharge.
Figure C1-11: A comparison of measured and simulated groundwater heads.
Note that the measured head data presented in Figure 11 is an assimilation of historic and more recent water levels which are not necessarily temporally or spatially consistent. Furthermore, the spatial distribution of these monitoring points across the Site is not presented. However, it appears as though the model is reasonably well calibrated to observed field conditions.
Pit Inflow Model Calibration
No additional calibration was undertaken for the pit inflow modelling.
TSF Seepage Model Calibration
No additional calibration was undertaken for the 2D or 3D modelling. Generally, material properties and boundary conditions from the baseline model were assumed.
Baseline Groundwater Model Results
The result of the baseline groundwater flow model is illustrated by the flow field in Figure 12. Generally, groundwater flow divides are shown to follow topographic highs and surface water features act as groundwater discharge zones.
Figure C1-12: Simulated hydraulic head and groundwater flow vectors.
The baseline model provides the relative depth to water table for the highlands and lowlands of the project site as well as the direction and magnitude of hydraulic gradients.
The steady state flow solution was used to perform a particle tracking exercise which is discussed in the Transport Modelling section.
Furthermore, the results of the baseline numerical model were used to verify the groundwater discharge assumptions used in the site-wide water balance.
Pit Inflow Model Results
A combination of steady state numerical modelling, analytical estimation, and discharge estimates from a separate numerical model (that explicitly included the TSF) were used to estimate pit inflow over the mine life. The results are provided in Table 3.
Table C1-3: Pit inflow estimates based on numerical and analytical methods.
These results are based on conservative assumptions regarding the location and transmissivity of an intersecting fault zone as well as the discharge contributions from the adjacent TSF.
The steady state results were interpolated using a Theis solution to provide estimates of the annual pit inflow rates (Figure 13).
Figure C1-13: Annual pit inflow rates.
TSF Seepage Model Results
Both 2D and 3D modelling were conducted to provide estimates of seepage from the tailings storage facility. For the 2D modelling, a sensitivity analysis was conducted with respect to the range of plausible hydraulic conductivity values assigned to the materials beneath the TSF. Prominent units were assigned plausible high, medium, and low hydraulic conductivity values and plausible scenarios were evaluated (Table 4).
Table C1-4: Results of 2D seepage analysis (values in L/s).
Values for flow to pumping towers within the TSF were also determined and reported for the various sensitivity runs. The 2D VADOSE/W results were acknowledged as overestimates, as radial flow is not accounted for in section and recharge is overestimated when summing the sections over the area of the TSF.
The results of the 3D numerical model are presented Table 5. The results cover a range of plausible scenarios with respect to variations in meteoric recharge, tailings characteristics, beach geometry, foundation materials, internal pumping, and pond management.
Table C1-5: TSF seepage estimates based on 3D numerical modelling.
Impacts to the receiving environment were also quantified using the 3D seepage model. Table 6 provides estimates of seepage to the local environment and open pit over time.
Table C1-6: Estimates of TSF seepage to the local environment and open pit.
MODPATH was used to visualize the travel time of groundwater across the Site. Simulated particles were released from various points of interest and their migration plotted at 30-day intervals (Figure 14).
Figure C1-14: Particle tracking results for baseline groundwater model.
No additional transport modelling was undertaken at this stage of application for an environmental assessment certificate.
Model limitations were not explicit in the model documentation reviewed for this case study, except where addressed by sensitivity analysis or other approaches to uncertainty.
Prior field investigations have been acknowledged in the development of these models and data referenced where appropriate. Generally, the calibration data is limited. Hydraulic testing was conducted to establish hydrogeologic properties; however, the data is limited spatially.
Like most sites in northern British Columbia, the Site likely exhibits complex hydrogeologic conditions that may not be adequately reflected in a conceptual model based on such a limited site data. However, the sensitivity of model predictions to the location of transmissive fault structures has been addressed using a simple, hypothetical, analytical calculation. The conceptual model for this site did incorporate a site-wide water balance for surface and groundwater. Furthermore, the conceptual model was updated during the modelling process to reflect new data from drilling investigations; this conceptual model was applied to more detailed modelling of TSF seepage.
The 3D numerical model code was appropriately selected based on the spatial complexity of the site, including the distribution of hydrostratigraphic units, boundary conditions, and the level of risk of mine-affected groundwater to the receiving environment. Discretization of the model domain appropriately reflected the conceptualization without creating an overly complex model. Analytical solutions were used appropriately for preliminary estimates of flow and to supplement more rigorous numerical modelling. Steady state models were appropriately used to represent various discrete phases of the mine plan and transient flow models were applied for prediction of cumulative seepage effects.
Transport modelling was restricted to pathline analysis with limited comments regarding the results of that analysis and their impact on risk to environmental receptors. The lack of site specific data precluded the use of a more sophisticated transport modelling approach. As a conservative approach, the assumption was made that contaminants would arrive "immediately" in the streams and the predicted impact locations and that there would be no loss of mass along the pathway.
Calibration statistics were reported, however limiting the dataset. While the calibration statistics are generally acceptable, the scarcity of calibration targets leaves uncertainty in the applicability of the model to meet the modelling objectives. An attempt was made to constrain the solution and reduce this uncertainty by making full use of stream flow measurements in order to meet the modelling objectives.
Generally, sensitivity analyses were applied appropriately; however, where a range of results are provided, the selection of the most plausible result is not always clearly justified in the documentation reviewed for this case study (e.g. the selection of most probably TSF seepage scenario after 15 years). In some cases, this results in the exclusion of the most conservative scenario, without documented justification.
While the series of modelling techniques applied in this study have met the modelling objectives, the project would greatly benefit from more hydrogeologic data for calibration to conditions of temporally variable baseline heads, aquifer stresses induced by mine development (e.g. pit dewatering), and the location, orientation, and transmissivity of high permeability features (e.g. especially faults). Water quality and tailings seepage potential could be investigated with predictive transport modelling given data on the physical and chemical nature of the tailings and more detailed construction and management plans for the TSF; however, a conservative mass balance approach was adopted to meet the modelling objectives at this stage of the project.