9.2 Open Pit Mines
At the end of an open pit operation, sump pumps and/or dewatering wells are decommissioned. The geometry of the pit and the void volume may be modified if partially backfilled with mine waste. Following cessation of pumping, in most climatic regimes, both surface water and groundwater will inflow to the pit, creating a pit lake (Figure 19). These pit lakes become part of the closure landscape and attention must be directed toward their hydrology and especially, water quality trends in the lake. Golder Ltd. (2017) provides a literature review of pit lakes in a number of different geographic settings. This report includes discussion of numerous case histories.
Figure 19 – Pit lake forming at a closed mine site. The pond level was rising at a rate of several meters per year, with the water level below the elevation of the subsurface spill point for the pit.
Figure 20 illustrates the water balance for a closed open pit; in this case a situation is depicted in which waste rock has been placed back in the pit as part of the mine closure plan. If rates of evaporation exceed rates of groundwater inflow, precipitation, and runoff of surface water into the pit, then the open pit remains in a dry state or perhaps a small seasonal pool of water forms on the pit floor. This situation ensures hydraulic containment of contaminated water. If the cumulative inflow rates exceed evaporation rates, a pit lake develops and the water level rises over time. The long-term equilibrium condition determines the ultimate elevation of the pit lake. One of three possible conditions occur: (i) if the water level stabilizes at an elevation everywhere below the level of the water table in the bedrock surrounding the pit, a permanent hydraulic sink is created; (ii) if the water level in the pit rises to a “spill point elevation” where at one or more locations on the pit perimeter the water level in the pit exceeds the water table elevation in the surrounding terrain, subsurface outflow of pit water commences once the spill point elevation is reached; and (iii) if the inflows exceed the outflows to the extent that the water level rises to a topographic low point on the pit rim, then a flow-through lake is established with surface water outflows.
Figure 20 – Schematic of the water balance for a closed open pit. From Linklater, C., et al., (2017), published by Sustainable Mineral Institute, University of Queensland Brisbane.
It often takes many decades or more for a pit lake to approach a dynamic equilibrium level if the controlling factor on the rate of rise of the lake level is the rate of groundwater inflow to the pit rather than surface water inflows. Groundwater inflow rates to the pit diminish over time as the hydraulic gradient driving groundwater toward the open pit decreases as the pit fills with water. When the dynamic equilibrium condition is established, lake levels fluctuate with annual changes in the climate cycle. Solute concentrations could exceed permissible standards if the quality of the water in the pit is impacted by sulfide mineral weathering on the walls of the pit exposed above the water line, or solutes are released from any mine waste placed in the pit before or at closure.
The assessment of the time for a closed pit to re-flood, impacts on the surrounding hydrologic regime, and groundwater flow paths to offsite receptors, is commonly developed using a transient, three-dimensional numerical groundwater flow model in conjunction with a surface water hydrology model (e.g., Linklater et al., 2017). For pits where the water level rises above the groundwater spill point, particle-tracking techniques can be used to identify the groundwater pathways, the eventual receiving surface water bodies, and to estimate solute travel times by advection. At some mine sites, decisions have been made that require a commitment to long-term pumping and water treatment following mine closure to prevent the water level in the pit lake from rising to the spill point elevation where hydrodynamic containment would be lost. It is also necessary to assess the risk of the pit lake exceeding the spill point elevation during extreme rainfall events.
Figure 20 also points to a calculation sometimes undertaken to determine the region around the open pit that contributes to groundwater entering the open pit (labeled “seepage from out of pit dumps”). Loadings released from any mine waste stockpiles contained within this area will eventually report to the open pit. This seepage is considered within the water management plan for the open pit. Stockpiles located outside this region have solute loads reporting elsewhere on the site. This calculation is equivalent to identifying the capture zone of a contaminant interception well or a water supply well. Figure 21 shows such a calculation for a mine located in a semi-arid region (Birch et al., 2006). Solute loads generated by mine waste stockpiles located within the red contour (capture zone of the open pit) report to the open pit. Loadings to groundwater from any stockpiles outside this zone would be projected to report to other drainages.
Figure 21 – Projected capture zone of groundwater reporting to an open pit. Groundwater recharge occurring within the region mapped by the red contour is predicted to report to the pit. The capture zone shaded purple accounts for the local-scale flow systems around the pit while the green contour represents the surface water drainage boundary. Modified from Birch et al., 2006).
An option sometimes considered as part of the closure plan for an open-pit mining operation is re-location of waste rock or deposition of tailings into the open pit following completion of mining. In other cases, an in-pit waste rock stockpile developed during operations may be feasible (see Figure 20). Several of the advantages gained in this approach are a reduced footprint of mine waste in the closure landscape and submergence of some of the waste rock or tailings below the re-bounding water table, which can have a long-term geochemical benefit. However, the quality of the water in the pit might be degraded in the near-term if the waste rock contains secondary mineral products that formed while the waste rock was stored on the ground surface, which may then mobilize upon re-wetting. To obtain permit approval, it may be necessary to document the potential impact of backfilling the pit on groundwater flow patterns in the nearby area, to assess impacts to the receiving environment, and to confirm the final landscape is stable. This is normally undertaken using a three-dimensional groundwater model of this closure scenario, combined with particle tracking to delineate the capture zone.