Use of groundwater models to design mine dewatering systems and estimate their effects
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Mine Dewatering Groundwater Model
The purpose of this non-technical web page is to present a groundwater model that demonstrates the use of dewatering wells for underground mine dewatering. Reconnaissance-level modeling using a relatively simple modeling technique is used for the demonstration. A sub-surface mine with two adits (tunnels to ore veins) was selected to use as an example. The locations of the adits relative to the land surface are shown on a topographic map in Figure 1. The contour lines on this map show land surface elevation. The land surface is sloping steeply northwestward. The lower adit (red) is at an elevation of 9930 feet above sea level and extends 1100 feet southeastward into the mountain. The upper adit (blue) is at an elevation of 10100 feet and extends 1740 feet into the mountain. Groundwater flowing into the tunnels is reacting with minerals exposed to the water and oxygen to produce acid mine drainage. The contaminated mine water flows to the stream downslope from the adits and causes undesirable surface water contamination.
Figure 1. Location of mine adits.
Pre-Mining Water Table
The computer technique used to model the groundwater system is called analytic element modeling (AEM), which is described on a separate web page. To see a more detailed and realistic MODFLOW model of this same system go to MODFLOW AEM Comparison (NT). The first stage of the AEM modeling procedure was to simulate the water table prior to mining. This was done by putting the elevation and slope of the water table into the model as the first element simulated. It is approximated as being close to the land surface. The result is shown in Figure 2. Since the simulated water table is planar and the land surface is irregular, the simulated water table is above the land surface in an area near the north end of the lower tunnel. This is an artifact of the simplifications required by the model.
Figure 2. Pre-mining water table.
Post-Mining Water Table
The second stage of the modeling procedure was to add the mine adits. These tunnels extend below the water table and act as drains (AKA line-sink elements). Groundwater flows into the tunnels and reacts with rock minerals in the presence of oxygen in the atmosphere to become acidic and contain dissolved metals. This produces acid mine drainage that flows to the stream downslope from the mines. Figure 3 shows the configuration of the water table after it declines to the elevation of the tunnels. Groundwater flows in the direction of the slope of the water table and into the tunnels.
The rate groundwater is flowing into the tunnels can be determined from (1) the slope of the water table shown by the contours, (2) the permeability (AKA hydraulic conductivity) of the rock, (3) the thickness of the bedrock layer that contains the groundwater flowing toward the mine and the stream, and (4) the width of the capture zone. The hydraulic conductivity was estimated from the specific capacity of water wells in the vicinity. The capture zone is drawn by utilizing the fact that the direction of groundwater flow is perpendicular to the water table contours. The capture zones for both tunnels are shown in Figure 3. The groundwater between the blue pathlines is flowing to the upper tunnel, and the groundwater between the blue and red pathlines is flowing to the lower tunnel. the discharge to the lower tunnel is 35 gpm (gallons per minute) and the discharge to the upper tunnel is 49 gpm. The total discharge to the tunnels is 84 gpm. This flow represents 84 gpm of acid mine drainage flowing from the two adits to the stream.
Figure 3. Groundwater capture zones for the upper and lower mine tunnels.
Post-Dewatering Water Table
The groundwater flow into the mine tunnels can be eliminated by using water wells to lower the water table enough so the tunnels are above the water table. Figure 4 illustrates how this might be accomplished by constructing wells located near the subsurface (south) ends of the tunnels. The simulated well at the end of the lower tunnel is extracting 35 gpm to lower the water level to 9875 feet at the well, which is 30 feet below the tunnel elevation and 275 feet below the land surface. So the well is over 275 feet deep. The well at the end of the upper tunnel is extracting 85 gpm to lower the water level to 9800 feet at the well, which is 300 feet below the tunnel and 750 feet below the land surface. So the well would be over 750 feet deep. These extraction rates produce water table elevations that are below the entire length of both tunnels. Each well is entered into the model as a separate element.
Figure 4. Water table elevation produced by dewatering wells.
Mine Dewatering Results
This quick analysis using AEM suggests that two dewatering wells pumping (or siphoning) a total of about 120 gpm might eliminate 84 gpm of acid mine water discharging to the stream from the tunnels. The only water entering the tunnels might be seepage from the unsaturated rock above the water table. The 120 gpm from the wells would not be affected by reaction with debris in the mine tunnels or with mine waste downslope from the adits. Consequently, the quality of the well water discharging to the stream would likely be better than the mine water it replaces. Water treatment costs could be reduced or eliminated. These quick reconnaissance-level results suggest that dewatering with wells might be feasible. More detailed numerical modeling can be used to refine the analysis.
Posted: August 8, 2018. Revised December 12, 2018.