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The rate of movement of the plume of lower quality water will be dependent on the configuration of the <br /> potentiometric surface in the postmining situation and the permeability of the undisturbed overburden units. As <br /> indicated earlier, recovery of potentiometric levels is expected to take about 350 years. The relatively high <br /> permeability of the rubblized and fractured zones, which are likely to exist over mined areas, should result in <br /> lower hydraulic gradients through these zones than existed in the pre- mining situation. Potentiometric levels <br /> within thisarea under fully recovered conditions is expected to show a relatively small drop from the major <br /> recharge area south of the underground mine workings to the undisturbed area to the west of the workings. As a <br /> "worst case" we could expect a postmining potentiometric level of about 6,900 feet (the potentiometric level in <br /> the spoils to the south of the mine) to occur at the down -dip extremities of mining. The postmining <br /> potentiometric level in the discharge area for the overburden unit is equivalent to the land surface elevation in <br /> this area, which is at about 6,600 feet. The predicted potentiometric levels, at the recharge and discharge areas <br /> govern the hydraulic gradient across the undisturbed part of the overburden unit. As it turns out, these "worst <br /> case" predicted potentiometric levels are about the same as presently exist in the unit. Consequently, the flow <br /> rates through this part of the unit, once potentiometric levels are fully recovered, are expected to be similar to <br /> baseline conditions. This flow rate has been calculated to be on the order of 11,000 gpd over approximately an <br /> 8,000 -foot flow width and 75 -foot thickness, which yields an average flux of about 0.0025 ft/day. If the <br /> effective porosity of the unit in this part of the basin were taken as 0.01, then actual flow velocities on the order <br /> of 0.25 ft/day would be expected. <br /> This calculated ground water flow velocity is the maximum rate that the plume of lower quality water from the <br /> disturbed part of the unit would be expected to move into the undisturbed part of the unit. The closest proximity <br /> of mining to the discharge area is about 10,000 feet, so that once potentiometric levels have been re- established, <br /> which may take 360 years, it may take an additional 110 years before any discharge of the lower quality water <br /> occurs to alluvial deposits on surface streams. <br /> Effects on Alluvial and Surface Water Quality As a Result of Bedrock Discharge <br /> • Discussions in Effects on Ground Water Quality of Aquifers indicate that the Wadge overburden unit will be the <br /> only bedrock unit in which water quality may be affected by the proposed mining operation. The calculations in <br /> the previous sections predict that it will take about 360 years for potentiometric levels in the Wadge overburden <br /> unit to fully recover and an additional 110 years before lower quality water in this unit (which results from <br /> leaching of caved and fractured overburden material) would discharge to alluvial units and surface streams. <br /> During mining and recovery of potentiometric levels following mining, it is likely that discharge from the <br /> Wadge overburden will be minimal. This reduction in flow, primarily to Fish Creek as discussed in a previous <br /> section, is not expected to have any significant effect on water quality characteristics of the alluvial deposits or <br /> the surface streams. <br /> Once discharge from the Wadge overburden is re- established, there will be a period, calculated to be about 110 <br /> years, when the quality of the bedrock discharge will be similar to premining conditions. After this time, <br /> discharge of poorer quality water, which has been affected by increased solute leaching near the underground <br /> workings, will start to occur. The quality of this discharge is likely to be characterized by high sodium and <br /> sulfate concentrations with conductivities of 4,000 to 7,500 µmhos /cm. The rate of discharge of this water is <br /> expected to be similar to premining conditions as discussed in a Effects on Ground Water Quality of Aquifers. <br /> This rate has been calculated to be about 11,000 gpd (12.3 acre -ft per year), which is equivalent to about 0.02 <br /> cfs or about 0.5 percent of the average low flow in Fish Creek. This level of discharge is not expected to have <br /> any significant effect on the overall water quality of surface water since it constitutes such a small percentage of <br /> the total flow in the creek even under low flow conditions. <br /> Effects of Mine Discharges on Alluvial Water Quality <br /> • The release of the mine discharge water into Foidel and Fish Creeks can be expected to affect the water quality <br /> in the alluvium in the creeks, downstream of the discharges. Due to mixing and diffusion, it would be expected <br /> that the water quality in the alluvium will change in accordance with the long -term average quality of in- stream <br /> water recharge. <br /> PR09 -08 2.05 -145 04/27/09 <br />