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Basin to the major discharge area along the northeast margin of the basin will also tend to be re- established. <br />This will result in movement of lower quality ground water from areas directly overlying mined zones into the <br />undisturbed overburden units to the east of the proposed mine area. <br />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 310 years for the Wadge <br />overburden unit and an additional 100 years for the WadgelWolf Creek interburden unit, for a total of 410 years. <br />The relatively high permeability of the rubblized and fractured zones likely to exist over mined areas should <br />result in lower hydraulic gradients through these zones than existed in the pre - mining situation. Potentiometric <br />levels within this area under fully recovered conditions are expected to show a relatively small drop from the <br />major recharge area south of the underground mine workings to the undisturbed area to the west of the <br />workings. As a "worst case" we could expect a postmining potentiometric level of about 6,900 feet (the <br />potentiometric level in the spoils to the south of the mine) to occur at the down -dip extremities of mining_ The <br />postmining potentiometric Ievel in the discharge area for the overburden unit is equivalent to the land surface <br />elevation in this area, which is at about 6,600 feet. The predicted potentiometric levels, at the recharge and <br />discharge areas govern the hydraulic gradient across the undisturbed part of the overburden unit. As it turns out, <br />these "worst case" predicted potentiometric levels are about the same as presently exist in the unit. <br />Consequently, the flow rates through this part of the unit, once potentiometric levels are fully recovered, are <br />expected to be similar to baseline conditions. This flow rate has been calculated to be on the order of 11,000 <br />gpd over approximately an 8,000 -foot flow width and 75 -foot thickness, which yields an average flux of about <br />0.0025 ft/day for the Wadge Overburden. Under similar conditions, the average flux for the Wadge/Wolf Creek <br />Interburden and Wolf Creek Seam is calculated to be about 0.0001 ft/d_ If the effective porosity of the units in <br />this part of the basin were taken as 0.01, then actual flow velocities on the order of 0.25 ft/day and 0.01 ft/day <br />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 410 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 Dischare <br />Discussions in Effects on Ground Water Quality of Aquifers indicate that the Wadge and Wolf Creek <br />overburden/interburden units will be the only bedrock units in which water quality may be affected by the <br />proposed mining operations. The calculations in the previous sections predict that it will take about 410 years <br />for potentiometric levels in the Wadge and Wolf Creek overburden/interburden units to fully recover and an <br />additional 110 years before lower quality water in these units (which results from Ieaching of caved and <br />fractured overburden material) would discharge to alluvial units and surface streams. During mining and <br />recovery of potentiometric levels following mining, it is likely that discharge from the Wadge and Wolf Creek <br />overburden /interburden will be minimal_ This reduction in discharge, primarily to Fish Creek as discussed in a <br />previous section, is not expected to have any significant effect on water quality characteristics of the alluvial <br />deposits or the surface streams. <br />Once discharge from the Wadge and Wolf Creek overburden/interburden is re- established, there will be a <br />period, calculated to be about 110 years, when the quality of the bedrock discharge will be similar to premising <br />conditions. After this time, discharge of poorer quality water, which has been affected by increased solute <br />leaching near the underground workings, will start to occur. The duality of this discharge is likely to be <br />characterized by high sodium and sulfate concentrations with conductivities of 4,000 to 7,500 µmhos /cm. The <br />rate of discharge of this water is expected to be similar to premising conditions as discussed in a Effects on <br />Ground Water Quality of Aquifers. This rate has been calculated to be about 11,500 gpd (12.8 acre -ft per year), <br />which is equivalent to about 0.02 efs or about 0.5 percent of the average low flow in Fish Creek. This level of <br />discharge is not expected to have any significant effect on the overall water quality of surface water since it <br />constitutes such a small percentage of the total flow in the creek even under low flow conditions. <br />TR13 -83 2.05 -145 11/03/14 <br />