2008-04-02_REVISION - C1981044 (5)

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constituents in. the alluvial water would see similar increases. None of the increases would be enough to impair the use of alluvial water. Compliance with Colorado Water Quality Control Commission's Regulation No. 41, Basic Standards for Ground Water. Alluvial ground water monitoring at the Eagle Mines has not: detected a negative impact to water quality. The F coal seam, downdip from the bond release area, is the stratigraphic unit most likely to be negatively impacted by mining in the bond release area. No water quality data has been collected for the F- seam in the area downdip from the workings. Baseline TDS in groundwater in the E coal seam at the rriine was 778 mg/1 (permit application Table 111- l Og). TDS of migrating mine water (l~eachate) is expected to be at least 1,000 mg/1, and possibly significantly higher. Assuming the F seam premining water quality was similar to the E seam, the migrating leachate from the F seam could degrade native ground water in its path. This migration path would extend downdip from the downdip edge of the mining workings, at a depth of 800 feet below the ground surface. Seepage from the workings into the bedrock expo:;ed in the workings would occur at a rate on the order of less than 100 feet per year based on a calculation ground water flow velocity using the Darcy equation (Table 1 in the hydrology evaluation memorandum). Dispersion would limit the areal extent of degradation along the migration path. The slow seepage velocity, combined with the dispE:rsion, supports a conclusion that the Basic Standards for Ground Water probably have not been exceeded in the vicinity of the bond release area. Compliance with requirement to minimize ground water hydrologic impacts inside the permit area [CDRMS regulation 4.05.1(2)J. The approved post-mining land use in the permit area i s rangeland/wildlife habitat, pastureland, and cropland. As previously discussed, ground water quality in bedrock and alluvial aquifers has not been adversely impacted by mining and reclamation operations at the Eagle Mines. The approved post-mining land uses have not been impaired by the changes to the ground water hydrologic balance caused by mining operations. Aquifers probably have been partially dewatered inside the bond release area. These changes are the minimum that can be expected from. an underground mining operation. Surface Water Impacts: Compliance with Instream Numeric Standards (Colorado Water Quality Control Commission .Regulation 3.3.0). The Eagle Mines are on Segment 13b of the Lower Yampa/Green River Sub-basin of the Lower Colorado River Basin. CWQCC Regulation 3.7.0 lists numeric standards for Segment 13b. The permittee monitors water in the Williams Forl: River at a site that is upstream from Eagle's mining (WF-1), and a site downstream from mining (WF-2). Data from the upstream site indicate the Williams Fork River naturally exceeds the iron numeric standard frequently, and the lead and manganese standards occasionally. The operator also monitors underground mine water quality where it is discharged into the Williams Fork River at NPDES outfalls 003 and 024. There have been no exceedances of discharge limitations at either outfall for at least 10 years. (They operation has been in temporary cessation since December 3, 1995.) The discharge lirriitations applicable to the outfalls were based on which chemical