2021-12-09_REVISION - C1981010 (7)

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reclaimed vegetation are suitable for the approved post-mining land uses. The Division has observed livestock grazing and substantial wildlife use of previously reclaimed areas. The revegetation success standards were based on what would be necessary for the land to properly function to accommodate the post mine land uses. Based on the achievement of these requirements,the Division finds that the revegetation on the Trapper Mine reclaimed areas is capable of supporting post-mining land use. Ground Water Impacts TMI monitors the quality and levels of ground water in various wells throughout the site. Mine backfill aquifers are monitored from wells GD3, GFS, GF7 and GF 11. These four wells were not included within the SL23 Phase III bond release area. However,because they are used as ash monitoring wells, they are sampled and are representative of backfilled aquifer conditions. Historical data from the backfill wells indicate leachate forms in the spoil/ash backfill. This leachate is alkaline, and high in dissolved solids (TDS),mainly sulfates. This concentration may continue for several hundred years, as predicted for the Seneca II Mine in Routt County, Colorado(USGS Water-Resources Investigations Report 92-4187). The alkalinity(pH near 7), appears to have prevented significant mobilization of metals in the spoil/ash backfill. At the northeast end of active mining GLEV-1 monitors the R coal aquifer and Well GC-3 Middle Flume monitors the Q seam alluvium. TDS concentration in GLEV-1 appear steady. Well GC-3 exhibited a gradual decline in water levels as mining moved into the area of response. Up gradient mining has moved east of this well. Well GP-9,Trapper's groundwater point of compliance monitors the Third White Sandstone immediately downgradient from Trapper's pits at a location where a leachate plume can be expected to form, as explained in the PHC. The 2020 data from well GP-9 does not reflect any new exceedances of the Basic Standards for Ground Water for a domestic use classification based on the parameters analyzed. There continues to be an exceedance of the drinking water standard for Mn; however,Mn exceedances also occurred in well GP-9 prior to mining in the area up gradient of this well. Other groundwater quality factors (e.g., concentrations of TDS and sulfate)continue to remain within historical sampling ranges and indicate that coal spoil leachate has not reached GP-9. The Coy well is the groundwater point of compliance for the Flume Gulch alluvium, as explained on page 4-242 of the permit application. Sampling data from 2020 indicates no exceedances of the Basic Standards for Ground Water for the classification of agricultural use at the Coy well. F, and Z pits have mined the Q and R coal seams and therefore the QR aquifer constitutes the major aquifer of interest for this bond release as the bond release block comprises parcels from former pits F and Z. Mining in A Pit utilized H and I coal seams making up the HI aquifer. Well GC-3 monitors the Q seam alluvium, GF-6,monitors the Q coal,GP-5, and GP-9,monitor the QR and third white sandstone respectively. GMP-, GP-3 and GP7 monitor the HI aquifer. Water levels in GP-5 and GF- 11(a backfill aquifer), generally trend up. F pit mining has not significantly affected water levels in GP- 5. TDS concentration in the QR aquifer are monitored downgradient at well GF-6 and have shown little influence from up gradient mining other than a very slight overall increasing trend,well within natural variations. TDS concentration in wells GP-5 and GLEV-1 appear steady with a slightly lower concentration in GLEV-1. The Probable Hydrologic Consequences (PHC)section of Trapper's mining and reclamation permit(Section 4.8.3), discusses the possibility of a high TDS plume of coal spoil leachate forming in the Third White Sandstone downgradient from the Trapper Mine in Section 29-T6N- Permit Number. C1981010 Page 13 of 16