2020-06-08_REVISION - C1982057 (31)

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concentrations downstream of the mine were conservatively estimated and no significant impacts to adjacent Alluvial Valley Floors were predicted. Post mine TDS concentrations support these predictions as TDS contributions remained below predicted values within the Dry Creek watershed. Furthermore, contributions from mining did not exceed the pre-mine downstream TDS concentrations in Dry Creek which were attributed to dry land farming in the watershed. The predicted post mine TDS concentration for Sage Creek was less than the recently measured concentrations. However,the prediction in the Seneca II-W permit was made prior to the development of the Yoast Mine which also contributes to this stream. The post mine concentrations remain well below the projected increases as described in the Yoast Mine PHC (see Tab 17 Permit C-1994-082). Therefore, it is unlikely that the contributions from Seneca II-W have been significant. Impacts from runoff of the reclaimed mine areas and sediment ponds were projected to be of minimal significance. This is in agreement with the water quality observed in the receiving stream and at the NPDES outfalls (see Section B and C above). G. Completion of the Hydrologic Reclamation Plan Seneca II-W has been reclaimed utilizing the approved practices and measures described within the C-1982-057 permit. The final remaining measure within Hydrologic Reclamation Plan to be addressed is the abandonment of the thirty-four remaining monitoring wells within or adjacent to the requested Phase III bond release area. Thirteen are located in the alluvium, six within the Wadge Coal seam, six within the Wadge Overburden, three within the Wolf Creek Coal, one with the Wolf Creek Coal Overburden, one within the Wolf Creek Underburden, one within the Sage Creek Coal, one within the Sage Creek Overburden, and two within the Trout Creek Sandstone (see Table 3 of the Seneca II-W Annual Hydrology Report for the list wells and their construction details). All thirty-four remaining wells will be removed using well abandonment procedures approved by the CDRMS upon approval of this bond release application. As described in above in Part A and in previously approved Technical Revision 63, alluvial well DCAL-02 is the only groundwater point of compliance(GWPOC)well for the Seneca II West Mine. The well is screened within the Dry Creek Alluvium and is located downgradient(north) of the mines permit boundary. A GWPOC for the Sage Creek Alluvium was deemed unnecessary due to the small portion of the ridgeline that was mined within the Sage Creek watershed. GWPOC bedrock wells were also deemed unnecessary based on the absence of the potential for the mine to negatively impact the quality of bedrock groundwater. See Part A above and TR-63 in Appendix 15-B of the Seneca II-W permit for additional justification. The groundwater quality at GWPOC well DCAL-02 meets all existing water quality standards except for dissolved iron. Over the last five years the dissolved iron has ranged from 0.14 — 10. 8 mg/L (mean: 6.74 mg/L) and is above the 8.06 mg/L standard established in TR-63. DCAL-02 has exhibited elevated iron since it was installed in 1998. No other groundwater quality standard has been exceeded and it is probable that the dissolved iron is the ambient background concentration for this area. In 2018 water levels at all monitored bedrock and alluvial wells were within historic ranges. A cursory review indicates the water levels remain within the historic range in 2019 as well. Additional evaluation 22