2018-11-05_GENERAL DOCUMENTS - C1981044

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Williams Fork Mines Prepared by: R. Reilley M.S. GISP C1981044 5 November 2018 23 Williams Fork alluvium. The seepage would be driven by a maximum pressure developing in the subcrop equal to a head of 100 ft. above the ground surface. This head would cause maximum seepage of 20 gpm. In a worst-case scenario, the 19.5 gpm discharge would raise SAR in the Williams Fork River alluvial water from the historical mean of 3.6 to 9.4. The alluvial water is naturally higher in dissolved solids and metals than the mine discharge water; therefore, increased SAR is the only expected impact. 12) The No. 9 Mine will not refill to the surface, but will refill to an equilibrium level between the Twentymile Sandstone and the overlying White Sandstone. 13) The No. 9 Mine portal backfill area is too small to generate enough leachate to have a measurable effect on nearby aquifers. Observed Hydrologic Impacts The monitoring plan contained in the permit application has been designed to verify the permittee's projected hydrologic impacts of mining. Section 2.05 of the permit application includes a description of the observed hydrologic impacts caused by mining at the Williams Fork Mines. Each year, MCM assesses the on-going impacts to the hydrologic system in its annual hydrologic report. Observed impacts are summarized below. Observed Ground Water Impacts Mine Inflows and Discharges Water was pumped from the underground mine workings to the ground surface at the No. 5 Mine well and the 7 North Angle well. Annual hydrology reports show the total discharge for the 5 and 6 Mines was a fairly constant 600 gpm during active mining in the early 1990s. After mining ceased in 1995, sumped water was pumped down. In 1997 and 1998, pumping became intermittent, averaging less than 1 gpm. In 2013, pumping was approaching an average quarterly rate of 300 gpm. The pumped mine water is discharged to the Williams Fork River at NPDES outfalls 003 (5 Mine well) and 024 (7 North Angle well). A total of more than 110 million gallons of water was discharged in 2013. The No. 5 Mine pump was then turned off in July of 2013; no further pumping at this site is anticipated in the near future. Monitoring data through 2013 collected at the outfalls indicate the mine water is consistently alkaline, with total dissolved solids less than 1800 mg/l, and low concentrations of iron and manganese. Trout Creek Sandstone The water level in the Trout Creek Sandstone monitoring well (No. 5 Mine well) dropped 200 ft. during 1989 and 1990. The drop was probably caused by pumping of the three mine water supply wells completed in the Trout Creek, rather than by mining in the overlying E and F coal seams. The water level fully recovered in 1991 and then fluctuated 200 ft. in 1998. The fluctuation is probably not related to dewatering in the overlying E and F seams because the base