2018-11-19_PERMIT FILE - C2010089A (6)

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Seepage into the backfill is equal to: (10' average backfill height) x (1,900' wide seepage area) x (hydraulic conductivity of 2.1'/day) x (hydraulic gradient of 0.053) x (1/43,560 cu. ft. per ac- ft.) x (365 days per year) is equal to 17.7 ac -ft per year, or an average of about 11 gpm. It is strongly believed that spoil water seepage into the backfill will not have any significant impact on water quality, flow rates, well usage, etc. due to four reasons: 1) As described in this section, the water quality of the spoil water will be at a maximum 6% to 10% higher in TDS than the existing overburden water quality, which is relatively poor. Regardless of the seepage rates into the sandstone zones, the water quality will be very similar and consistent with the overburden, with a TDS from 2,800 to 3,500 ppm. 2) Due to the low seepage rates, the movement is very slow. 3) The sandstones above the Dakota coals have historically been too poor in quality and too low in flow rates to provide for useful wells. For this reason, no known wells in the vicinity of the mine area have been completed in the Dakota Formation. It is very unlikely that any new wells will be completed in the Dakota Formation in the vicinity since the Brushy Basin Member of the Morrison Formation provides significantly better water quality and flow rate and is located only 160 feet deeper than the Dakota LDx Seam at NHN permit area. 4) The local wells are completed in the Brushy Basin Member of the Morrison Formation, which is below the Dakota coals. Although this zone has significantly better quality water, these sandstones are separated from the spoil by shale layers with very low permeability; therefore, these aquifers cannot be affected by the spoil water. 6) Impact of Backfill Water Oualitv on Surface Water Ouality. Spoil Water Quality The chemistry of the water interacting with the spoil is described in the previous section. In order to determine the impact of the spoil water quality on the surface water, it is first necessary to predict the expected quality of the spoil leachate for the NHN Mine. The principal impact from a quality perspective is that any irrigation water will seep rapidly through the spoil, increase in TDS and then discharge through a spoil spring. This is analogous to the old Peabody Nucla Mine and the water quality should be similar to that of SS #1, (see New Horizon #1 Area permit). The NHN spoil spring will issue to a tributary of Tuttle Draw which will impact the quality of Tuttle Draw. Timeframes of Elevated TDS in Spoil Water The time period that these slightly elevated levels of TDS in the spoil water and spoil spring discharges is difficult to calculate. In 1994, the USGS did a detailed study of the impacts of infiltration into spoil at the Seneca II Coal Mine in Routt County, CO. This study is Water Resources Investigations Report 92-4187 titled Hydrolopy and Geochemistry of a Surface Coal Mine in Northwest Colorado. Lysimeters were installed to measure infiltration rates into the spoil, and samples of inflow water, spoil water and spring discharge was analyzed for the entire area. It was determined that pyrite oxidation was the Section 2.05.6(3) Page 23 March 2018 (TR -16)