2016-01-07_PERMIT FILE - C2010089A (11)

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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 2800 to 3500 ppm. 2) Due to the low seepage rates, the movement is very slow (400 feet every 10 years). 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 permeabilities, therefore these aquifers cannot be affected by the spoil water. 6) Impact of Backfill Water Quality on Surface Water Quality. 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 Hydrology 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 principal cause of elevated TDS, and that the percent of pyrite in the spoil was the determining factor in the length of time that the TDS would be elevated in the spoil water. A spoil pyrite content of I% by weight, for example, was predicted to fully oxidize in 1600 years (their Table 18). TDS levels in the spoil water were approximately 4500 ppm, which was a significant increase over the overburden aquifer water in the area. The coal mine overburden at that site is somewhat younger (upper Cretaceous) in age but is similar in lithologic character to that of the Section 2.05.6(3) Page 26 Sept. 2015 (TR -11)