2016-10-28_PERMIT FILE - C2010089A (27)

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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 1 % 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 adjacent New Horizon Mine. The USGS study methodology was used as a basis to predict the time frames of slightly elevated TDS in the spoil water at the New Horizon Mine, (see New Horizon #2 Area permit Section 2.05.6(3)). The sulfur content in the overburden at NHN is similar to that at the New Horizon Mine (see Table 2.04.6-2, Section 2.04.6 of this application). The result of that prediction indicated that about 800 years would be required to oxidize the sulfur in the overburden at the New Horizon Mine and therefore about 800 years to oxidize the sulfur at NHN Mine. As the oxidation of the pyrite is the main source of increased TDS in backfill water quality, elevated TDS from spoil water springs could remain for several centuries. The NHN Mine will have two significant differences to the results observed at Seneca II. First, the calcite present in the spoil at NHN permit area seems to react with acid produced by the oxidation of pyrite and take a substantial amount of sulfate from solution directly into solid calcium sulfate. This keeps the dissolved solids content somewhat constant no matter how high or low the inflow water quality is with regard to TDS. Second, the Seneca II site is a dry -land reclaimed area where the only recharge into the spoil is a minor amount from precipitation and seepage from the underburden aquifers. Total measured discharge from the spoil was only 3" per year. The NHN Mine will be partially irrigated with water of better quality over a large portion (45%) of the year. As described earlier, this results in a total movement through the spoil of approximately 13" per year (81 ac -ft). Since the amount of water moving through the NHN backfill is greater, it should oxidize the pyrite more rapidly and also flush other salts which are contributing to the increase in TDS at a higher rate. For this reason, a ratio of the discharge at Section 2.05.6(3) Page 24 April 2016 (PR -01)