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

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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 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 11 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 both sites could be used to predict the time frame of slightly elevated spoil water quality. This ratio - 3"/13", as applied to 1500 years results in a time frame of 346 years until all the pyrite is oxidized. When considering all the variables involved, this means that it could dissipate within a range of 200 to 500 years. The TDS should begin to drop before this time. Once the pyrite is fully oxidized and other salts are flushed out, the backfill water quality should approach the existing stream water quality, possibly dropping to 800 to 1200 mg/1 TDS. It is not likely that the backfill water will ever reach the low TDS of the ditch- run water as seepage from the overburden zones into the backfill will continue as long as the 2nd Park irrigation continues. Impacts To Receiving Waters - Tuttle Draw As described earlier, drainage through the NHN backfill material will move to the south following the path of least resistance along the pit floor and issue at the general location of Spoil Spring # 1, (see Map 2.04.7-1). Spoil Spring #1, is the spoil water discharge from the old Peabody Nucla Mine (New Horizon #1 Area permit) and has Section 2.05.6(3) Page 24 March 2018 (TR -16)