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

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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 been monitored since 1987. Spoil Spring #1 water mixes with irrigation runoff and return water from Nygren Draw and then discharges into Tuttle Draw located about 1600 ft. downstream from the spoil spring. Tuttle Draw enters the San Miguel River approximately 2 miles to the southwest. Historic sample data (since 1987) from surface water sites, NPDES 001, SW -N3, and the San Miguel River has shown that the concentration of TDS in these sources varies inversely with the flow volume. Total dissolved solids (TDS) increase during the low flow periods and decrease during the high flow periods. For NPDES 001 and SW -N3 high flows correspond with the irrigation season; however, the San Miguel River is typically in low flow condition during the irrigation season. During the non -irrigation season flows from NPDES 001 and SW -N3 are low and TDS is higher while the San Miguel TDS is lower due to the higher flow, (see Table 2.05.6(3)-4 of New Horizon #2 Area permit). This presents both a high and low flow case which are complicated by the fact that spoil spring flow predicted to occur after the New Horizon Mine has been reclaimed (2013-2014) has not yet occurred. Therefore, the impacts to the receiving waters will be evaluated for both high and low cases, with and without irrigation, and including the predicted spoil spring flows from New Horizon Mine and the existing spoil spring flow from the old Peabody Nucla Mine (New Horizon 1 permit area). The irrigation case corresponds to the September/October time period when flows are fairly low in the San Miguel River. The non - irrigation period corresponds to March/April when flows in the San Miguel are usually at peak. As described previously, irrigation of the reclaimed NHN Mine will produce a flow in the spoil of about 81 ac-ft./yr,( at full irrigation). Over the length of the irrigation season (165 days) this is an average of about 0.248 cfs. During the non -irrigation season the flow from the NHN backfill spring will mimic flow levels from the old Peabody Mine or about 0.041 cfs. The TDS values for the NHN backfill spring discharge were averaged from sample analyses from NPDES 001 discharges from 1987 to 2000 and reported on Table 2.05.6(3)-4 of the New Horizon #2 Area permit. The Tuttle Draw TDS and flow values were also averaged from analyses and Section 2.05.6(3) Page 25 April 2016 (PR -01)