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at the New Horizon Mine and therefore about 800 years to oxidize the sulfur at NHN Mine. As the <br />oxidation of the pyrite is the main source of increased TDS in backfill water quality, elevated TDS <br />from spoil water springs could remain for several centuries. <br />The NHN Mine will have two significant differences to the results observed at Seneca II. First, the <br />calcite present in the spoil at NHN permit area seems to react with acid produced by the oxidation <br />of pyrite and take a substantial amount of sulfate from solution directly into solid calcium sulfate. <br />This keeps the dissolved solids content somewhat constant no matter how high or low the inflow <br />water quality is with regard to TDS. Second, the Seneca II site is a dry -land reclaimed area where <br />the only recharge into the spoil is a minor amount from precipitation and seepage from the <br />underburden aquifers. Total measured discharge from the spoil was only 3" per year. The NHN <br />Mine will be partially irrigated with water of better quality over a large portion (45 %) of the year. <br />As described earlier, this results in a total movement through the spoil of approximately 13" per <br />year (81 ac -ft). Since the amount of water moving through the N14N backfill is greater, it should <br />oxidize the pyrite more rapidly and also flush other salts which are contributing to the increase in <br />TDS at a higher rate. For this reason, a ratio of the discharge at both sites could be used to predict <br />the time frame of slightly elevated spoil water quality. This ratio - 3"/13 ", as applied to 1500 years <br />results in a time frame of 346 years until all the pyrite is oxidized. When considering all the <br />variables involved, this means that it could dissipate within a range of 200 to 500 years. The TDS <br />should begin to drop before this time. Once the pyrite is fully oxidized and other salts are flushed <br />out, the backfill water quality should approach the existing stream water quality, possibly dropping <br />to 800 to 1200 mg /1 TDS. It is not likely that the backfill water will ever reach the low TDS of the <br />ditch - run water as seepage from the overburden zones into the backfill will continue as long as the <br />2 °d Park irrigation continues. <br />Impacts To Receiving Waters — Tuttle Draw As described earlier, drainage through the NHN <br />backfill material will move to the south following the path of least resistance along the pit floor <br />and issue at the general location of Spoil Spring # 1, (see Map 2.04.7 -1). Spoil Spring #1, is the <br />spoil water discharge from the old Peabody Nucla Mine (New Horizon #1 Area permit) and has <br />been monitored since 1987. Spoil Spring #1 water mixes with irrigation runoff and return water <br />from Nygren Draw and then discharges into Tuttle Draw located about 1600 ft. downstream from <br />the spoil spring. Tuttle Draw enters the San Miguel River approximately 2 miles to the southwest. <br />Historic sample data (since 1987) from surface water sites, NPDES 001, SW -N3, and the San <br />Miguel River has shown that the concentration of TDS in these sources varies inversely with the <br />flow volume. Total dissolved solids (TDS) increase during the low flow periods and decrease <br />during the high flow periods. For NPDES 001 and SW -N3 high flows correspond with the <br />irrigation season; however, the San Miguel River is typically in low flow condition during the <br />irrigation season. During the non irrigation season flows from NPDES 001 and SW -N3 are low <br />and TDS is higher while the San Miguel TDS is lower due to the higher flow, (see Table 2.05.6(3)- <br />4 of New Horizon #2 Area permit). This presents both a high and low flow case which are <br />complicated by the fact that spoil spring flow predicted to occur after the New Horizon Mine has <br />ection 2.05.6(3) Page 25 November 2011 <br />