2010-08-10_REVISION - C1981008 (13)

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For the New Horizon site, the USGS study methodology can be used as a basis to predict the time frames of slightly elevated TDS in the spoil water. The New Horizon Mines are similar to the Seneca II Mine in terms of depositional history and observed spoil leachate chemistry. These similarities indicate the oxidation of pyrite can be assumed to be the main source of increased TDS in spoil aquifer water at New Horizon, as at Seneca II Mine. The pyritic sulfur content in New Horizon's spoil averaged 0.52% (by weight) in the 72 overburden core samples whose analyses are reported in the permit application. Pyrite is 53% sulfur (by weight); therefore, the 0.52% pyritic sulfur content indicates that pyrite comprises roughly 0.98% of the mass of the subject mine's spoil. Applying Williams and Clark's 1,600 year exhaustion time for 1.0% pyrite, the subject mine's spoil can be expected to generate high sulfate concentrations for at least 1,500 years. The calculated 1,600 year duration is reduced to about 800 years when reductions are taken into account for piping through the spoil (assumed to be 25 %) and lower annual precipitation (12 inches at New Horizon versus 17 inches at Seneca). The calculated 800 -year duration is not meant to be a precise prediction of the duration of elevated dissolved solids, but indicates the elevated solids can be expected to last, possibly, several centuries. The New Horizon Mine has two significant differences to the results observed at Seneca II. First, the calcite present in the spoil at New Horizon seems to react with acid produced by the oxidation of pyrite and take a substantial amount of sulfate from solution into solid calcium sulfate. This keeps the dissolved solids content somewhat constant no matter how high or low the the inflow water quality is with regard to TDS. Second, the Seneca II site is a dryland 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 New Horizon Mine is principally irrigated with water of very good quality over a large portion of the year. As described earlier, this results in a total movement through the spoil of approximately 16" per year. Since the amount of water moving through the New Horizon spoil is so much greater, it should oxidize the pyrite much more rapidly and also flush other salts which are contributing to the increase in TDS at a much 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 - 3'716 ", as applied to 1500 years results in a time frame of 280 years until all the pyrite is oxidized. When considering all the variables involved, this means that it could dissipate within a range of 100 to 500 REVISED August 2010 2.05.6(3) -37