Laserfiche WebLink
The New Horizon Mines are similar to the Seneca II Mine in terms of depositional history and <br />observed spoil leachate chemistry. These similarities indicate the oxidation of pyrite can be <br />assumed to be the main source of increased TDS in spoil aquifer water at New Horizon, as at <br />Seneca II Mine. <br />The pyritic sulfur content in New Horizon's spoil averaged 0.52% (by weight) in the 72 overburden <br />core samples whose analyses are reported in the permit application. Pyrite is 53% sulfur (by <br />weight); therefore, the 0.52% pyritic sulfur content indicates that pyrite comprises roughly 0.98% <br />of the mass of the subject mine's spoil. Applying Williams and Clark's 1,600 year exhaustion time <br />for 1.0% pyrite, the subject mine's spoil can be expected to generate high sulfate concentrations <br />for at least 1,500 years. <br />The calculated 1,600 year duration is reduced to about 800 years when reductions are taken into <br />account for piping through the spoil (assumed to be 25%) and lower annual precipitation (12 inches <br />at New Horizon versus 17 inches at Seneca). The calculated 800 -year duration is not meant to be a <br />precise prediction of the duration of elevated dissolved solids, but indicates the elevated solids <br />can be expected to last, possibly, several centuries. <br />The New Horizon Mine has two significant differences to the results observed at Seneca II. First, <br />the calcite present in the spoil at New Horizon seems to react with acid produced by the oxidation <br />of pyrite and take a substantial amount of sulfate from solution into solid calcium sulfate. This <br />keeps the dissolved solids content somewhat constant no matter how high or low the the inflow <br />water quality is with regard to TDS. Second, the Seneca II site is a dryland 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 New <br />Horizon Mine is principally irrigated with water of very good quality over a large portion of the <br />year. As described earlier, this results in a total movement through the spoil of approximately <br />16" per year. Since the amount of water moving through the New Horizon spoil is so much <br />greater, it should oxidize the pyrite much more rapidly and also flush other salts which are <br />contributing to the increase in TDS at a much higher rate. For this reason, a ratio of the discharge <br />at both sites could be used to predict the time frame of slightly elevated spoil water quality. <br />This ratio - 3"/16", as applied to 1500 years results in a time frame of 280 years until all the <br />pyrite is oxidized. When considering all the variables involved, this means that it could dissipate <br />within a range of 100 to 500 years. The TDS should begin to drop before this time. Once the pyrite <br />is fully oxidized and other salts are flushed out, the spoil water quality should approach the <br />irrigation water quality, possibly dropping to 300 ppm TDS. <br />July 2016 (TR -74) 2.05.6(3)-32 <br />