Laserfiche WebLink
• For the New Horizon site, the USGS study methodology can be used as a basis to predict the time <br />frames of slightly elevated TDS in the spoil water. <br />The New Horizon Mines are similarto the Seneca II Mine in terms of depositional history and observed <br />spoil leachate chemistry. These similarities indicate the oxidation of pyrite can be assumed to be the <br />main source of increased TDS in spoil aquifer water at New Horizon, as at Seneca II Mine. <br />The pyritic sulfur content in New Horizon's spoil averaged 0.52% (by weight) in the 72 overburden core <br />samples whose analyses are reported in the permit application. Pyrite is 53% sulfur (by weight); <br />therefore, the 0.52% pyritic sulfur content indicates that pyrite comprises roughly 0.98% of the mass <br />of the subject mine's spoil. Applying Williams and Clark's 1,600 year exhaustion time for 1.0% pyrite, <br />the subject mine's spoil can be expected to generate high sulfate concentrations for at least 1,500 <br />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 at <br />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 can be <br />expected to last, possibly, several centuries. <br />f~ <br />The New Horizon Mine has two significant differences to the results observed at Seneca II. First, the <br />calcite present in the spoil at New Horizon seems to react with acid produced by the oxidation of pyrite <br />and take a substantial amount of sulfate from solution into solid calcium sulfate. This keeps the <br />dissolved solids content somewhat constant no matter how high or low the the inflow water quality is <br />with regard to TDS. Second, the Seneca 11 site is a dryland reclaimed area where the only recharge <br />into the spoil is a minor amount from precipitation and seepage from the underburden aquifers. Total <br />measured discharge from the spoil was only 3" per year. The New Horizon Mine is principally irrigated <br />with water of very good quality over a large portion of the year. As described earlier, this results in a <br />total movement through the spoil of approximately 16" per year. Since the amount of water moving <br />through the New Horizon spoil is so much greater, it should oxidize the pyrite much more rapidly and <br />also flush other salts which are contributing to the increase in TDS at a much higher rate. For this <br />reason, a ratio of the discharge at both sites could be used to predict the time frame of slightly elevated <br />spoil water quality. This ratio - 3"/16", as applied to 1500 years results in a time frame of 280 years until <br />all the 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 is <br />fully oxidized and othersalts are flushed out, the spoil water qualityshould approach the irrigation water <br />quality, possibly dropping to 300 ppm TDS. <br />• (revised 6/01) 2.05.6 (3) - 34 <br />