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Spoil Spring I discharge best represents the quality of the spoil leachate at the <br />New Horizon #1 Mine, since this flow is solely of water emanating from the spoil <br />and has very little surface water influence. The NPDES 001 discharge is comprised <br />of the flow from Spoil Spring 1, a second spoil spring above a tributary drainage <br />and very rarely, surface flows. From samples of Spoil Spring 1 discharge over the <br />past 12 years, an expected average TDS is 3300 ppm TDS, which is approximately <br />6% higher than average levels in the overburden samples taken over the same <br />period. <br />Time- rames of Elevated TDS in Spoil Water <br />The time period for these slightly elevated levels of TDS to occur in the spoil <br />water and spoil spring discharges is difficult to calculate. In 1994, the USGS did a <br />U Laiieu SLUG-y of the impacts of inUILIaLioii into spoil at the Seneca ii Coai ivliiie iii <br />Routt County, CO. This study is Water Resources Investigations Report 92-4187 <br />titled Hydrology and Geochemistry of a Surface Coal Mine in Northwest <br />Colorado. Lysimeters were installed to measure infiltration rates into the spoil, <br />and samples of inflow water, spoil water and spring discharge were analyzed for <br />the entire area. It was determined that pyrite oxidation was the principal cause of <br />elevated TDS, and that the percent of pyrite in the spoil was the determining factor <br />in the length of time that the TDS would be elevated in the spoil water. A spoil <br />pyrite content of 1% by weight, for example, was predicted to fully oxidize in <br />1600 years (their Table 18). TDS levels in the spoil water were approximately <br />4500 ppm, which was a significant increase over the overburden aquifer water in <br />the area. The coal mine overburden at this site was similar in age to that of the <br />New Horizon Mine. <br />For the New Horizon site, the USGS study methodology can be used as a basis to <br />predict the time frames of slightly elevated TDS in the spoil water. <br />The New Horizon Mine is similar to the Seneca II Mine in terms of depositional <br />history and observed spoil leachate chemistry. These similarities indicate the <br />oxidation of pyrite can be assumed to be the main source of increased TDS in spoil <br />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 <br />the 72 overburden core samples whose analyses are reported in the permit <br />application. Pyrite is 53% sulfur (by weight); therefore, the 0.52% pyritic sulfur <br />content indicates that pyrite comprises roughly 0.98% of the mass of the subject <br />mine's spoil. Applying Williams and Clark's 1,600 year exhaustion time for 1.0% <br />pyrite, the 'subject mine's spoil can be expected to generate high sulfate <br />concentrations for at least 1,500 years. <br />The calculated 1,500 year duration is reduced to about 800 years when reductions <br />are taken into account for piping through the spoil (assumed to be 25%) and lower <br />annual precipitation (12 inches at New Horizon versus 17 inches at Seneca). The <br />34