Laserfiche WebLink
tributary to Tuttle Draw and within 1 mile, will also enter Tuttle Draw. Increases <br />or decreases in pH have never been observed on the site. The Spoil Spring 1 <br />discharge best represents the quality of the spoil leachate at the New Horizon 91 <br />Mine, since this flow is solely of water emanating from the spoil and has very <br />little surface water influence. The NPDES 001 discharge is comprised of the flow <br />from Spoil Spring 1, a second spoil spring above a tributary drainage and very <br />rarely, surface flows. From samples of Spoil Spring 1 discharge over the past 12 <br />years, an expected average TDS is 3300 ppm TDS, which is approximately 6% <br />higher than average levels in the overburden samples taken over the same period. <br />Since the New Horizon 92 Mine is larger and has a longer flow path through the <br />spoil, it is estimated that TDS levels will be 10% higher than average levels in <br />the overburden, resulting in a TDS of 3425 ppm. <br />Timeframes of Elevated TDS in Spoil Water <br />The time period that these slightly elevated level of TDS in the spoil water and <br />spoil spring discharges is difficult to calculate. In 1994, the USGS did a detailed <br />study of the impacts of infiltration into spoil at the Seneca II Coal Mine in Routt <br />County, CO. This study is Water Resources Investigations Report 92 -4187 titled <br />Hydrology and Geochemistry of a Surface Coal Mine in Northwest Colorado. <br />Lysimeters were installed to measure infiltration rates into the spoil, and samples <br />of inflow water, spoil water and spring discharge were analyzed for the entire <br />area. It was determined that pyrite oxidation was the principal cause of elevated <br />TDS, and that the percent of pyrite in the spoil was the determining factor in the <br />length of time that the TDS would be elevated in the spoil water. A spoil pyrite <br />content of 1% by weight, for example, was predicted to fully oxidize in 1600 <br />years (their Table 18). TDS levels in the spoil water were approximately 4500 <br />ppm, which was a significant increase over the overburden aquifer water in the <br />area. The coal mine overburden at this site was similar in age to that of the New <br />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 Mines are similar to the Seneca II Mine in terms of <br />depositional history and observed spoil leachate chemistry. These similarities <br />indicate the oxidation of pyrite can be assumed to be the main source of <br />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 <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 />.o <br />