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irrigation water will seep rapidly through the spoil, increase in TDS and then dischazge through a spoil spring at the lower part of the mine azea. Since the pre-mine site did not have spoil springs, there is a potential impact to the quality of the receiving waters. The spoil spring at the New Horizon #1 Mine enters Tuttle Draw. Increases or decreases in pH have never been observed on the site. The Spoil Spring 1 dischazge best represents the quality of the spoil leachate at the New Horizon #1 Mine, since this flow is solely of water emanating from the spoil and has very little surface water influence. The NPDES 001 discharge is comprised of the flow from Spoil Spring 1, a second spoil spring above a tributary drainage and very razely, surface flows. From samples of Spoil Spring 1 dischazge over the past 12 years, an expected average TDS is 3300 ppm TDS, which is approximately 6% higher than average levels in the overburden samples taken over the same period. Time-frames of Elevated TDS in~oil Water The time period for these slightly elevated levels of TDS to occur in the spoil water and spoil spring dischazges is difficult to calculate. In 1994, the USGS did a detailed study of the impacts of infiltration into spoil at the Seneca Il Coal Mine in Routt County, CO. This study is Water Resources Investigations Report 92-4187 titled Hydrology and Geochemistry of a Surface Coal Mine in Northwest Colorado. Lysimeters were installed to measure infiltration rates into the spoil, and samples of inflow water, spoil water and spring dischazge were analyzed for the entire azea. It was determined that pyrite oxidation was the principal cause of elevated TDS, and that the percent of pyrite in the spoil was the determining factor in the length of time that the TDS would be elevated in the spoil water. A spoil pyrite content of 1% by weight, for example, was predicted to fully oxidize in 1600 yeazs (their Table 18). TDS levels in the spoil water were approximately 4500 ppm, which was a significant increase over the overburden aquifer water in the azea. The coal mine overburden at this site was similar in age to that of the New Horizon Mine. 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 Mine is 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 aze 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 Clazk's 1,600 yeaz exhaustion time for 1.0% pyrite, the subject mine's spoil can be expected to generate high sulfate concentrations for of least 1,500 years. The calculated 1,500 year duration is reduced to about 800 yeazs when reductions are zs