2001-12-21_GENERAL DOCUMENTS - C1981008

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1 ? p ? ','" L i D} impact of sp©?? water quality on surface water unlit ?oi? Water ?ca?it? The chemistry of the water interacting with the spoil is described in the previous section. In order to determine the impact of the spoil water quality on the surface water, it is first necessary to predict the expected quality of the spoil leachate for the New Horizon #1 mine.. The principal impact from a quality perspective is that irrigation water will seep rapidly through the spoil, increase in TDS and then discharge through a spoil spring at the lower part of the mine area. 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 Kline enters Tuttle Draw. Increases or decreases in pH have never been observed on the site. The Spoil Spring 1 discharge 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 rarely, surface flows. From samples of Spoil Spring 1 discharge over the past 1Z 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 Spoil Water The time period for these slightly elevated levels of TDS to accur in the spoil water and spoil spring discharges is difficult to calculate. In 194, the USGS did a detailed study of the impacts of infiltration into spoil at the Seneca Il foal Kline in Routt +?ounty, CG. This study is Water Resources Investigations report 92-4187 titled Hydralo?v and' ?eoc?emistry ova ???cr?ace ?©a? Mine ?? 11?arthwest Colorado. Lysimeters were installed to measure infiltration rates into the spoil, and samples of inflow water, spoil water and spring discharge were analyzed for the entire area. 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 100 years (their Table 1 ?). TDS levels in the spoil water were approximately 4500 ppm, which was a significant increase aver the overburden aquifer water in the area. 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 Mines are similar to the Seneca II Mine in terms of depositional history and observed spoil leachate chemistry. These similal-ities 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%a Eby weight) in the 7? overburden core samples whose analyses are reported in the permit application. Pyrite is 53°Io sulfur Eby weight}; therefore, the o.52°Io pyritic sulfur content indicates that pyrite comprises roughly 0.95°Ia of the mass of the subject mine's spoil.. Applying Williams and dark's 1,bQ0 year exhaustion tune for 1.0°?? pyrite, the subject mine's spoil can be expected to generate high sulfate concentrations for at least 1,500 years. The calculated 1,500 year duration is reduced to about $00 years when reductions are taken into account for piping through the spoil assumed to be ?5 ?©} and lower annual precipitation ?1 ? Inches at New Horizon versus 17 inches at Seneca}. The calculated S00-year duration is not meant to be a precise prediction of the duration of elevated dissolved solids, but indicates the elevated solids can be expected to last, possibly, several centuries.