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• f) Impact of spoil water quality on surface water quality. Spoil Water Quality 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 and #2 mines. 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 each 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 Mine enters Tuttle Draw while the predicted spring for the New Horizon #2 Mine will enter a tributary to Tuttle Draw and within 1 mile, will also enter 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 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. Since the New Horizon #2 Mine is larger and has a longer flow path through the spoil, it • is estimated that TDS levels will be 10% higher than average levels in the overburden, resulting in a TDS of 3425 ppm. Timeframes of Elevated TDS in Sooil Water The time period that these slightly elevated level of TDS in the spoil water and spoil spring discharges is difficult to calculate. In 1994, the USGS did a detailed study of the impacts of infiltration into spoil at the Seneca II Coal Mine in Routt County, CO. This study is Water Resources Investigations Report 92- 4187 titled Hvdroloav and Geochemistry of a Surface Coa! Mine in Northwest 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 1600 years (their Table 18). TDS levels in the spoil water were approximately 4500 ppm, which was a significant increase overthe overburden aquifer water in the area. The coal mine overburden at this site was similar in age to that of the New Horizon Mine. • (Revised 6/01) 2.05.6 (3) - 33