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6) Impact of spoil water quality on surface water quality. <br />Spoil Water Quality <br />The chemistry of the water interacting with the spoil is described in the previous section. In order <br />to determine the impact of the spoil water quality on the surface water, it is first necessary to predict <br />the expected quality of the spoil leachate for the New Horizon #1 and #2 mines. The principal <br />impact from a quality perspective is that irrigation water will seep rapidly through the spoil, increase <br />in TDS and then discharge through a spoil spring at each mine area. Since the pre -mine site did <br />not have spoil springs, there is a potential impact to the quality of the receiving waters. The spoil <br />spring at the New Horizon #1 Mine enters Tuttle Draw while the predicted spring for the New <br />Horizon #2 Mine will enter a tributary to Tuttle Draw and within 1 mile, will also enter Tuttle <br />Draw. Increases 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 #1 Mine, since <br />this flow is solely of water emanating from the spoil and has very little surface water influence. <br />The NPDES 001 discharge is comprised of the flow from Spoil Spring 1, a second spoil spring <br />above a tributary drainage and very rarely, surface flows. From samples of Spoil Spring 1 <br />discharge over the past 12 years, an expected average TDS is 3300 ppm TDS, which is <br />approximately 6% higher than average levels in the overburden samples taken over the same <br />period. Since the New Horizon #2 Mine is larger and has a longer flow path through the spoil, it <br />is estimated that TDS levels will be 10% higher than average levels in the overburden, resulting <br />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 spoil spring <br />discharges is difficult to calculate. In 1994, the USGS did a detailed study of the impacts of <br />infiltration into spoil at the Seneca I1 Coal Mine in Routt County, CO. This study is Water Resources <br />Investigations Report 92-4187 titled HydroloQy and Geochemistry of a Surface Coal Mine in <br />Northwest Colorado. Lysimeters were installed to measure infiltration rates into the spoil, and <br />samples of inflow water, spoil water and spring discharge were analyzed for the entire area. It was <br />determined that pyrite oxidation was the principal cause of elevated TDS, and that the percent of <br />pyrite in the spoil was the determining factor in the length of time that the TDS would be elevated <br />in the spoil water. A spoil pyrite content of 1 % by weight, for example, was predicted to fully <br />oxidize in 1600 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 area. The coal mine <br />overburden at this site was similar in age to that of the New Horizon Mine. <br />For the New Horizon site, the USGS study methodology can be used as a basis to predict the time <br />frames of slightly elevated TDS in the spoil water. <br />July 2016 (TR -74) 2.05.6(3)-31 <br />