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by a value twice that of mean low flow (summer flow = 2 (mean low flow)). <br />Both peak and mean low flow spoils discharges were then adjusted for low <br />recharge during low flow years by dividing discharges by a factor of 3.5. <br />This value reflects the difference between surface water flows in 1985 (high <br />flow) and 1981 (low flow). Spoils discharge for all the surface mines in the <br />Yampa River Basin are predicted in TABLE 7. <br />TABLE 8 summarizes the water quality impacts from underground mines in the <br />Yampa River drainage. The Foidel Creek Mine shows the most significant <br />impacts to water quality from mine inflows, but as the mine is located on the <br />upper reaches of the drainage, downstream cumulative impacts will be minimized <br />by dilution. The Foidel Creek Life-of-Mine permit review has provided the <br />most detailed projections of impacts of drainage from the flooded workings. <br />Although their projections show very high concentrations of TDS and 504, the <br />quantities are so low that the impacts should not be significant. <br />Water quality parameters have units of mg/1 and when substantial historical <br />information is available it is listed in the following format: <br />Average <br />Min-Max <br />TABLE 9 tabulates the existing documentation on quantitative impacts from <br />underground mines. Again, the Foidel Creek permit provides more detailed <br />projections than other permits; it predicts impacts of of longer duration and <br />comparably greater quantitative impacts, due to the size of the stream. The <br />Eagle Mine depletions will have minimal impact on the Yampa River where <br />historical low flow is 305 cfs. The Foidel Creek Mine has transformed Foidel <br />Creek from an intermittent stream to a perennial stream, albeit a stream with <br />low average base flows (0.28 cfs). Foidel Creek Mine impacts to Fish Creek <br />will be insignificant. <br />TABLE 10 summarizes the calculations used to evaluate the potential for <br />material damage on Fish Creek, Trout Creek and two sites on the Yampa River, <br />using the assumptions described in the paragraphs on spoils aquifers and the <br />projections of impacts from underground mines. Existing standards for salt <br />loading are based on the potential for material damage to crops through <br />sub-irrigation or flood irrigation. Moderately sensitive species can <br />withstand being irrigated by water with EC levels less than 1 mmho/cm (this <br />water is equivalent to 730 mg/1 TDS for sulfate-dominated waters in Twentymile <br />Park). Twentymile Coal Company submitted vegetation data documenting that <br />irrigated lands in the upper Trout Creek drainage could withstand alluvial EC <br />concentrations of 3 mmhos/cm and flood irrigation EC levels of 2 mmhos/cm <br />without a significant decrease in crop productivity. Consequently, the <br />standard of 1 mmhos/cm was utilized in assessing compliance with material <br />damage standards in all reaches of the Yampa River Basin except in the Upper <br />Trout Creek Basin. All tributaries of Yampa except lower Middle Creek have <br />TDS and EC levels lower than the existing standards. (TABLES 4 and 5). As <br />water from lower Middle Creek will not be used for flood irrigation, this <br />reach is not out of compliance with the existing standard. <br />-25- <br />t -- <br />