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Colowyo, C1981-019, PR3 adequacy No. 2 15 April 22, 2011 <br />inches per year, calculated as 18% X 17 inches per year average precipitation. The 18% factor <br />is based on Williams and Clark's (1994) spoil lysimeter study which found that approximately <br />18% of annual precipitation recharge spoil at a Routt County mine. The annual recharge <br />values used in the submittal (0.2 to 0.35 inches, and 1.1 inches) are less than 7% of annual <br />precipitation in the Collom area and apply to undisturbed bedrock, whose permeability, and <br />consequently annual recharge, would be a fraction of the value for spoil. <br />6. To be consistent with the 15% swell factor noted in Table 2.05.6-5, please use in the spoil <br />resaturation calculations a spoil porosity value of 15%, rather than 35%. <br />7. The predicted spoil recharge rate of 3.1 inches per year (see previous comment 5) is <br />significantly greater than the maximum bedrock recharge rate of 1.1 inches per year; therefore, <br />it is probable that as spoil backfill resaturates, the spoil leachate-saturated zone at the downdip <br />pitwall will rise in elevation faster than the recovery of the potentiometric surface in the <br />adjacent unmined bedrock. The more rapid development of leachate head in the backfill than <br />ground water head in the downdip pitwall probably will enable postmining seepage of spoil <br />leachate into the downdip pitwall at elevations below the top of the pre-mining saturated zone. <br />(The submitted evaluation assumes leachate can migrate into unmined bedrock only at <br />elevations above the pre-mining saturated zone.) Please revise the calculation of leachate <br />seepage into the pitwall to recognize leachate probably will seep into the pitwall at elevations <br />below the top of the pre-mining saturated zone. <br />8. Please check if the 7,275-foot elevation noted on page 136 as the low point on the highwall <br />should be 7,290 as the low point is noted on page 131 <br />9. The words "no significant change to water quality" in the eighth paragraph on page 134 appear <br />to contradict the statement in the second paragraph on page 132 that spoil leachate will likely <br />have the same characteristics as water in the Streeter fill well or Streeter pond, or Seneca II <br />Mine spoil springs described by Williams and Clark, 1994. The Streeter well and Seneca II <br />Mine spoil springs have TDS concentrations above 3,000 mg/1(page 119 of the Rule 2 section <br />of Colowyo's PR-02 volume); undisturbed Williams Fork ground water in the Collom area <br />have TDS concentrations generally below 2,000 mg/1(Table 2.04.7-41 of Colowyo's PR-03 <br />submittal). Please change the wording in the eighth paragraph on page 134 so that it is <br />consistent with the statement in the second paragraph on page 132. Also, please change on <br />page 134 the wording "base environment" to "basic environment". <br />Rule 2.05.6(3)(b) Hydrologic Controls <br />The applicant has presented a plan for monitoring surface and ground water in accordance with Rule <br />2.05.6(3)(b)(iv). Any deficiencies with regard to the plan are noted under Rule 4.05.13 below. <br />1. Please add to Section 2.05.6(3)(b)(iii) a calculation of annual seepage of spoil water into the <br />downdip pitwalls of both pits, using the Darcy equation for flow through a porous medium. <br />The equation is Q = KA(h/1), where Q is volume of flow per unit time, K is hydraulic <br />conductivity, and h/1 is the hydraulic gradient (USGS Ground Water Manual, 1977, page 20).