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Colowyo, C1981 -019, PR3 adequacy No. 3 20 July 22, 2011 <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 tmmined 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 />The response is adequate. <br />8. 111 case chanec the 7, foot elceanoil noted in the nest to lust paragraph on pagc 13 ; as the <br />low point on the CAluut Lite lit ghwalI should he 7,275 as the loin point is noted ebew [let C .m <br />Page 133. <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 11 <br />Mine spoil springs described by Williams and Clark, 1994. The Streeter well and Seneca If <br />Mine spoil springs have TDS concentrations above 3,000 mgil (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 />Page 137_ in referring to mining impacts, says: "no signiticant change to local or regional <br />wafer quality is expected ". 10 support your conclusion of no significant change, please add to <br />the pcmtit application a mass- balance calculation of the TDS concentration that will result from <br />the mixing of pit spoil spring discharges with alluvial ground water in Little Collom Gulch and <br />Collom Gulch. <br />10. Please provide a calculation of potential spoil spring dischmge from the downdip cnd of the <br />spoil in Little Collom X Pit that will result t }om ,no"llielt and rain infiltrating from the surface <br />into the spoil. <br />Rule 2.05.6(3)(6) Hvdrolo¢ie 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/l), where Q is volume of flow per unit time, K is hydraulic <br />conductivity, and hQ is the hydraulic gradient (USGS Ground Water Manual, 1977, page 20). <br />