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At the outset, Denver Water was comfortable with a reduction in the mine pool <br />dewatering requirements from 500 feet below Steve Level to 150 feet below Steve Level with <br />the understanding that Cotter's proposed dewatering plan was going to involve a simultaneous <br />approach of dewatering and in -situ treatment as opposed to the decoupled approached now <br />being pursued. Because decoupling mine pool dewatering from in -situ treatment may result in <br />the in -situ treatment no longer being a viable option as discussed below, Cotter should not be <br />allowed to pursue mine dewatering without in -situ treatment, absent a detailed contingency <br />plan that provides for off -site disposal of RO concentrate. For this reason, the mine dewatering <br />plan should not be approved without addressing items 3 through 5 in the August 29, 2012 <br />adequacy review letter. <br />With regard to item 5, in particular, DRMS requested a contingency plan for disposing <br />RO residuals outside the mine pool, should pilot scale testing or full -scale testing indicate <br />returning the residuals to the mine pool is not feasible. The likelihood of success of Cotter's <br />proposed Dewatering Plan is highly uncertain. As discussed below, Denver Water is of the <br />opinion that it will be difficult and time consuming for Cotter to obtain a UIC permit from EPA, <br />and the decoupling of in -situ treatment from mine dewatering may decrease the future viability <br />of in -situ treatment. In addition, as the mine pool water becomes more concentrated it is likely <br />that the RO system will require increased maintenance. Because Cotter is being required to <br />reduce the mine pool only to 150 feet below Steve Level, there is less of a buffer, so it is <br />necessary that Cotter include a backup plan for alternate disposal of RO concentrate if injection <br />to the mine pool is not feasible. <br />2) Pumping Requirements: The Dewatering Plan does not address whether mine pool <br />pumping will continue after the mine pool is reduced to 150 feet below Steve Level. <br />Figure 4 in the Dewatering Plan outlines a schedule under which Cotter will initiate mine <br />pool dewatering by January 28, 2013, and reduce the mine pool to 150 feet below Steve Level <br />by April 30, 2013. Under the schedule, Cotter will not initiate in -situ treatment until December <br />10, 2013. Neither the schedule in Figure 4, nor the Dewatering Plan describes whether Cotter <br />will continue to pump and operate the RO system during the period of time between April 30, <br />2013 and December 10, 2013. Based on the information contained in the Dewatering Plan <br />Denver Water assumes that Cotter will cease pumping once it reaches 150 feet below Steve <br />Level and allow the mine pool to rise until in -situ treatment occurs. Because Cotter will now be <br />pumping down to only 150 feet instead of 500 feet, there is less of a buffer as the mine pool <br />refills. Because of the uncertainty of how the mine pool may change as a result of the discharge <br />of RO concentrate to the mine pool, the Dewatering Plan should require that once Cotter <br />reduces the mine pool to 150 feet below Steve Level, Cotter must continue to pump in order to <br />keep the mine pool 150 below Steve Level. Please note that it may only take 20 to 30 gpm of <br />pumping to keep it lowered to 150 feet. <br />Page 2 of 4 <br />Denver Water Comments Re: Cotter Dewatering Plan (Permit No. M- 1977 -300) <br />