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3) Evaluation of Effect of Returning RO Concentrate to Mine Pool: Before Mine Pool <br />Pumping is initiated there must be an evaluation of the effect of returning the RO concentrate <br />on the mine pool, particularly with regard to microorganisms which may be necessary for <br />successful in -situ treatment in the future. <br />Under the Dewatering Plan, RO treatment and in -situ treatment have been decoupled, <br />and returning the RO concentrate to the mine pool is risky. There needs to be some evaluation <br />of geochemical reactions and resultant water chemistry in the mine pool, prior to the start of in <br />situ treatment. At the very least, an increase in TDS is expected, as well as uranium <br />concentrations, and there may be some effect of the change in mine pool chemistry on <br />dissolution of wall rock, leading to a change in concentration of other constituents in the mine <br />pool. These conditions may make it difficult to support the microorganisms that will be <br />necessary for effective in -situ treatment. If conditions in the mine pool fail to support in -situ <br />treatment, the situation could be significantly worse if in -situ treatment does not work and if <br />water quality is further degraded due to return of the RO concentrate to the mine pool. <br />4) RO Concentrate will Degrade Water Quality: Injection of RO concentrate will further <br />degrade water quality. <br />RO concentrate placed in the mine pool will likely decrease water quality in the mine <br />pool. Although core holes are primary conduits for water to the alluvium, other features exist, <br />related to the local structural geology of the mine, including faults, that can communicate <br />water to Ralston Creek downstream from the mine. Poorer quality, untreated water may pose <br />an increased risk to the creek due to this plan. <br />The mine pool always needs to be pumped down to well below the creek level to reduce <br />the head in the mountain; grouting and sealing the core holes will help minimize <br />communication of mine pool water with the alluvium but other conduits for water flow may <br />result if head is at its highest. <br />5) Unrealistic Assumptions for UIC Permit: Unless the permit application process has <br />already been initiated with EPA, it is unlikely that a UIC permit will be obtained within the time <br />frame set forth in Figure 3. <br />The UIC permit timeline is optimistic /probably unrealistic. Has the EPA been involved to <br />date? Denver Water anticipates that EPA will likely consider this injection to potentially impact <br />underground sources of drinking water and require a permit application rather than the fast <br />track that is briefly described in the Dewatering Plan. A Class V permit will likely be necessary <br />and take additional time. <br />Page 3 of 4 <br />Denver Water Comments Re: Cotter Dewatering Plan (Permit No. M- 1977 -300) <br />