2010-01-04_REVISION - M1977285 (7)

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Response to DRMS Comments Environmental Protection Plan, Denison Sunday Mines Group capacity. The design capacity and the design storm event were not found described in the EPP or in the appended SWMP. For all such permanent stormwater control structures, please confirm that they are designed to handle 100-year, 24-hour events. If such statement cannot be made, please provide a commitment to construct and certify the necessary structure capacity, with a timeframe. (Please be aware that approval to leave this type of structure may also have to be obtained from the BLM. If so, please provide copy of such approval.) Response Design capacity information regarding permanent stormwater control structures is presented in the Final Drainage Report for the Sunday Mines Group (Attachment P of the EPP). As discussed in the drainage report, permanent control structures were designed based on the 100-year, 24- hour storm event. Denison is currently working with the BLM to obtain approval for drainage structures to remain following reclamation; however, this will likely be a lengthy process and BLM approval has not yet been obtained. Comment 6 Stormwater detention ponds. A similar concern exists for the design and capacity of the runoff detention ponds at the site. The specifications of the ponds were not found in the EPP or SWMP. Regardless of whether the ponds are to be temporary or permanent, please verify that the slopes of the embankments are constructed at a maximum gradient of 2:1, and what the size of storm event they are designed for (e.g., interval and duration). Response Section 9.3 of the EPP (p. 9-2) discusses the stormwater runoff detention basins and provides general pond design sizing/capacity information. Detailed design capacity information regarding runoff detention ponds is presented in the Final Drainage Report for the Sunday Mines Group (Attachment P of the EPP). As discussed in the drainage report, runoff detention ponds were designed based on the 100-year, 24-hour storm event. Detention pond embankment slopes were designed to have slopes between 1.5 Horizontal (H) to 1 Vertical (V) and 3(H):1(V). Comment 7 Existing underground water monitoring wells. The Division agrees that continued sampling from the monitoring well "nest" is necessary to establish baseline water quality and to track future changes in the quality. The Division does not believe that collection of valid baseline samples from these wells will be problematic because of impacts from past mining, since because of the historical dewatering, the water collected from these wells will be representative of the mineralized background with lowprobability of anthropogenic influence. The Division will require that quarterly monitoring be performed for the full suite of analytes that are listed under applicable CDPHE regulations. The Division may later approve that the set of analytes be abbreviated if certain ones return non- detectable or if other justification is provided. The Division will require that the samples be taken quarterly and reported timely to this office. Final RTC Sunday Mines EPP.doc 5