Laserfiche WebLink
Stormwater detention ponds. A similar concern exists for the design and capacity of the runoff detention <br />ponds at the site. The specifications of the ponds were not found in the EPP or SWMP. Regardless of <br />whether the ponds are to be temporary or permanent, please verify that the slopes of the embankments are <br />constructed at a maximum gradient of 2:1, and what the size of storm event they are designed for (e.g., <br />interval and duration). <br />Existing underground water monitoring wells. The Division agrees that continued sampling from the <br />monitoring well "nest" is necessary to establish baseline water quality and to track future changes in the <br />quality. The Division does not believe that collection of valid baseline samples from these wells will be <br />problematic because of impacts from past mining, since because of the historical dewatering, the water <br />collected from these wells will be representative of the mineralized background with low probability of <br />anthropogenic influence. The Division will require that quarterly monitoring be performed for the full suite <br />of analytes that are listed under applicable CDPHE regulations. The Division may later approve that the set <br />of analytes be abbreviated if certain ones return non-detectable or if other justification is provided. The <br />Division will require that the samples be taken quarterly and reported timely to this office. <br />Proposed "compliance" monitoring wells. The Division agrees with Denison's choices for the proposed well <br />locations and geologic strata to be intercepted, being hydrologically downgradient from the underground <br />mine workings. The groundwater quality compliance levels that Denison proposes to be defined statistically <br />on the ambient water quality levels that are established in the existing underground well nest, appear to <br />satisfactorily meet the Division's requirements, as long as the wells generate enough data for valid analyses. <br />At this time, Denison appears to have satisfactorily demonstrated through modeling and analyses that there is <br />minimum probability of offsite migration and impact. However, to ensure validity, continuity and <br />acceptability of the baseline sampling, the Division wishes to see dewatering continue throughout the period <br />of collecting baseline data. Denison must perform the quarterly monitoring for the full suite of analytes that <br />are listed under applicable CDPHE regulations. The Division may later approve that the set of analytes be <br />abbreviated if certain ones return non-detectable or if other justification is provided by Denison. The <br />Division will require that the samples be taken quarterly and reported timely to this office. Before the wells <br />are drilled, however, please provide the proposed specifications and preparatory steps taken prior to the <br />commencement of sampling (e.g., casing, screens, purging, etc.). Also, please provide the plans for sealing <br />and closure of the wells. (Please be reminded that review and approval of these wells may be required from <br />BLM.) <br />Post-mining mine pool. There is a history of standing water. in the mine, but the potential for the existence of <br />and impacts from a post-mining mine pool have not been adequately discussed. This office feels that if there <br />is an offsite impact from a mine pool, it will probably be a direct result of leaching of toxic-forming material. <br />There are numerous questions that should be identified and, as much as possible at this point, addressed by <br />Denison, such as: Does Denison anticipate that there will be standing water in any workings post-mining? If <br />so, what is the anticipated water quality (are there any water quality data from the mine pool before the <br />current dewatering program)? What is the potential for migration of the mine pool offsite and subsequent <br />potential for offsite groundwater degradation? Besides the proposed compliance wells, how does Denison <br />plan to monitor these possibilities? Is there a mitigation plan in the event of offsite impacts? The baseline to <br />be created by sampling of the "compliance wells" will help in determining possible impacts from <br />groundwater migration, but if sampling of the "well nest" reveals degradation from toxic-forming materials <br />of the more nearby strata, the underground sumps will have to be considered as EPFs and a different <br />approach to long-term groundwater management and reclamation will have to be considered. Please be