2010-01-04_REVISION - M1997032 - Laserfiche WebLink

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Response to DRMS Comments Environmental Protection Plan, Denison Van 4 Mine August through October, which suggests that precipitation during those months is delivered via thunderstorms - potentially high intensity events. It is known that plumes can form beneath waste rock facilities in and climates in response to seasonal patterns of such high intensity events. Please address the following: Does the model account for high-intensity, short-duration precipitation events such as thunderstorms? If not, what effect would these have on percolation estimates? Response Additional assessments using the UNSAT-H model have been completed to address this comment. An addendum to the Environmental Geochemistry Investigation of Rock and Soil Material Van 4 Mine (Attachment B of the EPP) has been developed; see attached addendum for review. Comment 5 Permanent stormwater structures. The Division requires that stormwater runoff originating from the affected areas (pad surfaces and slopes, stockpile surfaces, and roads) be controlled adequately. In light of the thorough treatment of these items, indicated in the Stormwater Management Plan (SWMP) prepared for CDPHE-WQCD, the Division considers such runoff and its control during the mine's active and inactive phases to be adequately addressed. However, the EPP states that certain stormwater control structures will be permanent, and remain on the site after final reclamation. The SWMP document, included in this EPP within Attachment F, reiterates the statement that certain Swales and Diversion Ditches will be permanent and remain onsite, though which specific structures are not identified. It is the Division's concern that there may be some such structures onsite that are planned to remain after reclamation (understood to be after monitoring and maintenance by the operator have ceased) that may not be able to continue functioning adequately to control, convey or divert their designed working capacity. [Note: The design capacities and the design storm event (100-year, 24-hour storm event) were described in the Drainage Report.] Likewise, the plan indicates that Retention Berms are to remain permanently, and the Division wishes to ensure that these are designed for longterm stability. The construction, upgrades and maintenance of these structures will require earthwork, and possibly riprapping or seeding, which the Division needs to closely monitor. Please identify which such stormwater structure are intended to be permanent. If certain structures will require construction or improvements to meet the SWMP plans, please identify which ones they are, plus a construction timeframe. (Please be aware that approval to leave these types of structures may also have to be obtained from the BLM. If so, please provide copy of such approval.) Brief description of permanent diversion ditches, berms and water bars is found in Section 16 If further description or identification of structures is needed, please provide it. Response Section 16 states that the Surface water diversion ditches to the south and up-gradient of the surface facility areas will remain in place following reclamation. These are existing ephemeral drainages (south) and a DRMS RTC Van4 Mine EPP.doc