2005-07-13_REVISION - M2000016 (27)

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FEB-II-65 IT:65 FROM-Colorado Division of Minerals b G¢olo¢y +I366866356i i-655 ? 60i/D!I F-526 M-2000-016, Lafarge West, Ine.: Riverbend Operation, AM-O1 February 1 i, 2005 AQepoacy Memo Page 2 these structures. The applicant also needs to explain the effect of discharges to Big Dry Creek. This ltem also relates to Item 13 under "Rule 6.4.7 Exhibit G -Water Information." 5. Tne slope stabiliry analyses conductetl to determine mining and stream offsets - minimum distances to be maintained between mining excavations and neighboring structures and streams - used a %::1 slope angle, whereas the Mining Plan indicates mining may occur up to vertical. The analysis also did not model surcharge loads from topsoiUoverburden piles. Offsets need to be recalculated with these inputs included. e. The applicant needs to provide the Division with drill hole records so we can check the slope stabiliry modeling. 7. On page 15 of 81, the first paragraph of the Phase Overview states. "The first tailings pond will eventually encompass all of Phase f, Cell 2. Phase I Cell 7 will be utilized as a clean water pond Throughout the duration of the project." Exhibit F1, Reclamation Plan map shows Phase !Cell 1 as 6.3 acres of silt storage and Phase 1 Cefl 2 as 39.6 acres of water storage. The applicant needs to clarify, or correct these items so that they are consistent. 8. Much of the site is Located in the 100-year flood plain of the South Platte River. Wherever mining will occur within 400 feet of the river channel, cells will need to include inflow and outflow channels, or other appropriate measures. The applicant must submit engineering designs and updated maps to address this situation, where applicable. Rule 6.4.5 EXHIBIT E -Reclamation Plan 9. The Reclamation Plan ident~es five cells and one existing pond that will be used for sill storage and reclaimed as wetlands. Since these silt storage cells have the potential to act as barriers to groundwater flow, the applicant must assess the potential impact and commit fo specific mitigation measures. Rule 6.4.7 EXHIBIT G -Water Information t0. The applicant needs to provide the basis for the statement on page 24 of 81, paragraph 3, "No impact to the South Platte River is expected as a result of mining." 11. Rote 6.4 7(1) requires the operator to directly state if surface water or groundwater will be affected by the mining operation. The applicant located wells within 600 feet of the affected land. However, a ground water study may reveal wells outside the 600-foot- radius that may be impacted. If so, these will need to be addressed and exhibits amended, including updating EXHIBIT C per Rule 6.4.7(2)(a). In accordance with Rule 6 4.7(2)(b), the Division requests that the applicant identify and characterize the aquifer to be mined in or through. The Division also requests that the applicant define or predict the cone of depression or shadow and mounding effect expected for any areas to be dewatered and dry mined or as a result of installation of clay liners. The cone of depression and assessment of other effects should define, a[ a minimum, the horizontal and vertical extent of expected impacts. If, during the assessment, the applicant determines that a drawdown or shadow effect will occur offsite that may impact a current ground water user; the applicant must explain any mitigation measures to be implemented and trigger points that would put mitigation measures into effect.