2005-07-26_REVISION - M2000016 - Laserfiche WebLink

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M-2000-016, Lafarge West, Inc.: Riverbend Operation, AM-01 July 25, 2005 Adequacy Memo No. 4 Page 3 13. In the April 22, 2005 adequacy review, regarding number of unlined pits, the Division noted, "If the model did not adequately represent the applied-for mining conditions, analyses and conclusions based upon this model cannot be validated. The applicant must address this discrepancy, and, if the applied-for conditions were not modeled, provide a reconstructed, reanalyzed hydrologic model " After receipt of the larger hydrologic model output maps on July 12, 2005 (the Maximum Disturbance maps), and further review of the AMEC report, additional discrepancies have been identified which must be addressed. Table 1 of the AMEC report, the Mine Plan that is the basis of the ground water model, shows Phase 5, Cell 2 as "tailing filled° while the Reclamation Plan maps show fl as an open water pond. Table 1 also shows Phase 6, cells 1 and 2 as "storage sealed" while the Reclamation Plan maps indicate it will be an open water pond. Phase 7 is shown on Table 1 as two cells composing 82.7 acres, while the Mine Plan maps show Phase 7 as one 99.9-acre cell. There are additional discrepancies in pond areas from Table 1 versus the Mine Plan maps, most involving two to three acres, but Phase 9 Celf 1 is 19.8 acres in Table 1 and 32.9 acres on the Mine Plan maps. Since there are some significant differences between the setup of the hydrologic model and the proposed Mine Plan and Reclamation Plan maps, the applicant must address the impacts of these differences by providing a reconstructed, reanalyzed hydrologic model, or adequately assessing, reporting on, and incorporating these impacts into new hydrologic model maps. 14. Regarding impacts due to clay-lined and silt storage ponds, the adequacy response indicates the maximum impact to ground water will be plus or minus 0.5 feet. Why does the Maximum Disturbance at Reclamation map show a contour with a range to 0.8 foot? What do the red numbers on the "contours" mean? Please explain why, per the map, the largest area of mounding occurs amidst the open water ponds of Phases 5, 6, 7, and 8? Please descnbe-flow Tfie assump~on that impacts frdm die-silt storage ponds were - equivalent to those from clay-lined ponds was incorporated into the modeled data and cumulative effect of the final configuration of clay-lined and silt-storage ponds determined and depicted. 15. Based upon review of the ground water Maximum Disturbance maps submitted on July 12, 2005, a number of wells outside the proposed pennit boundary, including some east of the South Platte River, will see drawdown effects in excess of the 2-feet trigger point for mitigation designated by the operator. There are also a number of structures evident from the aerial photos that likely have wells (unregistered or otherwise unident~ed by the applicant), that are expected to see a greater than 2-feet drop in static water level. Based upon these data, the applicant needs to do the following: • All wells potentially affected by this operation must be ident~ed on the map and a table provided that lists the structure type and the name, address, and phone number of the structure owner. • The "Groundwater Monitoring Plan" needs to be amended to provide adequate monitoring to protect these wells. This may require the construction of additional monitoring wells. • Identify all possible targets, including vegetation, that could be damaged or impacted by the ground water effects predicted for this operation and depict them on the Maximum Disturbance maps. Vegetation includes, but is not limited to, cottonwoods and wetlands, especially offsite vegetation. Please specify additional monitoring wells to be selected and/or installed to protect these targets. • Please show all monitoring wells on-the Maximum Disturbance maps so the Division can determine whether they are appropriately placed to monitor impacts.