2005-01-03_REVISION - M1999120 (5)

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Ms. Lynn Mayer January 3, 2005 Page 3 The monitoring wells will continue to be measured on a monthly basis by LGE throughout the life of the proposed amendment area. The existing 14 monitoring wells will be used. Some of these wells will be relocated in the future if they are in an area that will be mined. LGE will propose new locations and work with DMG on approvals, as needed. Water Ouality Negative impacts on water quality aze not expected. LGE will continue to use required measures for materials containment and their discharge permit. #8 Effects of proposed operation on local groundwater table and nearby wells. Effects of the existing lined pit on groundwater, based on monitoring by LGE, were presented in the previous response (#7 and Drawing 1). Data indicate a shadow effect (drawdown) with a maximum reduction of aoound 4 feet immediately north of the existing pit, attenuating to almost zero aoound 1,400 feet to the north. There are no known complaints of effects on water wells from the existing Ft. Lupton operation since the pit has been in operation. Likewise, no effects from mounding on the upgradient side of the pit have been observed, and no complaints have been received regarding mounding. Groundwater has also been monitored in the vicinity of the Heit Pit several miles north, as required by the DMG permit. The Heit Pit has not been constructed, but Weld County's Koenig Pit, which is lined, exists in the azea. This data set indicates mounding of aoound 2 feet on the upgradient side of the pit, and a shadow or decrease in groundwater level of approximately 1 foot downgradient of the Koenig Pit. The shadow extends approximately 800 feet north of the pit. The proposed LGE expansion will mine into the South Platte River alluvium. The expansion will result in a series of lined cells that will be used for water storage. Although existing monitoring data indicate relatively small and limited effects on the groundwater table from existing slurry wall operations, and there have been no complaints, a cumulative impact assessment was completed to determine effects of the proposed amendment area. This assessment also considered the effects of the other permitted aggregate pits in the area The cumulative impact assessment was completed using the USGS's MODFLOW groundwater model The study area extended approximately 2 miles south of the existing LGE Ft. Lupton operation and approximately 2 miles north of the Heit Pit. MODFLOW was calibrated and used to predict changes in groundwater levels with all the other permitted pits in place, and then with all other permitted pits and the proposed LGE amendment. There will not be any impacts from dewatering since LGE will install the slurry wall around each cell prior to mining (this is a modification to the original amendment application). Information on aquifer transmissivity, depth to bedrock, and shallow groundwater levels in the study area were obtained from the report by Robson, et al. (2000) entitled "Geohydrology of the Shallow Aquifers in the Fort Lutpon-Gilcrest Area, Colorado." The