2005-01-03_REVISION - M1999120 (5)

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Ms. Lynn Mayer Januazy 3, 2005 Page 4 model was calibrated in a two-step process. First, the model was calibrated to the water level information in the Robson, et al. report by varying hydraulic conductivity. A good match to the water surface contours was obtained with reasonable hydraulic conductivity values. The resulting hydraulic conductivity values ranged from 200 to 500 feet per day, which compare favorably to values reported for the area in the report by Hurr, et al. entitled "Hydrologic Chazacteristics of the Valley-Fill Aquifer in the Brighton Reach of the South Platte River Valley, Colorado." A second calibration run was then made to the average groundwater surface from the LGE monitoring data (Drawing 1}. The results of this effort showed good comparability between observed and predicted groundwater contours. The following model runs were then conducted All permitted mine sites in place, with the following assumptions based on current permit information: • Existing LGE Ft. Lupton site (one pit, lined) • RJ Campbell site (several pits, unlined) • NCCI site (one pit, lined) • Bearson site (two pits, unlined) • Koenig site (one pit, lined) • Heit site (one pit, lined) 2. All permitted pits, plus the proposed Ft. Lupton Pit amendment area. LGE plans to mine the amendment area in phases, with each phase lined prior to mining, and with setbacks between phases. However, it was assumed for simplicity in the model that the amendment area was one lined pit. This is a worst case scenazio since the phasing will allow for some groundwater flow around the individual cells and less impact on groundwater. Permitted water wells, wetlands, cottonwood ripazian areas, and existing ditches were identified within the study area Locations of water wells were obtained from the State Engineer's database for Weld County.. Locations of wells in the vicinity of the proposed amendment azea were verified based on the knowledge of wells in this area. Wetlands and cottonwood azeas were identified by a combination of aerial photograph interpretation and limited fieldwork. The wetland boundaries have not been approved by the U.S. Army Corps of Engineers (Corps); only the wetlands within the amendment azea have been officially delineated and approved by the Corps. Drawing 2 shows the pits, wells, wetlands, cottonwood riparian areas, and ditches in the study azea.