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Table 6 Boundary Conditions Area Boundary Notes North Model extends 2.5 miles to the north See Fi ure 2 South Platte River located River bounds the western East approximately 2000 feet east of site side of the model, see Fi ure 2 South Model extends 2.5 miles to the No flows and constant head south nodes, see Fi ure 2 West Steeper Slopes to a ridge, wells See Figure 1 chan a from shallow to dee 3.10 Site Groundwater Hall-Irwin installed six piezometers at the Site at the locations shown on Figure 3. Monthly groundwater levels were measured and are presented on Figure 6. The data indicates a slight hydraulic gradient toward the South Platte River to the northeast. The Site piezometers are monitored monthly in order to further evaluate the seasonal fluctuations in the groundwater surface. 3.11 Surrounding Site Groundwater Approximate surrounding Site groundwater levels were obtained from reported depths on the Colorado Division of Water Resources list of permitted wells. Reported levels ranged between 4 feet and 30 feet below the ground surface. A well list is attached in Appendix A. 4.0 MODEL SIMULATIONS 4.1 Existing Conditions Steady-state modeling of the Site was used to calibrate the system equilibrium to the measured water levels mentioned in Section 3.11. Monthly water level readings and other reported data can be seen in Appendix B. The aquifer was modeled as consisting of alluvium from the existing ground surface to the tap of bedrock. Civil Resources evaluated the claystone bedrock as an aquitard, thereby forming the bottom of the model (sea Figure 2a). The observed water levels used to calibrate the steady-state model are shown on Figure 6. Calibration is the process of refining input parameters and boundary conditions within reason so the model reflects observed water levels. Parameters including: 1) hydraulic conductivity; 2) bedrock elevations outside of the Site boundaries; 3) recharge rates; and 4) river slopes were adjusted during calibration. The groundwater levels to the east and west of the Site were estimated from the reported depths on the list of permitted wells in the area. The river slope was estimated from the USGS Quadrangle and the river elevation was extrapolated upstream and downstream from the Site. Hydraulic conductivities were adjusted within the range stated in Section 3.6 on Table 2. The recharge rates discussed in Section 3.8 (Table 3) were used in the calibration runs. Depth io bedrock at the Site was approximately mapped from the 20 plus geotechnical borings and generally ranges from 20 feet to 40 feet in depth. Bedrock was assumed to be approximately 35 feet below existing grade and was contoured with the existing land slopes towards the north northeast. Groundwater Model Report-Hall-Irwin Parker-DershamSite -6- Apri12006