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Mr. 1etTKeller May 3, 2005 Page 2 of the model remains the entire width of the alluvial aquifer north and south of Wattenberg, being approximately 2 miles wide on average. The input data for the model, including alluvial aquifer characteristics such as water table depth, saturated thickness, and transrnissivity, were derived from USGS maps and reports for the South Platte River basin, from water table information from Asphalt Paving, and from site specific boring logs conducted by Ground Engineering Consultants of Englewood, Colorado. We increased the number of cells within the model from 2604 to 5921 cells, in a configuration of 70 columns by 101 rows, in order to increase the sensitivity of the simulations azound these new pit areas. The model grid is variable in size, with cell dimensions ranging from a maximum of approximately 820 feet in the areas outside the general town area to a minimum of approximately 63 feet in the areas of the pits where greatest definition is required. Figure 2 presents the final active model grid, while Figure 3 presents the grid on the USGS topographic background that was used as a base map. Our next step in developing the model was to run a steady state simulation without the pits or the Town of Wattenberg's well in place in order to calibrate the modeled starting heads such that they accurately represent the water table elevation across the model. We modified water table elevation data from USGS maps and our prior Wattenberg model to reflect current conditions in the pit azea as determined from actual data obtained from the recent borehole data obtained at the pit sites. We compared the resultant head configurations to the mapped water table contours by USGS and noted good general agreement, indicating that these heads were thereby suitable for executing the pit simulations. These head values then became the initial heads for the subsequent predictive model runs. The calibration head contours aze presented in Figure 4. Our first set of simulations incorporated the Wattenberg municipal supply well into the model. We initially simulated pumping of the well at a rate of 75 gallons per minute ("gpm") for a continuous period of 365 days. Very little drawdown was noted in the vicinity of the well under this pumping scenario and it was at first thought that there might be a problem with the model. Further research into other wells producing from the aquifer in the Wattenberg vicinity revealed that wells pumping as much as 1,500 gpm exist nearby. This gave us comfort that a 7S gpm well would be expected to induce very little drawdown and that the model was operating properly. We next ran simulations incorporating the intermittent pumping that we understand reflects how the well is normally operated. Under these scenarios, even less drawdown was indicated at the well but this observation comports nicely with reports from representatives of the Wattenberg Improvement Association that the well, when in operation, draws down only a few feet and then goes into essentially a steady state condition with no further drawdown. The scenarios run simulated pumping cycles of 10, 12, and 16 hours per day. The drawdowns induced by these various pumping cycle scenarios were essentially identical, again matching the reported response of the aquifer to the actual well pumping. Since completing the modeling work, we were informed that the system may actually be pumping at only 45 gpm; we will be confirming this in the neaz future. A lower pumping rate for the well would be a positive factor in that even lower predicted drawdowns would be observed thus reducing the susceptibility of the well to changes in local water table caused by the presence of the lined pits. The slurry walls were then incorporated into the model, both with and without the Wattenborg well pumping. Figure 5 presents the model grid and basemap with the slurry walls Martin and Wootl Water Consultants, Inc.