PERMFILE103529 - Laserfiche WebLink

Home Browse Search

Mr. Jeff Keller May 3, 2005 Page 3 defined. The slurry walls were simulated by inactivating the cells contained within the perimeter of the slurry wall, preventing water from moving through the pit azea. The resultant mounding and shadowing up- and downstream of the slurry walls reflects the impacts to the ground water head contours from the presence of the lined pits. As presented in Figure 6, the predicted change in water levels resulting from the lined pits generally falls into the 1- to 2-foot range, with the lazger changes (mounding, indicated as negative drawdowns on the figure) observed closest to the pit walls. At the Wattenberg municipal well site, and throughout most of the central azea of the town, there is minimal predicted head change. The fact that the configuration of the pits allows for an open corridor of recharge and ground water flow though the town azea is felt to be an important factor in mitigating any potential impacts on the water levels that could impact the ability of the well to pump or to impact, via a rise in the water table, the septic systems that aze in use throughout the town. The model runs incorporating the well pumping with the slurry walls in place predicted the same minimal drawdowns at the well as observed in the scenarios excluding the slurry walls. Once again, these minimal drawdowns are in line with the reported measured changes when the well is pumping. Once the pit runs had been completed with the nominal aquifer characteristics simulated, we re-ran a number of the simulations to determine whether the model was overly sensitive to any of the input pazazneters and to analyze the potential for error in the predictions. Past experience and the sensitivities tamed out on the previous version of the Wattenberg model indicated that hydraulic conductivity was the most sensitive pazameter. Accordingly, we carried out simulations with values ranging from 340 to over 2,000 feet per day. Very little change in the head contours was observed over this range. Figures 7 and 8 present the predicted drawdowns with hydraulic conductivity values set at 340 feet per day and 1,380 feet pre day, respectively. The hydraulic conductivity value derived from the USGS transmissivity and saturated thickness mapping for the Wattenberg azea was in the 600 to 700 feet per day range. The final modeling runs applied a value of 680 feet per day. We found relatively minimal sensitivity to changes in the specific yield of the aquifer and chose a value of 18 percent as for the predictive runs. It is concluded from the results of the modeling study that there will be minimal ground water impacts resulting from the combined presence of the slurry walls for the five pits planned for the areas west, south, and east of the town of Wattenberg. While some mounding to the south of the Wattenberg Lakes pits to the south of town could result in azeas where saturated ground may be observed, it does not appear that the actual town area will be impacted to any significant degree. We strongly recommend, in spite of the model predictions, that as much data on the current water table conditions and pumping data on the Wattenberg municipal well be obtained and reviewed. This data can be compazed to the model predictions to check for compliance or to allow for upgrading of the model to facilitate more accurate prediction. In addition, the installation of monitor wells at selected locations may be required to provide adequate data coverage over the entire azea of concern. Martin and Wood Water Consultants, Inc.