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Derr Pit - Groundwater Modeling Report <br />January 31, 2020 Page 8 of 24 <br />McGrane Water Engineering, LLC <br />1669 Apple Valley Rd. Lyons, CO 80540 Phone: (303) 917-1247 <br />E-Mail: dennis@mcgranewater.com Web: ttp://www.mcgranewaterengineering.com <br />The predicted depth to water of 9.4 feet at the Harrell well is not a concern because he does not <br />have a basement (per J&T Consulting 12-2-19). However, BAI should continue to monitor the <br />Harrell well after the additional slurry walls are installed to validate the accuracy of the model. <br />Model Sensitivity <br />The modeled mound and drawdown impacts are insensitive to the permeability of the aquifer, <br />referred to as the aquifer hydraulic conductivity (K). K is calculated by dividing the <br />transmissivity (T) by the aquifer thickness. An increase in K causes a proportional increase in <br />model inflows, but also increases the hydraulic connection with the Poudre River which offsets <br />any additional mounding or shadow effects. Therefore, we would not expect any different results <br />if the aquifer permeability were different to what we used in the model. <br />The model results are likely very sensitive to the presence of the Poudre River, but there is no <br />realistic chance that the Poudre river will cease flowing due to strict river administration by the <br />State. Model results are insensitive to streambed leakance due to the relatively large distance <br />that the Derr pit is away from the river. Even using a conservatively low Ksb value of 36 ft/day, <br />the Poudre river bottom is sufficiently permeable to quickly respond to changes in groundwater <br />levels caused by slurry walls. <br />Model Uncertainty <br />There is error in the predictions of any groundwater model. Models include three types of error: <br />1) conceptual error (how the model is set up and what boundary conditions are used); 2) <br />parametric error (how aquifer properties are measured and calculated); and 3) predictive error <br />(which includes other influences such as seasonal recharge or climate change variations). It was <br />beyond the scope of this project to quantitatively evaluate how the sum of these errors could <br />affect the accuracy of our predictions. However, we feel the model is accurate because: <br />The model input data sets including aquifer parameters and water levels were carefully <br />created using the most recent data and historical research, modeling and USGS reports; <br />The aquifer boundary conditions and model conceptualization are simple and intuitively <br />reasonable; <br />We spent a considerable time “calibrating” the model water levels to measured values; <br />The results were consistent with other vicinity models created for other clients, and <br />The results are insensitive to a wide variation to input parameters. <br />Conclusions <br />We conclude: <br />The model was constructed after considering all available hydrogeologic data. <br />The model is “calibrated” to private well data located near the vicinity of the Loloff, and <br />Derr pits as shown by Figure 8 and in the calibration plot above. <br />Table 2 shows that if the Loloff and Derr slurry walls are installed, then ten (10) wells <br />could be impacted. The maximum increase in water levels to upgradient wells is