2023-05-25_PERMIT FILE - M2022048 (16)

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L.G. Everist West Farm Groundwater Modeling model is sufficiently calibrated to be used for predicting water levels after construction of the slurry walls and following mitigated conditions with a drain. PREDICTIVE SIMULATIONS Using the steady state model for pre-slurry wall condition as the base model, predictive simulations were performed for the following conditions: 1. Groundwater mounding after the slurry walls were constructed. 2. Mitigated groundwater mounding with an underdrain along a portion of the south property boundary. Predicted Unmitigated Groundwater Mounding Groundwater mounding was predicted for the reclaimed condition representing the proposed slurry walls. To understand the magnitude and extent of potential groundwater mounding upgradient of the slurry walls, a steady state simulation including slurry walls was performed. The pre-slurry wall model was changed by inputting the slurry wall as a no-flow boundary. All other aquifer parameters and boundary conditions were unchanged. Initial heads were the model simulated heads from the pre-slurry wall steady state model. The steady state model for the post-slurry wall conditions produced higher groundwater elevation heads than those produced for the pre-slurry wall steady state condition. The difference between these two groundwater surfaces are the predicted mounding levels shown on Figure 2. For the predictive simulation, positive residuals are reported as values of groundwater mounding (warm colors) and negative values represent groundwater shadowing (cool colors). The magnitude of the maximum groundwater mounding is approximately 3 feet upgradient in the southwest corner of the model area. The magnitude of the maximum groundwater shadowing is almost 1 foot, downgradient of the proposed slurry wall. Due to the terrace on the west, the change in topography on the site is significant, so the location of highest groundwater mounding is not where the groundwater is closest to the surface. Because of the terrace and change in topography across the site a figure (Figure 3)showing the depth to groundwater from the surface was created. The groundwater is closest to the surface (approximately 3 feet) in the southeast corner of the model, near the South Platte River. Predicted Mitigated Groundwater Mounding with an Underdrain The predictive model showing groundwater mounding for the reclaimed conditions was changed by adding a drain along a portion of the southern property boundary. The drain would most likely be a slotted PVC pipe surrounded by washed gravel bedding. The beginning elevation of the drain in the model represents the approximate elevation of the unmitigated groundwater mound upgradient of the property. The ending elevation is based off a drain slope of 0.25%. These elevations are approximate and set higher than measured groundwater levels in the monitoring wells. The modeled drain begins near the toe of the slope of the terrace and discharges to the east into the South Platte River. The actual design of the drain, if deemed necessary, may differ from these parameters and would go through any required permitting process. The simulated drain was given the following parameters: • Drain width, w= 1 foot(based on a 1-foot wide trench filled with a gravel bedding) • Drain bed thickness, dl= 1 foot • Hydraulic conductivity, K= 500 feet per day (same as aquifer K, which is assumed to be smaller than gravel bedding K and therefore controlling) • Drain conductance= 75,500 square feet per day, C=K*(di/w)*L Project 20C26026.02/May 19,2023 Page 5 Deere&Ault,a Schnabel Engineering Company