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

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L.G. Everist West Farm Groundwater Modeling Calibration Targets and Goals The calibration targets for the model include the measured groundwater elevations observed in seven monitoring wells measured at the end of August 2022. The calibration goal is to adjust model inputs such that the steady state groundwater heads for the pre-slurry wall conditions are similar to measured heads in the field. Of the seven wells, two have been installed upgradient from the proposed slurry wall (Wells MW 07 and MW-06). Therefore, these two wells are the best targets for the predictive mounding simulations. Although well MW-06 is located inside the slurry wall, a surrogate target was used just south of MW-06, outside the proposed slurry wall. This target was modeled with the same head value as MW-06. These two targets (Wells MW 07 and MW 06) were used to estimate groundwater mounding upgradient of the proposed slurry walls. The goals of the predictive simulation targets are: 1) to show how field measured groundwater heads differ from those in the predictive simulations, 2)to show how pre-slurry wall groundwater heads differ from those in the predictive simulations, and 3) to compare post-mitigation heads to pre-mitigation mounded groundwater heads. Boundary Conditions The boundary conditions used for the model include three constant head boundaries and one no-flow boundary. Specified groundwater head values(elevations) from the input groundwater surface were assigned to the constant head boundaries and do not vary during the simulations. Two constant head boundaries are present at the southern and northern ends of the model domain and a third one at the South Platte River. The no-flow boundary is the hydraulic divide between the South Platte basin and the Saint Vrain Basin. This boundary is based on topography which divides the two drainage basins and roughly follows the Meadow Island Ditch. These boundaries are shown on Figures 2 through 5. The proposed slurry walls are also modeled as no-flow boundaries. These boundary conditions define the sources and sinks for the water budget of the model. The sinks in the model are the northern boundary where alluvial underflow leaves the model, and the South Platte River constant head boundary. The water source into the model is alluvial flow through the southern boundary. No other recharge sources are included in the model, as precipitation is considered a negligible source within the domain and irrigation records were not reviewed. The system is assumed to be in equilibrium under pre-slurry wall conditions. The model domain is inactive outside of the defined boundary conditions. This includes the cells within the slurry wall no-flow boundaries. CALIBRATION The model is simple and homogeneous, containing heads that are well constrained by measured values for boundary conditions as well as a reasonable estimate of hydraulic conductivity. The calibration targets used for the pre-slurry wall condition steady state model illustrate that the input groundwater heads are generally within two feet of measured heads where mounding is expected. Monitoring wells MW 01, MW-4 and MW 7 showed a residual of less than one foot. The mass balance reported by MODFLOW for the steady state pre-slurry wall model is as follows: Inflows =236,094.43cfd Outflows =236,091.98 cfd Difference =2.45cfd (0.001%) This illustrates that the initial steady state model is accurately solved. Because the pre-slurry wall groundwater table represents data from measured groundwater levels, and the mass balance is accurate, this suggests the Project 20C26026.02/May 19,2023 Page 4 Deere&Ault,a Schnabel Engineering Company