Agenda - Council Work Session - 09/13/2022

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5.1.8 Model The groundwater flow and cone of depression calculation for the City of Ramsey were determined using an existing regional MODFLOW model that was developed by Barr Engineering Company for the Metropolitan Council (Metro Council, 2014). MODFLOW is a 3D, cell -centered, finite difference, saturated flow model developed by the U.S. Geological Survey (McDonald and Harbaugh, 1988; Harbaugh et al., 2000). MODFLOW was developed by the United States Geological Survey and is publicly available. The specific software code used for this delineation was MODFLOW-2005 (Harbaugh, 2005). The program has been thoroughly documented, is widely used by consultants, government agencies, and researchers and consistently accepted in regulatory proceedings. MODFLOW is also an extremely versatile program capable of simulating groundwater flow in up to three dimensions while offering a variety of boundary condition options, confined or unconfined aquifer conditions and allowing for vertical discretization through the use of layering. The Metro Model consists of nine layers that represent the major aquifers and aquitards within the seven -county metropolitan area. These layers represent, from top to bottom (youngest to oldest), the following units: (1) surficial aquifer of glacial deposits; (2) St. Peter Sandstone or Quaternary Buried Artesian Aquifer; (3) Prairie du Chien Group; (4) Jordan Sandstone; (5) St. Lawrence Formation (aquitard); (6) Tunnel City Group; (7) Wonewoc Aquifer, (8) Eau Claire Formation (aquitard); and (9) Mt. Simon Sandstone. The regional groundwater model was calibrated to steady-state water levels and river base flows. Model parameter development and error is discussed in the Metro Model report. A local model limited to an approximate radius around the city limits was extracted from the regional seven -county model using telescopic mesh refinement with the Groundwater Vistas software. Constant and general head boundaries around the limits of the model along with wells, rivers and lakes, and infiltration, provided the model boundary conditions. The model grid was refined around the City of Ramsey wells. Variable grid spacing was used, ranging from approximately 2 meters near the City of Ramsey wells to approximately 500 meters at the edge of the grid. Prior to their use in the delineations, the following modifications were incorporated in the refined models: • Local areas of modified horizontal conductivity were included in the model. • The pumping rates for baseline (no pumping), maximum present-day use, and projected 2040 demand were inputted into scenarios of the model. To determine the water contours of the aquifer and the resulting cone of depressions multiple model runs using multiple flow rates were inputted into the city wells. Baseline conditions were established creating a model that input no pumping from the City wells. This represents static aquifer water levels without influence of the City wells. Water elevations from this baseline condition is depicted on Figure 12 in Appendix D. The results from this model run are verified and match MnDNR hydrogeologic atlas potentiometric surface predictions depicted in Figure 7 and Figure 9 in Appendix D. The second model run input pumping values from June 12, 2019 to predict the cone of depression caused from 8 hours of pumping 4 wells. The resulting head values from this model FEASIBILITY STUDY RAMSY 154354 Page 23