Agenda - Council Work Session - 09/13/2022

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5.1.8.1 5.2 were subtracted from the baseline model. Results from this calculation are shown in Figure 13 in Appendix D. This Figure depicts the cone of depression created on June 12, 2019. To check accuracy of the model results Well 3 a non -pumping observation well saw water levels drop approximate 12-15 feet from baseline conditions, essentially matching modeled results. The third model run adjusted June 12, 2019 pumping wells to be increased to projected 2040 demand. The resulting cone of depression is depicted on Figure 14 in Appendix D. Results indicated almost double the drawdown depicted from the second model run. Well 3 was again input as a non -pumping observation well and would observe 40 feet of drawdown under these conditions. Model Calibration A qualitative evaluation of the calibration can be made by comparing the simulated potentiometric surface (Figure 12 in Appendix D) with observed water level targets obtained from the MWI database and Minnesota Department of Natural Resources Potentiometric Surfaces (Figure 7 and Figure 9 in Appendix D). Upon review, the calibrated flow model generally captures the major features of the groundwater flow system along with the elevation, shape, magnitude, and gradient of the MWI database observed flow field. A quantitative measure by which to evaluate the success obtained during calibration is to compare the root mean square of the residuals (RMS) and the maximum observed head difference of the calibration dataset. The calibration dataset included water level information from wells in an approximate 16-mile radius of the city's wells. The root mean square residual of the calibration for layers 6 and 7 for the model was approximately 5.15 meters with a Normalized Root Mean Squared of 5.0 percent. It is noted that this error is less than the calibration target of 15 percent (Anderson et al., 2015). Groundwater Modeling Conclusions The Source water aquifer that the City of Ramsey currently utilizes is a deep -confined aquifer comprised of two geologic units, the Tunnel City and Wonewoc Aquifers. Throughout the region, numerous other unconsolidated and bedrock aquifers exist along with substantial surface water bodies such as the Mississippi River. Overall, the area surrounding the Twin Cities has substantial surface and groundwater resources to support present and long term portable water. At present, there is no reason to assume that the current source water aquifer for the City of Ramsey will not be able to supply potable water for the foreseeable future. The City of Ramsey's source water aquifer and wells are able to meet present day demand and appear to have a noticeable but temporary radius of influence on the surrounding aquifer. The wells are able to support high pumping rates with specific capacity showing acceptable drawdown alongside the aquifer's ability to recharge to static levels within a day of pumping. The City of Ramsey will need to balance water demand with drawdown to meet Minnesota Department of Natural Resources drawdown thresholds described in MN Rule 6115.0630 Definitions Subps.15 and 16. Two thresholds are in place and regulate that wells must not drawdown MnDNR assigned static water levels to within 50-percent and 25-percent to the top of aquifer. These threshold values are set by a MnDNR observation well and would typically be enforced if long term issues are observed. Thresholds for the City of Ramsey could become a concern if there is extended pumping within a single well or pumping by multiple wells in close proximity. FEASIBILITY STUDY RAMSY 154354 Page 24