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CHAPTER TWO WELLHEAD PROTECTION AREA AND DRINKING WATER SUPPLY MANAGEMENT AREA DELINEATION (4720.5205) Physical setting and subsurface hydrogeology– The Centerville municipal wells draw water fromthe Prairie du Chien and Jordan aquifers. Groundwater flow and the hydrogeologic setting are described in City of Centerville Wellhead Protection Plan, Part 1 (MDH, 2001).A discussion of hydrogeologic conditions across the full model domain is presented in the reportTwin Cities Metropolitan Area Regional Groundwater Flow Model Version 3.00(Metropolitan Council, 2014). Delineation of the Wellhead Protection Area Porous FlowDelineation Method The delineation of the porous-flow aquifercapture zones was conducted utilizing the modified Metropolitan Model. Original model construction detail, data files, and calibration results are outlined in the Metropolitan Council report (2014). Modifications to the model included: Refinement of the model grid was doneto reduce model grid size from500 x 500 meters to 7.81 x 7.81 meters around the Centerville municipal wells. Updating modeled flow rates for Centerville municipal well to match wellheadprotection rule requirements. Modeled rates are shown in Table 2. Updating average modeled flow rates for nearby high capacity wells (both 2012 to 2016. municipal and private) to reflect the period from The Prairie du Chien (layer 3) parameters around the Centerville wells was adjusted to match local values, as determined fromnearby aquifer tests. Horizontal hydraulic conductivity was adjusted to 147 ft/day (44.8 m/day) in the Prairie du Chien aquifer. Additional model runs were completed with a 50% increase and 50% decrease of hydraulic conductivity as part of the uncertainty analysis. A global porosity of 0.056 was chosen to represent the Prairie du Chien aquifer. The Jordan aquifer was modeled with a porosity of 0.20. The thickness of the Prairie du Chien aquifer was changed to 102 feet around the Centerville wells to reflect the observed values. The thickness ofthe Jordan aquifer was modeled as 89 feet. Calibration of the modified Metropolitan Model was checked using target wells located in Layer 3 (Prairie du Chien aquifer) and Layer 4 (Jordan aquifer). Calibration results are presented in Appendix B for the area in the immediate vicinity of the City of Centerville. Calibration results showed a Root Mean Squared Error of 3.47m and a calculated mean deviation of -1.96. Calibration results are consistent with the original Metro Model 3.00. Thedelineations were created using particle trackinganalysis in MODPATH. A circle of particles was established at a radius of 15 meters from each wellhead and tracking backwards(upgradient) with 0 particle lines per well. MODPATH was set to have four release points between 0.2 and 1.0 offset. The path line groupings were then outlined to develop capture zonesfor 1-year and 10-year time-of-travel delineations. Figure 1 shows the outlined result of the delineation effort. 9