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2020-09-09 CC Packet
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2020-09-09 CC Packet
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CHAPTER TWO <br />WELLHEAD PROTECTION AREA AND DRINKING WATER SUPPLY MANAGEMENT <br />AREA DELINEATION (4720.5205) <br />Physical setting and subsurface hydrogeology– The Centerville municipal wells draw water <br />fromthe Prairie du Chien and Jordan aquifers. Groundwater flow and the hydrogeologic setting <br />are described in City of Centerville Wellhead Protection Plan, Part 1 (MDH, 2001).A discussion <br />of hydrogeologic conditions across the full model domain is presented in the reportTwin Cities <br />Metropolitan Area Regional Groundwater Flow Model Version 3.00(Metropolitan Council, <br />2014). <br />Delineation of the Wellhead Protection Area <br />Porous FlowDelineation Method <br />The delineation of the porous-flow aquifercapture zones was conducted utilizing the <br />modified Metropolitan Model. Original model construction detail, data files, and calibration <br />results are outlined in the Metropolitan Council report (2014). Modifications to the model <br />included: <br />Refinement of the model grid was doneto reduce model grid size from500 x <br />500 meters to 7.81 x 7.81 meters around the Centerville municipal wells. <br />Updating modeled flow rates for Centerville municipal well to match <br />wellheadprotection rule requirements. Modeled rates are shown in Table 2. <br />Updating average modeled flow rates for nearby high capacity wells (both <br />2012 to 2016. <br />municipal and private) to reflect the period from <br />The Prairie du Chien (layer 3) parameters around the Centerville wells was <br />adjusted to match local values, as determined fromnearby aquifer tests. <br />Horizontal hydraulic conductivity was adjusted to 147 ft/day (44.8 m/day) in the <br />Prairie du Chien aquifer. Additional model runs were completed with a 50% <br />increase and 50% decrease of hydraulic conductivity as part of the uncertainty <br />analysis. <br />A global porosity of 0.056 was chosen to represent the Prairie du Chien aquifer. <br />The Jordan aquifer was modeled with a porosity of 0.20. <br />The thickness of the Prairie du Chien aquifer was changed to 102 feet around the <br />Centerville wells to reflect the observed values. The thickness ofthe Jordan <br />aquifer was modeled as 89 feet. <br />Calibration of the modified Metropolitan Model was checked using target wells located in <br />Layer 3 (Prairie du Chien aquifer) and Layer 4 (Jordan aquifer). Calibration results are <br />presented in Appendix B for the area in the immediate vicinity of the City of Centerville. <br />Calibration results showed a Root Mean Squared Error of 3.47m and a calculated mean <br />deviation of -1.96. Calibration results are consistent with the original Metro Model 3.00. <br />Thedelineations were created using particle trackinganalysis in MODPATH. A circle of <br />particles was established at a radius of 15 meters from each wellhead and tracking <br />backwards(upgradient) with 0 particle lines per well. MODPATH was set to have four <br />release points between 0.2 and 1.0 offset. The path line groupings were then outlined to <br />develop capture zonesfor 1-year and 10-year time-of-travel delineations. <br />Figure 1 shows the outlined result of the delineation effort. <br />9 <br /> <br />
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