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<br />water levels and, hence, groundwater flow. Accordingly, wells from the DNR's SWUD database <br />are included throughout the model domain. Seaberg and Hansen (2000) present a detailed <br />discussion of the conceptual model upon which the Ramsey-Anoka-Washington County bedrock <br />model is based and, in addition, provide a detailed summary of how the model input was <br />assembled. <br /> <br />The aquifer parameters used in the groundwater flow model are as follows: <br /> <br />Prairie du Chien Group (Laver 3) <br />Base elevation: <br />Hydraulic conductivity: <br />Thickness: <br />Porosity: <br /> <br />165 meters <br />12 meters/day <br />40 meters <br />0.09 <br /> <br />Jordan Sandstone Aquifer (Laver 5) <br />Base elevation: 130 meters <br />Hydraulic conductivity: 12 meters/day <br />Thickness: 30 meters <br />Porosity: 0.30 <br /> <br />Aquifer boundary conditions (including recharge and discharge features), distribution of <br />hydraulic conductivity changes, and definition of leaky areas around wells are patterned after the <br />MPCA Metro Model (Seaberg and Hansen, 2000) and have been refmed as described in EOR <br />and KDB Consulting (2000). <br /> <br />The delineations were done using particle tracking analyses starting near the wells of interest and <br />tracking backward (upgradient). This analysis was conducted using starting elevations near the <br />base of the aquifer and also near the top. The results of these analyses are shown in Figure 1. <br /> <br />Uncertainties relating to the accuracy ofthe calculated wellhead protection area <br />boundaries - The main source of uncertainty that had to be managed in making the WHPA <br />delineation analysis had to do with the nature of the hydraulic interaction between the Prairie du <br />Chien and the Jordan. For the purposes of this analysis, it was assumed the two units are <br />hydraulically separate locally in the Centerville area, a conclusion supported by the water <br />chemistry data. <br /> <br />Delineation of the DWSMA - The drinking water supply management area is shown on Figure <br />1. This area includes all of the wellhead protection areas and was delineated using parcel <br />boundaries and transportation corridors. <br /> <br />CHAPTER THREE <br />VULNERABILITY ASSESSMENT <br /> <br />This chapter documents the vulnerability assessments of the wells and drinking water supply <br />management area for the public water supply system operated by the City of Centerville. This <br />assessment was performed in accordance with rules (Minnesota Rule 4720.5210) for preparing <br />and implementing wellhead protection measures for public water supply wells. <br /> <br />6 <br />