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EXECUTIVE SUMMARY This report documents the amended delineation of the wellhead protection area (WHPA) and drinking water supply management area (DWSMA) and the vulnerability assessments for the wells and DWSMAfor the City of Centerville (PWSID 1020036) drinking water supply wells (Table 1). The amendedplan covers in Table 1 . Well logs are presented in Appendix A. The delineationwas performed in accordancewith rules (Minnesota Rules 4720.5100 to 4720.5590) for preparing and implementing wellheadprotection measures for public water supply wells. The rules are administeredby the MinnesotaDepartment of Health (MDH), and the results described in this report were prepared by Stantec. The City of Centerville currently obtains its drinking water supply from one primary well completed inthe Prairie du Chien aquifer, with one emergency backup well completed in the Jordan aquifer. TheWHPAand DWSMA delineations and vulnerability assessments for the City’s wells was initially prepared in May 2001 by the MDH in the report City of Centerville Wellhead Protection Plan,Part 1.Since the time that plan was completed, the Minnesota Department of Health (MDH) has updated their guidance for delineating wellhead protection areas in setting where bedrock aquifers (suchas the Prairie duChien) are fractured and can potentially rapidly transmit water (and contaminants).Also, the 10-year delineation update deadline has passed, necessitating an update to the City’s wellhead protection delineations. As such, a full update is being undertaken to bring the Part 1 plan up to date. Acomputer groundwater modeling platform was utilized for this project. The porous-flow portion ofthe wellhead protection areas (WHPAs) for Centerville’s wells was delineated using a modified versionof the Metropolitan Councils Metro Model 3, released in 2014. This model is a steady state MODFLOWmodel built using Groundwater Vistas. The model was updated to reflect current pumping and geological conditions in and near the area around the Centerville well field. The modelwas used to delineate one-year and ten-year porous-flow capture zones for Centerville (Figure 1). A fracture flow analysis was then undertaken using current MDH methodology to predict the area of the Prairie du Chien aquifer that is capable of rapidly transmittingwater to the City’s primary water supply well (Well 2). This fracture flow delineation was created by utilizing MDH guidance developed for fracture flow settings. The calculated area is shown in Figure 2. Theporous-flow and fracture-flow areas calculated werecombined to create the composite wellheadprotection area (WHPA). The drinking water supply management area (DWSMA) was determined for the WHPA by using property parcels and roadways as boundaries. Figure 3 shows the boundaries of both the WHPA and the DWSMA. The amount of geologic protection documented in well logs from the water supply wells and regionalinformation,along with water quality information was used to determine well vulnerability.Centerville’s primary well (Well 2) is considered vulnerable to contamination, with elevated tritium levelsbeing the main reasons for the vulnerable classification.Tritium is harmless in water, but indicatesthat some portion of the water is less than 60 years old. Centerville’s wellsappear to meet the constructionstandards of the State Well Code, however, and the wells themselves are not considered a likely avenue for contamination to reach the aquifer from which they pump. 3