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2020-09-09 CC Packet
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2020-09-09 CC Packet
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EXECUTIVE SUMMARY <br />This report documents the amended delineation of the wellhead protection area (WHPA) and drinking <br />water supply management area (DWSMA) and the vulnerability assessments for the wells and <br />DWSMAfor the City of Centerville (PWSID 1020036) drinking water supply wells (Table 1). The <br />amendedplan covers in Table 1 <br />. Well logs are presented in Appendix A. The delineationwas performed in <br />accordancewith rules (Minnesota Rules 4720.5100 to 4720.5590) for preparing and implementing <br />wellheadprotection measures for public water supply wells. The rules are administeredby the <br />MinnesotaDepartment of Health (MDH), and the results described in this report were prepared by <br />Stantec. <br />The City of Centerville currently obtains its drinking water supply from one primary well completed <br />inthe Prairie du Chien aquifer, with one emergency backup well completed in the Jordan aquifer. <br />TheWHPAand DWSMA delineations and vulnerability assessments for the City’s wells was <br />initially prepared in May 2001 by the MDH in the report City of Centerville Wellhead Protection <br />Plan,Part 1.Since the time that plan was completed, the Minnesota Department of Health (MDH) has <br />updated their guidance for delineating wellhead protection areas in setting where bedrock aquifers <br />(suchas the Prairie duChien) are fractured and can potentially rapidly transmit water (and <br />contaminants).Also, the 10-year delineation update deadline has passed, necessitating an update <br />to the City’s wellhead protection delineations. As such, a full update is being undertaken to bring <br />the Part 1 plan up to date. <br />Acomputer groundwater modeling platform was utilized for this project. The porous-flow portion <br />ofthe wellhead protection areas (WHPAs) for Centerville’s wells was delineated using a modified <br />versionof the Metropolitan Councils Metro Model 3, released in 2014. This model is a steady state <br />MODFLOWmodel built using Groundwater Vistas. The model was updated to reflect current <br />pumping and geological conditions in and near the area around the Centerville well field. The <br />modelwas used to delineate one-year and ten-year porous-flow capture zones for Centerville <br /> (Figure 1). <br />A fracture flow analysis was then undertaken using current MDH methodology to predict the area of the <br />Prairie du Chien aquifer that is capable of rapidly transmittingwater to the City’s primary water supply <br />well (Well 2). This fracture flow delineation was created by utilizing MDH guidance developed for <br />fracture flow settings. The calculated area is shown in Figure 2. <br />Theporous-flow and fracture-flow areas calculated werecombined to create the composite <br />wellheadprotection area (WHPA). The drinking water supply management area (DWSMA) was <br />determined for the WHPA by using property parcels and roadways as boundaries. Figure 3 shows <br />the boundaries of both the WHPA and the DWSMA. <br />The amount of geologic protection documented in well logs from the water supply wells and <br />regionalinformation,along with water quality information was used to determine well <br />vulnerability.Centerville’s primary well (Well 2) is considered vulnerable to contamination, with <br />elevated tritium levelsbeing the main reasons for the vulnerable classification.Tritium is <br />harmless in water, but indicatesthat some portion of the water is less than 60 years old. Centerville’s <br />wellsappear to meet the constructionstandards of the State Well Code, however, and the wells <br />themselves are not considered a likely avenue for contamination to reach the aquifer from which <br />they pump. <br />3 <br /> <br />
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