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CHAPTER THREE VULNERABILITY ASSESSMENT This chapter documents the vulnerability assessments of the wells and drinking water supply management area (DWSMA) for the City of Centerville wells listed in Table 1. This assessment was performed in accordance with rules (Minnesota Rule 4720.5210) for preparing and implementing wellhead protection measures for public water supply wells. The vulnerabilities of the wells were determined by evaluating available information on the 1) geology, 2)well construction, 3) pumping rates, and 3) chemical composition of the well water and comparing these results with the criteria in Minnesota Rule 4720.5550. The vulnerability of the DWSMA was determined by evaluating available information on 1) the lateral continuity of protective geologic materials overlying the aquifer and 2) the chemical and isotopic composition of well water from the aquifer. DWSMA vulnerability was calculated using a combination of the Anoka County Geologic Atlas, observed confining layers from well log data, and available geochemistry data. A.Well vulnerability assessment - A vulnerability score was calculated for each well based on factors such as well construction, geology at the well site, and chemical data; higher scores correlate to greater perceived vulnerability. A numeric cutoff (of 45 points) is used to identify vulnerable from non-vulnerable wells (MDH, 1997). Vulnerable wells are also identified based on the presence of contamination, such as nitrate-nitrogen in excess of 10 mg/l, or young (post- 1953) water, as indicated by the presence of 1 tritium unit or greater in the well water. The completed well vulnerability assessment worksheets are provided in Appendix C. Well 1, which is an emergency use well, was calculated to be non-vulnerable based on a tritium sample from 1997 that indicated tritium was below the detection limit of 0.8 TU. This indicates that water in Well 1 is more than 60 years old. This sample result overrode the worksheet score of 45 which would have normally resulted in a vulnerable classification. Well 2, the City’s primary well, was scored at 30 points, which would normally indicate a non- vulnerable well. However, the tritium sample indicated a concentration of 2.6 TU, showing that some portion of the water supplying Well 2 is relatively young. Therefore, the well was given a “vulnerable” ranking. There is nothing that was discovered in the well vulnerability assessment that indicates that the wells themselves are a likely avenue for contamination to reach the aquifer. The wells appear to meet construction standards set forth in the State Well Code. Well vulnerability in Centerville is mostly indicative of overall aquifer vulnerability. B.Drinking Water Supply Management Area Vulnerability Assessment - The vulnerability of groundwater underlying the land parcels located within the DWSMA for Centerville was evaluated primarily on the basis of the geologic sensitivity, as determined from the Anoka County Geologic Atlas (Minnesota Geologic Survey, 2013, and Minnesota Department of Natural Resource, 2016) and well records in County Well Index. Geologic cross-sections in the atlas that run east west through Centerville in the area of the DWSMA indicate of thick layer of till and sandy till overlying bedrock. For a refined look at confining layers, an L-score analysis was conducted to look for clay or other confining layers in local well logs, concentrating only on confining units that are above the Prairie du Chien aquifer. For each 10 feet of continuous confining geology, an L-Score of “1” unit is applied. A well with an L-score of 4, for example, has at least 40 feet of confining 12