2020-09-09 CC Packet - Laserfiche WebLink

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materials between the land surface and the aquifer. Wells with an L-score of 1 and above are considered to have low geologic sensitivity. Wells with an L-score of “0” require examination of other details to determine sensitivity, including looking for potential confining units that might be classified as “sandy clay” or look for bedrock layers that are confining, such as the basal St. Peter sandstone. In these cases, a “moderate” geologic sensitivity is assigned. Wells with no evidence of geologic confining units are assigned a “high” geologic sensitivity. Figure 4 shows the geologic sensitivity of wells mapped within the Centerville DWSMA. Of the 313 wells mapped in and around the DWSMA, 293 wells (or approximately 94%) were assigned a low geologic sensitivity. Moderate geologic sensitivity was assigned to 19 wells, with high sensitivity assigned to only one well. The findings of the L-score analysis suggest that the Centerville DWSMA is largely low in geologic sensitivity. A second aspect of the DWSMA vulnerability assessment is to look at water chemistry data for indicators of the aquifer being recharged by younger water. The tritium reading of 2.6 TU from Well 2 in 2011 indicates that younger water is reaching the Prairie du Chien aquifer in the vicinity of Well 2. Elevated tritium increases the designated vulnerability of the DWSMA. In this case, the “low” vulnerability based on geologic sensitivity is increased to “moderate” vulnerability based on the tritium results. Figure 4 shows the designated “moderate” vulnerability designation for Centerville’s DWSMA. The implications of the assigned vulnerability level will be addressed in greater detail in the amended Part 2 Wellhead Protection Plan. In general, moderate vulnerability rankings will necessitate a thorough inventory of all wells that penetrate the geologic confining layers, along with an inventory storage tanks that contain high concentrations of potential pollutants. C.Recommendations – Assessing vulnerability requires periodic sampling for age dating compounds and potential contaminants to ensure that the water meets established standards and that the designated vulnerability levels are appropriate. The following steps could help in establishing more accurate vulnerability designations for future plan updates: a.Examine possible avenues for younger water to reach the Prairie du Chien aquifer through Well 2. If Well 2 is rehabilitated over the course of the next ten years, it is recommended that a video camera survey be conducted, looking for possible casing breaches or leaks that could be allowing younger water to reach the aquifer. Video inspections are standard for many well rehabilitation projects. b.Request that the MDH collect their standard assessment monitoring package of water quality samples from Well 2 by the fifth year of plan implementation. This sampling includes tritium, nitrates, ammonia, sulfate, chloride, and bromide. This sample will help determine if the previous sample result remains consistent. 13