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2020-09-09 CC Packet
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2020-09-09 CC Packet
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materials between the land surface and the aquifer. Wells with an L-score of 1 and above are <br />considered to have low geologic sensitivity. Wells with an L-score of “0” require examination <br />of other details to determine sensitivity, including looking for potential confining units that <br />might be classified as “sandy clay” or look for bedrock layers that are confining, such as the <br />basal St. Peter sandstone. In these cases, a “moderate” geologic sensitivity is assigned. Wells <br />with no evidence of geologic confining units are assigned a “high” geologic sensitivity. Figure 4 <br />shows the geologic sensitivity of wells mapped within the Centerville DWSMA. Of the 313 <br />wells mapped in and around the DWSMA, 293 wells (or approximately 94%) were assigned a <br />low geologic sensitivity. Moderate geologic sensitivity was assigned to 19 wells, with high <br />sensitivity assigned to only one well. The findings of the L-score analysis suggest that the <br />Centerville DWSMA is largely low in geologic sensitivity. <br />A second aspect of the DWSMA vulnerability assessment is to look at water chemistry data for <br />indicators of the aquifer being recharged by younger water. The tritium reading of 2.6 TU from <br />Well 2 in 2011 indicates that younger water is reaching the Prairie du Chien aquifer in the <br />vicinity of Well 2. Elevated tritium increases the designated vulnerability of the DWSMA. In <br />this case, the “low” vulnerability based on geologic sensitivity is increased to “moderate” <br />vulnerability based on the tritium results. <br />Figure 4 shows the designated “moderate” vulnerability designation for Centerville’s DWSMA. <br />The implications of the assigned vulnerability level will be addressed in greater detail in the <br />amended Part 2 Wellhead Protection Plan. In general, moderate vulnerability rankings will <br />necessitate a thorough inventory of all wells that penetrate the geologic confining layers, along <br />with an inventory storage tanks that contain high concentrations of potential pollutants. <br />C.Recommendations – Assessing vulnerability requires periodic sampling for age dating <br />compounds and potential contaminants to ensure that the water meets established standards and <br />that the designated vulnerability levels are appropriate. The following steps could help in <br />establishing more accurate vulnerability designations for future plan updates: <br />a.Examine possible avenues for younger water to reach the Prairie du Chien aquifer <br />through Well 2. If Well 2 is rehabilitated over the course of the next ten years, it is <br />recommended that a video camera survey be conducted, looking for possible casing <br />breaches or leaks that could be allowing younger water to reach the aquifer. Video <br />inspections are standard for many well rehabilitation projects. <br />b.Request that the MDH collect their standard assessment monitoring package of water <br />quality samples from Well 2 by the fifth year of plan implementation. This sampling <br />includes tritium, nitrates, ammonia, sulfate, chloride, and bromide. This sample will <br />help determine if the previous sample result remains consistent. <br />13 <br /> <br />
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