7A - Wellhead Protection Plan Update

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Chapter 3 - Delineation of the Wellhead Protection Area, Drinking Water Supply Management Area and Vulnerability Assessments A detailed description of the process used for 1)delineating the WHPA and the DWSMA,and 2)preparing the vulnerability assessments of the city water supply well(s)and DWSMA is presented in the first part of the WHP Plan (See Appendix I). The City of Pequot Lakes requested that MDH do this work and it was performed by Gail Haglund, a licensed geoscientist by the State of Minnesota. 3.1 WHPA and DWSMA Delineation Figure 1 shows the boundaries of the WHPA and the DWSMA. The WHPA was delineated using computer simulations of groundwater movement to generate the underground capture zones for city Wells 3 (Unique No. 241514)and 4(Unique No. 626480). The DWSMA boundaries were designated using the following criteria: • Center-lines of highways, streets,roads; • Public Land Survey coordinates; • Property lines/parcel boundaries; and • Political boundaries. 3.2 Well Vulnerability Assessment Well construction information and water quality data obtained for each well used by the City of Pequot Lakes is considered and evaluated in the assessment of well vulnerability. The city wells are considered vulnerable in part because of differences found in the well drillers logs for wells 3 and 4 (the drillers log for well 4 describes the till material as having a larger component of sand than the log for well 3)and previous tritium results for well 4 indicating that the aquifer contains a measureable component of post 1953 water or more recent recharge. 3.3 DWSMA Vulnerability Assessment The moderate vulnerability assigned to the DWSMA(Figure 1)was determined using geologic, soils, and groundwater chemistry information. Existing well logs indicate the aquifer is covered by a layer of clay rich till varying in thickness from< 10 feet to more than 50 feet. Similarly,the tritium detected in well 4 indicates there is a component of young water recharging the city wells so a moderate vulnerability was assigned to the DWSMA. (See Appendix 1 for the Part I Wellhead Protection Report for further information on the vulnerability assessment for the City of Pequot Lakes wells and aquifer.) 5