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The vulnerability ofCenterville Well Nos. 1 and 2 (Unique Well Nos. 511091 and 512748, respectively) was determined by evaluating available information on the 1) geology, 2) well construction, and 3) chemical and isotopic composition of the well water and comparing these results with the criteria in Minnesota Rule 4720.5550. This process was automated using the well vulnerability database at the MDH. The vulnerability of the drinking water supply management area for Centerville 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. Well vulnerability assessment - The MDH has developed a database of community and non- community, nontransient public water supply wells in Minnesota which stores information pertinent to well vulnerability and rates the vulnerability of individual wells. A score is 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 40 and below) is used to identifY vulnerable from nonvulnerable wells (MDH, 1993). 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 results of this assessment for the Centerville wells are described below. Printouts from the MDH vulnerability database are shown in Appendix D. On the strength of the review criteria mentioned above, Centerville Well No.1 is considered not vulnerable to contamination from activities at the land surface. At this well, this determination is based on a sequence oflow-permeability protective materials above the aquifer in which the well is completed, no chemical data indicating influence by land surface activities, and documentation of sound construction. In addition, water samples from Well No.1 were subjected to a tritium age-dating analysis. The tritium test results indicate the water from the well, and presumably water in the aquifer in the vicinity of the wells, entered the ground prior to about 1953. Wells that produce water with non-detectable levels of tritium are generally not considered susceptible to anthropogenic contamination. Well No.2, on the other hand, is considered vulnerable to contamination from activities at the land surface. Water samples from Well No.2 contained tritium (when collected in 1997), which is an indication that water pumped from the well entered the ground after about 1953. Well No.2 is completed in the Prairie du Chien Aquifer system, which is covered by about the same amount of protective geologic material as Well No.1. The fracture flow dominated flow system in the Prairie du Chien differs from the Jordan by having a lower porosity and having a closer hydraulic connection with overlying materials. The combination of these two factors may provide for a mechanism by which the well captures water with tritium. Drinking Water Supply Management Area Vulnerability Assessment - The vulnerability of land parcels located within the DWSMA for the Centerville wells was evaluated primarily on the basis of 1) geologic logs from wells located in the vicinity of the well fields and 2) regional studies. The Centerville city wells draw from two different geologic units. While each of the units are covered by low permeability deposits that are considered confining units, the Prairie du Chien produces water with tritium and the Jordan does not. The confining units are present throughout the area of the DWSMA for Well Nos. 1 and 2. Accordingly, the DWSMA for Well No.1 is considered nonvulnerable and the DWSMA for Well No.2 is considered vulnerable. The two DWSMAs are shown on Figure 1, and the smaller one, near Well No.2, is nonvulnerable. 7