Pennsylvania has a large population that relies on private water wells, but no statewide regulations govern the location or construction of the wells. Bacterial contamination occurs in nearly half of these water supplies, representing a health risk to rural residents. The role of large-scale aquifer contamination versus wellhead contamination from poor construction is poorly understood. The authors disinfected and installed sanitary well caps on 16 wells that contained coliform bacteria and retested them after 30-60 days and one year. Coliform bacteria were present in seven of the 16 wells within 30-60 days and all but two wells within one year. The occurrence and prevalence of bacteria in wells appeared to be influenced by weather conditions during the study. The few wells where disinfection and installation of a sanitary well cap were successful had low initial coliform bacteria and no E. coli. Samples were also collected from 24 new wells that were constructed with a grout seal and sanitary well cap. Twenty-nine percent contained coliform bacteria, and 17 percent contained E. coli. The results from this limited number of wells suggest that bacterial contamination of private wells in Pennsylvania may often originate from areas away from the wellhead.