Preservation and Survival of E. Coli in Well Water Samples Submitted for Routine Analyses

Total coliform testing to determine potability of drinking water has been performed routinely on Wisconsin well waters since the beginning of the 20th century. The data derived from this testing has driven the development and continuous improvement of the administrative codes that regulate well construction and maintenance. The quality assurance program associated with laboratory testing for coliforms establishes maximum sample holding times (APHA, 1998). For public water supply testing, these holding times are established by the United States Environmental Protection Agency (USEPA, 1989). The agency uses published data and expert opinion as the basis for setting the required holding times. Currently the holding time is set at 30 hours. In Wisconsin, the Laboratory of Hygiene (WSLH) did an extensive study that demonstrated coliforms actually survive quite well for up to 48 hours at ambient temperatures in typical Wisconsin well water samples (Standridge, 1983). Subsequent to this study, the USEPA granted an exception for the WSLH to allow compliance testing for total coliforms up to 48 hours after collection. This exception has permitted a high compliance rate for required testing of drinking waters, even from remote areas of the state where delivery to the laboratory can often take two days.