In 1986, Congress amended the Safe Drinking Water Act (SDWA) requiring the U.S. Environmental Protection Agency (USEPA) to promulgate disinfection requirements for all public-water supplies.  The USEPA promulgated the Surface Water Treatment Rule (SWTR) in June 1989 to establish disinfection requirements for surface-water supplies and for ground water under the direct influence of surface water.  To complete the requirements of the SDWA, the USEPA plans to propose and promulgate disinfection requirements for ground water not under the direct influence of surface water to protect the public health of persons served by those systems (U.S. Environmental Protection Agency, written commun., 1992).

Preliminary investigations by the Department of Health and Human Services, Centers for Disease Control, and the USEPA indicate that illnesses related to drinking water may be more prevalent than previously reported.  From 1971 to 1979, an estimated 57,970 persons in the United States were involved in waterborne disease outbreaks (Craun, 1986).   The consumption of contaminated ground water was responsible for 45 percent of all reported cases of waterborne disease in the United States (Gerba, 1988). As many as 65 percent of the documented outbreaks of waterborne disease in the United States could be attributed to illness of probable viral etiology (Keswick and Gerba, 1980).

More than 120 different types of potentially harmful enteric viruses are excreted in human feces, and they are widely distributed in type and number in domestic sewage, agricultural wastes, and septic drainage systems (Gerba, 1988).  The health significance of these viruses in humans ranges from poliomyelitis (polio), hepatitis, and gastroenteritis to innocuous

Missouri is widely dependent on ground water as a source of drinking water for its public-water systems, businesses, farms, and rural homes.  Ninety-five percent of the public-water systems in the State depend on ground water, and about 3,700 public-water-supply (PWS) wells are located within the State (fig. 1).  Historically, water provided from the deep bedrock aquifers in the Ozark Plateaus (most of the southern one-half of Missouri) generally has been free of microbiological contamination.  Years of bacteriological monitoring has confirmed that water drawn from properly constructed wells in this area are free of indicator bacteria (Kenneth Duzan, Missouri Department of Natural Resources, written commun., 1997).

The U.S. Geological Survey (USGS), in cooperation with the Missouri Department