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contaminants must become more proactive so that a broader range of feasible response options can be developed.

In the United States, regulation of drinking water contaminants has increased rapidly in the past decade since 1990. However, enforceable regulations were first instituted in 1914 to reduce the spread of disease across state lines. Under the Interstate Quarantine Act of 1893, municipalities that used their water in interstate carriers (e.g., buses, ships, and trains) were required to meet a federal regulation for fecal coliform bacteria. In 1925, 1942, 1946, and 1962, the federal Public Health Service added more contaminants to this regulation. Evidence from the late 1960s and early 1970s indicated that many previously undetected contaminants were in public water supplies and posed potential human health effects. Following the publication of these environmental data and Silent Spring (Carson, 1962), public awareness and concerns increased. Although most states adopted the earlier federal standards for the regulation of local water supplies, federally mandated regulations for large-scale water supplies did not exist until 1974.

Before the Safe Drinking Water Act (SDWA) was passed in 1974, most drinking water contaminants were controlled through guidelines and non-enforceable mechanisms. A major goal of the SDWA was to protect public health by ensuring that contaminants in public water systems met national standards. Environmental surveys following passage of the SDWA revealed the presence of additional contaminants. As a result, in 1986 Congress required that 25 more substances (chemicals and pathogens) be regulated every three years. This requirement proved to be unachievable, especially without new resources. The lessons learned from the efforts to meet these requirements led to a Congressional mandate to regulate contaminants on the basis of occurrence in water, level of risk posed, and the probability of adverse health effects (NRC, 1997).

For most of the twentieth century, the United States identified hazardous substances and microbial pathogens in drinking water through investigations of disease outbreaks. Although this approach has been recognized as limited (Balbus and Embrey, 2002), it has been the traditional method used by epidemiologists. In 1970, the U.S. Environmental Protection Agency (EPA) was charged with controlling hazardous substances in drinking water and other media (United States Code Annotated, 1970). In response, the agency began developing procedures to identify and prioritize chemical contaminants. A standardized chemical risk assessment framework published in 1983 was used on a substance-specific basis (NRC, 1983). However, this process proved to be too resource intensive and time-consuming to prioritize the increasing number of contaminants needed for research and regulatory purposes.

In the mid-1980s, EPA reported that the classification of substances into categories based on the weight of scientific evidence was an important part of the process of setting public health priorities and began developing more efficient ways to do so (EPA, 1987). In 1996, Congress directed the EPA to develop



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