has completed more than 27,000 preliminary assessments, and more than 9000 sites have been investigated in detail (EPA, 1988). As of June 1988, EPA's National Priorities List (NPL), included 1236 sites, about 30 percent of which have had initial actions to reduce immediate threats. The number of identified sites represents a small proportion of the sites that are expected to be identified in the future (OTA, 1989).

HAZARDOUS-WASTE SITES

Within the past decade, estimates of the number of potential NPL sites have shifted dramatically. The Office of Technology Assessment (OTA, 1989) concludes that there could be as many as 439,000 candidate sites. This is more than 10 times that estimated earlier by EPA. These sites include Resource Conservation and Recovery Act (RCRA) Subtitle C and D facilities, mining waste sites, underground leaking storage tanks (nonpetroleum), pesticide-contaminated sites, federal facilities, radioactive release sites, underground injection wells, municipal gas facilities, and wood-preserving plants, among others.

One recent EPA survey found that more than 40 million people live within four miles and about 4 million reside within one mile of a Superfund site. Residential proximity does not per se mean that exposures and health risks are occurring, but the potential for exposure is increased. As of December 1988, the Agency for Toxic Substances and Disease Registry (ATSDR) concluded that 109 NPL sites (11.5 percent) were associated with a risk to human health because of actual exposures (11 sites) or probable exposure (98 sites) to hazardous chemical agents that could cause harm to human health. These NPL sites were listed in the categories of “urgent public health concern” or “public health concern.”

The states with the largest number of NPL sites are New Jersey, Pennsylvania, California, Michigan, and New York. They accounted for 464 of 1236 (37.5 percent) sites as of 1991 (Figure 3-1). The activities associated with these sites are shown in Table 3-1. Figure 3-2 depicts the observed contamination of various media as a percentage of 1189 final sites on the NPL as of February 1991. Note that a site can have more than one type of contamination.

Data derived from the 951 ATSDR health assessments at hazardous-waste sites indicate the existence of more than 600 different chemical substances. Some of them are listed in Table 3-2. The documented migration of substances into water, soil, air, and food also is listed in Table 3-2. Most of the identified agents are toxic and represent potential threats to the public health, depending on the degree of expo-



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