Under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), which created the Superfund program in 1980, the Environmental Protection Agency (EPA) assesses uncontrolled hazardous waste sites and places those posing the greatest risks to human health and the environment on the National Priorities List (NPL) for cleanup. As of September 1995, this list included 1,232 sites. Cleanup standards and the degree of cleanup needed for Superfund sites are discussed in section 121(d) of the CERCLA statute, as amended by the Superfund Amendments and Reauthorization Act of 1986 (SARA). This section requires that Superfund sites be cleaned up to the extent necessary to protect both human health and the environment. In addition, cleanups must comply with requirements under federal environmental laws that are legally “applicable” or “relevant and appropriate” (ARAR) as well as with such state environmental requirements that are more stringent than the federal standards. Furthermore, Superfund cleanups must at least attain levels established under the Safe Drinking Water Act and the Clean Water Act, where such standards are relevant and appropriate as determined by the potential use of the water and other considerations. The federal standards most frequently considered relevant and appropriate for groundwater cleanups at Superfund sites are set under the Safe Drinking Water Act. This act establishes standards, called maximum contaminant levels (MCL), for certain contaminants in water delivered by public drinking water systems. As of March 1996, the MCLs included numeric limits on about 70 contaminants. The MCLs take into account estimates of the human health risks posed by contaminants. They also consider whether it is technically and economically feasible to reduce the contamination to a level that no longer poses a health risk. Although MCLs are legally applicable to drinking water systems, section 121(d) of CERCLA generally requires that they be considered relevant and appropriate standards for cleaning up contaminated groundwater that is a potential source of drinking water. For example, the MCL for benzene is 5 micrograms per liter. This concentration would generally be the cleanup level for benzene in groundwater that is a potential source of drinking water unless the state has promulgated a more stringent standard or other requirement that is relevant and appropriate. There are few federal standards for contaminants in soil that are considered potentially applicable or relevant and appropriate except those for certain highly toxic contaminants, most notably polychlorinated biphenyls (PCB) and lead. Under the Toxic Substances Control Act, EPA sets requirements for cleaning up PCB contamination. In addition, EPA has issued guidance for cleaning up lead in soil. Early in its investigation of a site, EPA determines, on the basis of the contamination present and the conditions at the site, which chemical-specific and other standards may be considered applicable or relevant and appropriate. As EPA proceeds with the selection of a cleanup method, it adjusts the list of standards to be considered on the basis of information gained during its investigation. Among the potential standards considered are any state environmental standards that are more stringent than the federal standards for the same contaminants. In addition to numeric standards for specific contaminants, some states have set more generalized standards or policies that may have to be considered when cleaning up Superfund sites. For example, some states have established “antidegradation” policies for groundwater that could require more stringent cleanups than cleanups based on health risks. These policies are intended, among other things, to protect the state’s groundwater as a potential source of drinking water. If federal or state standards do not exist for a given contaminant, the party responsible for cleaning up a Superfund site may use a site-specific risk assessment to help establish a cleanup level for that contaminant. A risk assessment evaluates the extent to which people may be exposed to the contaminant, given its concentration and the physical characteristics of the site. For example, the type of soil and the depth of the groundwater may affect whether and how quickly waste will migrate and reach a population. A risk assessment uses exposure and toxicity data to estimate the increased probability, or risk, that people could develop cancer or other health problems through exposure to this contamination. A risk estimate can be used along with the proposed waste management strategy to help determine the extent of the cleanup necessary at a site. EPA has published guidance for conducting risk assessments, a set of documents referred to collectively as the Risk Assessment Guidance for Superfund. These documents outline the well-established risk assessment principles and procedures that can be used to gather and assess information on human health risks. The documents also include information on mathematical models that can be used to estimate health risks at a site, given the contaminants present and the means of exposure to them. In addition to this guidance, EPA maintains an Integrated Risk Information System (IRIS), an on-line database on the toxicity of numerous chemicals, and publishes the Health Effects Assessment Summary Tables (HEAST), another source of information on contaminants’ toxicity. EPA uses this guidance in conducting baseline risk assessments at Superfund sites, which it uses in deciding whether the human health and environmental risks posed by the contaminants are serious enough to warrant cleaning up the sites. Some states also use EPA’s risk assessment guidance in setting their standards for specific chemicals. States that have set environmental standards have made decisions about what levels, or concentrations, of chemical contaminants can remain at hazardous waste sites after cleanups. We analyzed the processes that the states in our survey said they went through, as well as the factors that they said they took into consideration, in developing their soil and groundwater standards. In this section, we first summarize (1) the extent to which the states based their soil standards on estimates of the human health risks posed by contaminants at the sites and (2) the methods that the states used to estimate these risks. We then report on the factors other than health risks that the states said they considered when developing their soil standards. Since the bases for the states’ standards for groundwater differed somewhat from those for soil, we summarized the information on groundwater standards separately. Finally, since federal drinking water standards are frequently used as cleanup standards for groundwater, we compared the states’ groundwater standards to the federal standards for the same contaminants to determine the extent of their correspondence. We have included the information we obtained from the 33 states in our survey. In all, 21 of the 33 states had set their own standards for either soil or groundwater, or for both media. (See table 2.1.) Thirteen of the 21 states had set their own soil standards, and 20 had set some groundwater standards that were in addition to or different from the MCLs for drinking water, as discussed in the remainder of this section. All 13 of the states with soil standards indicated that they considered risks to human health when developing their standards. The number of chemical-specific standards per state ranged from about 10 to nearly 600. All but one of these states generally relied on EPA’s guidance for estimating health risks from contaminants (Missouri had developed its soil standards before EPA issued its guidance). These states said that they had used EPA’s guidance, either alone or in combination with their own methodologies and policies, to estimate health risks. (See table 2.2.) For example, Pennsylvania said that it had used EPA’s guidance to estimate the toxicity of contaminants and its own model to estimate how much contamination from the soil might travel into groundwater. These estimates are two of the major components in the health risk calculation. uses at Superfund sites, which extends from 1 in 10,000 to 1 in 1 million. As shown in table 2.2, eight states chose the more stringent risk level of 1 in 1 million for individual carcinogens in soil, while five states chose the somewhat less stringent risk level of 1 in 100,000. For noncarcinogens in soil, 11 states used the same measure that EPA uses at Superfund sites, while 2 states used a somewhat more stringent measure. Ten of the 13 states considered factors in addition to health risks when setting their soil standards. As a result, their standards could be either more or less stringent than those based solely on estimates of health risks. These other factors included the following: Chemical levels that occur naturally in the environment. In some locations, certain contaminants may exist naturally in the soil in concentrations differing from those that would be allowed under standards based on risks to human health. For such contaminants, the states typically set their standards at the naturally occurring levels rather than at the levels based solely on risk. In some cases, this practice would result in less stringent cleanups than would be necessary to meet the risk-based standards. However, since some chemicals do not occur naturally in the environment, this practice would in some instances result in more stringent cleanups than would otherwise be required. Detection limits and practical quantification limits. When the concentrations of some contaminants that could remain in the soil without posing health risks fell below the levels that can be accurately measured or detected by current technology, the states said that they typically adopt less stringent, but measurable, concentrations as their standards. Secondary, or aesthetic, criteria. Some chemicals cause unpleasant odors or other problems at levels that do not pose human health risks. The states may set their standards for these chemicals below risk-based levels to protect the public from such problems. Twenty of the 33 states we surveyed said that they had set some chemical-specific standards that would limit the concentrations of various toxic chemicals that could be present in groundwater at Superfund sites. These states not only adopted some of the existing federal standards, such as MCLs, but also set some standards in addition to or different from them. The number of chemical-specific standards per state ranged from about 30 to nearly 600. While the remaining states that we surveyed had not developed any of their own groundwater standards, the federal MCLs are typically used as Superfund cleanup standards for groundwater. Nineteen of the 20 states had based their groundwater standards, at least in part, on estimates of the human health risks posed by exposure to chemical contaminants. (See table 2.3.)