LETTERS Soil cleanup goals Dear Editor: Although I appreciate the contributions made by Terry Bowers and Neil Shifrin in determining soil remediation levels at an EPA Region 5 Superfund site ("Statistical Approach to Meeting Soil Cleanup Goals," May 1996, pp. 1437-44), three points need to be emphasized about their article. First, the identification of the exposure unit is a critical aspect of any risk-based remediation decision. We believe that averaging can only be done if the exposure unit adequately represents the geographic area to which a given individual would be exposed. Therefore, use of the confidence response goal (CRG) method requires that the exposure unit be defined in a manner that is protective. Second, while we supported the use of CRG on the Region 5 Superfund Site, we were extremely sensitive to the possibility that hot spots of contamination would be left in the exposure unit. This did not happen when CRG was applied to our site. However, we recommend that any health-protective soil remediation approach be site specific and ensure that unacceptable hot spots of contamination are addressed. Finally, the exposure unit described in the article specifically addressed human health risk. For ecological risk assessments, an exposure unit based on ecological receptors would have to be defined for the CRG approach to be effective. We do not believe that averaging across an exposure unit that is defined by human activity would necessarily protect ecological endpoints. Many site-specific considerations are critical to successful and protective remediation. EPA continues to be concerned about individuals who may be exposed to contaminant levels that are above a health-based cleanup goal. This situation could theoretically occur if exposure units designated as individual residential properties are not cleaned up to the
"Health-protective soil remediation approaches [should] be site specific and ensure that hot spots of contamination are addressed." risk-based level. If this were to happen, individuals could continue to be at risk of adverse health effects from the site. In addition, because contaminants left in place may continue to be a source for recontamination of remediated properties, EPA also considers the need to protect the remedy from recontamination when establishing site-specific remediation decisions. EDWARD J. HANLON U.S. Environmental Protection Agency Region 5 Chicago, IL 60604-3590
Authors' response We are in substantial agreement with Edward Hanlon's comments on our article. We also believe that identification of the exposure unit is a critical first step to any risk assessment, and one for which littie guidance exists and enough thought is not often given. An exposure unit should be developed from a minimum of the following considerations: It should be more or less homogenous with respect to prior waste practices; differences in land use, terrain, and accessibility should be assessed; geographic barriers such as rivers, roads, and fences should be noted; and it should be an area that an exposed individual may reasonably move randomly over its entirety during the long time assumed for exposure in a risk assessment. We considered all these things in the case study described in our article. The exposure units were developed by
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and agreed upon by a combined group of EPA personnel and industry representatives. We all agree that hot spots should not be left behind after remediation, and the CRG method described in our article provides a method to determine how hot is too hot. In practice, we have found that calculated CRGs are often a factor of 3 to 10 times the cleanup goal (CUG), which means that to achieve an average risk target of 10"6, no single location after remediation would have a risk greater than 10"5 (for a CRG/CUG of 10). As an added assurance at the site described in our case study, we agreed early on with EPA that no CRG would be set at a value higher than 100 times the cleanup goal, corresponding to a 10"4 risk, a level that is considered "safe" under many EPA programs and regulations. However, this additional assurance was not necessary because no CRGs were calculated to be that high. The CRG method can be used to address areas of either ecological or human health concerns, although the definition of the exposure unit may change depending on the receptor. We appreciate EPA's continued attention to the CRG method and expect that the general principle behind it of cleaning up to achieve a risk target on average will become more standard in the future. TERESA S. BOWERS Gradient Corporation Cambridge, MA 02138
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