How RBCA works Tier 1 analysis assesses site conditions and contamination. The main compounds of concern, extent of contamination, and potential exposure pathways to affected populations are all identified. The site is then classified as an immediate threat, short-term threat, long-term threat (greater than two years), or no demonstrable threat. Then, site conditions are compared with conservative risk-based screening levels (RBSL) contained in a reference table. At that point, it is decided whether to institute remediation, do nothing, or move on to a Tier 2 analysis. Tier 2 replaces the non-site-specific, generic RBSLs with site-specific target levels (SSTLs) and compares them with conditions at the site. Generating the SSTLs requires more involved data collection and analysis. Screening-level predictive models may be used. During Tier 1, the implication is that an exposed individual is located at the point of contamination. In Tier 2, default exposure assumptions may be replaced with alternative exposure points such as site boundaries, which usually result in a lower risk estimate. Again, a decision is made about which way to proceed. If no further action is deemed necessary risk management procedures are established for monitoring to ensure that site remediation targets are kept Tier 3 raises the level of sophistication up to and including a full risk assessment. Toxicity information, probabilistic evaluations, and complex analysis of chemical fate and transport may all be considered. This gives another, even more sitespecific, set of SSTLs, which leads to another decision about how to proceed. In practice, a Tier 1 analysis can help determine which compounds at a site actually pose a threat and through which potential pathways of exposure they do so. An RBCA reference table of contaminants and site conditions gives guidance on what degree of risk might be posed. The location of contamination and its potential pathways might include surface soil, subsurface soil, or groundwater, all of which can be compared with expected land use. That further narrows down specific risks.
policy decisions each state makes regarding their corrective action program," such as determining what degree of cleanup they decide is acceptable.
"Super RBCA" for chemical cleanups The still-evolving ASTM guidelines for chemicalcontaminated sites are currently in review and expected to be completed as early as next year; they will expand the RBCA framework of tiered decision making to cover the wide variety of situations involving hazardous chemicals. The initiative offers the promise of the risk-based approach, but the states still face tough decisions about how to handle chemical contamination. It requires that each state and EPA grapple with often-controversial details such as determining relevant exposure pathways and adopting toxicological values. "It's easier to address underground storage tanks.
The sites have similar characteristics, so the standards are easier to write," commented Paul Johnson of the University of Arizona-Tempe, lead author of the original ASTM standard and co-chair of the group drafting the chemical contamination protocol. "It is important to understand that the standards prescribe a way to go about handling a site; they do not prescribe what chemicals to look at or what values to adopt. That is up to the states." Pennsylvania has proposed regulations stemming from the state's Land Recycling and Environmental Remediation Standards Act, passed in July 1995, which was designed to put contaminated industrial sites back into productive use. The proposed regulations establish an RBCA-like assessment of almost 500 regulated substances. They allow remediators to choose whether to do a cleanup to background levels, to statewide health standard levels, or to site-specific cleanup levels, using a 1 in 100,000 cancer risk factor. The Pennsylvania regulations also address a major issue in the chemical RBCA standards: ecological risk. Largely ignored by the petroleum standards, the ecological risk posed by contaminated sites is of growing importance in state regulations. The ASTM guidelines being developed call for determining whether a pathway to ecological exposure is present at a site. If it is, a site-specific ecological risk assessment is recommended. The chemical standards feature the tiered methods of the original RBCA process but cover thousands of organic and metallic chemicals. Although the standards will probably be used first at voluntary cleanup sites, EPA staff from the Superfund and Resource Conservation and Recovery Act (RCRA) corrective action programs are closely watching the success of the standard. The RBCA approach may influence pending revisions to Superfund and RCRA, according to Bill Gulledge, vice president of Environmental and Commercial Insurance and chair of ASTM's subcommittee on environmental regulatory performance standards. "We saw Congress considering changes to RCRA and Superfund, and we felt that the provisional chemical standard would be good input." "In principle, it's possible to build RBCA into the reauthorization of Superfund," said Larry Froebe, manager of risk assessment and toxicology with International Technology Corporation. The process, he says, is effective in getting sites back into use. As more state and federal policy makers and their constituents demand greater consideration of risk in regulatory decision making, RBCA and similar approaches can be expected to become even more popular. Whether or not a given risk-based approach follows the ASTM model, "it's where we're all headed," Froebe believes.
References (1) American Society for Testing and Materials. Standard Guide for Risk-based Corrective Action Applied at Petroleum-Release Sites; ASTM: West Conshohocken, PA, ,995 (E1739-95))
Ronald Begley is a freelance journalist based in Richmond, Va. He is former Washington bureau chief at Chemical Week magazine. VOL.30, NO. 10, 1996/ENVIRONMENTAL SCIENCE & TECHNOLOGY / NEWS • 4 4 1 A
