Risk-Based Corrective Action: A Practical Approach
To Site Evaluation And Remediation
by Ijaz S. Jamall, Ph.D., DABT Risk-Based Decisions, Inc.

Introduction
For most of the last decade since its introduction by the U.S. EPA in the late 1980s, risk assessments have been largely limited to the status of regulatory compliance tools for evaluating the need for cleanup at federal and state superfund sites. The U.S. EPA guidance placed risk assessment as a component of the chemical characterization of hazardous waste sites known as the Remedial Investigation.

Over the past few years, there has been a growing recognition that hazardous waste sites are not being cleaned up as quickly or as cost effectively as was expected when Superfund and Resource Conservation and Recovery Act (RCRA) were first enacted. In particular, the practical application of “worst-case” analyses has resulted in the lack of distinction between sites that pose a serious threat to human health or the environment and sites that do not present significant risks. The original intent of the Superfund law, which was that sites were to be cleaned up to the extent that residual contamination was protective of human health, appears to have gotten blurred in practice.

There is increasing recognition within government and industry that the cleanup of hundreds of thousands of hazardous waste sites across the country to pristine background conditions is not scientifically defensible or economically feasible. The recession of the past few years has impressed this upon the general public who typically want to know, “Will this site impact my health or my children’s health?” A question that can only be answered through the risk assessment process.

The Evolution of Risk-Based Analysis
The move towards more pragmatic site evaluation and cleanup is gaining impetus with the re-emphasis of risk-based approaches. In 1994, the American Society for Testing and Materials (ASTM) issued its Emergency Standard Guide for Risk-Based Corrective Action Applied at Petroleum Release Sites (ASTM, ES 38-94, July 1994) commonly referred to as RBCA. This has been revised and updated by the Standard Guide for Risk-Based Corrective Action at Petroleum Release Sites, ES 1739-95 (ASTM, 1995). Also in 1995, the U.S. EPA issued its Use of Risk-Based Decision-Making in UST Corrective Action Programs, OSWER Directive 9610.17 (U.S. EPA, 1995). Several states (e.g., Texas, Ohio, Illinois, Massachusetts, Georgia, and Michigan among others) have adopted some version of the risk-based approach, at least for petroleum contamination resulting from leaking underground storage tanks (USTs).

What is Risk-Based Correction Action?
The basic premise to the ASTM’s RBCA process is to ensure that all characterization, evaluation and remediation efforts are targeted towards answering the question, “Do contaminants at the site pose a potential health risk, above levels of regulatory concern, to current or future occupants of the site?”

The RBCA process consists of three tiers. The ASTM Standard defines a Tier I evaluation as:
“A risk-based analysis to develop non-site-specific values for direct and indirect exposure pathways utilizing conservative exposure factors and fate and transport for potential pathways and for various property use categories (for example, residential, commercial, and industrial uses). Values established under Tier I will apply to all sites that fall into a particular category.”

Essentially, the Tier 1 risk-based screening levels (RBSLs) assume human exposure to contaminants at the source and, therefore, are very conservative values. If the maximum concentrations detected at a petroleum contaminated site are below the Tier 1 RBSLs, then that site is considered to pose no significant health risks and the RBCA process would recommend a no further action. If, however, the maximum concentrations exceed the Tier 1 RBSLs, then the site fails the Tier 1 evaluation and there are two possible options: (a) cleanup to Tier 1 RBSLs or (b) proceed with a Tier 2 evaluation.

The Tier 2 evaluation uses site-specific data both in terms of contaminant concentrations in soils and groundwater and in terms of pathways of human exposure. Once these data are available, the site-specific target levels (SSTLs) are calculated and the average, or upper bound estimate of average, concentrations are compared to the SSTLs. Again, one is faced with the option of cleaning up the site to the Tier 2 SSTLs or collecting more data and performing a Tier 3 evaluation.

A Tier 3 evaluation involves a more sophisticated evaluation of human exposure and perhaps even a Monte Carlo simulation. In the vast majority of petroleum hydrocarbon contaminated sites, a Tier 2 analysis would suffice to determine both the need for remediation and the levels to which any such remediation should be conducted.

The data needs and the level of effort and sophistication of the user increases in progressing from Tier 1 through Tier 3. Naturally, costs also escalate. Therefore, before proceeding to a higher tier, the user should consider the costs of the data needs for that tier of analysis, the costs of the analysis, the time costs and the greater time for regulatory review and approval against the costs of cleaning up the risk-based levels that correspond to the particular tier. Thus, the value of the RBCA approach is the cost savings obtained from a more focused investigation at each stage of the process with an explicit emphasis on protection of human health and the environment. It is important to note that the ASTM RBCA process maintains the same level of protection of human health through each tier of evaluation. This level of protection can be customized based on state policies. While the ASTM RBCA approach does not address the quantification of risks to ecological species, it does lay out a rational process to address such impacts qualitatively. In addition, the RBCA process lends itself to alterations of state-specific regulatory requirements and policies without losing the technical underpinnings of the risk-based approach.

The ASTM RBCA Standard addresses sites contaminated with fuel hydrocarbons. However, there is nothing inherent in the RBCA approach that would preclude its application to a wide range of hazardous environmental problems. In fact, sites contaminated with chlorinated solvents and other compounds would benefit from the application of the same philosophical approach, albeit with different technical considerations.

Case Studies: RBCA in Practice
Case Study #1: At a former industrial site, located in a semi-rural area about 1,500 feet upgradient of a slough that emptied into a river. The soils and shallow groundwater were contaminated with gasoline from a former UST that had leaked, the ASTM RBCA approach was used to obtain a no further action determination from the appropriate regulatory agency. The shallow groundwater (3 to 7 feet bgs) averaged about 8,500 ppb benzene (8.5 ppm), and had similar concentrations of the noncarcinogenic gasoline constituents, toluene, ethylbenzene and xylenes. The shallow groundwater was not used as a source for drinking water and a thick clay aquitard precluded significant downward migration of hydrocarbons to any useable aquifer. Five years of quarterly monitoring data indicated plume stability. And, thus, the primary pathway of human exposure was to benzene, toluene, ethylbenzene and xylene (BTEX) vapors in indoor and outdoor air. Contaminant fate and transport modeling was used to demonstrate that BTEX constituents would not adversely impact aquatic organisms in the river.

Case Study #2: At another UST site, the RBCA approach was used to demonstrate that BTEX levels in groundwater were not at concentrations that might present a risk to human health, and groundwater was not migrating offsite. The evaluation revealed that residual concentrations of benzene (45 ppm) located some 30 feet below ground surface could present risks to workers inhaling benzene vapors in indoor air at levels above regulatory thresholds. In this case, the RBCA process focused the investigation to a potential problem which identified the appropriate scope of remediation. Following this remediation and verification sampling, a no further action determination was obtained.

Ijaz S. Jamall, Ph.D., DABT, is a President of Risk-Based Decisions, Inc., in Sacramento