2
Review of Risk-Based Methodologies

Introduction

This chapter reviews EPA and American Society for Testing and Materials (ASTM) methodologies for risk-based cleanup at hazardous waste sites. These two methodologies, while not the only available, are the most widely used. Other frameworks considered by the committee include the Air Force's Enhanced Site-Specific Risk Assessment (ESSRA), the EPA Science Advisory Board's Integrated Risk Project, the Lawrence Livermore National Laboratory/University of California studies on leaking underground fuel tanks, and numerous state variations on the practices and methods of ASTM and the EPA.

A risk-based methodology is defined as a process that combines environmental data obtained for a hazardous waste site, risk assessment calculation(s), and a series of risk management decisions. The goal of such a methodology should be to determine how much and what kind of cleanup is necessary at the site, taking several factors into account including the extent of contamination, the risk posed by the contamination, the cost of cleanup, the values of the local community, and others. The risk-based methodologies reviewed by the committee were evaluated for their risk assessment and risk management components. Before turning to those reviews, the terms ''risk," "risk assessment," and "risk management" are discussed.

Risk, Risk Assessment, and Risk Management

This report follows the definitions of risk, risk assessment, and risk management that have been developed by several previous NRC committees (NRC, 1983,



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--> 2 Review of Risk-Based Methodologies Introduction This chapter reviews EPA and American Society for Testing and Materials (ASTM) methodologies for risk-based cleanup at hazardous waste sites. These two methodologies, while not the only available, are the most widely used. Other frameworks considered by the committee include the Air Force's Enhanced Site-Specific Risk Assessment (ESSRA), the EPA Science Advisory Board's Integrated Risk Project, the Lawrence Livermore National Laboratory/University of California studies on leaking underground fuel tanks, and numerous state variations on the practices and methods of ASTM and the EPA. A risk-based methodology is defined as a process that combines environmental data obtained for a hazardous waste site, risk assessment calculation(s), and a series of risk management decisions. The goal of such a methodology should be to determine how much and what kind of cleanup is necessary at the site, taking several factors into account including the extent of contamination, the risk posed by the contamination, the cost of cleanup, the values of the local community, and others. The risk-based methodologies reviewed by the committee were evaluated for their risk assessment and risk management components. Before turning to those reviews, the terms ''risk," "risk assessment," and "risk management" are discussed. Risk, Risk Assessment, and Risk Management This report follows the definitions of risk, risk assessment, and risk management that have been developed by several previous NRC committees (NRC, 1983,

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--> 1993, 1994). A risk is the probability of an undesired event whose occurrence is uncertain. Risk assessment is the evaluation of a risk in terms of the nature and consequences of the undesired event, the potential causes of the event, and the probability that the event will occur. The assessment can be purely quantitative (as in actuarial analysis or engineering failure analysis), purely qualitative, or some combination of the two. Risk management is the implementation of measures to reduce either the probability that the event will occur or the consequences of the event if it does occur. Risk management decisions generally consider societal values, economic cost, and other factors that are outside the formal risk assessment process. Risk Assessment Risk assessment is generally applicable to a wide variety of scenarios, including environmental cleanup of hazardous waste sites. The goal of a risk assessment is to determine the inherent level of risk posed by contaminated sites. This is accomplished by quantitatively linking the contaminant source to potential biological targets. Risk assessments integrate information on the physical conditions at the site, the nature and extent of contamination, the toxicological and chemical/physical characteristics of the contaminants, the current and future land use conditions, and the dose-response relationship between projected exposure levels and potential toxic effects. In 1983, the NRC defined the overall science of risk assessment by subdividing it into four major steps (NRC, 1983). Hazard assessment1 is the process of determining whether exposure to an agent can cause an increase in the incidence of a health condition. This step involves evaluation of epidemiological data, animal bioassays, short-term in vitro studies, and other data relevant to determining the nature and severity of effects that might be caused by chemical exposure. Dose-response assessment is the process of characterizing the relation between the dose of an agent administered or received and the incidence of an adverse health effect. This step estimates the probability that an individual will be adversely affected by a given chemical dose, relying primarily on data obtained from animal studies. Exposure assessment is the process of measuring or estimating the intensity, frequency, and duration of human exposures to an agent currently present in the environment or of estimating hypothetical exposures that might arise from the release of new chemicals into the environment. Exposure assessment includes estimation of concentrations of chemicals in environmental 1   Hazard assessment is the analytical part of a larger process during risk assessment (sometimes known as Hazard Identification or Source Assessment) in which sources of contamination are evaluated, the site is described, and potential receptors are identified.

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--> media to which humans are exposed, and the dose received by the exposed individuals. Risk characterization is the process of estimating the incidence of a health effect under the various conditions of human exposure described in the exposure assessment. It combines the exposure assessment with the dose-response curve. For carcinogenic compounds, the risk estimate is expressed as the probability of additional lifetime cancers (e.g., one in a million or 10-6). For noncarcinogens, risk is described by the hazard quotient (HQ), which is the ratio of the dose of a contaminant over a specified time period to a reference dose for that contaminant derived for a similar exposure period (e.g., HQ = 0.8). The framework described above has been widely accepted and today serves as the basis of the great majority of chemical-related risk assessments performed by federal agencies, state agencies, and private companies. In 1993, the NRC extended the framework to include ecological as well as human health risks. The fact that risk assessment has been embraced by the federal agencies does not mean, however, that risk assessments go without criticism, particularly in the arena of environmental cleanup. Because of uncertainties in the risk assessment calculation regarding the sources of contaminants, their transport to potential receptors, and their interaction with those receptors, there can be disputes over the value of the estimated risk among the regulatory agencies, the responsible parties, and the local community. The toxicological and exposure assumptions, as well as the quality of the data used in the risk assessment, are often the source of considerable disagreement. Risk Management The goals of risk management are to answer the questions: "What risk is acceptable?" and "How should we appropriately reduce, control, or eliminate risks to human health and the environment?" Risk management compares the risk assessment calculation to a defined level of acceptable risk and then describes the processes that will be used to reduce risk, if necessary. Acceptable Risk. Defining an acceptable risk level gives meaning to the risk estimate generated from the risk assessment. There are few legislative, public policy, and judicial guidelines on how to define acceptable risk. Although "safe" has not been found to necessarily mean zero risk (State of Ohio v. EPA 997 F.2d 1520, 1533, D.C. Cir. 1993), the courts have not provided (1) a risk level above which risk management action must occur, (2) specific guidance as to what might be done to determine whether a risk is acceptable, or (3) workable definitions of acceptable, safe risk levels. The EPA currently "endorses" a risk range from 10-6 (one in a million) to 10-4 for one's lifetime risk from exposure to carcinogens and a hazard quotient of 1.0 for noncarcinogens. As our state survey shows, acceptable risk levels across the state regulatory agencies tend to mirror EPA guidance.

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--> Risk Reduction. Although great strides in scientific methodologies have improved the science of risk assessment, improvements in the risk management decision-making process have not been as forthcoming. Risk management must balance the nation's interests in public health and environmental protection against the limits of public and private resources. Unfortunately, little guidance for balancing environmental protection and cost effectiveness in making cleanup decisions has been provided by the EPA. When such factors as feasibility and cost are not considered in an analytic paradigm similar to risk assessment, risk management frequently results in the misdirection of resources, incomplete protection of public health and the environment, and the loss of institutional credibility, public trust, and standing. One of the most important risk management decisions to be made at hazardous waste sites is the selection of a remedy. The remedial option specifies how the risk from a contaminated site will be reduced through a combination of cleanup technologies, containment strategies, and institutional controls. Cleanup technologies include natural and engineered physical, chemical, and biological processes that remove or reduce sources of contamination. Containment strategies, also referred to as engineering controls, consist of technologies that are designed to prevent contaminant migration into otherwise uncontaminated areas. Finally, institutional controls refer to restrictions on use of or access to contaminated land in order to minimize exposure to contamination. Ideally, the process of choosing the remedial option should not only consider issues uncovered during the risk assessment phase but also integrate considerations such as engineering feasibility, financial resources, community needs, and real or potential benefit of the proposed risk reduction solutions. The limitations of certain remedies, including both engineering and institutional controls, are discussed in Chapter 4. Universe of Risk-Based Methodologies In reviewing a variety of risk-based methodologies, the committee noticed a central principle common to all—the source-pathway-receptor paradigm. This paradigm states that for a risk to exist there must be (1) a source of chemical release, (2) a human or ecological receptor that is potentially exposed to the released chemicals, and (3) an environmental pathway connecting the source and the receptor(s). If a risk is present, it may be reduced or eliminated by removing the source or the receptor, or by interrupting the pathway. The source-pathway-receptor paradigm provides a straightforward approach to performing risk assessments at contaminated (or potentially contaminated) sites and linking the results of the risk assessments to risk management actions. In the following sections, this paradigm will be used to explain and compare the risk-based approaches developed by various state and federal agencies and the ASTM. Table 2-1 lists sources, pathways, and receptors that may be associated with a hazardous waste site.

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--> TABLE 2-1 Examples of Sources, Pathways, and Receptorsa Sourcesb     leaking tanks (above-ground or underground) tank appurtenances (e.g., pipes, distribution) surface spills free product contained in the vadose zone or aquifer soils saturated with chemicals of concern (CoCs) soils or waste piles lagoons or ponds injection wells landfills sources associated with facilities operation (e.g., weapons training, manufacturing) Transport Pathways     migration of free product in vadose zone and saturated zone dissolution of free product into ground water partitioning of free product between water and subsurface solids (sorption/desorption) leaching to ground water (desorption from soil) leaching to surface water (desorption from sediment) ground water to surface water (and vice versa) volatilization from ground water to outdoor air volatilization from ground water to indoor air or other confined space volatilization from vadose zone (free product or soil) to outdoor air, indoor air, or other confined space volatilization from surface water to outdoor air erosion and surface water runoff fugitive dust (wind erosion) Receptorsc     humans through:  dermal contact with soils, sediments, or contaminated water outdoors (e.g., swimming) or indoors (e.g., showering)  ingestion of soils, sediments, or contaminated water  ingestion of food sources (e.g., plants, aquatic species) that have bioaccumulated CoCs from contaminated water, air, or soil  indirect ingestion (e.g., baby exposed to breast milk)  inhalation of indoor air (or in other confined space)  inhalation of vapors from contaminated water (e.g., showering, running hot water)  inhalation of outdoor air  inhalation of fugitive particulates (e.g., dust) ecosystems (e.g., wetland, marsh) animals and other living species (e.g., endangered or protected species, fish, birds) ground water ground water wells a References include ASTM (1995, 1998); Farr et al., (1996); Rice et al., (1997). b Some are classified as "primary" and some as "secondary." c It should be noted that there is considerable controversy about considering non-living entities as receptors.

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--> Environmental Protection Agency The Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) established a national program for the management of potential public health and environmental threats posed by properties that are contaminated by hazardous wastes. CERCLA is implemented by regulations entitled the National Oil and Hazardous Substances Pollution Contingency Plan (NCP) (40 CFR Part 300) and enforced by the EPA. From CERCLA and the NCP, a regulatory process has evolved that has historically focused on the central metric of risk. Initial Steps of CERCLA The steps that make up the CERCLA cleanup process are shown in Figure 2-1. After discovery of a contaminated site, a preliminary assessment (PA) is conducted during which site-specific data are evaluated to determine the need for further cleanup action at the site. This is followed by a site inspection (SI) that collects additional data on air, soil, and water from the site and surrounding areas. Based on the information obtained in these two information gathering phases, the entire facility receives an initial ranking based on the degree of hazard presented using the Hazard Ranking System (HRS). If the facility receives a score of 28.5 or greater, it is listed on the National Priorities List (NPL), necessitating a Federal Facilities Agreement between the responsible party and the EPA and state regulatory authorities. Once contamination is demonstrated, a more detailed remedial investigation (RI) is conducted. The RI characterizes conditions at the site, identifying the sources of contamination, the extent of contamination, and the environmental characteristics and conditions contributing to unwanted exposure. During this phase, human health and ecological risk assessments are conducted, following the guidance provided by the EPA known as Risk Assessment Guidance for Superfund (EPA, 1989, 1991a, b, 1998a). Risk Assessment Guidance for Superfund EPA guidance documents covering risk assessment and management at hazardous waste sites are referred to as the Risk Assessment Guidance for Superfund (RAGS). RAGS estimates the risk to human health and the environment based on data generated during the RI, information that is essential for establishing the need for a remedial response and the extent of the action. These documents guide regulators, responsible parties, and stakeholders through the risk-based portion of the investigation, characterization, and remedy selection and implementation at a hazardous waste site. The first document, Risk Assessment Guidance for Superfund, Part A, focuses on developing a baseline risk assessment. The risk assessment contains a

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--> Figure 2-1 Steps in the CERCLA process. Each box describes the actions taken during the sequential phases of the CERCLA process. SOURCE: EPA, 1992a.

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--> mathematical description of complete exposure pathways linking the contaminant source through the environmental compartments and media to the biological receptor. This requires information about the concentrations and location of the contaminants, actual or potential mechanisms of release, migration, and fate of the contaminants, the environmental compartment or media through which the contaminant is transported or acts as a reservoir, points of potential receptor exposure, exposure conditions at the point of contact, integration of multi-media and multi-pathway exposures into a comprehensive scenario, and toxicity of the contaminant of concern. The end result is a numerical value of potential additional risk (e.g., 10-5 for carcinogens) from the contaminant source for the biological receptor at the exposure point. The risk assessment can be updated during the remedial investigation by integrating new site data into the risk calculation. Part A of RAGS is used for an initial estimation of risk at a hazardous waste site that can be compared to some acceptable target risk level. The risk estimate is generally calculated for both soil and ground water contamination. If the Part A risk estimate is greater than the acceptable target risk level, the second document, Risk Assessment Guidance for Superfund, Part B, is used to develop initial goals for risk-based cleanup. The document gives standard, detailed procedures for using risk assessment calculations to establish site cleanup levels that reflect the site's objectives. This process is similar to the risk assessment calculation specified under RAGS Part A, except it is carried out in the reverse direction. First, an acceptable target risk level that takes into account the desired land use scenario is specified. Mathematical equations are then used to calculate the concentration of a contaminant that will give rise to that risk, assuming certain transport and exposure pathways. The resulting concentration is set as the preliminary remediation goal (PRG). A PRG is a risk-based concentration limit for an individual contaminant in a specific environmental medium (e.g., soil) that is associated with a specific land use and exposure pattern. Values for PRGs can evolve with the characterization and understanding of a hazardous waste site. Generic PRGs can be developed initially based on general land use and exposure assumptions and minimal consideration of current or proposed site-specific conditions. The PRGs are refined as more site-specific data are gathered about the types of contaminants present, the nature and extent of contamination, and the potential site-specific exposure scenarios. RAGS advocates repeating the risk assessment calculation done during the remedial investigation. In practice, however, responsible parties rarely

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--> conduct more than one risk assessment for the major toxic or carcinogenic compounds identified at a site because of the time and cost involved in collecting the data. It should be noted that site-specific risk assessment under RAGS Part B, which is meant to generate a PRG, is conducted for soil and sediment contamination more frequently than for ground water contamination. This is because statutes often force cleanup of ground water to "applicable or relevant and appropriate requirements" (such as MCLs) regardless of the site conditions. However, for soils and those contaminants for which no MCLs exist, site-specific risk assessment is necessary. EPA Soil Screening Guidance Because site-specific risk assessment for contaminated soils under RAGS Part B requires substantial data collection, the EPA created a methodology, the Soil Screening Guidance, that can be used to screen soil contamination quickly before doing a full-scale risk assessment (EPA, 1996a). The intention of the Soil Screening Guidance is to focus resources on sites that pose the greatest risk. Another reason for using the Soil Screening Guidance is to eliminate from further consideration low-risk sites containing soil-only contamination. The Soil Screening Guidance provides a methodology to calculate risk-based, site-specific soil screening levels for a very specific subset of contamination problems. Only contamination problems that are similar to those used in the Soil Screening Guidance can be considered. The guidance assumes an acceptable risk of 10-6 for carcinogens and a hazard quotient of 1.0 for noncarcinogens, and it encompasses 110 chemicals. Only residential land use is considered, and six exposure pathways are specified, including direct ingestion of soil and ground water contaminated by soil, inhalation of volatiles and dust, dermal absorption, ingestion of produce that has been contaminated by soil, and migration of volatiles in basements. These criteria are used to formulate generic Soil Screening Levels (SSLs). The soil screening process is a tiered approach with seven basic steps (Figure 2-2). First, a site conceptual model is developed to collect, organize, and analyze data from the site. The site must be confirmed to have conditions similar to those described above (residential land use, similar exposure pathways, etc.). Second, the concentration data from the site are compared with the generic SSLs. If those data are below SSLs, the screening process ends (see below). If the concentration data are above SSLs, additional data collection is done to narrow down the areas in excess. More sophisticated soil analyses and sampling are conducted, and the generic SSLs are updated to include this new information, thus generating site-specific SSLs. Concentration data from the site are now compared to these site-specific SSLs. If they fall below these levels, the screening process ends. If not, another round of data collection, modeling, and further enhancement of the SSLs

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--> Soil Screening Process Step One: Develop a Conceptual Site Model (CSM)   Collect existing site data (historical records, aerial photographs, maps, PA/SI data, available background information, state soil surveys, etc. Organize and Analyze existing site data    Identify known sources of contamination  Identify affected media  Identify potential migration routes, exposure pathways, and receptors   Construct a preliminary diagram of the CSM Perform site reconnaissance    Confirm or modify CSM  Identify remaining data gaps Step Two: Compare Soil Component of CSM to Soil Screening Scenario   Confirm that future residential land use is a reasonable assumption for the site Identify pathways present at the site that are addressed by the guidance Identify additional pathways present at the site not addressed by the guidance Compare pathway-specific generic SSLs with available concentration data Estimate whether background levels exceed generic SSLs Step Three: Define Data Collection Needs for Soil to Determine Which Site Areas Exceed SSLs   Develop hypothesis about distribution of soil contamination (i.e., which areas of the site have soil contamination that exceed appropriate SSLs?) Develop sampling and analysis plan for determining soil contaminant concentrations    Sampling strategy for surface soils (includes defining study boundaries, developing a decision rule, specifying limits on decision errors, and optimizing the design)  Sampling strategy for subsurface soils, (includes defining study boundaries, developing a decision rule, specifying limits on decision errors, and optimizing the design)  Sampling to measure soil characteristics (bulk density, moisture content, organic, carbon content, porosity, pH)   Determine appropriate field methods and establish quality assurance/quality control (QA/QC)

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--> Step Four: Sample and Analyze Soils at Site   Identify Contaminants Delineate area and depth of source Determine soil characteristic Revise CSM, as appropriate Step Five: Derive Site-Specific SSL (if needed)   Identify SSL equations for relevant pathways Identify chemicals of concern for dermal exposure and plant uptake Obtain site-specific input parameters from CSM summary Replace variables in SSL equations with site-specific data gathered in Step 4 Calculate SSLs Account for exposure to multiple contaminants Step Six: Compare Site Soil Contaminants Concentrations to Calculate SSLs   For surface soils, screen out exposure areas where all composite samples do not exceed SSLs by a factor of 2 For subsurface soils, screen out source areas where the highest average soil core concentration does not exceed the SSLs Evaluate whether background levels exceed SSLs Step Seven: Decide how to address Areas Identified for Further Study   Consider likelihood that additional areas can be screened out with more data Integrated soil data with other media in the baseline risk assessment to estimate cumulative risk at the site Determine need for action Use SSLs as PRGs Figure 2-2 This figure describes actions taken during the seven steps of the soil screening process. Data collected from sites with contaminated soil are compared to generic Soil Screening Levels (SSLs). If the site data fall below generic SSLs, the screening process ends and site closeout is likely. If not, site-specific SSLs are generated considering relevant fate and transport and exposure pathways. Site data are then compared to site-specific SSLs. If site data do not fall below site-specific SSLs, these SSLs may be used as PRGs for the latter half of the CERCLA process. SOURCE: EPA, 19, 1996a.

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--> Figure 2-5 Framework for the tier 1 evaluation under chemical RBCA. The flowchart is used during the tier 1 evaluation in conjunction with Figure 1 from ASTM, 1998. This version of the tier 1 evaluation contains a consideration of relevant exposure pathways that can lead to early site closeout. SOURCE: ASTM, 1998.

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--> recommended. If they are above RBSLs or RESC, either remedial action or a tier 2 evaluation can commence. A tier 2 evaluation is also recommended for those exposure pathways for which tier 1 RBSLs or RESC are not available. Tier 2 Evaluation. In a tier 2 analysis, RBSLs and RESC are replaced with SSTLs and site-specific ecological criteria (SSEC), respectively. These are based on additional data collected at the site, revisions in the site conceptual model, and fate-and-transport modeling. RBSLs are applied at point(s) of exposure and then SSTLs for CoCs are calculated at source areas and points of compliance based on fate-and-transport modeling. SSEC can be developed by determining the toxicity of site media (e.g., ground water, soil, and sediment) to test organisms, by conducting biological surveys at the site, or by other lines of converging evidence. Cumulative risk, defined as the combined risk from multiple chemicals or multiple exposures on a single receptor, is mentioned for the first time as part of a chemical RBCA tier 2 evaluation. (Petroleum RBCA does not discuss cumulative risk.) In an attempt to capture the effects of uncertainty, a ''statistical data handling method" may be applied to concentrations of CoCs during tier 2. Some guidance on these issues is given in the appendixes; however, much is left to the user's discretion. Like petroleum RBCA, the chemical RBCA tier 2 evaluation requires many decisions and assumptions. Since many more chemical types are being evaluated, and for the most part our knowledge of the toxicity, fate, and transport of these chemicals is even more uncertain than for petroleum compounds, the decisions and assumptions associated with the calculated SSTLs and SSEC take on greater significance. As before, once SSTLs and SSEC are determined, they are compared with concentrations of CoCs for relevant complete and potentially complete exposure pathways. If concentrations of CoCs are less than SSTLs and SSEC, limited further action may be required. If concentrations of CoCs are greater than SSTLs or SSEC for one or more complete or potentially complete exposure pathways, the user may proceed to a tier 3 analysis or institute remedial action. Tier 3 Evaluation. The tier 3 evaluation of chemical RBCA involves much more complicated modeling (e.g., time-dependent numerical models) than tier 2, and it may include probabilistic evaluation (e.g., Monte Carlo analysis) of sources and model predictions, requiring more site-specific information. Values for SSTLs and SSEC may be revised based on this additional information. Once tier 3 SSTLs and SSEC have been developed for appropriate complete and potentially complete exposure pathways, they are compared with concentrations of CoCs. If concentrations of CoCs are less than the SSTLs and SSEC, then, as with a tier 2 evaluation, limited further action may be required. If concentrations of CoCs are greater than one or more SSTLs or SSEC, remedial action will be required.

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--> Remedial Action. Remedial action under chemical RBCA may include a variety of active and passive processes similar to those described in petroleum RBCA. Chemical RBCA more explicitly discusses the criteria that must be considered when choosing the remedial option. These criteria are very similar to the nine NCP criteria and include (1) effectiveness of the remedial action to protect human heath and the environment; (2) long-term reliability and probable success in meeting target levels now and in the future; (3) short-term risks associated with the remedial activities; (4) amenability of the remedial action to integration with property redevelopment plans; (5) acceptability of the remedial option to affected parties; (6) implementability and technical practicability of the remedial option; and (7) cost effectiveness of the options to meet the target levels. Unlike petroleum RBCA, chemical RBCA discusses options for revisiting the remedy selection if land use or other site conditions change. Monitoring requirements are more explicit in chemical RBCA. Chemical RBCA states that remedial action must continue until monitoring indicates that concentrations of CoCs are not above RBSLs or SSTLs for a "statistically significant number of monitoring periods." The goals of monitoring are to : (1) demonstrate the effectiveness of the remedial action; (2) confirm that current conditions persist or improve with time; and (3) verify model assumptions and conditions. If monitoring cannot confirm these accomplishments, the user should reevaluate the remedy or return to the appropriate tier evaluation. Like petroleum RBCA, there is no specific guidance for designing the monitoring system and establishing performance criteria for that system. Site Closure. Closure requirements are essentially the same as for petroleum RBCA except for the inclusion of ecological risk compliance requirements. Appendix X1. A major addition to chemical RBCA is Appendix X1, "Considerations for Development of a RBCA Program." Here general, although minimal, guidance is given regarding cumulative risk, the acceptable risk level, site characterization, ecological risk assessment, selection of remedial actions, monitoring network design, performance criteria, reopening of sites, site closure, and public involvement and risk communication. The appendix provides a matrix for use as an aid in making technical policy decisions and a checklist for implementing a RBCA program. It should be kept in mind that use of the appendixes is entirely up to the user. State Applications Like many responsible parties, state environmental cleanup programs are seeking methods that will allow available monetary resources to accomplish the greatest reduction in risk. This is particularly true for state UST programs, many of which are funded partially or wholly with gasoline taxes. Although consider-

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--> able effort has occurred over the years to incorporate risk into the states' cleanup programs, these activities have been particularly noticeable since the issuance of ASTM's petroleum RBCA standard guide. To understand the changes that states are making to adopt risk-based methodologies, such as the ASTM RBCA methodology, this committee created a survey to ask questions about state environmental cleanup programs. The survey form is illustrated in Appendix A. The survey was sent to 32 states known to have contaminated naval facilities (Department of the Navy, 1998). For the most part, the survey respondent was a member of the state's regulatory authority, but not always. Oklahoma is included in the survey although it does not have any contaminated naval facilities within its borders. The 19 states that completed the survey are Arizona, Connecticut, Florida, Georgia, Hawaii, Idaho, Indiana, Maine, Maryland, Massachusetts, Minnesota, New York, Pennsylvania, Oregon, Rhode Island, South Carolina, Tennessee, Virginia, and Washington. Questions were asked about each state's environmental cleanup program including whether it uses some type of risk-based approach during the cleanup process. Answers and any unique or insightful policies are summarized below. The survey used the terminology "risk-based decision-making" rather than "risk-based methodologies" or "risk-based corrective action'' to avoid any biases associated with the latter wordings. "Risk-based decision making" refers to the explicit consideration of the risks to human or ecological receptors when determining cleanup goals. General Use of Risk-Based Decision Making No state forbids the use of risk-based decision-making (RBDM) at sites contaminated with petroleum. Most states allow some form of this strategy, and seven states require its use. In almost all cases, risk-based decision-making facilitates prioritization of sites for cleanup by providing a metric that can be used to compare the relative risks of different contaminated sites. Almost all responding states allow the use of RBDM at sites contaminated with non-petroleum compounds, and three states (Florida, Oregon, and Rhode Island) require its use. Whether a state uses RBDM often depends on the non-petroleum contaminant present. There is considerable variation in the differences between each state's risk-based decision-making process and the ASTM RBCA standard guidance. Many states have begun using the ASTM standard guide for petroleum releases at storage tank sites and have made only minor changes to the methodology. Some states have modified RBCA to be more flexible and comprehensive. Others have allowed site-specific values to be incorporated into fate-and-transport parameters prior to a tier 2 evaluation. Efforts have also been made to simplify the system, sometimes by creating a "tier 0" for certain classes of contamination. For states that have not adopted the ASTM standard guide, there are greater

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--> This collection of drums recovered during a time-critical removal action shows the diversity of tank condition. Courtesy of the U.S. Navy. distinctions between that guide and the state's RBDM process. These states can be divided into those that use a tiered approach and those that do not. Of the states that use a tiered approach, many of these approaches are similar to the ASTM standard guide, but there can be fundamental differences. For example, some approaches focus more on source removal than ASTM RBCA, while other RBDM processes do not rely on a fixed risk level (e.g., 10-6) or they do not use the same risk level for all compounds. Generic soil and ground water screening levels developed by states represent a type of tiered approach, because they allow low-risk sites to be eliminated from further consideration in the same way that a RBCA tier 1 evaluation does. Table 2-2, which compares RBCA tier 1 RBSLs with state generic screening levels, reveals the substantial variability in the value of these screening levels for direct ingestion of soil. Ten of the states that completed the survey have developed generic screening levels for some types of contamination, while eight states do not have generic screening levels at all. Considerations of Source Characterization The main reason risk-based methodologies were developed initially for petroleum compounds is that these compounds were assumed to biodegrade natu-

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--> rally in most soils. The development of risk-based approaches has not been as rapid for other types of contaminants (non-petroleum compounds) because of their recalcitrance and the uncertainties associated with their fate, transport, and toxicity to humans. The states were asked to respond about three particular contaminant classes that can complicate implementation of a risk-based approach to cleanup. First, most states are moving away from requiring a determination of total petroleum hydrocarbons (TPH), which formed the basis for previous cleanup standards used for leaking USTs. Because the TPH result represents multiple chemicals that may have different chemical and toxicological properties, the risk assessment calculation for TPH is very complicated. States are devising alternative analytical and risk assessment strategies for the hydrocarbon range represented by TPH (e.g., use of surrogate hydrocarbon mixtures). Approximately half of the responding states require an evaluation of methyl tertiary-butyl ether (MTBE), especially at sites where gasoline has been released. MTBE is a fuel oxygenate that allows fuels to burn more cleanly. It is also an inexpensive way of improving the octane level of gasoline. The EPA has encouraged the use of fuel oxygenates in parts of the country that have failed to attain minimum air quality standards. In December 1997, the EPA issued a health advisory for MTBE that will prompt additional states to regulate the release of this compound. Because it is more mobile and less biodegradable than benzene, the compound generally used to characterize petroleum UST sites, MTBE must be explicitly considered during a site-specific risk assessment. This process is anticipated to be difficult because there is no accepted dose-response relationship or hazard quotient for MTBE. Most states require the removal of primary contamination sources during cleanup of a hazardous waste site, including any leaking tanks, pipes, and drums. Many states and the ASTM RBCA standard guide also consider free product to be a source. For many technical reasons, source removal of free product is particularly difficult (NRC, 1994). The presence of NAPLs or free product at a site will prevent the closure of a case in most of the states, and may often be the only impediment to UST site closure. Characterizing Contaminant Pathways Almost every state has ruled that all contaminant pathways must be evaluated (i.e., no pathway can be considered automatically insignificant). Pathways involving vapor migration appear to receive less attention than other pathways. Some states alter the allowable pathways depending on whether a generic or site-specific evaluation is being conducted.

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--> Cleanup of contaminated soil around an underground storage tank. At this enormous UST, water level was drawn down below tank level and contaminated soil was excavated. Courtesy of the U.S. Navy. Receptor Characterization Part of any risk-based approach to cleanup is a decision about whether ground water near a contaminated site can be used as a potential source of drinking water. Almost all states protect the quality of ground water with statutes that declare all ground water to be a potential drinking water supply. This assumption introduces many more potential exposure pathways into the risk assessment process. Most of these states, however, will give the responsible party an opportunity to prove that ground water in the area of its site is not used as a drinking water supply. For example, at a Navy site located in a zone of salt-water intrusion, the state may relax the cleanup goals for ground water resources.

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--> Risk Assessment In order to implement a risk-based approach to cleanup at a hazardous waste site, states must decide about the many elements of the risk assessment process. A majority of the states have decided that an acceptable target risk for carcinogens is 10-5 or 10-6 (the states are evenly split between the two). Acceptable risk across all respondents ranges from 10-4 to 10-6 depending on the type of carcinogen, whether generic or site-specific cleanup goals are used, and the completeness of current and future exposure pathways. The acceptable hazard quotient for toxicants in most states is 1.0, with values ranging from 0.2 to 3.0. Often, risk from individual chemicals is based on a hazard quotient of 0.2, while cumulative risk of multiple chemicals on a target receptor is based on a hazard index of 1.0. An assessment of cumulative risk during a tier 2 evaluation is one of the major differences between chemical RBCA and petroleum RBCA. Most risk-based approaches embraced by the states consider cumulative risk, defined as the combined risks of many chemicals on a target receptor. Most of the responding states consider the cumulative risk from exposure to multiple carcinogens or toxicants to be additive. In some states, risks from individual compounds are added only when the toxicants target the same organs, while in other states risks are considered cumulative regardless of the target organ. In some states, the acceptable levels of individual and cumulative risk are different, with the cumulative risk level often being 10 times less conservative than the individual risk level. Another major difference between petroleum RBCA and chemical RBCA is the inclusion in chemical RBCA of ecological risk assessment. The survey revealed that consideration of ecological risk is also permeating state cleanup programs, although the criteria for evaluating ecological risk vary widely across the country. In general, state guidance on ecological risk assessment is in its infancy. However, if it is known at a particular site that ecological receptors are affected, some states require further evaluation and possible remedial action. Remedy Selection The conditions under which each state will accept natural attenuation as a remedial strategy are somewhat variable and often not well documented. In a few states, the respondents were unaware of any policy regarding natural attenuation. In approximately half the responding states, natural attenuation is acceptable or may be considered if it can be shown that contaminant plume is stable or shrinking, and that the source has been or is being removed. Generally, these determinations must be made by monitoring the plume for contaminant concentration and breakdown products indicative of biological activity. The other responding states were less specific about the criteria needed to validate natural

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--> attenuation, although they will accept the strategy if it can be shown to achieve cleanup levels or reduce risk. States vary widely in how they deal with the technical feasibility of the proposed remedy. Few states have thought about the appropriate course of action to take when treatment technologies fail to meet cleanup goals. For those states that have developed, or are in the process of developing, guidance on this subject, there are three general categories of appropriate action: (1) search for better technology; (2) change the remediation goal to containment rather than removal and install long-term monitoring (and other engineering controls); and (3) impose institutional controls, such as restrictions on future land use. Depending on the affected media and other site-specific considerations, some states will allow certain contaminated sites to move to case closure or into long-term monitoring if technical impracticalities prevent those sites from meeting cleanup goals. Engineering and institutional controls were not mentioned in the responses of many states, although it is likely there are requirements for their use. It is clear from the survey that the states are moving toward risk-based approaches for addressing environmental contamination. It also appears that many states are developing their own risk-based decision-making process that will satisfy existing environmental statutes. This is especially true for petroleum contamination; in such cases, the ASTM RBCA standard guide has been widely adopted as the cleanup framework. It is not, however, apparent that the states have developed the appropriate tools to implement risk-based approaches. For example, the states are only now learning about ecological risk assessment. Few states were able to articulate the criteria for choosing natural attenuation as a remedial option and the appropriate monitoring that should accompany such a decision. Many states have yet to formally address the use of engineering and institutional controls, which inevitably must increase with the adoption of a risk-based cleanup approach. Each of these issues is characterized by significant uncertainty, which must be addressed to be confident that the risk-based approach is truly protective of human health and the environment. The EPA, recognizing the increased use of risk-based approaches, is currently drafting guidance for the states on some of these emerging issues, such as ecological risk assessment, natural attenuation, and institutional controls (EPA, 1997a, b, c, 1998b). It is hoped that the states will incorporate this advice into their risk-based decision making process in a timely manner. The danger with widespread and rapid adoption of risk-based approaches is that some sites may be closed prematurely and inappropriately. Not adequately addressing the uncertainties associated with leaving contamination in place may result in significant risk to future receptors.

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--> References American Society for Testing and Materials (ASTM). 1995. Standard Guide for Risk-Based Corrective Action Applied at Petroleum Release Sites (E 1739–95). Annual Book of ASTM Standards. ASTM, West Conshohocken, Pa. ASTM. 1998. Standard Provisional Guide for Risk-Based Corrective Action (PS 104–98). Annual Book of ASTM Standards. ASTM, West Conshohocken, Pa. Cooper, D. 1998. Environmental Protection Agency. Personal communication. Environmental Protection Agency (EPA). 1989. Risk Assessment Guidance for Superfund, Volume I, Human Health Evaluation Manual (Part A) — Interim Final (EPA/540/1-89/002). Washington, D.C. EPA. 1991a. Risk Assessment Guidance for Superfund, Volume I, Human Health Evaluation Manual (Part B) Development of Risk-Based Preliminary Remediation Goals — Interim, (USEPA ORD EPA/540/R-92/003 9285.7-01B). Washington, D.C. EPA. 1991b. Risk Assessment Guidance for Superfund, Volume I, Human Health Evaluation Manual (Part C, Risk Evaluation of Remedial Alternatives) — Interim (USEPA ORD EPA/540/R-92/004). Washington, D.C. EPA. 1991c. A Guide to Principal Threat and Low Level Threat Wastes. (USEPA OSWER 9380.3-06FS). Washington, D.C. EPA. 1992a. Understanding Superfund Risk Assessment. (USEPA 9285.7-06FS). Washington, D.C. EPA. 1992b. Health Effects Assessment Summary Tables (HEAST). (EPA OSWER OS-230) . Washington, D.C. EPA. 1993. Integrated Risk Information System (IRIS). Washington, D.C. EPA. 1996a. Soil Screening Guidance: Users Guide. (EPA/540/R-96/018). Washington, D.C. EPA. 1996b. Soil Screening Guidance: Technical Background Document. (EPA540/R-95/128). Washington, D.C. EPA. 1997a. Need to Develop Ecological Risk Management Guidelines. (EPA SAB-EPEC-ADV-97-002). Washington, D.C. EPA. 1997b. Ecological Risk Assessment Guidance for Superfund: Process for Designing and Conducting Ecological Risk Assessments. (EPA 540-R-97-006). Washington, D.C. EPA. 1997c. Use of Monitored Natural Attenuation at Superfund, RCRA Corrective Action, and Underground Storage Tank Sites. Interim Final. Washington, D.C. EPA. 1998a. Risk Assessment Guidance for Superfund, Volume I, Human Health Evaluation Manual (Part D, Standardized Planning, Reporting, and Review of Superfund Risk Assessments) — Interim (USEPA ORD EPA 9285.7-01D). Washington, D.C. EPA. 1998b. Institutional Controls: A Reference Manual (Working Group Draft). Farr, J., G. Apostolakis, M. Collins, R. C. Crounch, G. Fogg, M. Reinhard , and K. Scow. 1996. Senate Bill 1764 Advisory Committee's Recommendations Report Regarding California's Underground Storage Tank Program. Submitted to the State Water Resources Control Board pursuant to the requirements of section 25299.38 of the California Health and Safety Code. Sacramento, Calif. Federal Register. 1990. Corrective Action for Solid Waste Management Units at Hazardous Waste Management Facilities. Fed. Regist. 55(145):30865–30867. Judge, C., P. Kostecki, and E. Calabrese. 1997. State Summaries of Soil Cleanup Standards. Soil & Groundwater Cleanup. November 1997:10–34. Michigan Department of Natural Resources. 1998. Part 201 Generic Cleanup Criteria and Screening Levels Developed Under the Authority of the Natural Resources and Environmental Protection Act of 1994. Lansing, Mich. National Research Council (NRC). 1983. Risk Assessment in the Federal Government: Managing the Process. National Academy Press, Washington, D.C.

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--> NRC. 1993. Issues in Risk Assessment. National Academy Press, Washington, D.C. NRC. 1994. Science and Judgment in Risk Assessment. National Academy Press, Washington, D.C. Rice, D. W., B. P. Dooher, S. J. Cullen, L. G. Everett, W. E. Kastenberg, and R. C. Ragaini. 1997. Response to U.S. EPA Comments on the LLNL/UC LUFT Cleanup Recommendations and California Historical Case Analysis. Lawrence Livermore National Laboratory and the University of California.