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Environmental Cleanup at Navy Facilities: Risk-Based Methods (1999)

Chapter: 2 Review of Risk-Based Methodologies

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Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
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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,

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

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.

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

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.

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

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.

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

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.

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

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

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

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.

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

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

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

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

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

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)
Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

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.

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

can occur. When concentration data fall below either generic or site-specific SSLs, it is likely that the site will require no further action under CERCLA (EPA, 1996a).

For those sites that cannot be eliminated from cleanup consideration using the Soil Screening Guidance, the SSLs can be used as preliminary remediation goals (PRGs), provided appropriate conditions are met (e.g., conditions found at a specific site are similar to conditions assumed in developing the SSLs). This obviates the need for calculating PRGs using RAGS Part B. It should be kept in mind that the Soil Screening Guidance may only be used for a subset of contamination problems found at hazardous waste sites. It is likely that complex hazardous waste sites will exhibit conditions that require the calculation of PRGs under RAGS Part B.

The generic Soil Screening Levels developed by the EPA have many counterparts in regulatory programs at the state level. (Many states have devised generic screening levels for contaminated ground water as well). These generic soil screening levels may be more or less stringent than the Soil Screening Levels of the EPA, and generally are based on similar assumptions, such as residential exposure and particular exposure pathways (such as direct ingestion of soil).

CERCLA Risk Management

Information from the remedial investigation, including the results of all risk assessments and PRGs, is used to conduct a feasibility study (FS) to determine the ultimate remedy for contamination. The FS begins the risk management phase of the cleanup effort. During this phase, alternative remedial measures are evaluated for their risk reducing ability and effectiveness. Guidance for performing such evaluations is provided in RAGS Part C (which is also intended to evaluate the selected remedial alternative during and after its implementation) and RAGS Part D (EPA, 1991b, 1998a).

A remedy is selected from the suite of remedial alternatives using the nine evaluation criteria described in the NCP:

  • (1)  

    overall protection of human health and the environment;

  • (2)  

    compliance with the chemical-specific standards that are considered the statutorily required ''applicable or relevant and appropriate requirements" (ARARs);

  • (3)  

    long-term effectiveness and permanence;

  • (4)  

    reduction of toxicity, mobility, or volume through the use of treatment;

  • (5)  

    short-term effectiveness;

  • (6)  

    implementability;

  • (7)  

    cost;

  • (8)  

    state acceptance; and

  • (9)  

    community acceptance.

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

The first and second criteria (threshold criteria) must be met in all circumstances. For example, assume that the PRG for an individual chemical is not feasible as a cleanup value under criteria 6 and 7. To meet the threshold criteria 1 and 2, isolation of the contaminant from the receptor may be the only remedy. Where total containment is not feasible under criteria 6 and 7, changes in land use may have to be considered. It is often the case that cost and technical feasibility issues can be anticipated early on in the CERCLA process. In such situations, multiple PRGs are developed, with the expectation that some will be met using treatment technologies, some will be met with containment strategies, and some will be met using institutional controls (Cooper, 1998). The preamble to the NCP makes it clear that the EPA has a strong preference for treatment technologies over engineering and institutional controls, especially for "principal threat" wastes2 (Federal Register, 1990). The EPA does not encourage solutions in which institutional controls are the sole remedy and prefers that they be used in conjunction with containment strategies.

The remedial option chosen after consideration of the nine balancing criteria is described in the decision document, known as the record of decision (ROD) for facilities on the NPL. Following approval of the ROD, implementation of the remedy begins. Remedial design (RD) and remedial action (RA) encompass the design, construction, operation, and implementation of the final remedy.

The final stages of the CERCLA process include maintaining engineering and institutional controls and conducting long-term monitoring. These activities are evaluated by means of five-year reviews. If continued monitoring of all remedies demonstrates that the site no longer poses significant risk to human health and the environment, the site may be closed out.3

At any time during the investigation or cleanup phases, interim remedial actions (IRA) or removal actions may be taken to remove a source of contamination or block a contaminant pathway. These measures are not intended to be the final remedial action at the site. Rather, they are intended to stabilize the situation until a permanent remedy can be employed. IRAs can occur prior to, during, or after the RI/FS. IRAs and removal actions have been subject to controversy because they increase the possibility that actions may be taken without regulatory approval or public acceptance.

American Society for Testing and Materials

The American Society for Testing and Materials (ASTM) has developed two standard guides for risk-based corrective action (RBCA): (1) Standard Guide for

2  

Principal threat wastes are broadly defined by the EPA as being liquid or solid wastes and soil containing hazardous substances that constitute a risk of 10-3 or greater. More detail is available in EPA, 1991c.

3  

See Chapter 1 footnotes 3 and 4 for details.

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

Risk-Based Corrective Action Applied at Petroleum Release Sites (E 1739–95; referred to here as petroleum RBCA) and (2) Standard Provisional Guide for Risk-Based Corrective Action (PS 104-98; referred to here as chemical RBCA). The ASTM documents are meant to be used as a framework for developing risk management strategies and not as "cookbooks" to be rigidly followed. It is presumed that states will develop their own versions of RBCA using the ASTM standard guides as models.

Both ASTM standard guides cover the same environmental remediation activities that the CERCLA process does, from site discovery to site closure. For the cleanup of petroleum underground storage tanks, which are not regulated under CERCLA, the petroleum RBCA methodology can be adopted by states and be made to reflect state regulations (this has occurred already in 14 states). Chemical RBCA is intended for use at sites other than petroleum release sites, which may be subject to regulation under RCRA, CERCLA, or state statutes. For CERCLA sites, implementation of chemical RBCA would have to occur within the CERCLA framework, perhaps as an alternative approach to RAGS. Because the chemical RBCA standard guide is relatively new, it has not yet been implemented in any state.

Petroleum RBCA and chemical RBCA are similar to EPA guidance on risk assessment (CERCLA, RAGS, and SSLs) in many ways. They all rely on the source-pathway-receptor paradigm to quantify risk, using many of the same mathematical calculations and transport and exposure models. Specific differences between the methodologies that translate into strengths and weaknesses are discussed in detail in Chapter 3. In general, the ASTM standard guides are meant to apply to petroleum and non-petroleum compounds under circumstances that can be customized to the user. In the following paragraphs, a description of petroleum RBCA is given, followed by a shorter description of chemical RBCA with an emphasis on pointing out the differences between the two.

Petroleum RBCA (E 1739–95)

Petroleum RBCA provides a tiered approach for developing a remedial action plan for leaking USTs (similar to the tiered approach of the Soil Screening Guidance). Each tier is successively more complex and requires more extensive data and assumptions. The RBCA approach differs from traditional risk assessment by deciding up front an acceptable level of risk and then calculating the corresponding cleanup levels for chemicals of concern (CoCs). For a tier 1 analysis, these cleanup levels are termed risk-based screening levels (RBSLs); for tier 2 and tier 3 analyses, they are called site-specific target levels (SSTLs). RBSLs and SSTLs for soil contamination are comparable to the EPA's generic and site-specific SSLs, respectively. The general goal of the RBCA process is to compare concentrations of CoCs with RBSLs and SSTLs and take appropriate action.

The petroleum RBCA framework is summarized in Figure 2–3. First, a site

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

Figure 2–3

The three-tiered, 10-step petroleum RBCA flowchart. SOURCE: ASTM, 1995.

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

assessment is conducted that should result in an initial site classification with regard to interim remedial action. This is followed by a tier 1 analysis, which is followed by tier 2 and tier 3 analyses if necessary. Final and interim (if needed) remedial action plans are formulated from these analyses.

Initial Site Assessment and Classification. During this phase, sources, pathways, and potential receptors are identified and CoCs are quantified (concentration and extent of contamination). Sources include tanks, pipelines, and free product.4 Possible exposure pathways include movement with the ground water, sorption onto solids, and volatilization and migration to the surface. Petroleum RBCA defines receptors as "persons, structures, utilities, surface waters, and water supply wells that are or may be adversely affected by the release."

Information gathered in this phase is used to classify the site, to determine what initial response is appropriate, to compare with RBSLs in a tier 1 analysis, and to develop SSTLs for tier 2 and tier 3 analyses, if necessary. Petroleum RBCA provides examples of site classification and initial response actions in Table 1 of the standard guide. What constitutes sufficient data collection during this phase is left to the discretion of the user. State RBCAs may specify what level of detail constitutes sufficient data.

The site is classified as an immediate threat, a short-term threat, a long-term threat, or no demonstrable threat. Initial response actions will vary with the severity of contamination. Thus, outcomes can range from monitoring the site in preparation for a tier 1 analysis to removal of sources (such as tanks and pipes) with or without additional control measures (e.g., free product recovery).

Tier 1 Evaluation. In a tier 1 analysis, concentrations of CoCs for different exposure pathways are compared with RBSLs in a look-up table. RBSLs are generic and do not consider site-specific information. For example, the default RBSL for ground water ingestion might be the MCL (if one exists). There are eight exposure pathways given in the sample look-up table in Appendix X2 of the standard guide:

  • inhalation of vapors;
  • ingestion of ground water;
  • inhalation of outdoor vapors originating from contaminants in the ground water;
  • inhalation of indoor vapors originating from contaminants in the ground water;

4  

Free product refers to nonaqueous phase liquid (NAPL) contamination that has leaked from a primary source area into the subsurface and formed a pool of contamination.

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×
  • ingestion of surficial soil, inhalation of outdoor vapors and particulates emanating from surficial soils, and dermal absorption resulting from surficial soil contact with skin;
  • inhalation of outdoor vapors originating from hydrocarbons in subsurface soils;
  • inhalation of indoor vapors originating from subsurface hydrocarbons; and
  • ingestion of ground water contaminated by leaching of dissolved hydrocarbons from subsurface soils.

The exposure pathways listed are not all-inclusive; other pathways are possible. There are many assumptions made in developing the equations and calculating these RBSLs. It is noted that the materials presented in the standard guide are "solely for the purpose of presenting an example tier 1 matrix of RBSLs, and these values should not be viewed, or misused as proposed remediation 'standards."'

Development of RBSLs is a critical step in the RBCA process. First, the user must determine a level of acceptable risk. RBCA suggests using an acceptable risk of 10-4 to 10-6 for carcinogens and a hazard quotient of 1.0 for noncarcinogens. For mixtures of chemicals, a hazard index may be used where hazard quotients for different chemicals acting by the same mechanism (e.g., liver toxicity) are summed. In developing RBSLs, petroleum RBCA recommends use of toxicological information in the EPA's Integrated Risk Information System (IRIS) (EPA, 1993), Health Effects Assessment Summary Tables (HEAST ) (EPA, 1992b), or other peer-reviewed data. Petroleum RBCA allows consideration of non-technical issues, such as future land use and cost of likely remediation in relation to potential risk reduction, in developing RBSLs, although these issues are more likely to be incorporated in the SSTLs developed in a tier 2 analysis.

Because of the potential for their misapplication, considerable attention has been given to the RBSL values given in the appendixes of the petroleum RBCA standard guide. Table 2-2 compares these RBSLs to the RBSLs found in the chemical RBCA standard guide, the EPA generic soil screening levels, RCRA cleanup criteria, and several state generic screening levels. The table includes soil screening values for a variety of contaminants assuming direct ingestion of soil, residential land use, a carcinogenic risk level of 10-6, and a hazard quotient of 1.0. For most of the chemicals, the ASTM RBCA RBSLs are similar to the EPA values. The values for naphthalene and xylene are notable exceptions. The state generic screening values are highly variable and, in general, more conservative than either the EPA or the ASTM RBCA values.

Once RBSLs are developed for all appropriate exposure pathways, they are compared with site data. For a tier 1 analysis, exposure of receptors is assumed to occur at the source area, and generic exposure equations are used. This imparts a measure of conservatism on the tier 1 evaluation. If concentrations of CoCs at

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

TABLE 2-2 A Comparison of ASTM RBCA, EPA, and State Generic Soil Screening Levels

 

Petroleum RBCAa

Chemical RBCAb

Soil Screening Levelsc

RCRA Action Levelsd

Floridae

Michiganf

New Jerseye

Rhode Islande

Washingtone

Exposure Pathway

Direct ingestion

Direct ingestion

Direct ingestion

Direct ingestion

Direct ingestion

Direct ingestion

Direct ingestion

Direct ingestion

Direct ingestion and protection of ground water

Target Risk

10-6

10-6

10-6

10-6

10-6

10-5

10-6

10-6

10-6

HQ

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

1.0

Benzene

5.8

4.7

22

NG

1.1

88

3

2.5

0.5

Benzo(a)pyrene

0.13

0.13

0.09

NG

0.1

1.4

0.66

NG

NG

Cadmium

NG

365

78

40

NG

210

NG

NG

NG

Ethylbenzene

7830

7190

7800

8000

240g

140g

1000h

71

20

Lindane

NG

0.143

0.5

0.5

NG

NG

NG

NG

NG

Mercury

NG

16.1

NG

20

NG

130

NG

NG

NG

MTBE

NG

NG

NG

NG

350

850

NG

390

NG

Naphthalene

977

75900

3100

NG

1000

15000

230

54

NG

Toluene

13300

NG

16000

20000

300

250g

1000h

190

40

Xylene

1450000

NG

160000

200000

290g

150

410

110

20

NOTE: All values for chemical screening levels are given in ppm, or mg/kg. All State values were confirmed with the appropriate regulatory agency.

HQ = hazard quotient

MTBE = methyl tertiary-butyl ether

NG = not given

a ASTM, 1995

b ASTM, 1998g

c EPA, 1996b

d Federal Register, 1990

e Judge et al., 1997

f Michigan Department of Natural Resources, 1998

g Concentrations capped at the soil saturation limit. Different states used different limits for the same compound.

h New Jersey standards for toluene and ethlybenzene were capped at 1000 due to concerns over inhalation of these compounds.

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

the source area are below RBSLs and if there is confidence that RBSLs will not be exceeded in the future (there is no guidance as to how to arrive at this conclusion), then the site is deemed worthy of no further action. At this point the user and the regulatory agency may enter into some type of closure scenario, which varies from state to state. If, on the other hand, concentrations of CoCs at the source area are greater than RBSLs for one or more exposure pathways, the user has two options: (1) proceed to a tier 2 analysis or (2) institute remedial action. The major driver here is cost: will the cost of a tier 2 analysis and its projected outcome be less than remedial action to achieve RBSLs?

The tier 1 analysis does not consider uncertainty in the data used to make the decision. As currently formulated, the assumption is that RBSLs are conservative (for example, use of MCLs for ground water ingestion). Also, a quantitative determination of ecological risk is not part of petroleum RBCA. During the site assessment, environmental receptors are to be identified. However, there is no discussion about how the user is to decide whether remedial action is required for such receptors (i.e., no "ecological" RBSLs are determined).

Tier 2 Evaluation. In a tier 2 analysis, the point of compliance (where contaminant concentrations must be below target levels) is no longer the source area itself. Instead, contaminant concentrations are compared to target levels at areas away from the source area, where exposure might more realistically occur. Target levels in tier 2, termed SSTLs, are calculated by using the RBSLs from tier 1 in conjunction with site-specific fate-and-transport modeling. Additional site-specific data are required to develop parameters needed to adequately develop and apply fate-and-transport models. Obviously, a tier 2 evaluation will not reduce risk to human and ecological health, but it may result in significant savings in remediation cost.

Tier 2 evaluations require many decisions and assumptions about what types of data to collect, where to collect, and for how long. In order to make such decisions, the user must determine what transport processes and exposure scenarios may occur at the site. Possible fate-and-transport processes include advection, dispersion, sorption, and biodegradation. For example, if intrinsic (passive) bioremediation is thought to be occurring at a petroleum site, the user must collect sufficient data to establish the apparent rate at which biodegradation is occurring.

Once transport processes and exposure scenarios have been determined, the user must decide which fate-and-transport models to use. The examples given in petroleum RBCA (Appendix X3) are fairly simple, but they are not the only applicable models. These fate-and-transport models are more sophisticated than those used in calculating the EPA PRGs or SSLs, particularly for the migration of subsurface volatile organic compounds to indoor air. The increased level of modeling detail requires considerable site-specific environmental data for the calculations.

The development of SSTLs may also include the impact of non-technical

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

issues. Among the most common are potential future land use, potential cost of remedial action, and aesthetic concerns, such as odor.

Once SSTLs are determined, they are compared with measured concentrations of CoCs for relevant exposure pathways. If CoCs are less than SSTLs, limited further action may be required. For example, monitoring may be required for some time to ensure that SSTLs are not exceeded in the future. It is possible that a closure plan can be developed in some cases. If concentrations of CoCs are greater than SSTLs for one or more exposure pathways, the user must decide whether to proceed to a tier 3 analysis or to institute remedial action. Again, the major driver is cost. The complexity and expense of a tier 3 evaluation must be weighed against the cost of remediation to tier 2 SSTLs, or to whatever target levels are mutually agreeable to the user and appropriate regulatory agency.

As with tier 1, petroleum RBCA's tier 2 evaluation does not include a consideration of uncertainty. Individual state RBCAs could include provisions and guidance for inclusion of uncertainty in the development of SSTLs.

Tier 3 Evaluation. Tier 3 involves much more complicated modeling (e.g., time-dependent numerical models) and may include probabilistic evaluation (e.g., Monte Carlo analysis) of sources and model predictions. This requires more site specific information and more extensive data collection. Site-specific toxicological data may be developed. Petroleum RBCA does not include examples of a tier 3 evaluation.

Once tier 3 SSTLs have been developed for appropriate exposure pathways, they are compared with measured concentrations of CoCs. If CoCs are less than the SSTLs, then, as with a tier 2 evaluation, limited further action may be required. If CoCs are greater than one or more SSTLs, remedial action will be required.

Remedial Action. Remedial action may include a combination of active and passive processes. These include, but are not limited to, source removal, natural attenuation, a variety of engineering remedies (e.g., active bioremediation and soil-vapor extraction), containment technologies, and institutional controls. The remedial action plan may include delineation of monitoring requirements for assessing remediation success, but no specific guidance on monitoring is provided.

Site Closure. Once it has been demonstrated by monitoring or other measures that RBSLs or SSTLs have been achieved as described in the remedial action plan and further monitoring is not required, the site may be closed, "except to ensure that institutional controls (if any) remain in place."

Chemical RBCA (PS 104-98)

Chemical RBCA was written to provide an ASTM framework for sites contaminated with chemicals other than petroleum compounds (e.g., chlorinated or-

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
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ganics, phenolics, nitroaromatics, and heavy metals). The general approach is very similar to petroleum RBCA with some important additions. The major additions are a consideration of ecological risk and cumulative risk and details contained in the appendixes where examples of the framework are given.

The chemical RBCA framework is summarized in Figure 2-4, which is almost identical to Figure 2-3, except for (1) the inclusion of ecological risk assessment and (2) an option to return to the tier evaluations if the remedy is not effective. First, a site assessment is conducted that should result in an initial classification of the site with regard to interim remedial action. This is followed by a tier 1 analysis, which is followed by, if necessary, tier 2 and tier 3 analyses. Final and interim remedial action plans are formulated from these tier analyses.

Initial Site Assessment and Classification. Sources, pathways, and potential receptors are identified and CoCs are quantified (concentration and extent of contamination) in this phase in a manner consistent with petroleum RBCA. Major additions include (1) the identification of ecological receptors, (2) a section on data quality objectives, (3) language strongly suggesting that local government and communities be involved in the site assessment, and (4) use of the terms "complete" and "potentially complete" pathways. Data collected must be sufficient to assess the completeness of potential pathways for human, ecological, and habitat exposure.

Tier 1 Evaluation. The framework for a chemical RBCA tier 1 evaluation is different from petroleum RBCA in two major ways, which are summarized in Figure 2-5. First, a conceptual model of the site is developed that delineates "reasonably" potential sources, transport pathways, and "reasonably" potential receptors. Using site assessment data, an analysis of pathways completeness is made. If this analysis indicates that relevant exposure pathways are not complete, no further action is recommended. If one or more relevant exposure pathways are complete or potentially complete, tier 1 evaluation continues. The second major difference is the inclusion of ecological risk in addition to human health risk.

As with petroleum RBCA, concentrations of CoCs for different exposure pathways are compared with RBSLs (calculated from predetermined acceptable risk assumptions) in a look-up table. RBSLs are generic and do not consider site-specific information. Two additional possible pathways are included in the chemical RBCA appendix examples: migration of free-phase liquid in saturated soil and migration of free-phase liquid in unsaturated soil.

In chemical RBCA, a tier 1 evaluation is also conducted for ecological receptors and habitats by developing relevant ecological screening criteria (RESC). These criteria would typically be taken from the literature in a tier 1 evaluation. The document is vague as to how these criteria are to be developed and evaluated.

If concentrations of CoCs are below RBSLs and RESC, no further action is

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

Figure 2-4

The three-tiered chemical RBCA flowchart. Note the inclusion of ecological risk at each stage, and the option of returning to a tier evaluation if site conditions change.

SOURCE: ASTM, 1998.

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
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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.

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

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.

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

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-

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

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

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

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-

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

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.

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×

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.

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
×
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

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
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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.

Suggested Citation:"2 Review of Risk-Based Methodologies." National Research Council. 1999. Environmental Cleanup at Navy Facilities: Risk-Based Methods. Washington, DC: The National Academies Press. doi: 10.17226/6330.
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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.

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The fiscal and technological limitations associated with cleaning up hazardous waste sites to background conditions have prompted responsible parties to turn to risk-based methods for environmental rememdiation.

Environmental Cleanup at Navy Facilities reviews and critiques risk-based methods, including those developed by the U.S. Environmental Protection Agency and the American Society of Testing and Materials. These critiques lead to the identification of eleven criteria that must be part of any risk-based methodology adopted by the Navy, a responsible party with a large number of complex and heavily contaminated waste sites. January

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