2. Use of Risk Assessment in Site Cleanup

The use of risk assessmentAn organized process used to describe and estimate the likelihood of adverse health outcomes from environmental exposures to chemicals. The four steps are hazard identification, dose-response assessment, exposure assessment, and risk characterization (Commission 1997a). in the cleanupThe assessment and reduction, removal, or control of chemicals in environmental media. Cleanup is synonymous with other terms such as "corrective action" and "remediation" used in various state, local, and federal programs. process depends on the regulatory agency, regulatory program, purpose of the risk assessment, and phase of the project. In some cases, risk assessment may be used to estimate an incremental excess lifetime cancer risk (cancer risk) or hazard quotientThe ratio of a single substance exposure level over a specified time period (for example, subchronic) to a reference dose for that substance derived from a similar exposure period (USEPA 1989a). (noncancer hazard) from a chemical in one or more environmental media (forward risk assessment). In other cases, risk assessment may be used to back-calculate a chemical concentration in a single environmental mediumSoil, surface water, groundwater, indoor air, outdoor air, sediment, and other parts of the environment that may be impacted by the release of a chemical. that corresponds to a specified risk or hazard (backward risk assessment), as discussed in Section 2.1. Some regulatory programs use a tiered approach for risk assessment that requires the use of published screening criteria or simplifying and protective assumptions (or both) as an initial evaluation. This conservative risk assessment can be refined using more site-specific assumptions as the site investigation progresses, as discussed in Section 2.2. A baseline risk assessment, assuming no action and unrestricted land use, is discussed in Section 2.3. Risk assessments can also be based on deterministic approaches using reasonable maximum input parameters or on more complex probabilistic approaches to determine input parameters, as discussed in Section 2.4.

In many cases, a screening-level risk assessment may effectively and efficiently support risk-based decisions. In other cases, a more detailed site-specific risk assessment may be needed. The scope, type, and complexity of the human health risk assessment that would best serve risk managementThe process of identifying, evaluating, selecting, and implementing actions to reduce risk to human health and to ecosystems. The goal of risk management is scientifically sound, cost-effective, integrated actions that reduce or prevent risks while taking into account social, cultural, ethical, political, and legal considerations (Commission 1997a). decision making depends upon a number of factors, including the specifics of the project or site, the complexity of the issues at hand, and the types of risk management decisions that must be supported. The following sections provide a brief description of different types of risk assessments, their potential benefits, and their limitations.

2.1 Forward and Backward Risk Assessment Calculations

Risk assessment can be conducted using a forward or backward calculation. The forward and backward calculations use the same equations and assumptions but vary in the direction in which the risk assessment proceeds. The forward calculation starts with a concentration of a chemical in environmental media (exposure concentration), defines exposureContact of a receptor with a chemical. Exposure is quantified as the amount of the chemical available at the exchange boundaries of the organism (for example, skin, lungs, gut) and available for absorption (USEPA 1989a). assumptions, calculates dose, identifies toxicity valuesDerived values (for example, reference doses and slope factors) that can be used to estimate the incidence or potential for adverse human health effects in receptor (USEPA 2015h)., and estimates a cancer risk or noncancer hazard (see Figure 2-1).

Figure 2-1. Forward risk assessment process.

The forward calculation is used to determine whether chemicals in environmental media present a risk to human health and to guide remedial action selection. This calculation requires well-defined exposure concentrations and is easier for calculating cumulative risks (see Section 7.1.2) for complicated exposure pathways (for example, homegrown produce).

The backward calculation uses the same steps as the forward calculation, but reverses the order of the steps. The backward calculation begins with the selection of a target cancer risk or noncancer hazard, identifies a potential chemical in environmental media, defines the exposure assumptions, calculates a factor representing dose, identifies toxicity values, and determines the concentration of a chemical in environmental media that is protective of human health based on the acceptable cancer risk or hazard quotient (see Figure 2-2).

Figure 2-2. Backward risk assessment process.

The backward calculation is used in developing most health-based screening values such as the USEPA Regional Screening Levels (USEPA 2014e) or state program screening values. Assuming all applicable exposure routes/pathways are included, the backward calculation can provide a simple way to screen concentrations of a chemical in environmental media, particularly for large sites where the exact exposure areas are not yet defined. This calculation can also be used to develop chemical and environmental media-specific target levels for remedial action. For sites with multiple chemicals, the cumulative risks can also be estimated through this approach, as discussed in Section 5.14.2 of the USEPA’s Regional Screening Levels Users Guide (USEPA 2014e). It may be difficult, however, to identify contributing pathways, and screening values can become outdated as parameters and assumptions change (for example, if newer toxicity values become available or agency exposure assumptions change). When previously existing, health-based screening values are used, the user should understand the input parameters that were used to calculate the screening values.

2.2 Tiered Risk Assessment Approach

A tiered approach for risk assessment is a systematic progression that starts with a high degree of conservatism (but low complexity) and progresses to decreasing conservatism and increasing complexity in the values and information used to estimate risk. The tiered approach offers a balance between the benefits of conducting more complex site investigations and the cost of additional time, resources, and risk communicationRisk communication is the formal and informal process of communication among and between regulatory agencies and organizations responsible for site assessment and management, and the various parties who are potentially at risk from or are otherwise interested in the site. challenges. Each tier provides an opportunity to review and communicate results of the risk assessment and make decisions on subsequent actions or tiers in the risk assessment process.

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The tiered approach can be applied using either a forward or backward risk assessment calculation. This approach typically begins with a qualitative assessment to identify chemicals of concern and determine whether potentially complete exposure pathways are present (Tier 1). For potentially complete exposure pathways under Tier 1, concentrations of chemicals in environmental media are compared to the applicable predetermined screening values, or a conservative risk assessment calculation may be performed. The Tier 1 evaluation is generally based on default exposure scenarios, conservative parameters, and conservative exposure concentrations (for example, a maximum reported concentration of a chemical in an environmental medium). Subsequent tiers replace the conservative assumptions used in Tier 1 with more site-specific data and information to better represent the actual exposures/risks at a site. Further evaluation may involve more intensive investigation of chemicals present, exposure pathways, and receptorAn individual (for example, residential adult, residential child, worker, trespasser, or recreator) who has the potential to be exposed to a chemical in environmental media. characteristics. Additionally, the site may require more complex modeling, statistical analysis (including probabilistic evaluations), and introduction of alternative toxicity values (based on new information).

If a Tier 1 screening-level assessment indicates potential risk to human receptors, then managers must decide whether to move to the next tier or to conduct appropriate remedial actions. Some considerations for this decision may include:

• Are the conservative assumptions and exposure pathways used in the Tier 1 risk assessment representative of the conditions at the site?
• Were all appropriate exposure pathways considered in the Tier 1 risk assessment?
• Will a site-specific risk assessment result in a significantly different outcome for the risk assessment?
• Will a site-specific risk assessment conducted in Tier 2 or Tier 3 result in a significantly different or less extensive remedial action?
• Are resources and time available to support more study?

Many state underground storage tank programs use a risk-based corrective action (RBCA) approach to evaluate risks posed by underground storage tank sites. The ASTM published its first RBCA standard (ASTM E1739) to assist at petroleum cleanup sites (ASTM 2010b) and its second RBCA standard (ASTM E2081) for all corrective action sites (ASTM 2010a). RBCA incorporates a tiered evaluation process to classify and evaluate sites based on risk. Evaluations under the initial tier typically rely on screening values developed through a risk assessment and based on conservative scenarios and assumptions (see backward risk assessment calculation in Section 2.1). Subsequent tiers allow site-specific scenarios and assumptions to be incorporated into the risk assessment.

Other examples of tiered approaches include state voluntary action programs and corrective action programs. Many of these programs rely on screening values, developed using risk assessment based on conservative scenarios and assumptions, as an initial screening tool followed by site-specific risk assessments to develop remedial action target levels. For example, the Illinois Tiered Approach to Corrective Action Objectives (TACO) program is the Illinois Environmental Protection Agency’s (IEPA’s) risk-based method for developing remedial action objectives for soil and groundwater that take into account site conditions and land use (IEPA 2007). TACO is a three-tiered approach that uses risk-based screening values for the Tier 1 evaluation provided in tables in the appendix of the regulations and allows cleanup criteria to be developed by applying site-specific data to preestablished modeling equations in Tier 2. Tier 3 allows other fate and transport models and any modifications to exposure and toxicity criteria to be used.

2.3 Baseline Human Health Risk Assessment

The baseline risk assessment, commonly associated with the USEPA’s Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) and Resource Conservation and Recovery Act (RCRA) programs, is an analysis of the risks for potential adverse human health effectsTypically defined as an incremental lifetime cancer risk (for example, exceeding a range of 1E-4 to 1E-6) or a hazard quotient or hazard index (for example, one). caused by a release from a site in the absence of any actions to control or mitigate the exposure (USEPA 1989a). Baseline risk assessments commonly assume the most conservative land-use scenario in the absence of institutional or engineering controlsEngineered and constructed physical barriers to contain, prevent, or mitigate exposure to chemicals in an environmental medium. Examples of engineering controls include engineered caps and subslab depressurization systems, mitigation barriers, and fences. Similar to activity and land use restrictions, engineering controls also typically require a specific mechanism for noticing the presence of engineering control and related restrictions, as well as long-term maintenance and management of the control. The timing of a decision to use an engineering control, and the specific mechanism to be used, may be based on criteria outlined in statute, regulation, policy, or guidance. (for example, a residential land use). A baseline risk assessment is conducted when chemicals have been identified in environmental media during initial data collection for the remedial investigation. Baseline risk assessments quantify potential risks posed by chemicals in environmental media and assist in determining if these risks require action.

2.4 Deterministic and Probabilistic Risk Assessments

The type of risk assessment to be used is determined by site-specific conditions and the regulatory program under which the risk assessment is being conducted. There are two types of risk assessments methods:

• Deterministic risk assessments are the most common type of risk assessment and are conducted using a single value for each input parameter to calculate an estimate of the risk. Deterministic risk assessments can use established default assumptions (such as RME) as input parameters or use site-specific information, where available. The output of the deterministic risk assessmentA quantitative estimate of risk using single-point estimates for input parameters such as exposure factors. is usually a single point risk estimate (for example, 1 x 10^-6) or hazard quotient/index, with a qualitative discussion of variabilityA population’s natural heterogeneity or diversity, particularly that which contributes to differences in exposure levels or in susceptibility to the effects of chemical exposures (Commission 1997a). For example, workers may perform different functions that may affect time, frequency, and duration of contact with an environmental medium). Variability cannot be reduced by collection of additional data. and uncertaintyThe lack of perfect knowledge of values or parameters used in a risk assessment. Uncertainty may be reduced by collection of additional data..
• Probabilistic risk assessments use statistically derived distributions of input values to calculate a range of risk. The probabilistic risk assessmentA technique that uses statistically derived distributions of input values (for example, exposure factors) to calculate a range of risk. can address both variability and uncertainty, with the output of the assessment being a distributionA distribution describes the probability or likelihood of any potential value. of risk that identifies individual risks at the mean or 95th percentile, for example. As a result, these assessments can provide a more detailed understanding of the variability and uncertainty of potential risks and the sensitivity of the input parameters (identifying which parameters have the most effect on the risk results). A probabilistic risk assessment is typically conducted to support a quantitative uncertainty analysis. Because of the quantity and quality of data needed for a probabilistic risk assessment, the assessment is often conducted only for complex sites and for exposure pathways that contribute significantly to the risk estimate. The complexity decreases with fewer exposure pathways evaluated and decreases if the distribution data needed have already been developed.

Each type of risk assessment can affect the site cleanup decision making. A deterministic risk assessment provides a single risk estimate that can be used as a basis for decisions. A probabilistic risk assessment provides a range of values for the risk estimate, which must be interpreted in order to determine the action warranted.

2.5 Resources and Tools

The following resources and tools were not cited in the sections above and are included here for further information.

Probabilistic Risk Assessment to Inform Decision Making: Frequently Asked Questions (USEPA 2014j).

Risk Assessment Forum White Paper: Probabilistic Risk Assessment Methods and Case Studies (USEPA 2014l).

Policy for Use of Probabilistic Analysis in Risk Assessment at the U.S. Environmental Protection Agency (USEPA 1997e).

Guiding Principles for Monte Carlo Analysis (USEPA 1997c).

Use of Risk Assessment in Management of Contaminated Sites  (ITRC 2008)

Risk Assessment Guidance for Superfund (RAGS) Volume III - Part A: Process for Conducting Probabilistic Risk Assessment (USEPA 2001c).

Best Practices for Risk-Informed Decision Making Regarding Contaminated Sites: Summary of a Workshop Series (NRC 2014)

Publication Date: January 2015