3. Planning

Stakeholders may hold differing views on both the definition of the problem and on the list of possible options or problem evaluation methods (Adler and Kranowitz 2005). It is better to discover these differences during project planning than after communicating the risk assessment results. These differences may result from an individual’s risk perception on the basis of fear or feelings of inequality or distrust (see Section 9.2). Disagreements among stakeholders on critical aspects of the risk assessment may also pertain to issues not initially viewed as topics for discussion. Some examples include questions such as:

• Who sets the framework of the assessment? This question may include issues such as administrative site boundaries, sampling and assessment project time frames, and identifying appropriate regulation or guidance. For tribal stakeholders in particular, a parallel governmental structure exists with explicit or implicit rules.
• Who defines the time line for the project? The proposed schedule may not fit the time frame when some stakeholders are available to provide input. Stakeholders may also have a sense of urgency regarding their input that does not meet project management expectations. Where stakeholder involvement is key, deadlines should be set with sufficient time to allow stakeholder engagement and interaction.
• Whose has the power to make or influence decisions in 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).?  Risk management decisions are a balance of costs and benefits of remedial actions. The weighting of costs versus benefits is strongly affected by cultural values and other factors—including whether a stakeholder bears the costs or receives the benefits. But the costs and benefit of a remedial action may seem different to stakeholders who weigh prevention of ecological disturbance and prevention of potential human exposures differently.

Project managers must be aware of issues outside the perceived scope of the risk assessment that may derail project planning and risk communication.

3.1.1.1 Option – Clarify the Primary Regulatory Agency Jurisdiction

For many sites, one regulatory agency clearly has jurisdiction over a particular contaminated site cleanup. In some cases, however, multiple agencies and regulatory programs may have jurisdiction. Where multiple agencies or regulatory programs affect the planning and implementation of the risk assessment, appropriate jurisdiction and regulatory context (applicable statutes, regulations, policies, guidance, and project-specific approaches for a given risk assessment) should be determined and the roles and hierarchy for these agencies be identified. For example, in California, any one of several local, regional, or federal agencies may have jurisdiction on a given site, including USEPA, various California EPA agencies (such as DTSC, OEHHA, Air Resources Board, or RWQCB), county or city health departments, county or city fire departments, and other agencies. Many of these jurisdictions have their own risk assessment guidance, policies, or regulations, or a given jurisdiction may apply federal or state approaches. In some cases, a given governmental entity may not have jurisdiction in a strictly legal sense but may have the ability to influence the approach, methods, and interpretation of the risk assessment. Regardless of the number of agencies and regulatory programs, agree on an agency to take the lead in the oversight of the risk assessment and on the roles and hierarchy for other agencies.

3.1.1.2 Option – Identify Technical Expert and Resource Availability

Risk-based programs may require access to technical experts (for example, risk assessors) and appropriate technical resources (for example, modeling software and training). If the appropriate technical expertise or resources are not readily available in-house or readily accessible to a project manager, then outsourcing may be required to adequately staff the project. Private contractors, academia, and USEPA regional and state technical experts may be brought in to support the risk assessment. Many states may lack financial resources necessary to bring in contracted technical experts, but may find support from their USEPA regional offices, a state or federal (for example, ATSDR) public health agency, or other technical resources such as ITRC. Similar technical experts may be brought in representing the different agencies or organizations with varying levels of experience. This diversity of expertise can sometimes lead to power struggles. Identifying roles and responsibilities that include a hierarchy of technical experts helps to reduce power struggles.

3.1.1.3 Option – Engage all Appropriate Stakeholders

Community and other stakeholder input are important throughout the risk assessment process.

Early involvement of stakeholders often improves the quality of the risk assessment, expedites the risk assessment review, shortens the risk assessment schedule, and lowers the overall level of effort for finalizing the risk assessment. For example, Native American cultural/tribal exposure scenarios and issues are often different than those for the general public, vary by site, and may include unanticipated 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). media (for example, specific plants), exposure rates, and exposure pathways (for example, ceremonial use). Early involvement helps to identify site-specific exposure media, exposure points, 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. groups, exposure pathways, and data gapsMissing data or information needed to answer questions or allow a more refined analysis to be completed. that can be addressed during data collection and risk assessment. Stakeholders are also more engaged when they are part of the planning and review stages of the risk assessment. Community engagement during the planning stage demonstrates the commitment to addressing their interests and potential exposures.

Experience shows that decisions that are made during the risk assessment process in collaboration with stakeholders are more effective and durable. Stakeholders bring to the table important information, expertise, and knowledge that may increase the accuracy of the risk estimates (for example, a hill in the study area is often used by children for games). Additionally, stakeholders are more likely to accept the implementation of a risk management decision that they participated in shaping (Commission 1997a; Commission 1997b).

3.1.2.1 Option – Establish a Defined and Clear Communication Process

A clear process of communication and information sharing should include intra- and inter-disciplinary communications, technical to management communications, and communications to decision makers. Communicating with the community and other stakeholders throughout the process allows their input to be solicited and considered (see Chapter 9 for a discussion of risk communication).

3.1.2.2 Option – Engage Stakeholders

Keeping stakeholders engaged throughout the risk assessment process helps achieve a consensus outcome for the risk assessment. Communication between stakeholders supports clarity, transparency, and ultimately acceptance of the risk assessment outcome. Engagement begins with involving all stakeholders prior to initiating the risk assessment (and ideally before any field work is conducted) in defining roles and responsibilities and developing the purpose, scope, and technical approach of the risk assessment.

Inadequate coordination and communications among the disciplines involved in site cleanup can result in a rift between risk assessment and risk management because of the common perception that risk assessment should be completely separated from risk management. Agencies, however, often achieve successful integration between risk assessment and risk management. (Sedman, Reynolds, and Hadley 1992) present several examples of the successful integration of risk assessment and risk management functions. In addition to establishing clearer understandings and agreements about quantitative sampling objectives, which is where site characterization and risk assessment may overlap, risk assessors must be involved throughout the site cleanup process. Previous ITRC guidance recommends that all key players be identified up front, that they communicate and agree upon a common framework, and that the risk assessment process be iterative and modified as appropriate as new information becomes available. This process is necessary in order to achieve the best risk assessment and risk management possible (ITRC 2008).

In addition to the various levels of hierarchy that may be associated with a particular risk assessment, there may also be multiple jurisdictions (federal, state, local) each applying their hierarchy separately. Within this often complex set of requirements, multiple stakeholders may want additional social, environmental, or economic issues to be addressed in the risk assessment. A project manager should consider all of these issues in performing the risk assessment.

The laws, statutes, and regulations have the force of law, whereas policies and guidance are recommendations. Regulations may be the key driver in the risk process and can be either generic or specific (for example, specific exposure scenarios to consider, the 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). to be used). Although legislative statutes and regulations carry more authority, they often lack the detail necessary to prepare a risk assessment.

Project-specific judgment (at the bottom of the pyramid) is generated as a result of the regulatory project manager’s decisions on a specific site. The project manager may or may not have specific directives from legislation, regulation, and guidance that should be followed. Even if specific requirements are in place, there is often room for interpretation or site-specific conditions present that allow project-specific adjustments. If no requirements or only vague requirements exist, then project managers may need to use professional judgment on what may be allowed on a project. When using professional judgment, decisions should be based on data pertinent to the specific circumstances, rather than on personal opinions.

3.1.3.1 Option – Review Agency-Approved Work Plan, if Available

In some cases, regulatory requirements define the scope and contents that must be included in a work plan for the risk assessment; in other cases, a project-specific work plan is prepared and approved by the regulatory agency prior to preparation of the risk assessment. Where a project -specific work plan is prepared and approved, the scope and contents of the risk assessment should be compared to the agency-approved work plan in order to determine whether the risk assessment meets agency requirements. This approach, however, assumes that the agency staff that reviewed and approved the work plan were aware of the correct agency requirements, that agency requirements have not changed since the work plan was approved, and that the work plan was adequately detailed. The scope and level of detail presented in a risk assessment work plan can vary widely depending on the agency and can range from a brief overview of the major steps of the risk assessment process to detailed information on data groupings, exposure scenarios, models, input assumptions, and toxicity values to be used. In the absence of state-specific requirements for risk assessment content and reporting format, one resource to use is Risk Assessment Guidance for Superfund Part D (USEPA 2001b).

An iterative process can be used to modify/update the risk assessment scope presented in the work plan. When the risk assessment scope is prepared prior to data collection, it may be appropriate to later modify the scope based on findings of the site investigation and updates to the CSM.

The CSM is a working model that is refined and updated throughout the risk assessment as additional information and data are obtained that changes the understanding of site conditions or exposure scenarios. Changes may be based on information such as previously unidentified chemicals in environmental media or exposure media, screening results using risk-based concentrations and background data, and new information concerning potential activities and land uses. In addition, the CSM should be finalized during preparation of the risk assessment and included in the “Exposure Assessment” discussion of the risk assessment.

The presentation of the CSM is the basis for understanding the risk assessment process. A CSM presented in text format and accompanied by supporting diagrams or flow charts is generally the most useful. An example diagram from ITRC (2012a) is presented in Figure 3-2 and a flowchart CSM in Figure 3-3. In addition, the U.S. Navy offers guidance (2014a) that can be used for CSM development. Other example CSMs are provided in the USEPA RSL User’s Guide (USEPA 2014e) and USACE’s Conceptual Site Models (USACE 2012).

Figure 3-2. Example CSM.

Figure 3-3. Example pathway-exposure CSM.

Source: Appendix A1, DTSC 2008.

3.2.1.1 Option – Prepare and Refine a Site-Specific CSM 

The CSM is a dynamic element of the site investigation and the risk assessment process. The CSM may start out with a generic framework; however, as more information becomes available the CSM can be refined so as to better support decision making. In order to best support the development of an effective risk assessment, the CSM should be tailored to the circumstances and conditions at each particular site, rather than to a generic site or based on boilerplate language copied from other reports. An effective CSM can help to define overall project objectives and establish stakeholder agreement on key issues (including risk assessment scope and data needs), thus minimizing significant revisions or significant data gaps during stakeholder review of the risk assessment. All stakeholders should agree on the CSM before the project moves forward. In some cases, discussions with technical experts may be necessary to explain certain aspects of the CSM so that the entire team can reach consensus.

The CSM includes known and suspected sources of chemicals in environmental media, types of chemicals, affected environmental media, known and potential routes of migration, and known or potential human receptors (USEPA 1988a). The model incorporates available information regarding current and historical activities at the site, known or suspected chemicals in environmental media, current and reasonably anticipated future on-site and nearby off-site activity and land use, resources and their locations (for example, wetlands, water supply wells), populations (for example, residents, schools, hospitals), and physical and geological features (for example, groundwater hydrology, soil type, vegetation).

3.2.2.1 Option – Consider Unrestricted Use

An unrestricted use exposure scenarioA set of facts, data, assumptions, and professional judgment about how an exposure occurs or does not occur. An exposure scenario addresses the (1) chemicals in environmental media and their sources; (2) exposed populations (or receptors); (3) migration of chemicals in environmental media from sources to receptors; and (4) routes of exposure (ingestion, dermal contact, inhalation). (for example, residential) may be evaluated when the future site use is uncertain and a residential scenario is reasonably anticipated. This scenario can also be used to provide the scientific basis for possible restrictions on the land and groundwater use even when the future use is known. In this case, an unrestricted use evaluation can identify the chemicals and media warranting remedial action, in addition to those warranted for a reasonably anticipated alternative (other than residential) future land use and groundwater use. This information can be used to identify the incremental effort and investment needed to provide protection of human health for alternate future land uses. An unrestricted use scenario is not always a reasonable assumption. In some situations residential land use is not possible or not a realistic future site use (for example, at Department of Defense munitions sites with exclusions zones, within wetlands, and where physical features such as steep hillsides practically eliminate the likelihood).

In some cases, residential land use is not the most conservative land use evaluation for a risk assessment. For example, if bioaccumulative chemicals are present in soil and agricultural use of the land is possible, then an agricultural scenario may yield more conservative results than a residential scenario.

3.2.2.2 Option – Consider Reasonably Anticipated Future Land Use and Groundwater Use

When the current or reasonably anticipated future land use is other than residential, known existing exposure scenarios (for example, for workers who are currently employed at the property) and reasonably anticipated future exposure scenarios may be evaluated. Consider future land and groundwater use to determine the types of exposures that should be evaluated in a risk assessment and the frequency of exposures to any residual chemical concentrations that may occur. The evaluation of future land and groundwater use may be based on specific knowledge of the future use, such as when a property owner intends to maintain the current use or when future use decisions have been made and documented (for example, municipal zoning and planning ordinances).

Developing assumptions regarding reasonably anticipated future land and groundwater use can be challenging. A systematic process can be used to identify the reasonably anticipated future land and groundwater use for the site and adjacent properties. In some cases, the future land use may be determined by conducting a reuse assessment, which includes collecting and evaluating information to develop assumptions about reasonably anticipated future land use (USEPA 2001b).

Determining reasonably anticipated future land use should not be an “extensive, independent research project” (USEPA 1995c). Existing information can be used, much of which is available from local land use and land planning authorities (USEPA 1995c). The USEPA guidance can be used as a framework for identifying future land use (USEPA 1995c) and (USEPA 2001b). Sources of helpful information may include:

• current land use
• zoning laws and maps
• community master plans or development plans
• relevant chemical data
• population growth patterns and projections
• existing institutional controlsNon-engineered instruments that help minimize the potential for human exposure to contamination and/or protect the integrity of the remedy (USEPA 2001c). Examples include deed restrictions on land use, groundwater use restrictions, and city ordinances prohibiting private well installations. The use of these controls typically require a specific mechanism for placing the restriction and future compliance with the restriction. The timing of the decision to use an institutional control, as well as, the specific mechanism to be used may be based on criteria outlined in statute, regulation, policy or guidance.
• current land use in close proximity to the site
• wellhead protection information
• comprehensive groundwater protection plans
• historical or recent development patterns
• cultural factors (for example, Native American religious sites and historic sites)

This information, combined with interaction with stakeholders, increases the certainty of assumptions about reasonably anticipated future activity and land use, and thus increases the usefulness of the risk assessment.

USEPA defines institutional controls as non-engineered instruments, such as administrative and legal controls, that minimize the potential for exposure to chemicals in environmental media or protect the integrity of a response action (USEPA 2012d). The USEPA’s Institutional Controls: A Guide to Planning, Implementing, Maintaining, and Enforcing Institutional Controls at Contaminated Sites (USEPA 2012d) provides detailed information on the key activities of the life cycle for institutional controls, including planning, implementation, maintenance, enforcement, and termination. Regardless of whether or not institutional controls, engineering controls, or planned remedial action are incorporated as part of the risk assessment, the mechanism for how they are maintained and enforced after the corrective action is complete is crucial for long-term protectiveness of the site.

3.2.3.1 Option – Incorporate Institutional Controls, Engineering Controls, and Planned Remedial Action in the CSM

Incorporating existing or planned institutional controls, engineering controls, or planned remedial action allows streamlining of the risk assessment effort. This practice maintains focus on evaluating current and future exposure scenarios to assess whether additional actions are needed (on the basis of risk) beyond the controls already in place or planned. The USEPA guidance notes that a risk assessment should address current and reasonably foreseeable exposure scenarios rather than worst-case exposure scenarios. By doing so, the level of effort for the risk assessment is tailored to the site-specific issues and questions to be answered using risk assessment as a decision-making tool. The institutional and engineering controls can be used as a single action or part of a group of actions to render the exposure pathwayThe course a chemical takes from a source to a receptor. An exposure pathway describes a unique mechanism by which an individual or population is exposed to chemicals at or originating from a site. Each exposure pathway includes a source or release from a source, an exposure point, and an exposure route. If the exposure point differs from the source, a transport/exposure medium (for example, air) or media (in cases of intermedia transfer) also is included (USEPA 1989a). incomplete.

During the risk assessment planning process, incorporation of existing or planned institutional controls, engineering controls, or planned remedial action into the risk assessment should be confirmed with stakeholders. While planned institutional or engineering controls are important in determining reasonably anticipated future land use, the risk assessment should clearly communicate assumptions that rely on planned engineering controls, institutional controls, or remedial actions to eliminate certain land or groundwater use scenarios. The risk assessment should also, to the extent possible, assess how reliable controlling mechanisms will be in restricting land use in the future (USEPA 1995c); also see information regarding the Uniform Environmental Covenants Act (Kerr 2006). This assessment is critical because human health may not be protected if land use in the future does not reflect what was assumed in the risk management decision-making process.

At sites where risk-based justification of institutional controls, engineering controls, or interim remedial actions is needed, the risk assessment may not provide this information if the exposure scenarios are not quantified. Also, if existing controls are incorporated into the risk assessment, exposure scenarios excluded from the CSM that may ultimately be determined to be associated with acceptable risk (for example, site conditions may have improved and the restriction may no longer be needed), may not be evaluated. In addition, incorporating institutional controls and engineering controls may not identify land uses that may require only limited additional remedial action to achieve acceptable risk levels.

3.2.3.2 Option – Do Not Consider Institutional Controls, Engineering Controls, and Planned Remedial Action in the CSM

Conducting the risk assessment without incorporating institutional controls, engineering controls, and planned remedial action allows these controls or actions to be evaluated as part of the remedial action decision process. In some cases, institutional controls and engineering controls are viewed as remedial action and require specific mechanisms or actions that are appropriately conducted as part of the remedial action implementation process. When chemicals are persistent in environmental media, the effectiveness of institutional controls in mitigating exposure is less certain because of the time required (over 20 years) for these controls. In other cases, the responsible party or current landowner may not be able to ensure that institutional or engineering controls will be instituted and maintained. These cases are particularly problematic when future land use cannot be easily determined or the responsible party and landowner are not the same entity.

When a risk assessment is conducted without incorporating the existing institutional controls or engineering controls, an overly conservative site risk results and may lead to more extensive remedial action measures than are needed.

3.2.4.1 Option – Consider All Exposure Pathways in the CSM

While potentially complete exposure pathways are the focus of the risk assessment (see Section 3.2.4.3), it may be helpful to discuss incomplete exposure pathways in the CSM for documentation purposes, including any unique or site-specific exposure scenarios (see Section 3.2.4.2). If questions arise later regarding why certain environmental media, exposure pathways, and receptors were not included in the risk assessment, then this documentation will explain the exclusion of these items. In the presentation of results, incomplete exposure pathways can be graphically differentiated from potentially complete or complete exposure pathways.

3.2.4.2 Option – Incorporate Unique and Site-Specific Exposure Scenarios

For some sites, unique or site-specific exposure scenarios may need to be considered while performing an exposure assessmentThe determination or estimation (qualitative or quantitative) of the magnitude, frequency, duration, and route of exposure (USEPA 1989a).. Such scenarios may include consumption of homegrown produce; consumption of game animals (for example, deer, wild turkey, and fish); and residential nonpotable groundwater use exposures (for example, using groundwater in a swimming pool). These exposure scenarios can result in significantly more exposure (chemical intake) than assumed in typical default exposure scenarios (such as residential direct contact with soil or maintenance worker contact with groundwater). Failing to consider site-specific exposure scenarios may lead to unreliable risk assessment conclusions and thus inadequate risk management actions.

3.2.4.3 Option – Consider Only the Potentially Complete Exposure Pathways in the CSM

The risk assessment typically focuses on complete exposure pathways. With this focus, the exposure pathways included in the CSM may only be those that are determined to be complete or potentially complete based on the available information. In many cases, determining whether an exposure pathway is complete depends on the presence and location of an exposure mediumEnvironmental medium containing concentrations of a chemical that may be contacted by a receptor.. It is best to include potentially complete exposure pathways until adequate data is collected to eliminate pathways from the CSM.

Regulatory agencies vary as to whether an environmental medium is considered an exposure medium; for example, agencies differ in their definitions of surface soil and subsurface soil. Some regulatory agencies consider surface soil to be near surface or approximately the first few inches where an individual may come in contact with soil. Others may define surface soil to include the top two or three feet. In many cases, exposure media may be defined by statute, regulation, policy, or guidance, but it may also be determined as part of the planning process based on the current and reasonably anticipated future activity and land use. Exposure media may also be related to cultural or other activities unique to a site. For example, Native Americans who use clay for pottery may be exposed to chemicals in soil at varying depths, depending on where and how the clay is obtained.

The approach taken to identify receptors and define their specific activities on a site may vary based on the site and the type of activity. For example, it may be appropriate to address potential construction worker exposure through the use of Health and Safety Plans or other Occupational Safety and Health Administration (OSHA) criteria. Construction worker exposure may not need to be quantitatively evaluated if these risk management procedures are implemented. Certain sensitive receptors may require site-specific considerations.

3.2.4.4 Option – Consider Hypothetical Exposure Pathways in the CSM

In addition to the reasonably anticipated future exposure pathways, it may be helpful to assess the property for unrestricted use, which typically involves evaluating a residential land use scenario. In the remedial action stage, this evaluation may provide information that allows the responsible party and agency to identify the relative differences in remedial actions between the reasonably anticipated use and unrestricted use scenarios.

3.2.5.1 Option – Start with the Affected Media

When the original sources of chemicals are unknown, the CSM can start with environmental media that are known or suspected to have been affected by the original source and thus serve as both potential exposure media and secondary sources of chemicals. The secondary chemical sources can be represented by the environmental medium affected by a release. Examples include surface soil, subsurface soil, air, sediment, and surface water.

3.2.5.2 Option – Search for Additional Site Information

Historical site information and data can be used to learn about the potential sources at the site. This information may include site use, chemical storage and usage, geographic and topographic info, aerial photos, soil type, land records, Sanborn maps, as-built drawings, and community input. The site history helps to identify the primary and secondary chemical sources, the chemicals in environmental media, and potential exposure media. In some cases, this information can be obtained from a Phase I environmental site assessment or from the results of an appropriate inquiry.

3.3.1.1 Option – Incorporating Risk Assessment Data Needs During the Planning Process

Incorporating risk assessment data needs into the field investigation minimizes data gaps for conducting the risk assessment and costly additional data collection efforts. The data collection programs must define the type of data that will be collected, the quantity of data, and level of quality that the data must achieve to support and defend environmental decisions. The data collection programs can be scaled based on site-specific conditions (for example, the size of the site, the number of environmental media, the number of samples to be collected, the project scope, and the available budget).

3.3.1.2 Option – Identify DQOs

The DQO process described in USEPA guidance documents such as Guidance on Systematic Planning Using the Data Quality Objectives Process (USEPA 2006c), provides information to support the development of data collection programs and the evaluation of site data to decide whether the data are usable in the risk assessment. This process is iterative and flexible. As information is developed, the elements of the process can be reevaluated and revised to account for new information.

3.3.2.1 Option – Determine Exposure Areas

An exposure area (also called an exposure unit) is a geographic area over which a receptor is reasonably assumed to move at random and equally likely to come into contact with an environmental medium (for example, soil) at all sublocations (both spatially and temporally). An exposure area is further defined on the basis of observed or assumed patterns of receptor behavior, historical activity, and the nature and extent of chemicals in environmental media (USEPA 1989a).

Use of the exposure area concept can prevent bias in EPCs that might occur if data were averaged over an area not representative of the exposure area. Whether these areas are significant from a risk perspective depends on whether receptors and exposure areas have been accurately identified. Determination of exposure areas affects project costs because the number of samples needed (including duplicate and blank samples; see Section 4.3) depends on the variability in chemical concentrations within the exposure area (Hartmann et al. 1993). Section 6.2.2.1 of this document includes a discussion of exposure areas.

3.3.3.1 Option – Identify Hot Spots

Tools such as Gilbert’s Statistical Methods for Environmental Pollution Monitoring (Gilbert 1987) and Visual Sampling Plan (Pacific Northwest National Laboratory 2012) are available to evaluate the probability that a sampling design will find an area of some assumed size or, conversely, that one was missed. Various approaches can identify hot spots and how these areas should be treated in the quantitative risk assessment when making site management decisions. Section 3.2.1 of Use of Risk Assessment in Management of Contaminated Sites (ITRC 2008) identifies the characteristics and definition of hot spots used by various state agencies and presents a discussion of theoretical and practical considerations for hot spots. If concentrations are at levels that may pose acute toxicityAny poisonous effect produced within a short period of time following an exposure, usually 24 to 96 hours (USEPA 2013). issues, then the project manager should be notified so that appropriate action can be taken.

If preferential exposures may occur at hot spots, then the need for, and type of, hot spot evaluation should be discussed during project planning. If hot spots are evaluated in the risk assessment, exposure factors should be adjusted appropriately to distribute the frequency and rate of exposure between the hotspot and the rest of the site.

The issue of soil sampling to support risk assessment is discussed in Section 3.2 of Use of Risk Assessment in Management of Contaminated Sites (ITRC 2008). Data representativeness is key to the discussion of data quantity and quality and has received considerable attention in the literature (Crumbling 2002; Jenkins et al. 2005; Ramsey and Hewitt 2005). A representative sample should reflect the exposure concentrations to be considered for the pertinent exposure pathway. For example, exposure to shallow soil in a residential area is most often the average concentration throughout an area analogous to a residential lot. For a receptor that roams or spends time throughout an exposure area, a representative sample may reflect conditions over several acres.

For soil sampling in particular, and for other media as well, representativeness means that a sample is composed of the elements of interest in the population being sampled in the same proportion as they occur in the population of interest (the population being sampled). The population of interest should be established before actual sampling. The representativeness of a data set depends on the type, number, and locations of the samples collected as well as the value of collecting additional data (see Section 4.1.1.1).

3.3.4.1 Option – Identify Media to Sample

Samples should be collected from environmental media identified in the CSM. At many sites, particularly larger and more complex sites, samples are typically collected from various environmental media including soil, groundwater, soil gas, sediments, surface water, air, and biota. The principles that guide the process for collecting soil samples also apply to other environmental media, types of data, and information used in risk assessments. In particular, a clear understanding of the mechanism or algorithm by which the results are interpreted should be in place and should determine the sample design. Similarly, a visual presentation of the sampling results is recommended (as for soil sampling).

3.3.4.2 Option – Define the Number of Samples to Collect

The minimum number of samples needed to develop a representative concentration for chemicals in each environmental media for each exposure area is established as part of the development of the data collection program. For soils, the primary question is, “Over what volume of soil should I estimate the average?”  (Reynolds, Hadley, and R.M. 1990; Sedman, Reynolds, and Hadley 1992). For example, when evaluating residential soil ingestion, the appropriate volume reflects exposure in a residential setting, which could be on the order of many tons of soil. By analogy, the volume of groundwater of concern is estimated by the mass discharge that would be unacceptable for delivering suitable quality water from either a real or hypothetical well of a given extraction rate (Hadley and Newell 2012) and the screened interval of the well. Additional information needed to calculate appropriate groundwater EPCs for risk assessment are provided in USEPA’s Determining Groundwater Exposure Point Concentrations (USEPA 2014b), and useful information on groundwater statistics is provided in ITRC guidance (ITRC 2013). In many cases, the mass discharge (and thus the projected exposure concentrations) may be best estimated not by a series of groundwater measurements from monitoring wells throughout an aquifer, but rather by results of a pump test from one location. Estimating the mean in a well-defined exposure area, however, is not particularly useful in addressing stakeholder concerns about hot spots.

USEPA’s Web site “Resources for Planning New Data Collections” (USEPA 2008a) also offers useful planning guidance. Tools such as Pacific Northwest National Laboratory’s Visual Sample Plan (Pacific Northwest National Laboratory 2012) or the University of Tennessee’s Spatial Analysis and Decision Assistance (SADA) (University of Tennessee 2013) can assist in development of a sampling program that provides adequate sample density with an associated level of statistical confidence. Incremental sampling methodologies (ITRC 2012a) along with statistical sampling design may also be used to address many of the issues regarding the adequacy of sample numbers and spatial density for solid media (soil and sediment).

USEPA guidance (1992d) states the following:

Sampling data from Superfund sites have shown that data sets with fewer than 10 samples per exposure area provide poor estimates of the mean concentration (i.e., there is a large difference between the sample mean and the 95 percent UCL), while data sets with 10 to 20 samples per exposure area provide somewhat better estimates of the mean, and data sets with 20 to 30 samples provide fairly consistent estimates of the mean (i.e., the 95 percent UCL is close to the sample mean).

The number of samples to collect, however, is a site-specific decision that should be discussed by the project team during the planning stage. Defaulting to a minimum of 10 samples may not provide adequate characterization of a site or an appropriate data set for risk assessment, especially for sites with highly variable concentrations.

3.3.4.3 Option – Select the Appropriate Sampling Design

The applicable site-specific exposure locations, soil exposure depths, and sample types (for example, sieved versus unsieved for use in the integrated exposure uptake biokinetic (IEUBK) model (USEPA 2010a) should be identified during project planning and reflected in the site-specific CSM. The risk assessment data needs should include the site-specific exposure locations, soil depths, and sample types and should be incorporated into the planned sampling design. These data needs may be in addition to the sampling needs for other project objectives (such as nature and extent characterization of the site, which may require sampling at depths beyond those that may be contacted by site receptors).

Many risk assessments assume that human exposure can and will occur (no matter the locations or depths from which the soil samples were collected) or are based on depths established in regulations, policy, or guidance. This simplifying assumption minimizes the need to justify which depth intervals could be contacted either under current or potential use conditions. It is also reasonable, however, for a risk assessment to use only data that are relevant to current and potential exposures. Such assumptions may be appropriate if the soil is well characterized and if controls prevent or prohibit bringing subsurface soil to the surface, where receptors could contact the previously buried soil. In certain instances, assuming contact with soil only from certain depth intervals may require institutional or engineering controls to prevent exposures that were not evaluated.

3.3.4.4 Option – Evaluate the Benefits of Collecting Additional Data

Uncertainty is inherent in investigation and risk assessment, and an individual’s comfort level should not be entirely based on an arbitrary metric that X samples are required for an area of Y. Rather, the risk assessment process should effectively balance the value of spending more time and resources to obtain additional information to reduce uncertaintyThe lack of perfect knowledge of values or parameters used in a risk assessment. Uncertainty may be reduced by collection of additional data. with the need to make a timely decision. Project managers should guard against “paralysis by analysis,” when the desire to reduce uncertainty is used as an excuse to avoid or postpone decision making. Does additional information change the current understanding of the risks posed by the chemical or alter the nature of the eventual decision? If not, then no additional sampling and analysis is needed.

3.3.5.1 Option – Identify Types of Bias

In many cases, the only available data for the risk assessment are soil samples collected for purposes of source area characterization and delineation. These data are therefore biased to areas that are known or suspected to be contaminated or areas with the likely highest concentration of chemicals (for example, areas near the potential source of a chemical), even where those areas are small. In other instances, samples are sometimes collected from the perimeter of known source areas to define the extent of chemicals in environmental media (rather than characterize the maximum concentrations present on site). In the context of risk assessment, samples should be representative of the exposure concentration to be considered for the pertinent exposure pathway. For example, exposure to shallow soil in a residential area is most often the average concentration throughout an area the size of a residential lot. For a receptor that roams or spends time throughout an exposure area, a representative sample may reflect conditions over several acres. Biased samples may not represent the potential exposure area for a receptor, and may yield overly conservative EPCs.

3.3.6.1 Option – Consider Additional Analytical Parameters Where Appropriate

Site history and the CSM should be used to help determine additional analytical speciation and degradation products needed for the risk assessment. The potential presence of degradation products should be considered when developing a list of analytical parameters. For example, tetrachloroethylene (PCE) degradation products include trichloroethylene (TCE), dichloroethylene (DCE), and vinyl chloride. In addition, metals speciation may be needed (for example, hexavalent chromium versus trivalent chromium, inorganic arsenic versus organic arsenic). Chemists or other team members knowledgeable in biodegradation and biotransformation can be consulted as needed to identify potential degradation products.

3.3.7.1 Option – Addressing Background Concentrations During Chemical Screening

Some state environmental agencies acknowledge that naturally occurring background concentrations of some inorganics exceed risk-based screening values and do not require carrying a chemical forward in the risk assessment process if it is within background concentrations (DTSC 2008; MDEQ 2005; NJDEP 2012a; FAC 2013) Petroleum Contamination Site Cleanup Criteria, among others). This approach focuses the risk assessment on only those chemicals that are site related and have the potential to be addressed in remedial action. For example, Arkansas has mineral deposits throughout the state (galena, cinnabar, and vanadium, for example). It would be inappropriate, after confirmation of background concentrations, to require a responsible party to investigate those metals to a concentration smaller than naturally occurring mineral concentrations.

3.3.7.2 Option – Addressing Background Concentrations During Risk Characterization

USEPA’s Role of Background in the CERCLA Cleanup Program (2002e) addresses background concentrations in risk assessments. This document notes that USEPA’s preferred approach is that detected chemicals present at concentrations exceeding risk-based screening values are retained in the risk assessment, regardless of whether they are within background concentrations. The USEPA recommends discussing the contribution of background concentrations to the total site risk estimates in the risk characterizationThe risk characterization integrates information from the preceding components of the risk assessment and synthesizes an overall conclusion about risk that is complete, informative and useful for decision makers (USEPA 2000c). section of the risk assessment (see Section 6.2.5 for an example). With this approach, information is provided to the public regarding elevated risk levels attributable to background concentrations. This information allows the public to make informed decisions about exposure to affected environmental media. This approach, however, results in additional chemicals being carried through the risk assessment, beyond those chemicals that may be addressed in remedial action. In addition, at those sites where some or all detected concentrations are within background concentrations, the public may be left wondering how health will be protected when background concentrations exceed agency-acceptable risk levels.

3.3.7.3 Option – Addressing Background Concentrations During Uncertainty Analysis

In another approach to background concentrations, chemicals within background concentrations are eliminated in the data screening step, but a discussion of background risks is included in the uncertainty analysis section of the risk assessment. This approach requires less effort on non-site-related issues, while information is provided to the public regarding elevated risk levels attributable to background concentrations. The public can then make informed decisions about exposure to environmental media.