Appendix D. Common Risk Assessment Defaults and Potential Site-Specific Options

Section 7.1.1.1 includes discussion of alternatives to default assumptions for values in 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). calculations.

Table D-1. Common risk assessment defaults and site-specific options
Component of Risk Assessment	Route of Exposure	Chemicals	Common Default	Options
Characterization	All	All	Maximum detected or UCL on biased samples	Soil/sediment - perform Outlier test, address hot spot separately, calculate EPC that is true to the data distributionA distribution describes the probability or likelihood of any potential value. (area-weighted averages) Groundwater - use more reasonable/average EPC, use data from most recent rounds (where stabilized)
Characterization	All	PAHs, dioxins, pesticides, metals (commonly As)	All concentrations are presumed site-related	Use site-specific or literature values to quantitatively account for background contribution. Determine whether site-related using lines of evidence approach.
Exposure	All	All	Residential 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). may be possible anywhere	Selection of future land use through access planning documents or interview planners, evaluate feasibility of deed restrictions, identify areas of relatively lower concentrations
Exposure	All	All	Deterministic risk numbers	Probabilistic risk assessment may help bound the uncertainties and ranges of values
Exposure	Dermal absorption	Semivolatile organic compounds (SVOCs)	Pharmacokinetic properties– not from soil	Use literature based value from soil
Exposure	Fish consumption	All	Sport fish or subsistence fishers	Obtain site-specific/region-specific exposure data (creel study), camera observations for fishing activities
Exposure	Garden produce pathway	All	High bias, especially with elevated soil concentrations, deterministic plant:soil ratios	Use regression models where available (Bechtel Jacobs Company LLC 1998); sample existing vegetation or develop a test plot; compare modeled intakes of metals to background intakes published by USDA and ATSDR (USDA 2014; ATSDR 2014).
Exposure	Soil contact	All	Exposure to all depths may be assumed	Use a physically realistic mixing model if you assume deeper soil can be moved to the surface by excavation, use data relevant to receptors, implement 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.
Fate and Transport	Fish/biota uptake	All	Default/conservative bioaccumulation and uptake factors	Develop site-specific uptake factors, identify relevant region-/species-specific factors
Fate and Transport - Leaching to groundwater	Ingestion, dermal contact	Lead	Assumes default values for bioavailabilityThe fraction of an ingested dose that crosses the gastrointestinal epithelium and becomes available for distribution to internal target tissues and organs (USEPA 2007c). and levels in background environmental media	Run IEUBK model (USEPA 2010a) with site-specific values, obtain in vitro or in vivo site-specific bioavailability data, use published % bioavailability
Fate and Transport - Leaching to groundwater	Ingestion, dermal contact	Metals, pesticides and dioxins	Assumes 100% bioavailability	Determine % bioavailable and reestimate risk/hazard impacts (major issue for As due to conservative toxicity)
Fate and Transport - Partitioning from solid to liquid matrices	Ingestion, dermal contact	All	Default partitioning coefficient (Kd)	Obtain site-specific data, use relevant literature data
Fate and Transport - Partitioning from solid to liquid	Ingestion, dermal contact	All	Equilibrium partitioning and infinite mass	Use finite mass, Synthetic Precipitation Leaching Procedure (SPLP)-type leaching from Soil Screening Guidance (soil to groundwater) (USEPA 1996b)
Fate and Transport - Partitioning from solid to air matrices	Inhalation	All	Equilibrium partitioning and infinite mass	Use finite mass, Synthetic Precipitation Leaching Procedure (SPLP)-type leaching from Soil Use finite mass Jury model (soil to air) (Jury, Spencer, and Farmer 1983)
Fate and Transport - Vapor intrusion	Inhalation	Volatile organic compounds (VOCs)	Use models such as: Johnson and Ettinger, empirical, 3-D.	Consistent with other routes of exposure in the risk assessment, use all available data and good science to evaluate the relevance/significance of this inhalation pathway.
Toxicity	All	Chromium	Conservatively use hexavalent Chromium toxicity	Analytical speciation and application of appropriate toxicity value(s) for all environmental media
Toxicity	All	Emerging Chemicals	For chemicals with no standards or evolving standards, there is no default, which makes site decisions challenging	Identify emerging chemicals as early in the process as possible and get team agreement on how to evaluate and make decisions on these chemicals.
Toxicity	All	Noncarcinogens	For screening, and sometimes baseline assessments, sum all hazard quotients	Calculate separate hazard indexes for different toxic effects
Toxicity	All	Noncarcinogens	Treat all values identically for 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).	Focus on chemicals driving the risk estimates Focus on chemicals with low uncertaintyThe lack of perfect knowledge of values or parameters used in a risk assessment. Uncertainty may be reduced by collection of additional data. factors incorporated into the noncancer 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). Focus on most realistic exposure scenarios
Toxicity	All	All	Appendix values from Provisional Peer Reviewed Toxicity Value (PPRTV) in screening and identification of risk-drivers (USEPA 2013f)	per PPRTV - do not use Appendix values for 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. decisions, use literature values to provide ranges of outcomes
Toxicity	All - Ingestion	PCBs	Quantitatively evaluate noncancer hazards for exposure to Aroclors that do not have RfDs	Choose appropriate surrogate or evaluate qualitatively

Publication Date: January 2015

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