9.2 Other Considerations and Limitations
This section discusses key aspects of using site-specific RBA values in HHRAs, including the importance of communicating the limitations and assumptions, the potential for RBA values to change, how to account for bioavailability of background soil, soil and source characteristics, and variability and validity of bioavailability results.
9.2.1 Communicating Limitations and Assumptions
In addition to characterizing the risks associated with potential receptor exposure, an HHRA should discuss its key assumptions and uncertainties.
9.2.2 Potential for Bioavailability to Change in the Future
In order to support risk management decisions, HHRAs often seek to characterize current and reasonably expected future risks for each potential receptor. Because of the concern about potential future risks, HHRAs that use site-specific RBA values should account for potential factors that could alter the bioavailability of a chemical in soil in the future (Chaney, Basta, and Ryan 2008).
9.2.3 Accounting for Background Bioavailability
While many regulatory agencies offer specific guidance on how HHRAs should account for background soil concentration and background risk, few provide any guidance on how RBA could be considered in the characterization of background risk. This section describes one approach for how background RBA could be considered during the site investigation process and used in the HHRA.
9.2.4 Soil and Source Characteristics
The sampling design must capture the heterogeneity of the soils and meet data quality objectives. For example, the solubility of arsenic in soil varies depending on the origin of the soil, the source of the arsenic, and the chemical interaction of arsenic with other minerals present within the soil (see Arsenic). This variability can produce arsenic forms that are more tightly bound within the soil and less available for absorption. Site history also plays an important role in characterizing the soil contamination. Furthermore, the use of proper sampling techniques and methods could reduce uncertainty in the bioavailability results. Similarly, bioavailability could be influenced by particle size as demonstrated for lead (USEPA 2003b). (See the discussion of lead sources and soil type)
9.2.5 Variability and Validity of Bioavailability Results
Uncertainty can stem from the bioavailability tests and methods themselves, validation and quality assurance criteria used for testing, and the choice of correlation model. See Section 5.2.3. Other uncertainties include the extent to which bioavailability test results of a given protocol are reproducible within and between laboratories. If results are not comparable, uncertainties with the sample preparation and testing should be discussed in the uncertainty analysis.