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5. Moving Forward with Bioavailability in Decision-Making
Pages 356-410

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From page 356...
... To more rationally allocate limited environmental management and remediation resources, there is a need to improve risk assessment by including more explicit consideration of bioavailability processes. Inadequate scientific understanding has hampered the widespread consideration of bioavailability processes in remedial decision making to date.
From page 357...
... As some description of contaminant-solid interaction will usually be needed for assessment of risk associated with contaminated soils and sediments, the model and parameter uncertainty will transfer directly to the exposure assessment in a risk analysis. All models and parameters used in exposure assessment have a certain degree of uncertainty associated with them, including those used in bioavailability process considerations.
From page 358...
... Given these multiple sources of uncertainty, regulatory agencies have been cautious about moving away from default assumptions concerning bioavailability processes in risk estimates. It is not clear whether there is too much uncertainty associated with bioavailability tools for regulatory agencies to feel comfortable about more explicitly incorporating their results into exposure estimates.
From page 359...
... There have been many other cases, however, in which the level of uncertainty has been judged to be too high for bioavailability measurements to replace default assumptions. A prominent example is the case of the Times Beach, Missouri, Superfund site, where large amounts of dioxin-contaminated soil were excavated and incinerated (see Box 5-1~.
From page 360...
... Technologies Developed with the Intent to Decrease Bioavailability A number of treatment technologies have been reported that "decrease bioavailability" that is, treatment that impedes transfer of a contaminant from the soil or sediment matrix to a living organism. Although institutional controls and containment remedies would theoretically be encompassed by this definition, this discussion focuses on in situ treatments that aim to either (1)
From page 361...
... . Examples of such technologies include biostabilization (the use of bioremediation to reduce contaminant mobility and toxicity of contaminated soils and sediments)
From page 362...
... . Thus, thresholds can be affected by external mass transfer limitations, which often occur with aged pollutants in soils and sediments (Bosma et al., 1997~.
From page 366...
... may be insufficient, unless multiple exposure pathways and multiple receptors are considered. The challenge to all bioavailability assessment is to quantify the relevant bioavailability processes at work in a given situation, which requires an understanding of the importance of all exposure routes and receptors.
From page 367...
... Further, measurements that may approximate only certain bioavailability processes, such as chemical mobility measurements or the water-soluble fraction of a compound, have been employed to infer satisfactory treatment. Again using biostabilization as an example, a recent review of remedies for hydrocarboncontaminated soils from petroleum refining, wood treating, petrochemical manufacture, and gas and electric utility sites demonstrates the wide variety of surrogate measures of bioavailability utilized.
From page 368...
... Thus, some technologies attempt to increase pollutant removal or destruction by facilitating bioavailability processes. These technologies increase mass transfer from the sorbed phase via physical means (grinding or mixing to decrease diffusional paths, increasing temperature to increase mass transfer rates)
From page 369...
... from soil. Nonetheless, without a complete understanding of the bioavailability process and appropriate tools to measure the constituent steps, it is difficult to ascertain with certainty the impact of these bioavailability enhancement techniques on the long-term fate of the contaminants.
From page 373...
... In SOll. Remediation Strategies with Unintentional Effects on Bioavailability A number of technologies used for the remediation of contaminated soils or sediments operate through principles of increasing the mobility and consequently the bioavailability of contaminants.
From page 374...
... None of these potential outcomes is desirable or intentional, but all must be considered in the dredging of sediments. Similar concerns are considered in the excavation of contaminated soils, where particulate matter is prone to atmospheric transport, and volatile contaminants may be lost to the gas phase.
From page 375...
... WHEN WILL CONSIDERATION OF BIOAVAILABILITY PROCESSES MAKE A DIFFERENCE? Explicit consideration of bioavailability processes in site-specific risk assessment can be technically difficult, time consuming, and costly.
From page 376...
... considered. Redox Changes the chemical form of a No destruction of contaminant mass manipulation contaminant to decrease solubility, occurs such that long-term stability mobility, and bioavailability.
From page 377...
... when default assumptions about bioavailability processes or parameters are not appropriate for the site; 3. when a significant difference in the remediation goal is possible if bioavailability processes are considered; 4.
From page 378...
... Thus, decisions about the bioavailability of such chemicals in sediment or soil have important implications for bioaccumulation and food web transfer. As the concern at sites with contaminated soil or sediment is usually risk from long-term exposure, the chronic toxicity of chemicals is usually the focus of assessment.
From page 379...
... , the value of refining risk estimates to include bioavailability process information may be limited. Default Assumptions are Inappropriate As discussed in Chapter 2, risk assessment incorporates numerous assumptions that may be inappropriate or incorrect for a given site.
From page 380...
... With the flexibility to perform a site-specific risk assessment, the key regulatory issue then becomes the type of bioavailability process assessment allowed and the level of scientific rigor that must be associated with results for the assessments to be potentially acceptable. A Significant Difference in Remediation Goals is Possible Consideration of bioavailability processes in a risk assessment for a particular chemical is usually worthwhile only if there is potential for the revised exposure assessment to change the estimated risk (and thus the cleanup goal)
From page 381...
... This will be possible to do with confidence when the pathway of concern, site conditions, and key bioavailability processes are not likely to change with time. Obviously, there are many factors that may change the bioavailability or toxicity of a compound in the future.
From page 382...
... Regulatory and Public Acceptance is Possible The potential for results from bioavailability process analyses performed in risk assessment to support remediation decision-making depends on the regulatory domain and public acceptance. Before undertaking a bioavailability process assessment, the likelihood of acceptance of the results by regulators and the
From page 383...
... In contrast, some state environmental agencies and EPA regions have developed guidance for consideration of bioavailability processes in risk assessment. EPA Region 10, for example, developed guidance for bioavailability considerations in human health risk assessments for arsenic contaminated soil (see Chapter 2~.
From page 384...
... Various actions are needed to make progress in using bioavailability processes in risk assessment and decision-making at individual sites, in acknowledging bioavailability processes in regulations and creating appropriate guidance for management of contaminated soils and sediments, and in better understanding bioavailability processes on a mechanistic level. In Risk Assessment and Decision-making at Individual Sites In order for bioavailability processes to be considered more explicitly in risk-based management at individual sites, key issues that represent obstacles need to be addressed aggressively.
From page 385...
... Early attempts to explicitly consider bioavailability processes in risk assessment have frequently used inappropriate tools (that usually were not relevant and/or validated) , which has contributed to concern about inclusion of bioavailability in risk-based decision-making for contaminated soils and sediments.
From page 386...
... When it is possible to choose tools that will provide better mechanistic understanding, this opportunity should be exploited and not bypassed in favor of conventional empirical assessment approaches. Given the complexities of bioavailability processes identified in this report, it is likely that mechanistic tools and predictive model development will be a multi-decade effort.
From page 388...
... In the case of bioavailability processes, there is almost no guidance on approaches for long-term monitoring that specifically target the stability of the contaminant "form" instead of total contaminant concentration. Furthermore, monitoring may need to shift from classical site monitoring of total contaminant levels to include the activities of receptors, changes in site-specific processes (e.g., geochemistry)
From page 389...
... Community Concerns and Risk Communication Experience has demonstrated that communities often have concerns about consideration of bioavailability processes in risk assessments for decision-making at hazardous waste sites. Perhaps most importantly, bioavailability assessments may be viewed as a "do-nothing" or "do-less" approach.
From page 392...
... . Nonetheless, consideration of bioavailability process information for contaminated soils and sediments is inherently part of the risk assessment process, whether for protection of ecological receptors or human health.
From page 393...
... Into the Regulatory Arena In relatively few cases has the replacement of default assumptions about bioavailability processes with site-specific measurements been incorporated as a matter of practice into protocols that govern risk-based decision-making in the regulatory arena for contaminated soils and sediments. Consideration of physical transport processes is sometimes permitted, if these processes are adequately characterized.
From page 394...
... A viable way to move around these obstacles and achieve more widespread consideration of bioavailability processes in risk-based management of contami
From page 395...
... Adaptive management is receiving increasing attention and application to problems of regional ecosystem management (Gunderson et al., 1995; Lee, 1993; Walters, 1997~. It is being promoted for wider use in water management programs such as in the Florida Everglades and in Glen Canyon on the Colorado River (NRC, 1999, 2001b)
From page 397...
... The outcomes must be based on integrated scientific experimentation with attention to uncertainties and hypothesis testing to reduce these uncertainties. The adaptive management paradigm allows a way around the stakeholder, regulatory, and policy gridlock that characterizes cleanup at many contaminated soil and sediment sites.
From page 398...
... Assessment of risk can be performed simultaneously, and the influence on risk of the bioavailability process information developed can be elucidated. Into the Scientific Arena Expansion of bioavailability process considerations into risk assessment and remediation decision-making for contaminated soil and sediment sites requires improved scientific understanding and models for a number of key bioavailability processes.
From page 399...
... Those are usually, and understandably, not conducted in a way that advances understanding of fundamental underlying processes. Greater mechanistic understanding and predictive models of bioavailability processes are needed to improve the accuracy of risk assessments for contaminated soil and sediment sites.
From page 400...
... The vast majority of these research projects have involved mobility of chemicals in the environment, uptake relevant to assessing ecological risks, and bioavailability processes that might affect bioremediation. Despite this research investment, progress in understanding these bioavailability processes is quite limited.
From page 401...
... For understandable reasons, this objective does not include an exploration of factors that might influence bioavailability processes, and therefore it is difficult to determine the extent to which these observations can be generalized or used to predict the results that might be obtained at different sites or under different conditions. Unless a greater commitment is made to fund bioavailability process studies from more of a research perspective, progress in developing information that can be utilized to advance human health risk assessments will be slow.
From page 402...
... Thus, tools for measuring bioavailability processes that further mechanistic understanding and promote predictive model development are preferred over conventional empirical approaches. In the short term, empirical approaches are useful in generating sitespecific information provided that their results are analyzed using a weight-ofevidence approach and with an understanding that they will be replaced with more mechanistic tools as they are developed.
From page 403...
... Although bioavailability is not a unique risk communication problem, experience has demonstrated that communities often have concerns about consideration of bioavailability processes during risk assessments. In order to demonstrate the utility of explicitly considering bioavailability processes and to test new models and tools, adaptive management should be applied to select pilot bioavailability test sites.
From page 404...
... 1991. Toxicity reduction in contaminated soil bioremediation processes.
From page 405...
... 1993. PAH loss during bioremediation of manufactured gas plant site soils.
From page 406...
... 1998. Phytoremediation of uranium contaminated soils: role of organic acids in triggering uranium hyperaccumulation in plants.
From page 407...
... 1990. Effect of desorption and intraparticle mass transfer on the aerobic mineralization of oc-hexaclorocyclohexane in contaminated calcereous soil.
From page 408...
... 1987. Differential bioavailability of 2,3,7,8-tetrachlorodibenzo-p-dioxin from contaminated soils.
From page 409...
... 1988b. Bioavailability and cytochrome p-450 induction from 2,3,7,8-tetrachlorodibenzo-p-dioxin contaminated soils from Times Beach, Missouri, and Newark, New Jersey.


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