2.0 Overview of Processes Affecting Bioavailability
That Should Be Considered in Developing a Conceptual Site Model
Incorporating bioavailability into risk assessments and management decisions requires recognition during the scoping phases of how bioavailability fits within the state or federal regulatory process for characterizing the level of exposure to contaminants in sediments. Bioavailability concepts are implicit in USEPA’s risk-based approaches used for assessing both human health and ecological risks and are encouraged throughout the contaminated sediment management process (Greenberg and Sprenger 2008). Use of bioavailability in the sediment management process varies by state, although many use USEPA’s ecological risk assessment guidance for Superfund (USEPA 1998c, 1997b, 1992b) and for human health (USEPA 1989a, 1989d).

Consideration of bioavailability begins during the scoping or problem formulation step by linking the source release(s) to potential receptors through the development of a CSM. The CSM becomes the descriptive framework in which to insert specific exposure pathways and subsequent bioavailability measures and/or models into the site characterization and risk assessment process.

While consideration of processes affecting bioavailability should be an integral part of a risk assessment and/or the risk-based management of contaminated sites, it is often neglected due to a perceived greater burden of proof for incorporating bioavailability into decision making (NRC 2003). Bioavailability information can, however, help to alleviate concerns, provide greater transparency in decision making, and allow for setting more technically defensible cleanup goals and realistic cleanup priorities.

2.1 Processes Affecting Bioavailability
Management of COPCs requires an understanding of how they are released from the sediment matrix, transported, and taken up by a target organism. Bioavailability tools are methodologies applied to evaluate the magnitude of release of contaminants from the solid to dissolved phase (bioaccessibility) and assess the uptake of a COPC into receptor organisms (bioavailability). Figure 2-1 is a representation of some example contaminant transport pathways between environmental compartments and native ecological receptors in a freshwater system:

• movement or transport to and into an organism by absorption of the dissolved form from sediment pore water or by the degree of accumulation through the food chain
• transport of COPCs bound to sediment particles through the skin or gut following dermal contact or ingestion
• movement of the COPC across biological membranes
• subsequent transport to the site of biological response

For most aquatic risk assessments, COPC movement is either directly measured, estimated using models, and/or measured as tissue residues within the target organism.

Figure 2-1. Key exposure pathways for human health risk at contaminated sediment sites.

The complexities of the sediment and aquatic systems make accurate tracking impractical, but reasonable estimates can be made. Fortunately, the ability to measure the dissolved and particulate phases of COPCs has improved considerably, and the tools described in the specific pathway chapters have a documented basis for applying these values to exposure evaluation. The general scoping process described in this section is intended to help the user understand when relatively simple considerations are adequate and where more complex tools may be required.

2.1.1 Physical Transport Processes
Physical transport processes in sediments may include any of the following:

• advection/diffusion
• resuspension/deposition
• burial
• bioturbation
• ebullition (gas transport)

Physical processes (Figure 2-2) are generally responsible for the transport of a COPC within the sediment. Physical contaminant transport within sediment can be upward (advection/diffusion, ebullition), downward (advection/diffusion, burial), or lateral (resuspension/deposition). In water overlying the sediments, COPCs can move by the same advective and diffusive forces operating within the sediment, by sorption to/from sediments resuspended by currents or scour events, or through bioturbation caused by benthic organisms. Transport of COPCs to a site from sources such as groundwater, surface soil erosion, and outfall discharges can be considered physical processes. Hydrodynamics directly affects sediment and associated COPC transport in several ways. Sediment transport typically varies in a nonlinear fashion with hydrodynamic energy. This energy can be in the form of water flow in a river or tidal estuary or in wind-driven waves in a large lake or bay.

Advective flux in groundwater can transport COPCs directly into and through the sediment-associated pore water, as well as influence redox conditions in the sediments (USEPA 2008a). Diffusive mixing due to temperature, salinity, or pressure gradients can change COPC sorptive behavior (i.e., equilibrium partitioning) over short or long periods of time. COPC redistribution due to bed-shear caused by currents, ice scour, waves, propeller wash, or anchor drag can result in suspension of bulk sediments and can also alter partitioning relationships so that COPCs may be desorbed into the water column. These same processes can result in the release of buried nonaqueous-phase liquids (NAPLs). Finally, ebullition (gas bubble transport) may result in the upward movement of COPCs within the sediment.

Figure 2-2.Physical transport and ecological receptor processes in a freshwater system.

2.1.2 Chemical Processes
Just as physical processes impact the distribution of sediments and COPCs in aquatic systems, so do chemical processes. It is critical that chemical processes be considered in conjunction with physical and biological processes in developing a CSM and subsequent site investigations. The bioavailability of COPCs is directly affected by the interplay of these processes.

This document considers the following chemical characteristics or processes in sediments:

• sorption/desorption
• transformation/degradation
• oxidation/reduction

Sorption/desorption processes are important in defining what “compartment” of the sediment matrix a COPC will reside in. Sediment bioavailability is often mediated by the movement of a COPC from the bulk sediment matrix (desorption) into an aqueous phase. The transfer mechanism can be either via pore water or through the gut of the organism. An example of a sorption/desorption process affecting bioavailability is the binding of polycyclic aromatic hydrocarbons (PAHs) to black carbon at a former coal gas site, rendering these compounds less bioavailable than would have been predicted (Hawthorne et al. 2007). Geochemical processes (e.g., redox/pH) are generally more important when considering inorganic COPCs. An example is the reduction of soluble Cr(VI) to the less soluble Cr(III) species (Borch et al. 2010, Graham and Bouwer 2010).

Sediment geochemical parameters—such as the quantity and type/quality of organic carbon (OC), the presence of acid volatile sulfides (AVS), the redox state of the sediment, salinity, or pH—can also influence whether a COPC is tightly bound within the sediment and unavailable for uptake, or whether it is freely dissolved and can be absorbed into organisms.

Mixing in the hyporheic zone (i.e., the region where surface water and shallow groundwater mix) can result in a release of COPCs due to changes in pressure, oxygen, temperature, oxidation state, and/or salinity. In some cases this process has resulted in increased sequestration of metals or increases in the flux of reduced metals at higher concentrations than would otherwise be expected in oxygenated surface water (e.g., arsenic in a contaminated Montana stream) (Bencala 2005). Thus, the chemical environment is a controlling determinant of potential bioavailability associated with a specific physical transport process.

2.1.3 Biological Processes
As discussed above, infaunal benthic organisms may take up COPCs from sediments and then serve as prey for higher-level organisms. Biological processes in sediments discussed in this document include the following:

• uptake
• bioconcentration or bioaccumulation
• biotransformation

“Uptake” refers to processes whereby a COPC in sediment is transported into an organism. These processes can include movement of the COPC through the gut, external surfaces (e.g., gills), or the integument (skin) of an organism. The COPC must be bioavailable to be absorbed and subsequently transported to a site of action to have an adverse effect. The COPC may also be metabolized (biotransformed) by the organism, or it may bioaccumulate within the organism (generally within lipids or fatty tissues although some compounds preferentially bind to proteins). Finally, microbial activity within the sediment matrix may play a large role in the biotransformation or degradation of a COPC (e.g., inorganic mercury biotransformed to the more toxic and bioavailable form of methylmercury).

2.2 Scoping the Problem
During site assessment, one begins identifying the questions that need to be answered to evaluate and manage site risks. An initial site walkover reveals the nature of the aquatic habitat, which is important as the disposition of various

COPCs may differ with the type of water body and benthic substrate (e.g., freshwater vs. saltwater, lotic vs. lentic). The scoping process identifies the early CSM, initial site COPCs, and sometimes the initial narrative remedial alternatives for a site (USEPA 1988). Fundamental steps in the scoping process are as follows:

• determine site boundaries
• review site history
• develop an initial list of COPCs
• identify ecological and human receptors and exposure pathways
• develop the early CSM

Table 2-1 lists considerations within the scoping process that are introduced in this section and discussed in more detail in subsequent chapters.

2.2.1 Determine Site Boundaries
Boundaries for a sediment site include the source release area and the extent of flow-facilitated transport of COPCs, ice scour, wind, fetch, boat disturbance, construction activities, navigational dredging, and uptake and subsequent senescence of plants. It is recommended that, if not already performed, a wetland delineation be performed by a certified wetlands biologist at applicable sites to segregate operable units. Adjacent sites, particularly upgradient sites that may also have contributed COPCs, should be identified. In addition to identification of the types of COPCs present, the vertical and horizontal extent of the COPCs must be established.

The vertical and horizontal extent of the COPCs provides an indication of the areas that may contribute to an organism's potential exposure. In this sense, bioavailability could define a physical limit on the amount or duration of exposure that could occur. For example, at small sites, sessile organisms living within the sediments would be continually exposed to COPCs. Fish, birds, or mammals, all of which have larger home ranges, would be exposed to the contaminated area (or forage on contaminated prey) only during that time period that they (or their prey) are physically within (or foraging within) that contaminated area.

Table 2-1
Bioavailability considerations within the scoping process of a contaminated sediment site

¹The processes affecting bioavailability are defined in the Glossary (Appendix F). The tools and methodologies used to assess bioavailability are described in subsequent chapter and Appendix C.

2.2.2 Review Site History
The initial site scoping involves evaluation of existing information on the site background, site use, source release histories, potential COPCs released, area(s) of release, and site-specific and/or regional information available to help identify surface, subsurface, atmospheric, and biotic transport pathways (Ehrlich 1988). MacDonald and Ingersoll (2002) describe the type of information that should be identified from a review of the site history and that is needed for a site investigation. The following elements are helpful in formulating a CSM that includes an assessment of bioavailability:

• nature and quantity of COPCs released
• length of time contamination is estimated to have been in the sediment
• control or elimination sources of COPC(s)
• site or release conditions that may impact bioavailability
• other (i.e., non-site-related) potential source(s) of contamination
• physical and geochemical conditions of the site sediments
• hydrodynamic conditions at the site that may influence sediment transport
• historic and current use of the site by potential human and ecological receptors

Text Box 2-1 presents an example of the importance of site history review. At the Mocks Pond site in Muncie, Indiana, the use of a former quarry site as an effluent settling pond for a wire-galvanizing plant resulted in the deep accumulation of a fine, heavy metal–rich sludge. While the benthic conditions in the pond bottom were not conducive to support benthic infauna or fish, the overlying water supported a diverse and healthy fish and wildlife community. In evaluating the site history, it was determined that the spent sludge was treated with lime prior to discharge, which resulted in insoluble metal hydroxides. The CSM was developed and tested based on the hypothesis that these metal hydroxides were not biologically available through dissociation in pore water or surface water and that the sediment would not support aquatic life. Confirmation of the CSM allowed for a remedial alternative that included some removal, followed by capping to improve benthic habitat.

2.2.3 Develop List of Contaminants of Potential Concern
A critical element of the CSM is the identification of the chemicals that may have been released at a site or may occur for other reasons within or beyond the identified site boundaries. This initial list of COPCs is evaluated in more detail and refined during the screening process that is described in Chapter 3. Understanding the potential processes affecting bioavailability associated with different classes of compounds (e.g., PAHs, chlorinated organic compounds) may lead to a better understanding of analytical requirements, exposure pathways, COPC mobility testing, and relevant toxicity (e.g., critical body residue thresholds or species sensitivity distributions).

2.2.4 Identify Ecological and Human Health Exposure Pathways
A complete CSM requires an integration of the physicochemical conditions with each of the complete ecological and human health exposure pathways. The exposure pathways for screening assessment should be set up without consideration for bioavailability (Figure 1-2), then refined with bioavailability estimates in the baseline risk assessment. For example, fish could potentially ingest sediment in an impacted lake (so this would be a potentially complete pathway), but concerns of bioavailability of COPCs would be made in the text supported by empirical data. Considerations for bioavailability assessments in exposure pathway analyses are identified in Table 2-1 and described in more detail in Chapters 4–8.

2.2.4.1 Ecological exposure pathways
Numerous federal and state guidance documents that discuss the process for conducting sediment investigations are available to download (Table 2-2). These documents should be used to obtain background information on how to scope a sediment investigation. Additional references are also provided in each individual pathway chapter as it relates to how bioavailability may fit into the exposure evaluation.

Table 2-2
Links to guidance on conducting sediment investigations and risk assessments

While specific wildlife and exposure pathways are site dependent, four general receptor groups are evaluated (see Chapters 4–7 for more detail):

• Chapter 4: benthic invertebrates
• Chapter 5: fish and aquatic invertebrates
• Chapter 6: wildlife (aquatic dependent)
• Chapter 7: plants (aquatic)

Organisms that live in or on the sediment, generally referred to as “benthic invertebrates,” are in direct contact with the sediment and in some systems form the foundation of the aquatic food web. COPCs that are bound to sediment particles may partition to pore water and be absorbed across gill membranes or may be ingested and absorbed in the gut. Fish and other organisms that prey on benthic infauna are exposed in turn to those same COPCs. Processes that govern uptake by benthic infauna are discussed in Chapter 4.

Resuspension of bedded sediments can result in release of COPCs to the water column, where the chemicals can partition to a freely available (i.e., dissolved) form and be absorbed by phyto- and zooplankton. Fish and other water-column (pelagic) organisms that consume phytoplankton or prey on zooplankton are exposed in turn to these same COPCs. Processes affecting bioavailability and measures for pelagic food webs are discussed in Chapter 5. Aquatic-dependent wildlife (e.g., birds or mammals) are principally exposed through incidental ingestion of sediment (e.g., during foraging) or consumption of contaminated fish or shellfish. Processes affecting bioavailability and measures for these receptors are discussed in Chapters 6 and 7.

2.2.4.2 Human health exposure pathways
The primary and secondary exposure pathways for human health exposures from contaminated sediments typically include direct contact with sediments and surface water and ingestion of contaminated shellfish, fish, or less often plants (e.g., wild rice) and wildlife (waterfowl). Figure 2-3 shows an example of a human health food-web model. Chapter 8 discusses processes affecting bioavailability and measures for humans in more detail.

RAOs for human health exposure at contaminated sediment sites are most often based on the management of unacceptable lifetime cancer or noncancer risks from the consumption of fish or shellfish. Sediment cleanup levels are back-calculated from unacceptable tissue concentrations using models that range in complexity from simple BSAFs to sophisticated toxicokinetic models that are linked to systemwide fate and transport models. Because the primary risk pathway to humans is ingestion of organisms that have accumulated contaminants from the sediment, the most significant processes affecting bioavailability are addressed in that initial uptake. The processes and tools for evaluating this initial uptake are covered in more detail in this guidance in the chapters addressing risk to those receptors. Chapter 8 summarizes the human health exposure assumptions in more detail, with references to pertinent sections of Chapters 4–7.

Figure 2-3. Potential source media, chemical migration routes, receptors, and exposure pathways relevant to human health
(Lower Fox River).

2.2.5 Develop CSM
Assessing exposure and uptake requires formulating a CSM that considers the individual exposure pathways linking sources to potential receptors. Site-specific investigations of contaminated sites, as defined in USEPA’s Risk Assessment Guidance for Superfund (RAGS) (USEPA 1989d), are structured to incorporate measures of site-specific bioavailability through each step of the assessment process.

A CSM is especially important at sediment sites because the interrelationship of soil, surface water, groundwater, sediment, and human and ecological receptors can be complex. Sediment and soil may be subject to erosion, and both can be transported by natural events (e.g., floods) or man-made disturbances from engineering changes in a waterway. Because sediment is a dynamic medium subject to a wide range of natural phenomena, it is important to understand what chemicals may be bound and what may be bioavailable to human and ecological receptors.

The importance of a CSM in identifying the significance of bioavailability considerations in sediment management decision making is illustrated by the Lake Hartwell polychlorinated biphenyl (PCB) Superfund site in South Carolina (Text Box 2-2). The CSM identified the important physical, chemical, and biological processes for PCB transfer through the food chain that may cause risk to human health and ecological receptors. Based on the CSM, the natural burial of PCBs by clean sediment coming into the system was demonstrated to be reducing the bioavailable concentrations in bedded lake sediments. Based on this mechanism of decreased PCB bioavailability, USEPA selected monitored natural recovery (MNR) with long-term monitoring as the preferred remedy.

2.3 Data Quality Objectives
Incorporating bioavailability considerations into a CSM supports the development of testable hypotheses around which field data collection, laboratory testing, and modeling activities can be structured. Formulation of DQOs provides a systematic structure for defining the criteria that a data collection design should satisfy, whether or not they include bioavailability considerations. DQOs include when and where to collect samples, what parameters need to be tested, the tolerable level of decision error for the study, and how many samples to collect. Balancing risk and cost in an acceptable manner should be considered at this step in the process (see http://vsp.pnnl.gov/dqo/, DOE 2009). USEPA also provides an example DQO process germane to planning bioavailability assessments in the document Systematic Planning: A Case Study for Hazardous Waste Site Investigations (www.epa.gov/quality1/qs-docs/casestudy-final.pdf, USEPA 2006b).

2.4 Community Stakeholder Identification, Engagement, and Contributions
This section provides information to help affected individuals and groups participate more effectively in the site evaluation and remediation process for sites in which bioavailability processes are pertinent. In this document community stakeholders (stakeholders) include affected tribal sovereign nations, community members, members of environmental and community advocacy groups, and local governments.

Affected stakeholders are not necessarily limited to those on or immediately adjacent to the site. In the case of contaminated sediments in a river, affected stakeholders may include parties far removed from the source of contamination. Tribal sovereign nations may have treaties or other pacts with the federal government that grant them fishing, hunting, or access rights in places that are not necessarily near their present-day communities. This consideration is especially important in the identification of affected tribal sovereign nations.

For contaminated sediment sites, effective evaluation and treatment of environmental contamination require community engagement early in the process. Community involvement is particularly important for technically challenging situations where bioavailability assessments associated with contaminated sediments have not yet been well established and the approach will therefore necessarily involve some uncertainties. The contamination itself may involve a complex environment with many coupled systems and processes, and the relevant natural phenomena such as bioavailability, bioaccumulation, and biomagnification may be difficult to understand. Furthermore, the number of communities, states, and tribal sovereign nations affected by a contaminated sediment site may be large, encompassing a wide geographic area.

For effective community stakeholder identification and involvement, several important questions should be asked early in the process of investigating a site:

• Who are the stakeholders, and what is their interest in the site cleanup?
• What resources are the stakeholders trying to protect?
• What is the best method to communicate with the stakeholders?
• In what manner are these resources at risk, and can that risk be quantified?
• How will stakeholders be involved in deciding what is acceptable risk?
• Historically, what was the use of the site, and is this culturally significant to the stakeholders?
• What is the current use of the site, and what is the vision for the future use of the site?
• How will stakeholders be involved in the assessment, project planning, evaluation, and closure processes?

Individual states and the tribal sovereign nations may also recognize Native American tribes that are not currently recognized by the federal government. A list of federally recognized tribal sovereign nations can be found at http://www.gpo.gov/fdsys/pkg/FR-2008-04-04/html/E8-6968.htm. A list of Native American tribes not federally recognized can be found at www.kstrom.net/isk/maps/tribesnonrec.html.

2.4.1 Engagement Considerations
It is desirable to establish early and frequent communication between the project team and the stakeholders. As a project evolves and the scope of the project becomes better defined, stakeholder roles may change. Nevertheless, it is valuable to define roles and responsibilities early in the process and to agree on the methods that will be used for effective communication. Stakeholders must have an active role in the decision-making process.

Building effective relationships with stakeholders and involving stakeholders in collaborative decision making are essential components of a successful cleanup effort. All interested stakeholders must have access to critical information and the opportunity to be partners in technology development decisions during the planning, evaluation, and implementation processes. Stakeholders often have valuable information about site characteristics and history that can enhance the evaluation process and improve the quality of remediation decisions. Technology development and deployment decisions need to take into account the uses of affected water bodies, including fishing patterns, consumption of aquatic species, and cultural and ceremonial practices. It is important to achieve consensus with stakeholders about the intended end use of the site.

In the evaluation of the impact of the contamination and cleanup, it is important to keep in mind that not all of the affected communities may have comparable lifestyles. Lifestyle choices lead to different levels of risk, and communities may be disproportionately affected. For example, stakeholders with high ingestion rates of fish may be subject to greater risk than others. It is essential to establish what species are being fished and the methods used in their preparation for consumption. Certain stakeholders may have rights to fish by traditional methods that are prohibited for commercial fishermen. These may include dragnet fishing, which pulls up sediments. Thus, risks associated with contaminated sediments may be greater for those who fish by this traditional method than for commercial or sport fishermen.

2.4.2 Contribution Considerations
Stakeholders are likely to ask questions about the risks associated with specific technological solutions. Concerns might include questions about whether the remedy will lead to mobilization of the contaminants and how much contamination is being left in place. Stakeholders may also be concerned about making sure that there are sufficient data/support that any residual contamination will not pose an unacceptable risk (because it is not bioavailable) and will remain so in the future.

The most effective way to respond to such concerns and build public trust is to include stakeholders in the evaluation of the contamination problem and in the formulation of a remediation strategy. Open public discussion is particularly important in the evaluation of the problem and of potential technological solutions because the approaches to contaminated sediment problems are still under development and technological uncertainties remain. Stakeholders also need to be involved in the on-site observation and evaluation of the cleanup process.

Stakeholder involvement contributes to the overall success of a sediment management decision. Involvement of stakeholders in all stages of the process helps to build support for the cleanup effort and also helps to reduce the possibility of political and legal barriers to successful remedies.