The U.S. Environmental Protection Agency estimates that approximately 10 percent of the sediment underlying our nation’s surface water is sufficiently contaminated with toxic pollutants to pose potential risks to fish and to humans and wildlife that eat fish. This represents roughly 1.2 billion cubic yards of contaminated sediment (representing the upper five centimeters of sediment) where many bottom dwelling organisms live.
EPA’s Contaminated Sediment Management Strategy (USEPA 1998a)
According to current average costs for managing contaminated sediments, this volume of material could cost several trillions of dollars to dredge. In addition, state regulatory agencies are increasingly responsible for the identification, investigation, oversight, and management of contaminated sediment sites throughout the United States. More than 25% of the contaminated sediments sites addressed to date in the United States have had state drivers. As activity accelerates, this percentage will likely grow.
Methods to assess the potential effect of sediment contamination on human or ecological health are historically based on total contaminant concentrations in the bulk sediment. Unfortunately, the relationship between contaminant concentration in sediments and risk from exposure is not linear. Research conducted over the past 15 years has shown that the bioavailable concentration of many of these contaminants causing a toxic response in the receptors is much less than the total concentration of these contaminants in the sediment.
The National Research Council defines “bioavailability processes” as “individual physical, chemical, and biological interactions that determine the exposure of plants and animals to chemicals associated with soils and sediments” (NRC 2003). Only the bioavailable fraction of an environmental contaminant may be taken up and subsequently result in an effect on an organism. If contaminants are present but not physically accessible or chemically or biologically available, they should not be included in the calculation of risk. The principal objective of bringing bioavailability considerations into sediment risk management is to reduce the extent of cleanup required to that which is necessary to be protective of the environment. Incorporating bioavailability information in the calculation of risk can also be an important factor in balancing the risks caused by remedial action with the risks addressed by remedial action.
This web-based ITRC technical and regulatory guidance is intended to assist state regulators and practitioners in understanding and incorporating the fundamental concepts of bioavailability in contaminated freshwater or marine sediment management practices.
The objectives of this guidance are as follows:
- provide a basic understanding of bioavailability
- provide direction as to where bioavailability considerations may be pertinent in the human health and ecological exposure assessment processes
- describe the pertinence of assessing bioavailability during risk assessment process
- describe the tools available for bioavailability assessment and their application
- describe how bioavailability considerations can be used in risk management of contaminated sediment sites
- provide case studies that highlight the application of bioavailability assessment tools and methodologies in contaminated sediment site risk management
This guidance is constructed to assist the user in identifying the most relevant places within an exposure assessment that bioavailability can be assessed and which tools and methods are most useful and appropriate. As described in Chapter 2, the assessment of bioavailability is an iterative process that is carried forward through scoping and screening and in the evaluation of each applicable receptor pathway (benthic invertebrates, fish and aquatic invertebrates, wildlife, plants, and human health). Scoping activities are often revisited after completing a screening-level risk assessment (Chapter 3), as part of the planning for a remedial investigation and baseline risk assessment. Chapters 4–8 describe the ecological receptor pathways (benthic invertebrates, fish and aquatic invertebrates, wildlife, and plants) and the human health pathway. Bioavailability tools are identified in these chapters and organized according to whether they involve chemical analyses, biological analyses, or modeling. Case studies on their application are referenced as pertinent in these chapters as well as summarized in Chapter 9 and Appendix D. Chapter 9 describes how bioavailability assessment can be and has been incorporated into the contaminated sediment management process and indicates advantages and challenges to doing so.
Overall, this guidance establishes that bioavailability considerations should be incorporated in the exposure assessment process to obtain a clearer understanding of contaminant toxicity and exposure pathways such that remedy selection decisions can be focused and resources efficiently used. By incorporating bioavailability considerations into the early stages of site characterization, the risk assessment process, and remedy selection, a more effective remediation may be accomplished, which may well optimize overall cost. This web-based technical and regulatory guidance can help the user understand the proper application of these tools to assess bioavailability and more effectively protect human health and the environment.