THE CHALLENGE OF CONTAMINATED SEDIMENTS
Contaminated sediments in aquatic environments can pose health risks to many types of organisms, including humans. Exposure to the contaminants occurs by several routes, including direct contact and consumption of organisms that have accumulated contaminants from the sediments. The potential adverse effects on human health and the environment are compelling reasons to seek to reduce exposure.
Contaminated sediments can occur in small, localized areas or in vast areas, covering miles of river or harbor bottoms and associated floodplains. They occur in wetlands, coastal tidal flats and embayments, ocean basins, lakes, rivers, and streams. In some cases, contamination is relatively contained; in other cases, contaminated sediment exists throughout a watershed and may have multiple sources of contamination, including stormwater and sewer outfalls, industrial discharges, agricultural runoff, and atmospheric deposition.
The chemicals of concern in contaminated sediment sites vary; polychlorinated biphenyls (PCBs) are the most common, followed by metals, and polycyclic aromatic hydrocarbons (EPA 2005). The widely varied physical and chemical properties of contaminants markedly affect their distribution in the environment and their behavior (including transport, bioavailability, and toxicity) during and after remediation. The
degree of contamination can be severe in some areas with nearly unadulterated original products, such as PCB-containing oils, pesticides, or coal-tar residues. In other areas, contaminants occur at low concentrations in sediments among functioning ecosystems of fish, plants, and benthic invertebrates. The thickness of the contaminated sediment is highly variable and often poorly characterized but can range from a few inches to many feet thick with marked differences over small spatial scales. In addition, the nature of the sediments and particularly of the underlying substrate can vary widely on the basis of local geology, hydrology, and human activities that have altered the watershed characteristics.
Because of the highly variable nature of sediments, the environments in which they occur, and the type and degree of contamination, there are many approaches to their remediation. The techniques, which can be employed in combination, include removing the sediments from the aquatic environment (for example, by dredging), capping or covering contaminated sediments with clean material, and relying on natural processes while monitoring the sediments to ensure that contaminant exposures are decreasing, or at least not increasing (known as monitored natural recovery [MNR]). In-situ treatments that, for example, reduce the bioavailability of contaminants can also be used. The techniques, which are examined in greater detail in Chapter 2, differ in complexity, cost, efficacy, and time frame. That variability is driven by several factors, including site conditions (for example, variations in water flow and depth), underlying substrate characteristics, and implementation of the remedial approach. Regardless, achieving expected reduction in risk is of primary importance to regulators who require cleanup of a contaminated sediment site, parties responsible for funding the cleanup, and communities and user groups that are directly affected by the contamination and the remediation process.
Managing the risks associated with contaminated sediments has been an issue at the federal level since at least the middle 1970s (Johanson and Johnson 1976, as cited in EPA 1987), although it received substantially greater attention in the 1980s when the U.S. Environmental Protection Agency (EPA) sought to document the nature and extent of sediment contamination (Bolton et al. 1985; EPA 1987). The 1989 National Research Council report Contaminated Marine Sediments: Assessment and Remediation (NRC 1989) examined the extent of and corresponding
risk posed by marine sediment contamination and examined remedial technologies. EPA’s Assessment and Remediation of Contaminated Sediments (ARCS) program—an early effort to understand the extent of, associated risks of, and techniques for remediating contaminated sediments—published several useful reports and guidance documents dealing with the assessment of contaminated sediments and various treatment technologies (EPA 1994). Since then, additional National Research Council reports on managing contaminated sediments have been released (NRC 1997, 2001), and EPA has published its sediment quality surveys (EPA 1997; 2004), produced a contaminated sediment management strategy (EPA 1998), and issued comprehensive contaminated sediment guidance (EPA 2005). Yet, even after decades of analysis and review, managing and remediating contaminated sediments remains a major scientific and management challenge. Areas with contaminated sediments continue to be identified, and remediation efforts are increasingly large, expensive, and resource-intensive.
This report is one piece of the continuing dialogue and seeks to assess the effectiveness of environmental dredging for reducing risks associated with contaminated sediments, particularly at large, complex sites. Environmental dredging is of special interest because it can be expensive and technically challenging to implement. Dredging itself may create exposures (for example, through the resuspension of buried contaminants), but it removes persistent contaminants (and their associated potential for transport and risk) from the aquatic environment permanently. Whether to dredge contaminated sediments has proved to be one of the most controversial aspects of decision-making at sediment remediation sites.
THE CHARGE TO THE COMMITTEE ON SEDIMENT DREDGING AT SUPERFUND MEGASITES
This study was requested in the fiscal year 2006 appropriations bill for the Department of Interior, environment, and related agencies. The accompanying conference report (Report 109-188) states that “the managers believe that the appropriate role for the NAS [National Academy of Sciences] is to act as an independent peer review body that will conduct an objective evaluation of some of the ongoing dredging projects
underway at Superfund megasites. By undertaking such an evaluation, the NAS can serve as an objective voice on this issue.”1
In response, the National Research Council of the National Academies convened the Committee on Sediment Dredging at Superfund Megasites to consider the specific tasks provided in the statement of task (see Appendix A). In brief, the committee’s charge requests an evaluation of the expected effectiveness of dredging of contaminated sediments at Superfund megasites and of whether risk-reduction benefits are expected to be achieved in the expected period. The committee was asked to consider such aspects of dredging as the short- and long-term changes in contaminant transport and ecologic effects. The statement of task also directs the committee to evaluate monitoring strategies and whether those strategies are sufficient to inform assessments of effectiveness. Overall, the committee was charged to strive to develop recommendations that would facilitate scientifically based and timely decision-making for megasites in the future but not to recommend particular remedial strategies at specific sites.
One subject of great interest and concern at contaminated sediment Superfund sites is the risk-based comparison of remedial alternatives and selection of a remedy (Bridges et al. 2006; Wenning et al. 2006; Zeller and Cushing 2006). The committee briefly discusses this topic (Chapter 2) and addresses it in the context of improving future decision making at Superfund megasites (Chapter 6). However, the report does not develop specific procedures and recommendations for performing comparative risk analyses in selection of a sediment remedy. While that topic and type of analysis is quite important, it was not requested of the committee and it has not been undertaken.
NATIONAL RESEARCH COUNCIL AND THE COMMITTEE PROCESS
The National Research Council is a nonfederal, nonprofit institution that provides objective science, technology, and health-policy advice generally by producing consensus reports written by committees. It exists to provide independent advice; it has no government affiliation and
no regulatory role. There is no direct oversight of a committee by the study sponsor or any other outside parties. Thus, EPA and other interested parties have no more input or access to committee deliberations than does the general public. That arrangement gives the committee complete independence in conducting its study. The committee members have a wide variety of backgrounds and expertise. Members are selected by the Research Council primarily for their academic credentials and their knowledge, training, and experience relevant to the statement of task (see Appendix B for committee-member biosketches). Members conduct their work solely as a public service, volunteering to the Research Council and the nation, cognizant of the importance of providing timely and objective scientific advice.
In conducting its review and evaluation, the committee relied on the Superfund-site decision documents and supporting materials, other scientific studies, technical presentations made to the committee, other information submitted by individuals and interest groups, and the committee’s observations and personal expertise. All information received by Research Council staff that was made available to committee members is available to the public through the Research Council’s public-access records office.
The committee held five meetings. Three included open, information-gathering sessions in which the committee heard from invited speakers and from interested members of the public. The first meeting (in March 2006) was in Washington, DC; the second was held in Irvine, CA (June 2006); and the third was in Woods Hole and New Bedford, MA (July 2006), where the committee toured an active dredge site and sediment-handling facility. All of the public meetings included an open session where anyone was able to provide comment to the assembled committee. In addition, the committee was available to receive written materials throughout the study. The fourth and fifth meetings, held in September and October 2006 in Washington, DC, were closed, deliberative sessions attended only by committee members and staff.
Chapter 2 provides background on sediment management at Superfund megasites; it includes discussion of the concept of reducing
risk through environmental remediation and details on remedial techniques, particularly dredging. Chapter 3 describes the committee’s approach to considering effectiveness at various sites and developing conclusions from the analyses. Chapter 4 evaluates remedy performance and risk reduction on the basis of sites’ pre-dredging and post-dredging monitoring data and evaluates factors that affected performance. Chapter 5 looks at current practices for monitoring effectiveness at sediment remediation sites and considers the types of assessments and protocols that are needed to improve monitoring. Chapter 6 looks to the future: it considers the implications of the committee’s assessment of sediment management and identifies opportunities to advance the understanding of dredging and its effectiveness in improving the environment and public health.
Overall, the committee recognizes that the state of the science of environmental dredging is continually changing. New information is being gathered, research detailing the effects and effectiveness of dredging is being conducted, and technologies and performance continue to evolve. That process will continue for the foreseeable future. The committee does not consider its review to be the last word, but it hopes that its findings and recommendations will assist government agencies and other stakeholders in improving the approaches to contaminated sediments at large, complex megasites.
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