Sediment Dredging at Superfund Megasites Assessing the Effectiveness (2007) / Chapter Skim
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2 Sediment Management at Superfund Megasites
2 Sediment Management at Superfund Megasites
Pages 23-69
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From page 23... ...
Topics include the Superfund process and information available on contaminated sediment Superfund sites; evaluating and managing risks posed by contaminated sediments; and techniques for managing and remediating contaminated sediment with a focus on dredging technologies and their performance capabilities and limitations. The chapter is intended to provide a cursory overview of the topics while emphasizing other sources containing more detailed discussions.
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, or by some combination of the two. Selection of a remedy begins with a remedial investigation and feasibility study (RI/FS)
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Balancing Criteria • Long-term effectiveness and permanence. This criterion includes an evaluation of the magnitude of human health and ecologic risk posed by untreated contaminated materials or treatment residuals remaining after remedial action has been concluded (known as residual risk)
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• Implementability. This criterion is used to evaluate the technical feasibility of the alternative -- including construction and operation, reliability, and monitoring -- and the ease of undertaking an additional remedial action if the remedy fails.
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However, monitoring is not limited to sites where 5-year reviews are required. EPA guidance states that "most sites where contaminated sediment has been removed also should be monitored for some period to ensure that cleanup levels and RAOs [remedial action objectives]
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lists 5 66 sites while 60 sites are listed in output from EPA's internal database of tier 1 sites (EPA, unpublished data, "Remedial Action Objectives for Tier 1 Sites," Sept.
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The status of remediation at the sites varies. At some, such as Bayou Bonfouca and Marathon Battery, remediation has been completed; at others, such as Commencement Bay and Sheboygan Harbor, remedial activities are going on; and at still others, such as Hudson River and Onondaga Lake, remedial activities have not begun.
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, NY • Lipari landfill, NJ • Love Canal, NY • McCormick and Baxter Creosoting Co., CA • Nyanza chemical waste dump, MA • Wyckoff Co.–Eagle Harbor, WA Furthermore, as described below, large and expensive sediment remediations are conducted under authorities other than Superfund. A crucial question is how many additional major contaminated sediment sites are likely to be listed on the NPL.
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From page 31... ...
Other dredging projects -- such as those in the Fox River, New Bedford Harbor, and Commencement Bay -- are components of broader activities at large-scale megasites where remedial activities are going on and will take years or decades to complete. The $50million distinction for a megasite is not readily translatable into volume of materials removed.
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It appears that EPA has not allocated the resources needed to identify the scope of the problem and to develop a strategy to address issues related to contaminated sediments. To develop an effective long-term contaminated sediment strategy it is critical to know how much work remains to be done.
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. This project, described as "the largest environmental dredging project to be undertaken in North America," removed 786,000 cy of sediment from the Grand Calumet River (U.S.
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To the extent that other environmental dredging activities are conducted to address risk from contaminated sediments, many of the discussions and conclusions presented in the latter chapters of this report will be applicable. EVALUATING RISK REDUCTION AT CONTAMINATED SEDIMENT SITES Risks Posed by Contaminated Sediment As briefly described in Chapter 1, contaminants in sediment can pose risks to human health and the environment.
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. Because sediment contaminants typically are strongly associated with the sediment particles, contaminants buried below the biologically active zone are neither accessible nor available to sediment- or waterdwelling organisms.
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If contaminants buried below the biologically active zone are likely to remain buried, the potential exposure and risk may be so small that remediation of any kind is unwarranted. Remediation of deeply buried contaminated sediments that do not contribute to the exposure of aquatic systems now or under future conditions will not achieve risk reduction goals.
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Decision-Making in a Risk-Based Framework Principles for understanding and comparing risk reduction from various sediment remediation techniques are discussed briefly below, however, it should be noted that it is not the mandate or intent of the report to develop specific recommendations and procedures for performing comparative risk analyses of remedial alternatives in selection of a sediment remedy. While important, that type of detailed assessment was not requested or undertaken.
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. The commission ultimately developed a report that introduced a risk management framework "to guide investments of valuable public sector and private sector resources in researching, assessing, characterizing, and reducing risk" (PCCRARM 1997, p.
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. The document provides a useful evaluation of the various sediment management approaches and their advantages and limitations in attaining risk reduction.
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. Measuring Risk-Reduction Estimating the degree of risk reduction is central in considering the potential effectiveness of a remedial action.
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The report also suggests that a broader array of risks -- including societal, cultural, and economic risks -- should be evaluated comprehensively. The concept of net risk reduction has been embraced by EPA in its Contaminated Sediment Remediation Guidance for Hazardous Waste Sites (EPA 2005a)
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From page 42... ...
At Superfund sites, the overall goal of sediment management is reduction of risk to human health and the environment. That goal takes the form of remedial action objectives, which are used in developing and comparing alternatives for a site, and typically describe the desired effect of the remediation on risk (for example, reduction to acceptable levels of the risks to people ingesting contaminated fish)
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. The Conceptual Site Model: A Working Understanding of Processes Leading to Risk from Sediment Contamination The development of remedial action objectives and cleanup levels to reduce risk is based on a conceptual understanding of cause-effect
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Basing remedial action objectives on the best scientific understanding of the mechanisms that lead to site-specific risk maximizes the likelihood that remedial actions will meet the objectives. For most sites, but especially for the largest and most complex, a quantitative dimension must be added to the CSM to support development and selection of a remedy.
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. CONTAMINATED SEDIMENT MANAGEMENT TECHNIQUES Contaminated sediment is managed with various techniques, including source control, natural recovery, capping, and removal (dry excavation and dredging)
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The present committee's focus is on environmental dredging, which is conducted specifically to remove contaminated sediments, as opposed to navigational dredging, which typically is intended to maintain depth in waterways for navigation or other purposes. HISTORICAL PERSPECTIVE ON THE USE OF REMOVAL TECHNOLOGIES TO REDUCE RISK Although there has never been a presumptive remedy for sediments, the historical preference for removal is evident in the large percentage of sites whose remedy was based entirely or in part on dredging.
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Army Corps of Engineers experience with navigational dredging and disposal. Other remedies were typically viewed as less certain by regulators and the public with respect to long-term effectiveness or permanence.
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Navigational dredging techniques adopted for environmental dredging are designed to achieve a specific bottom elevation or the removal of a specific volume, often in the shortest possible time, whereas environmental dredging typically must achieve a specific final concentration while minimizing contaminant releases during dredging, handling, and disposal. As dredging remedies have been implemented at various sites, the effects of resuspension and transport of contaminated material off site and residual contamination in a remediated area have become apparent (Bridges et al.
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. BOX 2-6 Objectives of Environmental Dredging • Dredge with sufficient accuracy such that contaminated sediment is removed and cleanup levels are met without unnecessary removal of clean sediment.
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In addition, within dredge types, specific designs may differ and may have varied capability. In general, the dredge types listed above represent equipment that is readily available and used for environmental dredging projects in the United States.
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. Obviously, larger dredges are available for both mechanical and hydraulic equipment and can be used for environmental dredging if needed.
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52 Sediment Dredging at Superfund Megasites FIGURE 2-3 Categories of dredging and sediment removal equipment. Source: Francingues and Palermo 2006.
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In a complex project, large costs may be associated with the transport, dewatering, and ultimate disposition of the dredged material. Recent data, described below, support the premise that dredging accounts for 10-20% of the total cost of an environmental dredging project.
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TECHNICAL ISSUES ASSOCIATED WITH DREDGING Environmental dredging typically strives to achieve contaminantspecific cleanup levels set at each site. A number of technical issues can limit ability or efficiency in achieving those levels.
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Source: Data from EPA 2005d. Other Dredging Monitoring P M 4% 17% & Fees 15% Mob/Demob 15 % Wat er Management T ransport & 6% Disposal Sediment Handling 31% 12% FIGURE 2-6 Cost breakdown of components of environmental dredging at the head of the Hylebos Waterway (Commencement Bay, WA)
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From page 56... ...
Resuspension, Residuals, and Release of Contamination All dredging equipment disturbs sediment and resuspends some fraction of it in the water column. Resuspended sediment and the associated contaminants can settle back to the bottom in the dredge cut; finergrained materials can remain in the water column and be transported to other locations.
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From page 57... ...
Recent EPA guidance for sediment remediation states that When evaluating resuspension due to dredging, it generally is im portant to compare the degree of resuspension to the natural sedi ment resuspension that would continue to occur if the contami nated sediment was not dredged, and the length of time over which increased dredging-related suspension would occur.… Some con taminant release and transport during dredging is inevitable and should be factored into the alternatives evaluation and planned for in the remedy design.… Generally, the project manager should as sess all causes of resuspension and realistically predict likely con taminant releases during a dredging operation (EPA 2005a, pp.
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Resuspension Residual (Sediment) Removal Residual FIGURE 2-7 Conceptual illustration of environmental dredging and processes.
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2006. ment surface that have been uncovered but not fully removed as a result of the dredging operation (Bridges et al.
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Patmont (2006) compiled data on residuals from 12 environmentaldredging projects.
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That means that the mixed layer generated by the cutterhead or auger is not fully removed by the suction pipe and consequently there is a "spillage layer" left behind after dredging. • Another source of residual sediment is resuspension by the rotating cutterhead or auger, when sediment is displaced away from the cutterhead or auger into the water column.
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pore water. Contaminants in the generated or undisturbed residuals may be released to the water column by densification, diffusion and bioturbation (Bridges et al.
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2005. Environmental dredging pilot study successfully completed on the Lower Passaic River, NJ -- one of America's most polluted rivers.
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The Four Rs of Environmental Dredging: Re suspension, Release, Residual, and Risk.
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From page 65... ...
2005a. Contaminated Sediment Remediation Guidance for Hazardous Waste Sites.
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2006. Control of resuspended sediments in dredging projects.
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2003. Equipment selection factors for environmental dredging.
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2006. Guidance for Environmental Dredging of Contaminated Sediments, Draft.
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2004. Grand Calumet River Sediment Remediation Project Update.
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