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Sediment Dredging at Superfund Megasites: Assessing the Effectiveness 2 Sediment Management at Superfund Megasites A variety of subjects including environmental engineering, toxicology, environmental monitoring, human and environmental risk assessment, and risk management are relevant to evaluating remediation at contaminated sediment Superfund sites. In this chapter, a number of issues are briefly introduced to provide background for later discussions. 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. OVERVIEW OF SUPERFUND AND SEDIMENT MEGASITES Superfund and Environmental Remediation In 1980, Congress enacted the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA, 42 U.S.C. 9601-9675),
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Sediment Dredging at Superfund Megasites: Assessing the Effectiveness which authorized the establishment of the Superfund program. The goal of the program is to reduce current and future risks to human health and the environment at sites contaminated with hazardous substances. CERCLA established a wide-ranging liability system that makes those responsible for the contamination at sites liable for cleanup costs (see Probst et al. 1995 for greater detail). It also created the “Superfund,” a trust fund stocked primarily by a dedicated tax on oil and chemical companies, to fund cleanup activities where there was no financially viable responsible party. Since the taxing authority expired in 1995, the trust fund is largely depleted, and Congress now funds the program from general revenues through annual appropriations (Fletcher et al. 2006).1 The U.S. Environmental Protection Agency (EPA) implements the program through the National Oil and Hazardous Substances Pollution Contingency Plan (40 CFR § 300), commonly referred to as the NCP or the national contingency plan. Most of the Superfund program’s efforts are aimed at cleaning up sites on the National Priorities List (NPL). Typically, a site is proposed for inclusion on the NPL after being evaluated with a hazard-ranking system, which assesses the potential for hazardous-substance releases at a site to harm human health or the environment (40 CFR § 300 Appendix A). The Superfund process progresses from an initial site assessment through cleanup and eventually deletion of the site from the NPL. Site activities can be paid for by EPA (known as “fund-led” cleanups),2 by parties connected to the site (referred to as responsible parties), or by some combination of the two. Selection of a remedy begins with a remedial investigation and feasibility study (RI/FS). The RI is intended to determine the nature and extent of contamination and estimate the associated risk to people and the environment. The FS analyzes and compares remedial alternatives according to the nine NCP criteria (Box 2-1). The criteria require that the remedy, above all, be protective of human health and the environment and comply with all applicable or relevant and appropriate requirements 1 It is worth noting that, in the last few years, EPA has been in the position of not having enough funds to fund all the new remedies that are ready to be started at NPL sites (EPA 2004a). 2 For fund-lead cleanups, states are required to pay 10% of the costs.
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Sediment Dredging at Superfund Megasites: Assessing the Effectiveness BOX 2-1 Evaluation Criteria for Superfund Remedial Alternatives Before a remedial strategy is selected for a Superfund site, the options are evaluated on the basis of nine criteria (see below). The first two, overall protection of human health and the environment and compliance with applicable or relevant and appropriate requirements (ARARs), are termed threshold criteria, and a potential remedy must meet them to be selected as a final remedy.3 The next five criteria are termed balancing criteria and are used in weighing the advantages and disadvantages of potential remedies. The final two criteria are modifying criteria, and the agency is supposed to take them into consideration as part of the selection process. Threshold Criteria Overall protection of human health and the environment. This criterion is used to evaluate how the alternative as a whole achieves and maintains protection of human health and the environment. Compliance with applicable or relevant and appropriate requirements (ARARs). This criterion is used to evaluate whether the alternative complies with chemical-specific, action-specific, and location-specific ARARs or a waiver is justified. 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) and of the adequacy and reliability of controls to manage such risk. It also includes an assessment of the potential need to replace technical components of the alternative. Reduction of toxicity, mobility, and volume through treatment. This criterion refers to the evaluation of whether treatment processes can be used, the amount of hazardous material treated (including the principal threat that can be addressed), the degree of expected reductions, the degree to which the treatment is irreversible, and the type and quantity of treatment residuals. Short-term effectiveness. This criterion includes an evaluation of the effects of the alternative during the construction and implementation phase until 3 Except that specific ARARs can be waived.
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Sediment Dredging at Superfund Megasites: Assessing the Effectiveness remedial objectives are met. It includes an evaluation of protection of the community and workers during the remedial action, the environmental effects of implementing the remedial action, and the expected length of time until remedial objectives are achieved. 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. It also considers the administrative feasibility of activities needed to coordinate with other offices and agencies—such as for obtaining permits for off-site actions, rights of way, and institutional controls—and the availability of services and materials necessary for the alternative, such as treatment, storage, and disposal facilities. Cost. This criterion includes an evaluation of direct and indirect capital costs, including costs of treatment and disposal; annual costs of operation, maintenance, and monitoring of the alternative, and the total present worth of these costs. Modifying Criteria State (or support agency) acceptance. This criterion is used to evaluate the technical and administrative concerns of the state (or the support agency, in the case of state-lead sites) regarding the alternatives, including an assessment of the state’s or support agency’s position and key concerns regarding the alternative, and comments on ARARs or the proposed use of waivers. Tribal acceptance is also evaluated under this criterion. Community acceptance. This criterion includes an evaluation of the concerns of the public regarding the alternatives. It determines which component of the alternatives interested persons in the community support, have reservations about, or oppose. Source: Adapted from EPA 2005a. (ARARs).4 Remedies are also compared on whether they are technically feasible and cost-effective, provide long-term (permanent) effectiveness, and minimize deleterious effects and health risks during implementa- 4 ARARs pertain to federal, state, or tribal environmental laws relevant to a site.
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Sediment Dredging at Superfund Megasites: Assessing the Effectiveness tion. There is a preference for remedies that can reduce the toxicity, mobility, and volume of contaminants. Finally, there is a preference for remedies that have state and community support. EPA uses the FS to identify each alternative’s strengths and weaknesses and the trade-offs that must be balanced for the site in question (EPA 1988). The agency then selects a remedy and describes it in a record of decision (ROD). Additional studies may be conducted to support the design of the remedy. Once constructed and implemented, the remedy is maintained and monitored to ensure that it achieves its long-term goals. EPA may delete a site from the NPL when a remedy has been implemented, the cleanup goals have been achieved, and the site is deemed protective of human health and the environment (EPA 2000). If, after implementation of a remedy, contamination exists that could limit potential uses of the site, the site is subject to 5-year reviews even if it has been deleted from the NPL (EPA 2001). The reviews are intended to evaluate the performance of the remedy in protecting human health and the environment and are to be based on site-specific data and observations. 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] are met and will continue to be met” (EPA 2005a, p. 2-17). Post-remediation monitoring (required in conjunction with 5-year reviews or otherwise) is the basis for evaluating remedy effectiveness and adapting remedial strategies and risk management to achieve remedial action objectives (for further discussion, see Chapter 5). Sediment Contamination at Superfund Sites Contaminated sediment is a widespread problem in the United States (EPA 1994, 1997, 1998, 2004a, 2005a). Its wide distribution results from the propensity of many contaminants discharged to surface waters to accumulate in sediment or in suspended solids that later settle. Contaminants can persist in sediment over long periods if they do not degrade (for example, metals) or if they degrade very slowly (for example, polychlorinated biphenyls [PCBs] or polycyclic aromatic hydrocarbons
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Sediment Dredging at Superfund Megasites: Assessing the Effectiveness [PAHs]). Historically contaminated sediment can become buried or, if it is resuspended, can settle out eventually and lie on the sediment surface. At the national level, the geographic extent of areas with contaminated sediment is not fully defined. In the 2004 Contaminated Sediment Report to Congress (EPA 2004b), EPA reported on sediment sampling at 19,398 sampling stations nationwide, located in about 9% of the water-body segments in the United States. Of that nonrandom sample of sediment sampling stations, EPA classified 43% as having probable adverse effects, 30% having possible adverse effects, and 27% as having no indications of adverse effects. The 2005 Contaminated Sediment Remediation Guidance for Hazardous Waste Sites (EPA 2005a) cites EPA fish advisories covering all five Great Lakes, 35% of the nation’s other lakes, and 24% of total river miles as due partly to sediment contamination (EPA 2005b). EPA does not maintain a current list of NPL sites with contaminated sediments, nor does it compile a list of contaminated sediment areas that are potential Superfund sites. It also does not maintain a list of contaminated sediment sites that are being (or have been) remediated under another authority. EPA did report that “as of September 2005, Superfund has selected a remedy at over 150 sediment sites” (EPA 2006a). In addition, the EPA Office of Superfund Remediation and Technology Innovation is tracking progress at 66 sites, termed tier 1 sites, where the sediment-cleanup remedy involves more than 10,000 cubic yards (cy) of sediment to be dredged or excavated or more than 5 acres to be capped or monitored for natural recovery (EPA 2006b).5 Of the aforementioned 150 NPL sites where remedies have been selected, EPA considers 11 to be sediment megasites, defined as sites where the sediment portion of the remedy is expected to cost $50 million or more.6 Of these 11 sites, 10 were proposed for inclusion on the NPL in the very early years of the Superfund program (in 1982-1985), and one (Onondaga Lake) was proposed for inclusion in 1993. Thus, the overwhelming ma- 5 The exact number of tier 1 sites is not clear. EPA’s website (EPA 2006b) lists 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. 5, 2006). Seven sites listed in EPA’s internal database are not on the website; 13 sites listed on the website are not in the September 5 submittal. 6 Typically, megasites are defined as sites where the total cost of the remedy for the entire site (not just the sediment portion) is expected to be at least $50 million.
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Sediment Dredging at Superfund Megasites: Assessing the Effectiveness jority of the megasites have been on the NPL for over 20 years. Only one of the 11, Marathon Battery, has been formally deleted from the NPL. In addition to the 11 megasites on the NPL, EPA lists two megasites that have been proposed for the NPL but are not final (GE Housatonic River, MA and Fox River, WI) and one that has not been proposed (Manistique River/Harbor area, MI). The 14 sites are listed in Table 2-1. 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. Megasites are described only in terms of remediation cost (at least $50 million), so the size and volume of contaminated materials at the sites can vary greatly (see Box 2-2). One might ask, Why all this attention to contaminated sediment megasites if there are only 14 nationwide? There are two reasons. First, at 13 of the megasites mentioned above (no cost information was provided on the Triana/Tennessee River site), total remedial costs are estimated to be about $3 billion, a huge amount of money even by Superfund standards.7 Second, the 14 sites probably constitute only a subset of the contaminated sediment sites that will entail expensive remedies and will be cleaned up under the Superfund program. For example, the EPA list of contaminated sediment megasites does not include some well-known sites, such as the Bunker Hill Mining and Metallurgical Complex, ID, and Love Canal, NY. Both those tier 1 sites are megasites by the conventional definition (total remediation cost of at least $50 million), but the sediment portion alone is not expected to be $50 million. When comparing EPA’s list of tier 1 sites (EPA 2006b) with a somewhat dated list of megasites8 (that does not include federal facilities), one can find 11 “conventional” megasites on the tier 1 list. Alcoa–Point Comfort/Lavaca Bay, TX Allied Paper Inc./Portage Creek/Kalamazoo River, MI Bunker Hill Mining and Metallurgical Complex, ID 7 Based on data provided by EPA, “50M cost Query_091306.xls” (EPA, unpublished data, Sept. 18, 2006). 8 Based on the report to Congress, Superfund’s Future: What Will It Cost? (Probst et al. 2001), which lists megasites as of FY 2000.
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Sediment Dredging at Superfund Megasites: Assessing the Effectiveness TABLE 2-1 Sediment Megasites (Sites at Which Remediation of the Sediment Component Is Expected To Be at Least $50 million) Site Name, State NPL Sites New Bedford Harbor, MA Hudson River PCBs, NY Marathon Battery Corp., NY Onondaga Lake, NY Triana/Tennessee River, AL Sheboygan Harbor and River, WI Velsicol Chemical, MI Bayou Bonfouca, LA Milltown Reservoir Sediments, MT Silver Bow Creek/Butte Area, MT Commencement Bay, WA Non-NPL Sites GE Housatonic River, MA Fox River, WI Manistique River/Harbor area, MI Source: EPA, unpublished data, “$50M Cost query_091306.xls,” Sept. 18, 2006. Eagle Mine, CO EI duPont–Newport landfill, DE GM–Central Foundry Division (Massena), NY Lipari landfill, NJ Love Canal, NY Nyanza chemical waste dump, MA McCormick and Baxter Creosoting Co., CA 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. EPA does not designate “likely future megasites” in its tier 1 list of sites or NPL sites for which RODs have not been issued. According to EPA, the most likely future sediment megasites are the “tier 2” contaminated sediment sites (S. Ells,
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Sediment Dredging at Superfund Megasites: Assessing the Effectiveness BOX 2-2 How Large Is a Megasite? Contaminated sediment megasites are among the most challenging and expensive sites on the NPL. Megasites are conventionally defined as those with remedial activities costing at least $50 million, but there are large differences in the magnitudes and scales of these sites. A few megasites, such as Bayou Bonfouca and Marathon Battery, are relatively small, with dredging activities covering tens of acres and operations occurring over a few years. 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 $50-million distinction for a megasite is not readily translatable into volume of materials removed. For example, sediment remediation (including design, mobilization, marine demolition, dredging, water management, transportation and disposal, construction oversight and EPA oversight, without the upland-based removal costs) at the Head of Hylebos Waterway in Commencement Bay, WA, removed 404,000 cy at a cost of $58.8 million (about $145/cy) (P. Fuglevand, personal commun., Dalton, Olmsted & Fuglevand, Inc., May 11, 2007). In Manistique Harbor, MI, dredging operations removed 187,000 cy at a cost of $48.2 million (about $260/cy) (Weston 2002). Dredging operations in Bayou Bonfouca, LA, removed 170,000 cy at a cost of $90 million (about $530/cy) (EPA, unpublished information, “$50M Cost query_091306.xls,” Sept. 18, 2006). EPA, personal commun., Oct. 12, 2006). Tier 2 sites are designated for review by the Contaminated Sediments Technical Advisory Group because they are large, complex, or controversial contaminated sediment Superfund sites.9 There are 12 tier 2 sites. Three are on the earlier two lists provided, but nine are not. Of the nine, four are NPL sites (Ashland/Northern States Power, WI, Portland Harbor, OR, Lower Duwamish Waterway, WA, and the Pearl Harbor Naval Complex, HI), and five are not (Palos Verdes, CA, Kanawah River/Nitro, WV, Centredale Manor Restoration Sites, RI, Anniston PCB site, AL, and Upper Columbia River, WA). EPA also indicates that the Passaic River, NJ, Berry’s Creek at Ventron/Velsicol, NJ, and Tar Creek, OK, are likely future con- 9 Although, it should be noted that EPA indicates that “No quantifiable criteria were used to develop this list.” The list of sites is available at http://www.epa.gov/superfund/resources/sediment/cstag_sites.htm.
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Sediment Dredging at Superfund Megasites: Assessing the Effectiveness taminated sediment megasites, although they have not been designated as tier 2 sites (S. Ells, EPA, personal commun., Sept. 18, 2006). Because predicting future NPL listings is more an art than a science, in some ways, it is not surprising that there is no official list of likely future contaminated sediment megasites. That said, the committee was surprised that there is so little effort devoted to tracking and understanding likely future sediment megasites at the national level. Apparently, fewer than two full-time employees are assigned to contaminated sediment issues at Superfund headquarters. 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. To address that question, one needs to have three pieces of information: How much work remains at sites already categorized as contaminated sediment megasites. How many contaminated sediment sites already on the NPL are likely to be determined to be megasites. How many new such sites are likely to be added to the NPL in the coming years. None of that information is readily available from EPA. Clearly, EPA should not stop and wait until this information is collected. However, it is important that EPA obtain this information and update it regularly in order to be able to forecast likely future costs and needed resources, as well as to assess what kinds of research and monitoring improvements are likely to have the largest benefit to the program. Cleanup Under Authorities Other Than Superfund Remediation of contaminated sediments is also conducted under authorities other than Superfund and can be led by various parties, such as state or federal agencies or private entities, in combination or individually. For example, a 5-mile reach of the Grand Calumet River, a highly industrialized tributary to Lake Michigan in northwest Indiana, was dredged by U.S. Steel Corporation pursuant to a Clean Water Act
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Sediment Dredging at Superfund Megasites: Assessing the Effectiveness consent decree and a Resource Conservation and Recovery Act corrective-action consent order (Menozzi et al. 2003). 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. Steel 2004). State programs conduct and oversee sediment remediation under a variety of authorities. For example, the State of Washington Department of Ecology is charged with cleaning up and restoring contaminated sites under authority of the Model Toxics Control Act (MTCA) and Sediment Management Standards (SMS) (Washington Department of Ecology 2005). In 2005, 142 sediment cleanup sites were identified in Washington: 41 were being cleaned up under federal authorities, 48 were using state authority alone, 11 were under federal and state authorities, and the remaining 42 were either voluntary (conducted by the responsible party) or the authority had not been assigned (Washington Department of Ecology 2005). Contaminated sediments in many harbors and rivers of the Great Lakes are addressed in the Great Lakes Water Quality Agreement between the United States and Canada, which established 43 areas of concern (AOCs) in U.S. and Canadian waters. The U.S. EPA Great Lakes National Program Office administers funds from the Great Lakes Legacy Act of 2002 for the remediation of contaminated sediment at AOCs (EPA 2004c). The first Legacy Act cleanup was in 2005 at the Black Lagoon in the Detroit River AOC near Trenton, MI. At that site, 115,000 cy of contaminated material was dredged, and the area was capped. Hog Island, near Superior, WI, in the St. Louis River AOC of Lake Superior, was remediated with dry excavation (see Sediment Management Techniques in this chapter for a description of remedial methods). In 2006, two projects were under way with Great Lake Legacy Act funds. The Ruddiman Creek remedial action in Muskegon, MI, contains an excavation and dredging component and is expected to remove around 80,000 cy. Dredging will also occur at the Ashtabula River, near Cleveland, OH, where it is expected that about 600,000 cy of contaminated sediment will be removed from the lower portion of the river. Another program, the Urban Rivers Restoration Initiative, is a collaboration between EPA and the U.S. Army Corps of Engineers for urban-river cleanup and restoration (EPA 2003a). Eight demonstration pilot projects, including a dredging project in the Passaic River in New
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Sediment Dredging at Superfund Megasites: Assessing the Effectiveness BOX 2-8 Site-Specific Factors Affecting Resuspension, Release, and Residuals Sediment Physical and Chemical Properties Sediment Physical and Chemical Properties Grain size distribution (for example, percentages of silt, clay, and sand). Organic carbon content. Amount of sulfides. Spatial and vertical distributions of contaminants in the sediment (for example, layering). Site Conditions Water velocity and degree of mixing. Water salinity, hardness, alkalinity, and temperature. Type of substrate (for example, hardpan, bedrock or soft sediment). Type and extent of debris in sediment. Weather, such as storms that result in wind and waves. Wakes from passing vessels. Fluctuations in water elevation. Depth and slope of area to be dredged. Equipment Type of dredge (for example, cutterhead pipeline, open or closed bucket, and specialty dredgehead). Methods of dredging. Skill of operators. Extent of tender-boat activity. Methods of sediment transport and offloading. Source: Adapted from Palermo et al. 2006. ment surface that have been uncovered but not fully removed as a result of the dredging operation (Bridges et al. in press). Residuals may result from incomplete characterization, inaccuracies of dredging, mixing of targeted material with underlying materials during dredging, fallback (dislodged sediment not picked up), and resettlement of resuspended sediments (Palermo et al. 2006). Also contributing to residual contamination are such processes as sloughing of sediment into the dredging cut and sloughing induced by bank or slope
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Sediment Dredging at Superfund Megasites: Assessing the Effectiveness failures. Site-specific factors, such as debris or limitation of dredging by bedrock or hardpan can influence the amount of residuals. Box 2-9 describes specific processes during dredging that contribute to residual formation. The residual contaminant mass is typically limited to the upper few inches of sediment, which is populated and actively processed by sediment-dwelling organisms (although in the case of undisturbed residuals the depth can be substantially greater). That upper layer is subject to erosion and other physical and chemical processes that may promote release into the overlying water because of the entrainment of water into the dredged sediment, which causes physical (decreased consolidation) and chemical (redox) changes in the residuals. Residual contamination may also be attributable to sediment that was not dredged, because of the dredger’s failure to meet dredge cutlines (either depth or areal targets) or errors or incompleteness in site characterization that failed to identify appropriate depth and areal extent of contaminated sediment. Patmont (2006) compiled data on residuals from 12 environmental-dredging projects. Final generated residuals ranged from approximately 2 to 9% (average = 5%)11 of the mass of contaminant dredged during the last production cut. There is little research on the amount of generated residuals transported outside the dredge prism, but their presence has been documented analytically (EcoChem Inc 2005) and visually with sediment-profile imagery (Baron et al. 2005). Release Release is the process by which the dredging operation results in the transfer of contaminants from sediment pore water and sediment particles into the water column or air. Contaminants sorbed to resuspended particles may partition to the water column and be transported downstream in dissolved form along with contaminants in the released 11 More recently, Patmont and Palermo (2007) analyzed a similar (though not identical) dataset and found that final generated residuals ranged from approximately 2% to 9% (average = 4%) of the mass of contaminant dredged during the last production cut.
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Sediment Dredging at Superfund Megasites: Assessing the Effectiveness BOX 2-9 Specific Processes Contributing to the Residual Layer During Dredging For mechanical dredging, processes that contribute the residual layer are The erosion of sediment from around and within the bucket as it is placed on the bottom, closed, and raised through the water column. The erosion in the water column can be controlled with the use of enclosed buckets. However there can be significant resuspenion of contaminated sediment during the closing of enclosed buckets, as the bucket vents expel sediment at high velocity. The overflow of turbid water from the sediment haul barge, controlled with restrictions on barge overflow and associated capture and treatment of the turbid water. For hydraulic dredging, processes that contribute the residual layer are The spillage layer generated by hydraulic dredging associated with the turning of the cutterhead or auger in the sediment. Hydraulic dredges are normally configured with the inlet of the suction pipe well above the lowest reach of the rotating cutterhead or auger. 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. Dredging, either mechanical or hydraulic, can result in the formation of a residual layer through a variety of mechanisms including The sloughing of the sidewalls and headwall of the dredge cut face back on to previously dredged areas. This sloughing can be controlled through the use of relatively thin dredge lifts (few feet each) and by including a final cleanup pass of dredging once the bulk of sediment has first been removed (“two pass dredge approach”). If not controlled, this bank sloughing can result in a considerable residual layer forming on previously dredged areas. The remolding of soft fine-grained sediment by the dredging process can significantly reduce the strength of the material and generate a more liquid like flowable residual layer in the dredging area. This flowable material can be very difficult to capture with the dredge and result in a residual layer that is
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Sediment Dredging at Superfund Megasites: Assessing the Effectiveness difficult to manage and control once it is formed. The formation of this layer can be reduced (not eliminated) by a controlled and precise removal program using electronic, GPS-enabled dredge positioning and mechanical dredging. Once formed, capture of the flowable layer can be accomplished with overdredging into native substrate, provided that substrate is not hardpan or bedrock. Sources: Adapted from Dalton, Olmsted & Fuglevand, Inc. 2006; Fuglevand and Webb 2006, 2007; Hartman 2006. pore water. Contaminants in the generated or undisturbed residuals may be released to the water column by densification, diffusion and bioturbation (Bridges et al. in press). Releases of contaminants from the aforementioned sources and processes are considered to be up to about 5% of the contaminant mass in the sediment dredged, but larger or smaller releases may be observed, depending on site-specific factors and the type and operating characteristics of the dredge (Sanchez 2001; Sanchez et al 2002). The degree of contaminant release to the air and water is directly related to the degree of sediment resuspension (and pore water release) and chemical properties affecting the mass transfer of contaminants. Therefore, control of resuspension should have high priority at many dredging project sites that involve contaminated sediment. Contamination can also be released from sediment beds to the water column in soluble form without particle resuspension (Thibodeaux and Bierman 2003; Erickson et al. 2005). That suggests that the residual layer is also a contributor of contaminant release after dredging. Control of solids is important but is not always sufficient to prevent contaminant losses. Impact on Risk Risk can result from contaminant exposures driven by resuspension, production of residuals, and contaminant release. Those processes are important because they can alter the accessibility bioavailability of contaminants, create additional contaminant exposure pathways that
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Sediment Dredging at Superfund Megasites: Assessing the Effectiveness potentially affect the risk resulting from dredging, and may continue to influence risk after remedial operations cease. Surface-water concentrations and surface-sediment concentrations may increase during and after dredging and can result in adverse effects and accumulation of contaminants in organisms. The potential for volatile compounds to be released into the air may be an additional concern in connection with highly contaminated sites (EPA 2005a). Release, resuspension, and production of residuals will affect risk over different spatial scales and time frames depending on the site characteristics and nature of the dredging operation. As described by Bridges et al. (in press), “Characterizing how dredging will influence direct risks includes considering how the processes contributing to risk change with time, which elements or receptors in the ecosystem are affected by these changes, the spatial scales over which effects would be expected to occur, and the uncertainties associated with the predicted changes and risk reduction.” As will be discussed in much greater detail in Chapter 4, resuspension and release occur in a shorter time frame during dredging operations. Residuals will remain following dredging, however, their distribution, longevity, and effects are poorly understood. To the extent that release, resuspension, and production of residuals are present and contribute risk at a site, they detract from the overall or net risk reduction resulting from the remedial activity. As such, they are an important consideration in evaluating the effectiveness of a remediation. As noted in the 4 Rs workshop (Bridges et al. in press) and recent EPA sediment guidance (EPA 2005a), there is increasing recognition of the importance of these processes and of factors that influence their control. REFERENCES Averett, D.E., B.D. Perry, E.J. Torre, and J.A. Miller. 1990. Review of Removal, Containment, and Treatment Technologies for Remediation of Contaminated Sediments in the Great Lakes. Miscellaneous Paper EL-90-25. Prepared for Great Lakes National Program Office, U.S. Environmental Protection Agency, Chicago, IL, by U.S. Army Corps of Engineers Waterways Experiment Station, Vicksburg, MS. Baron, L.A., M.R. Bilimoria, and S.E. Thompson. 2005. Environmental dredging pilot study successfully completed on the Lower Passaic River, NJ—one of America’s most polluted rivers. World Dredging Mining and Construction
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Representative terms from entire chapter: