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2 Sediment Management at Superfund Megasites A variety of subjects including environmental engineering, toxicol- ogy, environmental monitoring, human and environmental risk assess- ment, and risk management are relevant to evaluating remediation at contaminated sediment Superfund sites. In this chapter, a number of is- sues are briefly introduced to provide background for later discussions. Topics include the Superfund process and information available on con- taminated sediment Superfund sites; evaluating and managing risks posed by contaminated sediments; and techniques for managing and remediating contaminated sediment with a focus on dredging technolo- gies 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 Re- sponse, Compensation, and Liability Act (CERCLA, 42 U.S.C. 9601-9675), 23
24 Sediment Dredging at Superfund Megasites 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 âSuperfund,â a trust fund stocked primarily by a dedicated tax on oil and chemical companies, to fund cleanup activities where there was no financially vi- able 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 Â§ 300), commonly referred to as the NCP or the national contingency plan. Most of the Superfund programâs 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 Â§ 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 âfund-ledâ 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 fea- sibility study (RI/FS). The RI is intended to determine the nature and ex- tent of contamination and estimate the associated risk to people and the environment. The FS analyzes and compares remedial alternatives ac- cording 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 It is worth noting that, in the last few years, EPA has been in the position of 1 not having enough funds to fund all the new remedies that are ready to be started at NPL sites (EPA 2004a). 2For fund-lead cleanups, states are required to pay 10% of the costs.
Sediment Management at Superfund Megasites 25 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 protec- tion 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 ad- vantages 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 â¢ Overall protection of human health and the environment. This criterion is used to evaluate how the alternative as a whole achieves and maintains protec- tion of human health and the environment. â¢ 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 â¢ Long-term effectiveness and permanence. This criterion includes an evalua- tion of the magnitude of human health and ecologic risk posed by untreated con- taminated 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. â¢ 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 ad- dressed), the degree of expected reductions, the degree to which the treatment is irreversible, and the type and quantity of treatment residuals. â¢ Short-term effectiveness. This criterion includes an evaluation of the ef- fects of the alternative during the construction and implementation phase until (Continued on next page) 3Except that specific ARARs can be waived.
26 Sediment Dredging at Superfund Megasites BOX 2-1 Continued remedial objectives are met. It includes an evaluation of protection of the com- munity and workers during the remedial action, the environmental effects of implementing the remedial action, and the expected length of time until reme- dial objectives are achieved. â¢ Implementability. This criterion is used to evaluate the technical feasibil- ity of the alternativeâincluding construction and operation, reliability, and monitoringâand 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 agenciesâsuch as for obtaining permits for off-site actions, rights of way, and institutional controlsâand the availability of services and materials necessary for the alternative, such as treatment, storage, and disposal facilities. â¢ Cost. This criterion includes an evaluation of direct and indirect capital costs, including costs of treatment and disposal; annual costs of operation, main- tenance, and monitoring of the alternative, and the total present worth of these costs. Modifying Criteria â¢ 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 stateâs or support agencyâs 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. â¢ Community acceptance. This criterion includes an evaluation of the con- cerns 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- ARARs pertain to federal, state, or tribal environmental laws relevant to a 4 site.
Sediment Management at Superfund Megasites 27 tion. There is a preference for remedies that can reduce the toxicity, mo- bility, and volume of contaminants. Finally, there is a preference for remedies that have state and community support. EPA uses the FS to identify each alternativeâs strengths and weak- nesses 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 imple- mented, 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 âmost sites where con- taminated 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 metâ (EPA 2005a, p. 2-17). Post-remediation monitoring (required in conjunction with 5-year re- views 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. Con- taminants can persist in sediment over long periods if they do not de- grade (for example, metals) or if they degrade very slowly (for example, polychlorinated biphenyls [PCBs] or polycyclic aromatic hydrocarbons
28 Sediment Dredging at Superfund Megasites [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 contami- nated 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 sedi- ment sampling stations, EPA classified 43% as having probable adverse effects, 30% having possible adverse effects, and 27% as having no indi- cations 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 nationâs 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 contami- nated sediments, nor does it compile a list of contaminated sediment ar- eas 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 âas of September 2005, Superfund has selected a remedy at over 150 sediment sitesâ (EPA 2006a). In addition, the EPA Office of Superfund Remediation and Tech- nology 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 sedi- ment 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- The exact number of tier 1 sites is not clear. EPAâs website (EPA 2006b) lists 5 66 sites while 60 sites are listed in output from EPAâs internal database of tier 1 sites (EPA, unpublished data, âRemedial Action Objectives for Tier 1 Sites,â Sept. 5, 2006). Seven sites listed in EPAâs internal database are not on the website; 13 sites listed on the website are not in the September 5 submittal. 6Typically, 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.
Sediment Management at Superfund Megasites 29 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 go- ing 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 stan- dards.7 Second, the 14 sites probably constitute only a subset of the con- taminated 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 conven- tional definition (total remediation cost of at least $50 million), but the sediment portion alone is not expected to be $50 million. When comparing EPAâs list of tier 1 sites (EPA 2006b) with a somewhat dated list of megasites8 (that does not include federal facili- ties), one can find 11 âconventionalâ megasites on the tier 1 list. â¢ AlcoaâPoint Comfort/Lavaca Bay, TX â¢ Allied Paper Inc./Portage Creek/Kalamazoo River, MI â¢ Bunker Hill Mining and Metallurgical Complex, ID 7 Based on data provided by EPA, â50M cost Query_091306.xlsâ (EPA, unpub- lished data, Sept. 18, 2006). 8Based on the report to Congress, Superfundâs Future: What Will It Cost? (Probst et al. 2001), which lists megasites as of FY 2000.
30 Sediment Dredging at Superfund Megasites 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, â$50M Cost query_091306.xls,â Sept. 18, 2006. â¢ Eagle Mine, CO â¢ EI duPontâNewport landfill, DE â¢ GMâCentral Foundry Division (Massena), NY â¢ Lipari landfill, NJ â¢ Love Canal, NY â¢ McCormick and Baxter Creosoting Co., CA â¢ Nyanza chemical waste dump, MA â¢ Wyckoff Co.âEagle 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 âlikely future megasitesâ 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 âtier 2â contaminated sediment sites (S. Ells,
Sediment Management at Superfund Megasites 31 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 Bon- fouca and Marathon Battery, are relatively small, with dredging activities cover- ing tens of acres and operations occurring over a few years. Other dredging pro- jectsâsuch as those in the Fox River, New Bedford Harbor, and Commencement Bayâare components of broader activities at large-scale megasites where reme- dial 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 mate- rials removed. For example, sediment remediation (including design, mobiliza- tion, marine demolition, dredging, water management, transportation and dis- posal, 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, per- sonal commun., Dalton, Olmsted & Fuglevand, Inc., May 11, 2007). In Manis- tique Harbor, MI, dredging operations removed 187,000 cy at a cost of $48.2 mil- lion (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, â$50M Cost query_091306.xls,â Sept. 18, 2006). EPA, personal commun., Oct. 12, 2006). Tier 2 sites are designated for review by the Contaminated Sediments Technical Advisory Group be- cause 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 (Ash- land/Northern States Power, WI, Portland Harbor, OR, Lower Du- wamish 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 Colum- bia River, WA). EPA also indicates that the Passaic River, NJ, Berryâs Creek at Ventron/Velsicol, NJ, and Tar Creek, OK, are likely future con- Although, it should be noted that EPA indicates that âNo quantifiable criteria 9 were used to develop this list.â The list of sites is available at http://www.epa. gov/superfund/resources/sediment/cstag_sites.htm.
32 Sediment Dredging at Superfund Megasites 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 sci- ence, 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 under- standing likely future sediment megasites at the national level. Appar- ently, 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 sedi- ments. To develop an effective long-term contaminated sediment strat- egy it is critical to know how much work remains to be done. To address that question, one needs to have three pieces of information: 1. How much work remains at sites already categorized as con- taminated sediment megasites. 2. How many contaminated sediment sites already on the NPL are likely to be determined to be megasites. 3. 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. How- ever, it is important that EPA obtain this information and update it regu- larly in order to be able to forecast likely future costs and needed re- sources, 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 indi- vidually. 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
Sediment Management at Superfund Megasites 33 consent decree and a Resource Conservation and Recovery Act correc- tive-action consent order (Menozzi et al. 2003). This project, described as âthe largest environmental dredging project to be undertaken in North America,â 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 re- maining 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 be- tween the United States and Canada, which established 43 areas of con- cern (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 con- taminated 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 pro- jects 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 col- laboration between EPA and the U.S. Army Corps of Engineers for ur- ban-river cleanup and restoration (EPA 2003a). Eight demonstration pi- lot projects, including a dredging project in the Passaic River in New
34 Sediment Dredging at Superfund Megasites Jersey, have been developed to coordinate the planning and implementa- tion of projects to promote clean water and sediment among multiple jurisdictions and federal authorities. This section is by no means a comprehensive listing of sediment projects or efforts outside of Superfund; rather, the intent is to convey that there are many sediment remediation projects outside of Superfund that are conducted by multiple groups and under several authorities. 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 applica- ble. 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. Apart from direct ex- posure to contaminated sediment during, for example, recreational ac- tivities, humans typically are exposed to contaminants through the in- gestion of fish or wildlife that have accumulated contaminants from the sediment. Fish and wildlife are exposed to contaminants in sediments through a number of pathways, including absorption from pore water or sediments, incidental ingestion of contaminated sediments, and con- sumption of contaminated organisms. Several of those processes are pre- sented graphically in Figure 2-1. Predicting effects of exposures can be complex. Variations in the sediment environments will alter the bioavailability of contaminants, and this can markedly affect their accu- mulation and effects on organisms (NRC 2003). For instance, the pres- ence of sulfide will greatly decrease the bioavailability of many metals, and organic carbon can decrease the bioavailability of organic pollutants, such as PCBs (EPA 2003b, 2005c). Accessibility is a primary factor in exposure to and effects of con- taminated sediments. A common problem in assessing risks posed by contaminated sediments is that the contaminants (or the highest contam-
Sediment Management at Superfund Megasites 35 FIGURE 2-1 Generic conceptual site model indicating contaminated sediment exposure pathways between sediment and ecologic receptors, including fish, shellfish, benthic invertebrates, birds, and mammals. Source: EPA 2005a. inant concentrations) can be buried beneath relatively clean sediment that has deposited over time (see Figure 2-2). 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 water- dwelling organisms. In such cases, a relatively small continuing source may pose a greater risk of exposure and associated injury than a large buried inventory of sediment associated contaminants. Risk due to sediments is usually limited to contaminants that are present in or can migrate into the biologically active zone, the upper layers of sediment where organisms live or interact. That layer typically ranges from a few centimeters to 10-15 cm deep, although some organisms (including aquatic plants) may penetrate more deeply (Thoms et al. 1995; NRC 2001). Sediment-associated contaminants tend to collect in relatively sta- ble depositional zones in water bodies. In such environments, buried contaminants (that is, those below 10-15 cm) may never be exposed to the biologically active zone. However, water bodies are dynamic systems and even in generally depositional and stable environments, high flow events and changes in hydrologic conditions may lead to short term
36 Sediment Dredging at Superfund Megasites ) FIGURE 2-2 Historical changes in sediment core profiles of mercury concentra- tions collected from Bellingham Bay, WA, during natural recovery (dredging or capping was not performed). Sediment cores were taken over time (1970, 1975, 1996) from the same vicinity of Bellingham Bay in an area with a stable sediment deposit. At this site, mercury was released from a nearby facility from 1965 until controls were put in place in 1971. Sedimentation has since continued to bury the contaminants. Source: Patmont et al. 2004. Reprinted with permission from the authors; copyright 2003, Battelle Press. erosion, exposure, and transport of these contaminants to the biologic active zone. In environments subject to such conditions, removal of con- taminant mass may be an effective remedial response to the risk posed by them. 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 bur- ied contaminated sediments that do not contribute to the exposure of aquatic systems now or under future conditions will not achieve risk reduction goals. In such cases, other contaminant sources, for example inadequately controlled surface discharges or atmospheric deposition, may control exposure and risk. A fair amount of effort in recent years has gone into developing approaches for assessing sediment column sta- bility and refining hydrodynamic models and linking them with fate and transport models to estimate contaminant transport under various condi-
Sediment Management at Superfund Megasites 37 tions (e.g., Bohlen and Erickson 2006). Output from these approaches and models are important in estimating the risks associated with reme- dial alternatives. 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 perform- ing comparative risk analyses of remedial alternatives in selection of a sediment remedy. While important, that type of detailed assessment was not requested or undertaken. The brief discussion provided here on risk- based remedy selection is intended to provide background for later dis- cussions on improving decision making. The process of managing risk at contaminated sediment sites was evaluated extensively in the 2001 National Research Council report A Risk Management Strategy for PCB-Contaminated Sediments (NRC 2001). Perhaps its most relevant conclusion is that all decisions regarding the management of PCB-contaminated sediments should be made in a risk- based framework. The report further suggests that the framework devel- oped by the Presidential/Congressional Commission on Risk Assessment and Risk Management provides a good foundation for assessing the risks and the management options for a site (see Box 2-3). The general framework exhibits several key features that make it appropriate for the management of contaminated sediment sites. It recognizes that risk- reduction should be the foundation of any decision-making process and the importance of the participation of interested and affected stake- holders in the decision-making process. It also provides a systematic and structured process for identifying and assessing risks, evaluating and implementing options, and monitoring the success of the overall process. The 2001 Research Council report also recommends that risk assessments and risk management decisions be site specific and concluded that cur- rent management options can reduce risks but cannot eliminate PCBs and PCB exposure from contaminated sediment sites. Because all reme- dial options will leave residual PCBs, the short- and long-term risks that
38 Sediment Dredging at Superfund Megasites they pose should be considered in evaluating management strategies. Those ideas also apply to other sediment contaminants. BOX 2-3 Presidential/Congressional Commission on Risk Assessment and Risk Management The Presidential/Congressional Commission on Risk Assessment and Risk Management was formed in response to the 1990 Clean Air Act Amendments in which Congress mandated that a Commission on Risk Assessment and Risk Management be formed to âmake a full investigation of the policy implications and appropriate uses of risk assessment and risk management in regulatory pro- grams under various Federal laws to prevent cancer and other chronic human health effects which may result from exposure to hazardous substancesâ (PCCRARM 1997, p. i). The commission ultimately developed a report that introduced a risk man- agement framework âto guide investments of valuable public sector and private sector resources in researching, assessing, characterizing, and reducing riskâ (PCCRARM 1997, p. i). The commission proposed a six-stage process: â¢ Define the problem and establish risk management goals. â¢ Assess risks associated with the problem. â¢ Evaluate remediation options for addressing the risks. â¢ Select a risk management strategy. â¢ Implement the risk management strategy. â¢ Evaluate the success of the risk management strategy. This process should be conducted â¢ In collaboration with all affected parties. â¢ In an iterative fashion when substantive new information becomes available. The proposed process, depicted in the schematic below, is a systematic method to manage risks that the commission defined as âthe process of identify- ing, evaluating, selecting, and implementing actions to reduce risk to human health and to ecosystems. The goal of risk management is scientifically sound, cost-effective, integrated actions that reduce or prevent risks while taking into account social, cultural, ethical, political, and legal considerationsâ (PCCRARM 1997, p. 2).
Sediment Management at Superfund Megasites 39 Key features of the framework are recognition of the importance of stake- holders in the process, the importance of defining risks in a broader context than single risks associated with single chemicals in single environmental media, and the importance of an iterative process where earlier decisions can be revisited when new findings are made. Remedy selection is a complex process with many considerations (see Box 2-1). In some cases, removal will be the best option for risk re- duction and satisfying the NCP criteria, in others, capping or monitored natural recovery will be preferable. An analysis of alternative remedies typically includes a comparison of both the short- and long-term risks to human and environmental receptors associated with a particular site. For example, the risks from dredging can include exposure to contaminants during dredging, rehandling, and transport, and contaminants that re- main after operations are completed. Those risks would be compared to other alternatives, including risks from unconfined contaminated sedi- ment and potential future resuspension and transport during storm and non-storm events. Risks beyond those related directly to exposure to contaminants are also considered in this process (see Net Risk Reduction below). Technical and policy guidance for making remedial decisions using a risk based framework at contaminated sediment sites was recently is- sued (EPA 2005a). The document provides a useful evaluation of the various sediment management approaches and their advantages and limitations in attaining risk reduction. It discusses in detail aspects of the
40 Sediment Dredging at Superfund Megasites Superfund decision-making process (site characterization, feasibility study, and remedy selection) particular to contaminated sediment, and it offers recommendations for implementing an effective monitoring plan. The guidance concludes that âThe focus of remedy selection should be on selecting the alternative best representing the overall risk reduction strategy for the site according to the NCP nine remedy selection crite- riaâ¦. EPAâs policy has been and continues to be that there is no pre- sumptive remedy for any contaminated sediment site, regardless of the contaminant or level of riskâ (EPA 2005a, p. 7-16). Measuring Risk-Reduction Estimating the degree of risk reduction is central in considering the potential effectiveness of a remedial action. Risk posed by chemical con- tamination is a function of the duration and intensity of exposure and the ability of the chemical or chemical mixture to exert adverse effects. There is not a direct measure of risk, so surrogate metrics are used to estimate risk. Environmental analyses have to use metrics that, in prac- tice, can be employed relatively easily, are not time- and cost-prohibitive, and have sufficient accuracy and precision to be reliable. Estimates of risk reduction at contaminated sites have often centered on measuring changes in the mass, volume, and concentration of contaminated sedi- ments. Those measures are related to the potential for exposure in the aquatic environment but do not provide information on effects. There- fore, although they are the most prevalent, they are not fully adequate to describe risk or to chart risk reduction. Toxicity testing, biologic commu- nity indexes, and tissue-residue analyses provide a fuller picture of ef- fects, although they too have limitations in their ability to describe risk. (See Chapter 5 for further discussion on the metrics and their advantages and disadvantages in estimating risk to aquatic biota and humans.) Temporal Scale In characterizing risk and evaluating risk reduction, it is necessary to consider the duration of time over which exposure and effects occur. After remediation, risk is usually predicted to decline over time rather than reach a protective level immediately on completion of the remedy,
Sediment Management at Superfund Megasites 41 so remedy selection involves a comparison of time profiles of predicted risks, often on scales of decades. Such a comparison of risk profiles over time is how the long-term effectiveness of dredging is evaluated relative to alternative technologies for the largest and most complex sites. Factors dictating the time to reduce risk are site specific and include the time required to design and fully implement the remedy, the time required to cleanse the food chain of existing contaminant body burdens, and the time for natural recovery processes to attenuate any residual surface sediment concentrations after implementation is complete. Net Risk Reduction The 2001 National Research Council report indicated that the paramount consideration for contaminated sites should be the manage- ment of overall or net risks to humans and the environment in addition to specific risks. The report concludes that the evaluation of sediment management and remediation options should take into account all costs and potential changes in risks for the entire sequence of activities and technologies that constitute each management option. (For example, managing risks from contaminated sediments in aquatic environments might result in the creation of additional risks in both aquatic and terres- trial environments.) The report also suggests that a broader array of risksâincluding societal, cultural, and economic risksâshould be evalu- ated comprehensively. The concept of net risk reduction has been em- braced by EPA in its Contaminated Sediment Remediation Guidance for Haz- ardous Waste Sites (EPA 2005a); it states that âProject managers are encouraged to use the concept of comparing net risk reduction between alternatives as part of their decision-making process for contaminated sediment sites, within the overall framework of the NCP remedy selec- tion criteria. Consideration should be given not only to risk reduction associated with reduced human and ecologic exposure to contaminants, but also to risks introduced by implementing the alternatives. The mag- nitude of implementation risks associated with each alternative generally is extremely site specific, as is the time frame over which these risks may apply to the site. Evaluation of both implementation risk and residual risk are existing important parts of the NCP remedy selection processâ (EPA 2005a, pp. 7-13, 7-14).
42 Sediment Dredging at Superfund Megasites Risk-Based Objectives and Cleanup Levels Each site has its own set of contaminants with different concentra- tions and distributions in its own particular geologic, geochemical, geo- graphic, social, ecologic, and economic setting. Therefore, management decisions based on the above framework are expected to differ among sites. At Superfund sites, the overall goal of sediment management is re- duction 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). Attainment of remedial action objectives can be difficult to quantify or might occur in a time frame or encompass a spatial scale that makes it difficult to link to reme- dial actions. Under such circumstances, cleanup levels, such as achieve- ment of a sediment concentration or removal of a given mass of con- taminant or sediment, which can be more easily used to evaluate remedial actions, are often adopted (EPA 2005a). Ideally, clean-up levels are tied to effects-based risk thresholds, and take into account effects of combinations of contaminants. That the application of a good risk management strategy is likely to result in significantly different cleanup levels at different sites makes it difficult for the committee to draw conclusions about the expected effec- tiveness of dredging. At some sites, cleanup levels are far less stringent than at others, and thus all other things being equal, a site with less stringent cleanup goals is more likely to be âsuccessfulâ than a site with more stringent goals. Geologic and site-specific conditions also differ, so even if the cleanup goals are similar at different sites, the technical ability to reach the goals may differ. Thus, one needs to be highly cautious in suggesting that success with a remedial option at one site necessarily means that the same success is likely at another. To help to ensure that remedial actions achieve their desired objec- tives, 11 principles have been developed by EPA to guide sediment remediation (Box 2-4). The principles were developed partially in re- sponse to the recommendations of the National Research Council Com- mittee on Remediation of PCB-Contaminated Sediments (NRC 2001).
Sediment Management at Superfund Megasites 43 BOX 2-4 Eleven Principles of Contaminated Sediment Management In February 2002, the EPA Office of Solid Waste and Emergency Response promulgated 11 principles of contaminated sediment management (OSWER Directive 9285.6-08): 1. Control sources early. 2. Involve the community early and often. 3. Coordinate with states, local governments, tribes, and natural-resources trustees. 4. Develop and refine a conceptual site model that considers sediment stability. 5. Use an iterative approach in a risk-based framework. 6. Carefully evaluate the assumptions and uncertainties associated with site-characterization data and site models. 7. Select site-specific, project-specific, and sediment-specific risk-manage- ment approaches that will achieve risk-based goals. 8. Ensure that sediment cleanup levels are clearly tied to risk-management goals. 9. Maximize the effectiveness of institutional controls and recognize their limitations. 10. Design remedies to minimize short-term risks while achieving long- term protection. 11. Monitor during and after sediment remediation to assess and document remedy effectiveness. The principles were designed to help EPA site managers to make scientifi- cally sound and nationally consistent risk-management decisions at contami- nated sediment sites. The principles are consistent with the recommendations of the National Research Council A Risk Management Strategy for PCB-Contaminated Sediments and the Presidential/Congressional Commission on Risk Assessment and Risk Management. They were incorporated into the Contaminated Sediment Remediation Guidance for Hazardous Waste Sites (EPA 2005a). 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
44 Sediment Dredging at Superfund Megasites relationships among contaminant sources, transport mechanisms, expo- sure pathways, human receptors, and ecologic receptors at each affected level of the food chain. That understanding of causal relationships is known as a conceptual site model (CSM) (EPA 2005a) and is typically developed for each site on the basis of site-specific conditions. The link between risk and the inventory of contaminants in sediments is not al- ways obvious. An accurate CSM is critical for identifying the processes and pathways that might lead to risk and appropriate means of interven- ing to reduce risk. For example, evaluation of the stability or potential instability of buried deposits and their potential for exposure in the bioavailable zone is a key component of a CSM because a CSM must be able to differentiate between important and unimportant routes of expo- sure. In addition to linking site contaminant sources to exposures and risks, the CSM must account for background conditions, including con- taminant distribution from offsite sources. Ecosystems may be highly stressed because of multiple watershed and atmospheric effects on con- ventional water quality measures, such as nutrients, suspended solids, acidity, dissolved oxygen, and temperature. The contribution of back- ground stressors or other background sources to site effects should be evaluated, including assessment of their importance relative to the con- taminants of concern, and recognized as potentially complicating factors in ecosystem restoration. The CSM should guide site investigation, and its hypotheses and assumptions should be tested and refined as site data are acquired. When the CSM has been accepted with a high degree of confidence, it is used to define remedial action objectives. 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 develop- ment and selection of a remedy. The result is a mathematical model or a set of models of the various component processes. Mathematical models are used to quantify the same cause-effect relationships that are embod- ied in CSMs so that magnitudes of predicted outcomes can be associated with specific causes or actions, such as contaminant loads, environ- mental conditions, and remedies. To be most accurate, the models
Sediment Management at Superfund Megasites 45 should be supported by and calibrated to site-specific data on the envi- ronmental media (such as sediments, pore water, and water), receptors (such as benthic organisms, fish, and humans), and processes (such as toxicity, bioavailabity, and bioaccumulation) that are being examined. Mathematical models range from simple to complex, including ana- lytic equations representing established scientific relationships between independent and dependent variables, statistical cause-effect relation- ships between site variables, and systems of differential equations repre- senting multiple fate and transport processes. With a mathematical model, quantitative versions of hypotheses can be tested and refined on the basis of site data, including data from field surveys of site conditions and pilot studies of remedial technologies, and then the relative effec- tiveness of alternative remedies in reducing exposures can be estimated, including the sensitivity of exposure to the remedies and the time needed to reduce exposure. The measures of predicted effectiveness are used to support remedy selection. Models are subject to uncertainty because of the uncertainty in pa- rameters and process representations. Model testing and refinement does not end with the selection of a remedy through a record of decision. It is important that the conceptual and mathematical site models also be used in designing monitoring of conditions during implementation and post-remedy phases and that the monitoring data be used to validate the modelsâ predictions. When risk reduction deviates significantly from a modelâs predictions, the model should be modified or recalibrated to improve its accuracy so that more reliable predictions can be available to guide midcourse adjustments in the remedy (EPA 2005a). CONTAMINATED SEDIMENT MANAGEMENT TECHNIQUES Contaminated sediment is managed with various techniques, in- cluding source control, natural recovery, capping, and removal (dry ex- cavation and dredging). Removal necessitates management of the re- moved material, which normally includes dewatering, transport, and disposal. Treatment of dredged material to remove or destroy contami- nants is an option, but cost and other factors usually lead to disposal in upland landfills or in near-shore confined disposal facilities. In some
46 Sediment Dredging at Superfund Megasites cases, dredged material can be returned to the aquatic environment through containment in confined aquatic disposal facilities (EPA 2005a). The National Research Council Committee on Contaminated Ma- rine Sediments (NRC 1997) and Committee on Remediation of PCB- Contaminated Sediments (NRC 2001) have reviewed and reported on a number of sediment management techniques. The committees stated that source control is advisable in all contaminated sediment management projects, notwithstanding the difficulties of identifying some sources of contamination. Beyond source control, interim controls (temporary measures to address exposures immediately) and long-term controls (such as in situ management technologies, sediment removal and trans- port, and ex situ management) may be needed to address sediment con- tamination. More recently, EPA (2005a) lists both in situ and ex situ remedial strategies for managing risks posed by contaminated sediment. The in situ strategies include monitored natural recovery (MNR), in situ cap- ping, hybrid (thin-layer placement) approaches, institutional controls, and in situ treatment; the ex situ strategies include dredging and dry excavation (following dewatering or water diversion). See Box 2-5 for an explanation of these approaches. The present committeeâs focus is on environmental dredging, which is conducted specifically to remove con- taminated sediments, as opposed to navigational dredging, which typi- cally 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 sedi- ments, the historical preference for removal is evident in the large per- centage of sites whose remedy was based entirely or in part on dredging. In an overview of Superfund sediment remediation, EPA presented in- formation from 60 tier 1 sites for which a remedy had been selected. Of the 60, 57% had only removal as the remedial action, 15% capping with removal, 13% removal with MNR, 5% only capping, 2% only MNR, and 8% all three remedies (Southerland 2006). The historical preference for
Sediment Management at Superfund Megasites 47 BOX 2-5 Remedial Approaches to Contaminated Sediment In Situ Approaches In Situ Approaches â¢ Monitored natural recovery (MNR) is a remedy for contaminated sediment that typically relies on naturally occurring processes to contain, destroy, or reduce the bioavailability or toxicity of contaminants in sediment. â¢ In situ capping refers to the placement of a subaqueous covering or cap of clean material over contaminated sediment, which remains in place. â¢ Hybrid approaches refers to placement of a thin layer of sand or other mate- rial to accelerate recovery. â¢ Institutional controls are controls on the use of resources. They typically in- clude fish-consumption advisories, commercial fishing bans, waterway-use or land- use restrictions (for example, no-anchor or no-wake zones and limitations on naviga- tional dredging), and agreements on maintenance of dams or other structures. Ex Situ Approaches â¢ Dredging and excavation are common means of removing contaminated bot- tom sediment from a body of water either while the sediment is submerged (dredg- ing) or after water has been diverted or drained (excavation). Ex situ approaches can include backfilling with clean material as needed or appropriate. Source: Adapted from EPA 2005a. removal is probably based on the perception (in both agencies and the public) of the permanence of the remedy. Dredging and excavation re- move the mass of contaminants from the aquatic environment, and this has historically been viewed as key to reducing human health and envi- ronmental risks. Technologies for removing sediment were already well established in the early years of sediment cleanup, in part as an extension of reme- diation technologies applied at upland sites. Most of the initial technolo- gies for managing sediment came from the U.S. Army Corps of Engi- neers 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. Leaving contamination in place under a capping or MNR remedy was often con- sidered more uncertain because of the residual risk posed by contami- nants left in place.
48 Sediment Dredging at Superfund Megasites The dynamic nature of aquatic environments has often led to the selection of removal as the preferred alternative in many areas of the country. Contaminated sediment is often associated with industrial, ur- ban harbors where operational and navigational constraints are viewed as limiting the feasibility of capping or natural recovery. Those environ- ments are often subject to disturbances, such as those caused by prop wash, seasonal flooding, ice scour, and storm surges, which were viewed as creating substantial risk if contaminants were left in place. Removal of contaminated sediment has brought unique challenges that were initially not well recognized. Navigational dredging tech- niques 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 re- leases 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. in press). The risks associated with the implementation of environmental dredging have received a great deal of attention in the last few years (EPA 2005a; Wenning et al. 2006). Greater experience with capping remedies has been gained over the last decade; cap performance can be better predicted and quantified, and this has led to greater acceptance among agencies. In addition, capping typically has been less expensive and can be implemented more quickly, so it is often preferred by responsible parties (Palermo et al. 1998). In re- sponse to the increasing experience with remedial technologies, recent guidance from EPA has called for a more equitable evaluation of all remedies with careful analysis of the short-term and long-term risks as- sociated with any remedy and thorough consideration of site-specific conditions (EPA 2005a). OVERVIEW OF ENVIRONMENTAL DREDGING Dredging refers to the removal of sediment from an underwater en- vironment. It involves dislodging and removing material on the bottom of a waterway. Dredges are normally classified according to the basic
Sediment Management at Superfund Megasites 49 operation by which sediment is removed, such as mechanical or hydrau- lic10 (EPA 1994). For purposes of this report, excavation in the dry using conventional equipment operating within dewatered containments such as sheet-pile enclosures or cofferdams is not covered. The term environ- mental dredging is more generally associated with removal of sediment from under water. Environmental dredging can be accompanied by backfilling of the dredged areas. Placement of clean material covers and mixes with dredging residuals and further reduces risk from contamina- tion that remains after dredging. Unlike capping, permanent confine- ment of underlying material is usually not the goal. Typical objectives of environmental dredging are shown in Box 2-6. Because the purpose of navigational dredging is to restore navigable depth to a waterway, the selection of equipment and operational ap- proaches considers economics, effectiveness, and environmental protec- tion (USACE/EPA 1992) in that order. Conversely, environmental dredg- ing has remediation as its stated purpose. The distinction results in reversing the order of importance of the selection factors for equipment and operational approaches; that is, one needs to consider environmental protection and effectiveness first before considering economics (Palermo et al. 2006). BOX 2-6 Objectives of Environmental Dredging â¢ Dredge with sufficient accuracy such that contaminated sediment is removed and cleanup levels are met without unnecessary removal of clean sediment. â¢ Dredge the sediments in a reasonable period of time and in a condition compatible with subsequent transport for treatment or disposal. â¢ Minimize and/or control resuspension of contaminated sediments, downstream transport of resuspended sediments, and releases of contaminants of concern to water and air. â¢ Dredge the sediments such that generation of residual contaminated sediment is minimized or controlled. Source: Palermo et al. 2006. 10Pneumatic systems, which use compressed air to pump sediment out of a waterway, have not gained general acceptance in environmental dredging pro- jects in the United States.
50 Sediment Dredging at Superfund Megasites Types of Environmental Dredges Selection of dredging equipment is sediment specific, site specific, and operations specific. Many textbooks and manuals describe the sci- ence and engineering principles of dredges, their selection, and their op- eration (Bray 1979; USACE 1983; Herbich 2000). This section provides basic definitions of dredging methods and equipment types normally considered for environmental dredging. There is no attempt to list all the possible types of dredge equipment that may be applicable to environ- mental dredging. Box 2-7 lists the equipment most commonly used for environmental dredging according to type (category) and definition (Palermo et al. 2004). Figure 2-3 shows the basic dredge types. More de- tailed descriptions of environmental-dredging equipment are available elsewhere (Averett et al. 1990; EPA 1994; EPA 2005a). Other dredge typesâsuch as hopper dredges, dustpan dredges, and bucket-ladder dredgesâare not included in Box 2-7, because they are used primarily for navigational dredging. 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 read- ily available and used for environmental dredging projects in the United States. A number of newer dredges, including some specifically designed for environmental dredging, are available. They have been termed spe- cialty dredges and are intended to provide benefits by reducing sedi- ment resuspension and contaminant releases. Other advantages may include operational efficiency for removal of sediment and transporta- tion, depending on the sediment and project conditions and the per- formance standards. Most specialty dredge designs originated outside the United States, but several U.S. companies have now formed partner- ships that allow use of specialty equipment from various countries. Field experience with specialty dredges in the United States is limited (Pal- ermo et al. 2003). The dredges have been proposed for use at contami- nated sediment sites, but little information is available about their sedi- ment-extraction efficiency or about the claimed improvements in innovations, such as improved solids capture and reduced resuspension. The equipment used for environmental dredging is usually smaller than that commonly used for navigation dredging because removal
Sediment Management at Superfund Megasites 51 BOX 2-7 Equipment Commonly Used in Environmental Dredging Mechanical Dredges â¢ Clamshell: Wire-supported conventional open clam bucket. â¢ Enclosed bucket: Wire-supported, nearly watertight or sealed bucket. In contrast to conventional open buckets, recent designs also incorporate a level-cut capability instead of the circular cut of conventional buckets (for example, the horizontal profiling buckets). â¢ Articulated mechanical: Backhoe design, clam-type enclosed bucket, hydraulic closing mechanism, all supported by articulated fixed arm. Hydraulic Dredges â¢ Cutterhead: Conventional hydraulic pipeline dredge with conventional cutterhead. â¢ Horizontal auger: Hydraulic pipeline dredge with horizontal auger dredgehead. â¢ Plain suction: Hydraulic pipeline dredge using a dredgehead design with no cutting action and plain suction (for example, cutterhead dredge with no cutter basket mounted, matchbox dredgehead, and articulated scoops). Pneumatic Dredges â¢ Pneumatic: Air-operated submersible pump, pipeline transport, and wire-supported or fixed-arm-supported. Specialty Dredges and Diver-Assisted Dredges â¢ Specialty dredgeheads: Other hydraulic pipeline dredges with specialty dredgeheads or pumping systems. â¢ Diver-assisted: Hand-held hydraulic suction with pipeline transport. Source: Adapted from Palermo et al. 2004. volumes and rates tend to be lower and water to be shallower. Mechani- cal-bucket sizes range from 2 to 8 m3 (about 3 to 10 cy), and hydraulic- pump sizes range from 15 to 30 cm (about 6 to 12 in.) (Palermo et al. 2006). Obviously, larger dredges are available for both mechanical and hydraulic equipment and can be used for environmental dredging if needed.
52 Sediment Dredging at Superfund Megasites FIGURE 2-3 Categories of dredging and sediment removal equipment. Source: Francingues and Palermo 2006. DredgingâOne Part of the Overall Process Train Physically removing sediments by dredging is only one component of the overall remediation process. The key processing steps shown in Figure 2-4 include (EPA 2005a): â¢ Mobilization and setup of equipment.
Sediment Management at Superfund Megasites 53 â¢ Site preparation including debris removal and protection of structures. â¢ Removal (environmental dredging). â¢ Staging, transport, and storage (rehandling). â¢ Treatment (pretreatment, solidification and stabilization of sol- ids, treatment of decant water and/or dewatering effluents and sedi- ment, and potentially separate handling and treatment of materials with and without special requirements under the Toxic Substances Control Act [TSCA]). â¢ Disposal (liquids and solids). Environmental dredging must be compatible with all later steps in the process train. For example, the production rate of a dredge (either mechanical or hydraulic) depends heavily on the mode of transportation and the ability to rehandle or directly manage the dredged material on the other end of the process. Compatibility must be considered with re- spect to the type of pretreatment, treatment, and disposal being planned, especially the availability, size, and capacity of disposal sites, the dis- tance from dredging site to treatment or disposal sites, and constraints associated with production rates for transport, storage, rehandling, treatment, or disposal. Inefficiencies in remedial dredging projects can result from constraints associated with components of the remedy other than dredging, such as dewatering capacity, water-treatment effective- ness, and disposal location and capacity (Palermo et al. 2006). Dredging accounts for only part of the overall cost of an environ- mental-dredging project. In a complex project, large costs may be associ- ated 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. For example, EPA Region 1 (EPA 2005d) reported on the costs of the 2004 New Bedford Harbor Superfund dredging project, Mobilization Mobilization/ Removal Site Removal Staging and Staging/ (Dredging or (Dredging or Transport and Transport/ Treatment Disposal Setup andSetup Preparation Rehandling Excavation) Excavation) Re-handling FIGURE 2-4 Dredging-process train. Source: Adapted from Palermo et al. 2006.
54 Sediment Dredging at Superfund Megasites as shown in Figure 2-5; dredging itself represents only 17% of the total yearly construction and operations cost. Similarly, in the Head of Hyle- bos remediation (Figure 2-6) dredging operations conducted from 2003 to 2006 (Dalton, Olmsted & Fuglevand, Inc. 2006), dredging represents 17% of the total cost. Dredging cost varies widely, depending on many factors, including site conditions, the nature of the sediments and con- taminants, the type and size of dredge selected, production rates, and seasonal construction windows. However, when dredging is selected as a remedy, all the other components of the process train will probably be required and will account for most of the overall cost. Only in those cases where transportation and disposal of sediments are relatively inexpen- sive (for example, where there is an existing in-water or upland disposal site, both suitable for the long-term containment of contaminated sedi- ment and in close proximity to the dredging) will the dredging be a ma- jor cost element. TECHNICAL ISSUES ASSOCIATED WITH DREDGING Environmental dredging typically strives to achieve contaminant- specific cleanup levels set at each site. A number of technical issues can limit ability or efficiency in achieving those levels. This section describes several of the issues, many of which are revisited in the context of site evaluations in Chapter 4. Accuracy of Dredging vs. Accuracy of Sediment Characterization The benefits of being able to position a dredge cut accurately may be achieved only if a corresponding degree of accuracy is reflected in the site and sediment-characterization data. The ability to map the precise location of chemical concentrations accurately both horizontally and ver- tically depends on the data density (grid density), accessibility of deeper sediments, and other aspects of site characterization (Palermo et al. 2006). In some cases, the ability to locate the dredge cut accurately ex- ceeds the accuracy of the knowledge of the location of the contaminated sediments (Palermo et al. 2006).
Sediment Management at Superfund Megasites 55 Monitoring P M & Dredging 17% Fees 25% Desanding 12% T ransport & Dewatering Disposal 22% 18% Water T reatment 6% FIGURE 2-5 Cost breakdown of components of environmental dredging at the New Bedford Harbor (New Bedford, MA) project in 2004. Full-scale operations cost about $800,000 per week. PM = project management. Source: Data from EPA 2005d. Other Monitoring P M Dredging 4% & Fees 17% 15% Mob/Demob 15 % T ransport & Wat er Management Disposal 6% 31% Sediment Handling 12% FIGURE 2-6 Cost breakdown of components of environmental dredging at the head of the Hylebos Waterway (Commencement Bay, WA) project. PM = project management; Mob/Demob = mobilization and demobilization. Source: Data from P. Fuglevand, Dalton, Olmsted & Fuglevand, Inc., personal commun., May 11, 2007. In the context of this discussion, vertical operating accuracy is the ability to position the dredgehead at a desired depth or elevation for the cut, whereas vertical precision is the ability to maintain or repeat the ver- tical position during dredging. The key to the success of an environ- mental-dredging project is the removal of the target layer, which is de-
56 Sediment Dredging at Superfund Megasites lineated by the cut line, without unnecessary removal of clean material (Palermo et al. 2006). The ability to dredge to a specified cut line in the sediment has been greatly improved by the advent of electronic positioning technolo- gies, such as differential global positioning systems (DGPSs) and kine- matic differential global positioning systems (KDGPSs). Depending on site conditions, dredge operator ability, size and type of dredge, and po- sitioning instrumentation and software, the dredgehead and cut eleva- tion may be locatable with vertical accuracy of less than 30 cm. Vertical accuracies of 10 cm for fixed-arm dredgeheads should be consistently attainable, whereas vertical accuracies of 15 cm should be attainable with proper operator training in the use of wire-supported buckets (Palermo et al. 2006). Notwithstanding the previous statements regarding accuracies and positioning of dredging equipment, there are numerous challenges in attaining them. For example, â¢ Effective use of sophisticated dredge positioning systems re- quires sophisticated operators and contractors in order to achieve the stated accuracies. â¢ In order to get effective positioning with any of the software packages, the operators must be specifically trained and capable of sys- tem operation, and the systems must be properly operated and cali- brated. â¢ Experience has shown that some systems are more difficult to operate than others, and some systems may experience difficulties main- taining calibration. Simply using an electronic positioning system on a dredge does not guarantee that the stated accuracy will be achieved. 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 associ- ated contaminants can settle back to the bottom in the dredge cut; finer- grained materials can remain in the water column and be transported to other locations. Those materials are deposited as residuals and result from dredging. Dissolved contaminants may also be released to the wa-
Sediment Management at Superfund Megasites 57 ter column during dredging from resuspended or exposed contaminated sediment. Figure 2-7 is a conceptual illustration of environmental dredg- ing and those processes. Dredged sediment resuspension, release, and residual and the re- sulting risk (the â4 Rsâ) were the focus of a recent workshop held at the U.S. Army Engineer Research and Development Center in Vicksburg, MS (Bridges et al. in press). Effective remediation by dredging requires minimizing the 4 Rs while maximizing the fifth R, removalâeither the dredging production rate or the volume removed (Francingues and Thompson 2006). The type and amount of sediment resuspension, con- taminant release, and residuals during a dredging operation depend on many site-specific project factors, as shown in Box 2-8. Resuspension Resuspension is the process by which dredging and associated op- erations result in the dislodgement of embedded sediment particles, which disperse into the water column. Resuspended particles may settle in the dredging area or be transported downstream. Recent EPA guid- ance 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.â¦ 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.â¦ 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. 6- 21, 6-22). Resuspension concerns related to dredging include the physical ef- fects of turbidity and burial that can result in seasonal restrictions on dredging operations (dredging windows). Sediment resuspension can
58 Sediment Dredging at Superfund Megasites Release (Air) Release (Water) Resuspension Residual (Sediment) Removal Residual FIGURE 2-7 Conceptual illustration of environmental dredging and processes. Source: Patmont 2006. Reprinted with permission; copyright 2006, Anchor Envi- ronmental LLC. result in chemical releases to the water column (for example, from pore water displaced from the dredged sediment or by desorption from re- suspended sediment particles) and residual contamination on the bottom after dredging. Resuspension can be caused not only by dredging equipment but by propwash of tenders (push boats or tugs used to move equipment) and during rehandling and transport operations, such as filling and overflowing of barges and leaky pipelines. Estimates of re- suspension from environmental-dredging projects range up to 10% of the mass of sediment dredged (Patmont 2006). Rates of resuspension depend on equipment, material, operator, and other site-specific factors. Residuals Residuals are contaminated sediment that remains after dredging. There are two general types of residuals: generated residuals, contami- nated sediment that is dislodged or suspended during dredging and later redeposited within or adjacent to the dredging footprint; and undis- turbed residuals, contaminated sediments found at the post-dredge sedi-
Sediment Management at Superfund Megasites 59 BOX 2-8 Site-Specific Factors Affecting Resuspension, Release, and Residuals Sediment Physical and Chemical Properties Sediment Physical and Chemical Properties â¢ Grain size distribution (for example, percentages of silt, clay, and sand). â¢ Organic carbon content. â¢ Amount of sulfides. â¢ Spatial and vertical distributions of contaminants in the sediment (for ex- ample, layering). Site Conditions â¢ Water velocity and degree of mixing. â¢ Water salinity, hardness, alkalinity, and temperature. â¢ Type of substrate (for example, hardpan, bedrock or soft sediment). â¢ Type and extent of debris in sediment. â¢ Weather, such as storms that result in wind and waves. â¢ Wakes from passing vessels. â¢ Fluctuations in water elevation. â¢ Depth and slope of area to be dredged. Equipment â¢ Type of dredge (for example, cutterhead pipeline, open or closed bucket, and specialty dredgehead). â¢ Methods of dredging. â¢ Skill of operators. â¢ Extent of tender-boat activity. â¢ 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, inaccura- cies of dredging, mixing of targeted material with underlying materials during dredging, fallback (dislodged sediment not picked up), and reset- tlement of resuspended sediments (Palermo et al. 2006). Also contribut- ing to residual contamination are such processes as sloughing of sedi- ment into the dredging cut and sloughing induced by bank or slope
60 Sediment Dredging at Superfund Megasites failures. Site-specific factors, such as debris or limitation of dredging by bedrock or hardpan can influence the amount of residuals. Box 2-9 de- scribes 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 sedi- ment-dwelling organisms (although in the case of undisturbed residuals the depth can be substantially greater). That upper layer is subject to ero- sion 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 dredgerâs 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 resus- pended particles may partition to the water column and be transported downstream in dissolved form along with contaminants in the released More recently, Patmont and Palermo (2007) analyzed a similar (though not 11 identical) dataset and found that final generated residuals ranged from approxi- mately 2% to 9% (average = 4%) of the mass of contaminant dredged during the last production cut.
Sediment Management at Superfund Megasites 61 BOX 2-9 Specific Processes Contributing to the Residual Layer During Dredging For mechanical dredging, processes that contribute the residual layer are â¢ 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. How- ever there can be significant resuspenion of contaminated sediment during the closing of enclosed buckets, as the bucket vents expel sediment at high velocity. â¢ 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 â¢ The spillage layer generated by hydraulic dredging associated with the turning of the cutterhead or auger in the sediment. Hydraulic dredges are nor- mally 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 conse- quently there is a âspillage layerâ left behind after dredging. â¢ 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 resid- ual layer through a variety of mechanisms including â¢ 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 (âtwo pass dredge approachâ). If not controlled, this bank sloughing can result in a considerable residual layer forming on previously dredged areas. â¢ 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 (Continued on next page)
62 Sediment Dredging at Superfund Megasites BOX 2-9 Continued 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 bioturba- tion (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 character- istics of the dredge (Sanchez 2001; Sanchez et al 2002). The degree of con- taminant 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 resus- pension 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 re- lease after dredging. Control of solids is important but is not always suf- ficient to prevent contaminant losses. Impact on Risk Risk can result from contaminant exposures driven by resuspen- sion, 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
Sediment Management at Superfund Megasites 63 potentially affect the risk resulting from dredging, and may continue to influence risk after remedial operations cease. Surface-water concentra- tions and surface-sediment concentrations may increase during and after dredging and can result in adverse effects and accumulation of contami- nants in organisms. The potential for volatile compounds to be released into the air may be an additional concern in connection with highly con- taminated sites (EPA 2005a). Release, resuspension, and production of residuals will affect risk over different spatial scales and time frames depending on the site char- acteristics and nature of the dredging operation. As described by Bridges et al. (in press), âCharacterizing 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 re- duction.â As will be discussed in much greater detail in Chapter 4, re- suspension 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 reduc- tion 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 Contami- nated Sediments in the Great Lakes. Miscellaneous Paper EL-90-25. Pre- pared for Great Lakes National Program Office, U.S. Environmental Pro- tection 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, NJâone of Americaâs most polluted rivers. World Dredging Mining and Construction
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