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Some of the key components to be evaluated when considering dredging or excavation as a cleanup method include sediment removal, transport, staging, treatment (pretreatment, treatment of water and sediment, if necessary), and disposal (liquids and solids). Highlight 6-1 provides an sample flow diagram of the possible steps in a dredging or excavation alternative. The simplest dredging or excavation projects may consist of as few as three of the components shown in Highlight 6-1. More complex projects may include most or all of these components. Efficient coordination of each component typically is very important for a cost-effective cleanup. Project managers should recognize, in general, fewer sediment rehandling steps leads to lower implementation risks and lower cost.
Sediment removal by dredging or excavation has been the most frequent cleanup method used by the Superfund program at sediment sites. Dredging or excavation has been selected as a cleanup method for contaminated sediment at more than 100 Superfund sites (some as an initial removal action). At approximately fifteen to twenty percent of these sites, an in-situ cleanup method [i.e., capping or monitored natural recovery (MNR)] was also selected for sediment at part of the site. When dredging is the selected remedy and hazardous substances left in place are above levels that allow for unlimited use and unrestricted exposure, a five-year review pursuant to the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA) §121(c) may be required (U.S. EPA 2001i).
Project managers should also refer to the U.S. Environmental Protection Agency’s (EPA’s) Assessment and Remediation of Contaminated Sediments (ARCS) Program Remediation Guidance Document (U.S. EPA 1994d), and Handbook: Remediation of Contaminated Sediments (U.S. EPA 1991c), the NRC’s Contaminated Sediments in Ports and Waterways: Cleanup Strategies and Technologies (NRC 1997), and Operational Characteristics and Equipment Selection Factors for Environmental Dredging (Palermo et al. 2004) for detailed discussions of the processes and technologies available for dredging and excavation.
Although each of the three potential remedy approaches (MNR, in-situ capping, and removal) should be considered at every site at which they might be appropriate, sediment removal by dredging or excavation should receive detailed consideration where the site conditions listed in Highlight 6-2 are present.
Highlight 6-2: Some Site Conditions Especially Conducive to Dredging or Excavation • Suitable disposal site(s) is available and nearby • Suitable area is available for staging and handling of dredged material • Existing shoreline areas and infrastructure can accommodate dredging or excavation needs;
maneuverability and access not unduly impeded by piers, buried cables, or other structures • Navigational dredging is scheduled or planned • Water depth is adequate to accommodate dredge but not so great as to be infeasible; or excavation in the dry is feasible • Long-term risk reduction of sediment removal outweighs sediment disturbance and habitat disruption • Water diversion is practical, or current velocity is low or can be minimized, to reduce resuspension and downstream transport during dredging • Contaminated sediment overlies clean or much cleaner sediment (so that over-dredging is feasible) • Sediment contains low incidence of debris (e.g., logs, boulders, scrap material) or is amenable to effective debris removal prior to dredging or excavation • High contaminant concentrations cover discrete areas of sediment • Contaminants are highly correlated with sediment grain size (to facilitate separation and minimize disposal costs) 6-2 Chapter 6: Dredging and Excavation
6.2 POTENTIAL ADVANTAGES AND LIMITATIONSOne of the advantages of removing contaminated sediment from the aquatic environment often is that, if it achieves cleanup levels for the site, it may result in the least uncertainty about long-term effectiveness of the cleanup, particularly regarding future environmental exposure to contaminated sediment. Removal of contaminated sediment can minimize the uncertainty associated with predictions of sediment bed or in-situ cap stability and the potential for future exposure and transport of contaminants.
Another potential advantage of removing contaminated sediment is the flexibility it may leave regarding future use of the water body. In-situ cleanup methods such as MNR and capping frequently include institutional controls (ICs) that limit water body uses. Although remedies at sites with bioaccumulative contaminants usually require the development or continuation of fish consumption advisories for a period of time after removal, other types of ICs that would be needed to protect a cap or layer of natural sedimentation might not be necessary if contaminated sediment is removed.
Another advantage, especially where dredging residuals are low, concerns the time to achieve remedial action objectives (RAOs). Active cleanup methods such as sediment removal and, particularly, capping may reduce risk more quickly and achieve RAOs faster than would be achieved by natural recovery. (However, in comparing time frames between approaches, it is important to include accurate estimates of the time for design and implementation of active approaches.) Also, sediment removal is the only cleanup method that can allow for treatment and/or beneficial reuse of dredged or excavated material. (However, caps that incorporate treatment measures, sometimes called “active” caps, are under development by researchers. See Chapter 3, Section 3.1.3, In-Situ Treatment and Other Innovative Alternatives.) There are also some potential sediment removal limitations that can be significant.
Implementation of dredging or excavation is usually more complex and costly than MNR or in-situ capping because of the removal technologies themselves (especially in the case of dredging) and the need for transport, staging, treatment (where applicable), and disposal of the dredged sediment. Treatment technologies for contaminated sediment frequently offer implementation challenges because of limited full-scale experience and high cost. In some parts of the country, disposal capacity may be limited in existing municipal or hazardous waste landfills, and it may be difficult to locate new local disposal facilities. Dredging or excavation may also be more complex and costly than other approaches due to accommodation of equipment maneuverability and portability/site access. Operations and effectiveness may be affected by utilities and other infrastructures, surface and submerged structures (e.g., piers, bridges, docks, bulkheads, or pilings), overhead restrictions, and narrow channel widths.
Another possible limitation of sediment removal is the level of uncertainty associated with estimating the extent of residual contamination following removal that can be high at some sites. For purposes of this guidance, residual contamination is contamination remaining in the sediment after dredging within or adjacent to the dredged area. The mass and contaminant concentration of residuals is generally a result of many factors including dredge equipment, dredge operator experience, proper implementation of best management practices, sediment characteristics, and site conditions.
Residual contamination is likely to be greater in the presence of cobbles, boulders, or buried debris, in high energy environments, at greater water depths, and where more highly contaminated sediment lies
near the bottom of the dredge thickness or directly overlies bedrock or a hard bottom. Residuals may also be greater in very shallow waters and when dredging sediment with high water contents. These complicating factors can make the sediment removal process difficult and costly. The continued bioaccumulation of residual contaminants can also affect the achievement of risk-based remediation goals. Dredging residuals have been underestimated at some sites, even when obvious complicating factors are not present. For some sites, this has resulted in not meeting selected cleanup levels without also backfilling with clean material.
Another potential limitation of dredging effectiveness includes contaminant losses through resuspension and, generally to a lesser extent, through volatilization. Resuspension of sediment from dredging normally results in releases of both dissolved and particle-associated contaminants to the water column. Resuspended particulate material may be redeposited at the dredging site or, if not controlled, transported to downstream locations in the water body. Some resuspended contaminants may also dissolve into the water column where they are more available for uptake by biota. While aqueous resuspension generally is much less of a concern during excavation, there may be increased concern with releases to air. Losses en route to and/or at the disposal or treatment site may include effluent or runoff discharges to surface water, leachate discharges to ground water, or volatile emissions to air. Each component of a sediment removal alternative typically necessitates additional handling of the material and presents a possibility of contaminant loss, as well as other potential risks to workers and communities.
Finally, similar to in-situ capping, dredging or excavation includes at least a temporary destruction of the aquatic community and habitat within the remediation area.
Where it is feasible, excavation often has advantages over dredging for the following reasons:
• Excavation equipment operators and oversight personnel can much more easily see the removal operation. Although in some cases diver-assisted hydraulic dredging or videomonitored dredging can be used, turbidity, safety and other technological constraints typically result in dredging being performed without visual assistance;
• Removal of contaminated sediment is usually more complete (i.e., residual contamination tends to be lower when sediment is removed after the area is dewatered);
• Bottom conditions (e.g., debris) and sediment characteristics (e.g., grain size and specific gravity) typically require much less consideration.
However, site preparation for excavation can be more lengthy and costly than for a dredging project due to the need for dewatering or water diversion. For example, coffer dams, sheet pile walls, or other diversions/exclusion structures would need to be fabricated and installed. Maneuvering around diversion/exclusion structures may be required because earth moving equipment cannot access the excavation area or double handling may be required to move material outside of the area. In addition, excavation is generally limited to relatively shallow areas.
6-4 Chapter 6: Dredging and Excavation
6.3 SITE CONDITIONS 6.3.1 Physical Environment Several aspects of the physical environment may make sediment removal more or less difficult to implement. In the remedial investigation, the following types of information should be collected, as they
can affect the type of equipment selected and potentially the feasibility of sediment removal:
• Bathymetry, slope of the sediment surface and water depth;
• Depth to and (un)evenness of bedrock or hard bottom (e.g., stiff glacial till);
• Sediment particle size distribution, degree of consolidation, and shear strength;
• Thickness and vertical delineation of contaminated sediment;
• Distance between dredging and disposal locations;
Additionally, sediment removal may change the hydrodynamics and slope stability of the remediation area. These changes should be evaluated to ensure that the removal activity does not cause significant bank or structural instability, shoreline facility damages, or other unacceptable adverse effects in or near the removal operation.
Data on both the horizontal and vertical characterization of the physical and chemical sediment characteristics are generally needed during the remedial investigation to evaluate the feasibility, cost, and potential effectiveness of dredging or excavation. The results of this characterization should help determine the area, depth, and volume to be removed, and the volume of sediment requiring treatment and/or disposal. Some aspects of sediment characterization are discussed in Chapter 2, Section 2.1, Site Characterization.
The project manager should refer to Evaluation of Dredged Material Proposed for Disposal at Island, Nearshore or Upland Confined Disposal Facilities - Testing Manual (USACE 2003) and Evaluation of Dredged Material Proposed for Discharge in Waters of the U.S. - Inland Testing Manual (U.S. EPA and USACE 1998) for further information. In addition, several guidance documents on estimating contaminant losses from dredging and disposal have been developed by the EPA and USACE.
For example, the project manager should refer to Estimating Contaminant Losses from Components of Remediation Alternatives for Contaminated Sediments (U.S. EPA 1996e).
6.3.2 Waterway Uses and Infrastructures Any evaluation of the feasibility of a dredging or excavation remedy should consider impacts to existing and reasonably anticipated future uses of a waterway. Waterway uses that may need to be
considered when evaluating a sediment removal alternative include the following:
• Navigation (e.g., commercial, military, recreational);
• Residential/commercial/military moorage and anchorage;
Evaluation of the feasibility of a sediment removal remedy should include an analysis of whether impacts to these potential uses may be avoided or minimized both during construction and in the long term.
6.3.3 Habitat Alteration The project manager should consider the impact of habitat loss or alteration in evaluating a dredging or excavation alternative. As is also discussed in Chapter 5, In-Situ Capping, while a project may be designed to minimize habitat loss, or even enhance habitat, sediment removal and disposal do alter the environment. It is important to determine whether the loss of a contaminated habitat is a greater impact than the benefit of providing a new, modified but less contaminated habitat. For example, a sediment removal alternative may or may not be appropriate where extensive damage to an existing forested wetland will occur. If the contaminated sediment in the wetland is bioavailable and may be impacting wildlife populations, the short-term disruption of the habitat may be warranted to limit ongoing long-term impacts to wildlife. Comparatively, if the wetland is functioning properly and is not acting as a contaminant source to the biota and the surrounding area, it may be appropriate to leave the wetland intact rather than remove the contaminated sediment. Deliberations to alter wetland and aquatic habitats should be considered in the remedial decision process. Appropriate coordination with natural resource agencies 6-6 Chapter 6: Dredging and Excavation will typically assist the project manager in determining the extent of impacts that a dredging project may have on aquatic organisms or their habitat, and how to minimize these impacts.
Another consideration is avoidance of short-term ecological impacts during dredging. This may involve timing the project to avoid water quality impacts during migration and breeding periods of sensitive species or designing the dredging project to minimize suspended sediment during dredging and disposal.
6.4 EXCAVATION TECHNOLOGIES
Excavation of contaminated sediment generally involves isolating the contaminated sediment from the overlying water body by pumping or diverting water from the area, and managing any continuing inflow followed by sediment excavation using conventional dry land equipment. However, excavation may be possible without water diversion in some areas such as wetlands during dry seasons or while the sediment and water are frozen during the winter. Typically, excavation is performed in streams, shallow rivers and ponds, or near shore areas.
Prior to pumping out the water, the area can be isolated using one or more of the following