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Silt curtains and silt screens are flexible barriers that hang down from the water surface. Both systems use a series of floats on the surface and a ballast chain or anchors along the bottom. Although the terms “silt curtain” and “silt screen” may be frequently used interchangeably, there are fundamental differences. Silt curtains are made of impervious materials, such as coated nylon, and primarily redirect flow around the dredging area. In contrast, silt screens are made from synthetic geotextile fabrics, which allow water to flow through, but retain a large fraction of the suspended solids (Averett et al. 1990). Silt curtains or silt screens may be appropriate when site conditions dictate the need for minimal transport of suspended sediment, for example, when dredging hot spots of high contaminant concentration.
Silt curtains have been used at many locations with varying degrees of success. For example, silt curtains were found to be effective in limiting suspended solids transport during in-water dike construction of the CDF for the New Bedford Harbor pilot project. However, the same silt curtains were ineffective in limiting contaminant migration during dredging operations at the same site primarily as a result of tidal fluctuation and wind (Averett et al. 1990). Problems were experienced during installation of silt curtains at the General Motors site (Massena, New York) due to high current velocities and back eddies. Dye tests conducted after installation revealed significant leakage, and the silt curtains were removed. Sheet piling was then installed around the area to be dredged with silt curtains used as 6-24 Chapter 6: Dredging and Excavation supplemental containment for hot spot areas. A silt curtain and silt screen containment system were effectively applied during dredging of the Sheboygan River in 1990 and 1991, where water depths were 2 m or less. A silt curtain was found to reduce suspended solids from approximately 400 mg/L (inside) to 5 mg/L (outside) during rock fill and dredging activities in Halifax Harbor, Canada (MacKnight 1992). At some sites, changes in dredging operating procedures may offer more effective control of resuspension than containment barriers.
The effectiveness of silt curtains and screens is primarily determined by the hydrodynamic conditions at the site. Conditions that may reduce the effectiveness of these and other types of barriers
include the following:
Silt curtains and screens are generally most effective in relatively shallow, undisturbed water. As water depth increases and turbulence caused by currents and waves increases, it becomes difficult to isolate the dredging operation effectively from the ambient water. The St. Lawrence Centre (1993) advises against the use of silt curtains in water deeper than 6.5 m or in currents greater than 50 cm/sec.
The effectiveness of containment barriers is also influenced by the quantity and type of suspended solids, the mooring method, and the characteristics of the barrier. To be effective, barriers should be deployed around the dredging operation and remain in place until the operation is completed, although it may need to be opened to allow transport of barges in and out of the dredge site, which may release some resuspended contaminants. For large projects, it may be necessary to relocate the barriers as the dredge moves to new areas. Where possible, barriers should not impede navigation traffic.
Containment barriers may also be used to protect specific areas, for example, valuable habitat, water intakes, or recreational areas, from suspended sediment contamination.
6.5.7 Predicting and Minimizing Dredging Residuals
All dredging operations leave behind some residual contamination in sediment, usually both within the dredged area and spread to adjacent areas. This residual contaminated sediment is often soft, unconsolidated, has a high water content, and may exist, at least temporarily, as a “fluid mud” or nephloid layer. The primary sources of the dredging residuals typically include: 1) contaminated sediment below the dredge line that was not removed, 2) sediment loosened by the dredge head or bucket, but not captured and removed, 3) sediment on steep slopes that fall into the dredged area, and 4) resettling of sediment from the dredging operation. Similar to resuspension releases discussed in Section 6.5.5, the
extent of the residual contamination is dependent on a number of factors including:
• Amount of contaminated sediment resuspended by the dredging operation;
• Extent of controls on dispersion of resuspended sediment (e.g., silt curtains, sheet piling);
• Extent of debris, obstructions, or confined operating area (e.g., which may limit effectiveness of dredge operation).
Project managers should factor a realistic estimate of dredging residuals into their evaluation of alternatives. Field results for some completed environmental dredging pilots and projects suggest that average post-dredging residual contamination levels have not met desired cleanup levels. However, aside from past experience, there is no commonly accepted method to predict accurately the degree of residual contamination likely to result from different dredge types under given site conditions. Additional guidelines are needed in this area and are likely to be developed in the future. Some preliminary research has shown that the residual concentration may be expected to be similar to the average contaminant concentration within the dredging prism (Desrosiers et al. 2005). In situations where more highly contaminated sediment is removed in a first dredging pass and deeper lower-level contamination is removed in a second dredging pass, lower residuals may be attainable. If the buried sediment is significantly more contaminated than the near-surface sediments, and if over dredging into “clean” sediment is not accomplished or feasible, the residual concentration may be greater than the average baseline surface concentration although significant contaminant mass may have been removed. When comparing alternatives and selecting of the best risk reduction alternative for the site, project managers should consider whether conditions are favorable for achieving desired post-dredging residual concentrations.
In cases where residuals may cause an unacceptable risk, additional passes of the dredge may be needed to achieve the desired results. Placement of a thin layer (e.g., 6–24 in) of clean material designed to mix with underlying sediment or the addition of reactive/sorptive materials to surface sediment can also be used to reduce the residual contamination. Project managers should consider developing a contingency remedy if there is sufficient uncertainty concerning the ability to achieve low cleanup levels.
Where a contingency remedy involves containment of residuals by in-situ capping, project managers should consider whether containment without dredging may be a more appropriate solution to manage long-term risks in that area.
It is generally important to conduct post-dredging sampling to confirm residual contamination levels. If resuspension and transport is expected, generally, it is also important to sample outside of the 6-26 Chapter 6: Dredging and Excavation dredged area to assess contaminant levels to which biota will be exposed from these areas. These data are often needed to assess the likelihood of achieving all RAOs.
6.6 TRANSPORT, STAGING, AND DEWATERING After removal, sediment often is transported to a staging or rehandling area for dewatering (if necessary), and further processing, treatment, or final disposal. Transport links all dredging or excavation components and may involve several different modes of transport. The first element in the transport process is to move sediment from the removal site to the disposal, staging, or rehandling site. Sediment may then be transported for pretreatment, treatment, and/or ultimate disposal (U.S. EPA 1994d). As noted previously, where possible, project managers should design for as few rehandling operations as possible to decrease risks and cost. Project managers should also consider community concerns regarding these operations (e.g., odor, noise, lighting, traffic, and other issues). Health and safety plans should address both workers and community members.
Modes of transportation may include one or more of the following waterborne or overland
• Pipeline: Direct placement of material into disposal sites by pipeline is economical only when the disposal and/or treatment site is located near the dredging areas (typically a few kilometers or less, unless booster pumps are used). Mechanically dredged material may also be reslurried from barges and pumped into nearshore disposal sites by pipeline;
• Barge: A rehandling facility located on shore is a commonly considered option. With a rehandling facility, dredging can be accomplished with mechanical (bucket) dredges where the sediment is excavated at near in-situ density (water content) and placed in a barge or scow for transport to the rehandling facility;
• Conveyor: Conveyors may be used to move material relatively short distances. Materials should be in a dewatered condition for transport by conveyor;
• Railcar: Rail spurs may be constructed to link rehandling/treatment facilities to the rail network. Many licensed landfills have rail links, so long-distance transport by rail is generally an option; and/or • Truck/Trailer: Dredged material can be rehandled directly from the barges to roll-off containers or dump trucks for transport to a CDF by direct dumping or unloading into a chute or conveyor. Truck transport of treated material to landfills may also be considered. The material should be dewatered prior to truck transport over surface streets.
In some smaller sites where construction of dewatering beds may be difficult or the cost of disposal is not great, addition of non-toxic absorbent materials such as lime or cement may be feasible.
A wide variety of transportation methods are available for moving sediment and residual wastes with unique physical and chemical attributes. In many cases, contaminated sediment is initially moved using waterborne transportation. Exceptions are the use of land-based or dry excavation methods.
Project managers should consider the compatibility of the dredge with the subsequent transport of the 6-27 Chapter 6: Dredging and Excavation dredged sediment. For example, hydraulic and pneumatic dredges produce contaminated dredgedmaterial slurries that can be transported by pipeline to either a disposal or rehandling site. Mechanical removal methods typically produce dense, contaminated material hauled by barge, railcar, truck/trailer, or conveyor systems. The feasibility, costs of transportation, and need for additional equipment are frequently influenced by the scale of the remediation project (Churchward et al. 1981, Turner 1984, U.S.
Temporary storage of contaminated sediment may also be necessary in order to dewater it prior to upland disposal or to allow for pretreatment and equalization prior to treatment. For example, a temporary CDF may be designed to store dredged material for periods when dredging or excavation is not possible due to weather or environmental concerns, while the treatment process may continue on a near 24-hour operating schedule. Storage may be temporary staging (e.g., pumping onto a barge with frequent off-loading) or more permanent disposal (e.g., moving the sediment to a land-based CDF where it may be dewatered and treated). A typical dewatering schematic is shown in Highlight 6-8.
Highlight 6-8: Sample of Dredging Dewatering Process
Depending upon the quality of the water after it is separated from sediment and upon applicable or relevant and appropriate requirements (ARARs), it may be necessary to treat water prior to discharge.
Where water treatment is required, it can be a costly segment of the dredging project and should be included in cost estimates for the alternative. Water treatment costs may also affect choices regarding dredging operation and equipment selection, as both can affect the amount of water entrained.
The project manager should consider potential contaminant losses to the water column and atmosphere during transport, dewatering, temporary storage, or treatment. For example, conventional mechanical dredging methods and equipment often rely on gravity dewatering of the sediment on a dredge scow, with drainage water and associated solids flowing into the surrounding water. Project managers should evaluate what engineering controls are necessary and cost-effective, and include these controls in planning and design. Implementation risks, both to workers and to the community, differ significantly between the various transport methods listed above. These risks should be evaluated and included when comparing alternatives. Best management practices for protection of water quality should also be followed.
6-28 Chapter 6: Dredging and Excavation The risks associated with a temporary storage or staging sites are similar to those associated with CDFs, as discussed in Section 6.8.2, Sediment Disposal. In particular, in-water temporary CDFs can prove to be attractive nuisances, especially to waterfowl, by providing attractive habitat that encourages use of the CDF by wildlife and presenting the opportunity for exposure to contaminants. For highly contaminated sites, it may be necessary to provide a temporary cover or sequence dredging to allow for coverage of highly contaminated sediment with cleaner sediment to minimize short-term exposures. This method of control has proven effective for minimizing exposures at upland sanitary landfills. In addition, because some holding areas may not be designed for long-term storage of contaminated sediment, the risk of contaminant transport to ground water may need to be evaluated and monitored.
6.7 SEDIMENT TREATMENT
For the majority of sediment removed from Superfund sites, treatment is not conducted prior to disposal, generally because sediment sites often have widespread low-level contamination, which the NCP acknowledges is more difficult to treat. However, pretreatment, such as particle size separation to distinguish between hazardous and non-hazardous waste disposal options, is common. Although the NCP provides a preference for treatment for “principal threat waste,” treatment has not been frequently selected for sediment. High cost, uncertain effectiveness, and/or (for on-site operations) community preferences are other factors that lead to treatment being selected infrequently at sediment sites. However, treatment of sediment could be the best option in some circumstances and innovations in ex-situ or in-situ treatment technologies may make treatment a more viable cost-effective option in the future.
The treatment of contaminated sediment is not usually a single process, but often involves a combination of processes or a treatment train to address various contaminant problems, including pretreatment, operational treatment, and/or effluent treatment/residual handling. Some form of pretreatment and effluent treatment/residual handling are necessary at almost all sediment removal projects. Sediment treatment processes of a wide variety of types have been applied in pilot-scale demonstrations, and some have been applied full scale. However, the relatively high cost of most treatment alternatives, especially those involving thermal and chemical destruction techniques, can be a major constraint on their use (NRC 1997). The base of experience for treatment of contaminated sediment is still limited. Each component of a potential treatment train is discussed in the next section.
Pretreatment modifies the dredged or excavated material in preparation for final treatment or disposal. When pretreatment is part of a treatment train, distinguishing between the two components may be difficult and is not always necessary. Pretreatment is generally performed to condition the material to meet the chemical and physical requirements for treatment or disposal; and/or to reduce the volume and/or weight of sediment that requires transport, treatment, or restricted disposal. Pretreatment processes typically include dewatering and physical or size separation technologies.