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Articulated Mechanical (Medium) - Ability to change bucket sizes for articulated mechanical is limited. Cutterhead (High) Capable of taking variable cut thicknesses by varying the burial depth of the cutter. Different cutterhead sizes or designs can be used to adapt to changing cut thicknesses or sediment stiffness. Horizontal Auger (Medium) - Designed for a set maximum cut thickness, and attempts to remove thick cuts may result in plowing actions with excessive resuspension and residual. Plain Suction/ Pneumatic (Low) - No cutting action limits ability to take thicker cuts or remove stiffer materials.
Specialty Dredgeheads (Low) - Specialty dredges are designed for a specific application and have limited flexibility. Diver Assisted (Low) - Removal is limited to thin cuts. Dry Excavation (High) - Allows use of a full range of conventional excavation equipment.
33 Thin Lift/Residual Removal - ability of a given dredge type to removal thin layers of contaminated material without excessive over dredging. Clamshell (Low) - Circular shaped cut not suited for efficient removal of thin layers. Enclosed Bucket/Articulated Mechanical (Medium) - Level cutting action is capable of removing thin layers, but the buckets would be only partially filled, resulting in inefficient production and higher handling and treatment costs. Cutterhead/Horizontal Auger (Medium) - Capable of removing thin layers, but the percent solids is reduced under these conditions. Plain Suction/Pneumatic (High) - Well suited for removal of thin lifts, especially loose material such as residual sediment.
Specialty Dredgeheads (High) - Some specialty dredges are designed specifically for removal of thin lifts. Diver Assisted (High) - Precision of diver-assisted dredging is well suited for removal of thin layers, especially residuals. Dry Excavation (High) - Allows for a precise control of cut thickness, amenable to removal of thin layers.
Source: Palermo et al. 2004 6.5.4 Dredge Positioning An important element of sediment remediation is the precision of the dredge cut, both horizontally and vertically. Technological developments in surveying (vessel) and positioning (dredgehead) instruments have improved the dredging process. Vertical control may be particularly important when contamination occurs in a relatively thin or uneven layer to avoid an unnecessary amount of over-dredging and excess handling of uncontaminated sediment. Video cameras are sometimes useful in monitoring dredging operations, although turbidity effects and lack of spatial references may present limitations on their use. The working depth of the dredgehead may be measured using acoustic instrumentation and by monitoring dredged slurry densities. In addition, surveying software may be used to generate pre- and post-dredging bathymetric charts, determine the volume of dredged sediment, locate 6-20 Chapter 6: Dredging and Excavation obstacles, and calculate linear dimensions of surface areas (see, e.g., St. Lawrence Centre 1993). Also available are digital positioning systems that enable dredge operators to follow a complex sediment contour (see, e.g., Van Oostrum 1992).
Depending on site conditions (e.g., currents, winds, tides), the horizontal position of the dredge may need to be continuously monitored during dredging. Satellite- or transmitter-based positioning systems, such as differential global positioning systems (DGPS), can be used to define the dredge position. In some cases, however, the accuracy of these systems is inadequate for precise dredging control. Where the accuracy of site characterization data or the high cost of disposal warrant very precise control, it is possible to use optical (laser) surveying instruments set up at one or more locations on shore.
These techniques, in conjunction with on-vessel instruments and spuds (if water depths are less than about 50 ft) and anchoring systems may enable the dredge operator to more accurately target specific sediment deposits. The effectiveness of anchoring systems diminishes as water depth increases.
The positioning technology described above enhances the accuracy of dredging. The accuracies achievable for sediment characterization should be considered in setting performance standards for environmental dredging vertical and horizontal operating accuracy (Palermo et al. 2004). However, project managers should not develop unrealistic expectations of dredging accuracy. Contaminated sediment cannot be removed with surgical accuracy even with the most sophisticated equipment.
Equipment may not be the only factor affecting the accuracy of the dredging operation. Site conditions (e.g., weather, currents), sediment conditions (e.g., bathymetry, physical characteristics), and the skill of the dredge operator are all important factors. In addition, the distribution of sediment contaminants may be only defined at a crude level and there could be a substantial margin for error. Accurately dredging to pre-established cut-lines is an important component of meeting remedial action objectives for sediment, but alone is not generally sufficient to show that the objectives have been met. Generally, post-dredging sampling should be conducted for that purpose. The section below describes the equally important factors of controlling dredging losses and residual contamination.
6.5.5 Predicting and Minimizing Sediment Resuspension and Contaminant Release and Transport During Dredging Sediment resuspension and the resulting unwanted contaminant release and transport in the water body arise due to a variety of activities associated with a dredging remedy. These frequently include resuspension caused by operation of the dredgehead, by operation of work boats and tug boats, and by deployment and movement of control measures such as silt screens or sheet piles. Contaminated sediment may also be lost from barges used during the dredging operation. In environments with significant water movement due to tides or currents, resuspended sediment may be transported away from a dredging site; therefore, limiting resuspension or increasing containment (so that resuspended sediment is later redeposited and dredged) can be an important consideration in remedy selection and design.
Storm events may also result in transport of contaminants beyond the dredging area. Use of containment barriers to limit transport of resuspended contaminated sediment is discussed in Section 6.5.6 of this chapter.
When evaluating resuspension due to dredging, it generally is important to compare the degree of resuspension to the natural sediment resuspension that would continue to occur if the contaminated sediment was not dredged, and the length of time over which increased dredging-related suspension
would occur. Typically, two types of contaminant release are associated with resuspended sediment:
6-21 Chapter 6: Dredging and Excavation particulate and dissolved. Particulate release refers to the transport of contaminants associated with the particle phase (i.e., sorbed to suspended sediment). Dissolved refers to the release of dissolved contaminants from the particles into the water column. This latter form of release can be significant because dissolved contaminants are the most readily bioavailable and are more easily transported away from the site. Consequently, resuspension can result in the release of bioavailable organic and inorganic contaminants into the water column, which may cause toxicity or enhanced bioaccumulation. Research is currently being performed to address the risk associated with resuspension at contaminated sites and some existing models have been developed by the USACE. Until further guidance is available, at most sites, the project manager should monitor resuspension during dredging and to evaluate its potential effects on water quality. Project managers should be aware that most engineering measures implemented to reduce resuspension also reduce dredging efficiency. Estimates of production rates, cost, and project time frame should take these measures into account.
Some contaminant release and transport during dredging is inevitable and should be factored into the alternatives evaluation and planned for in the remedy design. Releases can be minimized by choice of dredging equipment, dredging less area, and/or using certain operational procedures (e.g., slowing the dredge clamshell descent just before impact with the sediment bed). Generally, the project manager should assess all causes of resuspension and realistically predict likely contaminant releases during a dredging operation. The magnitude of sediment resuspension and resulting transport of contaminants
during a dredging operation is influenced by many factors, including:
• Physical properties of the sediment [e.g., grain size distribution, organic carbon content, Acid Volatile Sulfides (AVS) concentration];
• Vertical distribution of contaminants in the sediment;
To compare various remedies for a site, to the extent possible, the project manager should attempt to estimate the downstream mass transport and the degree of increase (if any) in downstream surface water and surface sediment contaminant concentrations. However, at present, no fully verified empirical or predictive tools are available to quantify the predicted releases accurately. As research in predicting resuspension and contaminant release associated with dredging progresses, project managers should watch for verified methods to be developed to assist in this estimate. Although the degree of resuspension will be site specific, recent analyses of field studies and available predictive models of the mass of 6-22 Chapter 6: Dredging and Excavation sediment resuspended range from generally less than one percent of the mass dredged (Hays and Wu 2001, Palermo and Averett 2003) to between 0.5 and 9 percent (NRC 2001). The methods contained in EPA’s Estimating Contaminant Losses from Components of Remediation Alternatives for Contaminated Sediments (U.S. EPA 1996g), may be useful to estimate the dredgehead component of resuspension losses. To the extent possible, the project manager should estimate total dredging losses on a site-specific basis and consider them in the comparison of alternatives during the feasibility study.
If conventional clamshell dredges may cause a high level of resuspension, a special purpose dredge may be considered. These dredges generally resuspend less material than conventional dredges, but associated costs may be greater, and dredges may not be usable in the presence of significant debris or obstructions. As in the case of conventional dredges, the selection of a special purpose dredge will be likely dictated by site-specific conditions, economics, and availability (Palermo et al. 1998b). Other factors unrelated to resuspension, such as maneuverability requirements, hydrodynamic conditions, or others listed in Section 6.5.3, Dredge Equipment Selection, may also dictate the type of dredge that should be used. The strategy for the project manager should be to minimize the resuspension levels generated by any specific dredge type, while also ensuring that the project can be implemented in a reasonable time frame. The EPA’s Office of Research and Development (ORD) and others are in the process of evaluating resuspension and its effects, both in field and modeling studies. The results of this research should help project managers to understand better and control effects of resuspension during future cleanup actions.
Another potential route of contaminant release during dredging or excavation may be the volatilization of contaminants, either near the dredge or excavation site or in a holding facility like a confined disposal facility (CDF) (Chiarenzeli et al. 1998). At sites with high concentrations of volatile contaminants, dredging or excavation may present special challenges for monitoring and operational controls if they may pose a potential risk to workers and the nearby community. This exposure route may be minimized by reducing dredging production rates so that resuspension is minimized. Covering the surface of the water with a physical barrier or an absorbent compound may also minimize volatilization.
At the New Bedford Harbor site, a cutterhead dredge was modified by placing a cover over the dredgehead that retained polychlorinated biphenyl (PCB)-laden oils, thus reducing the air concentrations of PCBs during dredging to background levels; see Report on the Effects of the Hot Spot Dredging Operations: New Bedford Harbor Superfund Site, New Bedford, MA (U.S. EPA 1997e and available through EPA’s Web site at http://www.epa.gov/region01/nbh/techdocs.html). In addition, the CDF that the dredged sediment was pumped into was fitted with a plastic cover that effectively reduced air emissions. To minimize the potential for volatile releases further, dredging operations were conducted during cooler weather periods and at night. During excavation, volatilization could be of greater concern as contaminated materials may be exposed to air. Care should be taken during dewatering activities to ensure that temperatures are not elevated (e.g., cautious application of lime or cement for dewatering), and other control measure should be taken as needed (e.g., foam).
6.5.6 Containment Barriers
Transport of resuspended contaminated sediment released during dredging can often be reduced by using physical barriers around the dredging operation. Barriers commonly used to reduce the spread of contaminants during the removal process include oil booms, silt curtains, silt screens, sheet-pile walls, cofferdams, and bubble curtains (U.S. EPA 1994d, Francingues 2003). Under favorable site conditions, these barriers help limit the areal extent of particle-bound contaminant migration resulting from dredging 6-23 Chapter 6: Dredging and Excavation resuspension and enhance the long-term benefits gained by the removal process. Conversely, because the barriers contain resuspended sediment, they may increase, at least temporarily, residual contaminant concentrations inside the barrier compared to what it would have been without the barriers.
Structural barriers, such as sheet pile walls, have been used for sediment excavation and in some cases (e.g., high current velocities) for dredging projects. The determination of whether these types of barriers are necessary should be made based on a thorough evaluation of the site. This can be accomplished by evaluating the relative risks posed by the anticipated release of contaminants from the dredging operation absent use of such structural barriers, the predicted extent and duration of such releases, and the potential for trapping and accumulating residual contaminated sediment within the barrier. The project manager should consult the ARCS program’s Risk Assessment and Modeling Overview Document (U.S. EPA 1993c) and Estimating Contaminant Losses from Components of Remediation Alternatives for Contaminated Sediment (U.S. EPA 1996e) for further information about evaluating the need for structural barriers.
Sheet pile containment structures are more likely to provide reliable containment of resuspended sediment than silt screens or curtains, although at significantly higher cost and with different technological limitations. Where water is removed on one side of the wall, project managers should be aware of the hydraulic loading effects of water level variations inside and outside of these walls. Project managers should also be aware of the increased potential for scour to occur around the outside of the containment area, and the resuspension that will occur during placement and removal of these structures.
In addition, use of sheet piling may significantly change the carrying capacity of a stream or river and make it temporarily more susceptible to flooding.
Oil booms are appropriate for sediment that may likely release oils or floatables [i.e., light nonaqueous-phase liquids (LNAPL)] when disturbed. Such booms typically consist of a series of synthetic foam floats encased in fabric and connected with a cable or chains. Oil booms may be supplemented with oil absorbent materials, such as polypropylene mats (U.S. EPA 1994d). However, booms do not aid in retaining the soluble portion of floatables [i.e., polycyclic aromatic hydrocarbons (PAHs) from oils].