Assessment and Remediation of Contaminated Sediments (ARCS) Program
REMEDIATION GUIDANCE DOCUMENT
US Environmental Protection Agency. 1994. ARCS Remediation Guidance Document. EPA 905-B94-003. Chicago, Ill.: Great Lakes National Program Office.
The removal or excavation of sediments from a water body, commonly known as dredging, is a process that is carried out routinely around the world. The term environmental dredging has evolved in recent years to distinguish dredging operations for the primary purpose of environmental restoration from those operations for the purpose of simply removing sediments. The most common purpose of dredging is to construct or maintain channels for navigation or flood control (Hayes 1992). Environmental dredging operations usually involve relatively small volumes of sediment, with the objective of effectively removing contaminated material in a manner that minimizes the release of sediments and contaminants to the aquatic environment. Other objectives may be established for specific projects.
As noted by Hayes (1992):
The primary purpose of routine dredging operations is usually to remove large volumes of subaqueous sediments as efficiently as possible within specified operational and environmental restrictions. Environmental dredging operations, on the other hand, would attempt to remove sediments with some known contamination as effectively as possible. An effective method would include complete removal of the desired sediment with as little environmental risk and consequence as possible. The important distinction is that economics play a secondary role to environmental protection in environmental dredging operations.
The loss of contaminants to the surrounding waters, or into the atmosphere, is of particular concern when dredging contaminated sediments. Because contaminants are generally bound to the fine particles, which are most easily resuspended, most efforts are focused on minimizing the amount of resuspension through innovative equipment design and operational controls. Further reductions in the transport of contaminants can be accomplished with barriers such as silt curtains, silt screens, and oil booms.
The various types of mechanical and hydraulic dredges, as well as barriers, are described in this chapter. Discussions of the factors used to select dredging equipment and techniques for estimating costs and contaminant losses (e.g., via resuspension) are also provided.
Different types of dredges were reviewed in the literature review prepared for the ARCS Program (Averett et al., in prep.). Other general references on the subject of dredging include the Handbook of Dredging Engineering (Herbich 1992), Fundamentals of Hydraulic Dredging (Turner 1984), and Dredging and Dredged Material Disposal (USACE 1983).
DESCRIPTIONS OF TECHNOLOGIES
Dredging involves mechanically penetrating, grabbing, raking, cutting, or hydraulically scouring the bottom of the waterway to dislodge the sediment. Once dislodged, the sediment is lifted out of the waterway either mechanically, as with buckets, or hydraulically, through a pipe. Thus, dredges can be categorized as either mechanical or hydraulic depending on the basic means of moving the dredged material. A subset of the hydraulic dredges employs pneumatic systems to pump the sediments out of the waterway. These are termed pneumatic dredges.
The most fundamental difference between mechanical and hydraulic dredging equipment is the form in which the sediments are removed. Mechanical dredges offer the advantage of removing the sediments at nearly the same solids content as the in situ material. That is, little additional water is entrained with the sediments as they are removed, meaning that the volume of the sediments is essentially the same before and after dredging. In contrast, hydraulic dredges remove and transport sediment in slurry form. The total volume of material is greatly increased, because the solids content of the slurry is considerably less than that of the in situ sediments. (The relationship between the volume of in situ sediment with various slurries is discussed in Chapter 6 in the Dewatering Technologies section.)
The two general types of dredges most commonly used to perform navigation maintenance and construction-related dredging, mechanical and hydraulic, are shown in Figure 4-1. Both dredges are available in a wide variety of sizes, including small, portable hydraulic dredges. The various types of dredges and dredging equipment, vessel positioning systems, contaminant barriers, and monitoring requirements applicable to sediment removal technologies are discussed below.
Mechanical dredges remove bottom sediment through the direct application of mechanical force to dislodge and excavate the material. The dredged material is then lifted mechanically to the surface at near-in situ densities (Averett et al., in prep.). As noted above, this feature is significant because it minimizes the amount of contaminated material to be handled. Mechanical dredges can be particularly effective for those locations where dredged sediment must be transported by a barge to a disposal or treatment facility (Zappi and Hayes 1991).
Production rates for mechanical dredges are typically lower than those for comparably sized hydraulic dredges. However, high productivity is typically not the main priority for environmental dredging projects. Mechanical dredges can operate in constricted areas and do not interfere with shipping to the same extent as hydraulic dredges (Zappi and Hayes 1991). Mechanical dredges are often selected for small dredging projects in confined areas such as docks and piers. They provide one of the few effective methods for removing large debris (Averett et al., in prep.) and are adaptable to land-based operations.
Major types of mechanical dredges include the following:
- Clamshell bucket
- Backhoe
- Bucket ladder
- Dipper
- Dragline
Although it has not been proven by field or laboratory measurements, it is commonly thought that the bucket ladder, dipper, and dragline dredges operate in a manner that would lead to high sediment resuspension rates, making them unsuitable for dredging contaminated material (Zappi and Hayes 1991). The clamshell bucket and backhoe dredges are described below.
Clamshell Bucket Dredges
The clamshell bucket dredge, also known as the grab dredge, is the most commonly used mechanical dredge in the United States, if not the world (Zappi and Hayes 1991). This dredge may consist simply of a crane mounted on a spud barge, although most bucket dredges have a crane/barge system specifically designed and constructed for dredging (Figure 4-1) (Zappi and Hayes 1991). Buckets are classified by their capacities, which range from <1 to 50 yd[3] (<0.8 to 40 m[3]), with 2-10 yd[3] (1.5-7.5 m[3]) being typical. Bucket dredges are available from a wide variety of sources throughout North America.
A bucket dredge is operated similarly to a land-based crane and bucket. The crane operator drops the bucket through the water column, allowing it to sink into the sediment on contact. The loaded bucket is then lifted, causing the jaws to close, and raised through the water column. Once above the water surface, the operator swings the bucket over the receiving container (usually a barge) and lowers the bucket to release its load (Zappi and Hayes 1991). The bucket dredge usually leaves an irregular, cratered sediment surface (Herbich and Brahme 1991). The bucket has been used at numerous sites throughout the Great Lakes for removing both contaminated and clean sediments. It is estimated that 77 bucket dredges are stationed in Great Lakes ports.
A variation of the conventional dredge bucket, the enclosed dredge bucket, has been developed to limit spillage and leakage from the bucket. Although originally designed by the Japanese Port and Harbor Institute and produced in Japan by Mitsubishi Seiko Co., Ltd., variations of this design have been produced by several U.S. manufacturers (Zappi and Hayes 1991). The operation and deployment of the enclosed dredge bucket is identical to that of the conventional clamshell bucket discussed above.
The original enclosed dredge bucket (Figure 4-2) features covers designed to prevent material from spilling out of the bucket while it is raised through the water column. The design also employs rubber gaskets or tongue-in-groove joints that reduce leakage through the bottom of the closed bucket. An alternative design, developed by Cable Arm, Inc. (Figure 4-2), offers several advantages over the standard clamshell design, including the ability to remove sediment in layers, leaving a flat sediment surface.
Enclosed bucket dredges have been used routinely in various Great Lakes ports for the maintenance of navigation channels. They have also been used in sediment remediation projects in the Black River near Lorain, Ohio, in 1990, and in the Sheboygan River, Wisconsin, in 1990 and 1991. The Cable Arm bucket was demonstrated by the Contaminated Sediment Removal Program (CSRP) on contaminated sediments in the Toronto and Hamilton Harbors in Canada in 1992 (Environment Canada 1993) and has been used for navigation maintenance dredging in the Cuyahoga and Fox Rivers.
Backhoes
Backhoes, although normally thought of as excavating rather than dredging equipment, can be used for removing contaminated sediments under certain circumstances. Backhoes are normally land based, but may be operated from a barge, and have been used infrequently for navigation dredging in deep-draft (20-ft [6-m]) channels. Backhoes have received limited use for removing PCB-contaminated sediments from the Sheboygan River. A backhoe was recently used to remove 13,000 m[3] of contaminated sediments from Starkweather Creek in Madison, Wisconsin. Sediment resuspension from the dredging was monitored and found to be no greater than that expected with other types of dredging equipment (Fitzpatrick 1994).
Specialized backhoes include closed-bucket versions and a pontoon-mounted model especially adapted to dredging applications (see WaterMaster described in St. Lawrence Centre 1993). The latter may be equipped with a suction pump as well.
Hydraulic dredges remove and transport sediments in the form of a slurry. They are routinely used throughout the United States to move millions of cubic meters of sediment each year (Zappi and Hayes 1991). The hydraulic dredges used most commonly in the United States include the conventional cutterhead, dustpan, and bucket-wheel. Certain hydraulic dredges, such as the modified dustpan, clean-up, and matchbox dredges, have been specifically developed to reduce resuspension at the point of dredging.
Hydraulic dredges provide an economical means of removing large quantities of contaminated sediments. The capacity of the dredge is generally defined by the diameter of the dredge pump discharge. Size classifications are: small (4-14 in., 10-36 cm), medium (16-22 in., 41-56 cm), and large (24-36 in., 61-91 cm) (Averett et al., in prep.). The dredged material is usually pumped to a storage or disposal area through a pipeline, with a solids content of typically 10-20 percent by weight (Herbich and Brahme 1991). Souder et al. (1978) indicated that slurry concentrations are a function of the suction pipeline inlet velocity, the physical characteristics of the in situ sediment, and effective operational controls. The slurry uniformity is controlled by the cutterhead (if one is employed) and suction intake design and operation. The cutterhead (both conventional and innovative) should be designed to grind and direct the sediment to the suction intake with minimal hydraulic losses. Water jets can also be used to loosen the in situ material and provide a uniform slurry concentration. The dredgehead and intake suction pipeline should be designed to maintain velocities that are capable of breaking the in situ sediment into pieces that the pump can handle while minimizing entrance and friction losses.
The dredge pump and dredgehead (e.g., cutterhead) should work in tandem so that the entire volume of contaminated sediment comes into the system, while maintaining a slurry concentration that the dredge pump is capable of handling. The pump must impart enough energy to the slurry so that the velocities in the pipeline prevent the solids from settling out in the line prior to reaching the next transport mode or remediation process. A properly designed and operated dredgehead, suction intake and pipe, pump, and discharge pipeline system can minimize sediment resuspension while significantly reducing system maintenance and the likelihood of pump failure.
Fundamentally, there are four key components of a hydraulic dredge:
- The dredgehead is the part of the dredge that is actually submerged into the sediment
- The dredgehead support is usually a ladder as shown in Figure 4-1, but may instead be a simple cable or a sophisticated hydraulic arm
- The hydraulic pump provides suction at the dredgehead and propels the sediment slurry through a pipeline (It may be submerged or deck-mounted.)
- The pipeline carries the sediment slurry away from the dredgehead to the receiving area (e.g., CDF, lagoon)
Dredgeheads
Various types of dredgehead configurations are used to facilitate the initial loosening and gathering of bottom sediment. Most hydraulic dredges are usually identified by the type of dredgehead (e.g., bucket wheel dredge). Various types of dredgeheads are discussed below.
Cutterhead Dredges--Conventional cutterhead dredges are the most common hydraulic dredges in the United States. According to Averett et al. (in prep.), there are 300 such dredges operating in the United States today. A conventional open basket cutterhead is shown in Table 4-1.
Cutterhead dredges are usually operated by swinging the dredgehead in a zig-zag pattern of arcs across the bottom, which tends to leave windrows of material on the bottom (Herbich and Brahme 1991). Innovative operating techniques, including overlapping dredge or step cuts, can reduce or eliminate windrows. Cutterhead dredges can be operated to reduce resuspension or losses of volatile contaminants using additional equipment such as sediment shields, gas collection systems, underwater cameras, and bottom sensors.
Innovative dredgehead designs have been developed specifically for removing contaminated materials. Such dredgeheads put a premium on minimizing sediment resuspension and on accurate control of the depth of sediments removed. Two such dredgeheads, the Clean-up and the Refresher, are shown in Table 4-1.
Suction Dredges--This category includes those hydraulic dredges that do not employ a cutterhead. Such dredges may use water jets to help loosen sediments. Examples of three dredgehead designs used for such dredges are provided in Table 4-2.
Hybrid Dredges--These dredges use a combination of mechanical action and hydraulic pumping, but would not be considered cutterhead dredges. Examples of dredgehead designs used by hybrid dredges are shown in Table 4-3, and include the bucket wheel, screw impeller, and disc-bottom dredgeheads.
Dredgehead Support
The physical support for the dredgehead, or ladder, is largely interchangeable among the various dredges and will not be discussed further in this document.
Hydraulic Pumps
The three main applications of hydraulic pumps in the dredging process are:
- Dredge plant pumps used to remove in situ sediments
- Booster pumps used to maintain slurry velocities
- Pumpout stations used to rehandle sediment from hoppers, barges, and railcars
Dredge plant pumps are discussed in this section. The other two types of pump applications are discussed in Chapter 5, Transport Technologies.
Fundamentally, pumps are used to convert mechanical or pumping energy into slurry energy. Usually they are driven by electric or diesel motors, although air-driven (pneumatic) pumps have also become popular. Energy put into a slurry by a pump is used to maintain pipeline velocities while overcoming elevation heads and friction and entrance losses.
The two general classes of dredge plant pumps are kinetic and positive displacement (Lindeburg 1992). A summary of the characteristics of selected examples of these pump types is provided in Table 4-4 [part i] [part ii] [part iii].
Pipelines
Details on slurry pipelines are provided in Chapter 5, Transport Technologies.
Portable Hydraulic Dredges
Portable hydraulic dredges are relatively small machines that can be transported over land. They are convenient for isolated, hard-to-reach areas and are economical for small jobs. These dredges are also capable of operating in very shallow water (approximately 0.5 m). Two such dredges are the horizontal auger dredge and the Delta dredge (Delta Dredge and Pump Corp.). These two dredges are shown in Table 4-5. Two examples of horizontal auger dredges are the Mudcat, manufactured by Ellicott Machine Co. and the Little Monster, manufactured by the H & H Pump and Dredge Co. A Mudcat dredge with several equipment modifications was demonstrated by the CSRP in November 1991 at the Welland River, Ontario (Acres International Ltd. 1993).
A third type of portable dredge is the hand-held hydraulic dredge. This dredge can be as simple as a hose connected to a vacuum truck, such as the one used to remove PCB-contaminated sediments from the Shiawasee River in Michigan (USEPA 1985b). In another example, diaphragm sludge pumps were used by the USEPA's Inland Response Team to remove PCB-contaminated sediments from the Duwamish River Waterway in Seattle, Washington (Averett et al., in prep.). The primary application of such dredges is the removal of small volumes of contaminated materials that can be easily accessed from the surface or by divers.
Self-Propelled Hopper Dredges
A self-propelled hopper dredge operates hydraulically, but it is often described as a separate type of dredge because the dredged material is retained onboard rather than being discharged through a pipeline (Figure 4-1). Self-propelled hopper dredges are well suited for dredging large quantities of sediments in open areas. They are not well suited for small dredging projects, especially in close quarters. For these reasons, they are not likely to be used for sediment remediation projects around the Great Lakes and will not be discussed in further detail in this document.
A critical element of sediment remediation is the precision of the dredge cut, both horizontally and vertically. Technological developments in surveying and positioning instruments have improved both aspects of dredging. Vertical control is particularly important where contamination occurs as a relatively thin or uneven layer. Video cameras can be used to continuously monitor dredging operations. The depth of the dredgehead can be measured using acoustic instrumentation and by monitoring dredged slurry densities. In addition, surveying software packages can be used to generate pre- and post-dredging bathymetric (water depth) charts, determine the volume dredged, locate obstacles, and calculate surface areas (St. Lawrence Centre 1993). A digital dredging method, which enables dredge operators to follow a complex sediment contour, has been developed in the Netherlands (van Oostrum 1992).
The horizontal position of the dredge may be continuously monitored during dredging. Satellite- or transmitter-based positioning systems (e.g., global positioning system, SATNAV, LORAN C) may be used to define the dredge position. In some cases, however, the accuracy of these systems is inadequate for precise dredging control. Very accurate control is possible through the use of optical (laser) surveying instruments set up at one or more locations onshore. These techniques, in conjunction with on-vessel instruments and control of spud placement, can enable the dredge operator to target specific sediment deposits.
The positioning technology described above may enhance the accuracy of dredging in some circumstances. However, planners and designers should not develop unrealistic expectations of dredging accuracy. Contaminated sediments cannot be removed with surgical accuracy even with the most sophisticated equipment. Equipment is not the only factor affecting the accuracy of a dredge. Site conditions (e.g., weather, currents), sediment conditions (e.g., bathymetry, physical character), and the skill of the dredge operator are all important factors. In addition, the distribution of sediment contaminants can, in many cases, only be resolved at a crude level and with a substantial margin for error. The level of accuracy required for environmental dredging should reflect the accuracy at which the sediment contamination distribution is resolved.
When dredging contaminated sediments, it may be advisable to limit the spread of contaminants by using physical barriers around the dredging operation. Such barriers may be appropriate when contaminant concentrations are high or site conditions dictate the need for minimal adverse impacts. A number of physical barriers commonly used in the construction industry may be adapted to this purpose. Structural barriers, such as cofferdams, are not generally applicable as temporary barriers, but are options for in situ containment (see Chapter 3, Nonremoval Technologies). The determination of whether these types of barriers are necessary, aside from regulatory requirements, should be made based on a thorough evaluation of the relative risks posed by the anticipated release of contaminants from the dredging operation, the predicted extent and duration of such releases, and the long-term benefits gained by the overall remediation project. The ARCS Risk Assessment and Modeling Overview Document (USEPA 1993a) and the Estimating Contaminant Losses from Components of Remediation Alternatives for Contaminated Sediment (Myers et al., in prep.) should be used to make this determination. More commonly, nonstructural barriers, such as oil booms, silt curtains, and silt screens, have been used to reduce the spread of contaminants during dredging. Oil booms are appropriate for sediments that are likely to release oils 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 (e.g., polypropylene mats).
Silt curtains and silt screens are flexible barriers that hang down from the water surface. Figure 4-3 shows a typical design of a silt curtain. 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 are frequently used interchangeably, there are fundamental differences. Silt curtains are made from impervious material such as coated nylon and primarily redirect flow around the dredging area rather than blocking the entire water column. In contrast, silt screens are made from synthetic geotextile fabrics, which allow water to flow through but retain a fraction of the suspended solids (Averett et al., in prep.).
Silt curtains have been used at many locations with varying degrees of success. For example, silt curtains were found to be ineffective during a demonstration in New Bedford Harbor, primarily as a result of tidal fluctuation and wind (Averett et al., in prep.). Similar problems were experienced when Dokai Bay (Japan) was dredged in 1972 (Kido et al. 1992). Barriers consisting of a silt curtain/silt screen combination were effectively applied during dredging of the Sheboygan River in 1990 and 1991. Water depths were generally 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). A silt curtain was employed during a dredging demonstration at Welland, Ontario (Acres International Ltd. 1993). The curtain minimized flow through the dredging area, although there were problems in the installation and removal.
Monitoring may be conducted during environmental dredging for a number of purposes, including:
- Measure contaminated sediment removal efficiency
- Determine dredged volumes
- Measure sediment resuspension at dredge
- Track contaminant transport
- Check performance of barriers and other controls
During maintenance dredging, monitoring is generally focused on the quantity of material dredged because the contractor is paid according to this quantity. The quantity of dredged material may be estimated from bathymetric surveys conducted before and after the dredging, or from other measurements, such as barge counts or pumping rates and duration.
Measurements of turbidity or suspended solids are made during sediment remediation and during some maintenance dredging operations to monitor the level of sediment resuspension caused by the dredge. Water samples are typically collected at one location upstream and several locations downstream from the dredging site. Additional water quality monitoring around the dredging site may be required by the State or other regulatory agencies. Monitoring programs for tracking contaminant transport and checking the efficiency of barriers and other controls are site-specific. During remedial dredging projects, sediment samples may be collected and analyzed after dredging to monitor the removal efficiency and to determine if additional passes by the dredge are needed.
SELECTION FACTORS
A number of publications on the selection of dredges for environmental applications have been published, including the Guide to Selecting a Dredge for Minimizing Resuspension of Sediment (Hayes 1986) and Selecting and Operating Dredging Equipment: A Guide to Sound Environmental Practices (St. Lawrence Centre 1993). Generally one of the key considerations in any dredging project involving contaminated sediments is the minimization of sediment resuspension. While this subsection focuses on the selection of dredging equipment, it should be noted that the operation of the dredge also has a profound effect on the rate of sediment resuspension (Hayes 1986). Selection of specialty dredges designed for minimal sediment resuspension does not guarantee superior results. The keys to an effective and environmentally safe dredging operation are:
- Selection of equipment compatible with the conditions at the site and the constraints of the project
- Use of highly skilled dredge operators
- Close monitoring and management of the dredging operation
Conventional dredging equipment, employed in a careful and efficient manner, can achieve results comparable to specialty dredging equipment.
The operational characteristics of selected dredges are summarized in Table 4-6. These characteristics may be used to help narrow the range of dredges potentially suited to a given remediation project. Other factors that can be used to guide the selection of an appropriate dredge for a site are discussed below.
Solids Concentration
There are two major factors that affect the desired solids concentration:
- Compatibility with Other Components In most cases, it is preferable to use a dredging system that is capable of delivering material at high solids concentrations. This tends to minimize the costs of handling, treating, and disposing of sediments. Mechanically dredged sediments do not require intensive dewatering, which is an expensive pretreatment process (see Chapter 6). Mechanical dredging keeps the volume of dredged material to a minimum and greatly reduces the costs of water treatment (see Chapter 9).
- Distance to Treatment/Disposal Site The feasibility of pipeline transport to the treatment/disposal site is discussed in Chapter 5, Transport Technologies. The ability to deploy pipelines, even temporarily, in highly urbanized areas can be limited. If access is unlimited, slurried sediments can be transported by pipeline several kilometers with the use of booster pumps. If pipeline transport is not feasible, sediments can be transported at high solids concentrations (e.g., as produced with mechanical or pneumatic dredges) by scows or barges.
Production Rate
For navigation dredging, the size of the dredge (and number of dredges) is largely dictated by the volume of sediments to be removed and the time allowed. The quantities of sediments dredged at remediation projects are small in comparison to navigation dredging, and factors other than sediment volume may influence the dredge size and production rates. Production rates may be deliberately reduced to minimize sediment resuspension or because of constraints caused by sediment transport, pretreatment, treatment, or disposal components.
Dredging Accuracy
Precise control of operational dredging depth is particularly important when dredged sediments are to be handled in expensive treatment and disposal facilities (Averett et al., in prep.). The vertical and lateral accuracy of the dredge is important to ensure that contaminated sediments are removed, while minimizing the amount of clean sediments removed. The accuracy of a dredging operation is only partially influenced by the type of dredge selected. Conditions of the site and sediments, the proficiency of the operator, and the rate of production all influence the accuracy of the dredge cut.
Dredging Depth
Dredges are limited to dredging areas with an adequate depth of water to accommodate the draft of the dredging vessel. This factor becomes important when contaminated sediments are located outside of navigable waterways. Some dredging equipment can be operated from land to access sediments in shallow waterways. The maximum depth to which dredges can reach is also limited. Some dredges are limited by the length of the dredging arm or ladder. Hydraulic dredging in very deep water (>20 m) may require submerged pumps or remotely operated dredges.
Ability to Handle Debris
Sediment, especially in urban areas, often contains large rocks, concrete, timber, tires, and other discarded materials. In cargo loading/unloading areas, pockets of coal, iron ore pellets, or other bulk materials may occur from spillage. Very large debris (e.g., greater than 0.5 m in any dimension) can only be removed mechanically (further discussion of specialized debris removal equipment is provided in Chapter 6). Mechanical dredges will generally remove large debris with the sediments, but are likely to produce greater turbidity in the process. Dredgeheads equipped with cutters are able to reduce the size of some debris such as wood. Although debris that is larger than the diameter of the suction pipe and not cut by the cutter simply cannot be removed by hydraulic dredges, smaller debris can also clog hydraulic pipelines and damage pumps.
Other Factors
In addition to the selection factors shown in Table 4-6, there are a number of other factors that may be significant in the selection of a dredge for a remediation project, including sediment resuspension, dredge availability, and site restrictions. These factors are discussed below.
Sediment Resuspension--In areas where sediments have high contaminant concentrations, toxicity, mobility, or a combination thereof, extraordinary care and expense may be required to minimize sediment resuspension or spillage. In such cases, releases of contaminants to the water are a primary concern, and may override other factors in selecting a dredge. As noted above, the degree of resuspension is influenced by both the type of dredge and its operation. Resuspension characteristics of dredges are discussed later in this chapter in regard to estimating contaminant losses.
Dredge Availability--A wide variety of dredging equipment is available throughout North America and in the Great Lakes region. A summary of dredges stationed in the Great Lakes is shown in Table 4-7. A summary of the availability of specialty dredges is provided in Table 4-8. As shown, many of the specialty dredges developed in Japan and Europe are not readily obtainable in the United States. The International Dredging Review publishes an annual directory of dredge owners and operators, which should be consulted for an up-to-date listing of dredging contractors and available equipment.
Site Restrictions--Channel widths, surface and submerged obstructions, overhead restrictions such as bridges, and other site access restrictions may also limit the type and size of equipment that can be used. For example, hopper dredges are ships that require navigable depths, cutterhead dredges require anchoring cables for operation, while bucket dredges can operate in confined areas. In some cases, it may be more appropriate to remove material from shore, as was done with contaminated sediments from Starkweather Creek in Madison, Wisconsin (Fitzpatrick 1994).
The effectiveness of nonstructural containment barriers at a sediment remediation site is primarily determined by the hydrodynamic conditions at the site. Conditions that will reduce the effectiveness of barriers include:
- Strong currents
- High winds
- Changing water levels
- Excessive wave height (including ship wakes)
- Drifting ice and debris
As a generalization, silt curtains and screens are most effective in relatively shallow, quiescent water. As water depth increases, and turbulence caused by currents and waves increases, it becomes increasingly difficult to effectively isolate the dredging operation 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 (JBF Scientific Corp. 1978). Typical configurations for silt curtains and screens are shown in Figure 4-4. To be effective, barriers are deployed around the dredging operation and must remain in place until the operation is completed at that site. For large projects, it may be necessary to relocate the barriers as the dredge moves to new areas. Care must be taken that the barriers do not impede navigation traffic. Containment barriers may also be used to protect specific areas (e.g., valuable habitat, water intakes, or recreational areas) from suspended sediment contamination.
A monitoring program for environmental dredging should be designed to meet project-specific objectives. Monitoring can be used to evaluate the performance of the dredging contractor, equipment, and the barriers and environmental controls in use. Monitoring may also be integrated into the health and safety plan for the dredging operation to ensure that exposure threshold levels are not exceeded.
The monitoring program must be designed to provide information quickly so that appropriate changes to dredging operations or equipment can be made to correct any problems. Simple, direct, and preferably instantaneous measurements are most useful. Measurements of turbidity, conductivity, and dissolved oxygen can be used as real-time indicators of excessive sediment resuspension. Project-specific guidelines for interpreting monitoring results should be developed in advance, as well as potential operational or equipment modifications.
ESTIMATING COSTS
The basic principles of cost estimating, and the use of cost estimates to support the decision-making process are discussed in Chapter 2. More detailed guidance specific to estimating the costs of dredging operations is provided in this section. This guidance is applicable to feasibility studies, but is not adequate for preparing a detailed dredging cost estimate.
This document discusses the removal (Chapter 4) and transport (Chapter 5) components of a sediment remedial alternative separately. However, these components are likely to be part of a single contract, and their costs would, in most cases, be estimated together. Virtually all costs associated with the removal component of a sediment remediation project are capital costs (direct and indirect). The elements of environmental dredging costs include:
- Mobilization/demobilization
- Dredge operation
- Contaminant barriers
- Monitoring
- Health and safety
- Equipment decontamination
Each of these elements is discussed below, and available unit prices are presented. Although many of these unit prices are obtained from navigation dredging experience, only the operational costs are likely to be increased significantly during sediment remediation dredging as a result of the more slowed operation and decreased production.
Cost information is available from some historical sediment remediation projects. A total of 13,000 m[3] of sediments was excavated from Starkweather Creek in Wisconsin by backhoe at a cost of approximately $10.00/m[3] (Fitzpatrick 1994). The Waukegan Harbor Superfund project in Illinois removed 23,000 m[3] by dredging at a cost of $1.1 million (Albreck 1994). However, these and other unit dredging costs from historical remediation projects should only be used when all cost items are known.
The first cost incurred in any dredging project is that of bringing the dredging equipment to the dredging site and preparing it for operation. This process is referred to as mobilization. Demobilization occurs at the end of the project operation and typically costs one-half the mobilization expense. Typical mobilization/demobilization costs for the Great Lakes region (provided by USACE Detroit District) are as follows:
Cost (per 100 km)*
Mechanical dredge (clamshell) $37,500
Hopper dredge (<4,000 m[3]) $75,000
Hydraulic (pipeline) dredge $18,750
* Distance the dredge must be transported to the project site.
Mobilization costs for backhoes (without the requirement for a floating platform) are typically less than $400 (USEPA 1985a). Portable dredges are often leased or purchased outright.
Mobilization/demobilization may represent the largest single cost element in the dredging project, especially for projects with small dredging quantities. Additional costs will be incurred if specialized pumps or unconventional dredgeheads are employed. Generally, specialty dredging equipment may be transported separately to the site and used with the conventional dredging equipment. The costs for specialty dredging equipment must be developed on a site-specific basis.
The costs of a dredging operation depend on the size of the dredge employed and the amount of time that the equipment is onsite (i.e., the cost of dredging is largely a function of the production rate). In conventional dredging, the rate of production is fairly predictable, based on the consistency of the sediments and the size of the dredge employed. Algorithms for predicting the production rates of different dredge types are provided in Church (1981).
During environmental dredging, additional time must be allowed for other factors, such as:
- Greater precision of cut
- Slower production rates to minimize resuspension
- Multiple passes needed to achieve cleanup goals
- Use of contaminant barriers
- Restrictions posed by other remedial components
In most cases, additional costs will be incurred as the production rates are lowered.
One of the goals of environmental dredging is to remove only those sediments that are contaminated. Because of the costliness of treating or disposing of contaminated sediments, the quantity of clean sediments removed must be minimized. The production rate of the dredge may be deliberately slowed so that downstream components such as sediment handling and transport, pretreatment, treatment, disposal, and/or effluent treatment are not overwhelmed. This is particularly true for hydraulic (pipeline) dredging, in which adequate time must be allowed for sediments to settle out in the receiving basin (see Chapter 8). In fact, it may be more cost effective, in such instances, to select a smaller dredge that can be operated at a constant rate close to its capacity, rather than a large dredge with an operating schedule that is frequently interrupted.
Typical unit costs for various types of maintenance dredges are provided in Table 4-9. They reflect the costs of dredge operation for rates of production typical of maintenance dredging in the Great Lakes. These costs should be adjusted to account for the lower production rates anticipated with environmental dredging. The adjustment for environmental dredging production rates may be as much as 2 3 fold (or more) for specific applications. For example, the hydraulic dredging of 23,000 m[3] of sediments during the Waukegan Harbor Superfund cleanup cost $1.1 million, or roughly $48/m[3] (Albreck 1994). This cost included the deployment of a contaminant barrier (silt curtain).
Several types of containment barriers are available to contain contaminants released during dredging. Current unit costs for oil booms and silt curtains and screens are summarized in Table 4-10.
The costs of a monitoring program for an environmental dredging operation may be significant. However, these costs are project specific, and few generalizations can be made. Among the potentially more costly items of a monitoring program are detailed bathymetric surveys (before and after dredging), post-dredging sediment contaminant analysis, and sediment resuspension monitoring. The cost of sediment analysis will depend on the contaminants analyzed and the turnaround time requested of the laboratory. The primary costs for resuspension monitoring are for field sampling, as turbidity and suspended solids analyses are relatively inexpensive.
The removal of contaminated materials from a waterway can be a hazardous activity, especially if contaminant concentrations are high. Depending on the types of contaminants present, the concentrations expected, and the degree of contact workers may have with the sediment, it may be necessary to provide workers with special PPE, such as respirators and Tyvek coveralls. Such gear can decrease the productivity of workers and thereby greatly increase operating costs. This is particularly true if workers are required to wear respirators or use supplied air. However, in most cases sediment contaminants are not volatile, and therefore respiratory protection is rarely needed.
Another health and safety consideration is the training of site workers. Workers at all Federal Superfund sites, as well as other hazardous waste sites, are required to undergo 40 hours of health and safety training (29 CFR 1910.120). This requirement may represent an additional expense not anticipated by the dredging contractor.
Reusable equipment that comes into contact with contaminated materials may have to be decontaminated prior to leaving the site. This is an expense not normally included with demobilization costs. The level of decontamination required will depend on the nature of the sediment contaminants and the laws and regulations governing the remediation. Large equipment such as dredges may have to be steam-cleaned or washed with detergents, unless it can be shown that contamination can be effectively removed using less intensive methods. It may be possible to clean pumps and pipelines by pumping clean water or clean sediment through them. All wash water from these operations would have to be captured and probably treated before being released.
ESTIMATING CONTAMINANT LOSSES
The loss of contaminants during dredging may need to be estimated for a number of reasons, including:
- Comparison and selection of dredging equipment
- Evaluation of the overall losses from remedial alternatives
- Determination of compliance with water quality requirements
- Determination of short-term impacts on sensitive resources
Factors that potentially affect contaminant losses from dredging are listed in Table 4-11.
A study conducted under the ARCS Program examined the available predictive tools for estimating contaminant losses from dredging (Myers et al., in prep.). The three mechanisms of contaminant loss from dredging are:
- Particulate contaminant releases
- Dissolved contaminant releases
- Volatile contaminant releases
Methods for predicting sediment resuspension have been developed for cutterhead and mechanical (bucket) dredges. These methods predict the resuspension of particulates as a function of dredging equipment, operation, and sediment properties. These techniques have not been field verified, and are therefore not fully developed (Myers et al., in prep.).
Limited field studies have indicated that the type of dredging equipment used may have less effect on sediment resuspension than how it is used. The care with which a dredge operator excavates material has a significant effect on sediment resuspension (Hayes 1992). For example, variables such as cutter speed, swing speed, and degree of burial (bank factor) have been incorporated into models for cutterhead dredges (Myers et al., in prep.). Decreasing each of these parameters can reduce the resuspension caused by hydraulic dredging. Similarly, smooth and controlled hoisting can limit resuspension during clamshell dredging (McClellan et al. 1989).
Sediment properties are site-specific variables that cannot be controlled. In general, fine-grained, less-cohesive sediments have the greatest potential for resuspension and will travel further before resettling to the bottom.
The resuspension characteristics of numerous dredge types have been measured at various locations. A summary of resuspension tests is provided in Table 4-12, as compiled by Herbich and Brahme (1991), Zappi and Hayes (1991), and others. The comparability of sediment resuspension results from different sites is highly limited due to differences in the monitoring programs, sediment types, site conditions, and other factors. As indicated above, the type of dredge used is not always the most significant factor affecting sediment resuspension.
Resuspension of sediment solids during dredging can impact water quality through the release of contaminants in dissolved form. Dredging exposes sediments to major shifts in liquid/solids ratio and reduction/oxidation potential (redox). Initially upon resuspension, the bulk of the contaminants are sorbed to particulate matter. As the resuspended particles are diluted by the surrounding waters, sorbed contaminants may be released, increasing the fraction of dissolved contaminants in the water. Changes in redox potential (i.e., from an anaerobic to an aerobic environment) can affect metal speciation. This may increase the solubility of metals (e.g., oxidation of mercury sulfides) or decrease metal concentrations (e.g., metal scavenging by oxidized iron flocs) (Myers et al., in prep.). Organic contaminants are largely unaffected by redox shifts.
Methods for predicting the release of dissolved contaminants during dredging are less developed than those for sediment resuspension. A method using equilibrium partitioning concepts has been proposed for estimating the concentrations of dissolved organic contaminants, and a laboratory elutriate-type test has also been evaluated (Myers et al., in prep.).
Dissolved organic chemicals are available at the air-water interface where volatilization can occur. Although the dissolved phase concentrations and therefore the evaporative flux are highest near the dredge, the mass release rate (flux times area) may be dominated by the lower concentration region away from the dredge.
Methods for predicting the rate of volatilization across the sediment-water interface are fairly well developed. To apply these methods at a dredging site requires the application of a mixing model to define both the area of the contaminant plume and the average dissolved-phase contaminant concentrations within that plume (Myers et al., in prep.). |
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