Dredging or excavation remedies remove contaminated sediment from freshwater or marine water bodies in order to reduce risks to human health and the environment. Removal is particularly effective for source controlThose efforts that are taken to eliminate or reduce, to the extent practicable, the release of COCs from direct and indirect ongoing sources to the aquatic system being evaluated. (mass removal of hot spots) but potentially less effective for overall risk reduction because of resuspensionA renewed suspension of insoluble particles after they have been precipitated. and residual contamination. Incorporating design features for resuspension control and residuals management can further reduce risk. After removal, the contaminated sediment can be treated or disposed in a controlled setting, such as an off-site landfill or other treatment, storage, and disposal (TSD) facility, an on-site aquatic or terrestrial confined disposal facility (CDF), or a facility that converts the sediment to a reusable product.
Under favorable circumstances, sediment removal can be effective in achieving RAOs, as illustrated in the case studies in Appendix A, which are summarized in Section 6.7. Removal has the potential, however, to disrupt the sediment and aquatic environment in the short term. Removing contaminated sediment can liberate a significant quantity of contaminants and leave residuals that may pose significant risks. Removal implementation costs are often higher than costs of other technologies, thus the selection process for this approach must balance costs, the site characteristics that drive applicability and limitations, and the net risk reduction that this approach can achieve. With a thorough site characterization, some of the removal challenges can be addressed through design and by using best management practices (BMPs) during operation.
Dredging of harbors and rivers for navigational purposes has been practiced for centuries and studied extensively. By comparison, environmental dredging (dredging for the sole purpose of removing contaminated sediment) is a relatively new development. While navigational dredging experience can be applied to environmental dredging projects, these applications have several key differences. For example, navigational and environmental dredging differ in their respective production rates (the amount of material dredged per hour). In navigational dredging, the production rate determines dredging effectiveness—a higher production rate results in a more successful project. In environmental dredging, production rate can affect the cost of the project, but not necessarily the success of the project. For environmental dredging operations, the removal operation is highly controlled, with efforts focused on minimizing the removal of clean material while, at the same time, controlling contaminant residuals and limiting the spread of contaminants. This level of control is often achieved at the cost of production rate. For an environmental project, remedial objectives can still be met despite a low production rate. Additionally, the controlled dredging needed for environmental projects results in a more resource-intensive operation than navigational dredging.
The two primary methods of contaminated sediment removal are mechanical dredging and hydraulic dredgingDredging by use of a large suction pipe mounted on a hull and supported and moved about by a boom, a mechanical agitator, or cutter head which churns up earth in front of the pipe, and centrifugal pumps mounted on a dredge which suck up water and loose solids.. A third method, excavation, is also described because it has been used at a number of sites in recent years. Dredging and excavation inevitably affect the aquatic and benthic environments, and this chapter presents some ways to minimize these effects. As with any type of removal operation, additional technologies are required to appropriately handle the removed sediment. Dredged material handling technologies may involve transport, dewatering, treatment, and or disposal of sediment.
Mechanical dredging removes sediment by capturing the sediment and then lifting the captured material to the surface. The dredged material is removed at near in situ solids content and density. A mechanical dredge usually consists of the following:
- a bucket equipped with a cutting and grabbing edge
- a crane or other means of lowering, manipulating, and retrieving the bucket (with the dredge material) through the water column1) The basic habitat and the medium through which all other fish habitats are connected; 2) a conceptual column of water from surface to bottom sediments. This concept is used chiefly for environmental studies evaluating the stratification or mixing (such as by wind induced currents) of the thermal or chemically stratified layers in a lake, stream or ocean. Some of the common parameters analyzed in the water column are: pH, turbidity, temperature, salinity, total dissolved solids, various pesticides, pathogens and a wide variety of chemicals and biota. Understanding water columns is important, because many aquatic phenomena are explained by the incomplete vertical mixing of chemical, physical or biological parameters. For example, when studying the metabolism of benthic organisms, it is the specific bottom layer concentration of available chemicals in the water column that is meaningful, rather than the average value of those chemicals throughout the water column.
- a means of transporting (usually a barge) the dredged material from the dredging site to a sediment handling and processing or disposal facility
Equipment typically used for environmental dredging includes environmental clamshell buckets or enclosed clamshell buckets. More detailed descriptions of each mechanical dredge types can be found in Section 5.1 of the USACE's technical guidelines for dredging (2008).
Depending on site conditions, mechanical dredging equipment can sometimes be operated from shore; however, most dredges are set up on a barge (floating platform) equipped with an anchoring system, such as spuds, to hold it in place. Dredged sediment from near-shore locations can sometimes be transferred to shore by the same mechanical dredge and barge. If the dredging site is further from shore, the dredged sediment may be transferred to a second barge that hauls the sediment to the handling and disposal facility. Access to shore-side facilities or infrastructure is often used to provide an off-loading area or staging area for treatment or dewatering of the dredged sediment.
Hydraulic dredging operations remove sediment by fluidizing and pumping the material to the handling location. A hydraulic dredge usually consists of a dredge head and a hydraulic pump. The dredge head is lowered into the sediment bed to fluidize the sediment by mechanical agitation and to draw the slurry into the suction pipe. Cutter heads and horizontal augers are the most common forms of dredge head design for environmental dredging. The hydraulic pump may be deck mounted or submersible.
Additional equipment needed for hydraulic dredging includes a ladder or cable used to support the dredge head and lower it into the water, as well as to swing the dredge head to advance into the sediment face. Most hydraulic dredges use spuds, which are devices driven into the sediment to stabilize the discharge line and the dredge, as they are operated or maneuvered using a cable system. A number of specialty hydraulic dredges are also available, including purely suction devices often used to dredge residuals or fluid sediments. These specialty dredges can also use water jets or pneumatic methods to fluidize the sediment, but these approaches are less common. Hydraulic dredges without mechanical agitators for fluidization are called "plain suction" dredges. A vacuum hose without an agitator can be used for dredging loose sediment at some sites. This operation is usually assisted by divers who guide the hose around obstacles.
Because the sediment must be fluidized and pumped, large volumes of water are mixed and transported with the sediment, resulting in the recovery of a slurry that is typically composed of between 10–15% (by weight) solids but may contain as little as 1–2% solids. The volume of water added to create a slurry that can be pumped (referred to as carrier water) depends on the in situ solids content of the sediment, sediment grain size, and pumping distance. For environmental dredging projects, the volume of carrier water needed is typically 5–10 times the in situ volume of sediment, which equates to 1,000–2,000 gallons per in situ cubic yard.
Hydraulic dredging is described in more detail in USEPA’s sediment guidance (USEPA 2005a) and in the USACE technical guidance (2008). When applicable, hydraulic dredging is economical for removing large volumes of sediment and is used in both navigational and environmental dredging.
Excavation refers to sediment removal conducted after the water above the sediment has been removed. In an excavation remedy, operators isolate a segment of the sediment and water column in an enclosure, dewater the enclosure, and remove the exposed sediment using conventional land-based excavation equipment. To isolate an area for dewatering, containment structures such as cofferdams, earthen berms and sheet piles are first installed to seal off the area and encircle the contaminated sediments. Once isolated, the interior of the enclosure can be pumped to remove water prior to sediment removal. Excavation equipment is often similar to that used in mechanical dredging and includes excavators, backhoes, and clamshells. In areas with large tidal swings, significant seasonal tidal changes, or intermittent streams and wetlands, excavation can be performed during low-water conditions and sometimes without an enclosure.
Excavated sediment usually contains less water than dredged sediment and thus is easier to handle. Excavated sediment, however, may still require additional land-based dewatering or solidificationTo make solid, compact, or hard, to make strong or united, or to become solid or united. followed by off-site transport and disposal. In general, improved access to target dredging areas, greater control on dredge cuts, reduced concerns regarding resuspension of residuals, and potentially reduced sediment dewatering needs are the primary factors for selecting removal by excavation rather than by dredging.
The most significant advances in environmental dredging in recent years have been the result of improved planning and operational efficiency, rather than the result of improved technology. Some conventional navigational dredging equipment has been customized to meet specific needs at larger sites. Enhanced planning and operational procedures, however, have been shown to improve removal efficiency and reduce the resuspension of sediment and generation of residuals for sites of any size. Residuals and resuspension are significant technical, environmental, and economic considerations for dredging (see Section 6.3.5). Reducing residuals and resuspension improves the overall effectiveness of removal and excavation technologies.
During removal planning, surface-weighted average concentrations (SWAC) may be used as targets to be met during dredging. In this method, a site is divided into cells (or "bands") of varying contaminant concentrations. Cells are removed by dredging or excavation to meet a site-wide SWAC that is below the remediationThe act or process of abating, cleaning up, containing, or removing a substance (usually hazardous or infectious) from an environment. goal. Often, the highest concentration cells are targeted for removal first because remediating these cells significantly reduces the SWAC.
The SWAC approach has proven effective as a target in field applications. At several recent mechanical and hydraulic dredging sites, dredging targeted sediments that were causing an exceedance of a SWAC equal to the cleanup goal. In a recent five-year review at the Continental Steel site at Kokomo and Wildcat Creek, USEPA Region 5 and the Indiana Department of Environmental Management (IDEM) affirmed that the SWAC approach is more representative of the exposure domain for receptorA plant, animal, or human that is typically the focus of a risk assessment following the direct or indirect exposure to a potentially toxic substance. populations than the small areas represented by individual samples (IDEM 2007). At the Army’s Natick site at Pegan Cove, the site achieved no further action (NFA) status after hydraulic dredging was conducted to achieve a cove-wide SWAC below the risk-based remediation goal of 1 ppm of PCBs. Backfilling was used in some areas with persistent residuals. At very large sites, such as New Bedford Harbor, SWAC goals are being assessed on different reaches of the river or harbor.
When low concentration goals are established for a site targeted for removal, residuals and resuspension or deposition may affect the attainment of these remedial goals. At a number of dredging sites, multiple dredging passes have been required to remove the residuals deposited and, in some cases, cappingTechnology which covers contaminated sediment with material to isolate the contaminants from the surrounding environment. has ultimately been required to achieve the remediation goal. At the GM Massena site (St. Lawrence River, NY), following more than 15 dredge passes, backfilling of dredged areas with clean material was required to achieve 1 ppm of PCBs in portions of the dredge prism. Consequently, residuals management plans are now being developed along with the removal plan in order to optimize the number of dredging passes and reduce resuspension, contaminant release, and erosion of residuals.
In planning for removal of contaminated sediments, site project managers must also consider biological factors. Fish reproduction or benthic community survival windows often permit removal only during certain times of the year (referred to as "dredging windows"). Additionally, benthic community structure may restrict the times during which removal can occur. While dredging does not usually damage fisheries, the effects of removal on the benthic community must be evaluated during planning. Additionally, the upland habitat of endangered species or sensitive wetlands habitat may be affected by sediment removal operations. Site evaluations must consider potential risks to these habitats when selecting access sites, lay-down areas, staging areas, and transfer areas.
Recent advances in dredge positioning and stability have improved the accuracy of environmental dredging. The accuracy of both mechanical and hydraulic dredges is affected by many of the same factors, such as wind speed (especially for an unanchored platform), currents, positioning system accuracy, and operator skill. A positioning system accuracy of ±1 corresponds to a mechanical bucket cut, or the arc of a hydraulic dredge cut, accuracy of ±1. At many sites, dredge operators have addressed accuracy limitations by over dredging (overlapping cuts). Over dredging materials, however, can become significant where the processing and disposal costs for removed sediments are high. For example, site managers who try to address a positioning accuracy of ±1 ft with a minimal overlap of 6 inches must target a mapped overlap of as much as 2.5 ft. The USACE guideline (2008) contains a more detailed discussion of vertical and horizontal dredging accuracy.
Although sophisticated positioning systems have been used at a few large sites, such as Fox River and Green Bay, at many moderately-sized sites, project teams have tried to incorporate some version of advanced positioning into dredging operations. Over-dredging usually proves to be an easier method to reach target depths and remove sufficient sediment. Bathymetric measurements before and after dredging are typically used to verify that target depths have been reached. This conventional method is seemingly crude but effective; however, as much as 20–25% more sediment than targeted may be dredged.
Several recent advances in dredging operations have improved targeted removal operations (Pastor 2012). One advanced positioning system, real-time kinematic global positioning, allows dredging to be focused on specific areas and depths, thus minimizing the requirement for over-dredging to achieve design goals. At some sites, this advanced positioning system can be an alternative to over dredging and its associated increased costs and materials handling. Finally, operator training and experience are other important variables in sediment removal that affect removal success (Pastor 2012).
Advanced Operational Controls
At the Fox River and Green Bay sites in Wisconsin, real-time kinematic global positioning system (RTK GPS) was used. A state-of-the art technology, RTK GPS indicates to the operator exactly where the dredge head is located while it is underwater (Pastor 2012). For each cut, the dredge is positioned in the water using RTK GPS and a series of electronic sensors measure tilt angle, acceleration, shock, vibration, and movement. The position of the cutter head is tracked and recorded in relation to the dredge. Special software uses input from the GPS and sensors to show the operator the exact position of the cutter head. The RTK GPS has been used at this site since 2004 and has improved the accuracy of dredging.
RTK GPS was developed specifically for this site. The technology cost several hundred thousand dollars, but it is expected to save money and time through improved efficiency. This system targets the neat line, a location identified during sediment characterization as the depth where PCB levels in the sediment drop from over 1 ppm to under 1 ppm (the target cleanup level). Before RTK GPS, the dredging plan was implemented using operator judgment. The operator reviewed the site map and make multiple dredging passes, often dredging more than was necessary.
A similar targeting system was also used at Ohio’s Ashtabula River. Although the RTK GPS was developed to work with hydraulic dredges, a similar system has been used in other places, such as Commencement Bay in Washington, with a clamshell dredge. According to USEPA staff, this system has not yet become standardized because of high development costs. In addition, the different sediment types (such as mud versus sand) and varying conditions and accessibility at different sites have also slowed the development of a standard system.
Conventional mechanical dredging equipment, such as dredges that use a clamshell bucket, bucket ladder, or dipper and dragline, are ineffective for environmental dredging. A variation of the conventional clamshell bucket, the enclosed dredge bucket, has been developed to limit spills and leaks from the bucket. An enclosed bucket reduces resuspension by improving the seal between the elements of a closed bucket. An enclosed bucket also reduces releases of water-soluble contaminants into the water column during dredging. Additional modifications to conventional mechanical dredging equipment based on site-specific conditions include:
- fitting the crane with longer boom (arm) for additional reach during dredging
- fitting an excavator with a longer arm for better access
- using a fixed arm bucket instead of a cable suspended bucket to increase the accuracy and precision of cuts and to provide greater bucket penetration in stiffer materials
- equipping the bucket with hydraulically operated closure arms to reduce bucket leakage
- installing a sediment dewatering and water collection and treatment facility on the barge or at a temporary staging site
- installing GPS and bucket monitoring equipment to the dredge to provide the equipment operator with precise coordinate control of the bucket during dredging operations
Often, backhoes can be modified or equipped with covers for the bucket to improve retention of the sediment and to minimize resuspension. Clamshell dredge buckets can also be fitted with baffles and seals to slow the movement of water and mud. USACE used this type of seal, which is similar to a rubber gasket, at the Fox River and Green Bay sites to minimize leakage of PCB-contaminated water and sediment from the bucket.
Recent developments in hydraulic dredging equipment have typically included project or site-specific modifications in order to achieve the following objectives:
- Increase solids content in the dredged material and lower water content.
- Prevent debris from entering the auger or pump intake.
- Pump dredged material over greater heights or distances.
- Improve on shore dewatering of dredged material.
- Reduce potential for releasing dredged sediment into the water column.
Because site conditions can vary greatly, many of these equipment and other modifications are not considered standard practice. For example, a screen that is installed on a hydraulic dredge to prevent debris from entering the auger or pump intake could also slow down production at a given site by reducing the sediment flow rate in the pump.
In evaluating, selecting, and designing a removal remedy, the effects of removal (particularly dredging) must be taken into account. Contaminated sediment removal actions resuspend sediment, generate residuals, and release contaminants as follows (USACE 2008):
- Resuspension is the fluidization and dispersion1) Pollutant or concentration mixing due to turbulent physical processes; 2) A distribution of finely divided particles in a medium. of the sediment particles into the water column due to dredging and associated operations. Resuspended sediment may eventually settle out in dredged areas or disperse and settle in surrounding areas.
- Residual is the disturbed, or undisturbed, sediment that remains in the dredged area (or local vicinity) following a dredging operation.
- Releases of contaminants from the sediment bed may occur due to dredging, and from the same processes that generate resuspension and residuals. Releases, however, may also include loss of pore waterWater located in the interstitial compartment (between solid-phase particles) of bulk sediment., NAPL (if present), and associated contaminants. Releases may further occur from the desorption of contaminants from resuspended sediment and residuals.
The potential risk reduction from the removal of contaminated sediment must be weighed against these potential increased risks from contaminant releases due to dredging.
Dredging-related resuspension, residuals, and releases can lead to increases in contaminant levels in fish tissue, difficulty in achieving sediment-based cleanup goals, and the need for additional postdredging site management or residuals management. The risk profile of a site can change following a dredging operation. While risk is potentially reduced by the removal of contaminants associated with the dredged material, residual risk may remain (and may need to be addressed) at the dredged site due to resuspension, residuals formation, or releases.
The degree of resuspension of sediment during dredging is determined by a number of factors, including:
- Sediment properties such as particle size, cohesiveness, and bulk density can affect resuspension. Silts are more easily resuspended than sands (which are larger and heavier than silt) and clays (which are smaller, but tend to be more cohesive or plastic than silts).
- Site conditions such as water depth, current velocity, waves, and underlying bedrock can make operational control difficult.
- Impediments such as debris, boulders, and pilings associated with piers can affect the operation of the dredge and lead to sub-optimal operating conditions.
- Operational factors such as design and planning of the dredge cuts, dredging equipment type, and operator skill can also influence resuspension.
Because these factors vary from site to site, a wide range of field data on levels of resuspension has been reported, ranging from less than 0.1% to as high as 5% (without losses from barges or hoppers). Resuspension rates from mechanical clamshell dredging operations typically range from 0.3 to 1.0% while losses from open bucket excavators tend to be as much as three times higher. Resuspension rates from hydraulic cutterhead dredging operations typically range from 0.1 to 0.6%, while losses from horizontal auger dredges tend to be about three times as high (USACE 2008). Characteristic (median) resuspension factors for hydraulic cutterhead dredges and closed mechanical environmental clamshell buckets are both estimated to be 0.5%, while resuspension factors for horizontal auger dredges and open buckets and excavators are two to three times higher (USACE 2008).
The performance of dredging equipment depends, in part, on sediment properties. Mechanical dredges limit resuspension of fines and contaminants from sandy sediments, while cutterhead and plain suction dredges limit resuspension of very soft, fluid sediments. Resuspension rates are based on navigational dredging and reflect the mass of fine particulates resuspended as a percentage of the fine-grained mass dredged, not the mass of contaminants adhering to or released with the sediment particles. Even in a well-managed operation, these suggested percentages may increase by a factor of two or three, depending on the presence of debris, debris removal operations, barge transport (tug operation), or any disturbance due to engineered controls such as silt curtains or sheet piling (USACE 2008).
Prediction models are available that can help designers estimate how much resuspension might occur and then plan for residuals. Risks and the need for engineering controls should also be considered during planning stages. A number of prediction models are available that are based on navigational dredging experience (USACE 2008; Bridges et al. 2008; and USEPA 2005a). These models, however, use variables that are not easily measured or estimated at many sites. In addition, factors such as operator experience or ability to maneuver the dredge around impediments may also make model predictions unreliable.
Despite their limitations, prediction models provide insight on the potential for resuspension and can guide the selection of site-specific BMPs and controls. BMPs may include operational controls, engineering controls, or both. Engineering controls should be carefully evaluated because these controls tend to be relatively expensive and may generate some resuspension during installation and removal. These controls may also result in other unintended consequences such as channel restrictions that cause resuspension of nontarget sediment, air releases, DOdissolved oxygen consumption and fish kills, or exacerbated residuals.
When contaminant concentrations are high or when sensitive aquatic environments are present, engineering controls can be used to minimize the effects of sediment removal. The most common engineering control used in navigational and environmental dredging operations is the silt or turbidity curtain. Silt curtains are vertical, flexible barriers that hang from floats at the water surface. Silt curtains are generally deployed from the water surface to a depth of one to two feet above the sediment bed; the curtain is not a complete enclosure. The resulting height of the deployed curtain is called the skirt depth. The curtain material is held in place by floats on top and a ballast chain at the bottom. Anchored lines are attached to hold the curtain in place. For navigational dredging, silt curtains are considered a BMP and are often successful in controlling turbidity in the surrounding water column.
USEPA (1994) and ERDC (2005) consider silt curtains ineffective at depths greater than 20 ft and at current velocities greater than 50 cm/sec (approx. 1 knot). Under these conditions, silt curtains can be reinforced to some extent with sheet piling at the corners or additional anchoring measures, but the effectiveness of any additional measures should be verified in the field. Adding sheet piling considerably increases the cost of the application.
A study conducted as part of USEPA's Assessment and Remediation of Contaminated Sediments (ARCS) Program concluded that silt curtains are most effective at relatively shallow sites in relatively quiescent water and wind conditions (USEPA 1994). Silt curtains should not be used at depths greater than 20 ft, where the water column pressure on the mooring system becomes excessive, and at current velocities greater than 50 cm/sec (approximately 1 knot), where billowing or flaring of the curtain in the flow direction may reduce its effectiveness (USEPA 1994; ERDC 2005). High currents lead to flaring, which can cause the bottom of the curtain to be raised several feet above the sediment bed (and above the installed lower depth). High currents can also cause curtains to tear.
A summary of case studies that address resuspension is included in Section 6.7. As shown in Table 6-2, silt curtain resuspension controls were used at all sites where mechanical dredging was done under a column of water. Excavation was generally done in a sheet piling enclosure. Some success with silt curtains was noted at the Kokomo Creek site, where mechanical dredging was conducted along a two-mile stretch of a creek in water depths of 1–4 ft. Problems were reported with silt curtains at the Formosa Plastics site, where mechanical dredging was done in 25–30 ft of water. At this site, soft, silty sediment kept flowing into dredged areas from under the curtain.
As shown in Table 6-2, resuspension controls (generally silt curtains) were used at most sites with hydraulic dredging. Success appears mixed. Among the hydraulic dredging sites examined, Pegan Cove (water depth of 0–10 ft) reported success in using double silt curtains to successfully keep turbidity out of the surrounding water. At the New Bedford Harbor site (hydraulic dredging, in the Lower Harbor) use of silt curtains was abandoned after the curtains were found to contribute to scouring from high current velocities and turbulence. Difficulties were encountered in water depths of more than 20 ft. This site is now relying on BMPs (operational controls) to minimize resuspension to the largest extent possible. At the Waukegan Harbor site (hydraulic dredging), water depth was 25 ft in some areas, and silt curtains failed due to wind and wind-induced currents. Shallower sites encountered some problems as well; at the Lavaca Bay site, for example, elevated contaminant levels occurred downstream of silt curtains.
For some sites, silt curtains must be supplemented or replaced with other engineering controls. At the Fox River and Green Bay Project 1 site, silt curtains were reinforced with sheet piling at the corners to avoid frequent maintenance. At the GM Massena St. Lawrence River site, silt curtains did not contain turbidity and were replaced with interlocking sheet piling. Sheet piling provides better containment, but tends to prevent both water and suspended particles from moving into and out of the enclosure. Sheet piling enclosures should be monitored to confirm that dissolved oxygen in the enclosure does not get depleted. Note that sheet piling has a much higher installation cost when compared to silt curtains. At some sites, sheet piling was used to shore up the banks of the water body being dredged, rather than as an alternative for silt curtains. For excavation sites, cofferdams and removable dams are generally used for containment.
Oil booms are also sometimes used as an engineering control for sediments that are likely to release oils when disturbed. These 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). These barriers are also effective for contaminants such as NAPLs, which can be readily released into the water column during removal.
No dredging operation removes all contamination, and contingencies for residual contamination must be addressed during design. The Reynolds site, for example, experienced particular difficulty with residuals, requiring multiple passes in several of the cells dredged. In some cells, the 1 ppm PCB cleanup goal could not be met, despite multiple passes. These cells were backfilled with clean material to meet cleanup goals.
Two types of residuals are expected at dredging sites:
- Generated residuals arise when sediment is disturbed by dredging, but is not collected by the dredge. Resuspension and subsequent settling of particles, sloughing along the sidewalls, and spillage from the dredge head, bucket, or clamshell are the primary causes of generated residuals
- Undisturbed residuals are contaminants that are neither disturbed nor collected by the dredge. Undisturbed residuals could arise from one or more of the following:
- insufficient characterization (as might happen at a large site)
- inadequate characterization of the depth of contamination (especially at sites with deep contamination or debris)
- limits of characterization methods (averaging of contamination in long sampling tubes)
- impediments (such as rock outcrops, boulders, debris, structures, pilings, or utilities)
- inaccuracies or insufficient control and precision in positioning during dredge operation
Additional factors that can cause residuals include slope failures, bucket over-penetration and overfilling (due to insufficient control or overly aggressive production rates), underlying bedrock, or an uneven sediment bed. Methods and calculations are available to predict the level of residuals, but as with resuspension, many site-related and operational variables can make prediction difficult (USACE 2008). One study showed that at several sites with PCBs, a family of contaminants that adheres strongly to sediment particles, 5–9% of the original PCB mass remained as generated residuals (Patmont 2006). At the other sites in this study, where contaminants were more soluble, the generated residuals ranged from 2–4%. The level of these residuals is greater than the level of resuspension (0.5–1 %) expected at a typical site. These results may indicate that spillage and fallback from dredging, sloughing, and slumping are major sources of residuals, contributing more to generated residuals than resuspension does.
Controls for residuals include equipment controls, operational controls, and postdredging controls. Equipment controls are modifications of the dredging equipment. Operational controls are implemented during dredging as a means of reducing residuals to the minimum amount feasible. Operational controls discussed in Section 126.96.36.199 for reducing resuspension, such as control of dredge cuts and production rates, are also useful in reducing residuals. The effectiveness of these operational controls has not been well documented, but in theory they should reduce residuals.
Resuspension and Residuals
Dredging generates resuspension and residuals. When postdredging residuals exceeded acceptable risk thresholds, sites have successfully used backfilling to efficiently achieve further risk reduction and cleanup goals.
Postdredging controls manage residuals after they have occurred. Over-dredging and the use of cleanup passes are the most common operational controls for residuals. These measures assume that there are limits to operational controls (such as positioning or depth of each cut), so the sediment is dredged to a greater depth or over a larger area than is warranted by the site characterization. A cleanup pass, made after the original target is reached, may help to gather residuals that have already accumulated and to mix the residuals with underlying clean sediment. The residual sediment that remains in the dredged area, however, may not have the same physical characteristics as the native sediment. In mechanical dredging, for example, resuspended residuals may settle in the dredged area at a lower dry-bulk density than the native sediment and may be more prone to fluidization and resuspension in subsequent passes. In this case, other dredging equipment such as a hydraulic suction dredge may be used to conduct additional cleanup passes and capture fluidized residuals. Note that these additional passes add expenses for a second mobilization with different equipment and operation.
Over-dredging is relatively common at sites where remediation goals are based on achieving a final cleanup concentration of contaminants in the sediment. Additional cleanup passes after initial dredging to required depths, however, result in increased cost. At some mechanical dredging sites, more sediment is dredged than planned (see Table 6-2). The excess dredged sediment may be a result of multiple dredge events or several passes over a single dredge area because confirmation samples indicated that project cleanup goals had not been achieved.
The available case studies show mixed results for dredging performance and postdredging sediment concentrations. Several mechanical dredging sites achieved clear success in meeting postdredging cleanup goals without backfilling (including sites with water depth greater than 20 ft). About half of the sites examined required backfill with clean material after dredging to help meet cleanup goals. Among the hydraulically-dredged sites (Table 6-3), at Gill Creek and Pioneer Lake cleanup goals were met. At Pegan Cove, cleanup goals based on SWAC were met after backfilling with clean sand. At Fox River and Green Bay, Operable Unit 1, and at GM Massena, meeting cleanup goals with hydraulic dredging was difficult, and some areas were eventually backfilled. At the Fox River and Green Bay Project 2 site, cleanup goals were not achieved after multiple passes. In postdredging sediment samples, concentrations were higher than pre-dredging samples in the same areas. These differences may be due to resuspension (and resettling), sloughing, heterogeneity of the sediments, or exposure of deeper contamination.
Postdredging management options and controls can also include backfilling or MNR. MNR as part of a technology train in dredged areas requires collecting data to establish natural recovery trends. This data collection may not be possible at all sites (such as for sediment in an erosional environment). Backfilling with clean material, sometimes called a "residuals cover" is often the quickest route to achieving target cleanup goals and has been used at many sites.
Backfilling of dredged locations with clean off-site material provides a cover over contaminant residuals at the newly created sediment surface. Backfilling is often a last resort after multiple dredging passes fail to achieve cleanup goals. A more efficient approach is to incorporate backfilling in the initial design at sites where residuals are expected and could hamper site closure. In this approach, backfilling is performed immediately after dredging has been completed to the targeted depths (as verified by a bathymetric survey). In shallow water systems, backfilling is also commonly incorporated into the remedial design to return the bed elevation to its original condition to support habitat functions and bank stability. Table 6-2 and Table 6-3 summarize several sites that used backfilling to help achieve cleanup goals.
Resuspension of sediment results in some short-term release of contaminants to the dissolved phase in the water column through release of pore water and desorption from suspended sediment particles. Additional releases may occur by erosion of the residuals or diffusion, mixing, or advectionBulk transport of the mass of discrete chemical or biological constituents by fluid flow within a receiving water. Advection describes the mass transport due to the velocity, or flow, of the water body. It is also defined as: The process of transfer of fluids (vapors or liquid) through a geologic formation in response to a pressure gradient that may be caused by changes in barometric pressure, water table levels, wind fluctuations, or infiltration. from the residuals. The release of dissolved contaminants yields the greatest risk because the dissolved phase drives biological uptake and volatilization. The fraction of the contaminant present in the dissolved phase of the water column increases with time as the suspension disperses and the contaminant desorbs. Depending on the contaminant, desorption may take hours or days to occur; therefore, control of sediment resuspension and residuals helps in control of contaminant release for contaminants normally associated with sediments—such as PCBs and PAHs, which tend to remain tightly bound to fine-grained sediment particles. For other forms of contaminants, such as NAPL, releases from the sediment during dredging can float to the surface as a separate phase. Appropriate measures may be required to control releases not related to the resuspended sediment particles or residuals.
Releases can substantially affect remedial efforts. For example, at the Grasse River site, resuspension and releases led to PCB levels in fish tissue that were 20 to 50 times higher than before dredging. Elevated fish tissue levels continued for three years. At the Shiawassee River site, samples of water, clams, and fish showed elevated levels of PCBs at all locations in the dredging area and downgradient. In all three media, PCB levels remained elevated over the six months that these levels were studied (Bremle and Larsson 1998; Rice and White 1987).
Management of sediment removed through dredging or excavation requires integration of the dredging technique with transportation, treatment, and final disposal or reuse of the dredged material in an approved location. Each of these steps influences available options for subsequent steps in the material handling chain. If any one of these critical steps is infeasible from a technical or cost standpoint, it may preclude dredging as a viable remediation strategy. During remedy selection, costs for multiple strategies for removed sediment handling should be reviewed as part of alternatives that include dredging or excavation. Removed sediment handling is often a sizable component of the total cost—often exceeding the cost of the in-water dredging. Table 6-4 lists the dredged material handling methods used at various sites. Reports prepared by USEPA (2005a) and USACE (2003) provide further discussion of previously implemented treatment and disposal technologies for dredged sediment.
Dewatering may be necessary to prepare dredged materials for disposal. Dewatering reduces the water content and hence the volume and weight of the disposed sediment. If the material is to be reused or further treated, dewatering also leads to reduced transportation cost and improves handling properties. The nature and extent of dewatering needed depends on the sediment characteristics and the type of dredging, transport, and disposal methods planned for the removed material.
Staging for dewatering operations varies depending on the resources available near the dredging site. Passive dewatering requires sufficient space to store the sediment during the separation process. Also, if the goal is to return the carrier water to the source river water body, dewatering relatively close to the discharge point would minimize piping costs. During passive dewatering, carrier water is removed primarily by gravity separation and to a lesser extent by evaporation. The more thinly the sediment can be spread at the dewatering site, the more effective passive dewatering by evaporation will be. Passive dewatering typically occurs in a CDF. Many other types of holding facilities can also be used, such as tanks or lagoons (USEPA 1994). Geobags with chemical conditioning have been used at many sites for efficient gravity dewatering of large volumes of sediment in relatively small spaces.
During active mechanical dewatering, equipment or materials are used to apply external pressure and can sometimes achieve a solids content of up to 70% by weight. Typical equipment used includes plate-and-frame presses, which are effective but operate in batch mode, and belt filter presses, which may be less effective but can be operated continuously. Water removed during mechanical dewatering must also be addressed. If the removed water contains contaminants at concentrations below regulatory thresholds, then it may be ready for immediate use or disposal. Otherwise, the water may require capture and treatment prior to disposal.
Disposal of dredged or excavated sediment is the placement of materials into a controlled site or facility to permanently contain contaminants within the sediment. Management is achieved through the placement of materials into facilities such as sanitary landfills, hazardous material landfills, CDFsconfined disposal facilities, or confined aquatic disposal (CAD) facilities. Table 6-3 shows that off-site landfilling has been the most common disposal method for dredged material. Off-site landfills are generally used for dredged material disposal when on-site disposal is not feasible or when off-site disposal is more cost effective.
Landfills have been used for sediment volumes of over a million cubic yards. Typically, some type of on-site or near-site disposal facility is used at sites where dredged material volumes greater than that 200,000 yd3 are generated. Landfilling is also favored at smaller or moderately sized sites, where transportation is feasible. The associated hazards and cost of transporting and landfilling large volumes of sediment make this disposal method somewhat less desirable than other solutions. Other considerations, such as public and stakeholderAffected tribes, community members, members of environmental and community advocacy groups, and local governments. acceptance, lack of access to suitable on-site land- or water-based disposal facilities, and proximity to an existing off-site landfill may support the landfilling option. The Fox River and Hudson River sites are two larger sites where the dredged material is being landfilled at commercial disposal facilities.
CDFsconfined disposal facilities are constructed to isolate dredged sediment from the surrounding environment. CDFs can be located upland, near shore, or in the water (as an island). Material staging or a temporary CDF may be necessary for dewatering dredged sediment. USACE (2003) and USEPA (2005a) describe CDFs in further detail. CDFs represent a common disposal method and typically are built for larger volume sites (200,000 yd3 or more of sediment).
The CAD method deposits dredged material within a nearby body of water. A pre-existing depression within the sediment surface is preferred, though one can be created if necessary. Dredged sediment is deposited in the depression and capped with clean material. This process carries with it the same risks associated with using capping as a remedy (see Chapter 5). The goal of moving the contaminated sediment to the aquatic disposal site is to reduce the risk of exposure to contaminated materials (USEPA 2005a). Some sites, such as New Bedford Harbor, are in the process of building CAD facilities. Ease of permitting and long-term management of the disposal site may be considerations in selecting this method, but this additional effort may be warranted for large sediment volumes.
Removed sediment is sometimes treated in order to facilitate reuse prior to aquatic or land disposal. Sediment is treated to meet disposal regulations, to reduce volume to be disposed of, or to facilitate beneficial use. On-site treatment is determined according to the planned subsequent use or off-site disposal method for the material. For a particular site, it may be more economical to treat dewatered sediment on site to stabilize heavy metals, and then transport the treated material to a Toxic Substances Control Act (TSCA)Enacted in 1976 this act requires premarket notification of EPA by the manufacturer of a new chemical. Based on testing information submitted by the manufacturer or premarket test ordered by EPA (including biodegradability and toxicity), a court injunction can be obtained barring the chemical from distribution or sale. EPA can also seek a recall of chemicals already on the market. This act prohibits all but closed-circuit uses of PCBs. compliant landfill for disposal of PCBs. On-site treatment techniques may include dewatering and physical size separation, followed by bioremediation, chemical treatment, extraction/washing, solidification/stabilization, or thermal treatment (USEPA 2005a). Information regarding on-site treatment is also available from the Federal Remedial Technologies Roundtable Technologies Screening Matrix and Reference Guide (FRTR ver 4.0).
If contamination levels, treatment methods, or economics permit, dredged or excavated sediment may be used for beneficial purposes (for example, as construction material for road building). As excavation plans are prepared, local needs should be reviewed and the beneficial use of excavated material should be considered. The potential for reuse of slightly contaminated or treated sediment is dependent upon the assurance that the planned use is protective of the environment and that future activities will not release unacceptable levels of contamination to the environment. The material can be reused either in aquatic or upland sites, depending upon the condition of the material and local needs. Further disposal costs can be avoided by not using a landfill, but additional treatment costs may be involved in making the material environmentally safe for the proposed use. Further information on reuse of sediment can be found in reports from USEPA (2005) and USACE (1987). Although many pilot studies have examined the beneficial use of removed sediment, few field studies are available.
Recently, the New Jersey Department of Transportation’s (NJDOT) Office of Maritime Resources teamed with Rutgers University’s Center for Advanced Infrastructure and Transportation (CAIT) to develop a comprehensive manual for integrating processed dredged material (PDM) into common construction applications (NJDOT 2013). This guide, Processing and Beneficial Use of Fine-Grained Dredge Material: A Manual for Engineers, covers research, development, and implementation of dredged material management techniques.
This section describes the physical characteristics, sediment characteristics, contaminant properties, and land and waterway use characteristics that should be considered when removal is evaluated as a remedial technology. Not all of these characteristics are critical for technology assessment at all sites; however, a thorough review of these characteristics will help to determine whether the removal is suitable for the site and which removal technologies will be most effective and implementable.
Physical site characteristics can determine whether removal is used at a given site, as well as the site zones that may be most promising for removal. In addition, site characteristics may influence how removal can best be accomplished. Inadequate site and sediment characterization for environmental dredging can potentially result in delays, higher costs, unacceptable environmental impacts, and failure to meet cleanup levels and remediation goals.
The data collected must be adequate to either determine whether removal should be selected as a remedy or to design a removal remedy. The timing and staging of the site characterization can also affect results. For example, during the early stages of an RI, there is less certainty as to which of the detected chemicals are COCs that require remediation. Therefore, the scheduling of site characterization often must be adapted based upon new information. These results determine the nature and extent of sediment contamination, inform remedy selection, and support remedial design. At many sites, a multi-phased characterization effort beginning during the RI and continuing into the FS and remedial design stage may be appropriate. The characterization must collect adequate site data to support decisions required during critical stages of the remediation process.
Sediment stability is not critical in the evaluation of removal as a remedial approach. In areas where sediments are unstable, however, natural disturbances would likely lead to significantly increased contaminant mobility and risk. These areas, therefore, may be good candidates for an active remedy such as removal.
The net deposition rateThe amount of material deposited per unit time or volume flow. is not a critical factor in selecting removal as a technology; however, zones with higher net deposition rates may provide adequate natural cover material for post-removal residual sediments. This process makes the installation of a residuals cover unnecessary, since deposition rates greater than 1 to 2 cm/yr provide a 10-cm cover in 5 to 10 years. Residuals cover or backfilling, described in Section 188.8.131.52, is often used at sites when sediment cleanup goals canot be met after a single or multiple passes with dredging equipment. Note that removal can result in creation of depressions in the sediment bed and therefore net deposition rates immediately following removal can be greater than rates prior to removal.
Erosional potential is not critical in the selection of removal as a remedial technology. Zones where erosion of the sediment bed would likely increase contaminant mobility and risks may be good candidates for engineered containment or removal, as long as erosion of dredge residuals are not a concern. Sediments with relatively low bulk density (less than roughly 0.7 gm/cm3 or 44 lb/ft3) or low cohesive strength have a greater potential for resuspension when disturbed during removal, resulting in generated residuals and releases (see Section 184.108.40.206 for more on magnitude of releases observed at completed projects), particularly at sites with high hydrodynamic shear stresses or steeper slopes. The potential for resuspension, which is further discussed in Section 220.127.116.11, should be considered on a site-specific basis when evaluating mechanical and hydraulic dredging options.
Site bathymetryThe measurement of or the information from water depth at various places in a body of water., and water depth in particular, are important for evaluating a removal approach. Generally, removal becomes increasingly more challenging as water depth increases. Removal experience to date has been limited to depths of about 50 ft or shallower; however, removal in water depths up to 75 ft is possible (for instance, using hydraulic dredge equipment with a ladder pump configuration or cable mounted buckets). Removal of contaminated sediment in water deeper than about 75 ft is generally impractical.
Note that as water depth increases productivity can decrease, releases to the water column can increase, and the accuracy of removal can decrease. Physical isolation controls (for example, silt curtains or rigid containment such as interlocking sheet piling) also have practical depth limitations for installation and effective operation (generally limited to about 20 ft of water or less). Mechanical dredges using fixed arm buckets are generally limited to about 20 ft of water unless a long-stick arm is used, which reduces the capacity of the bucket. Alternatively, shallow water can also restrict access for hydraulic and mechanical dredges by not providing sufficient draft for the equipment being used. Water shallower than 3 to 4 ft may limit access and removal to form a channel may be needed to facilitate access. Excavation is generally restricted to zones with shallow water depths (typically less than 10 ft) where the removal area can be isolated and dewatered (such as shoreline excavation or lower flow streams that can be bypassed).
Areas to be dewatered generally must be small enough to accommodate the dewatering operations. Larger areas and deeper water zones may still be considered for excavation in certain circumstances, but special engineering considerations may be needed, which complicate implementation and increase construction duration and cost.
All infrastructure (bridges, pilings, piers, utilities and even shoreline structures) adjoining the removal areas must be evaluated for stability before, during, and after removal. An adequate factor of safety should be built into the assessment. Safety offsets (leaving a buffer between the infrastructure and the removal area) or stabilization measures are often specified to avoid disturbance to the structures. Sediment located under structures such as piers may make removal impractical. For example, hydraulic dredges have limited access, maneuverability, and functionality to set cables and anchors to work around structures. A crane with a cable-mounted bucket has height requirements that can limit access, while fixed arm buckets can provide better accuracy in bucket placement and have the ability to reach under some structures.
Excavation generally poses concerns for shoreline slope and structure stability. Greater concerns for infrastructure integrity arise for deeper excavations, and structures and underwater utilities may limit effective containment, isolation, and dewatering of the removal area. In some cases removal and relocation of infrastructure may accommodate sediment removal, but in other cases moving the infrastructure may not be practical and may preclude sediment removal.
The presence of a hard bottom can limit effective containment during removal (if sheet piling is contemplated), depending on the composition and configuration of the hard bottom. Contaminated sediment overlying bedrock or glacial till may impede some dredging equipment. Contaminated sediment lodged in crevices in bedrock can be impractical to remove.
For hydraulic dredging, the presence of a hard bottom underlying the contaminated sediment limits over-dredging into a relatively clean surface and can also increase the magnitude of generated residuals and undisturbed residuals. For mechanical dredges, the presence of hard bottom typically leads to greater amounts of generated residuals and resuspension, due to over-dredging difficulties and the higher energy required to remove the consolidated underlying material. On the other hand, a hard bottom below contaminated sediment tends to limit over-excavation of material. Attempting to re-dredge residuals on top of a hard bottom using either mechanical or hydraulic dredges has been shown to be less effective in reducing contaminant concentrations, but plain suction dredges can more effectively capture generated sediments and residuals from a hard bottom. Mechanical leverage of an excavator during excavation results in more accurate removal and can remove hard material with less sediment loss.
Both large and small debris can slow some dredging equipment. Hydraulic dredges have inherent limits to the size of material that can be removed and are designed to only pass debris smaller than the diameter of the inlet pipe. As a result, a separate mechanical debris removal operation is often used to clear the area of large debris, logs, boulders, and cables prior to hydraulic dredging. Mechanical dredges are better suited to removing debris prior to sediment removal, but they also have some limitations depending on the specific equipment being used. For example, debris can become lodged in the bucket and allow sediment to discharge to the water body, thereby increasing turbidity. Special equipment may be needed to clear the debris. Debris removal activities, however, may disrupt the sediment structure and promote sediment erosion. In general, the presence of debris tends to result in increased resuspension and generation of residuals and, consequently, reduced production. Zones with extensive debris make removal less effective, and in some cases may make removal impractical.
Excavation techniques can generally accommodate debris removal without an increase in resuspension, release, and residuals.
Hydrodynamic characteristics such as water velocities, water depth changes (tides) and waves can affect the performance of removal operations. Experience has shown that higher water velocities can increase the release and transport of contaminants due to resuspension (both initial resuspension as well as resuspension of generated residuals) and can also affect the implementability of resuspension control technologies. Waves greater than 2 ft, currents greater than 1.5 fps, and fluctuating water levels greater than 3 ft complicate and may limit feasibility of removal and the effectiveness of more conventional resuspension controls like silt curtains.
The use of rigid resuspension containment structures, such as sheet piling, can also cause secondary effects such as flood rise and create the potential for erosion due to channel conveyance constrictions. This effect may also arise adjacent to isolation systems used for excavation. Excavation can be designed to accommodate a range of hydrodynamic conditions to mitigate concerns for resuspension, erosion of residuals, and release of contaminants. The design should consider the potential for containment over-topping events and potential for releases, as well as effects on production rate.
Sloping bathymetry of more than a few percent can affect removal operations. Each type of removal equipment has varying suitability to remove contaminated sediment on a slope. Navigational dredging equipment and operators are usually accustomed to performing removal operations to achieve a relatively flat bottom. Advances in equipment and operational procedures for environmental dredging, however, can now leave a more contoured bottom bathymetry after removal.
Steeper slopes can complicate dredging. These slopes are generally cut using a series of steps or box cuts progressing up the slope. These operations are less efficient and can result in greater removal of cleaner underlying sediments and slower production. A cut slope that is less than the angle of repose of the sediment promotes stability, because a higher angle may cause instability. For certain equipment, such as a cable mounted bucket and horizontal auger hydraulic dredges, slopes present difficulties in positioning and achieving a sloped cut elevation. Mechanical buckets mounted on a fixed arm operate much better on a slope but are typically limited to a water depth of about 20 to 25 ft. Since most mechanical equipment swings in an arc, improvements in slope dredging efficiency can be accomplished with the use of articulating buckets that better align with the slope. Some hydraulic operations rely on removal at the toe of slope to allow targeted sediment to fall or slide into the capture zone of the dredge. This operation can leave residuals on the slope that do not fall or slide into the cut area. Slopes with low factors of safety for stability (less than 1.5) or low undrained shear strengths (less than 20 psf or 1 kPa) can pose higher restrictions on dredging designs and offsets for structures, resulting in additional undredged sediment as well as potential losses during removal.
Groundwater infiltration into the surface water has little impact on hydraulic or mechanical dredging operations and is not a critical factor for selection. High groundwater discharge rates, however, hamper efforts to keep an area dewatered to facilitate excavation. Groundwater discharge rates can be particularly important if deeper excavations are needed to remove the contaminated sediment.
Sediment and pore-water geochemistry1) Science that deals with the chemical composition of and chemical changes in the solid matter of the earth or a celestial body (as the moon); 2) The related chemical and geological properties of a substance. parameters such as TOC, DOC, and POC can affect releases during dredging due to resuspension, as well as influence the management requirements for water generated during dewatering operations. In general, however, these parameters are not critical in the selection of removal as a technology.
One or more sediment properties such as particle (grain) size distribution, bulk density, porosity, water content, Atterberg limits (liquid and plastic limits and plasticity index), organic content, shear strength, and compressibility may influence the feasibility of dredging, dredging production rates, and contaminant losses during removal operations. Sediments with higher liquidity indices (indices greater than about 3 or 4) promote more resuspension, release, and generated residuals (fluid muds) and are more difficult to capture with hydraulic dredges, auger dredges, or mechanical dredging equipment. Plain suction dredges may be better suited for removal of highly liquid sediments.
Highly cohesive material may adhere to hydraulic auger dredges and mechanical dredging equipment, requiring frequent maintenance and slowing production. For excavation, low bearing capacity may pose concerns for supporting removal and transport equipment and for infrastructure (such as roads or support mats); low undrained shear strength may limit the support available for an enclosure and for preserving stable shoreline slopes.
Grain size distribution is not a critical factor in selecting removal as a remedial technology. The grain size distribution of the sediment, however, can be a factor in the selection and design of sediment processing (dewatering) and disposal methods.
Environmental dredging operations can result in some unavoidable contaminant releases. Sediments with a high potential for resuspension, release, and residuals (sediments with undrained shear strengths less than 0.5 kPa or 10 psf or a liquidity index greater than 4) pose concerns in selecting dredging as the remediation technology, particularly for mechanical dredging operations and horizontal auger dredges. Use of cutterhead dredges with articulated dredge heads and low rotational speeds can limit the resuspension, release, and residuals. These sediments can also be difficult to capA covering over material (contaminated sediment) used to isolate the contaminants from the surrounding environment.; therefore, resuspension, release, and residuals associated with removal need to be weighed against capping implementation challenges when selecting the remediation technology for sediments posing high risks. Deep water and high velocities or unfavorable wave conditions also increase the potential for losses. Consequently, the selection of the appropriate equipment is critical. Excavation is generally best for sediments with high potential for resuspension losses or for containing source materials such as NAPL, because losses can be readily controlled.
The presence of NAPL can lead to increased water and air releases during dredging, which may need to be mitigated. Studies have shown releases of 1-4% of the mass of contaminants dredged to the water column (frequently in the dissolved phase) even when resuspension controls are used. Increases in fish tissue concentrations of bioaccumulative COCs (such as PCBs) during dredging and for several years afterward have also been observed at environmental dredging projects.
Losses can be controlled, but not eliminated, by the proper selection of dredging equipment for the geotechnical properties and site conditions. Hydraulic dredges tend to control losses better for soft sediments. Plain suction dredges limit losses particularly for sediments with very low undrained shear strengths (less than 0.3 kPa or 6 psf) or a liquidity index greater than 4. Cutterhead dredges limit losses for sediments having greater strength and lower liquidity. Articulated cutterhead dredges produce lower losses and residuals, and auger dredges perform well for debris-free sediments with low liquidity and moderate shear strength. Mechanical dredges tend to control losses better for stiff and sandy sediments.
Closed buckets for environmental dredging have features to reduce resuspension, but generally do not perform as well as properly selected hydraulic dredges when removing sediments with low undrained shear strengths (less than 1 kPa or 20 psf) or a higher liquidity index (greater than 2.5). Environmental buckets can perform as well as hydraulic dredges for sediments with moderate undrained shear strengths (between 1 and 2 kPa or 20 to 40 psf), particularly in shallow water. Open buckets can perform very well for sediments with higher undrained shear strengths (greater than 2 kPa or 40 psf) or lower liquidity indices (less than 2).
Depending on equipment selection, site-specific geotechnical properties, presence of debris, and hard bottom characteristics, environmental dredging operations can leave behind disturbed residuals.
Generated residuals are estimated to be 1-12% of the mass of contaminants present in the last production pass based on past field measurements. Plain suction dredges, particularly for the cleanup pass, may help to limit residuals. The effects of residuals can be mitigated by placement of residual covers or caps.
Pore-water expression is not a critical factor in selecting removal as a remedial technology; however, it may be an important consideration in the selection and design of sediment processing (such as dewatering) and water treatment prior to discharge.
In general, benthic community structure and bioturbation potential are not critical factors in selecting removal as a remedial technology. These factors can be relevant, however, if rare or sensitive communities are present. Removal of contaminated sediment will remove the benthic community along with its habitat. If rare or sensitive benthic communities must be protected, then removal may not be appropriate.
If removed, benthic recolonization of the dredged surface (and any cover material placed over residuals) may require several years to fully recover all stages of the benthic community. Stage 1 recolonization tends to occur within a few months.
The horizontal and vertical distribution of contaminants influences the applicability of a removal remedy. The site must be characterized sufficiently to specify the areal and vertical extent of the COCs. Characterizing the horizontal and vertical distribution of COCs can aid in determining whether the zone is acting as an ongoing source of COCs to the environment. This parameter is significant for removal because a larger horizontal or vertical extent of contamination results in a longer implementation schedule and higher cost. Relatively higher concentration zones that are well defined (horizontally and vertically) and limited in extent (such as hotspots) are favorable for removal, while zones with a high degree of uncertainty in extent or with COCs that are dispersed are not suited for removal. In addition, areas with lower contaminant levels on the surface (in the bioactive zone) and with higher concentrations at depth can result in residual contamination in surface sediment that is higher in concentration than existed prior to removal.
Understanding the depth of contamination is critical to designing the removal limits and avoiding undisturbed residuals. At some sites, placement of residual covers have been installed immediately following removal. In areas of high variability in COC extent (horizontal or vertical), definition of the removal area can be inadequate because straight-line interpolation of results may not represent the true variability of the contamination. Postdredging sampling may show that additional excavation is needed because of this variability. Excavation requires a well-defined areal and vertical extent of contamination to avoid expensive and time consuming changes during field operations. Additionally, infrastructure must be designed prior to removal (cofferdams, dewatering systems) and changes to that design may not be practical once the area is dewatered. Excavation also induces infiltration gradients and may mobilize contaminants such as NAPL upward into the excavation area.
The mobility and potential risks posed by the contaminant depend not only on the concentration but also on the nature of the contaminants. For example, some metals and low hydrophobicity organics may be far more mobile than hydrophobic organics. The higher mobility can result in increased releases during removal activities. Assessment of the type of contaminant and its relative mobility is moderately important when selecting removal as a technology.
Contaminant type also determines the hazards that might be present at the site. For instance, dry excavation poses the greatest concern for loss of volatiles and air inhalation hazards for workers and the community. The presence of unexploded ordnance or munitions and explosives of concern (UXO or MEC, respectively) may also limit the application of removal technologies due to concerns regarding an unintentional detonation. Standard precautionary measures when UXO or MEC items are discovered are "recognize, retreat, and report."
Contaminant type can also affect available disposal options. The previously mentioned explosives and other types of contamination may require disposal in a specially permitted facility (such as a RCRA- or TSCA-compliant facility). In some cases, contaminants or contaminated media may require treatment prior to disposal.
The level of contamination is not a critical factor in selecting removal as a remedial technology. Well-defined areas with disproportionately higher concentrations and more mobile contaminants (hot spots), however, are good candidates for removal because erosion and re-deposition of high contaminant concentrations may contaminate surrounding areas The identification of removal areas is a site-specific determination and removal should justify the disruption to the ecosystem and community, short-term risks, use of landfill capacity, transfer of risk to the upland environment, implementation time, and costs for the effectiveness and permanence gained. Note that greater resuspension, release, and quantity of residuals are associated with removal of higher concentration areas and risk reduction may be limited, but can be improved with resuspension controls and residuals management.
Removal is compatible with all water exposure pathways, including those influenced by high contaminant mobility, high groundwater advection, NAPL presence, and deep bioturbation as well as hot spots. Areas with lower contaminant levels on the surface (in the BAZ) and higher concentrations at depth can result in residuals with contamination higher in surface sediment than existed prior to removal (increasing surface exposure concentrations). This issue has been addressed at some sites through placement of a residual cover immediately following removal. In addition, removal can create an airborne pathway by volatilization and fugitive dust, as well as other potential upland pathways at the processing and disposal site.
Presence of mobile source material such as NAPL in the sediment is moderately important in the selection of removal as a remedial technology. Each of the removal technologies accommodates NAPL removal in different ways. Hydraulic and mechanical dredging can remove NAPL material to the extent that it is retained by the dredge equipment; however, both can result in release of NAPL to the water column. Resuspension controls can be moderately, but not completely, effective in containing NAPL releases, and methods that work better in containing releases also can create secondary issues. For example, in sheet-piled areas there can be increased residuals and increased air emissions. Excavation can provide better containment and control in the removal of NAPL material, but NAPL present beneath the excavation area may be subject to upward migration during dewatering and can lead to increased air releases.
Contaminant mobility is an important factor in the assessment of potential for resuspension, release, and residuals. Typically, the more mobile contaminants (such as VOCs and BTEX) are not present in sediments. The presence of these contaminants may indicate an ongoing source. More mobile contaminants exhibit higher potential for releases to the water column and air during removal.
Contaminant bioavailabilityThe relationship between external (or applied) dose and internal (or resulting) dose of the chemical(s) being considered for an effect (NRC 2003). is not a critical factor in selecting removal as a remedial technology. Removal in areas with lower contaminant levels on the surface (in the BAZ) and higher concentrations at depth can result in residuals with contamination levels that are higher in surface sediment than existed prior to removal (increasing bioavailable exposure concentrations). This phenomenon has been observed at dredging sites and has been addressed at some sites by the placement of a cover over residuals immediately following removal.
Contaminant bioaccumulationThe accumulation of substances, such as pesticides, or other organic chemicals in an organism. Bioaccumulation occurs when an organism absorbs a toxic substance at a rate greater than that at which the substance is lost. Thus, the longer the biological half-life of the substance the greater the risk of chronic poisoning, even if environmental levels of the toxin are not very high. and biomagnification potential are not critical factors in selecting removal as a remedial technology.
Contaminant transformation and degradation are not critical factors in selecting removal as a remedial technology.
Watershed and ongoing sources must be identified during site characterization and effectively controlled. Ongoing sources can recontaminate treated areas, resulting in significant cost and resource losses. The effects of watershed sources, which are often beyond the control of those implementing the sediment remedy, must be accounted for and considered in defining the extent of sediment cleanup. If removal is the selected remedial technology, then the effects of ongoing sources can also help determine the level of post-removal residuals acceptable in the context of non-site-related releases or nonpoint source releases that will contribute to the future contamination of the site or surrounding sediment.
Significant sources must be identified and controlled in order to justify using removal technologies. When watershed and ongoing sources provide a source of contamination greater than the on-site source, dredging does not significantly reduce risks.
Ebullition is not a critical factor in selecting removal as a remedial technology. Sediments with higher ebullitionThe act, process, or state of bubbling up usually in a violent or sudden display. potential, however, may result in odors that require management during transport, processing, and disposal.
Just as ongoing sources limit the ability of a remedy to achieve objectives, the background levels of a contaminant may also limit the potential for remedy success. It is generally not feasible to sustain remediated sediment sites at concentrations below background levels even if complete removal is achievable. Background inputs should not be allowed to lead to recontamination that would exceed remediation goals.
As with any sediment-focused remedy, the effectiveness of removal can be offset by continued deposition of contaminated sediments to the sediment surface. Deposition of new contaminants can rapidly return the surficial layers to the pre-remedy conditions. The effects of such watershed sources (Section 2.2), which are often beyond the control of those implementing the sediment remedy, must be accounted for and considered in defining the extent of sediment cleanup. Control of watershed sources may require effort by multiple regulatory agencies and stakeholders. Complete control of ongoing watershed sources may not be possible, thus the long-term implications of any continuing source must be assessed.
The presence of cultural and archeological resources should be assessed when considering removal. Because removal operations disturb the ground, these operation may adversely impact cultural and archeological resources. The proper authorities should be consulted to determine the appropriate measures needed during removal operations, which may include a range of actions from removal of the resources along with the contaminated sediment, to recovery of artifacts, to avoidance of the area to protect the resource. A site-specific plan may be developed to address cultural resources. The use of excavation may facilitate more effective identification, documentation, removal, and preservation of cultural and archeological resources.
Site access determines the types of removal equipment that can be deployed and how removed sediment can be handled. In general, some access is needed to bring in labor and equipment for removal operations, possible staging and processing areas, water treatment operations, load-out facilities and disposal areas. Sites with ready access to the water body and ample upland space available in the vicinity of removal are more amenable to removal than sites with limited access to the water and limited upland space.
Site-specific access requirements vary depending on the removal method selected and disposal options available. A removal area or zone that is easily accessible in open water and from shore is favorable for removal. Hydraulic dredging operations typically require a larger staging area if off-site disposal is needed. A dewatering operation may be needed to process large volumes of sediment slurry, using equipment such as filter presses or Geobags coupled with water treatment, before transport to the disposal site. If a local CDF is available for disposal, then the staging area for hydraulic dredging may be reduced. Mechanical dredging generally requires a smaller staging area than hydraulic dredging because less carrier water is generated, but some space is needed for the transport of dredged sediment from the removal area to the processing and disposal location. For excavation, the removal area or zone should be easily accessible from shore, and the processing/staging area may be comparable to mechanical dredging. A safe, efficient means of transporting excavated sediment for disposal should be available, together with a suitable upland staging area.
Current and anticipated waterway uses are important considerations in selecting removal as a remedial technology. CERCLA requires that site remediation achieve a level of cleanup (and residual contamination) consistent with the reasonably anticipated future use of the site. Removal can be combined with other objectives for purposes unrelated to cleanup (such as navigation or construction). Removal viability and extent should account for current and future needs for navigation and infrastructure, including utilities. Removal of sediments can increase water depth, thereby improving navigability in the removal area; however, removal in an active navigation channel can temporarily obstruct navigation and recreational uses during removal operations (see Section 6.3.5).
For hydraulic dredging, submerged and floating pipelines as well as the dredge itself must be coordinated with marine traffic because these facilities may obstruct navigation. For mechanical dredging, barge locations (both material transport and the dredge barge) must be coordinated with marine traffic and with lock and bridge operations. For dry excavation, the isolation structure may also obstruct navigation.
Current and anticipated land use can be important factors in selecting removal as a remedial technology. An access area that is readily available, of adequate size, and compatible with current and future land use is favorable for removal (see Section 18.104.22.168). Upland areas, which may be incompatible with access requirements for removal, make removal less feasible. When evaluating compatibility for staging areas, overhead clearance should be considered. Current and future land use may influence removal design, type of removal equipment that can be deployed, and sediment handling.
Removal of sediment also removes any organisms present in the sediment as well as the habitat it may provide. Any unique or sensitive species and habitats present in the sediment targeted for removal may be removed or disrupted. The extent of impact and disruption must be assessed on a site-specific basis, but is generally directly related to the extent of removal and overall sensitivity of the species and habitats. Engineering controls can be evaluated to help protect surrounding areas, but the removal area is inevitably affected. Removal operations can often be restricted to periods in which endangered species are less prominent or when spawning activities are not occurring. In some completed projects, habitat restoration has been incorporated into the design (such as backfilling to appropriately designed elevation, plantings, and placement of a cover); however, time is needed for habitat recovery, and some habitats may be very difficult to restore. In cases where the risk of habitat loss is great, removal may be avoided or limited in order to protect the resources.
Increased water depths created by removal operations can also affect habitat quality. Removal can be favorable in areas where an increase in water depth does not degrade habitat. Conversely, removal is unfavorable in areas and zones where an increase in water depth degrades habitat (for instance, where removal converts historically shallow water habitat to unwanted deep water habitat).
The selection of sediment removal as a remedial approach should be based on an overall assessment using criteria appropriate for the specific site being investigated. Sometimes, a single site may use hydraulic dredging in some segments and mechanical dredging in another segment, in order to leverage the advantages of each. While CERCLA criteria (or similar) are commonly used to evaluate these approaches, each state may have its own set of evaluation criteria. Generally speaking the criteria fall into the three primary categories: risk, practical considerations, and cost.
Typically, the primary goal of the evaluation process is to select an approach that is permanently protective of human health and the environment, can be readily implemented, and is cost-effective. Often, alternatives developed for a site consist of multiple or combinations of approaches, such as varying degrees of removal, capping, in situ treatment, or MNR in different areas of the site. The evaluation process (Chapter 2) offers a consistent approach for selecting and applying these remedial technologies.
Sediment removal typically requires a higher initial monetary investment than capping or MNR. Therefore, to be cost effective, removal should provide a higher degree of effectiveness, permanence, or implementability than other approaches. When assessing protection of human health and the environment, the overall net risk reduction must be considered, including risks associated with implementing the remedy along with risks remaining after the remedy, as compared to baseline risks. The risks of implementing the remedy typically include resuspension and release of contaminants during removal, air emissions, worker-related risks, traffic risks, and others. Residual risks include risks from contamination that remains after removal activities are completed, such as residuals (both generated and undisturbed), areas not dredged, and inputs from continuing sources. Even when dredged materials are hauled to an off-site disposal facility, relatively large on-site infrastructure may be required for sediment dewatering and pretreatment operations.
Conditions at a site that may support sediment removal as a potentially viable remedy or a remedy component that is favorable for selection over capping, in situ treatment, or MNR include the following:
- zones currently acting as an unacceptable source of contamination to the water column and/or overlying biota (or could reasonably become an unacceptable source in the future)
- zones not reasonably amenable to capping, in situ treatment or MNR, such as navigational channels, high energy, or erosional environments
- isolated zones such as hot spots or high concentration areas, which present a much higher risk among larger areas of lower risk
- zones of contamination with a more mobile contaminant source, such as NAPL, which cannot be adequately contained using other remedial options
- zones with stable slopes along an accessible shoreline that can readily be isolated and dewatered for easier removal
- areas where water depth and other site conditions (such as wind and currents) are suitable for effective control of removal-related resuspension or releases
- sites located where relatively economical options for handling and disposal of the dredged material are available, such as a CAD facility, a CDF, or a local landfill
- removal activities that are acceptable to neighboring businesses and residences
Conditions at a site that are generally not favorable for selection of sediment removal over other technologies include the following:
- large zones with relatively low-concentration contamination, where a low net risk reduction would be expected from a removal remedy
- zones in low-energy (low erosive force) environments, which have low likelihood of resuspending or eroding surface sediments
- zones where higher contamination is buried beneath cleaner sediment, and where a relatively low likelihood exists for the buried contamination to be mobilized under a reasonable future event (such as a 100-year flood) at concentrations that would pose unacceptable risk
- zones with significant debris or shallow sediments resting on rock, which would exacerbate resuspension and residuals resulting in a lower net risk reduction
- zones in sensitive aquatic environments, where removal-related resuspension or releases would be undesirable
- zones that might receive contaminants from continuing sources after sediment removal
- zones that are difficult to access (for example, under bridges or piers with closely spaced pilings)
- deep water depths that may reduce the effectiveness of dredging and resuspension control equipment (such as silt curtains)
- zones that have utilities beneath the contaminated sediment, where damage to the utility may occur
A situation where dredged areas must be remediated again due to continuing sources of contamination should be avoided. Discussions with all parties, including community and tribal stakeholders and watershed management agencies may help resolve recontamination issues prior to large financial commitments. These discussions may lead to a more proactive regional management plan if a sustainable and productive resource can be recovered for use.
This list, while not comprehensive, provides general guidance on zones that may be amenable to removal when compared to capping, in situ treatment, or MNR/EMNR. A risk-based management decision should balance the predicted net reduction in risk, permanence, and implementability against overall costs (both implementation and long-term operation and maintenance costs), and the selection of a remedial technique should only occur following comparative evaluation of all potentially viable remedial techniques.
Dredging operations attempt to achieve protection of human health and the environment through removal of COCs from the aquatic environment. When assessing the degree of overall protectiveness, important considerations include:
- residuals that remain in the bioactive zone after dredging (incorporating any residual management like backfilling or capping dredged areas)
- releases which may cause contamination in nondredged areas
- other short-term impacts (described in this chapter)
- the loss of habitat in removal areas (incorporating time for restoration of such habitats)
- the degree of long-term protectiveness of the final disposition of removed sediments (such as CDF, landfill, or beneficial use)
Compliance with the chemical-specific ARARs is achieved to the degree that removal of sediment from the aquatic environment results in reductions of contaminant concentrations to specific ARAR concentrations. The act of dredging triggers a number of action-specific ARARs, such as Rivers and Harbors Act, Clean Water Act dredge and fill requirements (USACE, state water quality certifications), and depending on the methods used for processing and disposal, many others (NPDES, TSCA, RCRA, DOT, and others). Location-specific ARARs can include wetlands permitting, floodplain permitting, coastal zone management, and National Parks requirements. State historic preservation requirements and requirements under the Threatened and Endangered Species Act also must be considered. Waivers of some ARARs can be considered at some sites.
Dredging remedies attempt to achieve long-term effectiveness and permanence by removing contaminated sediment from the aquatic environment to achieve risk-based goals. These remedies manage the removed sediment in a manner that treats or contains the contamination for the long term. In cases where dredging has been unable to achieve the goals (residual contaminant concentrations are in excess of the goals), dredged areas at some sites have required subsequent placement of clean backfill or an engineered cap.
Dredging operations resuspend sediments, resulting in the release of contaminants. Operational controls and physical containment systems (silt curtains, sheet piling, and air curtains) can be used to reduce the release of contamination from the dredge area, but these controls do not completely eliminate the release. Tools and models are available to estimate those releases and their effects on the environment, and have been used at a number of sites (Hudson River, Fox River). In addition to resuspension, other potential short-term impacts must be considered, including air emissions (from water column releases, sediment transport releases, and dewatering or processing operations). Nuisance aspects such as noise, lighting, and odors should also be considered. Finally, the personnel safety risks associated with the construction, dredging operations, and transportation of sediments should also be considered.
Dredging removes sediment from the aquatic environment and therefore reduces the toxicity, mobility, and volume of contaminants contained in the removed sediment. Residuals and resuspension reduce some of the benefits of removal. Therefore, dredging should be undertaken when removal results in a net benefit and when site conditions are suitable for this approach.
Dredging and disposal services are commercially available for implementation of sediment removal. If the project is large or if specialized equipment is needed at a specific site, however, the availability of qualified contractors, facilities, and equipment must be closely assessed. Areas which present difficult or remote access, infrastructure, marginally stable slopes, shallow water, and sensitive habitats should be reviewed to determine the practicability of removing the sediments. Designers should assess whether the damage incurred to develop access and remove the habitat is warranted. In addition to the removal activities, the availability and proximity of property and facilities for sediment offloading, processing/treatment, and disposal should be reviewed. Permitting requirements for dredging projects can include assessments of rare, threatened and endangered species, cultural or historical resources, and other environmental factors.
Dredging is typically the most expensive remedial approach when compared to less intrusive approaches such as MNR, in situ treatment, and capping. In addition, the uncertainties of the potential costs tend to be higher due to the potential for sediment volume and removal depth to increase once removal operations begin, and uncertainties related to post-removal residuals contingency actions such as backfilling or capping. Typically, a large component of total removal cost is the cost for processing, transport, and disposal of the dredged material. A detailed site-specific cost estimate is vital early in the project (during the FS stage), and should consider all the components of the costs, including dredged sediment handling, long-term monitoring, and maintenance. An uncertainty analysis can be useful when weighing the costs of removal against other options, because many removal projects have experienced higher actual costs than expected. Project managers should consider cost data from other completed projects by incorporating project specific factors and conditions when developing site-specific cost estimates.
At many sites, removal is initially the preferred alternative among stakeholders because it has the potential to permanently remove contaminants from the sediment. Stakeholders should be engaged early in the assessment process and be provided with a full objective assessment of the benefits and costs of a removal approach. See Chapter 8 for additional information on tribal and stakeholder concerns.
Typically, removal requires more intrusive work and more construction equipment than other approaches with resulting consumption of more resources (fuel, energy, labor). The use of low sulfur fuels and biodegradable hydraulic fluids can reduce the potential environmental impacts, but these impacts cannot be eliminated. One important GSR consideration is beneficial use of the dredged material, which can reduce transportation (if re-use is closer to the site than the disposal facility), reduce processing needs (if less dewatering or processing is required for placement), develop usable land (for example, for a CDF), and minimize the use of commercial landfill capacity. ITRC (2011b) offers additional GSR guidance in Green and Sustainable Remediation: A Practical Framework.
Dredging removes existing habitat in the areas where removal is required and may also disrupt habitats in order to develop access and processing/handling facilities. Best efforts can be used to replace the habitat destroyed by the removal operations (if replacement is possible), but habitats need time to recover (in some cases, decades). Also, the removal of existing, mature habitats can make areas more vulnerable to invasive species infestation. These adverse effects should be examined along with the benefits to assess whether removal may result in more damage than benefit.
Developing an appropriate scope for monitoring a sediment removal remedy is best done on a site-specific basis. This section outlines the monitoring elements to consider when developing the scope of a monitoring program for a contaminated sediment removal project (see Table 6-1). Construction monitoring is typically conducted during remedy implementation. Operational monitoring is performed during sediment removal and post-remedy implementation. Performance and long-term monitoring aid in determining remedy effectiveness.
Water monitoring is typically used to provide data regarding resuspension and release of contaminants during removal operations.
Locations. Monitoring locations can include near-field and far-field monitoring. Near-field monitoring includes the immediate vicinity of removal operations and far-field includes locations further downgradient of operations at key monitoring points (beyond mixing zone, upstream of water intake, or upstream of confluence with receiving waters). The objectives for monitoring each location may be different and help to define the appropriate monitoring location.
Near-field locations may be used to provide ongoing feedback on the dredging operations. For example, turbidity is often monitored near the dredging operation to assess the effectiveness of silt curtains. Far-field locations may have a different purpose, such as monitoring contaminant levels to assess impacts of the removal operation on water quality (comparison with water quality criteria) or to protect a water supply intake.
Parameters: Parameters to be monitored can include field measurements (such as turbidity, dissolved oxygen, pHA measure of the acidity or alkalinity of a solution, numerically equal to 7 for neutral solutions, increasing with increasing alkalinity and decreasing with increasing acidity. The pH scale commonly in use ranges from 0 to 14., and temperature), physical parameters (such as total suspended solids and TOC), and chemical parameters (such as COCs and ammonia). The parameters can be different for the various locations.
Post-remediation monitoring evaluates the effectiveness of contaminated sediment removal in reducing or eliminating exposure and risk. At many sites the reduced or eliminated risk eventually results in a decreasing trend in tissue concentration of exposed organisms.
Physical and chemical monitoring is typically used to verify that removal has been adequately completed.
- Physical Monitoring. A physical survey, bathymetric survey, or both are often used during sediment removal operations to verify that removal has been completed in the target areas and that depths specified in the design have been reached. When the design objective is to both dredge and backfill a targeted volume of sediment, bathymetric surveys become the primary indicator that the removal operation is complete. Physical inventory of the volume or mass of sediment dredged can confirm completion of the target dredging. When sediment removal is designed to be followed by backfilling dredged areas with clean material, there is greater reliance on physical measurements. These measurements include bathymetry (depth) and dredged sediment inventory (volume), to establish performance.
- Chemical Monitoring. When the design objective is to dredge only (no backfilling), chemical monitoring verifies that concentration-based chemical goals have been achieved, that the exposed sediment does not pose an unacceptable risk, and that the dredging is complete. Sampling and analysis of sediment residuals remaining after removal operations is generally required for the chemicals of concern. The residual concentrations can be compared to cleanup goals established for the site to determine whether dredging is complete and to determine whether some additional measures are necessary (such as re-dredging or backfilling). When dredging is designed to be followed by backfilling with clean fill, chemical monitoring of the dredged area becomes less important.
ASTSWMOAssociation of State and Territorial Solid Waste Management Officials’s Sediment Focus Group has prepared a framework for long-term monitoring (ASTSWMO 2009) which describes monitoring of a sediment site, particularly long-term monitoring following a remedy. ASTSWMO recommends that decision rules for long-term monitoring should include site-specific criteria to continue, stop, or modify the long-term monitoring, or recommend taking an additional response action. The main elements of such a decision framework are likely to be the parameters of interest; the expected outcome; an action level; the basis on which a monitoring decision will be made; and monitoring decision choices (USEPA 2004). ASTSWMO recommends that the long-term monitoring strategy and decision framework be established early in the process of remedy selection, preferably in the FS discussion of various alternative remedies. The time required to attain long-term monitoring objectives under various alternatives should be clear to participants and stakeholders.
Long-term monitoring is required to determine whether the removal actions continue to effectively mitigate exposure and continue to meet site specific RAOs. The emphasis of long-term monitoring depends on whether RAOs are framed in terms of sediment concentrations or biota tissue concentrations. If the latter, then long-term monitoring typically includes testing the benthic infaunal community or collecting fish tissue samples to determine whether levels meet or are on a trend to meet RAOs. Depending on the exposure endpoint, other species (such as piscivorous birds or mammals) may be tested to evaluate the possibility of ongoing exposure. When residuals remain, chemical monitoring of pore waterfrom near surface sediments may be conducted to evaluate the potential for contaminant flux entering the water column at unacceptable levels. Bathymetry surveys can confirm that backfill remains in place.
Determine whether the established performance metrics for remedy implementation and construction are being met.
Performance: Determine whether the remedy has been successful in reducing concentrations of COCs in sediment to acceptable levels (RAOs) defined in the remediation decision documents, and whether specific criteria (such as cap thickness or dredge depth) have been achieved.
Effectiveness: Determine whether concentrations in affected media continue to meet RAOs (or continue on a decreasing trend expected to meet RAOs) and involve monitoring fish to determine whether tissue levels meet (or are expected to meet within some established time frame) the RAOs that are protective of human health as well as piscivorous birds and mammals.
Benthic infaunal survey
Air monitoring is sometimes conducted if air emissions during removal are expected to be of concern.
- Locations. Typically locations are selected upwind and downwind of operations of concern (for example, removal, transport of sediment, or processing of sediment) to assess potential net impacts from removal operations. Local meteorological data, such as wind speed and direction, are also used to in selecting appropriate monitoring locations.
- Parameters. The parameters to be monitored are determined based on the air emission concerns identified during remedy selection and remedy design. In addition, the type of sampler selected is based on the parameters to be measured and the required sensitivity of the measurements.
If water generated during sediment removal and processing must be discharged back to a waterway (or to a POTW), then monitoring of the water discharge must be considered. Typically, the specifics of this monitoring (location, frequency, and parameters) are determined on a site-specific basis in consultation with the agencies providing regulatory oversight.
Numerous sediment removal case studies, at different stages of completion, were reviewed for this document and are summarized in Tables 6-2 and Table 6-3. A summary of dredged material handling at sediment remediation sites is provided in Table 6-4. In many of these case studies, mechanical dredging was either used alone or in conjunction with other removal methods. A combination of mechanical and hydraulic dredging or hydraulic dredging alone was used at other sites.
At some sites, mechanical dredging was conducted dry, in a sheet pile enclosure that had been dewatered, sometimes aided by a bypass pump to divert water away from the enclosure. At other sites, both dredging and excavation were conducted on different segments of the same site. Compared to the studied hydraulic dredging sites, most of which were relatively shallow (water depth less than 20 ft), at least five of the mechanically dredged sites reported water depths of greater than 20 ft, indicating that water depth may be a factor in technology selection.
At most sites where detailed volume-of-dredged-sediment information was available, more sediment was actually dredged than planned. The reasons varied, resulting from later discovery of additional areas of contamination, multiple dredging passes, or events when confirmatory samples indicated that project cleanup goals had not been achieved. Several sites studied reported success in meeting postdredging cleanup goals without backfilling. Two of these successfully dredged sites were in relatively deeper water (water depth greater than 20 ft). At two of the successful sites, one in shallow and one in deeper water, cleanup goals were framed as surface weighted average concentrations (SWACs). Nearly half of the sites studied used backfill with clean material after dredging to help meet cleanup goals. At the Fox River and Green Bay OU 2 to OU 5 backfilled sites, the cleanup goal had been framed as a SWAC. Area average cleanup concentrations were also used for surface and deeper sedimentduring dry excavation at the Housatonic River site.
Experience shows that mechanical dredging can be effective for areas that contain large debris, where dredging will occur in small or confined areas, or where dredged sediment must be transported by a barge to a disposal or treatment facility. Production rates for mechanical dredges are typically lower than those for hydraulic dredges when sized for a given project. Mechanical dredges were often selected for dredging projects in confined areas such as areas near docks and piers. Mechanical dredges provided one of the few effective methods for removing large debris and are adaptable to land-based operations. As expected, mechanical dredging captured less water with the sediment, as compared to hydraulic dredging. While dependent on sediment composition, minimal dewatering was generally required for mechanically dredged material before treatment and transportation for disposal. As a result, mechanical dredging often required smaller staging areas for on-shore support operations, compared to hydraulic dredging, which limited effects on current land use.
For mechanical dredges, a conventional clamshell dredge (crane with a cable-suspended bucket) has been shown to work well in the field with sediment that is easy to penetrate. These dredges can remove thin or thick faces of sediments effectively. Backhoes can be used for removing contaminated sediments when more conventional buckets are less effective. Field experience also shows that backhoes can be used when debris is present that would prevent conventional clamshell buckets from closing. Backhoes are often considered when there are hard bottoms or the sediments are more consolidated and are harder to penetrate. When sediments have high shear stresses or contain stiff clays or highly cohesive sediments, they can reduce a clamshell’s ability to penetrate the sediment. If the clamshell performance diminishes, then backhoes may be a better alternative. A clamshell bucket mounted on a backhoe arm has sometimes been used to dredge stiff sediment. Backhoes are normally land based, but may be operated from a barge; however, their use is predominantly in shallower depth channels rather than deep draft channels. Backhoe excavators also have better location control and accuracy over the penetration depth since they can use the mass of the equipment and the rigid arm to achieve the required depth in more consolidated sediments.
Mechanical Dredging Site Experience
Mechanical dredging has a relatively slower production rate, but has been particularly useful at sites with stiffer sediment and/or sites that are spatially difficult to access (such as near piers or wharfs). Mechanical dredging has also been used in the field as a first step to clear debris and prepare for faster higher production hydraulic dredging.
Mechanical dredges with clamshell buckets suspended by wires have some difficulty in digging slopes since they tend to “stair step” the slope whereas backhoes can more neatly dress the slope. Clamshell buckets can have difficulty on steep slopes where the bucket tends to fall over or slide down the slope. Since mechanical dredging is often slower than hydraulic dredging, the effects of shoaling, deposition, or erosion on the removal operation are more likely and warrant consideration. Typically removal does not begin until after the source of contamination has been eliminated. Therefore, any shoaling or deposition during operations is most often clean sediments and can readily be considered during design and planning.
At the sites studied, sediments that were more consolidated and required some cutting action to dislodge were particularly suitable for mechanical dredging. Additionally, mechanical dredging was better suited for higher precision dredging, such as when working around in-water infrastructure or when removing small deposits. Often, a safety setback was used around such structures to reduce the risk of undermining or damaging the structure. Additionally, sediments did not always behave as expected, so in order to reduce the risk of slope failure or bank instability, mechanical removal sometimes included buffers. Challenges encountered during mechanical dredging at these sites tended to include the need for management of residual contamination left behind after dredging and resuspension control during dredging.
At the Messer Street site, the flexibility of the dredge operator to change the dredge type and vary depth of in-river operation demonstrates that mechanical dredging is one of the most adaptable sediment removal methods in environmental dredging.
At the sites described in Table 6-3, the primary advantages of hydraulic dredging over mechanical dredging (and the reasons for its selection) were higher production rates, less resuspension of fluid sediment, and more efficient transportation of solids in a single step from the dredge site to the on-shore processing area. A hydraulic dredge and slurry pipeline system eliminates the need for transfer of material from the dredge to barges, which reduces energy use, noise, and vessel traffic, and keeps the sediment contained. At suitable sites, these are substantial advantages. Where hydraulic dredging is at a disadvantage relative to mechanical dredging is in its limited ability to handle adverse site conditions, such as sediment with large debris or proximity to infrastructure (such as sediment under piers, between pilings, or closely overlying bedrock). The larger volume of water generated that typically requires treatment is another disadvantage of hydraulic dredging.
As with mechanical dredging, the physical characteristics of the sediment in its native environment are important factors in the selection of the hydraulic dredging, dewatering, and disposal equipment. The smaller hydraulic dredges used in environmental applications are capable of removing relatively soft to medium stiff sediment. Larger hydraulic dredges used in navigation applications are capable of removing very stiff sediment, but may have higher mobilization costs. Hydraulic dredges are not suited for dredging in areas with debris larger than the diameter of the pump impeller inlet or the hydraulic cutter clearance.
Hydraulic Dredging Site Experience
Hydraulic dredging offers the potential for a higher production rate at sites that are suitable (for example, sites without significant debris or stiff sediment). Sediment dredged with this method typically has a higher water content and may require larger staging areas, in part to support more extensive dewatering operations.
At suitable sites, such as the New Bedford Harbor site, a major advantage of hydraulic dredging was that the dredge pump could transport sediment to a reasonably distant discharge point on shore. To facilitate pumping over larger distances, however, considerable water was entrained with the sediment, compared to other sediment removal methods. Dewatering was a significant effort and cost driver at hydraulic dredging sites and a large volume of excess water often was treated before discharge or reuse.
Smaller hydraulic dredges appear to have worked well in relatively shallow waters that may have been inaccessible to larger hydraulic dredging equipment (or to barges with mechanical dredging equipment). Standard hydraulic dredges can operate in water depths of 30– 50 feet and special modifications or equipment (such as a ladder pump) may be included in dredging at greater depths (not common at the sites studied). Larger or specialty hydraulic dredges could be economical when large volumes of sediment need removal, whereas a relatively shallow cut over a large area can make a larger dredge inefficient.
Hydraulic dredging appears to have been used primarily at relatively shallow sites, with water depths reported at 20 ft or less for all of the sites studied. Many of the sites used hydraulic dredging in conjunction with capping or MNR. For the sites evaluated, capping and MNR were usually conducted for the less contaminated areas surrounding the dredged sediment.
Dewatering was a major operation at hydraulically dredged sites (Table 6-4) because the sediment was retrieved with higher water content (to keep the solids fluidized during pumping). Dredged material was often pumped large distances to be dewatered in isolated cells, coffer dams, filter Geotubes, hydraulic separation, filter presses, or on-site CDFs. Following dewatering, the dredged material was transported by road or rail to an appropriate landfill. Sometimes the sediment had to be stabilized on site with fly ash or cement before transport. At Formosa Plastics, hydraulic dredging was replaced with mechanical dredging because of severe on-shore limitations in conducting the required dewatering operation.
At some sites, excavation may offer better control over the dredging-related risks of resuspension and release of contaminated sediment with the use of proper enclosures. Six of the sites summarized in Table 6-1 were excavated after draining the overlying water column in a sheet pile enclosure. At two more sites, both wet dredging and excavation were conducted on different segments of the site.
If appropriate for the site, excavation can be less costly than dredging if land-based transportation infrastructure can facilitate better access and more timely removal operations. Typically, draining the water column above near-shore sediment provides easier access to underlying sediment at the sites where excavation is conducted. In the case of the Brookhaven Lab, Peconic River site in Upton, NY, near-shore sediments were removed by terrestrial excavators and placed on barges, hauling trucks, or railcars. The sediments were transported to transfer points, landfills, treatment sites, or designated reuse sites. The Housatonic River case study also illustrates the use of excavation.
Publication Date: August 2014