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ALTERNATIVES FOR CONTROL/TREATMENT
OF CONTAMINATED DREDGED MATERIAL
M. John Cullinane, Jr., Daniel E. Averett, Richard A. Shafer
Clifford L. Truitt, and Mark R. Bradbury
U.S. Army Engineer Waterways Experiment Station
and
James W. Male, University of Massachusetts
ABSTRACT
As concern over dredging and disposal of contaminated
sediments increases, unconfined open-water disposal of
dredged material from harbors and navigation channels is
closely scrutinized by state and local governments and numer-
ous federal agencies. Although control of potential contami-
nant release from dredging and disposal of contaminated sedi-
ments is a relatively new concern, practical methods are
available for handling such materials in an environmentally
sound manner. Alternative technologies for dredging, trans-
port, and disposal of contaminated dredged material are
reviewed in this paper. Cont~mi~nant control/treatment
during three basic operations are discussed: dredging,
material transport, and disposal operations.
BACKGROUND
Because many contaminants become attached to sediment particles,
concentrations in sediment are generally much greater than in water.
As the concern over dredging and disposal of contaminated sediments
increases, unconfined open-water disposal of dredged material from
harbors and navigation channels is being closely scrutinized by state
and local governments as well as numerous federal agencies. This paper
presents recent concepts and technologies for handling contaminated
dredged material
CONTAMINANT CONTROL DURING DREDGING OPERATIONS
Dredge Selection
During dredging operations all dredge plants disturb bottom
sediment, creating a plume of suspended solids around the dredging
221
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222
operation. Limitations may be placed on levels of suspended solids
even during normal dredging operations (Lunz et al., 1984~. Contami-
nated sediment may release contaminants into the water column through
resuspension of the sediment solids, dispersal of interstitial water,
or Resorption from the resuspended solids. Control of sediment resus-
pension during dredging reduces the potential for release of contami-
nants and/or their spread to previously uncontaminated areas.
Selection of dredging equipment and method in general depends on
the following factors: physical characteristics of material to be
dredged, quantities of material, depth, distance to disposal area, phys-
ical environment of and between the dredging and disposal areas, con-
tamination level of sediment, mobility of contaminants, method of dis-
posal, production required, and type of dredges available. Dredging of
contaminated sediments requires the additional consideration of con-
taminant loss during the extraction process and meeting of applicable
criteria pertaining to removal efficiencies and/or environmental
protection.
Different dredging methods appear more appropriate for certain con-
taminant classes. For volatile contaminants, mechanical dredges are
likely to produce less loss than hydraulic dredges. Soluble contamin-
ants can be removed more efficiently by a hydraulic dredge, but are
difficult to control at.the disposal site and treatment of the effluent
water may be required.
Equipment and Operational Controls
Hopper Dredge Operation
The rate of solids loss in the overflow (which may determine if
overflow is acceptable) will vary with amount of water in the hopper,
hopper capacity and drainage characteristics, material characteristics
(settleability), pumping rate, and elapsed time of overflow. Reduction
of sediment resuspension can be accomplished by reducing the flow rate
of the slurry being pumped into the hopper during the latter phases of
the hopper-filling operation, reducing the solids concentration in the
plume by reducing the sediment concentration in the overflow. By using
this technique, the solids content of the overflow can be reduced by as
much as 50 percent while the loading efficiency of the dredge is simul-
taneously increased. In extreme cases, pumping past overflow may be
prohibited. Another approach is a submerged discharge system for hop-
per dredge overflow, called an antiturbidity overflow system (ATOS)
(Ofuji and Naoshi, 1976~.
Cutterhead and Suction Dredge Operation
Concentrations of suspended sediments from a cutterhead dredging
operation range from 200 to 300 mg/liter near the butterhead to a few
mg/liter at 1,000-2,000 ft from the dredge. Resuspension of sediment
during butterhead excavation is dependent on the operating techniques
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223
used. The sediment resuspended by a Butterhead dredge depends on thick-
ness of cut, rate of swing, and cutter rotation rate (Barnard, 1978~.
Proper balance of these operational parameters can decrease sediment
resuspension while having little or no effect on production (Hayes et
al., 1984~. Modifications to Butterhead and suction dredges have
improved their production capabilities and reduced dredged sediment
resuspension. Greater production rates are achieved by pumping a
higher solids concentration, reducing the quantity of return water that
may be contaminated and require treatment. Recent modifications in-
clude matchbox heads, walking spuds, ladder pumps, flow and density
instrumentation, underwater video and sensor equipment, shape of the
Butterhead, and rake angle.
Dust Pan Dredges
Dust pan dredges are not well suited for dredging contaminated
materials. However, when used in this application, the angle of the
water jets on the head and the water pressure from these jets should be
adjusted to achieve the minimum amount of sediment resuspension.
Special Purpose Dredges
Special-purpose dredging systems have been developing during the
last few years in the United States and overseas to pump dredged mate-
rial slurry with a high solids content and/or to minimize the resuspen-
sion of sediment. Most of these systems are not intended for use on
typical maintenance operations; however, they may provide alternative
methods for unusual dredging projects, such as contaminated sediments.
Clamshell Bucket Dredge Operations
Resuspension of sediments during clamshell dredging operations can
be reduced by implementing operational controls and/or altering the
bucket design. Operational controls can be applied to hoist speed,
placement of the dredged material in the hopper barge, loading the
hopper past overflow and dragging the bucket along the bottom. Equip-
ment design includes the fit of the bucket and the use of enclosed
clamshell buckets. Watertight buckets have been developed in which the
top is enclosed so that the dredged material is contained within the
bucket (Barnard, 1978 ~ . Comparisons between standard open clamshell
bucket and a watertight clamshell bucket indicates that watertight
buckets generate 30 to 70 percent less resuspension in the water column
than the open buckets. The enclosed bucket did, however, produce
increased resuspension near the bottom, due to a shock wave that
precedes the watertight bucket.
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224
Additional Control Techniques
Several additional techniques and/or considerations have been sug-
gested to assist in controlling resuspension and contaminant release
during dredging. Typical control techniques that are commonly eval-
uated include silt curtains; barriers, such as dikes, weirs, and sheet
pile enclosures; and operational controls, such as dredging only during
a specific time in the tidal cycle. Success with these has been varied
and their application is very site specific.
CONTAMINANT CONTROL DURING TRANSPORT
Primary transportation methods used to move dredged material in-
clude pipelines, barges, scows, trucks, and rail. The primary emphasis
during this phase of the overall dredging process is toward spill/leak
prevention. Accidental release of contaminated materials into a pre-
viously uncontaminated environment has extremely costly consequences in
monetary and public relations aspects. Thus, each step in the trans-
port system must be carefully evaluated.
Controls for Pipeline Transport
Pipelines are commonly used to transport bulk materials over rela-
tively short distances. During the design stage, planners should care-
fully consider pipeline routes, climatic conditions expected, corrosion
resistance of the material, redundancy of safety devices (i.e., addi-
tional shutoff valves, loop/by-passes, pressure relief valves), cou-
pling methods and systems to detect leaks. Souder et al. (1978) out-
lines specific pump and pipeline design procedures.
Controls for Scow/Barge Transport
Barge/scow transport of dredged material has historically been one
of the most used methods to move large quantities over long distance.
Controls to prevent spread of contaminated materials when utilizing
barge transport are primarily concerned with loading/unloading proce-
dures, fugitive emissions, route and navigation hazards, and decontam-
ination of equipment.
Loading and unloading operations present the greatest potential for
uncontrolled release of contaminated materials. Use of clamshell and
dragline attachments at the dredging site will release substantially
more dredged material into the water column than vacuum/suction sys-
tems. However, when planning for pumping dredged material into barges,
planners should consider how the material will be transferred from the
dredge onto a barge. Overflow during such operations can cause a sig-
nificant return of contaminants to the water column. Flexible connec-
tions from dredge to barge will reduce the possibility of pipe damage
due to wave action. If the dredged material is tremied into the barge,
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225
then movement of the boom between barges or dredge and barges must be
carefully controlled to prevent material from falling directly into the
waterway.
Controls for Truck/Rail Transport
Trucks are used for dredged material when the distance from the
dredging site is beyond the range normally used for overland pipelines
and less than the distance for rail car transport (> 50 to 100 ml).
Controls associated with transporting dredged material by truck/rail
parallel those for barge/scow transport. Primary concerns include
weight restrictions, routing, and loss in transport, loading, and
unloading operations. Loading and unloading operations present the
greatest potential risk of contaminating nearby clean areas. Controls
suggested for consideration are drainage of water from loading and
unloading area into central sump for periodic removal, daily removal of
spilled material, specially designed loading ramps to collect spilled
material, use of watertight clamshells for transferring materials from
barges into truck. Decontamination of truck/rail under carriages may
be necessary to control contaminated materials from falling onto public
roadways when leaving loading/unloading areas.
CONTAMINANT CONTROL DURING UPLAND/NEARSHORE DISPOSAL OPERATIONS
Six categories of contaminated media may be associated with the dis-
posal of contaminated sediment. These include dredged material slurry,
dredged material solids, site effluent, site runoff, site leachate (in-
cluding flow-through dikes), and residual solids.
Upland disposal of contaminated dredged material must be planned to
contain the dredged material within the site and restrict contaminant
mobility out of the site in order to control or minimize potential envi-
ronmental impacts. Francingues et al. (1985) identified and described
five possible mechanisms for transport of contaminants from upland dis-
posal sites:
1. release of contaminants in the effluent during dredging
operations,
2. surface runoff of contaminants in either dissolved or suspended
particulate form following disposal,
3. leaching into ground water and surface waters,
4. plant uptake directly from sediments, followed by indirect
animal uptake from feeding on vegetation, and
5. animal uptake directly from the sediments.
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TABLE 1 Site Characteristics Affecting the Need for Control/Treatment
Technologies
Site area
Site configuration
Dredging method
Climate (precipitation,
temperature, wind, evaporation)
Soil texture and permeability
Soil moisture
Topography
Drainage
Vegetation
Site Selection
Depth to aquicludes
Direction and rate of
groundwater flow
Existing land use
Depth of groundwater
Ecological areas
Drinking water wells
Receiving streams (lakes, rivers,
etc.)
Level of existing contamination
Nearest receptors
Site Control Strategies
Site location is an important, if not the most important, consid-
eration in minimizing the cost of required restrictions. Selection of
a technically sound site can reduce or eliminate the need for applying
contaminant control/treatment technologies. Site characteristics that
may affect the need for, or type of, treatment/control are listed in
Table 1.
Covers
Covers are control measures designed to seal or isolate the surface
of contaminated dredged material from physical, chemical, or biological
processes that could release contaminants from a confined upland or
nearshore disposal site. Surface covers can be as simple as a 1- to
3-ft thick layer of clean dredged material or as complex as a multi-
layer cap that includes impermeable membranes, filters, gas channels,
biobarriers, and top soil. Functions of a cover could include one or
more of the following:
prevent or minimize surface water infiltration,
promote aesthetics,
reduce water erosion and dissolution of contaminants in surface
water runoff,
reduce wind erosion and fugitive dust emissions,
contain and control gases and odors,
provide a surface for vegetation and/or site reclamation, and
prevent direct bioturbation (human and animal)
Since these functions address all of the migration pathways (i.e.,
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TABLE 2 Normal Duration of Surface Water Diversion and Collection
Measures
Technology
Duration of normal use
Dikes and berms Temporary
Channels (earthen and CMP) Temporary
Waterways Permanent
Terraces and benches Temporary and Permanent
Chutes Permanent
Downpipes Temporary
Seepage ditches and basins Temporary
Sedimentation basins Temporary
Levees Temporary
Floodwalls Permanent
SOURCE: U.S. Environmental Protection Agency (by. EPA, 1985~.
surface water, groundwater, air, and direct contact), some type of
surface cover will likely be a component of any upland or nearshore
disposal system.
Surface-Water Controls
The overall objective of surface water controls is to minimize the
volume of water that becomes contaminated via contact with the contami-
nated sediment. Surface-water controls accomplish this objective by pre-
venting surface water runon from areas adjacent to the disposal site,
by draining the disposal site efficiently to reduce infiltration and
leachate generation, and by preventing erosion and sediment loss from
the cover of the site. Surface-water controls also aid in collecting
and transferring water that may be contaminated to treatment or dis-
posal systems. Surface-water control methods are well established and
are familiar to the engineering and construction industry. Lee et al.
(1985a) provides a detailed discussion of management practices of U.S.
Army Corps of Engineers (COE) construction sites. Table 2 lists
surface-water control measures and their duration of use at disposal
sites (U.S. EPA, 1985~.
Groundwater Controls
Liners
Lining a site is a technique designed to contain leachate within
the site and minimize groundwater contamination. A variety of liner
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materials are available for use in confined disposal operations. Soil
liners are suitable for use as the only liner in most dredged material
upland and nearshore sites. However, in certain upland applications, a
combination of synthetic membrane and soil liner may be required to
achieve maximum containment of contaminants. To ensure continued effec-
tiveness of the liners whether soil or flexible membrane, they must be
compatible with the dredged material and leachate they are to contain
and be properly installed (Phillips et al., 1985~.
Groundwater Recovery
Groundwater recovery technologies are usually considered as reme-
dial actions where sites containing hazardous materials have released
contaminants to the groundwater. Control of groundwater contamination
involves one of four options:
1. containment of a plume;
2. removal of a plume after measures have been taken to halt the
source of contamination;
3. diversion of groundwater to prevent clean groundwater from
flowing through a source of contamination or to prevent con-
taminated groundwater from contacting a drinking water supply;
or
4. prevention of leachate formation by lowering the water table
beneath a source of contamination.
Ideally, adequate site investigation and installation of appropriate
controls at a newly selected disposal site will prevent groundwater
contamination and hence the need for groundwater controls. The reader
is referred to other documents such as U.S. EPA 1982a and 1985 for more
detailed information.
Leachate Collection
Disposal sites for dredged material must accommodate the inter-
stitial water associated with the sediment, dilution water that may be
mixed with the sediment by the dredging operation, and precipitation or
other sources of water added to the disposal area surface. A leachate
collection system is usually a network of perforated pipes placed under
and around the perimeter of the site. The pipes drain to a sump or
series of sumps from which the leachate may be withdrawn either by grav-
ity, if topography allows, or by pumping. Spacing and sizing of the
pipes depends on the allowable leachate head in the site and the rate
at which water must be removed. Detail design of a collection system
for leachate control is described in U.S. EPA, 1985.
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229
Dewatering
Two mechanisms exist for dewatering and densifying fine-grained
dredged material using pervious underdrainage layers: gravity under-
drainage or vacuum-assisted underdrainage. The gravity underdrainage
technique consists of providing free drainage at the base of the
dredged material. Downward flow of water from the dredged material
into the underdrainage layer takes place by gravity. Vacuum-assisted
underdrainage is similar to gravity underdrainage, but a partial vacuum
is maintained in the underdrainage layer by vacuum pumping.
Site Security
Any time contaminated sediment is being dredged, transported, or
disposed, site security for the protection of safety and health of the
public and of workers must be addressed. In addition to the time when
the site is being filled, site security must be considered for the time
after disposal is completed. The extent of security measures will de-
pend on the nature and concentration of contaminants, the migration
pathways affected by the contaminants, the risk to humans and wildlife,
and future use of the site. For unusual conditions, where justified by
the risk presented by the nature and location of the site, a site-
specific safety plan may be developed in accordance with guidance pre-
sented in EM 1110-1-505 (U.S. Army COE, 1986~.
Treatment of Dredged Material Slurries
Solids Separation and Classification Processes
The objective of separating solids from slurries is to attain two
distinct waste streams: a substantially liquid waste stream that can
be subsequently treated for removal of dissolved and fine suspended
contaminants, and a concentrated slurry of solids and minimal liquid
that can be dewatered and treated. The most appropriate solids separa-
tion method for a given site depends upon several factors, including
the following:
volume of contaminated solids;
composition of sediment, including gradation, percent clay, and
percent total solids;
types of dredging or excavation equipment used, which determines
the feed rate to solids separation and, in the case of slurries,
the percent solids; and
site location and surroundings.
Types of available solids separation equipment includes settling
basins, clarifiers, impoundment basins, screens, and cyclones.
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Solidification/Stabilization
Solidification and stabilization are terms which are used to des-
cribe treatment that accomplishes one or more of the following objec-
tives (U.S. EPA, 1982b): improves waste handling or other physical
characteristics of the waste, decreases the surface area across which
transfer or loss of contained pollutants can occur, limits the solubili-
ty or toxicity of hazardous waste constituents. Methods involving
combinations of solidification and stabilization techniques are often
used (U.S. EPA, 1982b; Cullinane and Jones, 1985~.
Thermal Destruction Processes
Thermal destruction is a treatment method that uses high tempera-
ture oxidation under controlled conditions to degrade a substance into
products that generally include gases, vapors, and ash. The most
common incineration technologies applicable to the treatment of dredged
material slurries include rotary kiln, fluidized bed, and multiple
hearth. Because of the cost of incineration and the extremely low fuel
value of most dredged material slurries, it is doubtful that thermal
destruction technologies would ever be an economically viable option
for treating dredged material slurries. However, projects involving
small volumes of highly contaminated material may be candidates for
application of thermal destruction technologies.
Treatment of Dredged Material Solids
Dredged material solids are those solid materials remaining after
initial or final dewatering of the dredged material slurry. Treatment
of the dredged material solids can be accomplished before or after
placement in a disposal area. Conceptually, dredged material solids
can be treated with 8 variety of technologies. Among these are inciner-
ation, solidification/stabilization, extraction, immobilization, degra-
dation, attenuation, and reduction of volatilization. Incineration,
although a demonstrated technology for organics destruction is believed
to be far too costly for the treatment of contaminated dredged mate-
rial. In addition, the technology has limited application for treating
dredged material solids contaminated with heavy metals. Solidifica-
tion/stabilization technologies have been demonstrated at the field
scale for hazardous wastes and at the laboratory scale for dredged mate-
rial. However, this technology has not been proven for the containment
of organics or in the marine environment. The remaining technologies
are in various stages of development for application to hazardous waste
sites and, although they may have some potential for application to
dredged material solids, are many years away from being demonstrated
technologies.
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Treatment of Site Waters
A variety of physical, chemical, and biological processes have been
developed for municipal and industrial water and waste treatment
requirements. Many of these processes have potential in treating site
waters generated by the disposal of contaminated dredged material at
confined nearshore and upland disposal sites. However, few processes
have actually been required or applied to dredged material disposal.
Among the processes widely applied in confined disposal operations are
plain sedimentation for solids and sediment-bound contaminant removal,
and chemical clarification and filtration for enhanced removal of par-
ticulate (suspended solids) and sorbed metals and organics. Use of
activated carbon for removal of soluble organics has received some
limited application to dredged material. Other processes not previous-
ly applied to dredged material include organics oxidation, dissolved
solids removal methods (e.g., distillation), and volatiles stripping.
A comparison of the relative efficiencies of the treatment levels is
given in Table 3.
TABLE 3 Contaminant Removal Efficiency of Water Treatment Levelsa
Class of
Level contaminant
Percent Water concentration
removal remaining
I Solids 99.9+ mg/liter range
Metals 80 to 99+ ppb to ppm ranged
Organics 50 to 90+ ppb to ppm rangeb
II Metals 99+ ppb rangeb
Organics 50 to 90 ppb to ppm ranged
III Metals 99+ ppb range
Organics 95+ ppb range
IV Nutrients 90 to 98+ mg/liter range
V Metals 99+ highest quality attainable
Organics 99+ highest quality attainable
VI Pathogens 90 to 99+
NOTES:
aAssumes influent strength defined by dredged sediment that are not
classifiable as "extremely hazardous waste" under RCRA (i.e., low
saturation influents).
bConcentrations based on capability of best-available treatment
technology.
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Reuse of Contaminated Dredged Material
Reuse has been proposed as a potential alternative for long-term man-
agement of contaminated dredged material. Reuse of contaminated dredged
material serves at least two beneficial functions: continued use of con-
fined sites located close to dredging areas and creation of a potential
construction material resource. The concept of a reuse alternative may
also incorporate beneficial uses of materials such as sand and gravel
reclaimed by classification/separation processes. The development and
evaluation of reuse alternatives is extremely site specific and will de-
pend on several factors: physical and chemical characteristics of the mate-
rial to be dredged, availability of temporary storage and/or treatment
sites, and identification of long-term disposal sites or suitable bene-
ficial uses.
CONTAMINANT CONTROL/TREATMENT FOR RESTRICTED OPEN-WATER DISPOSAL
Restricted open-water disposal as used here simply suggests that one
or more controls beyond those normally applied in conventional projects
are required to address either known risks or uncertainties associated
with disposal of contaminated sediments. Most positive control measures
are based on the concept of isolating the contaminants from the water
column or benthic environment. Recently, concepts based on either the
separation of contaminants from the dredged material slurry or chemically
stabilizing the contaminants in the dredged material have also been pro-
posed.
Site Characteristics as a Control Technology
A level of increased control or restriction can be achieved during dis-
posal simply by taking advantage of the best features of the site, by con-
sidering natural mixing processes, and by using conventional techniques
and equipment to their best potential. At least six considerations can be
identified that are important in evaluating the engineering acceptability
of a proposed open-water disposal site: currents (velocity and structure),
average water depths, salinity/temperature stratifications, bathyme try
(bottom contours), dispersion and mixing, and navigation and positioning
(location/distance, surface sea state, etc.~.
Engineered Control Technologies
Submerged Discharge
The use of a submerged discharge or closed conduit of some type to
place dredged material is a second level of restriction or control avail-
able. In general, a conduit is used primarily to ensure more accurate
placement of the material and to reduce the exit velocity during formation
of the surge phase. A conduit extending from the surface to the bottom
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233
will isolate the material from the water column during descent, reduce
entrainment, and negate the effects of currents or stratifications. Sub-
merged diffusers have been successfully field tested in the Netherlands at
Rotterdam Harbor and as part of an equipment demonstration project at Calu-
met Harbor, Illinois (Hayes et al., 1984~. The diffuser minimizes upper
water column impacts, and especially improves placement accuracy, and con-
trols sediment spreading, reducing benthic impacts.
Some hopper dredges have pump-out capability by which material from
the hoppers can be discharged like a conventional hydraulic pipeline
dredge. In addition, some have further modifications that allow pumps to
be reversed so that material can be pumped down through the dredge's ex-
tended dragarms. Because of the expansion at the draghead, the result is
similar to use of a diffuser section.
Lateral Confinement at the Site
An increased degree of positive control over the movement of the mate-
rial placed at a site can be achieved by using lateral barriers to confine
the disposed material. Such confinement can be accomplished by using
depressions or contour irregularities existing at a site, by excavating
such depressions, or by construction of subaqueous dikes. Lateral con-
finement addresses the short-term benthic impact by ensuring accurate
initial placement and attenuation of the spreading dredged material. It
also addresses long-term benthic and water column impacts by providing an
inherent degree of isolation from the aquatic environment, reducing the
effects of convective currents, and increasing the ease and effectiveness
of capping when used.
Capping
Capping is simply the addition of a layer of some type of material
over the mass of dredged sediment at the disposal site to effect isolation
from the environment. The long-term impacts associated-with soluble diffu-
sion, convective transport, and bioturbation are reduced when a capping
control measure is used. Physical stability of the disposal mass over
time is also increased by capping, although short-term instability may be
a concern if capping material is applied too rapidly over weak underlying
dredged material.
Phillips et al. (1985), using the technologies discussed previously,
described five conceptual designs for restricted open-water disposal
sites: deep-water mound, deep-water confined, shallow-water mound,
shallow-water confined, and waterway confined. The general features of
these concepts are shown in Figure 1.
Dredged Material Treatment and Open-Water Disposal
Restricted open-water disposal is necessitated by the presence of
contaminants associated with the sediment. On a mass basis, these
