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9
Environmental Issues
INTRODUCTI ON
Dredging and the disposal of dredged materials have the potential to
cause physical and biological effects, and this potential,
particularly when sediments removed by dredging are contaminated by
toxic substances, has raised concerns about the environmental effects
of dredging and disposal. Among the potential physical effects,
implied in Chapter 8, is that a dredged channel or maneuvering area
represents a change in the geometry of a tidal body of water, and
local circulation and other patterns of flow are sensitive to such
changes. Dredging and disposal activities directly disrupt
bottom-dwelling communities; remove sediments from the bottom that may
have collected toxic and other hazardous materials from upstream
runoff and discharges; and transfer these sediments to other areas,
with the possible consequence of mobilizing and dispersing the
associated contaminants. These represent the potential physical and
biological effects of greatest concern.
A great deal of research has been undertaken in the past decade to
improve our understanding of the actual physical, biological, and
public health implications of dredoino and the disposal of dredged
materials.
-
_ _
_ _ This chapter reviews the accumulated knowledge and what it
suggests for existing and future policies adopted to protect the
marine and coastal environment, living marine resources, and public
health.
. . . .
. . .
SEDIMENTS
Deposits of sediments found within most ports can be divided into two
primary classes: deep sediments, typically representing the major
fraction forming the lower layers of the sediment column and known to
have been in place for times that are long compared to the local
history of industrialization; and surficial sediments, the more mobile
fraction, found at or near the surface of the sediment column and
typical of incoming sediments. The latter group includes the
materials of primary concern for most dredging projects: The rate of
deposition of surficial materials governs the extent and frequency of
117
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118
maintenance dr edg ing .
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119
downstream transport dominates, and maximum flux of sediments is
associated with periods of peak streamflow. As a result, sediment
distributions in this region often display significant temporal
variability and relatively high degrees of sensitivity to the
placement and orientation of fixed structures. This sensitivity has
been used to reduce the downstream flux of sediment through the
construction of dams or similar sediment-retention structures, or to
flush materials from piers and mooring areas.
In the estuarine region, transport routes display both spatial and
temporal variability in response to varying streamflow and tidal
conditions. In most estuaries, mixing of fresh and salt waters
produces density distributions favoring net seaward movement of
near-surface waters and their suspended loads, and a corresponding net
landward displacement of near-bottom water and associated suspended
sediment. This circulation system favors retention within the estuary
of a large percentage of the solids that can settle (introduced either
upstream or within the adjacent offshore), with maximum deposition
occurring in the vicinity of the "null zone," or area in which the
near-bottom downstream movement of river water encounters the upstream
flow of seawater (Ippen, 1966~. This convergence results in a
significant reduction in horizontal velocity and favors an increased
rate of deposition of suspended sediments. Changes in riverflow,
tidal flow, or cross-sectional geometry lead to a relocation of the
null zone. The positioning of port facilities relative to this null
zone represents an important determinant governing the frequency of
dredging required to maintain desired depths. Consideration of this
factor often provides at least partial explanation for the substantive
difference in the dredging frequency required to maintain one port as
compared to another despite both having apparently similar flow and
sediment supply characteristics.
The combination of factors affecting sediment transport within
coastal port facilities favors establishment of a controlling channel
depth representing a condition of equilibrium between flow-associated
transport energy and sediment supply. Dredging to increase water
depth beyond the controlling values disturbs this equilibrium by
modifying the flow regime and generally causes an acceleration in
deposition rates to force the system's return to equilibrium. With
the characteristic controlling depth for the majority of the U.S. port
facilities equalling 30 ft (10 m) or less, maintenance of the federal
navigational channels to depths approaching 45 ft (14 m) typically
requires dredging to establish the desired depth followed by a
continuing cycle of maintenance dredging to maintain channel depth and
to counter accelerating deposition as the system attempts to regain
equilibrium.*
*Significant in-channel forces may also be generated by the vessels
themselves, particularly larger vessels. The effects of all in-
channel forces for sedimentation can be estimated with a model and
local data (Hochstein, 1980~.
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120
Annually, dredging activities in the United States result in the
removal of approximately 300 x 106 cubic meters of sediment. The
largest-volume operations are in the southern states, where sediment
yields are high because deep weathering produces a deep soil profile,
and along the Mississippi River (Figure 7, Appendix G). The majority
of these operations are classed as routine maintenance intended to
remove deposits of surficial sediments. As a result, the displaced
materials are dominantly clays and silts with lesser amounts of sand,
and a moderate to high water content and organic fraction (Figure 8,
Appendix G).
Approximately 20 to 25 percent of these materials are disposed of
in ocean or ocean-fringing sites. The remainder is deposited within
or adjacent to project areas or at less proximate inland sites.
Dredged sediment dominates the materials dumped in the oceans of the
United States (see table below). Along several areas of the
continental shelf with large estuaries, the disposal of dredged
materials represents the dominant mechanism for transporting sediments
from the continent to the oceans (Goldberg, 1975; Gross and Palmer,
1979).
Ocean Dumping in the U.S. in 1983.
Waste Type
Amount
(103 tons)
Dredged material
Industrial wastes
Sewage sludge
Construction debrisa
Solid wastea
Explosivesa
Wood incin.
Chemical incin.a0
Total73807.5
awhile no materials in this
category were dumped in 1983,
they have been in prior years.
SOURCE: Dredged Material: U.S.
Army Corps of Engineers. All
other materials: U.S. EPA
65,160
304.5
8,312
o
o
o
The fine-grained nature of the majority of surficial sediments, in
combination with their sedimentation history and associated exposure
to the variety of anthropogenic inputs discussed can cause the chemi-
cal composition of this fraction to differ significantly from the
deeper sediments and the more general average crustal materials (see
Table 20, Appendix G). The variations in constituent concentrations,
above those produced by natural inputs, can display significant
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variability in both quality and quantity, and can be expected to be
highly site-specific.
In general, concerns about dredging and the disposal of dredged
materials center on elevated concentrations of selected
trace-elements, principally cadmium, mercury, and lead, and the
synthetic organics, with recent emphasis on the polychlorinated
biphenyls (PCB) and polynuclear aromatic hydrocarbons (PAH). Other
constituents of concern include the nutrients, phosphate, nitrate, and
ammonia, oil and grease, pathogenic microorganisms, and on occasion,
the sediment itself. Because of the relatively large volumes of
surficial sediments being dredged, the presence of elevated levels of
these constituents prompted more stringent controls on dredging and
disposal and the initiation of a variety of field and laboratory
studies to assess the range of potential effects and to establish
procedures to mitigate adverse effects.
DREDGING PROCEDURES
With the increasing incidence of sediment contamination by toxic
compounds, a variety of advanced dredging systems has been developed.
Mechanical systems employing closed buckets and hydraulic systems
using skirted horizontal augers in shallow water and pneumatic pumps
in deeper areas have been used, in combination with a variety of
electronic, microprocessor-based, control and monitoring arrays, to
dredge highly contaminated materials both in the U.S. and abroad.
Studies have indicated that such systems have the potential to effect
significant reductions in the turbidity associated with dredging while
providing increased removal efficiency relative to the more
conventional systems (Herbich and Brahme, 1983~. Although such
systems are finding general application in selected areas, notably
Japan, their use is not widespread, and the majority of available
dredges are "classic" or well-established systems. This situation
appears to be the result of the conservative character of the dredging
industry (Linssen and Oosterbaan, 1978~; uncertainty about the future
needs for advanced dredging techniques and the availability of the
required funding; and the acceptability of conventional dredges for
most projects. As detailed in Chapter 8, congressional action
instructing the Corps of Engineers to increase the percentage of
federal projects contracted to private firms stimulated the
development of more modern, high-efficiency hopper dredges; similar
improvements could be made in other dredging technologies if they were
considered necessary.
DREDGED MATERIAL DISPOSAL PROCEDURES
Since the enactment of the variety of laws favoring reduction in the
use of the ocean as a receiving area for wastes in the 1970s, the
management philosophy governing disposal of dredged materials has
emphasized selection of sites and procedures so as to minimize the
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dispersion of sediments discharged at offshore sites and to reduce the
leakage of particulates and associated contaminants from alongshore
containment sites. This containment policy was intended to (1)
minimize the area in which adverse effects might occur; (2) complement
evaluations of the adverse effects; and (3) permit possible future
removal of the materials if the effects proved unacceptable. The
selection of this protocol did not represent a universally held value
judgment that in all cases containment was to be preferred to
dispersal. The relative merits of containment versus dispersion
remain a matter of continuing discussion (see, e.g., Rhoads, et al.,
1978; Kamlet, 1981~.
Satisfaction of the containment policy has been a continuing
consideration in the selection and ultimate use of dredged material
disposal areas. In the Great Lakes, this policy (as embodied in the
River and Harbor Act of 1970), and consideration of the composition of
the dredged materials and the chemical environment characterizing the
open-water disposal areas, has resulted in the essential elimination
of open-water disposal in favor of diked containment areas. Within
the marine coastal region, diked structures are increasingly
employed. Facilities are now in use for several ports, including
Norfolk and Baltimore, and additional units have been proposed for
Long Island Sound (U.S. Army Corps of Engineers, 1979~.
In contrast to the care exercised in the design and specification
of diked containment areas, procedures for their operation, and
procedures for the selection and designation of ocean disposal sites
appear haphazard. Prior to passage of the Ocean Dumping Act,
approximately 160 sites were used for the disposal of dredged
materials within the open coastal waters or inner continental shelf of
the United States. The majority of these sites are on the Atlantic
and Gulf coasts (see following table). Positioning and selection of
Regional Distribution of Disposal
Volumes and Sites
,
Total Volume Ocean Dumped (100m~)
1976 1977 1978 1979
.
Atlantic 18 11 17 12
Gulf 24 10 15 36
Pacific 8 11 8 8
Total 50 32 40 56
Number of Active Dumpsites
1976 1977 1978 1979
-
Atlantic 28 20 23 20
Gulf 20 18 23 16
Pacific 24 25 21 14
Total 72 63 67 50
From: Kamlet, 1983.
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these sites was with few exceptions a simple function of proximity to
the project area. Minimizing project costs favored locating disposal
sites close to the dredging project. In 1977 (following enactment of
the Ocean Dumping Act), the Environmental Protection Agency reduced
the number of ocean sites from 160 to 127 (subsequently increased to
131), issued interim designations for each site, and initiated a
series of investigations that was intended to lead to final
designations (if appropriate) for the sites. With few exceptions, the
sites retained their historical positions on the assumption that
extending the effects of direct dumping to previously unused areas was
unjustified in the absence of more detailed data.
The site-designation process for ocean disposal remains unfinished
today, and the majority of the sites retain their interim
designation. Owing to a series of legal settlements (Kamlet, 1983)
and interagency agreements, the Environmental Protection Agency is
committed to the timely completion of the designation process at 29
sites and has recently proposed a protocol to be used during these
evaluations (Bierman and Reed, 1983~. No final completion date has
been established for the remaining sites.
Throughout the period of site designation, the disposal of dredged
materials at open-water ocean disposal sites has continued. In the
absence of site-specific data detailing dispersion and other important
environmental characteristics, the operational criteria followed by
the Corps of Engineers primarily emphasize the accurate placement of
dredged material within the boundaries of the designated site.
Procedures employing precision navigational systems (including
loran-C) have been incorporated within routine disposal operations and
detailed bathymetric surveys have been initiated at several sites to
monitor the results. These survey data indicate that for the case of
scow discharge of mechanically dredged materials, the consistent
release of sediments at designated navigational coordinates or
adjacent to a defined dumping buoy can produce coherent deposits of
dredged material at specified points in the disposal area (see Figure
9, Appendix G). Similar results can be achieved with hydraulically
dredged materials discharged from hopper dredges. Placement accuracy
tends to degrade progressively for pipeline discharge of muds because
of increasing water content or sediment fluidization and associated
increased potential for dispersion. For coarser materials, however,
even pipeline discharge can result in coherent placement of dredged
materials.
The availability of precision navigation and high-resolution
acoustic profiling systems permits the management of ocean disposal
sites to a degree not previously attainable. In combination, these
systems allow sequenced placement of dredged materials at a number of
specified points within the disposal area, avoiding development of
prominent mounds or shoals, and permit quantitative determination of
the amounts of materials actually placed within the disposal site
during a given project. These data allow estimates of the volume of
material loss (but not necessarily of contaminant loss) occurring
throughout the dredging and disposal operation and during the
immediate post-disposal period as the materials settle and become
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compacted. Such calculations assist both engineering and
environmental determinations, and in addition, provide a measure of
surveillance which serves to discourage the "short-dumping" or
"off-site" disposal practices that were common prior to 1970.
Finally, the development of precise placement procedures and
associated follow-up surveys promises to provide a means of reducing
the potential for biotic exposure or contaminant release from
contaminated dredged materials by allowing placement of a clean
"cover" or "cap" of sediments over these materials. This procedure is
discussed in a succeeding section ("The Disposal Area".
ENVIRONMENTAL EFFECTS
A large number of investigations have been carried out in the last 15
years to assess the environmental effects of dredging and
dredged-material disposal. These include (1) the Marine Ecosystems
Analysis (MESA) Program initiated in 1974 by the National Oceanic and
Atmospheric Administration (NOAA) with particular emphasis on the
disposal of wastes (including dredged materials) in the New York Bight
and lower New York Harbor (Ecological Stress..., 1982~; (2) the
Dredged Material Research Program (DMRP) a five-year, $30-million
program mandated by Congress specifically to study the effects of
dredging and the disposal of dredged materials, and to develop
improved dredging systems and alternative disposal schemes (see U.S.
Army Corps of Engineers, 1980, for publications list); (3) a variety
of site-specific studies of dredging and the disposal of dredged
material often associated with the preparation of a required
Environmental Impact Statement (EIS), and studies by individual
divisions and districts of the Corps, such as the Disposal Area
Monitoring System (DAMOS) sponsored by the New England district to
permit continuing environmental evaluations of the active disposal
sites in the region (Science Applications Intl., 1984~.
Reviews of the literature resulting from these investigations
provide reasonably clear indications of the short-term effects of
dredging and disposal activities, but often raise as many new
questions about long-term effects as they provide answers for old
ones. The data suggest that it is possible, using existing equipment
and procedures, to design and carry out a dredging project in which
the short-term effects are both minimal and acceptable. Specification
of the associated long-term effects is more difficult. This body of
information provides a useful first-order picture of the range of
environmental effects associated with dredging and disposal processes
and serves to highlight the areas needing further elaboration to
complement environmental management.
THE DREDGING S ITE
Of the large number of studies intended to detail the environmental
effects of dredging and disposal, a relatively small percentage have
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focused on the dredging site itself. The studies that have been
conducted in this area have placed primary emphasis on the extent and
character of the sediment resuspension induced by dredging and the
influence of these materials on local pelagic fish populations, or the
benthic community found in the areas adjacent to the channel being
dredged, or both. Additional studies have examined the effects of
dredging-induced resuspension on local water quality, with particular
emphasis on the release of particulate-associated contaminants, and
have detailed the extent to which dredging affects local circulation
and sediment transport by modifying channel depth and cross-sectional
characteristics. Data from these studies provide a basis for the
development of quantitative predictive models.
Both mechanical and hydraulic dredging operations introduce
significant quantities of sediment into the water column immediately
adjacent to the operating dredge. For mechanical operations in areas
of moderately fine-grained cohesive sediments, concentrations of
suspended materials adjacent to the dredge have been observed to
exceed background levels by more than two orders of magnitude, as
shown in Figure 10 (Appendix G). Similar variations have been
observed adjacent to an operating Butterhead dredge with
concentrations varying as a function of the size and relative
production of the dredge (Figure 11, Appendix G). Hopper dredge
overflows appear to have the potential to produce the maximum
perturbation of suspended material: observations at several locations
indicate concentrations adjacent to the overflow port in excess of 100
gm/l, or more than five orders of magnitude above background (Figure
12, in Appendix G).
The materials suspended by the operating dredge are distributed
downstream by the local transport field, and display concentrations
varying as a function of mass-settling properties, free-stream
velocity, and associated turbulent diffusion characteristics.
Observations indicate that for representative estuarine conditions,
this combination of factors favors rapid deposition of the resuspended
materials. The sediment plume represents a relative near-field
feature displaying characteristic longstream spatial scales of less
than 2000 m (see Figures 13 and 14, in Appendix G. for example).
Comparisons between distributions observed at a variety of sites, and
for several different dredge systems, indicate clear similarity and
have permitted development of reasonably accurate predictive modeling
requiring only definition of the initial concentrations adjacent to
the dredge and an estimate of free-stream diffusion and particulate
settling velocities (Cundy and Bohlen, 1980~. These models have
proved useful for evaluation of the potential effects of dredging.
In addition to the solid particulate phase, the operating dredge
also directly and indirectly alters the concentrations of dissolved
nutrients and selected trace elements within the waters in the
immediate vicinity of the dredge. Studies of these constituents
indicate elevated concentrations above background within an area
representing approximately 30 percent of the total suspended material
plume. Over the remaining area of the plume, dilution and particle
scavenging favor a return to background levels (Tramontano and Bohlen,
1984).
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126
The limited spatial extent of the suspended material plume produced
by the typical estuarine dredging operation effectively limits the
associated effects to areas immediately adjacent to the operating
dredge. Within this region, the elevated suspended material
concentrations serve to (1) increase turbidity, which reduces the
penetration of light and associated photosynthetic activity; and (2)
provide a continuing supply of sediment for deposition along and over
adjoining benthic areas. The potential effects associated with these
material concentrations appear to be limited by a combination of
factors. Within the water column, the effects of particulates on the
drifting biotic community, including zooplankton--although difficult
to evaluate--are considered negligible because of the limited area
affected and the characteristically short exposure time. For the more
mobile, free-swimming organisms, potential effects are further reduced
by their ability to avoid the resuspension area. The benthic
biological community not affected directly by dredging can be affected
by the rain of resuspended sediments. The rapid settling of these
materials serves to confine the primary effects to the immediate
vicinity of the operating dredge, resulting in zones of influence
having characteristic spatial scales ranging from 100 to 1000 m2.
The deposition of suspended sediments within this area affects
particularly the filter-feeding organisms, including several species
of commercial value such as oysters, scallops, and blue mussels. The
extent and character of the effects varies as a function of the
concentration levels of suspended sediments, sedimentation rate, and
exposed species. Persistent concentrations in excess of 2 gm/1, or
deposition sufficient to produce deep burial (~20 cm), or both, can
prove lethal to a majority of benthic organisms. Such conditions,
however, exist only within the areas immediately adjacent to the
operating dredge where the effects are generally negligible compared
to those induced directly by the bucket or hydraulic intake. Beyond
this area, over the undisturbed region flanking the dredged channel,
the increase induced by dredging in suspended material concentration
over background seldom exceeds 100 mg/1, typically representing a
potential deposition of less than 1 cm. In these conditions, the
ultimate effects become primarily a function of the tolerance of the
exposed species. Epifaunal suspension feeders such as oysters and
mussels display maximum sensitivity. A variety of investigations has
shown that these organisms as adults can tolerate suspended material
concentrations in the range of 100 to 1000 mg/1 over reasonably short
exposure times and that on occasion such exposure can serve to
stimulate pumping activity and increase growth by increasing nutrient
supplies (Lunz, 1938; Loosanoff and Tomars, 1948; Loosanoff, 1961;
Stern and Stickle, 1978~. Nevertheless, persistent exposure to high
concentrations of suspended sediments, or shallow burial (<1 cm), or
both, is generally lethal (Kranz, 1974~. For the larval and juvenile
stages of these organisms, effects appear to be negligible at
concentrations below 200 mg/1, and slowly increase to critical at
approximately 750 mg/1 (Davis and Kidu, 1969~. Although such
concentrations occur only in the immediate vicinity of the dredge, the
degree of uncertainty in the available data on the biological effects
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127
of those concentrations appears sufficient to justify the management
practices applied in many areas limiting dredging activity during the
critical spawn-and-set periods of the commercially valuable species of
shellfish. Restrictions based on finfish sensitivity, however, appear
to be seldom justified, except perhaps if the channel and dredge
occupy a large fraction of the waterway's cross-section, and the
waterway is a major passage for migrating species.
An additional factor often limiting the environmental effects of
dredging is the natural degree of variability in the sediment
transport system of the majority of shallow-water lakes, estuaries,
and coastal embayments, as well as inland waterways. In many
estuaries, near-bottom concentrations of suspended material vary by
more than an order of magnitude over each six-hour half-tidal cycle
(where there are semi-diurnal tides) as fine-grained organic and
inorganic materials are alternately suspended or deposited in response
to the varying tidal velocities (Meade, 1972~. Over longer periods,
the suspended material field within each of these systems will be
perturbed aperiodically by short-term, high-energy events sufficient
to increase concentrations by several orders of magnitude above
background. Such events display a typical recurrence interval of less
than twelve months and often represent the primary determinant
governing the flux of sediments to a given system and through it.
Less-frequent events can have major effects on coastal sedimentary
systems. The effects of tropical storm Agnes on the sedimentary
system of Chesapeake Bay present a particularly clear illustration of
the potential of these less-frequent, aperiodic events (Schubel, 1974;
Zabawa and Schubel, 1974~. Perturbations occurring over a range of
temporal scales will each tend to affect significantly larger areas
than those affected by routine dredging operations. This factor, in
combination with the amount of sediment displaced by events suggests
that against such perturbations, the system-wide influence of sediment
suspension produced by dredging will generally be negligible (Bohlen,
1980).
In addition to the variety of relatively short-term effects,
dredging operations may induce a number of longer-term effects
associated primarily with modifications in local circulation and
sediment transport following changes in channel depth and
cross-sectional area. These effects are most likely to be significant
within estuarine areas, where altered channel contours can increase
the degree of salinity intrusion and alter vertical mixing, leading to
a modification in the density structure and associated gravitational
circulation, and causing repositioning of the areas of maximum
sediment accumulation (Simmons and Brown, 1969~. Changes in mixing
and gravity circulation can also affect the distribution of dissolved
oxygen and other water-quality parameters.
The relationships between changes in channel geometry and changes
in circulation and channel shoaling have been detailed in a variety of
investigations (Harleman and Ippen, 1969~. The investigations
indicate that while modification in channel configuration has the
potential to alter local circulation characteristics, the physical
effects can be predicted with reasonable accuracy using appropriate
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equivocal nature of the effects that can be directly associated with
all but the most toxic materials complicates justification of these
added costs. Moreover, success in achieving and maintaining total
leachate control has been marginal.
If a terrestrial containment site is used, it must be chosen
carefully and should not be located in an unsuitable area such as atop
an aquifer, in a wetland, or in an area of high runoff (Gordon et al.,
1982~. To the extent possible, the soil beneath the site should be
predominantly fine-grained material to ensure a chemical capacity to
adsorb and bind contaminants to particles, and be of high porosity and
low permeability.
The best strategy for the disposal of contaminated dredged material
is one that contains the particles, confines the contaminants to the
particles and isolates the deposit and associated contaminants from
plants and animals, and particularly from man. These conditions can
perhaps be approached most closely by burial beneath the seafloor
(Bokuniewicz, 1983), under a cap of clean sediment (Morton, 1983~. All
the major elements of a subaqueous burial operation have been
demonstrated in the field including the intentional construction of a
compact deposit (e.g., Morton, 1983; Bokuniewicz 1982) and the
successful capping of fine-grained dredged sediment under a sand cap
(e.g., Morton 1983; O'Connor, 1982~. Indeed, a small operation to
bury contaminated dredged mud in a submarine pit under a sand cap has
been successfully completed (Sumeri, 1984~. Available field studies
and continuing laboratory tests indicate that the caps are apparently
effective in containing contaminants (O'Connor, 1982; Brannon et al.,
1984~. Although the limiting criteria for a successful burial
operation are not well known, a successful large-scale operation could
be carried out so long as the conditions, materials, and techniques
are not significantly different from those of the capping operations
that have already been completed. Before the burial options could be
routinely used in a wide range of conditions and materials, however,
generally applicable criteria need to be developed concerning, for
example, the spread of dredged sediments along the seafloor during the
discharge process, the geotechnical conditions that allow capping, and
the migration mechanisms of specific contaminants.
It is probably neither possible nor appropriate at this time to
conclude categorically that either upland containment or subaqueous
disposal is universally preferable for the management of contaminated
dredged materials. As was pointed out by a Corps of Engineers
scientist (Engler, 1981) following the DMRP, "containment of highly
contaminated or toxic dredged material (at an upland disposal site)
...can be an environmentally sound and preferred alternative,but
cannot be categorically considered better than (other management or
disposal techniques)...."
The best, most appropriate, choice of a subaerial or a subaqueous
disposal site will vary with the quantity and quality of material to
be disposed of, the characteristics of the terrestrial and aquatic
environments in that region, the uses society makes of these
environments, and the availability of sites.
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Open Water Sites
The placement of dredged materials in open-water disposal sites has
the potential to induce a variety of short-term, acute, and
longer-term, chronic environmental effects. The short-term effects
are confined to the period of disposal and result primarily from
direct burial of marine organisms or their exposure to increased
concentrations of suspended materials, trace elements and other
contaminants, and nutrients. The majority of these effects can be
reduced or eliminated by proper site selection and project timing.
Studies of longer-term effects have considered rates of recolonization
and the character of the subsequent biological community, variations
in contaminant body burdens within these organisms, reproductive
success, and a variety of sublethal but persistent effects, such as
alterations in genetic structure. This latter set of effects is by
far the most difficult to assess, and consequently, is the least well
known.
As in the case of dredging-induced resuspension, a number of field
studies have shown that the open-water disposal of dredged materials
by hydraulic pipeline or hopper barge produces increases in
suspended-material concentrations that are short-lived, and that the
primary effects of these short--lived increases are confined to the
immediate vicinity of the discharge point. During hydraulic placement
of materials by an outfall pipe, suspended-material concentrations
vary as a function of mean grain size and production rate, with values
decreasing rapidly with distance downstream. Typically, the perturbed
suspended-material concentrations return to background within
approximately 2000 m of the point of discharge (Figures 10 and 12 in
Appendix G), and within a few hours after the discharge operation ends.
The discharge of materials from a hopper or scow creates a
descending jet of sediment with a trailing wake of entrained waters
and suspended particulates (Figure 15 in Appendix G). The
water-column distributions of these latter materials will vary as a
function of the sediment mass characteristics, particularly the degree
of cohesion, and for water depths in excess of 100 m or so, the
density structure of the water column. On impact with the bottom, a
fraction of the descending mass will be redirected upwards, and an
additional volume of sediment will be introduced into suspension from
disturbance of the bottom. The energies associated with the
combination of descending and ascending sediments then slowly
dissipate and the cloud of materials settles toward the sediment-water
interface. In water depths of approximately 20 to 50 m, this process
typically results in a well-defined pile of sediment having a conical
core and displaying symmetrical axial dimensions equal to
approximately 30 percent of the water depth (Gordon, 1974~.
Investigations have shown that the distributions of suspended
sediments resulting from both hydraulic discharge and barged disposal
can be predicted reasonably well by analytical models (Koh and Chang,
1974; Wilson, 1979~.
The sediments suspended during disposal operations have the
potential to produce the same range of effects as sediments
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132
resuspended by the operating dredge. Although the potential is
greater, the majority of the effects produced by ocean disposal of
dredged material are considered negligible, except in areas dominated
by sensitive species such as corals, or filter-feeding organisms such
as oysters, clams, and mussels. Efforts are generally made in the
selection of disposal sites to avoid sensitive areas, including those
that support submerged aquatic vegetation and significant
concentrations of commercially important shellfish.
The direct burial of the variety of benthic organisms resident
within the disposal area represents the primary short-term
environmental effect of dredged material disposal in open water. With
few exceptions, organisms buried during large-volume disposal
operations will not survive, resulting in nearly azoic conditions on
completion of the project. Colonization of the dredged-material pile
begins within a relatively short time, producing initially a benthic
community displaying limited diversity and dominated by opportunistic,
stress-tolerant species (Rhoads et al., 1978~. Times associated with
the development of this assemblage are typically short, ranging from
weeks to less than a year. The rate and degree of subsequent change
varies with the nature of the sediment, particularly its texture and
cohesiveness; the relief of the mound above the seafloor and the
sediment transport field. This combination of factors results in
significant variability in substrate characteristics and benthic
communities. Times associated with establishment of an equilibrium
community vary from months to years (Obrebski and Whitlatch, 19811.
Beyond the obvious mortality produced by initial burial, the
adverse environmental effects of dredged material disposal cannot be
specified. The presence of the dredged material can alter local fish
habitat, resulting in a local shift of the dominant species.
Available data suggest, however, that while deposits of dredged
material may inconvenience local fisherman, they do not necessarily
reduce total yield or the landed value of commercial species
(Chesapeake Biological Laboratory, 1970; Oppenheimer, 1984~. Mounds
of dredged material can, for example, interfere with nets that are
towed or set to drift at specific depths. Some investigations suggest
that the disturbance of the equilibrium state produced by some amount
of dredged material disposal increases productivity, and can on the
whole, be beneficial. These results form the basis for a recent
proposal to test modification of the prevailing scheme (based on a
small number of relatively large-volume dumpsites) to one favoring a
larger number of smaller sites distributed throughout the estuary, or
offshore, or both (Rhoads et al., 1978~. The similarity between the
proposed scheme and the spatial distribution of disposal areas
prevailing prior to 1970, although obviously sited for substantially
different reasons, raises some interesting questions concerning the
optimum management of dredged-material disposal in estuaries and open
coastal waters.
Coincident with the physical and biological variations occurring
during and immediately after the disposal operation are a number of
chemical processes that affect the distribution and ultimate
bioavailability of the variety of organic and inorganic compounds
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133
associated with the dredged materials. Since many of these materials
are known to be potentially toxic, the character and extent of
chemical processing typically receives particular attention in efforts
to detail the effects of disposed materials. A number of studies,
representing a major portion of the effort to determine the
environmental effects of dredged material disposal have considered
contaminants found within both dissolved and particulate phases. The
general approach used in both laboratory and field studies has been to
establish a reference or control (station or sample), if possible, and
to collect some series of pre-project baseline data, and then with the
onset of disposal, to initiate analyses comparing disposal-site
conditions to those prevailing in the control.
Reviews of the large body of literature resulting from these
investigations indicate general agreement that the availability and
ultimate biological uptake is higher for contaminants associated with
the dissolved phase than for those found within the particulate
phase. This availability is associated primarily with the release of
interstitial waters, and favors maximum uptake during and for some
short time after the completion of the disposal operation. The
subsequent effects vary with a variety of factors, including time of
year, class and age of the organism, and the particular
contaminants. The principal adverse effects are generally
associated with well-known contaminants, including halogenated
hydrocarbons, such as PCB, and mercury (see Table 21 in Appendix G).
Beyond this class of essentially short-term effects associated with
dissolved-phase uptake, evaluations rapidly become more difficult.
Considerations of particulate-phase contaminants often show weak
correlation between sediment concentrations and body burden levels
within the local biological community (Pequegnat, 1983~. A variety of
studies conducted during the DMRP both in the laboratory and the field
provided similar results and lead to the conclusion that for
short-term effects "...impacts of dredged materials are primarily
associated with physical effects and....biochemical interactions are
infrequent and bioaccumulation of metals and hydrocarbons negligible"
(Engler, 1981~. The data imply that the availability of the
contaminants associated with the particulate phase is limited by
electrochemical binding that requires major changes in pH or Eh for
dissociation (Gambrel! et al., 1976~. For all but the most severe
contamination involving moderately to highly toxic materials,
short-term biological effects are essentially limited.
Despite the large body of data developed by the DMRP supporting
these conclusions, acceptance of the minimal-effect view is far from
widespread. Our conclusion based on review of these data, as well as
the variety of information developed within other programs (MESA,
DAMOS, etc.), is consistent with the view, but additional, more
sophisticated, and longer-term studies are required for unequivocal
assessment. Until such information is available, an environmentally
conservative course appears prudent. The determination of uptake of
contaminants and ultimate biological effects are both complicated by
variety of fundamental unknowns--the factors governing an adequate
control or reference station; the life histories of the selected
a
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134
indicator organisms; the mechanisms used by the indicator organism to
metabolize contaminants; and the physiological effects of continued
exposure to toxic contaminants, including consideration of genetic
modifications. Compounding the difficulties associated with these
unknowns is the high degree of variability associated with the inshore
biological community (Livingston, 1982~. This combination of factors
generally precludes simple determination of cause and effect using
short-term data sets. Based on these factors, the prevailing opinion
among experts is that the effects associated with long-term exposure
to moderate or low levels of contamination are, for the majority of
the marine biological community, largely unknown and that therefore
any potential for adverse effects should be minimized through proper
management practices based on the best available information.
Regulatory Procedures
Environmental legislation and regulation are discussed in Chapter 7.
From the environmental standpoint, the primary difficulties associated
with procedural and institutional matters are the lack of
responsiveness to the flow of information about environmental
effects--both positive and negative--and lack of assessment of the
implications for present criteria. In the case of dredging and
dredged material disposal, it appears that far more is known about
environmental effects and probable causes than is incorporated in
regulatory criteria and environmental practices. Streamlining the
regulatory process has the potential to improve not only port
management but also the incorporation of scientific results in
environmental criteria.
SUMMARY
Port dredging and disposal operations have the potential to induce a
variety of short- and long-term environmental effects. The majority
of these effects can be predicted, and efforts are proceeding to
resolve the remaining unknowns. Even within the category of unknown
effects, sufficient data exist to permit definition of the potential
range of effects that might occur in extreme conditions and to select
management strategies that minimize the probability of adverse
effects. Overall, the effects associated with a proposed dredging
project can be reasonably well defined and controlled. This review
suggests that the major concerns remain with the disposal of
contaminated sediments containing moderate to high concentrations of
toxic materials. Since typically this contaminated fraction
constitutes a relatively small percentage of the materials removed
during maintenance of existing berths, channels, and maneuvering
areas, and an even smaller percentage of the sediments associated with
new construction dredging, their presence should not represent a major
impediment to future port management or development plans if dredging
and disposal methods can be matched to their location, type, and
amount.
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135
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Representative terms from entire chapter:
dredged materials
