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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
118 maintenance dr edg ing .
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~.
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
121 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
122 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.
123 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
124 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
125 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).
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
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
128 hydraulic and numerical models (Thatcher and Harleman, 1972; Festa and Hansen, 1976; Officer, 1980~. Increasing salinity intrusion by channel deepening may lead to encroachment of salt into local supplies of groundwater and surface water. Increases in salinity associated with channel deepening may affect the viability of adjoining freshwater wetlands and tidal marshes, which may in turn influence local aquatic resources, including the range of freshwater, anadromous, estuarine, and coastal fish populations. These implications appear to be of particular concern in the Gulf of Mexico, where they have been the subject of discussion for more than 30 years (Morgan, 1973~. There is no doubt that undesirable effects can accompany salinity intrusion in specific conditions. These are associated primarily with new construction dredging, and not with routine maintenance dredging. As a result, plans proposing major alterations in channel depth and cross-section should include consideration of the associated modification in salinity intrusion in sufficient detail to permit resolution of changes induced by dredging and the short-term natural variations associated with fluctuations in river flow and astronomical and meteorological tides (Morgan 1973~. The character and extent of biological recolonization within the dredged channel varies as a function of the post-project hydrographies and sedimentological conditions, and the frequency of dredging. A significant increase in salinity above preproject levels and an associated increase in sedimentation rates, particularly of the finer-grained materials, will favor a permanent modification in the composition of the benthic community, a possible shift to more salinity-tolerant organisms, reductions in diversity, and slow rates of initial recolonization (Kaplan et al., 1974; Taylor and Saloman, 1968~. The rates at which these alterations proceed vary substantially from region to region: times for reestablishment of a stable community range from 1.5 to 12 years. In some areas, recovery times are long compared to dredging intervals, resulting in a continuing state of instability within the benthic community. The over-all result of these variations for estuarine productivity has not been demonstrated and appears negligible in most cases, owing to the small areas affected. For large new construction dredging projects that would significantly alter channel cross-sectional areas, the potential for such changes should be carefully assessed. The Disposal Area Upland Sites and Sites Fringing the Shoreline Since the initiation of dredging in the United States in the late 1800s, upland sites and sites fringing the shoreline have been primary receiving areas for dredged materials. Materials placed in these areas have served as construction fill for airports, footing for recreational areas and flood-control structures or dikes; and for the coarser fraction, as replenishment sands for beachfront restoration.
129 In recent years the use of such areas for disposal has tended to decrease because of increasing population pressure and the resultant decrease in available open space, legislation prohibiting the filling of wetlands and marshes, and concerns about the release of contaminants associated with dredged materials. As a result of these factors, a reasonably coherent policy concerning the use of terrestrial sites has evolved sufficient to effect a general elimination of the haphazard disposal of dredged materials that had been common in many areas. As implemented, this policy favors the use of fringing or upland sites if secondary benefits can be realized--the construction of a tidal marsh, creation of a wildlife habitat, or beach replenishment--or if the degree of contamination exceeds established levels for open-water disposal. Marsh and habitat development with uncontaminated sediments and beach replenishment have been studied in some detail and shown to have relatively short-lived adverse effects, these occurring principally during the placement operation (Lunz et al., 1978~. Determining the adverse effects associated with terrestrial disposal of contaminated materials and the advisability of using land sites rather than open-water sites is more difficult and controversial. Arguments favoring the use of terrestrial sites as receiving areas for contaminated dredged materials emphasize the combination of containment, the ability to observe closely any negative effects, and the relative ease with which corrective actions, such as removal and relocation, could be taken if unacceptable effects are observed. Countering arguments point to the inherent difficulty of realizing absolute containment of dredged materials and the enhanced potential for release and mobilization of a variety of contaminants associated with placement of anaerobic sediments in an aerobic environment. The increased availability of oxygen results in the alteration of the phase of some sediment-associated heavy metals from the insoluble sulfide form (favored in reducing conditions) to more soluble sulfates (Kester et al., 1983~. In addition, these reactions affect the pH of the interstitial waters generally leading to more acidic conditions and the potential for additional release of particulate-bound contaminants. The extent and character of contaminant release resulting from this combination of oxidation reactions varies as a function of the redox potential (Eh) and pH. Increasing Eh and an associated decrease in pH relative to natural in situ values appears to favor release of a progressively wider range of trace metals (Gambrel! et al., 1976~. The potential for contaminant release from dredged materials placed in terrestrial sites and the associated probability of surface water or groundwater contamination, as well as increased availability to the local biological community complicates the management of these sites both during and after receipt of contaminated sediments. Effective leachate control presumably can be achieved by the placement of impermeable liners to contain the materials and the use of settling and retention basins sufficient to permit evaporation or effective depuration. These procedures are expensive and significantly increase the area required for a containment site. The often
130 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.
131 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
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
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
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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