4
Site-Specific Considerations

An initial characterization of the source(s), type, and extent of site contamination and bioavailability needs to be conducted coincident with, or even before, an evaluation of regulatory realities is made and identification of the stakeholders and their particular interests is established. The range of factors governing transport and contaminant concentrations in marine systems requires that assessment procedures and methods be site specific. A reasonable understanding of site dynamics is also necessary to evaluate the proposed methods of characterization and methods of site assessment in terms of cost effectiveness and scope.

This chapter deals with contaminant sources, transport processes, and methods of site characterization. Site-specific considerations are important, in the committee's judgment, because inadequate source control and site assessment can undermine the best management practices. This chapter outlines how appropriate attention to these issues can help control costs and enhance the effectiveness of sediment management. However, the discussion is not intended to provide comprehensive, step-by-step guidance or to evaluate all methods that may be applicable. The emphasis is on the need for a systematic approach that couples site-specific information with remedial efforts. For each project, the time and resources required for source control and site assessment need to be weighed against the projected benefits of these activities, the availability of quantitative data, and the need to proceed with site management.

SOURCES OF CONTAMINATION

Source control refers to measures undertaken to identify and curtail continuing sources of contamination. Source control is advisable in all situations. It may



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--> 4 Site-Specific Considerations An initial characterization of the source(s), type, and extent of site contamination and bioavailability needs to be conducted coincident with, or even before, an evaluation of regulatory realities is made and identification of the stakeholders and their particular interests is established. The range of factors governing transport and contaminant concentrations in marine systems requires that assessment procedures and methods be site specific. A reasonable understanding of site dynamics is also necessary to evaluate the proposed methods of characterization and methods of site assessment in terms of cost effectiveness and scope. This chapter deals with contaminant sources, transport processes, and methods of site characterization. Site-specific considerations are important, in the committee's judgment, because inadequate source control and site assessment can undermine the best management practices. This chapter outlines how appropriate attention to these issues can help control costs and enhance the effectiveness of sediment management. However, the discussion is not intended to provide comprehensive, step-by-step guidance or to evaluate all methods that may be applicable. The emphasis is on the need for a systematic approach that couples site-specific information with remedial efforts. For each project, the time and resources required for source control and site assessment need to be weighed against the projected benefits of these activities, the availability of quantitative data, and the need to proceed with site management. SOURCES OF CONTAMINATION Source control refers to measures undertaken to identify and curtail continuing sources of contamination. Source control is advisable in all situations. It may

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--> be impractical in navigation projects but needs to be an integral component of environmental remediation projects, except in the most unusual circumstances. Failure to control the source of contamination leads to the recontamination of newly exposed sediments, in which case remediation efforts have to be considered unsuccessful. Source control is not, however, easy or inexpensive. In some cases, contaminant source(s) cannot be identified. Even if they can be pinpointed, some types of contamination, such as atmospheric fallout, are difficult or impossible to control. Another difficulty is the question of who is responsible for source control. From the standpoint of both economics and fairness, the costs of prevention and control ought to be borne by the polluter(s) and internalized into their production costs. Indeed, U.S. environmental law is generally based on the principle that the polluter pays. But those responsible for sediment contamination are not always (and sometimes cannot be) held to that standard. Thus, under current regulations, the burden for source control is not distributed equitably, which means that some sources of contamination are not controlled at all. Source control is used more often in environmental remediation projects, which are usually funded by the government (i.e., taxpayers), than in navigation dredging projects, which are partly financed by commercial navigation users through various tax assessments established by the WRDA of 1986. In Superfund site cleanups, a legal mechanism may be available to force upstream sources of contamination to bear an appropriate share of remediation costs and even to require the abatement of ongoing releases. However, in navigation dredging projects, the local port authority or other dredging proponent usually has little leverage over upstream polluters and, in the case of atmospheric deposition, virtually none over polluters outside the watershed. Thus, contamination may persist, leading to a continuing need to dredge and redredge contaminated sediments, which is costly and politically unacceptable. Source control could be encouraged in navigation dredging projects through regulation, as long as the question of who pays is resolved in a manner that is acceptable to all parties. A port cannot be expected to finance source control as well as sediment remediation (allocation of remediation costs is discussed in Chapter 3) when it is not responsible for the initial contamination. The primary focus needs to be on the development and implementation of state and federal pollution prevention programs aimed at reducing or eliminating the sources of sediment contamination. Regulators have long recognized that the identification of upstream sources of contamination is essential for the progressive improvement of water quality. CWA §303 emphasizes control of point-source discharges using technology-based measures but allows especially stringent discharge limits to be imposed by states based on the water quality in a given area. The logic of this approach applies equally to contaminated sediments.

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--> Section 303 could be amended to require EPA and delegated states to consider the impact on the quality of sediment downstream in setting total maximum daily loads (TMDLs) for waterway segments and developing load allocations for contaminant sources.1 In addition, congressional initiatives (i.e., CWA reauthorization legislation) could require watershed-specific inventories (including the identification of contaminant sources) of upstream contaminant contributions to sediment contamination downstream in port areas. In situations where watershed planning has failed and identifiable upstream sources have contributed disproportionately to sediment contamination downstream, the EPA could be authorized to recover an appropriate share of cleanup or disposal costs from the responsible parties. Part of the EPA's draft document on a strategy for managing contaminated sediments (EPA, 1994) outlines the agency's use of sediment quality criteria (SQC). One chapter describes how the water program (Office of Water) will permit municipalities and industrial facilities to meet SQC. The EPA has also initiated an inventory of sites and sources of sediment contamination using information from national databases. 2 These initiatives will be very useful, if not critical, to the understanding and control of sources of contaminated marine sediments. CONTAMINANT TRANSPORT AND AVAILABILITY Decision makers must understand the factors affecting contaminant transport and availability to develop a site characterization plan and, eventually, to evaluate site management alternatives. Understanding these factors can help minimize project costs, foster the development of efficient and effective sampling plans, and assist in the selection of optimum remedial schemes. This section outlines the primary factors. The distribution of contaminants in the coastal marine environment is determined by complex interactions among meteorological, hydrodynamic, biological, geological, and geochemical factors. Interactions within and among these factors result in a transport system with wide variations, both spatial and temporal. This variability complicates site assessment surveys and requires that care be taken to specify the frequency and location of field samples. Usually the time scales range from hours to months and are reasonably 1   This approach, although it might be difficult to implement, could be designed to address sources of sediment contamination. Although resolution of source control problems is outside the scope of this report, these issues warrant further attention. 2   See the National Sediment Contamination Point Source Inventory: Analysis of Facilities Release Data for 1994 and National Sediment Quality Survey: A Report to Congress on the Extent and Severity of Sediment Contamination in Surface Water. Both documents are in development as of this writing.

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--> regular. The patterns are sometimes disturbed, however, by high-energy storms, which can displace large amounts of sediments and significantly alter the distribution and availability of contaminants. Thus, comprehensive site assessments need to include consideration of the effects of both long-term, periodic variations and infrequent, but often high-energy, aperiodic events. Beyond the issue of spatial and temporal variability, assessment of coastal marine sites can be further complicated by the inherently nonlinear behavior of the transport system affecting the distribution and availability of contaminants. System response seldom displays simple functional dependence on force magnitude. Thus, evaluations typically need to consider other factors, such as the history of disturbances or antecedent conditions. For example, the effects on water-column mixing, of winds of identical velocity and duration vary greatly depending on the direction and magnitude of tidal conditions. These and other nonlinear tendencies are particularly pronounced in the processes that govern the transport of fine-grained, cohesive sediments. Fine-grained silts and clays (inorganic particles less than 60 micrometers in diameter), because of their relatively large surface-area-to-volume ratio and electrochemical character, are the favored adsorption sites for most contaminants found in coastal areas (Gibbs, 1973; Moore et al., 1989). These sediments enter the system from a variety of local, upstream, and offshore sources and can be transported initially as discrete particulates suspended in the water column. Larger, sand-sized particles are moved closer to sources by sedimentation, whereas fine-grained particles are readily dispersed. With time, individual particles come together to form larger-diameter aggregates as a result of either physicochemical coagulation or biologically mediated agglomeration. In the water column, the sizes of these aggregates and their associated settling velocities are controlled by the balance between collision and breakup forces induced by flow-associated shear. This force balance continuously changes as the particles migrate through differing flow regimes caused by horizontal advection and turbulent mixing. The process continues as long as flow energy and the associated boundary shear stresses are high. As energies decrease (as in many estuaries and dredged channels), aggregates settle to the sediment-water interface, forming a loosely consolidated, high-water-content surficial deposit (NRC, 1987), often referred to as a "fluff layer." This layer is highly porous with minimal shear strength, so subsequent tidal cycling typically results in the resuspension of some or all of the deposit, favoring low rates of net deposition. Cohesive sediments tend to consolidate slowly because of the weights imposed by the cyclic loading of surficial materials and because of a response to the increasing burden imposed by persistent net deposition acting in combination with the varying surficial load. This process favors the development of a column of sediment in which physical strength and associated erodibility vary significantly with depth.

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--> In addition to physical loadings, the vertical structure of the sediment column in a fine-grained deposit is affected by a variety of chemical and biological factors. The sediment-water interface represents a relatively distinct chemical boundary separating the generally oxygenated water column from an anoxic sediment column. This transition, typically occurring within a few centimeters of the interface, favors reducing conditions within the body of the sediment column and the dominance of facultative and anaerobic bacteria. Changes in pH (an indicator of acidity) and Eh (a measure of oxidizing potential) associated with this transition can directly affect sediment contaminant availability, altering the degradation of organic matter and providing a sink for selected trace metals. The latter process can be particularly pronounced in sulfate-rich seawater, resulting in the precipitation of trace metals by sulfides in the anaerobic pore waters and the subsequent down-gradient diffusion from surficial, aerobic sediments to the deeper anoxic pore waters. The rates of degradation of organic matter are also affected by the shift from oxidizing to reducing conditions within the upper levels of the sediment column (the redox gradient), with more effective microbial degradation of bioavailable compounds of concern (e g., polyaromatic hydrocarbons) possible within the oxic region. These processes slow significantly within the deeper anoxic regions of the sediment column, often resulting in contaminant half-lives on the order of years. The tendency of fine-grained materials to assimilate and concentrate nutrients and organic substances attracts a diversity of macrobiota, particularly within the upper 20 to 40 centimeters (cm) of the sediment column. The activities of these deposit and filter-feeding organisms significantly modify sediment fabric by burrowing and altering surface roughness, internal porosity, and physical strength. These modifications, known as bioturbation, can be expected to alter contaminant transport pathways and the overall erodibility of the sediment deposit. The combination of physical transport, chemical interactions, and biological processing results in sediment deposits typically characterized by horizontal gradients that are weaker than vertical gradients. The depositional sequence described above favors the formation of a mobile, near-surface layer of material overlying a reasonably well-consolidated and virtually immobile interior (Hayter, 1989; Ross and Mehta, 1989, Bohlen, 1993). The mobile layer, which is subject to diffusive or advective processes, is generally confined to the immediate sediment-water interface and is seldom more than 2 to 4 cm thick. Boundary shear stresses produced by the prevailing flows are sufficient to displace only the upper portions of this region, including the fluff layer and a thin underlayer no more than 1 to 2 millimeters (mm) thick. Displacement of the entire mobile layer requires boundary shear stresses that occur only during major storms. The deeper interior region, below the mobile layer, is even more resistant to transport and can be considered immobile in the absence of loadings extreme enough to produce mass failure of the entire deposit. This vertical gradient in erodibility has profound implications

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--> for evaluating sediment-associated contaminant availability, particularly in projects where natural remediation can be considered. The variety of factors affecting sediment erodibility makes it difficult to predict the response of a given deposit to a specified range of forces. Deposition rates, chemical environment, and biological activity can vary significantly, both spatially and temporally. This complexity directly affects the fabric of the sediment column and typically precludes the development of a generally applicable transport algorithm. As a result, erosion rate models require site-specific data. The application of site-specific formulas can be complicated further by the sensitivity of a given region to disturbances. Typically, the first storm of the season, acting on a sediment surface formed during an extended period of low transport energy, displaces a significantly larger mass of sediment than subsequent events. despite similarities in peak energies. These differences in response are often difficult to specify quantitatively, complicating the development of predictive numerical models for site assessment or management. SITE ASSESSMENT: APPROACH, METHODS, AND PROCEDURES The variety of factors affecting the distributions and availability of sediment-associated contaminants require the site manager to use a well-structured, tiered approach to site assessment (see Figure 4-1). (This approach expands on the one outlined in Chapter 2, Figure 2-1.) The remainder of this chapter outlines the elements of this approach. The committee views this kind of approach as having the best potential for achieving overall cost effectiveness and for clearly focusing on survey and remediation efforts. Focus means having a clear definition of project objectives, the satisfaction of which is the sole purpose for acquiring survey data, characterizing contaminant distributions and availability, and designing and selecting remedial schemes. None of these activities is an end in itself; each is justified only to the extent that it contributes to the fulfillment of project objectives (see Box 4-1). Use of Historical Data To ensure the cost-effective management of contaminated sediments, site characterization needs to begin with a review of the past and present uses (residential, commercial, and industrial) of waterways and adjoining lands. An understanding of past uses can place some bounds on the range of contaminants stored within the sediment column and highlight important geographical or archeological features of the site. The knowledge of present contaminant discharges and local transport dynamics can provide an immediate indication of the long-term effectiveness of a contaminant removal strategy and the overall advisability of proposed uses of the site. Source control is an important element in the management of contaminated sediments. Acquisition of historical data requires a careful

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--> FIGURE 4-1 Conceptual site assessment protocol. search of a variety of repositories, including state and federal permit files and water quality data; municipal planning, zoning, and land-use records; and assessors' files of deeds and titles dating back to the preindustrial period. Although data gathering requires resources, failure to identify the historical features of a site can also result in wasted time and money. This lesson was evident from the

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--> BOX 4-1 Basic Tenets of Site Assessment After 20 years of cleanup experience, some basic tenets of site assessment have emerged. The committee developed the following list: An understanding of site history, existing conditions, and dynamics is needed for the design and implementation of a successful management plan. The process of site assessment is complex and expensive, but it is possible to obtain the information necessary for making informed decisions. There is always some uncertainty associated with any decision; if one waits until all uncertainty has been eliminated, then no decision will ever be made. Data gathering must focus on meeting specific needs; data gathering is not an end in itself. Good site assessment results in minimum-cost projects that meet cleanup objectives. Marathon Battery case history, in which remediation plans had to be redesigned to accommodate the late discovery of an old gun-testing platform (see Appendix C). The area to be included in the historical survey depends on the transport dynamics and routes, both hydrologic and atmospheric, that affect the contaminants of concern. The project area may extend well beyond the immediate confines of the site, out to, and sometimes beyond, the boundaries of the watershed. An understanding of past operations affecting the area and the range of previous management concerns can define the character and loadings of a contaminant and can provide a qualitative indication, at least, of probable areal distributions, both horizontal and vertical. This information is essential to the design of subsequent field surveys and, if carefully gathered, can increase significantly the cost effectiveness of field surveys. The experience of committee members suggests that the value of historical reviews for enhancing cost effectiveness is often overlooked. Historical data may be set aside on the assumption that past analyses do not meet current standards. Although quality assurance and quality control problems may limit the value of quantitative historical data, several recent studies suggest that they do not justify outright rejection. In the ongoing cleanup of Boston Harbor, for example, the U.S. Geological Survey (USGS) reviewed available historical data in an effort to increase the resolution of the sediment characterization study. Although the majority of these data would have failed current quality assurance/quality control

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--> criteria, computer-based batch-screening procedures keyed to internal consistency and wild point minimization allowed recovery of nearly 3,000 useful data points (F. Mannheim, USGS, personal communication to Marine Board staff, November 5, 1995). This figure represents a sixfold increase in the original data file. The resulting improvement in data density may significantly improve qualitative evaluations of the degree of contamination, complementing evaluations of toxics transport and flux and assessments of the natural recovery rate of the system. Historical site data, combined with reviews of governing regulations and stakeholder interests and the definition of ongoing contaminant discharges, enhance the quality of the evaluation of the proposed uses of the site and the scope of the associated management efforts Consideration of these issues complements the specification of remediation end-points and the criteria for field surveys. Honest, reasoned considerations of site use can provide early indications of project advisability, save significant time and money, and foster goodwill. For example, early reviews may indicate that expansion of port facilities is inadvisable because the area is characterized by high sedimentation rates that require frequent dredging and that efforts would better be directed or confined to another site that needs less maintenance. Evaluation of Site Dynamics After a consideration of site use, the next step is an initial evaluation of site dynamics. The purpose is to determine the extent to which contaminants entering the study area are retained and the probable location of major repositories or sinks. Because most contaminant transport is associated with the displacement of fine-grained sediments, these evaluations place particular emphasis on factors that affect sediment erosion, transport, and deposition. Depending on the location, it may be possible to define the majority of these factors using existing information. Data are needed concerning the topography of adjoining lands and ground cover characteristics; local tidal height and currents; stream flows; meteorology, particularly wind speed and direction and concurrent air temperatures; water depths; and surficial sediment characteristics. Data should extend over the range of seasonal conditions and include indications of system response to aperiodic storms. The majority of these data can be obtained from federal agencies, including the National Weather Service for meteorological data, the National Ocean Survey for tidal and bathymetric observations, the U.S. Geological Survey (USGS) for stream flows and regional topography, and the Soil Conservation Service for ground cover characteristics. Other information, including data on surficial sediment, may be available from the USACE, the EPA, or local, state, or municipal regulatory groups or agencies, such as departments of transportation responsible for the construction and maintenance of road and railway bridges. The latter group is an often-neglected source of information. Foundation designs for roads and

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--> bridges often require deep, drill-hole data detailing soil characteristics. These data often provide a unique view of the vertical structure of the sediment column down to bedrock at several points across a waterway. Such perspectives are difficult and expensive to obtain but, when available, the may be of great value in investigations of sediment transport dynamics and associated contaminant availability. Transport data (e.g., tides, winds, and stream flows), in combination with information detailing municipal and industrial outfall locations and numbers, permit an initial evaluation of the probability that contaminants and sediments entering the waterway may be retained, in whole or in part, within the project site. These evaluations place particular emphasis on surficial sediment characteristics and water depths. Contaminant retention within a basin where most sediments are sands or other coarse materials is likely to be less than within a basin dominated by fine-grained materials, all other factors being equal. Similarly, retention within shallow basins with smooth, regular features is generally less than in deeper systems, particularly those with abrupt discontinuities in water depth. The remaining factors detailing the regional flow regime permit evaluation of the sensitivity of the transport system to changes in meteorological and hydrological conditions, particularly aperiodic storms. Initial site evaluation, in combination with data detailing sediment contamination, enables decisions to be made concerning the need for and form of supplemental field surveys. It is possible, although unlikely, that the initial evaluation of the transport system will indicate minimal contaminant retention, which would eliminate the need for additional surveys. More often, initial evaluations highlight deficiencies in the existing data and help define the most probable sites of contaminant retention within the project area. This is extremely valuable information and, if carefully developed, can reduce field survey costs significantly Field Surveys Initial field surveys3 are intended to address any obvious deficiencies in data indicated in the preliminary reviews and to provide quantitative data on the extent and character of contamination in the project area. If no site information is available, then the field work typically begins with a survey of water depths and evaluations of surficial sediment characteristics. Surveys focus on areas adjoining known or suspected contaminant outfalls. Depths typically are measured acoustically along surveyed transects. Gross characterizations of sediment might be made with simple rod probes to ''feel" the bottom, supplemented by occasional mechanical grabs to recover 3   The discussion of field surveys relates to both navigation and environmental cleanup projects in which contaminated sediments are present.

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--> masses of sediment for laboratory analysis. Alternatively, surficial sediment characteristics might be mapped acoustically and verified by direct mechanical sampling or visual surveys. The latter technique has the potential of providing high-resolution spatial coverage of both water depth and surface sediments, thereby significantly reducing survey time and costs. The initial sampling locations are selected based on the definition of surface characteristics of the study area and adjoining outfall locations, as well as the purpose of the project (e.g., environmental cleanup, port maintenance, new construction). Field surveys need to focus on both depositional and sensitive areas. The intent is to characterize the degree and type of contamination in the area so as to provide a basis for evaluating the need for more detailed, high resolution, higher-cost surveys. Sites may be located within expected contaminant source or sink areas, in channels or slips to be dredged, and at one or more points upstream and downstream of the limits of the project area. In addition, areas where there are abrupt changes in sediment character, in shoreline use, or in the hydrodynamic regime typically warrant sampling. The initial sampling locations are generally selected through a collaborative effort by representatives of a variety of regulatory agencies and the project applicant or site manager. At each designated location, core samples of the vertical sediment column can be obtained by a variety of mechanical methods (e.g., push corer, gravity corer, box corer, vibra corer). Typical sampling depths are on the order of 1 to 2 meters. In dredging projects, the vertical extent of the desired dredging establishes the required length of the sediment core. Core samples are retained in contaminant-free liners and returned to the laboratory for analysis. The number of samples extracted from an individual core is generally a function of the extent of stratification. If the sediment column is relatively homogeneous, then the entire core often is mixed to produce a composite, resulting in a single subsample for analysis. Significant stratification over the vertical tends to limit compositing to individual strata, and the number of samples generally is at least as high as the number of strata. Analyses of samples typically include qualitative visual logging to detail stratification and grain size and a number of quantitative physical and chemical analyses. The character and extent of these bulk-sediment analyses depend on the project. In dredging projects, analytical protocols follow the guidelines specified in the so-called Green Book (EPA and USACE, 1991). Simple sediment quality surveys might be less comprehensive, with the analysis focusing on a particular contaminant or class of contaminants. Bulk testing of composite sediments provides an indication of the "average" degree of contamination in the study area. The mixing in compositing is considered representative of the mixing that occurs during dredging. The procedure is not intended to yield high-resolution spatial data detailing contaminant concentrations throughout an area. Efforts to obtain such data would be initiated only if justified by the results of the initial field survey(s).

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--> After the initial field survey(s) and the associated laboratory analyses, including evaluations of contaminant concentrations and biological availability, the character and extent of sediment contamination in the study area can be determined. If the results indicate contaminant concentrations and/or bioavailability above defined "action levels," or if there are special stakeholder concerns, then a more detailed survey of contaminant distributions may be required. Detailed site surveys usually place particular emphasis on a single contaminant and require more dense physical sampling than the initial survey(s). Although physical coring is the most common and reliable method for detailed mapping of contaminant distributions, it is a slow and expensive process and, depending on the heterogeneity of the sediment column and the number of potential contaminant sources, it tends to provide limited spatial resolution. Contaminant concentrations are often interpolated horizontally, resulting in an overestimation of the mass or volume of sediment that needs to be removed. It is important, therefore, to develop and implement more cost effective site assessment technologies to replace physical coring. Recent Survey Innovations In recent years, several systems have been developed that appear to have the potential to supplement and, in some cases, replace physical coring. The most promising extends acoustic sub-bottom profiling techniques to permit high-resolution mapping of acoustic reflectivity. The resulting data can be related directly to a variety of geotechnical properties, including porosity, bulk density, and grain size. Samples from physical coring are still necessary as a baseline, but these new systems have the potential to reduce overall project costs and significantly increase the spatial resolution of field surveys. Increased resolution is needed if the full value of precision dredging technologies (described in Chapter 5) is to be realized. Tests conducted by the USACE (1995) have shown that acoustic profiling techniques can provide accurate, high-resolution characterization of both surficial and sub-bottom sediments (McGee et al., 1995). At the current stage of development, acoustic profiling cannot identify chemical contaminants or measure their concentrations in sediment. However, acoustic profiling surveys can help define the thickness and distribution of disparate sediment types, as was demonstrated in the Trenton Channel of the Detroit River (Caulfield et al., 1995). In this demonstration, acoustic profiling produced the following results: Contaminated sediment surface reflection coefficients exhibited high spatial variability. This variability added to the natural heterogeneity of the fine-grained sediment deposits and increased the resolution required of the field surveys.

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--> Spectral changes in reflectivity occurred across the boundaries between contaminated and uncontaminated sediments. In short, the results suggested that the acoustic signature of contaminated sediments differs substantially from that of uncontaminated sediments. Committee members had different views regarding the potential of acoustic profiling for differentiating between contaminated and uncontaminated sediments. Initial research results suggest that the use of acoustic profiling in site assessment, combined with precision dredging, has the potential to reduce the costs of contaminated site remediation. In addition, acoustic profiling might be more effective and cost less than current techniques for evaluating and monitoring capped contaminated sediment sites. However, the ultimate utility of acoustic profiling remains to be demonstrated. Questions remain about how effectively acoustic techniques will be able to identify specific compounds and their concentrations in sediments containing a wide range of contaminants (N. Francingues, USACE, personal communication to Marine Board staff, December 15, 1995). Complementing the development of remote, in situ sensing of physical sediment properties is a growing interest in the use of real-time or near-real-time chemical sensors for use in the field. These sensors can provide both point measurements and long-term, time-series observations. Although the majority of in situ sensors currently under development are intended for surveys of soils or groundwater (e.g., Lieberman et al., 1991; Apitz et al., 1993), many could be adapted for use in the marine environment. Sensors that measure pH, Eh, and pore pressure are already used routinely. Microelectrodes, providing millimeter-scale measurements of pH, oxygen, carbon dioxide, and ammonia, are also commercially available. Currently, these sensors are not capable of measuring contaminants of concern in sediments. Examples of near-real-time sensors include X-ray fluorescence for the detection of selected metals in sediments and masssensing piezoelectric transducers suitable for both solid and liquid phase determinations of a variety of contaminants, ranging from hydrocarbons to selected metals (Ward and Buttry, 1990). Particularly promising is the development of a range of fiber-optic chemical sensors and systems for in situ use (Tebo, 1982; Seitz, 1984; Smutz, 1984-1985). Fiber-optic chemical sensors make use of either direct optical measurements down a fiber or one of many immobilized membranes or reagents at the fiber tip that reversibly or irreversibly bind with specific analytes, producing a response that can be sensed optically. A simple example is fiber-optic-guided fluorescence, which provides a direct measure of concentrations of polyaromic hydrocarbons and other compounds that fluoresce at the wavelength sent down the fiber (e.g., Inman et al., 1989, 1990). Ligands that fluoresce when bound to an analyte (such as a dissolved metal) can be either pumped to the fiber tip or immobilized on the fiber, providing a signal when the contaminant is encountered. A number of these sensors use biological coatings that can be selected for sensitivity to particular

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--> pollutants. To date, biosensors have been used primarily in the medical sciences, but they are also being used to monitor food quality and environmental conditions (Keeler, 1991; Schultz, 1991). Fiber optics are also used in light-addressable potentiometric sensors, which are solid-state devices sensitive to a variety of biochemical reactions (Hafeman et al., 1988). If suitably configured, biosensors may be particularly good for long-term monitoring of biological responses to selected contaminants, an area of special importance and considerable research difficulty. In addition to applications dealing with a specific contaminant or reaction, fiber optics have been incorporated into a miniature spectrometer, permitting high-resolution measurements of fluorescence, absorbance, reflectance, and radiance in a variety of materials (Ocean Optics, 1996). Although demonstrated in a number of industrial applications, the system has not yet been tested in the marine environment. These new systems represent a promising beginning, but there is still a clear need for the identification, development, and demonstration of new and improved chemical sensors (both remote and in situ) for measuring contaminant concentrations in marine sediments. The availability of such sensors would contribute significantly to the development of improved management protocols for contaminated sediment sites. Survey Design: Numerical Simulation Methods Designs of sediment sampling strategies, and identification of optimum remediation methods, increasingly rely on computer-based numerical models. In concept, these simulations have the potential to highlight the key parameters requiring field measurement, assess the sensitivity of the study area to aperiodic storms, and forecast changes resulting from a specified remediation scheme. These models fall into four general categories: hydrodynamic, sediment and chemical transport, biological toxicity, and ecosystem response. Each has a characteristic range of strengths and weaknesses. Numerical modeling of coastal and estuarine hydrodynamics has been a subject of interest for more than 30 years (Ward and Espey, 1971; Fischer, 1981 ). In combination with increasingly sophisticated field observations (e.g., acoustic Doppler current meters [Brumley et al., 1991]) and advances in computing power and capability, hydrodynamic models provide reasonably accurate simulations of a variety of flow conditions. Many of these models are available through commercial vendors. To produce reliable results, however, the models need to be used by professionals who are familiar with coastal dynamics and numerical methods; they also require a comprehensive set of field data for calibration and verification.4 Two-dimensional representations of the flow field are routinely available, 4   Predictive modeling requires a research strategy, not simply a monitoring approach.

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--> with improvements in computational efficiency favoring increasing availability and the development of three-dimensional models. Such models can accurately evaluate the effects of extreme events, such as floods and hurricanes, on local hydrodynamics, and make quantitative estimates of any effects of remediation projects, such as changes in water depth as a result of dredging or the placement of sediment caps to isolate contaminated deposits. Although the modeling of coastal hydrodynamics is relatively advanced, efforts to couple the resultant flow field to the underlying sediment column have been hampered by the complexity of the factors governing the erosion and entrainment of fine-grained sediment. As described above, the structure and strength of a sediment deposit can be expected to display significant spatial and temporal variability. To date, this variability has precluded the derivation of a generally applicable transport algorithm. As a result, accurate simulations of sediment and chemical transport require that site-specific formulations be developed for each project. As a minimum, measurements to assess sediment erodibility under varying boundary-shear stress conditions and estimates of particulate settling velocities over a range of concentrations and water temperatures are required. Several models, such as the USACE-developed TABS-2 (Thomas and McAnally, 1985) and HEC-6 (Hydrologic Engineering Center, 1993), that can predict coarse-sediment transport are available. Both TABS-2 and HEC-6 have been used for contaminated sediment sites. Similar models for the transport of cohesive sediments would be useful for estimating contaminant dispersion while remediation alternatives are being explored or even implemented. Unfortunately, transport and contaminant partitioning are much more complicated in cohesive sediments than in coarse sediments, and such models are not readily available. The uncertainties associated with the numerical predictions of fine-grained sediment transport complicate the quantification of the exposure of resident biota to sediment-associated contaminants. Suspended sediment concentrations, resuspension, and deposition rates represent primary input data for the commonly used numerical physical-chemical fate models (Thomann and Mueller, 1987). Inaccuracies in these data affect the estimates of contaminant concentration governing biotic exposure. In turn, exposure data typically serve as essential input for numerical models of species toxicity and subsequent ecosystem response. The lack of accurate data, combined with the limited understanding of the effects of an assemblage of contaminants acting individually and synergistically on selected species, significantly limits the accuracy of numerical models of toxic response. Ecosystem models are limited further by difficulties inherent in predicting system responses to the addition or removal of particular classes of contaminants, factors that govern the partitioning of contaminants within the food chain and prey-predator contaminant transfers. Considerable research will be needed before numerical models of species toxicity and ecosystem response can be widely used.

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--> SUMMARY Source control is advisable in all contaminated sediment management projects. There are, however, impediments, including the difficulty of identifying certain sources of contamination. Even if sources cannot be controlled, remediation efforts may still be warranted. At the same time, regulatory steps could be taken to improve source control. For example, the EPA and the states could consider the impact on sediment quality downstream in setting TMDLs for waterway segments and in developing load allocations for contaminant sources. Site assessment represents an essential element in the design and implementation of every contaminated sediment management plan. Carefully conducted site assessments can accurately define the nature and extent of contamination and facilitate effective remediation, thereby maximizing overall project cost effectiveness. Given the complexity of the coastal and estuarine environment and the number of factors affecting contamination, a well-structured and systematic approach to site assessment is needed. The methods and procedures associated with detailed field surveys are in an early stage of development. Selected field tools are available but have not been widely used. As dredging methods and practices improve and pressure for cost reduction increases, the demand for increased survey resolution and more accurate short- and long-term predictions will force the development of improved survey methods and complementary numerical models. The cost effectiveness of site assessment could be enhanced by the continued identification, development, and demonstration of innovative survey approaches. Acoustic profiling, if the state of the art evolves as some believe it will, has the potential to reduce the costs of site characterization and, perhaps, the costs of evaluating and monitoring capped sites. However, additional fundamental research is needed on the acoustic properties of marine sediments to determine if acoustic profiling can accurately define areas of contamination. In addition, chemical sensors (both remote and in situ) for measuring contaminant concentrations in marine sediments need to be developed and tested. Site evaluation and assessment of remedial alternatives require conceptual, analytical, and numerical models that can predict hydrodynamics and contaminant transport, transformation, and biological effects. At this point, however, there is no fundamental understanding of cohesive sediments transport and the effects of contaminants on ecosystems. Models that enable comparisons between environmental effects can be used to gain some insights on site assessment. REFERENCES Apltz, S.E., L.M. Borbridge, K. Bracchi, and S.H. Lieberman. 1993. The fluorescent response of fuels in soils: Insights into fuel-soil interactions. Pp 139-147 in International Conference on Monitoring of Toxic Chemicals and Biomarkers. K. Cammann and T. Vo-Dinh, eds. SPIE Proceedings, vol 1716. Bellingham, Washington: Society of Photo-Optical Instrumentation Engineers.

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--> Tebo, A.R. 1982 Sensing with optical fibers: An emerging technology. Pp 1655-1671 in Proceedings of the Instrument Society of America Annual Meeting. Philadelphia: Cahners Publishing Company. Thomann, R.V., and J. A. Mueller 1987. Principles of Surface Water Quality Modeling and Control. New York: Harper & Row. Thomas, W.A., and W. H. McAnally, Jr. 1985. Open Channel Flow and Sedimentation. TABS-2 User Manual (rev. 1990) HL-85-1. Vicksburg, Mississippi: U.S. Army Engineer Waterways Experiment Station. U.S. Army Corps of Engineers (USACE). 1995. Micro-Survey Acoustic Core and Physical Core Inter-Relations with Spacial Variation. Washington, D.C.: USACE. Ward, G.H. , and W. Espey. 1971. Estuarine Modeling: An Assessment of Capabilities and Limitations for Resource Management and Pollution Control. Project 16070DZV Water Quality Office Washington, D.C.: EPA. Ward, M.D., and D.A. Buttry 1990. In situ interfacial mass detection with piezoelectric transducers. Science 249 1000-1007.