C Transport and Fate of Pollutants in the Coastal Marine Environment

INTRODUCTION

This appendix presents an assessment of current knowledge of the various physical, chemical, and biological processes that determine the transport and fate of pollutants associated with wastewater and stormwater inputs to coastal waters, and how well the behavior of these inputs can be modeled and predicted for engineering purposes. Specifically, how do the quantity, quality, and method of discharge of the wastewater to the coastal ocean affect the ambient water-quality and the quality of the sediments? With increasing knowledge of environmental engineering and marine sciences, it is now possible to design a waste management system by the water-quality and sediment-quality driven approach, namely finding the most cost-effective combination of source control, wastewater treatment, and outfall configuration. This process is explained in a later section on Overall Design following the next two sections which address Mechanisms of Input and Transport and Fate.

Coastal areas include a continuum from poorly-flushed small estuaries to the well-flushed open coastlines. This study focuses on larger estuarine and coastal systems subject to major urban impacts and that have significant exchanges of marine water with also the possibility of internal recirculation and entrapment of pollutants in the sediments within these bodies.

Federal law classifies inputs into point and nonpoint sources, according to whether discharge permits are required or not. As the regulations have changed (e.g., storm drains for cities over 100,000 people now require per-



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Managing Wastewater in Coastal Urban Areas C Transport and Fate of Pollutants in the Coastal Marine Environment INTRODUCTION This appendix presents an assessment of current knowledge of the various physical, chemical, and biological processes that determine the transport and fate of pollutants associated with wastewater and stormwater inputs to coastal waters, and how well the behavior of these inputs can be modeled and predicted for engineering purposes. Specifically, how do the quantity, quality, and method of discharge of the wastewater to the coastal ocean affect the ambient water-quality and the quality of the sediments? With increasing knowledge of environmental engineering and marine sciences, it is now possible to design a waste management system by the water-quality and sediment-quality driven approach, namely finding the most cost-effective combination of source control, wastewater treatment, and outfall configuration. This process is explained in a later section on Overall Design following the next two sections which address Mechanisms of Input and Transport and Fate. Coastal areas include a continuum from poorly-flushed small estuaries to the well-flushed open coastlines. This study focuses on larger estuarine and coastal systems subject to major urban impacts and that have significant exchanges of marine water with also the possibility of internal recirculation and entrapment of pollutants in the sediments within these bodies. Federal law classifies inputs into point and nonpoint sources, according to whether discharge permits are required or not. As the regulations have changed (e.g., storm drains for cities over 100,000 people now require per-

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Managing Wastewater in Coastal Urban Areas mits), distinctions on the basis of physical characteristics have become blurred. Traditional point sources at the time of passage of the Clean Water Act of 1972 included only outfall discharges from defined municipal and industrial installations; these sources, the focus of most control efforts heretofore, are generally well characterized now by types and fluxes of pollutants, although that was not the case before the Clean Water Act was passed. Outfalls (with very few exceptions) are submarine pipelines or tunnels discharging from a few hundred meters up to 15 kilometers (10 miles) from shore depending on the volume and character of discharge and the nature of the receiving water body. The term nonpoint sources is a poor descriptor because this term includes all inputs that are not point sources. Also, the definition of point sources changes with new laws and regulations. Here, the broad classification of diffuse sources is used to include all sources except the traditional point sources. This category includes (but is not limited to) streams, storm drains and flood control channels, combined sewer overflows (CSOs), discharges from boats, ground water seepage, and atmospheric deposition. These sources have three common features: 1) the original pollutant sources are widely distributed, 2) the rates of delivery to coastal waters are highly irregular depending primarily on the occurrence of rain, and 3) control measures other than at the original sources are limited. In some locations, the release of pollutants from existing contaminated sediments can be a significant diffuse source. Inputs of storm runoff, CSOs, and streams occur in a very unsteady manner at, or close to, the shoreline. Storm drains and flood channels (separate from sewers) discharge significantly when it rains, bringing as pollutant loads whatever wastes have accumulated in the drainage basin since the last storm; but also, smaller dry-weather flows may be highly polluted by illegal or unregulated waste disposal practices. Combined sewer overflows occur when runoff combined with sewage flows exceeds the capacity of a system, which then discharges at numerous predesignated places into various bodies of water in an urban area, including into streams and estuaries as well as the open coast. Natural streams and rivers may bring other pollutants from upstream areas, such as agricultural chemicals, atmospheric deposits, and nutrients washed off the land. Mathematical and conceptual models are used extensively to explain processes that disperse and modify pollutants in the ocean and to predict their effects on ecosystems. Various submodels may be combined to produce an overall model to relate pollutant inputs to water and sediment quality for single and multiple sources. These models are fundamental to management by the environmental-quality driven approach because the limits on emissions for any outfall discharge or diffuse source may be back calcu-

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Managing Wastewater in Coastal Urban Areas lated through the models. These models are analogous to the emissions-to-air-quality models used in developing air pollution control programs. The main purpose of this appendix is to assess the knowledge of all the relevant processes and evaluate the modeling capability for management of the quality of coastal waters and sediments by environmental-quality driven approaches. To be successful there must be good predictive capability for the dominant factors that determine the engineering choices for satisfying the standards. These factors can be determined based on sensitivity analyses and the experience of the modeler. Thus, for engineering purposes it is not necessary to understand every process if more knowledge would have no effect on the choice of control strategy. For example, it is not necessary to understand the behavior of a certain pollutant at a location where the exposure is far below any possible threshold value of concern. Since modeling for design of a management plan for pollution control always has some uncertainty covered by safety factors, it is cost-effective to implement a system (such as a waste treatment plant and an outfall) in a stepwise flexible way to allow for continuous feedback of the operating experience and the observed impacts on the coastal waters. In fact, there are very few situations where there is not already an existing discharge that serves as a prototype to study before and during upgrading the system. For example, the full effect of upgrading primary treatment on coastal water quality might well be observed before proceeding to secondary treatment levels if there is significant uncertainty about the need for secondary treatment. Or source control efforts for specific chemicals can be focused on those observed to be too high. This approach is always self-correcting as the discharger commits itself to take as many steps as necessary to solve any known problems. This incremental approach is one of the important features of integrated coastal management as proposed in the main body of this report. MECHANISMS OF INPUT Outfalls An outfall is a pipeline that discharges liquid effluent into a body of water. In the last four decades, there have been great advances in technology for ocean outfalls to achieve high initial dilutions and submerged plumes that are trapped beneath the pycnocline (or by the density stratification of the ambient water). Outfalls have advanced from simple open-ended pipes not far from shore to long outfalls with large multiple-port diffusers discharging in deep water. Figure C.1 provides an example of a deep water ocean outfall with a long multiport diffuser. The characteristics of major

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Managing Wastewater in Coastal Urban Areas FIGURE C.1 Schematic plan and profile of the 120-inch outfall, County Sanitation Districts of Orange County, California. (In metric units, the overall length is 8.35 kilometers, the diffuser length is 1.83 kilometers, the diffuser depth is 53-60 meters, and the pipe diameter is 3.05 meters). (Source: Koh and Brooks 1975. Reproduced, with permission, from the Annual Review of Fluid Mechanics, Vol. 7, © 1975 by Annual Reviews Inc.)

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Managing Wastewater in Coastal Urban Areas outfalls on the Pacific coast of the United States constructed prior to 1978 are summarized in Fischer et al. 1979. The construction of large outfalls in the marine environment has most commonly been accomplished with reinforced concrete pipe (RCP) with flexible joints. Recently, steel pipes have become more common because of improved manufacturing processes, better corrosion protection technology, proven constructibility (from technology transfer from the offshore oil industry), and construction costs for steel pipes, which can be significantly less than for RCP. One reason is that steel pipes are made into much longer lengths, requiring fewer junctions to be made in the marine environment. Two steel pipes of 64-inch diameter were used for the two outfalls of the recently built Renton outfall system in Puget Sound (Metro Seattle). They discharge through 500-foot long diffusers at a depth of about 185 meters (600 feet), which is probably beyond the capability of RCP construction. Tunnels have also become more competitive because of great advances in tunnel boring machines in the last 15 years. For example, the Boston outfall now under construction will be a 15 kilometer (9.4 mile) long tunnel, 7.39 meters (24.2 foot) in diameter, including a 2,000 meter (6,600 foot) long diffuser section with 55 vertical risers, each with 8 discharges ports. Three new successful outfalls in Sydney, Australia, are also tunneled. The combination of source control, treatment plant, and outfall is an engineering system that has achieved often dramatic improvements in coastal water quality. Even today, however, while many major discharges have state-of-the-art systems, there are still others that discharge through short outfalls with poor initial dilution. Figure C.2 shows schematically a typical multiport diffuser at the end of an ocean outfall discharging buoyant effluent into a density-stratified receiving water. Sewage effluent, being effectively fresh water, rises in the ocean, mixing intensely with the receiving water. The ocean is also usually density-stratified due to temperature and/or salinity gradients. Thus, the effluent mixing with the near-bottom denser ocean water can give rise to a mixture that is neutrally buoyant before the rising plume reaches the surface, leading to the formation of a submerged waste field, which is in turn advected by the prevailing currents (Figure C.2). This region of initial mixing is often called the near-field. The mixing that occurs in the rising plume is affected by the buoyancy and momentum of the discharge and is referred to as initial dilution. It is typically completed within a matter of minutes. Dilution as used in the engineering community is defined to be the ratio of the volume of the mixture to that of the effluent (i.e., the reciprocal of the fraction of effluent in the mixture). This initial dilution phase of the mixing process is under some control by the design engineer since it depends on the diffuser details such as length of diffuser, jet diameter, jet spacings, and discharge depth.

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Managing Wastewater in Coastal Urban Areas FIGURE C.2 Formation of a submerged effluent plume over a multiport diffuser in a stratified ocean with a current perpendicular to the diffuser. For clarity, only a few ports are shown, as typically there are hundreds for a large outfall.

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Managing Wastewater in Coastal Urban Areas The initial dilution is also controlled partially by nature since it depends on the density stratification and currents in the receiving water. A typical large discharge diffuser (for a flow of 5 m3/s) might be a kilometer in length and located in 60-meter water depth at a distance of 10 kilometers offshore. There might be several hundred discharge jets (typical diameter 10 centimeters) spaced along the 1-kilometer-long diffuser. The initial dilution obtainable for such a diffuser would be expected to be in the hundreds to a thousand depending on details (mainly flow rate and density stratification). The initial dilution and waste field submergence can now be estimated with a fair degree of confidence as a result of three decades of engineering research on the mixing processes in buoyant jets and plumes (Koh and Brooks 1975, Fischer et al. 1979). A number of computer models can provide such estimates of sufficient reliability as to make decisions on design choices. The most commonly available ones in the United States are the ones published by the U.S. Environmental Protection Agency (EPA) (Muellenhoff et al. 1985; Baumgartner et al. 1992, based in part on laboratory tank experiments at the EPA by Roberts et al. 1989a, b, and c). All the models for dilution and submergence calculations are based on analyses of buoyant jets discharged into a large receiving body of water. Usually the equations of conservation of mass, momentum, and buoyancy fluxes are integrated across the plume cross-section, having first assumed similarity of cross-plume profiles for the velocity and density deficiency (usually Gaussian). The resulting equations in this integral method are a system of nonlinear ordinary differential equations with the auxiliary conditions being in the form of initial conditions. Such systems are readily solved numerically. This is indeed the backbone of the available models. For approximate start-up calculations, it often suffices to use the formula for a simple line plume in a linearly stratified environment based on assuming that the multiple-port diffuser is well approximated by a line plume-a source of buoyancy flux only. For this case, Brooks and Koh (1965) derived simple formulas as follows:

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Managing Wastewater in Coastal Urban Areas where: q = discharge rate per unit length of diffuser g = gravitational acceleration ρ = density of discharge Δρ/ρ = relative density difference between effluent and receiving water ymax = maximum height of rise of the plume S = initial dilution (centerline or minimum time-averaged dilution) ∂ρa/∂z = average ambient density gradient These formulas can be applied to the typical case described in the previous paragraphs. If we assume that the ambient density gradient is (which can be due to a temperature difference of about 2°C over a depth of 50 meters), then ymax = 31 m (halfway from the discharge depth of 60 meters to the surface) and the initial dilution would be 200. Note that as the stratification increases, the plume rise and dilution are both reduced. For actual design calculations with mathematical models, one also needs to examine many different ambient conditions such as density profiles and current speeds and directions. The effluent flow also varies. Thus the initial dilution for an outfall is not a constant value but fluctuates considerably depending on ocean conditions and the effluent flow rate. It is important to point out here that dilution, being the ratio of the volume of the mixture to that of the effluent, can be converted to concentration c of a particular pollutant provided we know the concentration of that pollutant in both the effluent ce and the receiving water cb. Thus, If cb, the concentration of the pollutant in the receiving water were zero, then The value of cb includes the increase of the regional background concentration (background buildup) in the receiving water due not only to the continuous discharge from the outfall itself but also to all other sources. Discharges from Barges and Ships Ocean dumping from vessels has been practiced in the past by many coastal communities in various countries. It is still being practiced by some

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Managing Wastewater in Coastal Urban Areas and is being planned by others. Nations are not in total agreement regarding ocean dumping, although the practice has seen a dramatic decline, particularly in the developed countries. By far the largest amount of material involved in ocean dumping is dredged material (formerly known as dredge spoil). In the past, other materials dumped have included digested sewage sludge, various industrial wastes (including acids), oil well drilling mud and cuttings, coal ash, and mine tailings. Refuse has also been dumped in the past, but the practice has ceased (except for occasional illegal acts). Bilge water and ballast water are also discharged by ships in coastal waters. Ocean dumping has been mandated by law to cease in the United States, with the exception of dredged material. Other developed countries have also largely agreed to stop dumping of sewage sludge. In less developed countries, the status of ocean dumping is unclear. Rules and regulations may not exist. It is unrealistic to expect ocean dumping of nonhazardous polluted materials to be eliminated worldwide any time soon or even in a few decades. Clandestine dumping and dumping where not allowed are difficult to police in most areas of the world's ocean, usually for the simple reason that the necessary infrastructure is inadequate or nonexistent. Procedurally, the barge (or ship) is loaded with the waste material by placement into the ship's compartments. The vessel is moved to the designated dump site, which is generally a rectangular area with typical linear dimension of several kilometers. As long as the vessel is in the dump site, the material is allowed to be discharged into the ocean. Frequently a bottom-opening hopper barge is used. Here the barge bottom is equipped with doors, which can be opened to permit the material to fall out by gravity. Sometimes, the material is pump-discharged into the wake of the moving vessel to take advantage of the high turbulent energy that increases the initial dilution. Modeling of the mixing, transport, and fates of materials after disposal from barges and ships is less well developed and much less well verified than the corresponding models for outfalls. While the physical processes involved in the two cases are similar, the situation for ocean dumping is less amenable to analyses because the discharge conditions (discharge rate, bulk density of the material, and characteristics of its contents) may be ill-defined. This has the most effect on near-field predictability but extends also to the intermediate and far-field because the near-field equilibrium vertical location of the discharged material depends on the discharge condition. A detailed discussion of ocean disposal of digested sewage sludge has been presented with policy recommendations in a previous NRC report (NRC 1984).

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Managing Wastewater in Coastal Urban Areas Diffuse Sources The term diffuse sources describes the inputs other than municipal and industrial wastewater to coastal water bodies. These pollutant sources include urban storm drains, combined sewer overflows, natural streams and rivers, ground water outflow (under the sea), discharges from recreational boats and commercial shipping, and atmospheric deposition. The input of pollutants into these delivery pathways is widely distributed, and more challenging to control at the points of origin. Furthermore, there is little opportunity to manage the hydraulics of the inputs to achieve high dilution far from shore as for wastewater from publicly owned treatment works (POTWs). Nonetheless, the same principles of transport and fate apply. For modeling the water and sediment quality, it is, of course, important to include all these diffuse sources along with the outfall discharges from publicly owned treatment works. TRANSPORT AND FATE Following plume rise and the attainment of initial dilution, the diluted effluent cloud (often submerged below the thermocline) is advected with the currents and undergoes a variety of physical, chemical, and biological processes, referred to as transport and fate of pollutants. These processes occur in the natural environment and are beyond the direct control of the engineers, other than the initial conditions determined by the characteristics of the outfall and the effluent. For example, if a plume is kept submerged below the surface mixed layer, the subsequent transport, fate, and effects may be greatly different from a surface plume in the near-term (on the order of days to weeks). This region, which is dominated by natural processes beyond the near-field, is called far-field. This section describes the major processes affecting the behavior of pollutants in the coastal ocean, that is, transport and fate. Far-Field Transport and Dispersion of Contaminants Scientific knowledge of far-field transport and dispersion of contaminants has advanced significantly in the last several decades. When this knowledge is coupled with modeling and site-specific programs to measure currents, density stratification, and dispersion, engineering designs for outfall diffusers can be made by the water-quality driven approach. Far-field transport and dispersion can be modeled for design purposes with reasonable factors of safety to cover uncertainties. This section addresses the current knowledge and gaps in science and modeling. While predictions now are adequate for project design, increased knowledge will lead to improved management techniques and predictions.

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Managing Wastewater in Coastal Urban Areas Beyond the near-field region where the delivery of the effluent has a dominant influence on its dispersal, the subsequent transport and dilution depends primarily on the currents in coastal waters. Persistent currents cause advection away from the outfall site, while currents that fluctuate over short time and space scales result in dispersion of the effluent. Dispersion results in dilution of the effluent, while advection carries it away from its point of entry. The exposure of the receiving waters to the environmental hazards introduced in the effluent depends sensitively on the advection and dispersion rates. Higher concentrations, and hence greater exposures, occur in regions of sluggish transport, and lower concentrations and exposure occurs in rapidly flushed environments. There has been considerable effort mounted over the last 20 to 30 years to measure, model, and better understand transport and dispersion processes in coastal waters with application to the siting of outfalls and assessing the risks of oil spills and other toxic contamination, as well as developing an understanding of the interaction between the physics and the ecology of coastal waters. Because of the diversity of coastal water bodies and the complexity of the interactions between topography, density stratification, freshwater inflows, tidal motions, and the wind, it is not possible to predict a priori the magnitude of advective and dispersive transports at a given location. However, as will be discussed in more detail later in this section, it is possible to combine our general understanding of coastal processes with site-specific measurements to yield quantitative estimates of these processes that are accurate at least to an order of magnitude and often within a factor of two. This level of confidence is usually adequate to support water- and sediment-quality based analyses, with suitable safety factors to cover any errors of prediction. A general discussion of the transport and dispersion in coastal waters must first acknowledge the great diversity in the physical characteristics of coastal environments, from lagoons to estuaries and bays of various sizes to continental shelves with widths that vary from several kilometers along the southern California coast to more than 100 kilometers on the east coast of the United States. The driving forces vary tremendously from place to place as well. For example, the currents on the west coast of the United States are driven primarily by the along-shelf winds, while in other areas, such as the Gulf of Alaska and the South Atlantic Bight (Georgia, South Carolina, and part of North Carolina), the currents are strongly influenced by the input of freshwater from rivers. Tidal motions, which are more important with respect to dispersion than net transport, are also highly variable in strength and relative importance, being tremendously important, for example, in Puget Sound and the Gulf of Maine. Finally, the currents in the ocean margins adjacent to the continental shelf often influence the transport on the shelf-the most famous example being the Gulf Stream, which inter-

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Managing Wastewater in Coastal Urban Areas the outfall longer and deeper, which is the arrow from Box 10 back to Box 5. Also, if the chosen candidate outfalls and treatment seem to be excessive, i.e., run up more cost than is necessary to achieve the desired objectives, then revisions in the opposite direction might be made. Finally, if it appears that the objectives are impossible to satisfy at any reasonable cost, then the arrow from Box 10 back to Box 1 indicates a possible change of objectives. For example, a small kelp bed or shellfish bed near the proposed discharge might be closed to beneficial use at a small environmental cost compared to the possibly large monetary cost to do otherwise. Boxes 11 & 12. If the planned project appears viable and cost effective, then a major discharger would proceed typically with a year-long detailed environmental survey of the physical, chemical, and biological characteristics in the ocean in the area of the proposed outfall site and the target areas where water quality is to be protected. Included would be continuously recorded currents at different depths; profiles and transects of temperature, salinity, density, dissolved oxygen, and nutrients; measurement of toxicants in organisms and sediments; and biological assessments. Box 13. With this information then, a full-blown mathematical modeling is undertaken with the near-field and far-field behavior of the projected waste discharge with full consideration of the combination with other sources. The predicted results will include spatial variations and frequency distributions of various water-quality parameters in the water column and rates of accumulation of pollutants in sediments. However, the ability to do this is not perfect, but it is growing and it would be advantageous for the profession to have more post-construction evaluations of designs of systems that have been put into operation. The whole water-quality driven approach can easily assimilate new scientific information and oceanographic data as it becomes available for future adjustments or corrections of management plans. Box 14. By this time, a completely satisfactory design has been developed with perhaps only minor refinements needed. Box 15. These refinements are then carried in an iterative process by the arrows shown back to Boxes 1, 4, and 5 from Box 15. Box 16. The next question is whether the system proposed is the most cost-effective system to meet the objectives, and, if not, adjustments can be made as needed. Box 17. The proposed design and some viable alternatives can be presented to decisionmakers. Sometimes systems are developed that will go well beyond existing requirements in anticipation of future upgrading or tightening of requirements. Decisionmakers may often prefer such an alternative because the life of outfalls may be 50 to 100 years, far longer than the lifetime of many regulations.

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Managing Wastewater in Coastal Urban Areas Many coastal outfalls have been designed and built in the past two to three decades. Those that were designed generally using procedures outlined above have performed well and largely in accordance with predictions or better; some have probably been over designed because of large safety factors to cover uncertainties. High dilutions and plume submergence have been obtained when predicted. Present day upgradings are being driven mostly by more stringent water-quality requirements and increased loadings, rather than incorrect predictions of performance at the time of initial design. A new factor is sediment quality, which was usually not directly included in design considerations before about 1980. Quality driven approaches are not as well developed for nonpoint sources, but the principles are basically the same. Multiple point and diffuse sources can all be logically integrated into environmental-quality driven calculations as part of integrated coastal management. Discussion The Quality-Driven Approach The preceding sections address the range of scientific knowledge and engineering techniques related to the processes by which wastewater treatment plant effluents can be discharged to coastal waters safely. Much of this knowledge has evolved within the past three decades. Since scientists and engineers now have a good basic understanding of these processes, the management of coastal water and sediment quality can be addressed through a logical scientific framework. Predictions can be made of the benefits and costs of various control actions on a case-by-case basis or by classes. Future research will contribute to this improved understanding of coastal waters, and allow for a shift from mandated technologies to the water-quality and sediment-quality driven approach. It is only through the latter approach that the most technically and cost effective control measures can be identified. Because many uncertainties have been described, it may seem that the integration of all the necessary scientific information and engineering techniques for the purpose of making a decision is a hopeless task. But scientists, engineers, and other professionals working together with the public can sort out the key factors and focus on solving the most important problems in a cost effective manner through the integrated coastal management process discussed in Chapters 3, 4, and 5. It is clearly possible to design pollution control systems to achieve water and sediment quality sufficient to meet specified standards with appropriate safety margins. The outlines of this approach are provided in this

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Managing Wastewater in Coastal Urban Areas appendix with additional information presented in Appendix A on nutrients, Appendix B on pathogens, and Appendix D on wastewater treatment and stormwater management. The water-quality and sediment-quality driven approach is the only one that can be applied logically to multiple point and diffuse sources and rapidly assimilate new research and monitoring results. While water quality modeling is well developed, sediment quality modeling is new within the past decade and is not fully developed yet. While currently there is limited agreement on the way in which sediment quality standards should be specified, it can be anticipated that a consensus will develop over the next few years. Toxicants Toxicants have received great attention during the last two decades (but little before that), and strong control measures have been implemented in many areas. Source control and source reduction have proved to be effective measures for many POTWs. As effective source control programs are implemented the toxics problems will evolve toward one primarily associated with either sediment beds contaminated with past deposits or diffuse sources that are still unregulated or uncontrolled. Further work on the chemical speciation of metals in relation to toxicity will help to focus and refine requirements. In the meantime, toxicity limits for metals are established without regard to speciation. Many POTWs have found source control programs to be easier to implement than expected. Still illegal discharges continue to pour into many storm drains and are polluting the shorelines. Particles Residual particles (or suspended solids) in the effluent may be a concern for several reasons: they may be carriers of adsorbed pollutants; they may reduce light levels; they may contribute to nutrient enrichment; and they may, by settling, decrease the dissolved oxygen in the water column and sediments. But whether these problems actually exist depends on the circumstances. For example, if toxics are well controlled by source control in the sewer system, then toxics transport by particles may be below the levels where any standards would be violated. Similarly if the wastewater plume is confined below the thermocline where light levels are already low, then there is no effect on the euphotic zone above the thermocline. Also, when a region is well-flushed, then nutrient buildup will not likely be a problem. Finally, for outfalls producing high dilutions, all of the effects are reduced. Thus, acceptable limits for suspended solids concentrations are site-

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Managing Wastewater in Coastal Urban Areas specific, but they can be worked out by the water quality/sediment quality approach explained in this appendix. Nutrients The need to limit nutrient inputs to coastal waters is also site or region specific. Coastal waters that are impacted by excessive nutrients (usually nitrogen) usually receive these inputs from a variety of sources, including some natural inputs (for example Long Island Sound). In such cases, it is absolutely essential to follow an integrated coastal management plan (as explained in the main report) in order to achieve any results. Tightening up on minor sources may be a real waste of effort if major sources are left uncontrolled. An overall water-quality modeling including all sources is necessary to first understand the system then to devise the most effective control measures. In the long run, the nutrient enrichment problems in some areas may be the most difficult and expensive problems to solve; by comparison, toxics appear to be coming under control, with the residual in sediments being the remaining issue. Better Integration of Field with Laboratory and Computer An existing outfall discharge (or multiple discharges) is a full-scale prototype that can be studied and compared with mathematical and laboratory models. If models can reproduce what occurs now, then they reduce the uncertainties in planning the next level of improvement of environmental quality. Furthermore, post-construction field investigations are valuable to compare predictions with the actual performance. Such information provides a valuable feedback for planning future control measures. SUMMARY Integrated coastal management requires the use of the water-quality and sediment-quality driven approach to model and manage the effects of single or multiple discharges and diffuse pollution sources and to make effective regional control strategies. Predictive models have a number of uncertainties and need improvement, but nonetheless appropriate engineering systems for wastewater disposal and diffuse source control can be designed to meet prescribed water-and sediment-quality objectives. There is much to be learned from existing problem discharge situa-

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Managing Wastewater in Coastal Urban Areas tions that is useful to support modeling and engineering efforts for designing new or upgraded facilities. Our ability and effort to develop and use mathematical and conceptual models is ahead of our field confirmation of the accuracy of models. More effort is needed to study prototype systems after construction to evaluate the pre-construction modeling and analysis. A continuous, responsive approach is needed for future management of major discharge areas, including on-going ocean studies and flexibility of management to modify the discharge system as needed in response to new research findings, new problems, or new environment objectives. Coastal water-quality management must be site (or region) specific because of widely varying conditions along the coastline of the United States. Because of the wide range of length and time scales of various ocean processes, and the various time scales of various water-quality problems, different modeling approaches are required for different pollutants. REFERENCES Alldredge, A.L., and C.C. Gotschalk. 1989. Direct observations of the mass flocculation of diatom blooms: Characteristics, settling velocities, and the formation of diatom aggregates. Deep-Sea Research 36:159-171. Allen, J.S. 1980. Models of wind-driven currents on the continental shelf. Annual Review of Fluid Mechanics 12:389-433. Andreae, M.O. 1977. Determination of arsenic species in natural waters. Anal. Chem. 49:820. Andreae, M.O. 1979. Arsenic speciation in seawater and interstitial waters: The influence of biological-chemical interactions on the chemistry of a trace element. Limnol. Oceanogr. 24:440. Andreae, M.O., and J.T. Byrd. 1984. Determination of tin and methyltin species by hydride generation and detection with graphite-furnace atomic absorption or flame emission spectrometry . Anal. Chim. Acta 156:147. Andreae, M.O., J.F. Asmod, P. Foster, and L. Van't dack. 1981. Determination of antimony (III), antimony (V), and methylantimony species in natural waters by graphite furnace atomic absorption spectrometry with hydride generation. Anal. Chem. 53:287. Baker, J., S.J. Eisenreich, and B.J. Eadie. 1991. Sediment trap fluxes and benthic recycling of organic carbon, polycyclic aromatic hydrocarbons, and polychlorophenyl congeners in Lake Superior. Environmental Science and Technology 25:500-508. Baptista, A.M., E.E. Adams, and K.D. Stolzenbach. 1984. The solution of the 2-d unsteady convection-diffusion equation by the combined use of the DE method and the method of characteristics. Proc. 5th International Conf. on Finite Elements in Water Resources. Burlington, Vermont: University of Vermont. Baumgartner, D.J., W.E. Frich, P.J.W. Roberts, and C.A. Bodine. 1992. Dilution Models for Effluent Discharges. EPA Draft Report. Becker, D.S., R.A. Pastorok, R.C. Barrick, P.N. Booth, and L.A. Jacobs. 1989. Contaminated Sediments Criteria Report. Olympia, Washington: PTI Environmental Services, for the Washington Department of Ecology. Berner, R.A. 1980. Early Diagenesis. Princeton, New Jersey: Princeton University Press.

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