Managing Wastewater in Coastal Urban Areas (1993) / Chapter Skim
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C TRANSPORT AND FATE OF POLLUTANTS IN THE COASTAL MARINE ENVIRONMENT
C TRANSPORT AND FATE OF POLLUTANTS IN THE COASTAL MARINE ENVIRONMENT
Pages 231-294
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From page 231... ...
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.
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From page 232... ...
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.
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From page 233... ...
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.
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From page 235... ...
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.
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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)
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From page 238... ...
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.
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From page 239... ...
. Bilge water and ballast water are also discharged by ships in coastal waters.
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From page 240... ...
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)
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From page 241... ...
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.
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From page 242... ...
Transport in coastal waters varies as a function of depth, so the fate of the effluent may depend sensitively on the vertical location of the discharge. Surface waters tend to be more energetic than deep waters, hence the advection and dispersion tend to be more rapid there.
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. Dispersion has been notoriously difficult to predict, whether in estuaries, coastal waters, or the deep ocean, due to the complexity and wide range of scales of motion that may contribute to the mixing.
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From page 244... ...
This so-called estuarine circulation is generally the most important flushing mechanism in estuarine systems. The well-documented variations in water quality in San Francisco Bay as a function of freshwater input clearly indicate the important role of estuarine circulation to the flushing and hence the water quality of estuarine systems.
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From page 245... ...
The wind is often the most important driving force of net transport on the continental shelf, and it is often a major contributor to exchange between embayments and coastal waters. In the upper few centimeters of the water surface, the flow tends to proceed in the same direction as the wind at approximately 3 percent of the wind speed (Wu 1983~.
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From page 246... ...
On the west coast, instabilities in the southward flowing California Current result in a complex field of eddies adjacent to the continental shelf. These eddies result in strong offshore flows between the outer shelf and the ocean, which carry cold, upwelled water from the shelf into the ocean interior.
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From page 247... ...
Even without the aid of a numerical model, such a measurement program can provide the basis for predictions of effluent transport (e.g., Koh 19889. By combining current measurement results with analyses of water column and sediment distributions of various measured constituents,
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From page 248... ...
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, the water- and sedimentquality driven approach incorporating transport and dispersion is feasible with reasonable safety factors.
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From page 249... ...
A similar approach, in which all of the particles and contaminants are assumed to remain in the water column, could be used in addressing water quality criteria. Particles in Marine Environments Particulate matter in the ocean is comprised largely of aggregates of algae, bacteria, organic detritus, and inorganic particles (including natural sediments)
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From page 250... ...
The sorption of uncharged organic chemicals to particles is dominated by hydrophobic interactions and depends primarily on a chemical's affinity for water, typically described by an octanol-water partition coefficient, and on the organic carbon content of the solid sorbent phase. An estimate of the sorption of an uncharged organic pollutant in a wastewater or sludge can be made on the basis of the chemical's octanol-water partition coefficient, the organic carbon content of the solid phase, and the concentration of solids in the water.
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From page 251... ...
Representative studies are given by Schwarzenbach and coworkers (Schwarzenbach and Westall 1981, Imboden and Schwarzenbach 1985~. These studies and others show that the sorption of many organic compounds is proportional to the organic carbon content of the solids when this is greater than 0.1 percent and indicate that sorption can be predicted from the octanol-water partition coefficient of the nonpolar organic solute and the organic carbon content of the natural solid sorbent.
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From page 252... ...
The Outfall Region. The sedimentation of particles from sewage sludge discharged from ocean outfalls to coastal waters off southern California was modeled by Koh (1982)
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(1991) provides a paradigm for pollutant cycling at the sediment-water interface in coastal waters.
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From page 254... ...
Discussion and Conclusions A hypothetical description of the behavior of particles and particlereactive pollutants in wastewater effluents and sludges prior to and after discharge to coastal waters is as follows. · Particles in both wastewater effluents and sludges can be enriched in hydrophobic organic contaminants such as PAHs and PCBs.
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From page 255... ...
An example of this type of calculation is provided in the subsequent section on Sediment Quality Modeling. With respect to water quality criteria, all of the particles and particlereactive contaminants can be assumed to remain in the water column and be transported by wind, tides, and large-scale circulation.
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From page 256... ...
Speciation-based models of the environmental fates of potentially toxic trace metals and organic compounds couple equilibrium representations of the reversible reactions with rate laws for the transport and transformation processes. The importance of speciation lies in the fact that different species behave differently, e.g., particulate species settle and those associated with dissolved or colloidal organic matter are generally unavailable to biota.
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From page 257... ...
It also assumes that pore water concentrations of the chemical correlate best to biological effects. The partitioning coefficient between the organic carbon and pore water (KoC)
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From page 258... ...
Trace Elements and the Importance of Speciation Although knowledge of the concentrations and distributions of trace elements in coastal waters has advanced dramatically, it has become increasingly clear that information on just total concentrations is insufficient for providing an adequate understanding of a trace element's biological and geochemical interactions. Much of the uncertainty about the relationship between total metal concentrations and their toxicity to aquatic organisms results from a lack of definitive knowledge of the chemical forms of these metals in natural waters.
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1992~. Water quality criteria need to take into account the chemical form or species of the various elements in both source waters and receiving waters, together with a knowledge of the potential transformations that can occur in natural water systems.
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The factors controlling the biological availability of trace metals influence both their uptake by the food chain and their involvement in the biologically mediated reactions of their biogeochemical cycles. Because most biologically mediated processes require uptake of the element, either into cells or by surface sites of cells, biological availability can often be reduced to the relative rates of uptake of different metal species.
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From page 261... ...
Nutrients are elements required for plant growth and include nitrogen, phosphorus, silicon, and sulfur as well as trace metals such as iron and molybdenum. Damage caused by excess growth may simply include reduced transparency or, at the extreme, hypoxia or anoxia caused by respiration and decay of dying plants and animals.
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From page 262... ...
Only a fraction of the nitrogen input from the Hyperion Treatment plant is expected to enter the mixed layer in Santa Monica Bay. Again, there has been no evidence of damaging excess phytoplankton blooms in Santa Monica Bay over the past two decades (no dissolved oxygen depression, no green epicenter, no depression of secchi disc depth readings (light penetrations)
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From page 263... ...
To control biostimulation due to excessive nutrients in areas where it is or may be a problem, it is necessary to use an integrated management approach using the water- and sediment-quality driven approach to devise appropriate control strategies. Sediment Processes Many of the chemicals that have caused environmental impacts in estuarine or marine systems readily partition to particles, either suspended in the water column or on the bottom in sedimentary deposits.
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From page 264... ...
Reasonably well-accepted methods for estimating the bottom stress from oceanographic measurements are now available and have been used in attempts to predict sediment resuspension in relatively open coastal waters (Grant and Madsen 1986~. To date, these efforts seem to have been limited in predicting the long-term sediment accumulation by the variation in the critical shear stress as a function of grain size and depth in the sediment as well as by interactions between different grain sizes (armoring of fine sediment by larger grains)
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From page 265... ...
Sediment Mixing, Contaminant Reactions, and Release to the Water Column The subsurface distribution of contaminants associated with deposited sediments may be altered by biogenic or physical mixing. In the marine environment, sediment mixing by organisms, where present, usually exceeds that resulting from waves and currents.
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From page 266... ...
Changes in the contaminant profile in the sediments and release of contaminants to the water column may also involve the processes of molecular diffusion and colloidal advection (Gschwend and Wu 19851. A variety of models of varying complexity have been developed to describe such reactions in conjunction with bioturbation and sediment accumulation (see Berner [1980]
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In the latter case, concentrations that result in acute or chronic toxicities to aquatic organisms (called Water Quality Criteria) are determined through laboratory exposures, and these are used to establish acceptable levels, called Water Quality Standards.
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the partition coefficient between organic carbon and pore water. Experimental evidence indicates that for many hydrophobic organic compounds, the partition coefficient of the substance between n-octanol and water approximates the partitioning between sediment organic carbon and water.
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From page 271... ...
, the next question is: what fraction of the organic carbon in the sediment is derived from a particular POTW discharge; again to be conservative, assume 100 percent. If it is now conservatively assumed that there are no losses of the organic pollutant adsorbed on the suspended particles from point of discharge to sediment deposition, then the concentration of acenaphthene on the suspended solids in the effluent should be limited to 110 ,ug/gOC.
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From page 272... ...
. The calculations to derive a corresponding effluent limit from a total sediment concentration standard would be different from the example above because the rate of deposition of organic carbon of sewage origin must be predicted, along with the losses from the sediments back into the water column.
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From page 273... ...
This is particularly true of the elements associated with the enrichment of biological productivity in receiving waters, such as carbon, nitrogen, and phosphorus, but is also true of such potential toxicants as trace metals. Furthermore, some substances of concern, such as chlorinated hydrocarbons, can be broken down chemically into compounds of no concern and thereby have an ultimate fate, while the carbon atoms of which they are composed continue to be recycled.
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From page 274... ...
The other 80 percent would continue to drift with the water. They argued that in the region underneath the actual discharge site, the anticipated impact of municipal dumping would be an increase in the benthic sedimentation rate of about 10 percent and in organic carbon sedimentation rate of about 20 percent.
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From page 275... ...
(1974) estimated the fraction of different trace metals associated with anthropogenic sources that were settling within three inner basins of the Southern California Bight: Santa Barbara, Santa Monica, and San Pedro.
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OVERALL DESIGN OF DISPOSAL SYSTEMS, CONTROL OF DIFFUSE SOURCES, AND USE OF MODELS For a coastal community planning an ocean discharge, the task of choosing an outfall location, depth, design, and allowable mass loadings is basically an iterative one subject to definite but somewhat elastic boundary condi
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From the overall systems point of view, the more treatment and source control implemented, the less that is required of the outfall to achieve the same water quality; conversely, a really good outfall reduces the need for
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From page 278... ...
. The design of engineering systems based on water quality objectives is called the water-quality driven approach, shown schematically in Figure C.4.
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From page 279... ...
I Outfall location and design , ~ Effluent objectives for POTW Aes~e~ac Enjoyment Ocean Environmenti Level of Reagent at POIW Influent objectives for POTW . ~ Source control FIGURE C.4 Overview of water- and sediment-quality driven approach for design of municipal wastewater disposal system.
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From page 280... ...
However, the three kinds of required information apply equally well to all other types of pollutant sources and the approach to devising an engineering system. For example, in the case of combined sewer overflows, there is a set of water quality objectives; some knowledge of the environment; and information on the amount, quality, frequency, and distribution of existing combined sewer overflows.
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already met by initial dilution l 81 Approximate transport and fates modeling (WQ & SQ) 9 Check off more objectives satisfied.
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Box 1. Water quality and sediment quality objectives are established.
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Box 7. At this stage, it is useful to do a quick scan to see which water quality objectives and sediment objectives are already met.
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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.
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From page 285... ...
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.
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From page 286... ...
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.
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From page 287... ...
2. 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 waterand sediment-quality objectives.
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From page 288... ...
1991. Sediment trap fluxes and benthic recycling of organic carbon, polycyclic aromatic hydrocarbons, and polychlorophenyl congeners in Lake Superior.
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From page 289... ...
1990. California Ocean Plan, Water Quality Control Plan, Ocean Waters of California.
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1979. Mixing in Inland and Coastal Waters.
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From page 291... ...
Environmental Science and Technology 25(10)
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From page 292... ...
east coast continental shelf-II. Modelling suspended concentration and transport rate during storms.
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From page 293... ...
1990. Evolution of a three dimensional curvilinear-grid hydrodynamic model for estuaries, lakes and coastal waters: CH3D.
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From page 294... ...
Pp. 871-891 in Trace Metals in Seawater, C.S.
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