Handbook of ECOTOXICOLOGY - Section 4 doc

© 2003 by CRC Press LLC SECTION IV Methods for Making Estimates, Predictability, and Risk Assessment in Ecotoxicology 31 Global Disposition of Contaminants Roy M. Harrison, Stuart Harrad, and Jamie Lead 32 Bioaccumulation and Bioconcentration in Aquatic Organisms Mace G. Barron 33 Structure Activity Relationships for Predicting Ecological Effects of Chemicals John D. Walker and T. Wayne Schultz 34 Predictive Ecotoxicology John Cairns, Jr. and B. R. Niederlehner 35 Population Modeling John R. Sauer and Grey W. Pendleton 36 Ecological Risk Assessment: U.S. EPA’s Current Guidelines and Future Directions Susan B. Norton, William H. van der Schalie, Anne Sergeant, Lynn Blake-Hedges, Randall Wentsel, Victor B. Serveiss, Suzanne M. Marcy, Patricia A. Cirone, Donald J. Rodier, Richard L. Orr, and Steven Wharton 37 Ecological Risk Assessment Example: Waterfowl and Shorebirds Feeding in Ephemeral Pools at Kesterson Reservoir, California Earl R. Byron, Harry M. Ohlendorf, Gary M. Santolo, Sally M. Benson, Peter T. Zawislanski, Tetsu K. Tokunaga, and Michael Delamore 38 Restoration Ecology and Ecotoxicology John Cairns, Jr © 2003 by CRC Press LLC CHAPTER 31 Global Disposition of Contaminants Roy M. Harrison, Stuart Harrad, and Jamie Lead CONTENTS 31.1 Introduction 31.2 Environmental Transport Mechanisms 31.2.1 Atmospheric Transport 31.2.2 Freshwaters 31.2.3 Marine Transport 31.2.4 Soils 31.3 Transfer Mechanisms and Fluxes between Environmental Compartments 31.3.1 Atmosphere–Land Surface Exchange 31.3.2 Air–Plant Exchange 31.3.3 Air–Sea Exchange 31.3.4 Sediment–Water Exchange 31.3.5 Solid–Solution Exchange 31.4 Chemical and Microbiological Breakdown 31.4.1 Rate Expressions 31.4.2 Environmental Lifetimes 31.5 Spatial Distribution of Contaminants 31.5.1 Microscale 31.5.2 National and Regional Scales 31.5.3 Hemispheric and Global Scales 31.6 Temporal Trends in Contaminant Concentrations 31.7 Summary References 31.1 INTRODUCTION In most instances, pollutant sources are relatively easy to identify. Point sources especially present few problems of quantification, while diffuse sources (e.g., runoff from agricultural land) are more difficult to determine with certainty. The source is, however, only the first part of the picture, and the period that exists between emission/discharge of a pollutant and contact with the receptor may contain many varied and interesting processes. It is the aim of this chapter to describe © 2003 by CRC Press LLC some of the more important processes involved in pollutant transport and removal from the envi- ronment and to demonstrate how such processes influence the distribution of pollutants within the environment. Of particular interest are processes leading to the transfer of chemical substances between environmental compartments, i.e., water to air, air to soil, etc. Environmental cycles of chemical elements and compounds are generally termed “biogeochem- ical cycles.” Ideally, such cycles include quantitative estimates (however uncertain) of the fluxes between compartments and the total inventory of substance within a given compartment. Such quantification is difficult even for chemical elements, especially in relation to the flux component. For chemical compounds subject in some cases to rather rapid chemical change, estimation of fluxes is even more problematic. In this chapter, the transport mechanisms responsible for pollutant transfer within and between environmental compartments are first considered. Mathematical treatments allowing calculation of transfer fluxes and lifetimes are then described. Some examples of pollutant distributions are given, indicating where possible the processes responsible. Finally, some examples are provided of the use of present-day environmental measurements to infer historical concentrations of pollutants. 31.2 ENVIRONMENTAL TRANSPORT MECHANISMS 31.2.1 Atmospheric Transport Atmospheric motions occur on a number of spatial scales, most notably: 1. Global 2. Synoptic, or large-scale (thousands of kilometers) 3. Mesoscale, or intermediate (tens and hundreds of kilometers) 4. Microscale (ten kilometers and less) In addition to horizontal transfer processes, movements in the vertical are important, especially for substances with long atmospheric lifetimes. The atmosphere, viewed in the vertical (Fig - ure 31.1), divides readily into discrete regions. The lowermost part, known as the troposphere, is characterized by decreasing temperature with increasing altitude. This region is the most accessible to us and consequently is the part most thoroughly observed in scientific terms. Above the tropo - sphere lies the stratosphere, a region within which temperature increases with altitude. This is the region in which ozone mixing ratios peak, as discussed later. The atmospheric regions above the stratosphere are of little concern in relation to pollution phenomena. The troposphere is typically also thermally stratified (see Figure 31.1). The main regions are the boundary layer, typically about 1 km in depth during the daytime but often reducing to only around 100 m at night, and the free troposphere, which lies between the boundary layer and the tropopause. These regions are separated by a temperature inversion, which severely limits exchange between them. The extent to which a pollutant is subject to either vertical or horizontal movement in the atmosphere is a function of its atmospheric lifetime, def ined in section 31.4. For substantial movement of a substance between compartments, the approximate minimum lifetimes τ are indi - cated below: Boundary layer to free troposphere τ > 5 days Entire tropospheric hemisphere τ > 1 month Global troposphere τ > 2 years Troposphere to stratosphere τ > 10 years © 2003 by CRC Press LLC To cite some examples, nitrogen dioxide has a chemical lifetime of about a day, and little of that emitted in the tropospheric boundary layer transfers to the free troposphere. Aerosol emitted from Chernobyl, which had a lifetime of around 1 month, led to contamination of much of the northern hemisphere. Methane, with an atmospheric lifetime of about 9 years, is rather well mixed between northern and southern hemispheres and penetrates in modest amounts into the lower stratosphere. Chlorofluorocarbons and nitrous oxide, with lifetimes in excess of 100 years, have no significant tropospheric sinks and mix appreciably into the stratosphere. Horizontal atmospheric motions are driven by gradients in temperature and pressure that lead to general circulation, as described in Figure 31.2. Wind speeds in the boundary layer vary greatly Figure 31.1 Vertical structure of the atmosphere indicating approximate temperatures and the main regions. Figure 31.2 The general circulation of the atmosphere. 50 40 30 20 10 8 6 4 2 0 STRATOSPHERE TROPOSPHERE Stratopause Tropopause High-level cloud (Stratus) High-level cloud (Cirrus) Storm clouds (Cumulonimbus) Intercontinental airliner 1 10 10 2 2 3 4x10 10 Boundary layer 200 220 240 260 280 Temperature (K) Altitude (km) Pressure (mb) Polar fronts Polar fronts Polar easterlies Westerlies Westerlies Horse latitudes Trade winds Trade winds Equatorial doldrums 0 ° 30° Polar easterlies 60° Subpolar lows Subpolar lows © 2003 by CRC Press LLC from place to place but are typically of the order of 5 m/sec, implying transport distances of around 400 km/day. It is thus possible for a pollutant to travel around the globe at a given latitude in a matter of days, but more substantial north–south mixing takes months. Atmospheric circulation at low latitude is dominated by the Hadley circulation, which involves updrafts of air in equatorial regions, with subsequent movement both north and south at high tropospheric altitudes and sub - sidence to form the subtropical high–pressure regions. The existence of two such circulatory cells on either side of the equator ensures inefficient mixing between northern and southern hemispheres, and only substances with lifetimes measured in years mix appreciably between the hemispheres. This is particularly important in limiting movement of contaminants from the heavily populated northern hemisphere into the far cleaner southern hemisphere. Mesoscale circulations involve such processes as land–sea breeze circulations that can pro- foundly influence pollutant movements in coastal regions. Mountain upslope and downslope winds driven by thermal convective motions may also act as an important transfer route. Processes occurring on the micro- and mesoscale are important in influencing concentrations of locally generated pollutants but play only a minor role in transferring pollutants on a larger scale. 31.2.2 Freshwaters Pollutants entering freshwater may be in the dissolved (< 1nm), colloidal (1 nm – 1 µm), or particulate (> 1 µm) form. The exact nature of the pollutant species has important implications for both bioavailability and environmental transport. For instance, biological impact of trace metals is often related directly to the dissolved (free) metal concentration. 1 In addition, dissolved and colloidal forms of the pollutant will tend to remain in the water column due to mixing processes, while particulate forms will tend to settle out of the water column and be incorporated in the sediments. 2 Once in a river or lake, they may remain in their original form or repartition between these different forms in response to a changed matrix. Important parameters affecting the distribution of pollutants include microbiological activity, concentration, and nature of organic matter, pH, ionic strength, redox potential, and so on. These changes can be illustrated by the following examples: • Rain water (pH ~ 5) falling on lake water (pH ~ 7–8). As the pH increases, metals in the dissolved form will tend to bind to solid phases. Over this pH range many metals change from 0 to 100% bound to solid phases. • Freshwater mixing with seawater in estuaries. Increased ionic strength will cause colloids, and subsequently sediment, to aggregate out of the water column. The fate of associated pollutants will also be affected. • Oxic–anoxic boundary in lakes. The oxidized form of iron (Fe [III]) exists as the solid phase in oxygenated surface waters. The particles sediment from the water column (with any associated pollutants) and come into contact with the deoxygenated bottom waters. At this point the iron is reduced, forming Fe (II), which exists in the dissolved phase. Particulate-phase pollutants may therefore pass into the dissolved phase. In reality, the processes occurring are much more complex than indicated, with many competing reactions occurring on different spatial and temporal scales. For instance, in estuaries, both pH and chloride concentration as well as ionic strength increase dramatically, with many consequences for pollutant behavior. In the case of lake waters, as the iron is reductively solubilized across the oxic–anoxic interface, released trace pollutants may bind to other solid phases and not be released into the dissolved phase. Processes such as sorption, precipitation, microbiological activity, and others may affect both organic and inorganic pollutants. Therefore, the assumption that a substance entering a river in an effluent will maintain its original physicochemical form will often prove to be incorrect. Metals are important contaminants that may undergo a wide range of physical changes such as those outlined above and, in some instances, may also undergo changes in oxidation state leading © 2003 by CRC Press LLC to a complete change in physical form. The relevance of such changes to environmental transport is that they affect the size association and hence the mode of movement in the freshwater system. Figure 31.3 exemplifies the possible forms of trace metals and their size association. 3 Dissolved contaminants will move freely with flowing water and, although subject to diffusive movements caused by turbulence, are predominantly influenced by advective transport (i.e., they are mainly carried along by the water, rather than diffused within it). The behavior of particulate material is more complex, as it may deposit, entering the bottom sediment. This process is controlled by two processes. First, the tendency to deposit is a function of the size and settling velocity of the particles with which the contaminant is associated. However, this tendency is counteracted by turbulence forces that keep particles in suspension and make them available for movement with the water. Thus, in relatively static waters (lakes and ponds), there is a strong tendency for particles to be incorporated into bottom sediments. In fast-moving rivers and estuaries, the turbulent energy of the water keeps the particles in motion and may even lead to resuspension of bottom sediment. In such circumstances, contaminant transport may occur by three main mechanisms: • As truly suspended particles • By saltation, or “bouncing” of particles along the sediment surface • As bed load, or bulk motion, of surface sediment Changing flow conditions within a river may lead to rapidly altering transport properties. An example is given in Figure 31.4, which shows measurements of total (suspended plus dissolved) concentrations of chromium in the River Thames (U.K.) as a function of discharge (flow). As flow increases from very low values, concentrations of contaminant fall due to dilution of dissolved material in a greater volume of water. They reach a minimum and then begin to increase for flows of greater than about 70 m 3 s–1 . This flow is known (from other measurements) to correspond to that at which riverbed sediments begin to become resuspended. Concentrations of chromium continue to increase with increasing flow as more bed sediment enters the water. In slow-moving waters, where much of the suspended sediment enters the bottom sediment, a historical record of inputs to the lake may be preserved in the bottom sediment. For instance, the concentration of lead, zinc, and copper in recently deposited sediments in Lake Erie have increased by several-fold in comparison to sediments deposited ca. 1900. 4 Such records often reflect changes in atmospheric deposition to the lake surface, and for substances such as heavy metals 5 and poly- nuclear aromatic hydrocarbons, they have been used to reconstruct a record of air quality in the past. Figure 31.3 Typical physicochemical forms of trace metals in aquatic systems, as related to size association. (From de Mora, S. J. and Harrison, R. M., Water Res., 17, 723, 1983. With permission.) Metals Species Free Metal Ions Inorganic Ion Pairs Inorganic Complexes Low Molecular- Weight Organic Complexes High Molecular- Weight Organic Complexes Metal Species Adsorbed onto Inorganic Colloids Metals Associated with Detritus Metals Absorbed into Live Cells Metals Adsorbed onto or Incorporated into Mineral Solids and Precipitates Examples Mn 2+ Cd 2+ NiCl + HgCl 4 2- Zn-fulvates Pb-humates Co-MnO 2 Pb-Fe(OH) 3 Cu-clays PbCO 3(s) Soluble Colloidal Particulate © 2003 by CRC Press LLC The behavior shown in Figure 31.4 for chromium in the River Thames is not common to all contaminants. In the same study, measurements were made of nitrate and soluble reactive phosphate (SRP). For nitrate, concentrations were almost independent of flow, suggesting the dominance of inputs in waters making up the major part of the river flow (surface runoff and groundwater). In the case of SRP, concentrations of this dissolved species showed a monotonic decrease with flow rate reflective of dilution of effluent inputs (from sewage treatment works) and no input from resuspended sedimentary materials. 31.2.3 Marine Transport While rivers generally take only hours or days to flow to the sea, water has a very long residence time in the ocean. Consequently, transport and transformation processes control the distribution of contaminants. The oceanic surface layers to a depth of around 100 m are driven largely by the wind, the water motion being modified by the Coriolis force that arises from the rotation of the earth. Oceanic circulations in large part consist of gyres constrained by continental boundaries. Faster currents occur along western margins, leading to pronounced circulatory features such as the Gulf Stream, Brazil Current, and Kuroshio Current. At greater depths within the ocean, circulations are determined by the chemistry and temperature of the water. Dense waters (either cold or more saline) form in cold polar regions, where the sea ice formation leads to an increase in salinity. Antarctic bottom water flows north from the Weddell Sea into the south Atlantic, while North Atlantic deep water moves on a timescale of around 1000 years from the Norwegian Sea through the Indian Ocean into the Pacific. 6 Surface ocean waters are relatively well mixed and show little variation of temperature with depth. In the layer beneath, termed the “thermocline,” temperatures decline rapidly with depth, with the most dramatic temperature changes occurring down to about 1 km in equatorial and temperate latitudes. The deep layers beneath the thermocline show little change in temperature with depth. The thermocline presents a rather sharp boundary and a considerable barrier against mixing of the surface and deep layers. Owing to the relatively slow time scales of oceanic water movement, even in the surface layers, highest concentrations of contaminants occur in the coastal regions, where inputs are greatest — from rivers, direct coastal discharges, and atmospheric deposition. Much of this contaminant load may deposit to the sediments before mixing processes carry it far from coastal waters. Figure 31.4 Concentration of chromium (suspended plus dissolved) in the waters of the River Thames as a function of flow rate. © 2003 by CRC Press LLC 31.2.4 Soils Soils are by their nature rather immobile. If undisturbed, they may retain a record of contaminant inputs over a very long period. Many pollutants bind strongly to soils and, if there is input from the atmosphere, show a very strong surface enrichment. Mechanical mixing by plowing or other agricultural practices, or bioturbation by burrowing organisms, can cause some vertical and lateral mixing of pollutants in soils. Following deposition from the atmosphere, some organic pollutants, particularly polychlori- nated biphenyls (PCBs), readily undergo volatilization from soil to the extent that for many such compounds, volatilization represents the principal loss mechanism from soil. 7 This has important implications for understanding the origins of the continuing atmospheric presence of PCBs (the manufacture of which was ceased in most western countries in the late 1970s). Specifically, although accidental releases from PCBs remaining in use continue, such volatilization of previously deposited material is widely considered to represent the main contemporary source of PCBs in the atmosphere. 31.3 TRANSFER MECHANISMS AND FLUXES BETWEEN ENVIRONMENTAL COMPARTMENTS Every time it rains, small amounts of highly persistent compounds, such as chlorinated pesti- cides, are deposited to land and sea. Dry deposition of particles and gases also contributes to inputs. This occurs even for some compounds no longer in large-scale production or use. Residues in soils and waters in parts of the world where heavy usage has occurred are still evaporating into the atmosphere and are carried to locations remote from their sources, where deposition takes place. Exchange between environmental compartments is a major route of transfer for some substances and can lead to unexpected instances of pollution. One such phenomenon occurred in West Cumbria, U.K., where abnormally high levels of plutonium were found in coastal soils. Detailed investigation showed that plutonium discharged from the Sellafield reprocessing plant to the Irish Sea was incorporated into sea spray with an efficiency greater than that expected from its abundance in seawater and carried back to land in marine aerosol. 5,8 Some of the major processes involved in intercompartmental transfer will now be considered. 31.3.1 Atmosphere–Land Surface Exchange Transfer from air to land can occur by two major routes: 1. Rainfall (termed “wet deposition”) 2. As dry particles or gas, without the intervention of rain (termed “dry deposition”) A third pathway, involving “occult” deposition of fog and/or cloudwater droplets, may also be of localized importance. Concepts of dry deposition were originally developed to describe the transfer of radioactive gases and particles from the atmosphere into terrestrial systems. The process of deposition is first order; that is, the flux to the surface depends linearly upon the atmospheric concentration. The constant of proportionality is termed the “deposition velocity,” v g , defined as: v g flux to surface atmospheric concentration at 1 m = © 2003 by CRC Press LLC For gases, deposition velocities to soils and vegetation vary greatly according to the affinity of the gas for the individual surface. Thus, for chlorofluorocarbons, deposition velocities are essentially zero, while for highly reactive and water-soluble nitric acid vapor, v g has a typical value of 2 to 3 cm s –1 . The deposition velocity is not constant for a given gas or a given surface. While one can make approximate statements about the magnitude of v g ( such as that above for nitric acid), in reality v g v aries with the surface characteristics and atmospheric properties at the time of deposition. This may be expressed as follows for the total resistance to deposition, R. (31.1) where r a is the aerodynamic resistance, or the resistance to transfer downward through the atmo- sphere to within 1 to 2 mm of the surface; r b is the boundary layer resistance, which is the resistance to transfer through a laminar layer of air of about 1 mm thick over the individual roughness elements of a surface; and r c is the canop y, or surface resistance, which describes the resistance of the surface itself to take-up of the gas. For a sticky, reactive molecule like nitric acid, r c i s essentially zero, while for chlorofluorocarbons, it is almost infinite and accounts for the differing depositional behavior of these gases. For ozone, r c over vegetation typically varies with time of day according to the opening of stomatal apertures of the vegetation, necessary for rapid ozone deposition. Deposition velocities are not as strong a function of chemical properties in airborne particles as in gases. The main determinant of deposition velocity is the particle size (see Figure 31.5), with high values of v g appl icable to large particles due to their inertial properties and very small particles due to their high diffusivities, which cause them to behave more like gases. 9 Between the two lies a region of low dry depositional efficiency. For particles in this size range, around 0.1 to 1 µm in diameter, wet scavenging is also inefficient, and atmospheric lifetimes are long — of the order of 7 to 40 days. Chemical composition may affect dry deposition, as hygroscopic particles are liable to grow adjacent to a humid surface, leading to enhanced deposition. Some gases are released by soils or vegetation. If their concentration immediately adjacent to the surface exceeds that in the atmosphere above, the concentration gradient will cause them to diffuse upward, and the net flux will be from surface to atmosphere. In this instance, the concept of deposition velocity is of little value. A gas that normally exhibits upward fluxes from soil is Figure 31.5 Typical variation of aerosol particle deposition velocity with particle size over land. R 1 v g r a r b r c ++== © 2003 by CRC Press LLC nitrous oxide, N 2 O. Ammonia, NH 3 , shows ambivalent behavior; net fluxes from fertilized soils are commonly upward into the atmosphere, while over unfertilized soils, within which concentra - tions of ammonium (and by inference ammonia) are very low, deposition normally occurs. 31.3.2 Air–Plant Exchange Air-to-plant transfer of persistent organic pollutants (POPs), such as dioxins, PCBs, and poly- cyclic aromatic hydrocarbons (PAH), occurs readily, primarily as a result of vapor-phase deposition onto the leaf surface, either as an equilibrium partitioning process or as a kinetically-controlled process, although particle-bound deposition can be influential for some compounds. 10 Given the facile nature of air–plant transfer of POPs, it is important to evaluate the quantitative role that vegetation plays within the overall biogeochemical cycling of such pollutants. Several authors have speculated that because of the vast surface area covered by vegetation, this role could be significant. Indeed, there is ample evidence that vegetation, particularly forests, remove a substantial quantity of airborne POPs, with consequent reductions in their atmospheric residence times. A steady-state mass-balance model determined that vegetation removed 4% of PAHs from the atmosphere in the northeastern region of the United States. It is evident that vegetation plays a significant role in the biogeochemical cycling of PAHs and other POPs possessing similar propensity for atmosphere-to- foliage transfer. The implications of this phenomenon in terms of human and ecotoxicological impacts are at present unclear, and this research topic is likely to receive much attention in the future. 31.3.3 Air–Sea Exchange For many large bodies of water, the atmosphere is a major route of contaminant input. For example, for the North Sea, which is heavily influenced by adjacent industrialized countries, atmospheric deposition is the major source of some trace metal inputs. Particles deposit to water surfaces in much the same manner as to land. There are clearly differences arising from the resistance terms in Equation 31.1 that are not identical for the two surfaces or the air above them. In general, similar patterns of behavior are observed, although there is stronger wind-speed dependence in the case of the sea, since surface characteristics change appreciably as high winds break up the sea surface into waves and spray. Aerosol produced from the sea surface itself can be enriched in trace elements that are trans- ported over land. The extent of enrichment is controversial, with some research indicating a major impact on aerosol and rainwater composition and other papers suggesting little effect in the case of trace metals. One clear example of enrichment is plutonium, cited above. Plutonium is enriched in aerosol and deposition some 30- to 500-fold relative to its abundance in bulk seawater. Fortu - nately, marine aerosol is of relatively large particle size and deposits within a rather narrow coastal band of land; the majority of deposition occurs within 5 km of the coast. Incorporation of contaminants in rainwater leads to wet deposition to both land and sea surfaces. Scavenging of contaminants may occur either within the cloud (termed “rainout”) or by falling raindrops (termed “washout”). The overall efficiency of the process is often described by the scavenging ratio, W, also known historically as the Washout Factor: Values of W for particulate species are typically around 200 to 1000, implying crudely that substances present in air at microgram-per-cubic-meter concentrations will occur in rain at milli - gram-per-liter-levels, as a cubic meter of air weighs 1.2 kg at 25ºC and 1 atm pressure. An alternative W concentration in rain mg kg 1– () concentration in air mg kg 1– () = [...]... Organic Contaminants 32 .4 Biomagnification and Trophic Transfer 32 .4. 1 Overview 32 .4. 2 Dietary Absorption 32 .4. 3 Dietary Bioavailability 32 .4. 4 Aquatic-Based Food Webs 32 .4. 4.1 Biomagnification 32 .4. 4.2 Food-Web Models 32 .4. 4.3 Maternal Transfer 32.5 Summary References © 2003 by CRC Press LLC 32.1 INTRODUCTION Concern for the bioaccumulation of contaminants arose in the 1960s because of incidents such as... McCartney, H A., A comparison of the predictions of a simple Gaussian plume dispersion model with measurements of pollutant concentration at ground-level and aloft, Atmos Environ., 14, 48 9, 1980 20 Williams, M L., Atmospheric dispersal of pollutants and the modelling of air pollution, in Pollution: Causes, Effects and Control, 4th ed., Harrison, R M., Ed., Royal Society of Chemistry, London, 2001 21... Lewis Publishers, Boca Raton, FL, 19 94 13 Watson, A J., Upstill-Goddard, R C., and Liss, P S., Air-sea gas exchange in rough and stormy seas measured by a dual-tracer technique, Nature(London), 349 , 145 , 1991 14 Brendel, P J and Luther, G W., Development of a gold amalgam voltammetric microelectrode for the determination of dissolved Fe, Mn, O2, and S (-II) in porewaters of marine and freshwater sediments,... Determinants of Bioaccumulation Aquatic sediments are formed from the deposition of particles and colloids and can act as both a sink and a source of pollutants Long-term contaminant input leads to sediment concentrations that can exceed the water concentration by several orders of magnitude because of partitioning of chemicals onto sediment-binding sites Determinants of the bioavailability of sediment-associated... permeability of the absorbing membrane (e.g., gill, skin) and other tissues constitutes a series of barriers to absorption and transfer of chemicals.1, 24 The absorption of a chemical from water by an aquatic animal can be viewed as a series of steps, each of which can govern the rate of uptake as well as the extent of accumulation.22 The skin, gill, and digestive tract are potential sites of absorption of waterborne... concentrations of PCBs in plankton do not occur in the environment and should not be used in food-web modeling Multiple studies have reported biomagnification of chlorinated pesticides in marine mammals and birds. 64, 79 Varanasi and Stein33 concluded that trophic transfer of low-molecular-weight PAHs to higher vertebrates was probable because of low metabolism in fish Trophic transfer of higher-molecular-weight... the cell surface to affect chemical uptake .44 A reduction in bioavailability due to decreased absorption of chemicals with large molecular size or shape is termed stearic hindrance Opperhuizen et al .45 suggested that the structure of the phospholipid membrane of the gill epithelium can restrict uptake of hydrophobic molecules of long chain length or large cross-sectional area by imposing a physical barrier... biodegradation. 64 32 .4. 4.2 Food-Web Models Aquatic food-web models describe the transfer of contaminants from surface water and sediment to higher trophic levels and can be used to predict contaminant bioaccumulation in organisms exposed by multiple pathways. 64 The models are composed of a series of equations that calculate contaminant concentrations in predator organisms (e.g., fish) from intake of contaminated... and fish .46 ,50,51,57 The gill surface is negatively charged and thus provides sites for ionic binding with the metal .46 Reid and McDonald46 speculated that lower-binding-affinity metals such as copper have a greater likelihood of entering the fish rather than binding at the gill surface 32.3.5 Bioconcentration of Organic Contaminants Bioconcentration factors of organic contaminants can vary 100-fold with... 1993 41 Yang, R., Thurston, V., Neuman, J., and Randall, D J., A physiological model to predict xenobiotic concentration in fish, Aquat Toxicol., 48 , 109, 2000 42 Mackay, D., Correlation of bioconcentration factors, Environ Sci Technol., 16, 2 74, 1982 43 Petersen, G I and Kristensen, P., Bioaccumulation of lipophilic substances in fish early life stages, Environ Toxicol Chem., 1998, 1385, 1998 44 Campbell, . and Precipitates Examples Mn 2+ Cd 2+ NiCl + HgCl 4 2- Zn-fulvates Pb-humates Co-MnO 2 Pb-Fe(OH) 3 Cu-clays PbCO 3(s) Soluble Colloidal Particulate © 2003 by CRC Press LLC The behavior shown in Figure 31 .4 for chromium in. that substances present in air at microgram-per-cubic-meter concentrations will occur in rain at milli - gram-per-liter-levels, as a cubic meter of air weighs 1.2 kg at 25ºC and 1 atm pressure transport of contam- inants are the resuspension of particles, the entrapment of water in pore spaces, the upward flow of pore water due to hydrostatic pressure gradients, and diffusional fluxes of