Bioavailability of Contaminants in Soils and Sediments Processes, Tools, and Applications (2003) / Chapter Skim
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3. Processes
3. Processes
Pages 119-215
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From page 119... ...
To appreciate the importance of this interaction, it is worth noting that for many chemicals of concern the fraction of contaminant mass that resides in the released form is orders of magnitude less than that which may be present in the bound form. For example, in Lake Michigan only 3 percent of the total polychlorinated biphenyl (PCB)
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From page 120... ...
FIGURE 1-1 Bioavailability processes in soil and sediment. Circulation within organism, accumulation in target organ, toxicokinetics, and toxic effects
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From page 121... ...
A wide range of solids exists in natural systems that vary in their reactivity toward organic and inorganic contaminants. Before discussing retention processes themselves, it is useful to review the types of solids in soils and sediments and to define how the terms soils and sediments are used in this report.
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From page 122... ...
This section discusses both primary and authogenic minerals, focusing mainly on clay minerals and organic compounds which are often the most reactive phases and thus most important for influencing bioavailability.
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From page 123... ...
This smallest size fraction is typically dominated in volume by secondary (authogenic) minerals composing a mineralogical class known as the clay minerals (a chemical definition of layered aluminosilicate minerals)
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From page 124... ...
causes them to be one of the most important classes of materials controlling contaminant-solid interactions. Clay minerals are layered silicates in which sheets of silicon coordinated by oxygen anions are bound with sheets of aluminum and/or magnesium coordinated by hydroxyl anions.
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From page 125... ...
; manganese oxides, while less abundant, have a very high reactivity. Collectively, these phases are termed hydrated metal oxides, and they often control the dissolved concentrations of inorganic contaminants such as lead or arsenic through reaction with ionizable surface functional groups.
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From page 126... ...
appear to dominate hydrophobic organic compound sorption and Resorption in some systems and potentially dominate bioavailability, even when they make up a small proportion of total carbon. These types of carbonaceous materials arise from geologic processes such as sediment burial and associated elevated temperature that (1)
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From page 128... ...
A1though humic substances are usually the dominant form of carbonaceous material in soils and modern sediments, they have much lower sorption capacity for hydrophobic organic contaminants than the more condensed carbon forms. The methods used to identify and, when appropriate, quantify the forms of carbonaceous matter in soil and sediment are described in Chapter 4.
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From page 129... ...
Fulvic acid Chemical, Soils, Aquatic High Moderate Electrostatic, sediments Hydrophobic Humic acid Chemical, Soils, Aquatic High Moderate Electrostatic, sediments Hydrophobic Humin Hydrophobic Soils, Aquatic Moderate Moderate sediments Kerogen Hydrophobic Soils, Aquatic Low High sediments, Subsurface sediment Coal Hydrophobic Soils, Aquatic Low High sediments, Subsurface sediment Soot Hydrophobic Soils, Aquatic Low High sediments, Subsurface sediment Clay minerals Electrostatic, Ubiquitous High Low Chemical Metal oxides Chemical, Soils, High Low Electrostatic Subsurface sediment Metal Chemical, Alkaline Low to carbonates Electrostatic environments moderate Low Metal sulfides Chemical, Electrostatic Aquatic High Low sediments aChemical reactivity denotes material having functional groups that tend to form bonds with contaminants through the sharing of electrons (covalent/ionic bonds)
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From page 130... ...
Certain extraction techniques discussed in Chapter 4 can be used to determine what percentage of the total mobile contaminant mass of interest is colloid-bound as opposed to dissolved in the aqueous phase. The potential for colloid-enhanced contaminant transport and organismal uptake of colloids depends on many factors, as discussed later in this chapter.
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From page 131... ...
3 G VIABLE ORGANICS, Fe S2 it_ BIOTA ~~ 131 FIGURE 3-2 Diagrammatic representation of the important trace element sinks on the surface of an idealized kaolinite crystal.
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From page 132... ...
Organic Contaminants The United States produces and consumes enormous quantities of organic and inorganic chemicals, some of which enter the environment through accidental or purposeful releases. Approximately eight million synthetic and naturally occurring organic compounds have been widely disseminated since the late nineteenth century (NRC, 1994)
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From page 133... ...
Pyrene lubricating oils Coal tarb Nitroaromatics 2,4,6-trinitrotoluene (TNT) Trifluralin Benefin Eth al fl Ural in Methyl parathion Military installations Bombing ranges Bactericides Pesticides Phenols, anilines Pentachlorophenol Wood preservative Phenylamide herbicides: Biocide phenylureas, phenylcarba-mates, Dyestuff wastewater and acylanilides Phenylamide herbicides Halogenated Polychlorinated biphenyls (PCBs)
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From page 134... ...
Thus their potential to be highly persistent in soils and sediments is generally less than for hydrocarbons such as PAHs. Inorganic Contaminants At least nine of the top 25 most frequently detected hazardous substances in groundwater are inorganic compounds, primarily metals (NRC, 1994~.
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From page 135... ...
As many of these contaminants occur in multiple chemical forms, the most important isotopes in terms of toxicity, mobility, and bioavailability are noted. Inorganic contaminants can exist in soil and sediment systems in the aqueous phase, as part of a precipitated mineral, or adsorbed on the surface of a mineral.
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From page 136... ...
The association reactions of organic and inorganic contaminants may differ appreciably. Inorganic contaminants associate with solids through physical or chemical bonding or through the precipitation of a new solid phase.
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From page 137... ...
Both are important processes for metal and metalloid retention but generally do not contribute to organic compound retention in soils and sediments. Retention of Inorganic Contaminants Unlike organic molecules, inorganic species cannot be degraded.
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From page 138... ...
will also lead to a strong association with the solid phase. If chemical interaction or precipitation is not operable, then associations via a physical attraction of an ion and surface of opposite charge may arise.
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FIGURE 3-4 Adsorption reactions illustrating (a) chemical interactions and (b)
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From page 140... ...
A transition from an electrostatic to a chemical association with increased reaction time, as discussed below, will modify the availability of the contaminant. Inorganic contaminants vary considerably in their tendencies to bind on soil and sediment components, even with similar solution conditions.
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(1976) Soil Science Society of America Journal.
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From page 142... ...
Moreover, the stability of the phase increases with age and thus will lead to diminished dissolved concentrations of transition metal contaminants. In summary, association of inorganic contaminants with solids in soil or sediment is typically dominated by adsorption processes.
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Dense SOME Amorphous SOM`` Aged or Weathered NAPL —NAPL SOM Mineral Phase - ~......................................... ~;~ ;~ FIGURE 3-6 Conceptual model of association and dissociation of hydrophobic organic compounds with soils and sediments.
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From page 144... ...
The magnitude of the contribution of each of the two components depends upon the relative abundance of the types of carbonaceous materials present in the sediment. SOURCE: Data reprinted, with permission, from Xia and Ball (1999~.
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From page 145... ...
is essentially constant (Schwarzenbach et al., 1993~. Furthermore, KoC can be correlated to physicochemical properties, such as the octanol-water partitioning coefficient or inverse of water solubility, for a variety of low polarity organic compounds (e.g., Karickhoff, 1981~.
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From page 146... ...
A better understanding of the operative mechanisms will be important to understanding the relative importance of adsorption versus partitioning of nonpolar organic compounds onto these solids. Polar and Ionizable Organic Compounds Compared to nonpolar organic compounds, polar and ionizable organic compounds are involved in more diverse binding mechanisms, which for ionizable compounds are similar to those outlined for inorganic contaminants.
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From page 147... ...
Clearly, although the binding of many polar and ionizable organic compounds can be readily reversible, the extent and kinetics can vary significantly depending on the compound and the solid phase. Overall, the retention of polar and ionizable compounds such as trinitrotoluene, chlorinated phenols, and other common compounds on soils and sediments is governed by a complex set of physical-chemical processes making it difficult to generalize about trends in behavior.
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From page 148... ...
This in turn leads to a decrease in the bioavailability of the contaminant with increased reaction time in both soils and sediments. Inorganic Contaminants The state of an inorganic contaminant bound to the solid phase may change on a micropore scale with increasing reaction time.
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From page 149... ...
With increased reaction time, a chemical bond may develop between the ion and surface functional group. Despite the strong retention, the ion may migrate along the surface (surface diffusion)
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From page 150... ...
Organic Contaminants Although the aging processes that affect the retention of organic contaminants to solids over time are less well understood than for inorganic contaminants, there are two general types: diffusional or reaction processes of the organic solute, and diagenetic processes that change the properties of the soil or sediment sorbent. Solute-based aging processes include chemical oxidation reactions that lead to solute incorporation into natural organic matter (Richnow et al., 1994; Burgos et al., 1996; Karimi-Lotfabad et al., 1996~; slow diffusion into very small pores (similar to Figure 3-9 for lead)
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From page 151... ...
The rate of contaminant volatilization from soil or sediment to a gas phase depends not only on the specific contaminant but also on environmental factors such as temperature. Contaminant release to the bulk aqueous phase of pore water or surface water involves multiple steps as the chemical moves through different soil or sediment compartments.
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From page 152... ...
.* ~ .~ ~ ~ ~ .,.`,`.`,>`,,,~,~,~,~,~,,~,,^~.~` FICURE 3-10 Scbem~Uc Olus~Ung me deso~Uon of ~ PCB molecule Tom sediment into porew~ter, and divisive transport of me PCB molecule ~roupb me sediment m~cropore to me sediment interface.
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From page 153... ...
In cases of phosphorus deficiency, plants can secrete organic acids along with H+ to solubilize soil phosphorus. One side effect of this is that in arseniccontaminated soils, phosphorus deficiency will induce elevated arsenic uptake and potential phytotoxicity, because both elements share the same uptake system
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From page 154... ...
or via dissolution of iron oxides. This may occur as a result of rhizosphere acidification or root proliferation and secretion of organic acids.
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P ~ ' Cyt. Felll He'll -PS FIGURE 3-11 Plants utilize two distinct strategies to access solid phase soil Fe.
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156 BIOAVAILABIL[l Y OF CONTAMINANTS IN SOILS AND SEDIMENTS In addition to plants and microorganisms altering the soil and sediment environment in order to better access compounds for themsleves, their activities can also gratuitously affect the bioavailability of compounds to other receptors.
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From page 157... ...
PROCESSES 157 As discussed in Box 3-3, the microbial reduction of iron in sediment (which has led to the release of iron oxide-bound contaminants) has contributed, along with other important processes, to serious arsenic exposure to humans in Bangladesh.
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From page 158... ...
In the San Francisco Bay, mixing occurred over a 30-cm depth in some locations during a six-month period (Fuller et al., 1999~. Summary For inorganic contaminants, a variety of mechanisms exist by which ions associate with the solid phase.
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From page 159... ...
Furthermore, pores may become occluded, thus entrapping contaminants within the particle and helping to minimize their bioavailability. Rates of desorption for both organic and inorganic contaminants from soils and sediments are highly variable and dependent on the mode of uptake, the time of reaction (aging)
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From page 160... ...
Soil particles at the soil-air interface can be entrained in air flows moving over the ground surface, or they can be suspended in surface runoff following precipitation. These contaminant-bearing particles may be transported directly to receptors, e.g., through inhalation by animals or deposition on plants, or to other environmental media, e.g., via atmospheric deposition or runoff to surface waters.
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From page 161... ...
Sediment-borne Contaminants Contaminated sediment particles at the sediment-water interface can be transported via resuspension in water flows moving along the sediment surface (Figure 3-13~. Due to their size, larger and heavier particles may be suspended for just a short period of time, resulting in their deposition after lateral transport for a short distance.
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From page 163... ...
A similar process occurs for any contaminant mass that is volatilized and moves up and out of the soil in soil gas. As shown in Figure 3-14, when this contaminant mass enters the air flowing over the contaminated soil area, it will be transported with the air in the prevailing wind direction sometimes for long distances.
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From page 164... ...
This illustrates the importance of accounting for bioavailability processes that operate both locally and remotely. If the fluid into which the contaminant is released is not flowing or flowing only at very slow rates, such as groundwater in low permeability soil or porewater in fine-grained sediments, molecular diffusion will be the primary means of transport.
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From page 165... ...
Transformation processes fundamentally alter the chemical form of inorganic contaminants. Microorganisms can mediate the transformation of species of elements from one form to another, for example the transformation of dissolved Hg2+ to extremely toxic methylmercury (CH3Hg+)
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From page 166... ...
Environmental conditions, including pH, affect the rate and extent of aerobic PCB biodegradation (Williams and May, 1997~. Under anaerobic conditions such as typically found in PCB-contaminated sediments, reductive dechlorination can occur resulting in an increase in lesschlorinated PCBs, that is, mono-, di-, and tri-chlorobiphenyls (Brown et al., 1987, 1988; Natarajan et al., 1996)
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From page 167... ...
In summary, the chemical form of the contaminant released from soil and sediment, the geochemical environment where the release and transport take place, and the fluid properties of that environment will determine the form, delivery route, and delivery rate of contaminant to biological receptors. All of these factors must be considered in assessing the availability of a soil or sediment contaminant to biological receptors.
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From page 168... ...
The ability of membranes to serve as selective barriers is a function of their structure. Biological membranes are composed primarily of phospholipids arranged in a bilayer, with the hydrophobic portion of the molecules oriented toward the middle of the membrane and the hydrophilic portion toward the outside (Figure 3-15~.
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From page 169... ...
typically diffuse through the cellular membrane, such that the microbial uptake of many hydrophobic solvents (e.g., alkalies, mono- and polynuclear aromatic hydrocarbons) is often a simple passive diffusion process (Baseman et al., 1986; Sikkema et al., 1995; Bugg et al., 2000~.
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From page 170... ...
As with facilitated diffusion, there is specificity in the binding of chemicals to these carrier proteins. Active secretion of organic acids and bases by the kidneys, for example, utilizes membrane active transport processes.
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From page 171... ...
A colloid- or particle-bound contaminant can reside in the gastrointestinal tract for hours to days plenty of time for the unique environment of the gut to affect particlecontaminant associations. Although the membrane transport processes described above are universal, digestive processes result in more complicated membrane transport phenomena than occur, for example, across the gill in aquatic organisms or across the skin in mammals.
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From page 172... ...
and surfactants can occur in the gut fluids (Mayer et al., 1997) and are very effective in solubilizing sediment-associated metals and organic contaminants (e.g., PAHs)
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From page 173... ...
This can occur by passive diffusion, by carrier-mediated transport (active transport or facilitated diffusion) , or by pinocytosis, depending upon the chemical.
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From page 174... ...
The gastrointestinal absorption of most environmental contaminants probably occurs by passive diffusion, but there appear to be many exceptions. Many inorganics are nutrients, and specialized transporters exist to regulate and facilitate their absorption from the gastrointestinal tract.
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From page 175... ...
(1989) Ellis Horwood Ltd.
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From page 176... ...
The stratum corneum overlies the viable epidermis and in humans consists of several layers of flattened, keratinized, dead cells called corneocytes. Corneocytes are stacked together like over-lapping plates and bound together by adherent structures (called corneodesmosomes)
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From page 177... ...
Hair follicles and eccrine sweat glands offer pathways for chemicals to reach the dermis and hypodermic without having to cross the stratum corneum. Within the hair follicles, the space surrounding the hair shaft is filled with sebum, through
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From page 178... ...
Hair follicles and sweat glands, although offering means for chemicals to circumvent the stratum corneum barrier, have usually been regarded as minor pathways for dermal absorption because they comprise a very small percentage of the surface area of the skin. However, experiments using rat skin where hair follicles and sweat gland pathways have been eliminated suggest that, at least in some circumstances, their contribution to dermal absorption may be substantial (Zatz, 1993~.
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From page 179... ...
These conducting airways are lined with epithelial cells and mucous-secreting cells. The upper airways contain numerous ciliated cells.
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From page 180... ...
Particles in the alveoli may be absorbed directly into the lymphatics because the endothelial cells lining the alveolar lymphatic capillaries are porous, allowing relatively large molecules to enter. Finally, partial or complete dissolution of the particle in the alveolus can result in absorption into the blood or lymphatics, primarily through passive diffusion.
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From page 181... ...
In addition, it is not clear how well plants can distinguish between ions of similar size and charge. The size of solid particles precludes their entry into plant roots, even for very small particles like colloids, such that contaminant release from the solid phase is a prerequisite regardless of the underlying uptake mechanism.
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From page 182... ...
studied because consumption of plant tissue with elevated cadmium at levels below phytotoxic thresholds has resulted in human fatalities. Plant zinc concentrations, soil temperature and moisture status, soil solution chloride concentration, pH, total and extractable cadmium concentrations, and plant species and cultivar have all been found to affect plant uptake of cadmium (McLaughlin and Singh, 1999)
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From page 183... ...
Because soil and sediment play no role at this stage, accumulation and subsequent effects are not considered bioavailability processes per se. However, they are influenced by other bioavailability processes and thus are indicators of bioavailability, they are frequently measured endpoints, and they are of great concern to some stakeholders.
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From page 184... ...
seem to function primarily as transporters for certain metal ions (e.g., Scott and Bradwell, 1983~. Even relatively hydrophilic organic chemicals can be bound to plasma proteins such as albumin.
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From page 185... ...
In most instances, this incorporation of a contaminant into the lignin of the plant transforms the compound to a state in which it is no longer bioactive. Similarly, animals can bind both inorganic contaminants such as metals and organic compounds in such a way that the compounds are not available to interact with critical structural or functional biomolecules.
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From page 186... ...
It involves modification of existing reactive functional groups by combining either the original or an altered molecule with sugars, amino acids, or other compounds. In keeping with the above example, this might include the conjugation of the Phase I product benzoic acid with glycine to form hippuric acid (i.e., benzoylglycine)
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From page 187... ...
In theory, passive diffusion of chemicals from the plasma to the urine can also occur in the renal tubules, although this mechanism is probably a minor contributor to overall urinary excretion because concentration gradients typically favor reabsorption more than excretion (Wilkinson et al., 2001~. (Organic acids and bases, which at certain pH values are significantly ionized in the urine and, thus "trapped", are exceptions.)
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From page 188... ...
neither intestinal non-absorption nor biliary excretion can explain the concentration of toxicant found in the feces. In these cases direct passive diffusion from the blood has been proposed as the mechanism for fecal excretion (Dayton et al., 1983~.
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From page 189... ...
in that their structure resembles the natural ligand 1713-estradiol (Shi et al., 2001~. Many PCB congeners are metabolized in viva to more polar compounds that can further disrupt normal estrogen system activity (Bergman et al., 1994; Koga et al., 1992; Schultz et al., 1998~.
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From page 190... ...
At a higher level, the active removal of damaged cells (apoptosis or programmed cell death) can be disrupted by chemical contaminants.
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From page 191... ...
In order to capture processes important for higher order organisms, Figure 11 can be made more detailed to show food web transfer of contaminants from prey to predators and other higher order organisms (see Figure 3-21~. In fact, food chain transfer is probably a more important exposure pathway to contaminants in soils and sediment for higher order animals than is direct ingestion of the soil or
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From page 192... ...
Figure 3-21 captures some additional processes that control the bioavailability of contaminants in soils and sediment to higher order animals, in particular the extent of contaminant uptake through the biological membranes of each successive organism and the resulting bioaccumulation in each organism. If there is sufficient biomagnification through the food web, higher organisms can be exposed to contaminants that originated in soils and sediments at concentrations high enough to cause adverse effects.
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From page 193... ...
. Most studies have worked with anecic or epigeic species because they are thought to be more abundant and important in food webs (Diercxsens et al., 1985~.
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From page 194... ...
From the above it is clear that although geochemical processes may have broad effects relevant to bioavailability, biological processes determine how each organism is exposed to that geochemical milieu, and substantial differences in that exposure are possible among species and among contaminants. It is not practical to understand all biological factors for all species in the near term (e.g., contaminant assimilation from all combinations of food sources available to all benthos)
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From page 195... ...
. The food web concept defines interactions of interrelated food chains and takes into account species participation in multiple food chains over different trophic levels (see Figure 3-22)
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From page 196... ...
. I Omnivorous/Carnivorous Birds Gulls Terns Grebes Fish detritus-consumers plankton feeders herbivorous species 1 ~ Primary Producers Algae Exposure Media Water Column | PCBs in Sediments | · Suspended Sediment Plumes FIGURE 3-22 Food web with examples of representative species.
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From page 197... ...
PROCESSES 197 accumulation, and metabolic processes as the chemicals move from one trophic level to the next (Fisk et al., 2001~. This is described in Box 3-4 for the case of PCBs, in which certain congeners are more or less bioavailable depending on what trophic level is being considered.
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From page 198... ...
Bioavailability of a contaminant to a receptor will be determined by the combined effect of these processes, as well as by the properties of the soil or sediment, the contaminant, and the receptor of interest. In particular, the heterogeneity of soils and sediments has a profound effect on bioavailability processes.
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From page 199... ...
The following specific recommendations address the most pressing knowledge gaps deemed necessary for better understanding, predicting, and measuring bioavailability processes. An improved understanding of contaminant-solid interactions is needed, especially regarding the nature and effects of aging on contaminant release rates.
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From page 200... ...
2002. New modeling paradigms for the sorption of hydrophobic organic chemicals to heterogeneous carbonaceous matter in soils, sediments, and rocks.
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From page 201... ...
1995. Microbial reductive dechlorination of polychlorinated biphenyls.
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From page 202... ...
1994. Application of a permeant/ polymer diffusional model to the desorption of polychlorinated biphenyls from Hudson River sediments.
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From page 203... ...
1983. Elimination of drugs by passive diffusion from blood to intestinal lumen: factors influencing nonbiliary excretion by the intestinal tract.
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From page 204... ...
1998. Bioaccumulation of polychlorinated biphenyls from sediments to aquatic insects and tree swallow eggs and nestlings in Saginaw Bay, Michigan, USA.
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From page 205... ...
2000. Microscale location, characterization, and association of polycyclic aromatic hydrocarbons on harbor sediment particles.
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From page 206... ...
2000. Changes in food web structure affect rate of PCB decline in herring gull (Laws argentatus)
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From page 207... ...
2002. Characteristics of trophic transfer of polychlorinated biphenyls in marine organisms in Incheon North Harbor, Korea.
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From page 208... ...
1997a. Sequestration of hydrophobic organic contaminants by geosorbents.
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From page 209... ...
1992. Illustrated handbook of physical-chemical properties and environmental fate of organic chemicals.
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From page 210... ...
1988. Aqueous solubility of polychlorinated biphenyls related to molecular structure.
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From page 211... ...
1988. Arteriosclerotic plaque development is promoted by polynuclear aromatic hydrocarbons.
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From page 212... ...
1999. Role of chemical and ecological factors in trophic transfer of organic chemicals in aquatic food webs.
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From page 213... ...
1994. Reductive dechlorination of polychlorinated biphenyls in St.
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From page 214... ...
2002. Polycyclic aromatic hydrocarbons present in cigarette smoke cause endothelial cell apoptosis by a phospholipase A2-dependent mechanism.
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From page 215... ...
2000. Microbial reductive dehalogenation of polychlorinated biphenyls.
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