Benzo[a]pyrene bioavailability from pristine soil and contaminated sediment assessed using two in vitro models

A major route of exposure to hydrophobic organic contaminants (HOCs), such as benzo[a]pyrene (BaP), is ingestion. Matrix-bound HOCs may become bioavailable after mobilization by the gastrointestinal fluids followed by sorption to the intestinal epithelium. The purpose of this research was to measure the bioavailability of [14C]-BaP bound to pristine soils or field-contaminated sediment using an in vitro model of gastrointestinal digestion followed by sorption to human enterocytes (Caco-2 cells) or to a surrogate membrane, ethylene vinyl acetate (EVA) thin film. Although Caco-2 cells had a twofold higher lipid-normalized fugacity capacity than EVA, [14C]-BaP uptake by Caco-2 lipids and EVA thin film demonstrated a linear relationship within the range of BaP concentrations tested. These results suggest that EVA thin film is a good membrane surrogate for passive uptake of BaP. The in vitro system provided enough sensitivity to detect matrix effects on bioavailability; after 5 h, significantly lower concentrations of [14C]-BaP were sorbed into Caco-2 cells from soil containing a higher percentage of organic matter compared to soil with a lower percentage of organic matter. The [14C]-BaP desorption rate from Caco-2 lipids consistently was twofold higher than from EVA thin film for all matrices tested. The more rapid kinetics observed with Caco-2 cells probably were due to the greater surface area available for absorption/desorption in the cells. After 5 h, the uptake of BaP into Caco-2 lipid was similar in live and metabolically inert Caco-2 cells, suggesting that the primary route of BaP uptake is by passive diffusion. Moreover, the driving force for uptake is the fugacity gradient that exists between the gastrointestinal fluid and the membrane.     

Mobilization of chrysene from soil in a model digestive system

Accurate estimates for the oral bioavailability of hydrophobic contaminants bound to solid matrices are challenging to obtain because of sorption to organic matter. The purpose of this research was to measure the bioavailability of [14C]chrysene sorbed to soil using an in vitro model of gastrointestinal digestion and absorption to a surrogate intestinal membrane, ethylene vinyl acetate (EVA) thin film. The [14C]chrysene moved rapidly from soil into the aqueous compartment and reached steady state within 2 h. Equilibrium was reached in the EVA film within 32 h. Aging the spiked soil for 6 or 12 months had no effect on chrysene mobilization. This was supported by the finding that the data best fit a one-compartment model. Despite significant decreases in [14C]chrysene mobilization when water or nonneutralized gastrointestinal fluids were used in place of the complete medium, the equilibrium concentration of [14C]chrysene in EVA film remained the same in all conditions. Thus, the driving force for uptake was the fugacity gradient between the aqueous phase and the EVA film. Cultured human enterocytes (human colorectal carcinoma cell line [Caco-2 cells]) had a higher lipid-normalized fugacity capacity than EVA film, but the elimination rate constants were the same, suggesting that the rate was controlled by the resistance of the unstirred aqueous layer at the membrane-water interface.     

Bioavailability of Hydrophobic Organic Contaminants in Sediment with Different Particle-Size Distributions

Few studies have been conducted examining the distribution of different-sized particles in sediment and its potential impact on bioavailability of sediment-associated contaminants. In the current study, three sediments composed of different particle sizes, i.e., fine (0–180 μm), combined (0–500 μm), and coarse (180–500 μm), were used to evaluate the bioaccumulation potential and toxicokinetic rates of four hydrophobic organic contaminants (HOCs) including two polychlorinated biphenyls (PCB-101 and PCB-118), a metabolite of an organochlorine insecticide (p,p′-DDE), and a polybrominated diphenyl ether (BDE-47) to the benthic oligochaete Lumbriculus variegatus. Two chemical approaches, Tenax extraction and matrix-solid phase microextraction (SPME), were also used to measure bioavailability of the sediment-associated HOCs. The uptake and elimination rates of HOCs by L. variegatus from coarse sediment were greater than those from fine sediment, although the biota–sediment accumulation factors (BSAFs) were not significantly different among sediments with different particle sizes. The freely dissolved HOC concentrations measured by matrix-SPME were greater in coarse sediment, however, no difference was found in uptake and desorption rates for the matrix-SPME and Tenax extraction measurements. Although BSAFs in L. variegatus were the same among sediments, kinetic rates of HOCs for organisms and freely dissolved HOC concentrations were lower in fine sediment, suggesting that sediment ingestion may also play a role in organism uptake, especially for HOCs in fine sediment.     

Characterization of polycyclic aromatic hydrocarbon bioavailability in estuarine sediments using thin-film extraction

It is well documented that the bioavailability of hydrophobic organic chemicals (HOCs) can vary substantially among sediments. This makes risk assessments based on total sediment concentrations problematic. The present study investigates the application of thin-film solid-phase extraction to measure bioavailable concentrations of phenanthrene in estuarine sediment by comparing concentrations of phenanthrene in the amphipod Corophium colo and in thin ethylene/vinyl acetate films at different concentrations in three geochemically different sediments. For all sediment types, concentrations of phenanthrene in sediments and thin films followed linear relationships, indicating first-order exchange kinetics. Organism/thin-film concentration ratios did not vary systematically among sediment types but dropped significantly with increasing phenanthrene concentration in the sediments. While at low phenanthrene concentrations in the sediment fugacities of phenanthrene in the amphipods approached the fugacities in the thin films, they were significantly lower than those in the sediments at higher concentrations. While phenanthrene concentrations in the three sediment types were identical, biota sediment accumulation factors and concentrations in amphipods and thin films were consistently lower in sediments enriched with black carbon than in sediments with sedimentary organic matter bearing a more diagenetic organic signature. It is concluded that, for the range of concentrations tested, thin-film solid-phase extraction can be a useful tool in the characterization of differences in bioavailability of HOCs among sediment types.     

The sorption of polycyclic aromatic hydrocarbons in Zegrzyńskie Lake

The sorption of polycyclic aromatic hydrocarbons in aquatic sediments from Zegrzyńskie Lake was examined. Batch experiment was performed in order to determine sorption efficiency in different kinds of sediments from Zegrzyńskie Lake. Five polycyclic aromatic hydrocarbons (phenanthrene, fluoranthene, pyrene, chrysene benzo[a]pyrene) were chosen to this experiment and sorption process was examined on seven sediments of different properties. Chosen hydrocarbons are of different structure of molecule and different chemical and physical properties. Concentrations of polycyclic aromatic hydrocarbons in aquatic sediments and in water phase were measured in the following order: extraction with dichloromethane, concentration on rotary evaporator, silica gel clean up, n-hexane elution, concentration on rotary evaporator and in vials, GC/MS analysis. Chemical composition of aquatic sediments were examined using methods for sewage sludge and soils analysis. In every sediment concentrations of PAHs, organic matter and organic: carbon, phosphorus, nitrogen and sulphur were measured. Also fractional analysis of sediments was made. Isotherms of sorption were measured for these sediments and compounds. Equations of these isotherms were performed and were used in order to find relationships between sorption efficiency and sediments composition. Depending on sediment properties and composition different concentrations of polycyclic aromatic hydrocarbons were found in solid phase. Sediments of high quantities of organic matter and small particles were the best sorbents for PAHs. Fluoranthene, pyrene and chrysene were efficiently sorbed in sediments of high concentration of organic matter. And efficiency of phenanhrene and benzo[a]pyrene sorption were better in sediments with high quantity of organic sulphur.     

Impact of sediment manipulation on the bioaccumulation of polycyclic aromatic hydrocarbons from field-contaminated and laboratory-dosed sediments by an oligochaete

The accumulation kinetics of polycyclic aromatic hydrocarbons (PAHs) by the freshwater oligochaete Lumbriculus variegatus were measured for field-contaminated and laboratory-dosed sediment. In addition, sediment manipulations typically used for homogenization and dosing in bioaccumulation assays were compared. Rather than an asymptotic approach to steady state, both resident and dosed PAH accumulation exhibited a peak during the 14-d assays, with steeper declines being noted for the lower-molecular-weight compounds. Lack of evidence of a peak for higher-molecular-weight PAHs may be due to slower kinetics and the short length of the assay. Relative to minimally mixed sediment, slurried sediment enhanced the accumulation of less-soluble resident PAHs, did not affect moderately soluble PAHs, and reduced the uptake of the more-soluble PAHs, fluorene and phenanthrene. Aging sediment after mixing reduced the availability of highly to moderately soluble resident PAHs but had no effect on less-soluble PAHs. A similar effect was noted for dosed PAHs, though a larger reduction in bioavailability was observed. Dosed PAH uptake clearance coefficients (ks) exceeded those of minimally mixed resident PAHs by factors of 3 to 4 for pyrene and 26 for benzo[a]pyrene. These results demonstrate that sediment manipulations and contamination history need to be considered when measuring PAH bioaccumulation.     

Polycyclic aromatic hydrocarbon bioaccumulation by meiobenthic copepods inhabiting a superfund site: techniques for micromass body burden and total lipid analysis

Microtechniques for polycyclic aromatic hydrocarbon (PAH) body burden and total lipid analysis were developed and applied to determine the first lipid-normalized bioaccumulation factors for a hydrophobic organic toxicant in a meiobenthic organism (0.063-0.500 mm) living in field-contaminated sediments. The total lipid microtechnique combines the standard Bligh-Dyer extraction method with a colorimetric quantification method for analysis of samples containing I to 50 microg lipid. The microtechnique for body burden analysis quantifies PAHs from tissue samples containing as little as 10 pg PAH. Fluoranthene, benz[a]anthracene, and benzo[a]pyrene biota-sediment accumulation factors (BSAFs) were determined for the meiobenthic copepod Microarthridion littorale living in an estuarine U.S. Environmental Protection Agency Superfund site. Gravid female, nongravid female, and male BSAFs were 0.82, 0.54, and 0.36, respectively, for fluoranthene; 0.50, 0.44, and 0.40, respectively, for benz[a]anthracene; and 0.09, 0.12, and 0.15, respectively, for benzo[a]pyrene. Comparison of nonlipid-normalized bioaccumulation factors (BAFs) to BSAFs indicates that M. littorale bioaccumulated PAHs on a gram lipid basis. The BSAFs declined consistently with increasing PAH log K(ow) for all copepod sex and reproductive stages. Sex- and stage-specific comparisons of BSAFs suggest that differences in lipid content and quality may lead to differences in BSAF values depending on PAH molecular weight and/or hydrophobicity.     

Examining the role of temperature and sediment-chemical contact time on desorption and bioavailability of sediment-associated tetrabromo diphenyl ether and benzo(a)pyrene

The effects of temperature and sediment-chemical contact time on desorption and bioaccumulation of sediment-spiked (14)C-labelled 2,2',4,4'-tetrabromo diphenyl ether (BDE-47) and benzo(a)pyrene (BaP) were examined. Experiments were performed after 2 or 6 weeks and 23 or 24 months sediment-chemical contact time at 10 and 20 degrees C. Desorption was measured in a sediment-water suspension using Tenax extraction, and bioaccumulation was measured by exposing Lumbriculus variegatus (Oligochaeta) to BDE-47 and BaP-spiked sediments in a 10d kinetic study. Biota-sediment accumulation factors (BSAFs) ranged between 2.9 and 4.3 for BDE-47 and between 0.5 and 0.9 for BaP. Thus, temperature and aging had a minor effect on bioavailability estimates. On the other hand, the difference between the chemicals was clear and could not be interpreted solely by reference to the size of the desorbing fractions, although the rapidly desorbing fraction-revised estimate clearly reduced the difference. The remaining discrepancy may be related to methodological (Tenax extraction vs. worm exposure) and/or biological (digestive extraction) causes. However, the data support the role of diffusional forces in the bioavailability of sediment-associated organic contaminants. Therefore, desorption-revised bioavailability estimates would lead to more precise bioavailability estimates than the traditional sediment organic carbon-organisms' lipids-based equilibrium partitioning approach.     

Dependency of polychlorinated biphenyl and polycyclic aromatic hydrocarbon bioaccumulation in Mya arenaria on both water column and sediment bed chemical activities

The bioaccumulation of polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) by the filter-feeding soft-shell clam Mya arenaria was evaluated at three sites near Boston (MA, USA) by assessing the chemical activities of those hydrophobic organic compounds (HOCs) in the sediment bed, water column, and organisms. Polyethylene samplers were deployed to measure the activities of HOCs in the water column. Sediment activities were assessed by normalizing concentrations with sediment-water sorption coefficient values, including adsorption to black carbon in addition to absorption by organic carbon. Likewise, both lipids and proteins were considered in biota-water partition coefficients used to estimate chemical activities in the animals. Chemical activities of PAHs in M. arenaria were substantially less than those of the corresponding bed sediments in which they lived. In contrast, chemical activities of PCBs in M. arenaria often were greater than or equal to activities in the corresponding bed sediments. Activities of PAHs, such those of pyrene, in the water column were undersaturated relative to the sediment. However, some PCBs, such as congener 52, had higher activities in the water column than in the sediment. Tissue activities of pyrene generally were in between the sediment and water column activities, whereas activity of PCB congener 52 was nearest to water column activities. These results suggest that attempts to estimate bioaccumulation by benthic organisms should include interactions with both the bed sediment and the water column.     

Bioavailability of polycyclic aromatic hydrocarbons in field-contaminated Anacostia River (Washington, DC) sediment

Sediment-water partitioning behavior and bioavailability of five polycyclic aromatic hydrocarbons (PAHs; phenanthrene, pyrene, chrysene, benzo[k]fluoranthene, and benzo[a]pyrene) were measured in field-contaminated sediment collected from moderately polluted regions of the Anacostia River (Washington, DC, U.S.A.). Much of the sediment PAH burden was resistant to desorption: Effective partition coefficients were 2- to 10-fold greater than expected from literature values, and more than 80% of PAHs remained sorbed after treatment of the sediment with a nonionic polymeric adsorbent (Amberlite XAD-2) for 20 h. Bioaccumulation, elimination, and assimilation of each PAH in the deposit-feeding tubificid oligochaete Ilyodrilus templetoni were measured and compared with the equivalent measurements from laboratory-inoculated sediment. Ilyodrilus templetoni effectively accessed the desorption-resistant fraction of these organic contaminants, as exhibited by high single-gut passage assimilation efficiencies (ASEs) of the five PAHs (60% < ASE < 90%). However, steady-state accumulations of PAHs by I. templetoni were very low and consistent with low pore-water concentrations. The present results suggest that steady-state accumulation of PAHs is controlled by pore-water concentrations and is not necessarily related to route of uptake or assimilation efficiencies.     

Toxicokinetics of sediment-sorbed benzo[a]pyrene and hexachlorobiphenyl using the freshwater invertebrates Hyalella azteca,Chironomus tentans,and Lumbriculus variegatus

This study investigated the effect of long-term sediment aging on the toxicokinetics of benzo[a]pyrene (BaP) and hexachlorobiphenyl (HCBP) using three freshwater benthic invertebrates. Hyalella azteca, Chironomus tentans, and Lumbriculus variegatus were exposed to BaP- and HCBP-spiked sediments that were aged for 7 d or 1.5 years. The toxicokinetics of the two compounds were determined for each test organism using a two-compartment model. The modeling of BaP was more complex because biotransformation was included within the model. The results of this study showed that the HCBP uptake clearance rates (k(s)) for each species were generally an order of magnitude greater than those determined for BaP and this difference was most likely due to preferential and rapid binding of BaP to sediment particles. Overall, the bioavailability of HCBP in spiked sediments tended to decrease with duration of aging, based on k(s) values and bioaccumulation factors (BAFs). However, the decreases in bioavailability appear to be species specific. Benzo[a]pyrene did not decline in bioavailability for the species tested because it may resist movement into the micropores of the sediment due to its large size. In addition to the bioassays, this article outlines a method for toxicokinetic modeling of biotransformed compounds and methods for statistical comparisons of kinetic parameters (i.e., k(s), k(d)...) and BAF values.     

Ability of polycyclic aromatic hydrocarbons to induce 7-ethoxyresorufin-o-deethylase activity in a trout liver cell line.

Along with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 24 unsubstituted polycyclic aromatic hydrocarbons (PAHs) were evaluated for their ability to induce 7-ethoxyresorufin-o-deethylase (EROD) activity in the rainbow trout liver cell line RTL-W1. When the duration and cell density of exposure were increased, the EC(50) for EROD induction was relatively constant for TCDD, but increased for PAHs. Regardless of exposure conditions, EROD activity was not induced by 9 PAHs: naphthalene, phenanthrene, anthracene, pyrene, perylene, acenaphthylene, acenaphthene, fluorene, and fluoranthene. Two PAHs, benzo[g,h,i]perylene and coronene, induced EROD activity inconsistently. The remaining 13 PAHs consistently induced EROD activity. The EC(50)s for induction exhibited approximately a 110-fold range. The order of potency, from most to least potent, was benzo[k]fluoranthene, dibenzo[a,i]pyrene, dibenzo [a,h]anthracene, benzo[a]pyrene, indeno[1,2,3-cd]pyrene, benzo [b]fluoranthene, pentacene, benzo[b]anthracene, benzo[b] fluorene, chrysene, benzo[a]anthracene, benzo[e]pyrene, and triphenylene. When the induction potency was expressed relative to TCDD, the toxic equivalency factors (TEFs) ranged from 0.001 to 0.000 01. When expressed relative to benzo[a]pyrene, the TEFs ranged from 3.44 to 0. 03.     

Supercritical Fluid Extraction of Polycyclic Aromatic Hydrocarbons from Seaweed Samples Before and After the <Emphasis Type="Italic">Prestige</Emphasis> Oil Spill

Samples of seaweed which are used for human consumption were collected along the Galician coast (NW Spain), in order to determine the level of contamination from polycyclic aromatic hydrocarbons, by supercritical fluid extraction and liquid chromatographic analysis. No detection was made of benzo[a]pyrene, benzo[a]anthracene, benzo[b]fluoranthene, benzo[ghi]perylene and dibenzo[ah]anthracene. PAHs were found and quantified in only two samples. The PAHs found were the following: anthracene, chrysene, fluoranthene, fluorene and pyrene. The levels found were below maximum limits established by the Spanish Food Safety authority (<200 mg/kg dry weight). Here we show that no relevant effects were detected in terms of PAHs contamination in seaweed.     

Multiple mechanisms contribute to the biodegradation of benzo[a]pyrene by petroleum-derived multicomponent nonaqueous-phase liquids

The presence of multicomponent nonaqueous-phase liquids (NAPLs) on contaminated sites critically alters the biodegradation susceptibility of many target pollutants, including polycyclic aromatic hydrocarbons. This study investigated the effects of petroleum-derived multicomponent NAPLs on biodegradation of benzo[a]pyrene by a bacterial consortium in liquid culture. When high-boiling point diesel fuel distillate (HBD)-NAPL was added to liquid culture, the consortium initiated benzo[a]pyrene mineralization after a lag period of several days. This lag period was not observed in the mineralization of phenanthrene, anthracene, and chrysene by the same consortium with HBD. Nonaqueous-phase liquids added to cultures pregrown before experimentation largely affected the extent of benzo[a]pyrene mineralization and the duration of lag period in subsequent experiments, suggesting that NAPL presence was important for maintaining the efficiency of the mineralizing consortium. Experiments using further fractionated oil components suggested that stimulation of benzo[a]pyrene mineralization by NAPLs was fraction dependent; an alkylated aromatic fraction was more effective than aromatic and aliphatic fractions. The effect of NAPL on benzo[a]pyrene biodegradation was determined to be multimechanistic; that is, NAPL acted as a cosolvent for polycyclic aromatic hydrocarbon dissolution, as a substrate to induce cometabolic degradative pathways, and as an agent to formulate the effective microbial consortium. Data suggest that the third mechanism was of particular importance for rapid benzo[a]pyrene mineralization.     

Effect of bile salts, lipid, and humic acids on absorption of benzo[a]pyrene by isolated channel catfish (Ictalurus punctatus) intestine segments

Dietary absorption of lipophilic contaminants may be a significant route of exposure in aquatic organisms. Bile salts, lipids, and humic acids are important factors that may influence the intestinal absorption of a contaminant such as benzo[a]pyrene (BaP). We hypothesized that bile salts, monoglycerides, and free fatty acids would increase BaP intestinal absorption, while triglycerides, humic acids, and sediment would decrease BaP intestinal absorption. We have established and validated an in vitro model to examine modification of 3H-BaP absorption in everted intestinal segments from channel catfish (Ictalurus punctatus). Uptake of BaP into the everted intestinal segments continued to increase over the times examined in this study (60 min) and apparently occurs passively; thus, fugacity-based models of uptake are supported. Absorption of BaP into intestinal cells was significantly decreased by the addition of monoglycerides and free fatty acids to bile salts in the incubation media. Addition of triglycerides decreased BaP absorption even further. Humic acids may have decreased BaP intestinal absorption, while natural sediment may have increased BaP absorption. The results of this study suggest that all lipids may decrease intestinal uptake of lipophilic contaminants if they remain in unabsorbable excess in the intestinal lumen by retaining BaP in lipid/bile micelles. In contrast, if triglycerides are hydrolyzed into monoglycerides/free fatty acids prior to absorption, lipophilic contaminant uptake will likely be facilitated. Thus, it may be the hydrolytic state of lipids that determines its effects on BaP absorption. Humic acids alone may decrease dietary uptake of BaP, but our results suggest that other components in natural sediment may counteract this effect to cause a slight enhancement of BaP uptake. Further studies are needed to determine the dietary conditions necessary for bioaccumulation to contribute significantly to lipophilic contaminant body burdens in benthivorous fish. Finally, the everted intestinal segment technique has the potential to be used in other species and with different contaminants.     

Separation and characterization of gall bladder bile metabolites from speckled trout, Salvelinus fontinalis, exposed to individual polycyclic aromatic compounds

Speckled trout, Salvelinus fontinalis, were orally exposed to individual polycyclic aromatic compounds (PACs) represented by benzo[a]pyrene, carbazole, chrysene, dibenzofuran, dibenzothiophene, fluorene, phenanthrene, and pyrene. Fish were sacrificed 7 d after exposure and the gall bladder removed for bile analysis. High pressure liquid chromatography (HPLC) with fluorescence (F) and ultraviolet (UV) detection was used to determine the presence of PAC derivatives in the bile without pretreatment. Glucuronide conjugates were predominant in all exposures with variable amounts (0-53%) of phenols and starting material. Identification of compounds was confirmed by selective extraction of less polar nonconjugated PACs and enzymatic hydrolysis of water-soluble material. This was followed by HPLC and/or gas chromatography-mass spectrometry (GCMS) characterization of the produced phenols. Total metabolite levels varied widely among compounds.     

Relative Importance of Ingested Sediment Versus Pore Water as Uptake Routes for PAHs to the Deposit-Feeding Oligochaete <Emphasis Type="Italic">Ilyodrilus templetoni</Emphasis>

The relative role of sediment pore water and ingested sediment particles to the total uptake of sediment-associated hydrophobic organic contaminants was examined by estimation from a water-only exposure experiment and from a bioenergetic-based toxicokinetic model utilizing experimentally measured sediment ingestion rates, assimilation efficiencies, and elimination rates. Phenanthrene (PHE) and benzo[a]pyrene (BaP) uptake in the bulk deposit-feeding oligochaete, Ilyodrilus templetoni, was measured. Assimilation efficiencies (ASE) were measured using a pulse-chase technique, based on a single-gut-passage time. Sediment-associated phenanthrene exhibited a lower ASE (50%) compared to BaP (80%), possibly due to a general relationship between assimilation and compound logK _ow. Estimated uptake of phenanthrene from pore water alone was essentially equal to the observed total uptake from both ingested sediment and sediment pore water. Estimated contribution of sediment-bound phenanthrene accounted for less than 20% of the total uptake. For benzo[a]pyrene, estimated uptake from sediment ingestion accounted for essentially all of the total uptake and estimated absorption from pore water accounted for <5% of the total uptake. This research provides direct experimental evidence for a predicted increase in the importance of sediment ingestion relative to the pore water route of exposure as the hydrophobicity of organic contaminants increases.     

The Role of Ingestion as a Route of Contaminant Bioaccumulation in a Deposit-Feeding Polychaete

A mass-balance bioaccumulation model was used to examine the bioaccumulation of benzo[a]pyrene (BaP) from ingested sediment by the deposit-feeding polychaete Abarenicola pacifica over a series of experiments employing nine different sediments. Through selective ingestion of fine-grained material, the worm was able to increase the BaP content of ingested sediment by 10–35% above that of the bulk material. During digestion, an average of 5–21% of the ingested BaP was absorbed from gut contents. The relative importance of ingestion as a route of BaP uptake was dependent on the time period of observation. Initial uptake of BaP was postulated to come from absorption of dissolved BaP across the body wall since, after short periods of exposure (<24 h), only 3–38% of observed BaP tissue concentrations could have been derived from ingested material. With time and with increased feeding activity, however, ingested sediment became the major source of BaP to the organism. After 72 h of exposure, 36–119% of the tissue BaP (mean = 77%) was estimated to have been derived from dietary absorption of ingested material, and with continued exposure the dietary route is likely to be as great or greater as the relative significance of the initial uptake from the dissolved phase diminishes. This work and other studies indicate that for many deposit feeders, ingested sediment can be the primary source for the bioaccumulation of hydrophobic toxicants. Received: 27 April 1999/Accepted: 12 October 1999     

Characterizing and comparing risks of polycyclic aromatic hydrocarbons in a Tianjin wastewater-irrigated area

A probability risk assessment was conducted to characterize the ecotoxicity of three polycyclic aromatic hydrocarbons (PAHs), including benzo[a]pyrene, fluoranthene, and phenanthrene, to various species in a wastewater-irrigated area of Tianjin, China. The relative risk of these chemicals was investigated using joint risk probability distribution curves, which were generated based on the distributions of exposure and acute toxicity data. Risk at various exposure levels was discussed. The results indicated that among the three PAHs studied, the overall risk of phenanthrene was the highest, with that of benzo[a]pyrene the lowest, due mainly to their exposure concentrations. For lower exposure levels at which the percentage of species affected was less than 20%, the risk associated with benzo[a]pyrene was clearly higher than that of the other two chemicals.     

Effect of dissolved organic matter of various origins and biodegradabilities on the bioaccumulation of polycyclic aromatic hydrocarbons in Daphnia magna

As a preliminary study of the influence of urban organic matter on the bioavailability of polycyclic aromatic hydrocarbons (PAHs), the effect of different types of dissolved organic matter (DOM) on the bioaccumulation of fluoranthene, pyrene, or benzo[a]pyrene in Daphnia magna was studied. Commercial humic substances, DOM from the aeration basin of a wastewater treatment plant, and highly biodegradable DOM (algae or animal extracts) were tested. The bioaccumulation of benzo[a]pyrene was reduced by each DOM (up to 80% reduction with humic substances). Pyrene bioaccumulation was also decreased by each DOM to a lesser extent. Fluoranthene bioaccumulation was affected by the presence of humic acids only. In each experiment, the solution containing humic DOM led to the lowest bioaccumulation. Supposing that only dissolved PAHs were bioavailable, the reduction of bioaccumulation allowed a biological estimate of the partition coefficients of DOM and PAH, K(DOC). The estimated coefficients were positively related to the aromaticity of DOM and negatively related to its biodegradability.