In laboratory studies, Daphnia magna were exposed for 6 h to five polycyclic aromatic hydrocarbons (PAH) (0.1–2.0 μg/L) in water with and without Aldrich® humics (2 mg DOC/L). Compared to results in nonhumic water, accumulation of PAH by Daphnia in water with humics was significantly reduced for benzo[a]pyrene (−25%) while it was increased for methylcholanthrene (+210%). Humics did not significantly alter Daphnia accumulation of anthracene, dibenzanthracene or dimethylbenzanthracene. In additional studies, humics reduced Daphnia accumulation of benzo[a]pyrene (B[a]P) over a range of B[a|P concentrations (1.1–5.4 μg/L) exceeding the reported limit for water solubility (1.1–1.2 μg/L). Humics consistently increased Daphnia accumulation of methylcholanthrene (MC) over a range of humic concentrations from 0.2 to 10.0 μg DOC/L. Particulates and DOC (10–12 mg TOC/L) occurring naturally in two South Carolina streams reduced Daphnia accumulation of B[a]P by 38–66%, with about 40% of the overall reduction attributable to DOC. We conclude that dissolved refractory organics may significantly affect bioavailability and environmental transport of some PAH in fresh waters.
The exposure of benthic organisms to sediment-associated toxic organics is influenced by the sediment's organic carbon (OC) content because hydrophobic organic contaminants sorb to the organic and fine grain portions of sediments, and benthos ingest the OC associated with fine material. The effect of varying sediment composition, measured as the percent of combustible solids (CS), percent of OC, and percent of fine-grained material (FM) (the <63 pm fraction of the sediment), was examined by determining the accumulation of sediment-associated polychlorinated biphenyl and polycyclic aromatic hydrocarbon congeners by the Great Lakes arnphipod, Diporeia sp. Prepared sediments (3 and 5% CS) and native sediment were dosed with pairs of contaminants: 3H-pyrene and 14C-2,5,2',5'-tetrachlorobiphenyl, and 3H-benzo(a)pyrene and 14C-2,4,5,2',4',5'-hexachlorobiphenyl. Additionally, some of the dosed 5% CS sediment was recombined with the coarse material to recreate a 2% CS sediment where the fine material had been dosed preferentially. The accumulation of the radiolabeled compounds was followed for 27 days, and the partitioning between sediment particles and interstitial water was measured at the end of the experiment. When uptake rate coefficients (K,) and the depuration rate constants (K,) were estimated, the K, values were found to be similar to those previously measured in Diporeia. The K, values from the native and fine labeled sediments (both approximately 2% CS) were similar. K, values declined with increasing CS, OC, and FM. Correlations indicated that the percent of OC best accounted for the variation resulting from changes in the sediment composition for each compound, both for K, and for sediment-interstitial water partitioning. As in previous studies, the partition coefficient between sediment and interstitial water did not account for the changes in the bioavailability between different classes of compounds. Higher partitioning and higher uptake rate coefficients were found for the chlorinated hydrocarbons than for the polycyclic aromatic hydrocarbons, even when the hydrophobicity of the contaminants was accounted for. A linear solvation energy quantitative structure activity model was useful for describing the differences in the uptake rate coefficients by accounting for the differences in the molecular characteristics of the two compound classes.
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