Humic Acids Reduce Bioaccumulation of Some Polycyclic Aromatic Hydrocarbons

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.