Chemical and biological availability of sediment-sorbed hydrophobic organic contaminants

The ability to predict accumulation levels of sediment-sorbed hydrophobic organic contaminants (HOCs) by deposit-feeding organisms based on sediment concentrations is limited in part by an incomplete understanding of the chemistry that controls assimilation efficiency. This study was designed to test the hypothesis that desorption is an important process that controls the bioavailability of HOCs to deposit-feeding organisms; we planned to do so by conducting desorption and bioavailability experiments with field-contaminated sediments collected from New York Harbor, New York, USA. Three classes of organic contaminants, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and linear alkylbenzenes (LABs) were studied. In order to address the effects of contaminant aging, we compared the contaminant desorption rates from sediments collected from surface and at depth in an area of known high-sediment accumulation to retarded intraparticle model predictions. Measured desorption rates of the LABs and the most hydrophobic PCBs compare well with model predictions. However, the PAH and less hydrophobic PCB desorption rates range from one to four orders of magnitude slower than model predictions. We postulate that these compounds are present in a resistant sedimentary phase and may represent only a small fraction of what was originally sorbed. The fraction of PCBs, PAHs, and LABs desorbed after 48 h correlate well with measured biota-sediment factors (BSFs) in Yoldia limatula that were exposed to the same sediments, indicating that desorption rate-limited assimilation. Several studies have related field BSFs with log K-ow and have observed a maximum at intermediate K(ow)s (similar to 6.0-6.5). This maximum may be due to predictably slow desorption of high-K-ow compounds and may be lower than predicted rates and extent of desorption of the low-K-ow compounds because of association with resistant phases.