The mechanism of PCB bioavailability reduction in a freshwater oligochaete (<i>Lumbriculus variegatus</i>) after activated carbon amendment
Hydrophobic organic compounds such as polychlorinated biphenyls (PCBs) are the most frequently occurring contaminants at EPA Superfund sediment sites. These compounds degrade very slowly and are released into sediment pore water and overlying water by diffusion and can be readily taken up by benthic organisms that form the base of aquatic food webs. The cleanup process of sediment sites is complex and creates a unique challenge due to expensive cleanup strategies, large and diverse sediment sites, presence of ecologically valuable resources, and lack of good understanding of the relation between sediment geochemistry and biouptake. In this research, PCB bioavailability to a freshwater oligochaete (Lumbriculus variegatus) was studied using four freshwater sediment samples from the Great Lakes Areas of Concern sites with a wide range of sediment geochemical characteristics (0.83–5.1% TOC) and PCB concentrations (0.33 to 84.7 µg/g PCB). The work focused on understanding the impact of activated carbon (AC) amendment on PCB bioavailability. PCB aqueous equilibrium concentration and desorption rates were greatly reduced after AC amendment, indicating reductions in the two primary mechanisms of PCB bioavailability in sediments: chemical activity and chemical accessibility. AC addition at 0.5 times of sediment organic carbon reduced PCB bioaccumulation in all four study sediments. Steady state reduction in the aqueous PCB concentrations was a good predictor of PCB bioaccumulation reduction in deposit feeding organisms.
A PCB mass transfer model was developed to predict the bioaccumulation of PCBs by L. variegatus. This model accounts for PCB intraparticle mass transfer, desorption from the different particle types, and uptake by benthic invertebrates through two pathways, dermal absorption and sediment ingestion. The biological and physicochemical parameters of the model were measured independently. The model was evaluated by laboratory bioaccumulation experiments for native sediments and sediments amended with activated carbon to reduce PCB bioavailability. For most PCB congeners, the modeled and measured values agreed within a factor of two for Grasse River and Milwaukee River sediments. Because the model is generic and adjustable to reflect site-specific conditions, it can be used to predict PCB bioaccumulation in a broad range of sediments and to conduct long-term risk assessments for carbon amendment to sediment.