The objective of this research was to provide the basis for improving the accuracy of sediment exposure assessments by including explicit consideration of the functional ecology of benthic receptors, which determines the nature of their interactions with sediment particles, pore water, and overlying water. Much of the uncertainty that currently plagues sediment risk assessments, remedy selection, and design is due to limitations associated with characterizing exposure processes in sediment. The lack of consideration given to biological variation in feeding strategies, uptake responses, and physiology among benthos produces significant uncertainty, preventing accurate assessment of bioavailability and risk. The overarching hypothesis used to inform experimental design and analyses within the study was that functional attributes of benthic organisms explain differences in observed exposure, uptake, and bioaccumulation among benthic functional groups.

A series of laboratory experiments was conducted to test the hypothesis that the functional attributes of benthic organisms can explain differences in the organism’s exposure, measured in terms of bioaccumulation, to different contaminant compartments within sediment. Four benthic invertebrates (a polychaete, two functionally distinct amphipods, and a bivalve) with differing functional attributes were exposed, through manipulative experiments, to a series of conditions that were designed to assess the relative contribution to exposure made by contaminants in overlying water, pore water, and sediment particles using pathway isolation chambers (PICs). Two polychlorinated biphenyl (PCB) contaminated sediments (from New Bedford Harbor (NBH), MA and the Bremerton Navy Complex (BNC), WA) were used in the experiments, in addition to uncontaminated control sediments. Experiments were performed to define the bioaccumulation kinetics for the four organisms.

These experiments were complemented with tests that evaluated how organism behavior impacts the movement of sediment particles and solutes using radioactive and fluorescent tracers. Finally, organism exposure was evaluated under two remediation scenarios (application of a thin sand cap and activated carbon (AC)). Measurements of PCB bioaccumulation were related to bulk PCB concentrations in overlying water and sediment, as well as dissolved contaminant concentrations in overlying water and sediments that were estimated using polyethylene devices (PED). Data collected during experiments were used to enhance the capability and predictive reliability of an existing modeling framework (RECOVERY) for assessing exposure processes in contaminated sediment.

The evaluation of bioaccumulation kinetics in NBH sediment revealed significant differences among the benthic invertebrates. Lipid-normalized net body residues varied greatly among the species investigated, consequently, so did biota-sediment accumulation factor (BSAF) values. The measured BSAF was highest for L. plumulosus (5.4), intermediate for Y. limatula (2.9) and E. estuarius (2.4), and lowest for the polychaete N. arenaceodentata (2.0). The manipulative experiments using PICs resulted in a number of observations. As expected, exposures to whole sediment produced the greatest bioaccumulation for all four species. The amphipod, L. plumulosus, bioaccumulated more PCBs than the other organisms (particularly high log Kow homolog groups), whereas M. mercenaria generally accumulated lower concentrations of PCBs. While overlying water exposure had some importance to the amphipod L. plumulosus, which comes to the substrate surface often, pore water exposure and direct sediment particle exposure were by far more important factors for determining overall L. plumulosus tissue residues. The overlying water had lower importance for E. estuarius and was virtually irrelevant for the worm N. arenaceodentata. Exposure to PCBs sourced from the overlying water was clearly more important for the filter-feeding clam, M. mercenaria. The 2-centimeter sand cap significantly reduced PCB exposure by 78–95 percent for all the species except M. mercenaria. However, the use of in situ placement of AC in the top 2-centimeters did not result in significant reductions in bioaccumulation. PED estimations of pore water concentrations were a vast improvement over bulk sediment concentration for predicting bioavailability, but could not account for divergent animal behavior between species and substrate types.

Contaminant bioaccumulation from benthos to fish, wildlife, and humans is the exposure pathway responsible for the risks driving most contaminated sediments cleanup decisions. Risk assessments that have been performed at sediment cleanup sites to date have primarily relied on bulk sediment chemistry and a number of simplifying assumptions in order to model exposure processes and risk. However, the results of the present study document the complexity of exposure processes for benthic organisms. Because current risk assessment and performance monitoring practice does not generally consider the role of functional differences among benthic organisms, these assessments can produce inaccurate risk estimates, as well as inaccurate projections of the risk reduction benefits associated with proposed remedies. The combined use of passive sampling techniques, information about the functional ecology and physiology of the target receptors, and site-specific bioaccumulation data for the functional receptors of concern will lead to more accurate projections of risk and remedy performance. The results of the present study can be used to help design functional bioavailability assessments at contaminated sediment sites as well as provide the basis for development of future guidance.

Bridges, T. S., Kennedy, A. J., Lotufo, G. R., Coleman, J. G., Ruiz, C.E., Lindsay, J. H., Steevens, J. A., Wooley, A., Matisoff, G., McCall, P., Kaltenberg, E., Murgess, R. M., and Fernandez, L. A. (2017). The Biology of Bioavailability: The Role of Functional Ecology in Exposure Processes. Environmental Laboratory. ERDC/EL TR-17-2.