Deposit‐feeding marine clams (Macoma nasuta) were exposed for 119 d to three sediment types that varied in total organic carbon (TOC) from 0.8 to 2.5%. Sediments were spiked with equal concentrations of 13 polychlorinated biphenyl congeners and hexachlorobenzene. Tissue residues were measured, and steady‐state bioaccumulation factors (BAFs), the corresponding lipid, and TOC‐normalized biota sediment accumulation factors (BSAFs) were determined. The BSAFs were less variable than were the BAFs with the exception of compounds with log Kow > 7. Many of the BSAFs exceeded 1.7, which is a calculated maximum value based on partitioning alone. Although BSAFs varied with sediment type and compound, the use of a BSAF of 4 as a screening level for neutral organic compounds in assessing dredge materials is supported by the present study.

Current environmental models use organism lipid concentrations to estimate maximum pollutant bioaccumulation potentials. This collaborative study has shown that significantly different lipid concentrations (3.5X) are found when using common, but different, extraction solvents and methods. Based on these variable lipid values, models that estimate tissue pollutant concentrations normalized to lipid will give significantly different bioaccumulation estimates. To reduce that variability, a standard lipid method needs to be developed or adopted.

The uptake of sediment-associated hexachlorobenzene (HCB) by the deposit-feeding clam Mucoma nusuta (Conrad) was determined using a clam ventilation chamber. Clams were exposed to [ ''C]HCB-dosed sediment, and the 14C amounts were measured in inhalant and exhalant waters, fecal pellets and soft tissues. The volume of water the clam ventilated and the amount of fecal pellets produced were measured. The contributions of 10 possible uptake routes to HCB tissue residues were estimated using a bioenergetic-based bioaccumulation model. Mass balance results indicate that uptake of HCB by the gut from ingested solids was the single most important route, accounting for 63 to 84% of HCB tissue residues. The next largest route was uptake from interstitial water ventilated across the gills, which accounted for 11 to 12% of tissue residues. Uptake of HCB from overlying water was minimal. These results indicate that sediment-bound HCB is bioavailable to benthic deposit feeders such as Mucoma and supports the contention that ingested sediment is an important uptake route for highly lipophilic pollutants.

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