Analysis of Uncertainty in Estimating Dioxin Bioaccumulation Potential in Sediment-Exposed Benthos

Abstract : This technical note demonstrates two methods for analyzing uncertainty in estimating theoretical bioaccumulation potential (TBP) in benthic organisms residing in or on sediments contaminated with polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/F). An additional question addressed is whether congener-specific biota/sediment accumulation factors (BSAFs) in the TBP calculation provide a more certain estimate than simply using a generalized BSAF representing all congeners. BACKGROUND: Regulatory decisions regarding the suitability of dredged sediment for disposal in open waters of the United States are made using a tiered approach, as described in the implementation manuals for the Federal Clean Water Act, Section 404 and Section 103 of the Marine Protection, Research, and Sanctuaries Act (U.S. Environmental Protection Agency/U.S. Army Corps of Engineers (USEPA/USACE) 1991, 1998). TBP is a simple equilibrium partitioning-based model recommended in both manuals for estimating levels of neutral organic chemicals that could result in the tissues of organisms exposed to sediments. The model is used in a screening mode to indicate whether the sediments in question are either so clean or so contaminated that a decision regarding disposal can be made at that point, or to indicate the necessity of definitive (and more costly) bioaccumulation testing. The model delivers a point estimate, expressed as a probable organism tissue concentration of a chemical of interest, given a known concentration of the chemical in sediment and the organic carbon content of the sediment. TBP incorporates a partition coefficient, BSAF, which is the ratio of lipid-normalized concentration of a chemical in an organism to organic carbon- normalized concentration of the chemical in sediment to which the organism is exposed. BSAFs for a given chemical can vary over orders of magnitude in different species and sediments.