Reevaluation of dioxin bioconcentration and bioaccumulation factors for regulatory purposes

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Abstract
Bioconcentration factors (BCF) or bioaccumulation factors (BAF) reported for 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TCDD or dioxin) in aquatic environments encompass a wide range of values, from less than 1000 to 189,000 I/kg. These values are based on concentrations of TCDD in various environmental media including water, sediment, or food. Under the Federal Water Pollution Control Act and its enabling regulations (40 CFR 700–740, 400–470), point source discharge limits are established so that the nominal receiving water concentration will not exceed the water quality criterion. To be consistent with this regulatory process, the water quality criterion should also be calculated using an accumulation factor that is based on a nominal water concentration. The regulatory process for developing a water quality criterion for TCDD requires the selection of a BAF that describes the relationship between the source to be regulated and the fish tissue concentration. In this paper, TCDD concentrations in the environment are expressed as a nominal water concentration, defined as the total TCDD added to a system (i.e., the source) divided by the total volume of water in the system. TCDD levels in fish tissue are based on the fillet portion normalized to a common lipid content of 3%. Using these definitions, the BAF and BCF values reported in the literature were reevaluated. An alternative BAF, termed the regulatory bioaccumulation multiplier (RBM), employing these definitions was developed so as to be consistent with the regulatory process. When compared on this basis, traditional bioconcentration studies resulted in RBM values ranging from 720 to 7730 I/kg. The RBMs calculated from flow‐through studies resulted in an average RBM of 5280 I/kg. The average RBM based on all traditional BCF studies was 4280 I/kg. The analysis of field‐simulated studies resulted in RBM values 1–540, which were lower than those calculated from the traditional BCF studies. The following RBMs were calculated based on field data: 3890 I/kg based on Lake Ontario data, 4900 I/kg for Maine rivers below kraft pulp mills, 4459 I/kg based on experimental stream data, and 2650 I/kg for U.S. rivers below kraft pulp mills. The analysis presented in this paper indicates that an RBM of 5000 I/kg for 2,3,7,8‐TCDD is a conservative upper limit and appropriate for the calculation of a water quality criterion for TCDD.