8.3.1 Tier II Theoretical Bioaccumulation Potential Procedure

earthworms in the Tier II evaluation animal bioaccumulation in the UTM.

Methods for TBP calculations with metals and polar organic compounds are under

development and may be added to this manual in the future.

It is useful to calculate the TBP for nonpolar organic COC, because it may

show these compounds are not bioavailable and thus do not warrant further

evaluation in higher tiers. If further evaluation of any nonpolar organic COC is

warranted, TBP provides an indication of the magnitude of bioaccumulation that

may occur.

Nonpolar organic chemicals include all organic compounds that do not

dissociate or form ions. This includes the chlorinated hydrocarbon pesticides,

many other halogenated hydrocarbons, PCBs, many PAHs including all the

priority pollutant PAHs, dioxins, and furans. It does not include metals and metal

compounds, organic acids or salts, or organometallic complexes such as tributyltin

or methyl mercury.

The environmental distribution of nonpolar organic chemicals is controlled

largely by their solubility in various media. Therefore, in sediments they tend to

occur primarily in association with organic matter (Karickhoff 1981). In

organisms they are found primarily in the body fats or lipids (Konemann and

van Leeuwen 1980; Geyer et al. 1982; Mackay 1982; Bierman 1990).

8.3.1 Tier II Theoretical Bioaccumulation Potential Procedure

Bioaccumulation of nonpolar organic compounds from dredged material can

be estimated from the organic carbon content of the material, the lipid content of

the organism, and the relative affinities of the chemical for sediment organic

carbon and animal lipid content. The TBP calculation assumes that various lipids

in different organisms and organic carbon in different sediments are similar and

have similar distributional properties. Other simplifying assumptions are that

chemicals are freely exchanged between the sediments and tissues and that

compounds behave conservatively. In reality, compound size and structure may

influence accumulation, and portions of organic compounds present on suspended

particulates may have kinetic or structural barriers to availability. Another

important assumption implicit in the TBP calculations is that there is no metabolic

degradation or biotransformation of the chemical. Organic carbon normalized

contaminant concentrations are used such that the sediment-associated chemical

can be characterized as totally bioavailable to the organism. Calculations based

on these assumptions yield an environmentally protective (e.g., overestimate) TBP

value for the dredged material if the dredged material in question is the only

source of the contaminant for the organism. Note that TBP calculations are not

valid for sediments or soils with total organic carbon (TOC) content less than or

equal to 0.2 percent.

For each nonpolar organic COC, TBP is calculated for the dredged material

and the reference material according to the guidance in Appendix G. The TBP of

the dredged material is compared statistically to the reference TBP to determine

8-11

Chapter 8

Animal Bioaccumulation