Benthic impact-Theoretical bioaccumulation potential for nonpolar organic compounds

b. Contaminants that are released in the water can become associated with

dissolved organic matter in the water column and become less

bioavailable for accumulation by biota.

If the results of the screening level calculation exceed applicable WQS, the

mixing model can be rerun using the results from a standard elutriate test.

Professional judgment can be used to provide estimates of the range of

possible values for the mixing area. Uncertainty in the mixing model approach

can be evaluated by changing specific parameters in the model and evaluating the

impact on the results. Therefore, uncertainty in the ability of this screening-level

approach to make conservative predictions of concentrations of COCs in water is

assumed to be "easy" to quantify. The magnitude of uncertainty is ranked

"moderate" because such simple mixing zone modeling techniques yield order-

of-magnitude estimates (USEPA/USACE 1998). The accuracy of WQS values

was not assessed in these rankings.

Benthic impact-Theoretical bioaccumulation potential for nonpolar

organic compounds

The TBP is an approximation of the equilibrium concentration of nonpolar

organic contaminants in tissues of benthic invertebrates if the dredged material

were the only source of contaminant to the organism. In this tier, a comparison is

made between TBP calculated for the nonpolar organic contaminant of concern

in the dredged material and for the same contaminants in the reference sediment.

If the TBP for the dredged material exceeds that of the reference sediment,

further evaluation of bioaccumulation in Tiers III or IV is appropriate.

At present, the TBP can be calculated only for nonpolar organics, such as

polychlorinated biphenyls (PCBs). At equilibrium, a nonpolar organic chemical

would be expected to associate with organic matter in sediment and with lipids in

tissue. Therefore, the potential bioaccumulation of nonpolar organic chemicals

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 each of these phases. Biota-sediment accumulation factors (BSAFs)

are ratios that describe the relationship between the concentration of a nonpolar

organic chemical in the lipid phase in tissue of a sediment-dwelling organism and

the concentration in the sediment organic carbon phase to which the organism is

exposed. BSAFs are defined as:

BSAF = ( CB / fL )/( CS / fOC)

(1)

where

CB = concentration of contaminant in biota, mg/kg wet weight

fL = the fraction lipid of the biota, kg lipid/kg wet weight

Chapter 4 Uncertainty in Tiered Evaluation of Dredged Material