Equilibrium-partitioning sediment guidelines

by examining the standard error on the slope of the regression equation used to

derive the factor. However, these sources of uncertainty are irrelevant when TBP

results for dredged material are used only for comparison with the TBP estimated

for the reference sediment. The same BSAF equation is used to estimate TBP for

dredged material and sediment; therefore, this term, as well as the fraction of

lipid in the benthic organism, cancels out.

If the TBP is used only to compare dredged material with reference sediment,

there is little advantage to better specifying the factor of four. The fractions of

organic carbon and contaminant concentrations in sediment represent the key

parameters. Uncertainty in these parameters is easily quantified and may span

one or two orders of magnitude given the small sample sizes typically used

(ranking of "moderate").

The potential for trophic transfer and for biomagnification of COCs is not

considered in this tier. The importance of evaluating potential trophic transfer

and biomagnification is discussed in sections on Tiers III and IV evaluations.

Equilibrium-partitioning sediment guidelines

ESGs are numeric concentrations of nonpolar organic contaminants in

sediment that can be used to screen sediments for the potential to cause adverse

effects to benthic organisms (DiToro et al. 1991). ESGs have been established for

a suite of chemicals, and USACE and USEPA plan to use them in the future as

part of a Tier II evaluation.

The ESG approach assumes that an equilibrium exists between the chemical

sorbed to sediment organic carbon and chemical that is freely dissolved in the

pore water. Because the chemical activity of each phase is the same at

equilibrium, the effective exposure concentration is the same regardless of

exposure route (e.g., from sediment or pore water). Therefore, the equilibrium-

partitioning approach predicts that toxic effects will be observed at

concentrations of freely dissolved chemical in pore water that are equal to

concentrations that produce toxic effects in water-only exposures.

Because assumptions of the ESG approach are only approximately true,

prediction will be uncertain. For example, the distribution of the PAHs between

sediment organic carbon and pore water may not reach equilibrium if these

compounds are trapped within soot (McGroddy and Farrington 1995; Gustafsson

et al. 1997). Furthermore, some benthic organisms (e.g., tube-dwelling animals)

that are exposed predominantly to the overlying water, rather than the pore water,

may experience less exposure than predicted (Kane Driscoll, Harkey, and

Landrum 1997; Kane Driscoll, Schaffner, and Dickhut 1998).

The uncertainty associated with the use of ESGs to predict sediment toxicity

has been examined extensively (Kane Driscoll, Harkey, and Landrum 1997;

Ankley et al. 1994). Equilibrium-partitioning based predictions of sediment

toxicity can differ from experimentally determined toxicity by more than a factor

of ten (Hoke et al. 1995; Kane Driscoll, Schaffner, and Dickhut 1998). Therefore,

the magnitude of the uncertainty associated with the use of ESGs is expected to

be moderate.

Chapter 4 Uncertainty in Tiered Evaluation of Dredged Material