Bioaccumulation of COCs

(McCarthy and Jimenez 1985; Landrum et al. 1985) and invertebrates

(McCarthy, Jimenez, and Barber 1985). Although considerable uncertainty exists

in measured values for Kdoc, experimental data suggest that Kdoc is approximately

a factor of 10 less than the Kow of a chemical (Landrum et al. 1984).

Temporal considerations: Degradation of labile contaminants.

Approaches for estimating risk associated with disposal of dredged material often

do not consider the potential for microbial biotransformation or other degradation

processes (e.g., hydrolysis, photooxidation) that effect labile organic

contaminants. Numerous studies have demonstrated the degradation of organic

contaminants by microorganisms in pure culture and in natural sediments

(Herbes and Schwall 1978; Cerniglia and Heitkamp 1989). When dredged

sediments are removed from sources of contamination, concentrations of low

molecular weight PAHs may decline significantly over time due to

biodegradation that occurs at the disposal site. In this example, risk could be

overestimated at the disposal site.

Prediction of the rate of degradation is uncertain because rates of microbial

degradation can be affected by a variety of factors, including temperature,

salinity, and adaptation of the ambient microbial community (Shiaris 1989). For

example, rates of transformation of PAHS can be 3,000 to 125,000 times greater

in contaminated sediments than in uncontaminated sediments (Herbes and

Schwall 1978). This phenomenon can be quantitatively reflected by a

biodegradation rate constant in a fate and transport model.

Bioaccumulation of COCs

Organisms can bioaccumulate and biomagnify dredged material contaminants

from sediment and water. After the fate and transport of contaminants in the

environment are estimated, bioaccumulation potential is evaluated.

A variety of models are used to estimate the bioaccumulation of COCs. In

these models, exposures can be estimated for upper trophic level receptors. For

aquatic organisms, models that predict the concentration of COCs in tissues of

exposed organisms are usually used (Gobas 1993; Thomann 1989). These models

predict concentrations of COCs in benthic invertebrates and in higher trophic

level fish. For semiaquatic organisms, models that predict a daily dose of COCs

are typically used (Opresko, Sample, and Suter 1994). Models are also available

that relate contaminant concentrations in prey to concentrations in eggs of fish-

eating birds in the form of biomagnification factors. Human-based exposure

models typically predict a daily dose of COCs (USEPA 1992). Uncertainties

associated with each approach are described in the following sections.

Bioavailability of contaminants from sediment for ecological receptors.

Sediment bioaccumulation tests are used in Tier III to measure accumulation of

contaminants from samples of dredged material (USEPA/USACE 1991, 1998).

USACE has developed a database from these sediment bioaccumulation tests and

other sources that demonstrates the variability in reported BSAFs

framework exists for predicting bioaccumulation of metals or polar organic

chemicals from sediment. This could be a major source of uncertainty in the risk

Chapter 5 Uncertainty in Tier IV Risk Assessments