Bioaccumulation of COCs (Cont.)

Very little information is available on how often contaminants in the

environment reach equilibrium among phases. If equilibrium conditions

are not reached, time-variant models are more appropriate for predicting

contaminant concentrations. The distributions of contaminant

concentrations might differ from predicted concentrations if the system is

not in equilibrium because there is high temporal variability or because

biological processes maintain disequilibrium conditions. Many

ecosystem and physical processes are variable over time. The input of a

contaminant into an estuary, for example, can occur during episodic

events, such as large storms or periodic disposal of dredged sediments.

An example of biological processes that result in deviations from

equilibrium conditions would be the detoxification of metals in tissues of

animals by sequestration into granules. The quantitative importance of

the uncertainty associated with the use of equilibrium models, rather than

time-variant models, can be significant (Ludwig et al. 1993; USEPA

1996b), and is ranked MS in Table 1, because the significance will vary

from model to model.

b. Parameter uncertainty in uptake models. The Gobas (1993) and

Thomann (1989) exposure models require a variety of input parameters

that are derived from various sources. For example, some are based on

observed allometric relationships. Others are based on observed

relationships to the physical/chemical characteristics of the COC.

Uncertainty associated with these parameters will contribute to

uncertainty in tissue concentrations and risk estimates. An overall

estimate of the contribution of these models to uncertainties in the risk

assessment is provided at the end of this section. The individual

parameters have moderate magnitudes of uncertainty and moderate

difficulty for reducing uncertainty because they vary from site to site and

can be measured only with substantial effort. For humans, these

parameters can be very difficult to measure when appropriate biomarkers

are not available and they vary from one contaminant to the next.

The Gobas (1993) model assumes that fish take up chemicals from the

water (via the gills) and through the consumption of food. Loss of

chemicals occurs via the gills to the water, via egestion of fecal matter,

or by growth of the organism, which dilutes its concentration. The Gobas

model uses various assumptions to estimate these uptake and loss rates.

Uncertainty in these estimates will contribute to uncertainty in estimates

of exposure and risk to the fish and its consumers. The following section

discusses the importance of parameter uncertainty for the following

parameters of the Gobas model: uptake from water, uptake from food,

ingestion and elimination, and growth rates.

(1) Uptake from water. The rate at which fish take up chemicals from

water depends upon the gill ventilation rate and the rate of diffusion

of the chemical across the gills. The Gobas (1993) model uses

experimental data to derive uptake rates based on the following:

(a) Kow of the compound.

Chapter 5 Uncertainty in Tier IV Risk Assessments