Bioaccumulation of COCs (Cont.)

kE = 0.25 kD

(6)

where kD is the dietary uptake rate constant. In the environment,

there is variability between species, locations, and seasons in the

actual rates and uncertainty in applying rates from the literature to

field conditions at a particular site.

(4) Growth rates. No uncertainty is reported for the equations used for

generalized growth of fish (Thomann, Connolly, and Parkerton

1992):

kG = 0.00251 VF 0.2

for temperature around 25o C

(7)

kG = 0.000502 VF

for temperatures around 10o C

(8)

where

kG = the growth rate constant

VF = the wet weight of the fish, kg

(5) Sensitivity analysis for food-chain models. Each of the

bioaccumulation models uses a set of parameters to predict the body

burdens of organic contaminants in higher organisms. The

uncertainty associated with these parameters contributes to the

uncertainty of the risk estimate. Burkhard (1998) compares the

sensitivity of the Gobas (1993) and Thomann (1989) model outputs

to changes in input parameters. Sensitivity of the models was

determined by running each model once with nominal input values,

changing one input value by 10 percent, and running the model with

the altered input value. A sensitivity of 1.0 means that a 10 percent

change in the input parameter resulted in a 10 percent change in the

model output. In this case, the model output examined was the

Bioaccumulation Factor (BAF), which is equal to the ratio of the

lipid-normalized concentration of chemical in fish to the

concentration of freely dissolved chemical in water.

For both models, the input parameters with the largest influences

were

(a) Lipid contents of the organisms.

(b) Kow of the chemical.

carbon to the concentration in overlying water ΠSOCW.

(d) Feeding preferences of the organisms (only for chemicals with

log Kow exceeding 6).

Chapter 5 Uncertainty in Tier IV Risk Assessments