Bioaccumulation of COCs (Cont.)

(b) Weight of the fish VF, kg.

in units of L/day.

(d) Rate of chemical transport in the lipid phase of the gill QL, in

units of L/day.

The uncertainty associated with Qw and QL is expressed by the range

on the exponential term in the equation for Qw:

Qw = 88.3 VF 0.6 (+ 0.2)

(3)

QL = Qw / 100

(4)

Uncertainty in the uptake rates (QW and QL) will contribute to

uncertainty in fish body burden estimates and estimates of risk to

the fish and their consumers.

(2) Uptake from food. The rate at which fish take up chemicals from

food depends upon the food ingestion rate, the rate of diffusion of

the chemical across the intestinal wall, and the fecal egestion rate.

The Gobas model (1993) assumes that the efficiency with which

chemicals are taken up from food is related to the transport of

chemicals across aqueous and lipid phases of the gut:

1/ED = A Kow B

(5)

where

ED = the uptake efficiency from the diet (fraction)

= a constant relating to the transport rate of the chemical in the

aqueous phases of the fish

= a constant relating to the transport rate of the chemical in the

lipid phase of the fish

According to a nonlinear regression of available data, A is 5.3

(+ 1.5) 10-8), and B is 2.3 (+ 0.3). Uncertainty associated with

these estimates could contribute to uncertainty associated with

estimates of body burdens in fish and estimates of risk to the fish

and their consumers. The magnitude of uncertainty could be

evaluated by performing a sensitivity analysis on the input

parameters.

(3) Ingestion and elimination rates. The Gobas model (1993) expresses

the rate at which chemicals are eliminated through fecal egestion, as

the fecal elimination rate constant kE. Available data show that the

fecal egestion rate is approximately 3 to 5 times lower than the

ingestion rate. Therefore, the model assumes that

Chapter 5 Uncertainty in Tier IV Risk Assessments