Step 3: Food chain modeling (Cont.)

The choice of which expression of exposure to use depends on the toxicity data

available for a particular receptor.

For example, if the effects level for a given receptor is expressed as a dietary

concentration (the concentration of a contaminant in the food of a receptor), then a

dietary concentration associated with exposure at the disposal site should be

calculated.

The calculation of doses, body burdens, and dietary concentrations proceeds in a

similar manner to the prediction of exposure point concentrations. That is, the

assessment may make an initial estimate based on relatively simple and reasonably

conservative assumptions. The risk assessment must use a more sophisticated food

chain model if the initial estimates:

a. Result in potential risk.

b. Ignore an essential exposure route defined in the conceptual model.

c. Exceed some known biological or physical limitation governing body

burdens.

This is not to suggest continuous iterations. Rather, the risk assessor must

ultimately choose a model which most realistically reflects site conditions and uses

as much site specific information as possible.

Initial estimates of concentrations in infauna or fish

This text provides a simple calculation to estimate the concentrations of some

COC in infauna and fish which may inhabit the management area and the local field

of influence.

There are five classes of contaminants for which concentrations in infauna and

fish may be important in the exposure assessment. These include: metals (generally

only mercury biomagnifies), chlorinated organics (i.e., pesticides, PCBs,

dioxin/furans), and polycyclic aromatic hydrocarbon (PAH). At dredged material

management sites which have progressed to Tier III and Tier IV evaluations, the 28-

day bioaccumulation results modified according to Clarke and McFarland (1991) to

account for steady state provide estimates of invertebrate tissue concentrations. The

risk assessment may use these tissue concentrations as input to food chain models to

develop body burdens in higher trophic levels such as fish or piscivorous birds.

If a measured estimate of tissue concentration is not available for a COC, one

can estimate concentrations of bioaccumulative compounds in biota (invertebrates

or fish) based on a biota-sediment accumulation factor (BSAF) which expresses the

accumulation of contaminants from sediments to the biota. The BSAF depends upon

the concentration of the contaminant in the biota, Ca, the fraction lipid of the biota

(Fl), the concentration in sediments (Cs) and the fraction organic carbon of the

sediments (Foc). The relationship is:

Chapter 3 Ecological Exposure Assessment