Step 3: Food chain modeling

numerical and analytical solutions to differential equations that are theoretically and

empirically based. The USACE provides a technical note (USACE 1995a) which

describes the available ADDAMS models, their application to various management

technologies, a technical point of contact, and a request form for the models.

Output from the ADDAMS suite of models, which often provide contaminant

flux rather than concentrations, can be used as input to a number of USEPA fate and

transport models. These contaminant transport models are available from the

USEPA Center for Exposure Assessment Modeling (CEAM). These contaminant

transport models use mass balance principles and vary in complexity from simple

analytical estimates which are useful to make initial calculations to numerically

complex iterative models that predict time-varying contaminant fate and transport.

These hydrodynamic and sediment transport models predict water and sediment

concentrations. These include:

a. WASP4 - Predicts dissolved and sorbed chemical concentrations in

sediment and overlying water. The model is time variable and can simulate

three chemicals and three sediment size fractions simultaneously.

b. EXAMS II - This modeling system is also based on the WASP models.

EXAMS predicts dissolved and sorbed chemical concentrations and can be

run in a steady-state or quasi-dynamic mode. Unlike the WASP models,

EXAMS does not simulate solids settling and resuspension.

c. SMPTOX3 - This is a simplified analytical steady-state model that

calculates the distribution of contaminants in water and sediment. This

model is typically used for initial calculations.

d. The product of this section is a description of the fate and transport model

and its output. The description should include the equations which the

model uses, the constraints on the model, the source of the model (e.g.,

USACE, USEPA), the input parameters, and any modifications which may

have been made.

Step 3: Food chain modeling

The final step in the exposure assessment is to predict the amount of the

contaminants of concern which a receptor will ingest, contact, or concentrate in its

body. The risk assessment must express this exposure in the same manner as the

available toxicological information. There are essentially three expressions of

a. Dose - amount of a contaminant of concern ingested per unit body weight of

the receptor per day.

b. Body burden - concentration of a contaminant of concern per unit body

weight or per unit body lipid.

c. Dietary concentration - concentration of a COC in the prey organism of a

receptor.

Chapter 3 Ecological Exposure Assessment