Available Exposure Models (Cont.)

properties with site-specific biological characteristics or physical transport

mechanisms. The models may be the only way to predict the potential fate of

sediment contaminants or deposition of dredged materials and so will be critical

for developing exposures at sites where dredged material management options are

proposed.

Most of the discussion centered on the use of a food web model developed by

Frank Gobas, which is now applied to the aquatic food webs of the Great Lakes,

and the model first developed by John Connolly with versions now in use by

Robert Thomann, HydroQual, Quantitative Environmental Analysis, CLC, and

others. These predict the bioaccumulation of organic compounds. There appears

to be a paucity of models within the open literature which predicts the fate of

metals in food chains. One member of the group noted that the International

Atomic Energy Agency (IAEA) has developed models which address the fate and

transport of metals as part of their ocean disposal programs. The group felt that

the USACE should explore the use of these models at dredged material

management sites.

The models discussed included those which allow time-variable simulations

and those which permit steady-state simulations. The two key differences between

these are:

a. Time-variable simulation requires time-dependent exposure concentrations

and physiological input information.

b. The analysis of model results is more complex for a time-variable

simulation.

The choice between steady-state and time-variable applications of food web

models depends on the availability of information regarding temporal variability of

exposure concentrations and the physiological characteristics of the organisms, as

well as on the specific questions addressed. If concentrations and conditions are

changing slowly relative to the response time of the organisms, steady-state

simulations may be appropriate. Six examples of situations under which time-

variable simulations are appropriate follow:

a. Defining impacts of short-term, relatively large changes in exposure

concentration due, for example, to a temporary release of a contaminant

into the environment.

b. Defining response time of the biota to natural remediation or to

remediation activities. This is especially important if more than one

contaminant source contributes to the body burden of a species. For

example, if a contaminant originates in the sediments and in ongoing

sources to the water column, remediation activities may alter the relative

importance of each source. A time-variable model allows one to track the

changes in contaminant levels in the biota under these conditions.

Chapter 2 Exposure Assessment Workgroup Summary