Available Exposure Models (Cont.)

c. Early life stages are often more sensitive to contaminants than older

individuals. Full-life-cycle, time-variable models permit an analysis of

how levels change during early life.

d. A time-variable model permits assessment of within-year variation in body

burdens. Exposure concentrations, migratory behavior, growth rate, and

lipid content may vary seasonally. Seasonal variation in body burdens

may be of interest to aid in interpreting measured contaminant levels,

depending on the timing of sample collections. It may also be of interest if

planned activities (e.g., navigational dredging) occur at certain times of

year.

e. In some ecosystems, bioaccumulation may be affected by the physiological

condition of the organism. For example, the fat content of a fish species

may change over time, perhaps due to changes in prey availability. This

will cause the degree of bioaccumulation of hydrophobic organic

contaminants to change over time. Such a change may affect the

interpretation of temporal trends in fish contaminant levels.

Organisms integrate their exposure over time. This means that short-term

variation in the level of a contaminant in fish may be much less than short-

term variation in the water column. Water quality criteria are often

expressed as the frequency of exceedance of a critical contaminant level.

A time-variable model provides estimates of the frequency of exceedance

in the presence of variable exposure levels.

Members of the workgroup felt that the uncertainty associated with the structure

and parameterization of food web models for hydrophobic organic compounds are

low relative to the uncertainty associated with the site-specific information

required to perform site-specific simulations. One workgroup member noted that

calculations of body burdens were within a factor of 3 with a 95 percent

probability. The major sources of uncertainty tend to be the foraging area of the

biota, the migratory behavior of the biota, dietary composition and food web

structure, and exposure concentrations in sediment and water.

The workgroup addressed the issue of what factors need consideration in

setting spatial and temporal scales within a model. Temporal scales are often set

by the site recovery period and the recovery period of the biological community.

Site recovery periods depend on processes such as biodegradation and burial of

dredged material which occur in aquatic and terrestrial environments. The

recovery of the biological community is usually a recovery from the physical

placement of dredged material. Monitoring in the DAMOS program indicates two

biological recovery periods in aquatic systems: a 1- to 3-month first-stage

recovery due to colonization by opportunistic species; and, a 1- to 3-year end-

stage recovery which involves a return to ambient biological communities. Model

conditions generally address the end-stage receptors. The group noted that the

first-stage recovery involves the exposure of ecologically different species from the

end-stage organisms and that these opportunists encounter the "freshest" dredged

Chapter 2 Exposure Assessment Workgroup Summary