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Page Title: Available Exposure Models (Cont.)
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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.
f.
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
10
Chapter 2 Exposure Assessment Workgroup Summary

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