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Exposure Assessment
This section discusses uncertainties that are associated with the exposure
assessment portion of a risk assessment. The exposure assessment identifies
potential exposure pathways and estimates the extent and magnitude of exposure
in a defined field of influence.
Estimation of exposure point concentrations
The concentrations of COCs in various environmental compartments (e.g., air,
water, and sediment) are used in the exposure assessment phase of the risk
assessment to predict exposure to various human and ecological receptors. These
concentrations are measured, estimated from laboratory tests, or predicted from
models. When data are available, exposure point concentrations can be expressed
as arithmetic averages or 95 percent upper confidence limits. The 95 percent
upper confidence limit is an estimate of the upper bound on the mean with
95 percent confidence (i.e., there is a 95 percent chance that the true mean will be
less than the estimated number). In some cases, the calculated 95 percent upper
confidence limit will exceed the maximum detected concentration. Summary
statistics on available data provide an indication of the measured variability in
samples of dredged material. These estimates are typically less robust when
based on small sample sizes.
If the initial screening level estimate exceeds a criterion, standard, or known
toxic concentration, fate and transport models incorporating site-specific
parameters are typically used to refine the estimates. The following section
describes the uncertainties associated with the estimation of exposure point
concentrations using fate and transport models.
Fate and transport models: model uncertainty. A variety of fate and
transport models exist that estimate the amount of contaminant that can be
released from various dredged material disposal alternatives to the surrounding
environment. These models may predict new exposure point concentrations in
space, time, or in other media, (e.g., air), or may simply predict the mass of COC
that is lost from a source. For example, some contaminant may be lost in effluent
from an island CDF. The Assessment and Remediation of Contaminated
Sediments (ARCS) Program of the USEPA has published the guide, "Estimating
Contaminant Losses from Components of Remediation Alternatives for
Contaminated Sediments" (USEPA 1996a). This report presents techniques for
estimating contaminant losses associated with various sediment management
alternatives. The techniques include laboratory tests and contaminant transport
models. The models estimate contaminant losses during dredging, dredged
material transport, pretreatment, and effluent and leachate treatment, and from
CDFs. The report evaluates the relative reliability of the available predictive
techniques, but does not include a statistical analysis of the uncertainty associated
with using the various techniques.
Potentially one of the most significant sources of uncertainty in risk
assessment is error associated with fate and transport models (Suter 1993). Model
error arises from the inappropriate selection or aggregation of variables, incorrect
33
Chapter 5 Uncertainty in Tier IV Risk Assessments

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