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Uncertainties of Tier IV evaluation
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Development of conceptual model
5
Uncertainty in Tier IV Risk
Assessments
Problem Formulation/Hazard Identification
This section discusses uncertainties associated with the problem formulation
phase of the risk assessment process. Problem formulation is a planning phase
that evaluates the nature of the problem and identifies the major factors to be
considered in the assessment. Initial characterization of potential exposure
pathways and receptors occurs in this phase. Assessment and measurement end
points for ecological receptors are also identified. The outcome of the problem
formulation/hazard identification phase is a conceptual model describing
contaminant sources, migration pathways, exposure potential, and human and
ecological receptors of concern.
Uncertainty is considered in a qualitative sense at the problem formulation
phase, compared to the statistical or mechanistic approaches used to address
uncertainty during the exposure and effects assessments. For example, there may
be uncertainty in the selection of specific receptors or assessment end points,
which is more difficult to quantify than other areas of uncertainty. Consequently,
each source of uncertainty in the problem formulation phase is not ranked in
Table 1.
Development of conceptual model
The conceptual model is a representation, sometimes graphical, which
describes chemical sources, release mechanisms, exposure media and pathways,
uptake routes, and receptor populations. Conceptual model development may be
one of the most important sources of uncertainty in a risk assessment (USEPA
1998a). Uncertainty may arise from lack of knowledge about how the ecosystem
functions, failure to identify secondary effects such as increased levels of
predation, and lack of information about how stressors move through the
environment and cause adverse effects (USEPA 1998a). The conceptual model is
critical to defining subsequent analyses.
Characterization of the surrounding environment. The characteristics of
the environment surrounding the site must be understood and described so that all
potential exposure pathways and receptors are included in the conceptual model
(U.S. Army Engineer Division, Huntsville, 1995). The following characteristics
may influence exposures:
30
Chapter 5 Uncertainty in Tier IV Risk Assessments

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