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Development of conceptual model
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Exposure Assessment - trdoer30041
potentially sensitive organisms due to lack of data contributes to the uncertainty
of the dredged material assessment.
Assessment and measurement end points
Selecting assessment end points. Assessment end points are defined as
environmental characteristics or values that are to be protected, such as
abundance of a wildlife population, species diversity, or ecosystem productivity
(Suter, Barnthouse, and Bartell 1992). Lack of information with which to develop
assessment end points for some COCs is an important source of uncertainty for
the risk assessment.
In the conceptual model development stage, the uncertainty in selection of
appropriate assessment end points is not quantifiable, but clearly has the potential
to exert a significant influence on the direction of the analysis.
Selecting measurement end points. Measurement end points are quantitative
expressions of a biological response that are related to the assessment end points.
Measurement end points for ecological risk assessment are usually related to the
survival, growth, and reproduction of receptors of concern. These measurement
end points are assumed to be closely related to assessment end points, such as
changes in population structure and function. The correlation of stressor to
response is an indication of the strength of the relationship.
In some cases, assessment and measurement end points are closely related.
For example, fish that consume contaminated prey in the area of a dredged
material disposal site might accumulate body burdens of persistent chemicals,
such as organochlorine compounds, which might subsequently accumulate in the
eggs of fish-eating colonial birds. The amount of contaminant in the egg is a
measurement end point that has been linked to embryolethality (measurement
end point) and reproductive failure (assessment end point) in exposed
populations (Giesy, Ludwig, and Tillitt 1994). However, measurement end points
and assessment end points are not always so clearly related. For example,
appearance of lesions or tumors on older adult fish may have little impact on the
reproductive success of the population.
Uncertainty in the relationship between assessment and measurement end
points can contribute to significant uncertainty in estimates of risk to the
receptors. For example, consensus has not been reached on the appropriateness of
existing and proposed assays to detect the potential for endocrine disruption
properties of chemical substances and mixtures (Kendall et al. 1995). The results
of an expert panel on ecological risk assessment for sediments found that the
ecological relevance of measurement end points involving organic extracts and
elutriates was most uncertain relative to other toxicity tests (Ingersoll et al. 1997).
Lack of measurement end points. In some cases, measurement end points
will not be available. For example, although ERED provides information on the
critical body burden of some chemicals, it does not contain information for all
potential COCs. This information could be obtained through additional data
collection and analysis, but is typically beyond the scope of a site-specific risk
assessment.
32
Chapter 5 Uncertainty in Tier IV Risk Assessments

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