Uncertainty in Tier IV Risk Assessments

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:

Chapter 5 Uncertainty in Tier IV Risk Assessments