General Description of Uncertainty Analysis

decision to be made. Only a small fraction of dredged material disposal projects

reach Tier IV. This evaluation addresses chemical impacts resulting from

dredged material disposal. Physical impacts of disposal are not evaluated in this

work because they are considered earlier in the site selection and designation

process (USEPA/USACE 1992).

Tiers I, II, and III include elements of a complete ecological risk assessment.

Tier I analysis demonstrates whether potential environmental impact can be

determined based on existing information. Tier II provides rapid screening for

potential impacts. Tier III includes toxicity and bioaccumulation testing of

dredged material to determine whether it would be expected to cause

unacceptable impacts. When Tier III analysis results in a highly uncertain

conclusion, Tier IV may include a risk assessment.

A Tier IV risk assessment differs from analysis in earlier tiers by

a. Providing a comprehensive evaluation of human and ecological risk

within a standard structure.

b. Explicitly identifying uncertainties in risk estimates.

The following sections analyze and rank sources of uncertainty in the context of

this four-tiered evaluation. Future work will examine uncertainties associated

with the assessment of potential environmental impacts of upland disposal

alternatives and beneficial uses.

General Description of Uncertainty Analysis

Uncertainty analysis represents a collection of quantitative and qualitative

methods that can be used to increase understanding of uncertainties in

assessments of human health and ecological risk. Before describing how these

methods might be applied to dredged material management, it is important to

establish a vocabulary of uncertainty nomenclature.

Uncertainty nomenclature

In the environmental field, a nomenclature for uncertainty has evolved over

the last decade (USEPA 1997b; Frey 1992). Uncertainty refers to a lack of

knowledge, while variability refers to temporal-, spatial-, or population-level

heterogeneity. This report uses the term uncertainty to describe uncertainty and

the term variability where these sources have not been partitioned. In some

instances, available data do not facilitate partitioning of uncertainty and

variability.

Three principal types of uncertainty are recognized: scenario uncertainty,

model uncertainty, and parameter uncertainty.

a. Scenario uncertainty originates from a lack of knowledge needed to

specify a problem. For example, an exposure pathway that is important

Chapter 1 Introduction