Development of conceptual model

a. Hydrodynamics. The depth, tidal range, and flow characteristics of the

site may influence the potential for resuspension and transport of dredged

sediment.

b. Climate. The temperature and precipitation profile of the area may limit

the frequency of exposure, as well as the extent of chemical migration

(e.g., fewer people fishing near a disposal site during the winter months).

c. Meteorology. Wind may transport particles and airborne contaminants

(e.g., fugitive dust from a barge).

d. Sediment type. The grain size, organic carbon, and clay content may

influence the leaching potential, resuspension of material during filling,

and bioavailability of sediment contaminants.

e. Ecological. Characteristics of potentially exposed biota may include the

numbers and types of receptors, their life histories, foraging and

migratory habits, and the trophic structure of the food web. Food webs

with a greater number of trophic levels can exhibit greater

biomagnification to higher trophic level organisms, such as piscivorous

(fish-eating) birds (USAEWES 1995).

The magnitude of uncertainty associated with characterizing the surrounding

environment is ranked high given the complexity of ecosystems (Odum 1971).

Uncertainty associated with characterizing the environment is difficult to

quantify.

Identification of complete and incomplete exposure pathways.

Uncertainties in identification of complete and incomplete exposure pathways

can arise from lack of information about dredging operations or about the site.

For example, information about the presence of migratory species at a site may

not be available. Exclusion of any complete exposure pathways would result in

an underestimate of risk.

It is typically straightforward to assess the effect of an incomplete or excluded

pathway by rerunning the analysis to include the pathway to determine the

relative risk of each pathway.

Selecting and characterizing representative ecological receptors. Most

ecosystems are complex and not all trophic levels and species can be considered

during a risk assessment. Therefore, a few species are selected as indicators of

environmental conditions. Various criteria have been developed for selecting

ecological receptors of concern. For example, selected species should be

distributed in the area under consideration, sensitive to the effects of the

contaminants of concern, and representative of a group of important species to

humans or to the functional integrity of the ecosystem (USEPA 1993c).

However, significant uncertainty arises from the fact that little to no data are

available for some receptors in terms of sensitivity to contaminants, feeding

preferences, migration, etc. For example, contaminant dose response information

is unavailable for many marine mammals and marine reptiles. Exclusion of

Chapter 5 Uncertainty in Tier IV Risk Assessments