Exposure models for semiaquatic organisms and humans: Model uncertainty

Kow of the contaminant influence estimates of tissue concentrations

more than other parameters and are important to overall estimates of

risk in the assessment. The magnitude of uncertainty associated

with the use of these models for nonmetabolizable organic

contaminants is generally thought to be low.

Exposure models for semiaquatic organisms and humans: Model

uncertainty. Semiaquatic organisms (e.g., piscivorous birds) may consume fish

that have been exposed to contaminants from a disposal site. Inability of models

to predict concentrations of metals or readily metabolized organic contaminants

in tissues of higher trophic level organisms (e.g., eggs of piscivorous birds) is a

significant source of uncertainty for assessing risk associated with dredging and

disposal activities. Magnitude of the uncertainty will depend upon the specific

model under consideration. The "moderate" ranking is given because numerous

models are available to evaluate exposure but field studies would be needed to

substantially reduce the uncertainty.

Exposure models for semiaquatic organisms and humans: Parameter

uncertainty. Wildlife near open-water disposal sites could be exposed via oral,

inhalation, or dermal routes of exposure. The Wildlife Exposure Factors

Handbook (USEPA 1993c) tabulates data for various routes of exposure, with an

emphasis on oral exposure. Oral exposures might occur via ingestion of

contaminated food or water or incidental ingestion of soil or sediment during

foraging activities. The Handbook provides quantitative information on various

exposure parameters that can be used to estimate exposure of an animal to

environmental contaminants. The Handbook separates exposure parameters

conceptually into four types:

a. Normalizing parameters, such as body weight, growth rate and metabolic

rate.

b. Contact rate parameters for oral exposure (dietary composition and

ingestion rates of food, water, sediment/soil), exposure by inhalation

(inhalation rate), and dermal exposure (surface area).

c. Population dynamic parameters, such as home range size, population

density, fecundity, age at sexual maturity, mortality rate, average

longevity.

d. Seasonal activity parameters, such as mating season, hatching,

molting/metamorphosis, and dispersal/migration/hibernation.

Humans who work with the dredged material or consume impacted fish and

shellfish might also be exposed. Exposure factors for humans are readily

available (USEPA 1997a). Uncertainty associated with these exposure

parameters is described in the following section.

a. Physiological characteristics. Physiological characteristics such as body

weight, inhalation rate, and other parameters are necessary inputs for

mechanistic-based bioaccumulation models. For wildlife, allometric

relationships have been developed from empirical studies. For humans,

Chapter 5 Uncertainty in Tier IV Risk Assessments