Exposure models for semiaquatic organisms and humans: Parameter uncertainty

empirical studies and pharmacokinetic modeling provide information to

develop distributions and/or appropriate statistics for use in exposure

models. The variability in physiological parameters is well understood

and the magnitude of uncertainty is low.

b. Exposure duration. The following characteristics affect exposure

duration:

(1) Feeding preferences. Because of lack of information about actual

feeding preferences, screening-level risk assessments often

conservatively assume that a receptor of concern feeds exclusively

on a single prey item from the area of the disposal site. However,

different species may have very different exposures and estimated

body burdens of contaminants. For example, a fish that feeds on

benthic invertebrates could consume polychaete worms that feed on

highly contaminated sediment or a filter feeding clam that is

exposed to lower concentrations of hydrophobic contaminants in

overlying water. Uncertainty about feeding preferences will

contribute to uncertainty in estimates of exposure and risk.

(2) Food and water ingestion rates. Because information on rates of

ingestion of food and water will not be available for all receptors of

concern, rates may be estimated from allometric relationships

(USEPA 1993c). However, estimated values may not reflect actual

rates for some species. For example, some species obtain more of

their water from their diet and drink less water than other species.

When available, measured values for ingestion rates of food and

water should be used rather than estimated rates.

(3) Home range. Home range is defined as the geographic area

encompassed by an animal's activities (except migration) over a

specified time (USEPA 1993c). Home range size (also known as

territory size or home range) can be used to estimate the proportion

of time that an individual animal is expected to contact

contaminated environmental media. In a screening-level risk

assessment, the home range of the species of concern is often

assumed to completely overlap the site under consideration, so that

the predator takes all of its prey from the contaminated site. In

actuality, the home range of the predator can be larger than the area

of the site and only a fraction of the predator's total prey is taken

from the site. Uncertainties in the actual home ranges of particular

species can be significant. For example, home range size for

individuals within a population can vary with season or latitude or

as a consequence of changes in the distribution and abundance of

food or other resources.

Typical home range sizes for commonly encountered terrestrial

species, as summarized in the Wildlife Exposure Factor Handbook

(USEPA 1993c), are presented in Table 3.

Chapter 5 Uncertainty in Tier IV Risk Assessments