Risk assessment modeling estimates the likelihood that a chemical-related exposure (significant enough to cause adverse effects) may occur in a given human or animal population based on the estimated or measured concentrations of a given chemical in soil, air or animal tissue. Exposure can be modeled for any of three direct pathways by which a chemical enters into and interacts with an organism; typically expressed as doses (milligrams per kilogram of body mass per day). These doses are compared to toxicity thresholds to determine the likelihood of an adverse effect or risk.

Methodologies for conducting ecological risk assessments are less defined, but are based on United States Environmental Protection Agency (USEPA) guidance documents (USEPA, Framework for Ecological Risk Assessment, Risk Assessment Forum, EPA Headquarters, EPA/630/R-92/001, February 1992). A conservative approach to risk modeling for animal receptors is to choose a species with the following characteristics:

• A species known to be present in the location of probable or measured highest contaminant concentrations for several environmental media for all or most of its lifecycle (that is, with a small home range relative to the area of potential contamination)


• A species of small body mass relative to the probable or expected contaminant concentration (i.e., to maximize potential dose)


• A species with behaviors that offer the highest potential for contact with contaminated media (i.e., species that spend a lot of time digging, plant eating species, species that eat both plants and animals or species that acquire food and water in the modeled potential contamination zone)

These characteristics favor the selection of a species that would be expected to receive the maximum environmental dosage. If the results of the ecological risk assessment indicate a potential for toxic effects, it may be appropriate to conduct other assessments. However, if modeling reveals no significant risk to these types of species, toxic effects would not be expected in other species with larger ranges, larger body masses, diets that include sources outside the modeled potential deposition zone or dining habits that keep them aboveground most of the time.