5.8.1 Exposure Pathways

The exposure pathway is the course a chemical takes from the source to the exposed individual. The exposure pathway is characterized by the source (the contaminated medium), the mechanism of release, a retention or transport medium, a point of potential human contact with a contaminated medium, and an exposure route at the point of contact (i.e., ingestion). An exposure pathway is complete only when each of these elements is present. Air monitoring may be used to supplement theoretical calculations of air disper­sion pathways.

5.8.2 Sources

Surface soil and subsurface hydrogeological formations (i.e., soil/groundwater sys­tem) that have been affected by the site's former usage are the source areas that come under consideration.

5.9 Environmental fate and transport of copCs

The inorganic contaminants identified at the site may not be generally considered mobile in the soil/ground water system. Mobility is affected by soil pH and ground water for aqueous transport. Factors influencing dust generation and movement from the soil surfaces to the air migration pathway must also be considered. Inorganic contaminant mobility through storm water runoff and surface water transport are evaluated. Existing site conditions are evaluated to estimate the migration of contaminants to subsurface soil, groundwater, and surface water, and air.

5.10 Exposure points and exposure routes

Only complete exposure pathways involving current or future contact with contami­nated media are cause for concern. Exposure pathways, in that the potentially exposed population is not likely to experience significant contaminated medium contact or the envi­ronmental medium contacted is not significantly contaminated, will not be considered. Therefore, before determining that the pathways are complete and that the possible recep­tors are likely to be exposed at significant levels, site-specific information is gathered. This information will also be used to develop site-specific exposure parameters. An interim BLRA deliverable is prepared to propose the exposure scenarios that are used in the final BLRA.

5.11 Complete exposure pathways evaluated

A well survey is conducted to determine if any down-gradient domestic wells at the site may be affected. Based on the proximity of the wells, their depth, construction, and use, the potential for these wells to be affected by the site are determined. Irrigation wells, if within the site's vicinity, are a potential exposure pathway vector to the fields that are irrigated.

For the industrial exposure scenario, on-site workers and site visitors are considered for exposure to contamination through surface soil, subsurface soil, and air. The recre­ational exposure scenarios are considered for site trespassers and visitors to the site.

5.12 Ecological risk assessment

Pathways for terrestrial and aquatic environmental receptors regarding exposure to subsurface soil, surface soil, sediment, surface water, and air are considered in the ecolog­ical component of the risk assessment. The ecological component of the BLRA characteri­zation includes the following:

• Identification of habitats and predominant species occurring at the site including the river areas

• Selection of representative species that may have the greatest exposure based on feeding habits, habitat usage, and exposure duration (Receptors of concern will also be evaluated based on available published eco-toxicological data.)

• Verification that contaminants of potential ecological concern are limited to heavy metals

• Research into toxicological reference values in published material (Ambient water quality criteria [AWQC] are used for contaminants.)

• Evaluation of environmental data to adequately estimate existing and potential future ecological risks

• Development of a sampling and analysis plan (SAP) for data collection to evalu­ ate ecological exposure

• Determination whether background data are required or if other information such as surface water hardness is necessary to evaluate toxicity

• Quantification of exposure by estimating the magnitude and rate of exposure for receptors of concern. (This quantification will include evaluation of contaminant concentrations, bioavailability, bioaccumulation, bioconcentration, and biomag- nification potential; feeding rates; habitat usage; and food chain considerations.)

• Review of the results of the toxicity and exposure assessments

• Comparisons of exposure concentrations to appropriate toxicological reference values to complete a risk characterization (This characterization will include an evaluation of the spatial distribution of contamination with regard to ecological receptors.)

Decisions on whether an ecological impact exists are based on this risk characteriza­tion. Sampling to determine contaminant exposure potential then follows the same criteria as for the human health risk assessment, except that sampling routines may be specialized to deal with species-specific exposures. An example would be the exposure of burrowing animals to soils through ingestion, inhalation, and dermal contact.