Environmental Analysis Tools - Environmental Impact Analysis: Process and Methods - page 280

accomplished by identifying which of the primary exposure pathways apply:
dermal contact, inhalation, total immersion (e.g., fish in contaminated water),
or ingestion. The determination of applicability is based on the chemistry
of the contaminant (e.g., inhalation for a volatile compound or ingestion for
a compound such as methyl mercury that magnifies up the food web), the
media involved (e.g., soil, water, air), and the specific receptor (e.g., food items
for the ingestion pathway). The characterization of contact identifies not only
which pathways but also which media are involved in the exposure.
Once the media producing exposure are identified, the final step in the expo-
sure characterization is to determine the intensity of exposure, which in the
case of risk from contamination is the concentration or dose of the contaminant
in the environmental media of concern. The determination of concentration is
required for inhalation, dermal exposure, and total immersion pathways and
begins with a sampling and analysis program throughout the site for each envi-
ronmental media of concern. Frequently the sampling program is adequate to
identify the exposure concentrations for the receptors but sometimes it is neces-
sary to model the media to account for changes in concentration with time or
distance (Boucher 1993). Also if the size of the site is large and there is a low
density of sampling, a statistical estimate of exposure concentration is necessary
to represent uncertainty and distribution of contaminants throughout the site.
The determination of dose for ingestion pathways is a little more complex.
For this exposure pathway, a food web model must be constructed to represent
the diet of each ecological receptor that is vulnerable to risk from ingestion.
First the ingested material, usually a food source, must be identified, quanti-
fied, and the percent composition represented by each item is determined or
estimated. Then the concentration of contaminant in each item is determined,
and the total mass per day of contaminant is calculated as the sum of the con-
centration times the mass of each food item ingested. This calculation yields
exposure via the ingestion pathway, expressed as the total daily dose.
7.2.7 EffectsCharacterization
As described above, the exposure characterization step in the risk assessment
defines which stressors (e.g., contaminants at a hazardous waste site) contact
the receptor and at what level of intensity (e.g. contaminant concentration at
a hazardous waste site). The effects characterization identifies the effects to
the receptor following contact. The ecological benchmarks described above
as part of the screening-level assessment (Section 7.2.4 and Table 7.2) are a
simple expression of an effects characterization. The screening-level bench-
marks are concentrations below which no elevated risk to any life stage of
the most sensitive receptors is expected. In some cases, the same benchmarks
are applicable in the BERA and represent the final effects characterization for
specific receptors, exposure pathways, and endpoints.