Dames & Moore, 1999 - USDA Forest Service

environment. IHSs were selected for the HHRA based on a screening level risk assessment which
compared maximum detected concentrations to data evaluation criteria listed below:
Essential Nutrients. If the constituent was considered an essential nutrient. it was
eliminated as an IHS. This approach is consistent with USEPA risk assessment guidance
(USEPA, 1989) and MTCA (WAC 173-340-708[2][b][i]).
Frequency of detection. If a constituent was detected in less than 5 percent of the samples
of a medium (or in an exposure area) it was eliminated as an IHS. This approach is
consistent with USEPA risk assessment guidance (USEPA, 1989) and is recommended in
MTCA guidance (WAC 173-340-708[2][b][vi]).
Background values. If the maximum detected concentration was less than the
corresponding background concentration developed, the constituent was eliminated as an
IHS. This approach is consistent with USEPA risk assessment guidance (USEPA, 1989)
and is recommended in MTCA guidance (WAC 173-340-708[2]p][iv]).
Model Toria Control Act (MTCA) Method A Levels. If the maximum concentration of
a constituent was less than the relevant MTCA Method A level, it was considered to
contribute a small percentage of the overall risk and was eliminated as an IHS. Method A
levels are generic cleanup levels which were obtained fiom MTCA (WAC 173-340-720[2].
173-340-740[2]), amended January 1996.
MTCA Method B Levels. If the maximum concentration of a constituent was less than
the relevant MTCA Method B level, it was considered to contribute a small percentage of
the overall risk and was eliminated as an IHS. Method B levels are developed using
standard equations in MTCA and were obtained h m Model Toxics Control Act Cleanup
Levels and Risk Calcularions (CLARC II) Updare (Ecology, 1996). When a Method B
level was not available in CLARC I1 for a constituent detected at the site. it was calculated
using equations provided by MTCA, if toxicity data were available. For the particulate
emissions pathway, soil cleanup levels were calculated using the methodology presented in
Soil Screening Guidance. Technical Background Document (USEPA. 1996).
The selection of the IHSs is presented in the Screening Level Human Health Assessment in Section 5-3.1.
7.1.3 Screening Level Human Health Assessment
A screening level human health assessment was conducted to provide a preliminary evaluation of exposure
pathways and analytical data, select indicator hazardous substances (IHSs) to cany through the site-specific
human health risk assessment and define significant exposure pathways. lHSs are defined by Ecology as
those hazardous substances that can be used to define site cleanup requirements.
7.1.3.1 Preliminary Exposure Pathway Model
Based on site characteristics and land use, a preliminary exposure pathway model was developed for the
Site. The model illustrates our conceptual understanding of the chemical sources, release and transport
mechanisms and the potential exposure pathways, exposure routes, and receptors.
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17693-005-019Uuly 27.1999;5:16 PMDRAFT FINAL RI REPORT