Dames & Moore, 1999 - USDA Forest Service

Little Brown Bat
The little brown bat is a nocturnal species that feeds exclusively on insects captured in flight. emerging from
surface waters, or stationary on vegetation (Sample et al., 1997). Aquatic insects (e.g., Chironomodidae.
Diptera, Isoptera, and Trichoptera) are the primary types of insects capnrred in western Oregon (Whitaker et
al., 1977) and collected in Railroad Creek. Reported body weights range from about 3 to 12 g, but average
about 8 g (Sample et al., 1997). Bats are active chiefly from March to August, and hibernate during the
winter and early Spring.
Using the equations provided by Sample et al. (1997), this is a feeding rate of 0.162 kg/kg body weight per
day, and a drinking rate of 0.160 L/kg body weight per day. Although the forage range of this species is not
known, a related species, the gray bat may travel as far as 12 km from roost to foraging areas (LaVal et al..
1977).
It was assumed that little brown bats at the Holden Mine site feed on emerging aquatic insects. Therefore.
the predicted insect body burdens found in Table 7.2.3-6, were used to estimate the dose received by bats in
the area (Table 7.2.3- 18).
7.2.4 Risk Characterization
The risk characterization was first evaluated for locations where the highest concentrations of metals were
found using worst case exposure assumptions. If risk was found using these worst case exposure
assumptions, these assumptions were modified to better reflect more probable exposures. This more refined
level of analysis was only conducted where risk was found using the most conservative exposure
assumptions.
The hazard quotient (HQ) method of risk characterization was used to assess the potential for existing metal
concentrations at the Holden Mine site to pose risks to ecological receptor species. The worst case HQ for
each PCOC was determined by dividing the upper 95 percent confidence limit on the mean of the site
concentration or estimated dose data by the appropriate toxicity reference value (TRV) for each ROC in a
given potentially complete exposure pathway:
HQ = concentration or dose/TRV 7-23
The total risk for each ROC was determined by summing the hazard quotients for each exposure pathway
(XHQ). At this time the USEPA and Ecology have not provided specific guidance to interpret hazard
quotients. However, it is generally accepted that when CHQ > 1 .O, there may be a small potential risk of
adverse effects to ecological receptors. The British Columbia Ministry of the Environment, Land, and
Planning (BCMELP, 1998) has defined HQs > 1 but 400, as an intermediate risk, and HQs > 100 as high
risk. The specific HQ for each PCOC and ROC exposed to the UCL concentrations measured are shown in
Tables 7.2.4-1 to 7.2.4-14. Individual HQs were not added to produce hazard indices (HIS) because there is
insufficient evidence that metals act synergistically, and sufficient evidence to conclude that at least some
metals act antagonistically (Bremmer, 1979; Zmudzki et al., 1984; Edelstein et al., 1984; Carlson et al..
1985; van Barneveld et. al., 1985; Turecki et al., 1995).
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17693dOS-019Uuly 27.1999;5:16 PMDRAFT FINAL IU REPORT