Dames & Moore, 1999 - USDA Forest Service

lack of suitable habitat due to the presence of flocculent, although some potential risk for
adverse effects from metal flocculent concentrations was noted.
Under a reasonable scenario conditions, there is no risk due to metal toxicity to the birds or
&nmals associated with aquatic habitat near the site.
Terrestrial Exposure Pathway and Receptors of Concern
Tailings Pile Slope Stability
Plants may experience toxicity from cadmium, copper, lead, and zinc in Holden Village
surface soil and in the surface soils and subsurface soils of tailings piles 1, 2 and 3, the
lagoon and the maintenance yard; however, when compared to soil metals concentrations at
other mine sites where plants are successfirlly growing, only copper concentration in
subsurface soils, the lagoon, and maintenance yard may present a risk of phytotoxicity.
Earthworms may be at risk from cadmium, copper, lead andlor zinc in surface and
subsurface soils at Holden Village, 'tailings piles, dust, lagoon and the maintenance yard
under the worst-case scenario; however, suitable earthworm habitat may not exist due to the
physical qualities of the substrate at the sarnple'locations.
Robins could be at risk from cadmium in the subsurface tailings, lagoon and maintenance
and from zinc in the subsurface soils, tailings pile 3, the lagoon and the maintenance
yard, and from lead in the lagoon and maintenance yard based on the worst-case scenario.
However, under the reasonable scenario (median concentration), there was no risk from .
cadmium or zinc. It is highly likely that the input parameters for the'robin overestimate the
actual exposure conditions because a risk was also shown for robins feeding on earthworms
exposed to background concentrations of cadmium and the exposure assessment does not
account for the robins relatively large forage range.
Under normally expected conditions, there is no risk due to metals toxicity to mammals
associated with terrestrial habitat near Holden Mine.
The slopes adjacent to Railroad Creek vary in height between 50 and 120 feet, and are relatively steep.
The tailings pile slopes have the potential to release tailings to the creek during an
earthquake event with a recurrence interval of approximately 40 years. The event would
likely be limited to a maximum depth of approximately 15 feet and include only those
slopes of the tailings piles facing Railroad Creek that are steeper than 34 degrees.
The rock placed as Railroad Creek streambank protection (riprap) during the Site
rehabilitation,efforts performed by the USFS is weathering relatively rapidly. The height of
the rock placement, as well as the size of the rock, appears marginal to protect the base of
the tailings piles during a hypothetical 100-year storm event, and is likely not adequate to
protect the base of the tailings piles during a hypothetical 500-year storm event.
G:\WPDATAUX)S\REPORTSWOLDEN-~ECUTI- SUMMARY.~~:
17693-005-019Uuly 28, 1999;12: 16 PM;DRAFT FINAL RI REPORT ES-12 .