Dames & Moore, 1999 - USDA Forest Service

case scenario; however, suitable earthworm habitat may not exist due to the physical qualities
of the substrate at the sample locations.
Robins could be at risk from cadmium in the subsurface tailings, lagoon and maintenance yard,
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.
. .
9.2.8 Tailings Pile Slope Stability
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 releasekilings 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.
9.2.9 Windblown Tailings Material
The erosion of the tailings from the piles has resulted in the deposition of the materiqs on the ground surface
adjacent to and downwind of the Site. The majority ofthe windblown tailings deposiis-were found to be less
than several millimeters in thickness. Concentrations of all metals analyzed, other than iron, were below the
regulatory standard for soil. Based on the results of human health and ecological risk assessment, the
potential for adverse effects from the iron concentrations was low.
. .r
9.2.10 Riprap and Soil Source Evaluation ; I
An evaluation was completed to identify a source of riprap within the Railroad Creek drainage and sources of-, .- '
granular soil that may be needed for remedial actions. The results of the evaluation confirmed that the rock
quality within the existing quarry is relatively poor. However, a potential source of higher quality rock exists
nearer the Site as a talus deposit (cobble- to boulder-sized rock at the base of a bedrock outcrop). The riprap
source had been eliminated by the USFS during the Site work between 1989 and 1991 due to safety
considerations; however, it appears feasible'to design measures to mitigate the concerns. A potential source
of granular soil was identified near tailings pile 3.
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17693-405-019Uuly 28.1999;10:07 AMDRAFT FINAL RI REPORT