Dames & Moore, 1999 - USDA Forest Service

unoff into the lagoon (SP-16). Observations of the surface of the tailings piles and slopes within the Site
area indicate that there is little evidence of rilling or any overland runoff from the tailings piles. This
observation would include that infiltration to the tailings piles is more likely to occur as compared to
runoff. The only area where rilling from overland runoff was observed was in windblown tailings piles
and soils west of tailings pile 1, between the tailings piles, and the Copper Creek diversion in the area
immediately down slope of the access road, and in the windblown tailings and soils within the Copper
Creek channel and banks between tailings piles 1 and 2. Runoff from the west side of tailings pile 1
would flow into the SP-19 ditch or into the Copper Creek diversion channel, and runoff within the Copper
Creek channel would flow directly into Copper Creek.
Runon from upgradient sources during melt and heavy rainfall would tend to be routed in existing drainages
either to Copper Creek, the Copper Creek diversion, directly into Railroad Creek, or into the SP-16 lagoon;
the water which collects in the lagoon appears to infiltrate into the subsurface soils. Some of the runon
collected in the ditches and lower-lying areas on the surfaces of the tailings likely infiltrate into the
subsurface. The amount of infiltration resulting from run-on into the tailings is not expected to be large
because of the relatively small areas of infiltration and the relatively low permeabilities measured in
surfaces of the tailings historically by others (Hart Crowser, 1975) and Dames & Moore during the RI.
Runoff from the tailings piles and surrounding areas would be generated when either snowmelt or rainfall
exceeds the infiltration and holding capacity of the surface soils. The holding capacity is a function of the
slope and storage capacity or condition of the surface. Surfaces that are highly irregular and include a
porous material or dense leaf litter, tend to hold more water within interstices and in puddles. Smooth
surfaces made up of uniform materials will generate more runoff. Surface slopes on the tailings piles are
relatively flat and in some areas shallow ponds develop (as observed on the southern portion of tailings pile
2 during the May 1997 and 1998 sampling events). Additionally, loose gravel surfacing on the piles allows
for water to be stored within the surface soils, encouraging infiltration and decreasing runoff potential.
Surrounding soils on slopes are steeper; however, they include leaf litter and organic forest soils that would
store water and inhibit the generation of runoff. Neither the surface conditions on the tailings piles or on
surrounding areas are conducive to creating runoff and runon to adjacent areas.
Steep tailings pile slopes, areas that are not capped by the loose gravel, and compacted access road surfaces
are more likely to generate runoff and runon during snowmelt and during large rainfall events. Limited
rilling was observed during the RI in several of these areas. Runoff is also generated from areas that
become saturated and are sloped. These saturated areas occur adjacent to drainages and where snow or
rainfall accumulates. The saturated areas appeared to occur adjacent to drainage ditches on the upslope side
of the tailings piles and in lowland areas on the banks of Railroad Creek.
The infiltration potential of the tailings material beneath the gravel cap has been estimated to be on the order
of 4 to greater than 20 inches per day under saturated conditions (see Section 4.4). The saturated infiltration
capacity, under most conditions, represents the lowest potential infiltration rate. Exceptions to this occur
when extremely dry soil repels water due to surface tension of the dry dust. However, dryness under most
conditions will accelerate infiltration because of negative pressure, heads in the soil. Comparing the
saturated infiltration capacity to expected snowmelt rates, assumed to have averaged 1.6 inches per day (50
inches of water equivalent snowpack for 1997 melted over the time period April 1.6 to May 19) indicates
that melt rates are generally below potential infiltration capacities of the tailings pile material. Based on
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17693-005-0 19Wuly 19. 1999:4:5 1 PM;DRAFr FINAL RI REPORT