Dames & Moore, 1999 - USDA Forest Service

A 500-year event was not analyzed, with or without fire and landslide upstre& scenarios; however, it is
assumed that such an event would remove additional riprap and likely result in erosion and the release of
tailings materials to Railroad Creek.
8.2.3.2 Copper Creek
The primary tributary to Railroad Creek is Copper Creek which enters Railroad Creek at the Site between
tailings piles 1 and 2. Copper Creek provides less than 10 percent of the total flow of Railroad Creek at this
location. The source of Copper Creek is the basin to the south of the Site. Due to the relatively steep
gradient and irregular substrate, it is difficult to determine whether Copper Creek is losing water adjacent to
tailings piles 1 and 2.
Groundwater measurements collected from nearby wells installed in tailings pile 2 infer a losing condition
for Copper Creek adjacent to the tailings piles for portions of the year. However, it is reported that much of
Copper Creek is diverted to the hydroelectric plant during the winter months to provide electric power for
Holden Village. Consequently, the potential for loss of wa'ter to the tailings piles is likely limited during this
period of time.
8.2.3.3 Copper Creek Diversion
Copper Creek contains a concrete weir approximately one-half mile southeast of the Site. A pipe transports
a portion of the Copper Creek water to the Holden Village hydroelectric plant located immediately west of
tailings pile 1. The water exits the plant and flows above-ground to Railroad Creek. The drainage, known
herein as the Copper Creek Diversion, comes into contact with the western limit of tailings pile 1. A portion
of the stream flows through a pond utilized by Holden Village residents and visitors for cooling off after
using the sauna facility before it also exits into Railroad Creek.
8.2.3.4 Mine Portal Discharge
1500-Level Main Portal Drainage
After the operations ceased in 1957, the lower workings of the underground mine eventually filled with
groundwater. The water now flows out of the lowermost mine opening at the 1500-level portal near the
abandoned mill facility. Weekly flow measurements were collected during the RI between May and October
1997. In addition, a data logger was installed in a weir placed at the portal opening in early October, 1997 in
order to collect nearly continuous water level readings. The measured flow rates ranged from
approximately 0.10 to 0.20 cfs (approximately 45 to 90 gallons per minute) from about September to April,
with isolated peak flows as high as 3.5 cfs (approximately 1,570 gallons per minute) during spring melt in
May and June of 1997; in comparison, peak flows for May and June of 1998 were measured to be less than
approximately 1.8 cfs (approximately 808 gallons per minute).
An analysis of precipitation data collected during the RI when compared to the portal drainage flows for the
same period indicates a relatively rapid response (within approximately one to two days) in flow rates in the
portal drainage after a precipitation event in spring through early summer. From approximately mid-summer
through fall, the influence of precipitation on portal drainage flow rates appears minimal. This suggests that
the bedrock is saturated during the spring to early summer months, and unsaturated for most of the
remainder of the year.
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17693-005-019Uuly 28, 1999:10:24 AM,DRAF? FINAL RI REPORT
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