Dames & Moore, 1999 - USDA Forest Service

As the summer season progressed, the response to precipitation events became less evident.
This relatively rapid response to precipitation events suggests that any of the following may be occurring:
(I) the portal drainage reflects the pool level in the mine (it is not possible for the water levels in the mine to
increase fast enough to account for the increased discharge rates observed); (2) the bedrock in the mine is
relatively saturated during spring and early summer and, therefore, responds relatively quickly to
precipitation events; and (3) as the summer months continue, the bedrock becomes less saturated and,
therefore, responds less dramatically to precipitation events.
4.3.3.7 Seep, Surface Water Runon, Runoff, and Infiltration
This section addresses surface waters that contribute to flow in Railroad Creek from the Site that are
intermittent and that do not flow in a defined tributary channel. These surface waters include select seeps
which reflect surface water, runoff and runon. In addition, infiltration of runon and runoff into the tailings
and soils at the Site is also addressed.
Water sample locations that have been designated as seeps include direct groundwater discharges observed
at the ground surface, and drainage ditches which combine runoff generated on the Site and in some cases
runon from upgradient sources. Groundwater discharge may also contribute to flow in ditches. At the
request of the Agencies, ponded surface water that receives seepage and which was sampled during the field
investigation have also been designated as seeps. Table 4.3-6 lists all of the designated seep sample
locations, including seep water source, receiving water and measuredJestimated flow rates for the four
sampling rounds in 1997 and any additional sampling rounds that were collected at each seep location. All
seep sample locations are shown on Figure 4.3-8.
Generally, seeps emerging from the banks of Railroad Creek and from the base of the tailings piles
discharge groundwater directly, and do not include component'runon/runoff flow. The groundwater seeps
are expressions of general groundwater discharge from the valley sidewalls, and typically emerge along
seepage fronts, which become concentrated in specific areas where they were observed and sampled. The
flow in these seeps is greatest during spring immediately after snowmelt, and their highest flow coincides
with the highest groundwater levels (see Section 4.4). Most of the groundwater seeps cease flowing during
the late summer as groundwater levels drop; however, several were observed flowing, albeit at a low rate,
throughout the 1997 field season.
Throughout September and early October 1997, a series of rainstorms resulted in a rise in groundwater
levels with subsequent increase in seepage flow. However, seeps did not increase flow or begin to flow
again until the beginning of October after several weeks of rainfall. Nearly 4.5 inches of rain fell in
September and more than 3 inches at the beginning of October, including 6 storms exceeding 0.5 inch in 24
hours (Figure 4.3-9a), before seep flow was observed to increase. This relation suggests that there is a lag
time between recharge and seep response, and that a relatively large amount of recharge is necessary to
replenish groundwater levels before seepage flow is increased.
In general, flow from groundwater dominated seeps is low relative to flow in Railroad Creek. Groundwater
seeps located along the streambanks of Railroad Creek are also impacted by the water levels in the creek.
These seeps, including SP- I, SP-3, ~ ~14, SP-9, SP- I OE, SP- 1 OW, SP- I 1, SP- 12, SP-24, and SP-25, become
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