Dames & Moore, 1999 - USDA Forest Service

Referring to Figures 4.4- 15 through 4.4- 18, flowtube width and groundwater gradient are measured off of
the flow net. Saturated thickness of the aquifer is based on hydrostratigraphic unit thicknesses derived from
boring logs and geophysical data, and on water levels collected in 1997. The saturated thickness of the
alluvial/reworked till unit is estimated to be 10 feet throughout the Site, except in the vicinity of geophysical
survey line F-F' (north of tailings Pile 2; Figures 4.2-6b and 4.2-12, and Appendix A) where bedrock was
detected to be as shallow as approximately 10 feet below the surface. The saturated thickness of the
alluvial/reworked till unit is assumed to be similar to September to May 1997. While the alluviaVreworked
till unit becomes unsaturated beneath the southern portion of tailings piles 2 and 3 in September, the unit
appears to remain fully saturated adjacent to Railroad Creek.
The saturated thickness of the tailings in both May and September 1997 was estimated to be 10 feet, based
on groundwater elevations-measured in May and September 1997 and elevations of the base of the tailings
from boring logs completed by others (Appendix B). The quantity of water calculated using this approach
should be considered a conservative estimate because it includes potential discharge from seeps located near
Railroad Creek. . '
The parameters used for each flow tube and the calculated discharge for each tube are provided in
Appendix I. Separate flow net analyses were completed for groundwater within the native material (e.g.,
alluvial/reworked till unit), and the tailings. The analyses evaluated two reaches along Railroad Creek:
RC-4 to RC-7; and RC-7 to RC-2 (Figures 4.4-15 through 4.4-18). The flow net constructed for the
alluvium/reworked till unit in May 1997 is shown on Figure 4.4-15, and the flow net for the tailings unit in
May 1997 is shown on Figure 4.4-16. Flow net evaluation of groundwater recharge to Railroad Creek,
based on May groundwater levels, is summarized in Table 4.4-4. The discharges developed by flow net
evaluation range between 0.6 and 1 1.4 (mean 2.2) cfs for RC-4 to RC-7, and 0.3 and 5.2 (mean 1 .O) cfs for
RC-7 to RC-2; these values compare reasonably with spring baseflow estimates of 5 cfs and 1.7 cfs for the
respective reaches from surface water measurements provided in Section 4.3.7.1. The mean discharge
estimates represent approximately 50 percent of the estimated baseflow. The remainder of the baseflow is
likely provided by groundwater discharge from areas not included in the flow net analysis (i.e., the area
north of Railroad Creek).
The September 1997 flow nets for the alluvium/reworked till unit .and the tailings are shown on
Figures 4.4-17 and 4.4-1 8, respectively. The flow net evaluation of groundwater recharge to Railroad Creek
based on September 1997 groundwater levels is summarized in Table 4.4-5. The discharges developed fiom
the flow net evaluation of 0.3 to 6.1 (mean 1.2) cfs for RC-4 to RC-7 and 0.2 to 0.0 (mean 0.1) cfs for RC-7
to RC-2 are within the approximate range of those discharges measured during the September 1997
baseflow monitoring described in Section 4.3.7.2. The groundwater discharge appears to be less in
September than in May 1997 due to decreases in hydraulic gradient, especially beneath tailings pile 1.
As indicated on Tables 4.4-4 and 4.4-5, the contribution of tailings drainage to Railroad Creek as a ratio to
total groundwater contribution increases approximately 3 times fiom May to September 1997. In May,
mean tailings contribution is estimated at 0.2 cfs of a mean total of 3.2 cfs of groundwater, or approximately
3 percent of the total groundwater contribution. In September, mean tailings discharge appears to be 0.1 cfs
of a mean total of 1.3 cfs, or approximately 10 percent of total.
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17693905-019Uuly 19. 1999:4:51 PM;DRAFT FINAL RI REPORT