Dames & Moore, 1999 - USDA Forest Service

Referring to Table 4.3-6a and 4.3-6b, the results of the annual water budget yields a comparable value for
AET as was estimated from potential evapotranspiration (PET) and the monthly water budget data. The
long term monthly water budgets are considered to be reasonably representative of actual conditions. Based
on this, long-term and short-tern (1997 spring and summer) water budgets were developed for the Site
using the same estimating methods. The water budgets shown on Table 4.3-6b assume a Site area of 120
acres located between RC-4 and RC-2, including the tailings piles, the waste rock piles and the mill site.
The water budget totals were estimated by subtracting the estimated evapotranspiration (in inches) and
runoff (converted to inches by dividing the runoff totals for the month by the estimated 120 acres of
contributing area) from the total precipitation. If the precipitation was less than the evapotranspiration, then
evapotranspiration is limited to the available water, or the precipitation total.
The objective of the analysis is to evaluate whether climatic inputs (P) could account for the observed
groundwater and seep ~noff from the Site. As an average estimate of runoff, the Railroad Creek flow
relationship ratios were used. As stated previously, runoff between RC-4 and RC-2 averages between 0 and
3 percent of the flow at RC-4, excluding input from Copper creek (Table 4.3-4). Taking the average of this,
runoff from the Site w& assumed to be one half of the average (this assumes half of the estimated inflow
arrives from the south side of the valley), or 0.75 percent of the monthly average flow at RC-4.
Based on these assumptions there is a significant deficit of water at the Site based on the negative net
change in storage (Table 4.3-6b). The deficit in the Site water budgets indicate that a significant portion of
inflow (groundwater and seepage) between RC-2 and RC-4 is generated from areas upgradient of the Site.
On average, the upgradient source inflow could be as high as 40 percent (total deficit divided by total
runoff). The upgradient inflow source areas probably include groundwater recharge higher up in the basin
discharging through the bedrock and alluvial aquifers beneath the tailings and waste rock piles, down valley
groundwater flow through the alluvial floodplain sediments which underlie the tailings piles, and to a lesser
degree recharge upslope of the tailings and waste rock piles and the mill building from upgradient meltwater
runoff.
A discussion of the site-specific water balance is presented in Section 4.4.4.
4.3.6 Hydraulic Analysis and Synthesis
4.3.6.1 Predicted Flood Levels at Site
The climatic data, watershed characteristics determined from topographic maps, and historic streamflow
data were used to develop and calibrate a watershed model Hydrologic Engineering Center-l (HEC-1) of
the Railroad Creek basin. The model predicted the 100-year and lesser floods for comparison with previous
flood estimates (ORB, 1975), and as input to the HEC-2 hydraulic model of the channel adjacent to the
tailings piles.
The HEC-I and HEC-2 models of Railroad Creek watershed and Railroad Creek channel adjacent to the
Site provide estimates of water levels and flow velocities along the tailings piles. The results of the
modeling indicate areas of potential erosion for use in the development of remedial design options to
address the potential for erosion of the tailings piles. Geomorphologic observations and observations of
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17693-005-019Uuly 19. 1999;4:51 PM;DRAFT FINAL Rl REPORT