Dames & Moore, 1999 - USDA Forest Service

3.11.4.2 Two-Dimensional Groundwater Modeling
Numerical groundwater modeling (i.e.. computer modeling) was not included in the RI. Hydraulic and
hydrologic conditions at the Site are highly variable, which results in inherent difficulties in obtaining
representative results using complex groundwater models.
3.11.4.3 Site-Specific Water Balance
In comparison to the basin-wide climatic water balance discussed in Section 3.1 1.3.1. a site-specific water
balance was completed to evaluate the magnitude and importance of water source inputs to Railroad
Creek in the vicinity of the Site. The potential sources of water analyzed on the Site included
precipitation runoff, tributary, inflow, and groundwater inflow. Groundwater inflow included flow
through the tailings piles, flow through the alluvial aquifer associated with Railroad Creek, and
groundwater inflow from the bedrock and mine workings.
The component source inflows were quantified within the accuracy of the available data, and compared
with baseflow gain in Railroad Creek for the MayIJune 1997 and September 1997 timeframes. Average
flow conditions were used or estimated for the source inflows and baseflow gain in Railroad Creek, and
storage effects were not directly quantified. Railroad Creek adjacent to the Site was subdivided into two
reaches: Reach 1 from station RC-I to RC-4, and Reach 2 from station RC-4 to RC-2. These reaches
represent unique hydrologic environments, with Reach 1 transitioning from background conditions to the
impacted portions of the Site, including the effects of the Portal Drainage and groundwater from the
western portion of the site, and Reach 2 representing conditions potentially impacted by the tailings piles.
Tlie site-specific water balance approach is to reference all of the water balance components to gain or
loss of water in Railroad Creek. Flow measurements made in Railroad Creek provide the most complete
and accurate measure of the flow of water from the Site. The water balance analysis was completed for
the period of May 1 5IJune 15, 1997, and September 1997 using flow data obtained during the respective
sampling periods. These two sampling periods represent the range of seasonal conditions observed
during 1997.
The results of the site-specific water balance are presented in Section 4.4.4.
3.12 TASK 12 - BASELINE RISK ASSESSMENT
The methods utilized for the Human Health and Ecological Risk Assessments are described in Section 7.0
of this report.
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17693-005-019Uuly 19.1999;2:07 PM:DW FINAL IU REPORT