Dames & Moore, 1999 - USDA Forest Service

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I . Soil and Fill
Water flow within the Railroad Creek valley is believed to occur principally as surface water within
Railroad Creek and groundwater flow in the permeable materials within the valley. Limited groundwater is
anticipated to occur within the bedrock (Patmont, 1989). Areas of shallow bedrock force more flow above
ground and to Railroad Creek. As the depth to bedrock increases and surficial, higher permeability material
thickness increases. surface water returns to the subsurface.
Processes and conditions described below for the Site are presumably in operation throughout the Railroad
Creek watershed. Where groundwater elevations are greater than surface elevations, springs or seeps will
form. This occurs most commonly in spring, as snowmelt saturates near-surface materials. Seep flow may
continue later into summer where zones of discontinuity in surficial materials provide preferential pathways
for flow or where impermeable zones create perched water conditions.
Groundwater is not used at the Site as a potable source. Groundwater is used as a domestic water supply at
the Lucerne Bar USFS station, approximately 1 1 miles downstream.
4.4.3 Site Hydrogeology
The following sections describe the Site hydrostratigraphy, water levels, and groundwater flow.
4.4.3.1 Hydrostratigraphy
The characteristics of the geologic units at the Site affecting groundwater flow are summarized in
Table 4.4-1. Figures 4.4-2 through 4.4-5 are interpretive geologic and hydrogeologic cross-sections
through tailings pile 2 and tailings pile 3 for May and September 1997, respectively; the locations of the
cross-sections are shown on Figure 4.4- 1. The geologic and hydro-geologic cross-sections iire based on the
interpretation of geophysical survey results (Appendix A), review of site boring logs (Appendix B), and
groundwater monitoring well levels measured in May 1997 (Table 4.4-2). The characteristics of the
geologic ,materials as hydrostratigraphic units are based on groundwater occurrence as measured in
monitoring wells and by hydraulic conductivities. Groundwater levels were measured in wells during the
1997 field program. Hydraulic conductivities for the tailings piles were calculated from slug test data
(Appendix I) colle.cted from monitoring wells screened within and beneath the tailings and percolation tests
conducted by Dames & Moore (Appendix G) and others ar art Crowser, 1975; Appendix E). Hydraulic
conductivities for other native materials at the Site are based on literature (Freeze and Cherry, 1979; Daly,
1982). Slug test results for wells tested as part of the RI are compiled in Table 4.4-3.
Following the format provided in Table 4.4-1, each of the hydrostratigraphic units present at the Site is
discussed in more detail below. A hydrostratigraphic unit is composed of one or more types of geologic
materials that have similar properties related to groundwater flow and storage. The hydraulic conductivity
and occurrence of groundwater within the hydrostratigraphic units are described below in generally
descending stratigraphic order from the ground surface. The areal distribution of the principal geologic
units is presented on Figure 4.2-6a.
Native soil thinly covers (assumed to be often less than approximately one foot) all undisturbed portions of
the Site where bedrock is not exposed at the ground. Man-made fill, in addition to tailings and waste rock,
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17693-005-019Uuly 19. 1999,4:51 PM;DRAFT FINAL RI REPORT