Dames & Moore, 1999 - USDA Forest Service

Groundwater occurs at the base of the tailings piles. The thickness of saturated tailings ranges from
approximately 3 to 21 feet in May and 3 to 17 feet in September 1997, based on the boring logs
(Appendix B) and water levels collected during 1997.
Referring to Figure 4.2-6b, a test pit excavated at the northeast corner of tailings pile 3 (DMTP3-4; see
Appendix C) encountered a partially cemented layer at the contact between the tailings and native ground
with flowing water noted beneath the layer and dry tailings above. This suggests the potential for a lower
permeability layer at the contact between tailings and native ground. This layer was not logged on boring
logs (Appendix B) of wells completed beneath the tailings, as well as in exploratory borings completed by
Hart Crowser in 1975 as part of a geotechnical engineering evaluation (Appendix E).
Waste Rock
Waste rock piles are found associated with the six underground mine portals in the Honeymoon Heights
area and near the 1500-level main portal, on both the west and east sides of the abandoned mill building.
The hydraulic conductivity of the waste rock piles has not been measured; however, hydraulic conductivity
is estimated to be large (greater than 1 x 10" cmlsec) due to the relatively large size and poor sorting of
fragments which make up the piles. Groundwater is believed to occur intermittently (during the spring
snowmelt period only) within the waste rock piles, and based on the limited flow rates of seeps SP-6 and
SP-8 near the base of the west and east waste rock piles, respectively, is anticipated to be relatively small in
quantity.
Alluvium/Reworked Till
The alluviumlreworked till is considered herein to consist of the transition from the alluvium, noted above to
be associated with creeks, to the glacial till (discussed below) which has been reworked by glacial and post-
glacial processes. The hydraulic conductivity of the alluviurn/reworked till ranges from 9 x 1 o4 to 5.3 x 1 u2
cmlsec, based on slug tests performed at wells TPI-5, TP2-11, and TP2-7 (Table 4.4.3). Water level
measurements collected during slug tests and slug test calculations are provided in Appendix I. Vertical and
horizontal hydraulic conductivity within the alluviumlreworked till unit are likely comparable to a glacial till
that has been to some degree washed of fine material.
Groundwater appears to occur within the alluviaVreworked till unit throughout the Site. In May, the unit
apparently is hlly saturated, based on water levels collected during the RI, hence, the saturated thickness
ranges from 5 to 15 feet. As shown on Figures 4.4-4 and 4.4-5, in September some portions .of the
alluviaVreworked till unit become unsaturated beneath tailings piles 2 and 3.
Glacial Till
The glacial till consists of fine silt- to boulder-size material deposited and compacted by pre-historic glaciers
within the Railroad Creek drainage. No hydraulic conductivity data are available for the dense glacial till at
the Site. Hydraulic conductivity values of 1 x 10" to 1 x 10''~ cmlsec have been reported in the geologic
literature (Freezd and Cherry, 1979). The values above are for intergranular hydraulic conductivity.
Weathered and fractured till may have larger hydraulic conductivity values.
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17693-005-0 19Uuly 19. 1999:4:51 PM;DRAFT FMAL RI REPORT