GENERAL42548

• Permit No: M1988112
• IBM Index Class Name: General Documents
• Doc Date: 11/8/1999
• From: SHEPHERD MILLER
• To: DMG
• Media Type: D
• Archive: No

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Monitoring well PC-2, while not located neaz the proposed slurry wall, may be representative
of ground water chazacteristics in the Precambrian bedrock. A 25-foot interval of
' Precambrian gneiss is screened in monitoring well PC-2 at depths of 17 to 4E feet below the
surface. Hydraulic testing of monitoring well PC-2 was conducted during clevelopment of
' TR
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ydraulic conductivity for the Precambrian
' gneiss.
2.3 Santa Fe Formation
'
The Santa Fe Formation is a buff to light reddish-brown unconsolidated conglomerate.
Clasts of several rock types aze present in the Santa Fe Formation. The cl.ists ranging in
diameter from 2 to 10 cm and consist of gray-green quartzose gneiss, non-silicified biotite
gneiss, hornblende gneiss, biotite schist, granite, gray aphanitic volcanics, anc. other igneous
rocks. The conglomerate matrix is reddish-brown clayey silt and sand.
' Outcrops in the high wall of the West Pit, north of monitoring well BF-5, indicate that the
Santa Fe Formation is in unconformable contact with the underlying biotite gneiss. In these
' outcrops, the Santa Fe material has been deposited on a fresh gneiss surface; there is no
weathered zone or soil horizon developed on the biotite gneiss surface.

The geologic sample description logs for monitoring wells SF-2, M-20, SF-4, M-28, M-18,
' M-29, SF-5 and M-17 indicate that Santa Fe Formation was encountered but not completely
penetrated in each of these borings as shown on Figure 2. The two deeper monitoring wells,
' SF-2 and SF-5, penetrated 118 feet and 86 feet of Santa Fe Formation, respectively. The
thickness of Santa Fe Formation at the locations of the other monitoring wells listed above is
not known.. As shown on Figure 2, monitoring well M-16 encountered Precambrian gneiss
directly beneath alluvial gray sand and gravel. The Santa Fe Formation was not present.
'
Along the alignment of the proposed slurry wall, the Santa Fe Formation is Expected to be
' present beneath alluvium west of monitoring well M-16. East of monitoring well M-16, the
Santa Fe Formation is expected to be absent. The Santa Fe Formation to the east of M-16
' has likely been removed by erosion that preceded deposition of the Rito Seco alluvium.
Monitoring well SF-2 has an 80 foot screened interval of Santa Fe Formation. Hydraulic
'
Batrle bfounlain Resources, /nc. Sbepherd 6li/ler, /nc.
' v:uoole; ~sb,.~~worno~,~q~rtp~r.,trc 3 November 5. 1999