2008-04-15_REVISION - M1980244 ‎(325)‎

• Permit No: M1980244
• IBM Index Class Name: REVISION
• Doc Date: 4/15/2008
• Doc Name: VOL II Appendix 2, Hydrologic Evaluation Report
• From: CC & V
• To: DRMS
• Type & Sequence: AM9
• Media Type: D
• Archive: No

Grassy Valley hydrologic conditions 27
fluctuations. Hydrographs are included in Attachment D. Recent water levels in the wells
are shown on Figure 4.1 and presented in Table D1 (Attachment D). Water levels in
diatreme show some seasonal variation as a result of recharge. Seasonal variations in water
levels are less pronounced in granite wells, indicating less recharge to the granites compared
to the diatreme. Bedrock water levels in Grassy Valley for the diatreme and granite are
provided in Attachment D.
During the 2007 field program, three wells were installed in the Precambrian granites on the
north side of Grassy Valley. Field data indicated that the granites are extremely competent.
This is consistent with historical information regarding the geologic and hydraulic
characteristics of the granite. Water levels in the granite bedrock wells are generally shallow
and range between about 9,700 feet and 10,100 feet amsl, with water levels close to the
granite surface (e.g. GVMW-9A, -6A and -7A.) and showing little seasonality. Saturated
competent bedrock indicates low hydraulic conductivity and very limited recharge and
groundwater flow. Water infiltrating the granite is expected to migrate slowly downgradient.
The Grassy Valley diatreme feeder is recharged from the overlying alluvium as evidenced by
seasonality in bedrock and alluvial groundwater levels. Groundwater level data also indicate
that this portion of the diatreme is saturated to within about 100 feet of the diatreme surface
(CAM-1A). Some wells show much deeper water levels (GVMW-10, GVMW-11A, WL-1A) or
are dry. No pattern of groundwater flow is clearly discernable from the available data
indicating that the system is dominated by fracture flow and is structurally
compartmentalized.
Data from several of the well pairs within the Grassy Valley diatreme feeder (designated A/B
wells) demonstrate strong vertical gradients within this part of the diatreme. For example,
recent water levels in well pairs CAM-1A/CAM-1 B and GVMW-8A/GVMW-86, both located
within the diatreme feeder, are approximately 10,005 feet and 10,119 feet amsl, respectively,
and approximately 9,737 and 9,849 feet amsl, respectively. These gradients reflect a low
permeability and/or a compartmentalized groundwater flow system with limited and localized
recharge. Water levels in the diatreme feeder in Grassy Valley are significantly higher than
those in main diatreme to the south. This indicates hydraulic connection between the two
areas is limited (Adrian Brown, 2008) which further supports the theory of tow permeability
and/or strong compartmentalization within the diatreme feeder, limiting or slowing
groundwater flow.
Diatreme groundwater in Grassy Valley is expected to drain slowly toward the main diatreme
which ultimately drains to the Carlton Tunnel based on the topographic and geologic
constraints, and the strong hydraulic gradient from the Grassy Valley feeder of the diatreme
to the main diatreme (Adrian Brown, 2008).
4.3.5 Grassy Valley hydrogeology summary
The shallow and deep groundwater systems are summarized below:
• The composition of the alluvial material overlying Grassy Valley is highly variable and
contains a significant amount of clay. Flow within the alluvium is controlled by the
amount and distribution of clays.
• Alluvial groundwater above the granites is encountered close to the granite contact.
Water may flow along this contact towards Grassy Creek. The continual presence of
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