REV88066

• Permit No: M1980244
• IBM Index Class Name: Revision
• Doc Date: 9/29/2004
• Doc Name: Dr. Adrian Browns Responses
• From: CC&V
• To: DMG
• Media Type: D
• Archive: No

Memorandum
the base of the mine backfill will not have any incremental impact on the quality of the
regional ground water drained by the Carlton Tunnel.
Comment 2b: The notion that water quality changes have not occurred belies the high
zinc concentrations that resulted from upset conditions shortly after Amendment 8 was
approved.
Response: The commenter is incorrect that there was an upset zinc concentration
condition shortly after Amendment 8 was approved. There was an upset condition in
1999 prior to full implementation of Amendment No. 7 and before approval of
Amendment No. 8. There was also an elevated zinc concentration in Carlton Tunnel
water briefly in 2001, since the passage of Amendment No. 8. The 2001 concentration
was in compliance with relevant water quality permits, is precipitation related, and is not
the highest peak observed. It has been observed, and is stated in Amendment No. 8,
that peaks in concentration of (particularly) zinc occur when there is high flow exiting the
Carlton Tunnel. This phenomenon has been observed independent of significant
surface development resulting from current mining (TR-43 Exhibit F, Figure 5t). It has
also been observed and reported in Amendment No. 8 that flow exiting the Carlton
Tunnel is related to precipitation, both due to rainfall and snowmelt. Thus the
concentration of zinc in flows exiting Carlton Tunnel waters is a function of precipitation.
The observed peaks of zinc concentration in the period before and after Amendment
No.8 occurred in 1994, 1995, 1998, 1999, and 2001 (TR-43 Exhibit F, Figure 5). These
same five years are also the highest precipitation years since 1992 (TR-43 Exhibit F,
Figure 2). The highest flow exiting the Carlton Tunne{2, and the highest zinc
concentration in the Carlton Tunnel water, was measured in 1999. In 1999 a very wet
late winter period provided a significant late snow depth, and heavy early summer rains
melted this snow-pack and also provided more infiltration. This "upset" meteorological
condition was not caused by mining, nor was the resulting flow and zinc peak caused by
mining. The flow and zinc peak in 2001, upon which the commenter appears to base his
comment, occurred in the fourth wettest year since 1992, and produced the fifth highest
zinc concentration peak. Again, this peak was in compliance with relevant water quality
permits. There is no evidence that this is inconsistent with the "natural" flow-zinc
relationship in the regional groundwater flow system, and there is no evidence that this
zinc peak is the result of post-Amendment No. 8 mining, as suggested by the
commenter.
Comment 2c: The Division needs assurance that the additional waters, which should
flow along similar pathways as prior surface infiltration, will not appreciably change the
' It is noted [ha[ [he flow data reported in Figure 5 of TR-43contains a discontinuity in early 1996. This was
coincident with, and is considered to be a result of, a change in the location of the flow measurement point. Flow
from [he Carlton Tunnel is computed from calibrated flume measurements, and the resulting change in [he flume
location and calibration appears [o have resulted in a reduction in [he reported average flow rate since [ha[ time. II is
considered that the base flow (i.e. flow ignoring short term peaks) was similar in the years before and the years after
the flow measurement point change, and that the apparent reduction in base flow since 1996 is due to the change in
measurement point and calibration curve, rather than [o any actual reduction in flow.
z Allowing for the approximately 300-gpm-calibration difference between pre-1996 and post-1966 Flow
measurements described in Footnote 1 above. _ _
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