PERMFILE62071

• Permit No: C1982056
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: Exhibit 35 Backfill Hydrology Evaluation
• Media Type: D
• Archive: No

discharges was not available at the time so that it was difficult •
to calculate the average total discharge over a year for compari-
son with the average annual recharge. More recent data from the
1986 snowmelt period (given in the final report) indicates that
the spoil discharges respond very rapidly to snowmelt recharge.
The Pond F Spring discharges exceeded 1000gpm for a short time
during this period. The discharge rate at the spring drops
rapidly to about 30gpm during the latter part of the year. On
the basis of the more complete discharge data it may be estimated
that the average discharge rate for the Pond F Spring is probably
in the order of 100gpm. The discharge to the Area 2 pit is
believed to be very similar to that of the Pond F Spring on the
basis of data collected in June and July of 1985. Consequently,
the maximum discharge rate and the average annual discharge rate
for the eastern backfill area is now projected to be about
2000gpm and 200gpm respectively. These new estimates are
included in the final report and are higher than the estimates in
the original report of 200 and 120gpm.
A single discharge value of 1057gpm had been reported for
the Pond F Spring during 1985, but there was no documentation as
to how that rate had been measured. As this value initially
appeared to be anomalous it was not considered in the original
report. Data from 1986 now corroborate the 1985 high value so
that maximum spoils discharges and average annual discharge •
estimates have been increased accordingly.
The revised average discharge estimate of 200gpm is very
similar to the calculated annual recharge rate to the eastern
backfill area. This would indicate very little infiltration into
the lower bedrock from the spoils. This is not surprising
considering that the undisturbed bedrock permeabilities are one
or two orders of magnitude lower than the backfill material.
Some infiltration of spoil waters into the downdip bedrock does
appear to occur based on electrical conductivity measurements of
groundwater inflows into the Foidel Creek underground mine. These
data indicate mixing of Wadge overburden groundwater with typical
backfill groundwater.
A more exact water balance may be possible when a longer
data record is available and more comprehensive monitoring of
discharges is performed. At this time, it appears that less than
5 percent of the total recharge to the backfill infiltrates into
the downdip bedrock. This is based on relative permeability of
the bedrock and spoils, and the existing water balance data. For
the eastern backfill area this would be equivalent to less than
lOgpm over the 800,000 square foot area of potential recharge at
the spoil/bedrock interface.
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