1992-07-22_GENERAL DOCUMENTS - C1981017

• Permit No: C1981017
• IBM Index Class Name: GENERAL DOCUMENTS
• Doc Date: 7/22/1992
• Doc Name: Bid Documents ‎(IMP)‎ Proposed Decision & Findings
• Media Type: D
• Archive: No

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The quality and quantity of the alluvial aquifers of Fourmile Creek, Thompson
Creek and Coal Creek will not be cumulatively impacted during the first 5-year
permit period of the three mines. There is a possibility of quality and
quantity impacts on Coal Creek in the distant future when, and if, the north
Thompson Creek Mines progress into the Coal Creek Drainage.
There is a minor potential for the Coal Basin Mine load-out and the North
Thompson Creek Mine load-out to cumulatively impact the quality and quantity
of ground water in the Roaring Fork alluvium, since both load-outs are located
on this alluvial body. However, both of these load-outs have limited surface
disturbance, small amounts of disturbed drainage to handle, and much of the
disturbed drainage water is going into total containment lined ponds.
Therefore, that minor amount of polluted drainage which may escape from the
drainage control system will negligibly impact the quality of the vast amounts
of Roaring Fork alluvial water.
The greatest cumulative ground water impact will be the indirect depletion of
surface water flow through the ground water system to the mine workings.
Since the impacts are to surface water, they are quantitatively discussed
below in the Surface Water section of this Cumulative Impacts Study. The
mechanism of stream depletion has been discussed in the general description of
ground water.
Cumulative Impacts - Surface Water
One of the potential cumulative impacts that has been effectively mitigated is
unnatural sediment loads generated by the operations. The sediment control
systems in use at all mines and the strict effluent limitations imposed on all
discharges will keep sediment loads near natural levels. It also should be
noted that all three mines considered in this assessment are underground mines
with minimal surface disturbance. Total surface disturbance is only 0.06% of
the 1451 square mile Roaring Fork River Basin. Because of these facts,
cumulative sediment loads are not expected to be significant.
The cumulative effect of impounding water in sediment ponds at all three mines
will also be insignificant. Because of the small relative surface disturbance
at each mine, pond storage volumes are likewise small . Sedimentation ponds at
all mines are dewatered within 72 hours to minimize any water loss to the
hydrologic system.
The total worse case depletion to the Roaring Fork River by mine inflows is
small . If all mine inflows were consumed and not discharged back to the
receiving stream, a total of 456 acre-feet/year would be depleted from the
Roaring Fork. This is only 0.06% of the total flow. Even during the low flow
of 179 cfs for the period of record on Roaring Fork, the total depletion would
be 0.4%. This amount is insignificant to the surface water system. (Refer to
Table 4. )