GENERAL48922

• Permit No: C1980001
• IBM Index Class Name: General Documents
• Doc Date: 10/29/1981
• Doc Name: Proposed Decision and Findings of Compliance
• Permit Index Doc Type: FINDINGS
• Media Type: D
• Archive: No

_ r~,_
This amount of inflow is insignificant relative to the total .sulply of water
available in Trout Creek and the alluvium. The impact will also be temporary i.n
nature, occurring only For the duration of tfre mining of the box cut and adjacent
pits. Following reclamation of the pits and subsequent reestablishment of the normal
hydraulic gradient, the impact will cease.
A review of the permit application has shown that although the amount of water
extracted from the alluvial formation is negligible relative to the available
flow in Trout Creek, it is possible that water levels in the alluvium on the East
side of Trout Creek could be lowered as a result of the establishment of the reverse
gradient. This could conceivably reduce the subirrigation potential of the alluvium
(if water Levels were to drop below the rooting depth of suhirrigated species of
vegetation).
An analysis was done by the Division to attempt to ~luan ti fy drawdowns in the TrouC
Creek alluvium as a result of mining. A ground water model, as developed by Koch,
et al., 1981, was used for this purpose. Specifically, this model was developed
for finite line sinks and non-steady con di tion.s. The maximum drawdown that was
calculated occurs in the West Ridge area, where the alluvium is closest to the box
cut. The anticipated drawdown is 1.6 feet. The maximum drawdown calculated for
other points along the Trout Creek alluvium is 0.5 feet. Generally, this analysis
showed that maximum drawdowns will probably occur where the alluvium is closest
to the West Ridge box cut. Anticipated drawdowns are not as severe in the h7o ffa t,
or Center Ridge areas. These predictions are probably worst case situations because
of the following:
1. The model assumed that the entire pit len9tlr in both the West Ridge and
Moffat areas would be open at the same time and remain ooen fora year. This will
not entirely be the case (see the mine plan section of this document). Therefore,
inflows will probably be less.
2. The model assumed that the total length of the floor of the box cut will
remain lower in elevation than the alluvial water toble. This is not always the
actual situation (see page 7.5-03 of the ~iccmit alq,lication for a further discussion).
This could also lessen inflows.
3. The model assumed that the rock strata situated bet:/c~en the alluvium and the
box cut has a uniform permeability in all directions. This is probably not the
case. Because the strata are dippim7 benvat,h 'trout Creek (sec- Exhibit 2.?-3 of
the permit application), any flow frorn the nliuviwn woui.i have to cross bedding
planes. The composite permeability across the bedding planes would probably be
less than the permeabilities parallel to bedding. Therefore, the expected flow
from the alluvium to the mine mould be less than c/hat was actually calculated using
permeabilities measured parallel to the bedrliny (sec pa ye 2.5-93 of the permit
application).
As is discussed above, the only potentially significant drawdown will occur on the
East side of Trout Creek :/here the alluvium i.s closest to the box cut. The sub-
irrigation potential could be slightLl rerluced in the ircirlated cropland near the
West Ridge area. This reduction is not e:apected to significantly affect hay
production. A stipulation is proposed later in this documcnc to require monitoring
of hay productivity on the Trout Crech. AVE. The irrigated croplands on the western
bank of the Trout Creek AVF would probahly not Le affected, since the potential
lowering of the alluvial water table should he confined to the East side of Trout
Creek only. A stipulation is attached to the ground water section of this document
to require the applicant to install an additional alluvial uonitoring well in the
West Ridge area, the purpose of which is to monitor water level declines in the
alluvium nearest the mining limits.