2010-12-17_REVISION - C1981019 ‎(140)‎

• Permit No: C1981019
• IBM Index Class Name: REVISION
• Doc Date: 12/17/2010
• Doc Name: Exhibit 7 Item 22 Collom Pit Regional Hydrogeologic Model
• Type & Sequence: PR3
• Email Name: JRS
• Media Type: D
• Archive: No

Hydrological system and available data DRAFT 6
Most bedrock recharge is expected to occur in the higher elevations of the modeled
area to the south, and along the upper reaches of the streams that flow on an
intermittent basis. Potential recharge mechanisms for the bedrock units include:
Direct infiltration of rain and snowmelt into bedrock - this is mainly
expected to occur at elevations above 7,500 ft and in outcrop areas;
Seepage from ephemeral and intermittent streams that flow over bedrock
- in the upper reaches of streams where valley fill deposits are very thin
or absent; and
Seepage of valley fill groundwater to bedrock units - this may occur in
the upper reaches of the streams, in areas where valley fill deposits are
saturated and have higher potentiometric head than the underlying
bedrock units.
Surface water infiltrates into the valley fill in the upper reaches of the streams. Some of
the valley fill groundwater may recharge the bedrock units while the balance flows in the
lower valley fill. Groundwater stored in the valley fill is a source of stream base flow
after the spring runoff, as well as the source of many of the springs. Potential recharge
mechanisms to valley fill material include:
• Seepage from ephemeral and intermittent streams into valley fill - this is
expected in the upper to middle reaches of the major streams;
• Direct infiltration of rain and snowmelt into valley fill - because the areas
of valley fill are relatively small, this is a minor component;
• Discharges from bedrock groundwater into valley fill - this is expected in
the middle to lower reaches of the major streams; and
• Valley fill groundwater inflow - this is a potential source for streams (such
as Wilson Creek) that originate upstream of the model boundary.
2.2.4 Discharge
Bedrock groundwater in the Collom area is expected to discharge to the valley fill
deposits along the stream drainages. The steeply dipping northern limb of the Collom
Syncline on the northern side of the Collom area forms a boundary to groundwater flow.
As a result, most bedrock groundwater will discharge to the valley fill deposits along the
stream channels and there will be minimal bedrock groundwater flow to the north out of
the Collom area.
Valley fill groundwater discharges into the major stream drainages as base flow. Where
the groundwater level in the valley deposits intersects the base of the stream channels,
it contributes to stream flow or is lost to evapotranspiration. The majority of
evapotranspiration occurs where the water table in the valley fill is at or near the land
surface.
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