The City of Challis, located
in the Salmon River valley (Figure 1), derives domestic water
from a combination of surface and ground water sources. The
supply consists of water diverted from Garden Creek and from
three wells. Two wells, located west of the city, penetrate
volcanic rocks. The third, on the east side, obtains water from
sand and gravel deposits. This report describes the ground water
systems feeding the existing wells and analyzes targets for
future development. Specific objectives of this study include:
- Determine which ground water systems
supply water to the three wells.
- Establish the hydrogeology of each
ground water system.
- Ascertain other ground water systems
available for development and their reliability.
- Determine alternative ways to use
water from Garden Creek without dealing with regulations
required of a surface water system.
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Unconsolidated sediments that underlie the Garden
Creek and Salmon River valleys include gravel, sand, clay and silt.
These unconsolidated sediments fill depressions in older bedrock
formations. The older formations consist of layers of volcanic rocks
that overlie older sedimentary rocks. This study focuses on the
unconsolidated sediments and the volcanic strata because of their
importance to ground water flow.
Unconsolidated sediments in the study area
include sand and gravel layers deposited by flowing streams and clay
beds deposited in lakes and ponds. Sand and gravel from contemporary and
ancestral stream channels fill much of the Salmon River Valley. Challis
East Well #1 obtains water from this aquifer, which is connected
hydraulically to the Salmon River (Figure 2).
The oldest volcanic strata, basaltic lava flows,
emanated from nearby volcanoes more than 50 million years ago and
covered the existing landscape (Figure 3). Ground water occurs and moves
in fracture zones along contacts between flows or where faults have
disrupted the continuity of these rocks. An air-fall rhyolite ash
overlies the basaltic lavas. (The ash that covered the northwestern US
after Mt St Helens erupted was an air-fall ash). This unit probably
inhibits ground water movement because of limited fracturing and
abundant clay. A welded, rhyolite ash-flow tuff, the youngest and
uppermost volcanic rock in the Challis area, lies above the air-fall
tuff. (A welded ash flow tuff is formed when the ash is hot enough to
weld together after settling out of the air.) The welded ash-flow tuff
lies above the water table so does not produce ground water.
A rectilinear framework of faults cuts the
volcanic strata near Challis and controls ground water flow through
these strata. Faults offset and compartmentalize aquifers because ground
up rock material called fault gouge inhibits flow across the structures.
Ground water flows from mountainous recharge
areas, where precipitation is higher, via hydraulically conductive zones
(aquifers) to discharge areas such as springs or seeps. Three ground
water flow systems provide water to Challis. The first of these includes
aquifers within fractured volcanic rocks. The two Challis West Wells
penetrate these rocks. The second ground water flow system occurs in a
sand and gravel aquifer of the Salmon River Valley. The Challis East
Well penetrates this system. The third ground water flow system lies
within shallow sand and gravel zones underlying Garden Creek. None of
the city wells obtains water from these gravels; however, this shallow
flow system if developed by a well may adequately replace water
presently diverted from Garden Creek while better fulfilling regulatory
requirements. West Well #1 fails to produce adequate water. Plugging the
lower portion of the well and perforating the casing adjacent to the
bottom 30 feet of alluvium will tap water flowing through these shallow
gravels.
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An attempt to measure water
level in West Well #2 suggested a potentially serious problem
with water level decline. However, pumping rate measurements do
not show a steady drop, which is the norm during water level
decline. Collection of accurate static water level measurements
in West Well #2 will resolve this question. This should be a
high priority for the City of Challis. The Salmon River supplies
the aquifer tapped by East Well #1. This hydrologic system
contains more storage and has a more robust recharge capacity
than all others in the area. Although East Well #1 taps this
robust system, we recommend continuing to monitor water levels
to make sure demand does not exceed recharge.
- A shallow well that taps Garden Creek
gravel could probably replace the existing surface water
system. This resource would likely fall under fewer
regulatory restrictions than the current surface diversion
system.
- Drilling a new well along the east
edge of the city should tap the Salmon River gravels. East
Well #1 penetrates about 250 ft of unproductive gravels
before encountering the more productive Salmon River
gravels. We recommend investigating IDWR requirements
regarding water rights along the Salmon River before
exploiting new targets in the Salmon River Valley.
- Targeting a new compartment within the
volcanic strata will limit direct competition between West
Well #2 and the new well. Locating the new well at least 500
feet east of West Well #1 should place it in a different
compartment. This new aquifer may or may not receive
recharge, so monitoring the new well is a priority.
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