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The communities of Bloomington and Paris lie
along the west side of the Bear Lake valley in southeastern
Idaho Figure 1). Both communities derive domestic water from
springs that issue from sedimentary rocks to the west, in the
Bear River mountain range. The IWRRI Community Water Project
team completed a study in year 2002 that provides the
communities with hydrogeologic assistance to address a number of
concerns regarding their water supplies. The study achieves the
following goals:
- Evaluate the hydrogeology of the primary water supply
springs
- Delineate recharge areas for the springs
- Locate potential alternate groundwater sources
- Delineate the recharge zones for the alternate water
supplies
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The dominant rock types in the Bear River range include limestone and
sandstone, deposited in an ancient, Paleozoic sea (Figure 2). Later,
during Mesozoic time, regional compression folded and thrust faulted the
rocks. The deformed strata were exposed and deeply eroded at land
surface during Tertiary time by regional uplift. A thick pile of
fine-grained sediments then accumulated above the old erosional surface.
These sediments, called the Salt Lake formation, consist, in part, of
material eroded from the older strata. Most recently, formation of the
Bear Lake valley resulted from renewed faulting. Many of these faults
show recent movement. The conceptual model describing ground-water flow
involves precipitation entering the hydrologic system primarily in the
mountains, melting in the spring, flowing through the limestone, and
discharging through springs in the Bloomington formation just above a
shale unit (Figure 2). The shale acts as a barrier to ground water flow
and forces most of the ground water out of the limestone aquifers
through springs. Some water seeps through the shale into the Brigham
formation that lies above a thrust fault. The fault acts as a barrier to
ground water flow, forcing ground water to discharge from springs in the
Brigham formation. Most of the high yielding wells in the valley
completed in gravels within the Salt Lake formation, suggest this rock
unit forms a viable target as well.
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Based on these interpretations potential target
rock units include: 1) the Salt Lake formation 2) the
Bloomington formation and 3) the Brigham formation (Figure 2).
The gravelly portion of the Salt Lake formation hosts numerous
high-yielding wells, suggesting a priority target that occurs
close to both communities. This aquifer may be isolated,
however, from potential recharge areas, and wells developed in
the gravel will require an engineered screen. The Bloomington
formation contains several high-volume springs, and connects
directly to a recharge area. Fracture and cave distribution
controls water flow in the Bloomington formation; therefore, a
well must intersect these features to yield significant water.
The Brigham formation contains water, however, the Lead Bell
Shale apparently limits recharge. Geochemical analyses of water
issuing from springs in the Brigham indicate that the aquifer
does not connect to a regional ground-water flow system. Also
springs and wells in the Brigham formation typically yield less
than 40 gpm.
The following list prioritizes exploration targets for
additional ground water sources. It is based on the relative
merit of each target from an ease-of-discovery versus a
risk-of-failure point of view.
- Salt Lake formation
- Brigham formation
- Bloomington formation
We recommend drilling a test well prior to drilling a production
well to determine the suitability of the target aquifer. The
city should initiate a routine water level monitoring plan once
the new well is on line, to plan water usage and development. |
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