| REPORT OF GEOLOGIC/HYDROGEOLOGIC SERVICES |
| CITY OF KOOSKIA |
| IDAHO COUNTY, IDAHO |
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INTRODUCTION
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| The City of Kooskia (Kooskia) is located in the
Clearwater Plateau, in northwestern Idaho County, Idaho. There
is some evidence based on ground water levels measured in
selected municipal water wells within the Clearwater Plateau
area have shown long-term declines. The long-term water level
declines may be indicative of ground water withdrawal rates that
exceed the rate at which water is recharging the aquifer. If
this condition is allowed to proceed then the aquifer becomes a
limited and finite resource that either can be depleted or reach
a state that the cost associated with withdrawal becomes
prohibitive. There is also evidence of elevated nitrate
concentrations in ground water produced from water wells located
in the Clearwater Plateau. Kooskia supplies water to 675 people
with three water supply wells and has requested assistance in
evaluating the water supply wells in relation to long-term use
and elevated nitrate trends. |
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GEOLOGY
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| The geology within the vicinity of Kooskia is
part of a larger geologic setting defined as the Clearwater
Embayment. The Clearwater Embayment consists of basalt units
that formed when flows of molten lava filled a pre-existing
depression surrounded by a mountainous upland composed of
granitic and metamorphic rocks. The basalt flows filled in
low-lying areas over time. Geologic structures, drainage
patterns, stratigraphy, and exposed basement rock divide the
Clearwater Embayment into different basins. The numerous basalt
flows of the CRBG in the area resulted in thicknesses that can
exceed 3,000 feet. |
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The geologic setting for the community of Kooskia is defined by
its location, where the junction of the south and middle forks
of the Clearwater River meet in the northern part of the
Stites-Kooskia basin. The locations of these rivers are
controlled by north-south and east-west oriented geologic
structures (faults and folds) in the Kooskia area. The regional
geology suggests that over a thousand feet of basalt are present
in the Kooskia area. Geologic mapping in the Kooskia area is
planned to help define these faults and folds. These faults and
folds formed during and after emplacement of the basalts.
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HYDROGEOLOGY
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| As discussed earlier the Clearwater Plateau area
is composed of a series of basalt flows that consist of very
fluid basalt that flowed over great distances. The most
significant water-producing units occur at the contact zones
between overlying and underlying basalt flows. The middle
portion generally acts as a barrier to vertical flow and
separates the aquifers above and below. Most basalt flows are
approximately 100 to 200 feet in thickness, thus the aquifers at
the contacts are found approximately every 100 to 200 feet in
depth. |
|
Geologic and geomorphic boundaries define the extent of the
Stites Basin that is approximately 10 miles wide and 15 miles
long. The aquifer system is most likely defined by the lateral
extent of the basalt flows as defined by the basin boundaries
and interconnection with the Clearwater River. The extent of the
aquifer system outside the basin is not presently known.
Recharge to the basin is not well understood at this time and
could be a combination of precipitation and contributions from
the Clearwater River.
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|
Water usage records and water level data for Kooskia’s Wells
KW-1, KW-2, and KW-4 are available for the period from 1993 to
1994. It would appear from the limited data available that the
quantity of water pumped is dependent on usage rather than the
capacity of the well. Based on the available data Kooskia does
not appear to have a water supply problem at this time.
|
|
Analytical nitrate data for water samples obtained from Wells
KW-1 through KW-4 are available for some years between 1991 and
2002. The analytical data indicate that all the historic nitrate
concentrations are below the MCL of 10 mg/l. Two samples
obtained from KW-3 between 1991 and 1994 show concentrations
elevated relative to background levels of 2 mg/l indicating
potential anthropogenic (man induced) influences. Water samples
obtained from KW-4 showed increasing nitrate concentrations
until the well was remediated and the surface seal deepened. The
nitrate concentrations fell to negligible concentrations after
remediation of the well. The elevated nitrate concentrations are
likely caused by improper well construction.
|
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CONCLUSIONS
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| Ground water is the sole water-supply source for
Kooskia. We suggest that Kooskia monitor water level changes
associated with present uses and reduce water use if large-
scale, water level decline is identified. The water level and
pumpage data should be compiled and reviewed at least once per
year. |
|
The evaluation of nitrate concentrations in the available
analytical reports for Wells KW-1 through KW-4 indicate that
there does not appear to be any concentrations above the MCL of
10 mg/l, and there currently are no analytic results with
elevated concentrations above 2 mg/l indicating potential
anthropogenic influences. Kooskia should continue to monitor
nitrate concentrations in the municipal water supply wells.
Kooskia should develop a source water assessment/wellhead
protection plan and implement strategies to protect surface
areas and control activities with the contribution zones for
each water supply well.
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