| REPORT OF GEOLOGIC/HYDROGEOLOGIC
SERVICES |
| CITY OF CRAIGMONT |
| LEWIS COUNTY, IDAHO |
|
INTRODUCTION
|
| Craigmont is located on the
Clearwater Plateau in northwestern Lewis 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 can either 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. Craigmont supplies about 550 people from two wells and
has requested assistance in evaluating the water supply wells in
relation to long-term use and elevated nitrate trends. |
|
GEOLOGY
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| The geology within the
vicinity of Craigmont 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 low-lying
areas were filled in over time by a successive series of basalt
flows. The Clearwater Embayment is divided into different basins
based on evidence from geologic structures, drainage patterns,
stratigraphy, and exposed basement rock. The numerous basalt
flows of the CRBG in the area resulted in thicknesses that can
exceed 3,000 feet. |
|
The geologic setting for the community of Craigmont is defined
predominantly by its location near the margin of the CRBG and
the older basement upland areas that bound the town on the west,
southwest, and south. The regional geology and cross-sections
suggest that over 900 feet of basalt are present in the
Craigmont area. Some driller’s logs report thin (<15 feet)
interbedded clay sediments of the Latah Formation between basalt
flows. The Craigmont area also is capped with a surficial clay
unit that is most likely derived from surrounding basement
highlands and very thin deposits of wind blown loess. The clay
unit varies in thickness that ranges from approximately 2 to 120
feet in the Craigmont vicinity. Geologic mapping delineated
three monoclines, a syncline, and an anticline in the Craigmont
area. These folds formed during and after emplacement of the
basalts.
|
|
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. Basalt flow systems are generally recharged
from precipitation at higher elevations. At upper elevations,
the basalt aquifers can thin or by following the slope of the
underlying basement dip upward. The water flows through the
aquifers bounded above and below by the middle zone of the
basalt flow generally discharging in the form of springs at
lower elevations. The extent of the regional flow system is
defined by geologic and geomorphic boundaries. |
|
Groundwater recharge to the Clearwater area is predominantly
through precipitation that averages approximately 22-inches per
year, most of which is lost to evapotranspiration and overland
flow. The wells completed in the deeper aquifers would receive
relatively little recharge because most is intercepted by the
shallow aquifers and wells. It would be expected that water
pumped from wells completed in the deeper aquifers would be
replaced predominantly with water in storage because the
withdrawal rate would exceed the quantity and rate of recharge
water migrating from above. Ground water withdrawn under these
conditions contributes to declining water levels that may
eventually require deepening of area wells to remain completed
in saturated aquifers.
|
|
Water usage records and water level data for Craigmont’s well
CM-3 are available for the period of 1977 to 1988 with one water
level measured in 1958 at the time of completion. The original
static water level at the time of completion in Well CM-3 as
reported in the 1958 was 536 feet bgs. The static water level in
1977 was 550 feet bgs and by 1988 was 564 feet bgs. Although no
pumpage data is available before 1977 the period between 1977
and 1988 showed no significant increase or decrease in pumping
rates in CM-3. It would appear from the limited water level and
pumpage data available that there is a general decline in water
levels with time that is not due to increased pumping rates. The
decrease in water levels in Well CM-3 with time would most
likely be indicative of pumping rates that exceed recharge rates
to the aquifer.
|
|
Analytical nitrate-nitrogen data for water samples withdrawn
from Wells CM-1, CM-2, and CM-3 are available for some years
between 1962 and 2001. With the exception of the 1974 sample
obtained from CM- 2 the analytical data indicate that all the
historic nitrate concentrations are below the MCL of 10 mg/l but
the concentrations are elevated relative to background levels of
2 mg/l indicating potential anthropogenic (man induced)
influences. The elevated nitrate concentrations are most likely
caused by water wells with improper well construction
|
|
CONCLUSIONS
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| It would appear based on
limited pumpage and water level information that Craigmont’s
well CM-3 is experiencing long-term water level decline. The
static water level has declined by approximately 28 feet over a
30-year period and would be indicative of a pumpage rate that
exceeds the recharge rate to the aquifer. We would recommend
that Craigmont monitor and record water level and pumpage
information in Wells CM-1, CM-2, and CM-3. 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 CM-1, CM-2, and CM-3 indicate that
with the exception of one sampling event there does not appear
to be any concentrations above the MCL of 10 mg/l but there are
analytic results with elevated concentrations above 2 mg/l
indicating potential anthropogenic influences. Craigmont should
continue to monitor nitrate concentrations in the municipal
water supply wells.
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