| REPORT OF GEOLOGIC/HYDROGEOLOGIC
SERVICES |
| CITY OF FERDINAND |
| LEWIS COUNTY, IDAHO |
|
INTRODUCTION
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| Ferdinand is located on 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 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. Ferdinand supplies approximately 145 people from two
wells (FD-2 and FD-3) 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 Ferdinand 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. 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 Ferdinand is defined
by its location near the margin of the CRBG and the older
granitic basement upland areas that bound the town on the south,
southeast, and southwest. Based on limited data from water well
it would appear that the granitic basement rocks pass beneath
Ferdinand sloping to the north and northeast and are overlain by
approximately 220-260 feet of basalt. Interbedded clay sediments
of the Latah Formation are reported between basalt flows in some
driller’s logs. The Ferdinand area also is capped with a
surficial silt and clay unit that is most likely derived from
surrounding basement highlands and very thin deposits of wind
blown loess. The silt and clay unit varies in thickness that
ranges from approximately 5 to 45 feet in the Ferdinand
vicinity.
|
|
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.
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|
No water usage records or water level data are available for
Ferdinand’s water supply wells. The hydrogeological conceptual
model would suggest Ferdinand’s water supply wells would be
responsive to area precipitation events due to 1) proximity to
the recharge areas (granitic upland area), 2) presence of
interbeds that may originate from and interconnect the upland
recharge area and the basalt aquifers and 3) shallow well
completion. If no recharge events occurred, such as a drought,
then Ferdinand’s water supply wells would also respond quickly
with a decline in water levels. The conceptual model would
suggest that the water levels in Ferdinand’s water supply
wells would most likely fluctuate seasonally or under drought
conditions but would most likely not show long-term water level
decline.
|
|
Analytical nitrate-nitrogen data for water samples withdrawn
from Wells FD-1, and FD-2 are available for some years between
1972 and 1999. The range of nitrate concentrations as indicated
from available analytical reports range from 5.6 to 53.5 mg/l.
The indicated range of values demonstrate concentrations that
exceed the maximum contaminant level (MCL) of 10 milligrams per
liter (mg/l) or concentrations that are elevated relative to
background levels of 2 mg/l indicating potential anthropogenic
influences.
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CONCLUSION
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| It would appear at this time
that the available information is insufficient to determine any
long- term water level trends in Ferdinand’s water supply
wells. The evaluation of nitrate concentrations in the available
analytical reports for Wells FD-1and FD-2 indicate that
historically there has been elevated concentrations of nitrates
above 2 mg/l and concentrations above the MCL of 10 mg/l,
indicating potential anthropogenic influences. |
|
Ground water is the sole water-supply source for Ferdinand. We
suggest that Ferdinand monitor water level changes associated
with present uses and reduce water use if large- scale, water
level decline is identified. Ferdinand should continue to
monitor nitrate concentrations in the municipal water supply
wells. Ferdinand 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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