| REPORT OF GEOLOGIC/HYDROGEOLOGIC
SERVICES |
| CITY OF NEZPERCE |
| LEWIS COUNTY, IDAHO |
|
INTRODUCTION
|
| The City of Nezperce (Nezperce)
is located in the Clearwater Plateau, in central 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. Nezperce supplies
water to approximately 525 people with two water supply wells
(NP-1 and NP-4) and has requested assistance in evaluating the
water supply wells in relation to long-term use and elevated
nitrate trends. |
|
GEOLOGY
|
| The geology within the
vicinity of Nezperce 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. |
|
The geologic setting for the community of Nezperce is defined by
its location near the center of the Nezperce Plateau and older
basement upland areas that bound the town on the south,
southeast, and northeast. The regional geology and
cross-sections suggest that over a thousand feet of basalt are
present in the Nezperce area. Some driller's logs report thin
(<15 feet) interbedded clay sediments of the Latah Formation
between basalt flows. A surficial silt and clay unit caps the
Nezperce area that is derived from surrounding basement
highlands and very thin deposits of wind blown loess. The silt
and clay unit varies in thickness ranging from approximately 5
to 85 feet in the Nezperce vicinity. Geologic mapping delineated
two synclines and an anticline in the Nezperce area. These folds
formed during and after emplacement of the basalts.
|
|
HYDROGEOLOGY
|
| 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 Nezperce's Well NP-4 are available for the period of 1994 to
1997 and 2001 to 2002. 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 Nezperce does not appear to have a water supply
problem at this time.
|
|
Analytical nitrate data for water samples
obtained from Well NP-4 are available for some years between
1969 and 2000. The analytical data for Well NP-4 indicate that
all the historic nitrate concentrations are below the MCL of 10
mg/l; 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
|
| Ground water is the sole
water-supply source for Nezperce. We suggest that Nezperce
monitor water level changes associated with present uses and
reduce water use if large- scale, water level decline is
identified. We believe that recharge to the shallow aquifers is
small and that recharge to the deeper aquifers is even less. It
may be possible to pump water from the aquifer(s) at greater
rates than is supplied by annual recharge. 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. We would
recommend that Nezperce monitor and record water level and
pumpage information in Wells NP-1 and NP-4. 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 NP-1, NP-2 and NP-4 indicate that
there does not appear to be any concentrations above the MCL of
10 mg/l, there are analytic results with elevated concentrations
above 2 mg/l indicating potential anthropogenic influences.
Nezperce should continue to monitor nitrate concentrations in
the municipal water supply wells. Nezperce 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.
|
