| REPORT OF GEOLOGIC/HYDROGEOLOGIC
SERVICES |
| CITY OF HOPE |
| BONNER COUNTY, IDAHO |
|
INTRODUCTION
|
| This report summarizes the
results of the geologic/hydrogeologic services supporting the
development of new water resources for the city of Hope, Idaho.
The services are provided by the Idaho Water Resources Research
Institute's (IWRRI) Technical Assistance for Rural Ground Water
Development Within Idaho program. |
|
Hope is located on the north shore of
Lake Pend Oreille on State Route 200 in the northern panhandle
portion of Idaho. Hope supplies about 80 people from a spring
collection system located approximately 3/4-mile northwest of
the city. According to Hope personnel the spring runs year round
discharging approximately 200 gallons per minute (gpm). Hope has
requested assistance in evaluating potential water well sites in
the area to replace or supplement the spring collection system
and provide greater water system capacity. The water well may
preclude or minimize the need for treatment and reduce overall
operating costs. |
|
GEOLOGIC SETTING
|
| Hope is located on the north
shore of Lake Pend Oreille on State Route 200 in the northern
panhandle of Idaho. The Hope area is largely composed of three
geologic units: 1) bedrock, 2) glacial deposits and 3) recent
unconsolidated sediments. The predominant bedrock in the Hope
area is the Belt Supergroup, composed of metamorphosed
sedimentary rocks. Structural deformation of the Belt Supergroup
over time has resulted in moderately dipping and highly faulted
and fractured rocks that have been intruded by various types of
igneous rocks. |
|
Glaciation further modified this area
apparently during at least two ice ages approximately 10,000 to
15,000 years ago. The glaciation deposited a relatively thin
veneer of sediments consisting predominantly of silt, sand, and
gravel.
|
|
The glaciers created ice dams in the area
east of Hope when they blocked the paleo-Clark Fork river
channel. Structural failure of the ice dam resulted in the
release of large quantities of water creating significant flood
events. These floods eroded both the sediment and rock and
created the topography seen today.
|
|
Recent unconsolidated sediments are
derived from erosion of the upland area surrounding Hope. The
sediments are then transported and deposited downslope by
gravity or within area streams. Accumulations of the erosional
material appear immediately south of Hope along the north shore
of Lake Pend Oreille and at the mouths of Strong and Trestle
Creeks.
|
|
HYDROGEOLOGY
|
| Review of available
information for the Hope area indicated the presence of three
potential sources of ground water: 1) bedrock, 2) near-shore
colluvial/long-shore drift deposits, 3) alluvial fan deposits.
Bedrock is used as a domestic ground water source in the
vicinity of Hope. Water well reports in the area indicate that
wells completed to considerable depth (greater than 600 feet)
result in yields generally less than 15 gpm. Wells completed in
this type of bedrock throughout northern Idaho typically have
low yields. |
|
The near-shore deposits immediately south
of Hope consist of clay, silt, sand, and gravel and appear to be
derived from both long-shore drift deposits and colluvium. The
ground water produced from a well completed in the near-shore
deposits immediately south of Hope will most likely be derived
predominantly from Lake Pend Oreille.
|
|
Significant thicknesses of coarse fluvial
deposits can accumulate in the alluvial fan deposit forming a
significant aquifer. The ground water produced from a well
completed in the alluvial fan aquifer may be derived from
precipitation collected within the drainage and/or water flowing
from Pend Oreille Lake. The closer the well is drilled to the
lake, the more likely that larger contributions of water will
originate from the lake. The few shallow wells completed in the
alluvial fan deposits indicate relatively low specific
capacities (0.2 and 2.8 gpm/ft respectively).
|
|
CONCLUSIONS
|
| The potential locations for
water well siting that may acquire yields greater than 200 gpm
in the Hope area are very limited. The best location for a new
water well is in the alluvial fan deposits. The relatively low
specific capacity reported from the few shallow wells completed
in the alluvial fan indicate it is unlikely that a new well with
similar completion characteristics would obtain yields of 200
gpm. Wells completed in different areas and/or different depths
may produce increased yields. Given the very limited subsurface
data regarding the alluvial deposits, it may be necessary to
conduct an exploratory drilling program to determine the best
locations and depths for completing a well with the greatest
yields. |
