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INTRODUCTION
This report summarizes the results
of the geologic/hydrogeologic services supporting the development of new
water resources for the Little Black Tail Ranch Development (LBTR). The
services are provided by the Idaho Water Resources Research Institute’s
(IWRRI) Technical Assistance for Rural Ground Water Development Within
Idaho program.
LBTR is located approximately 15
miles southeast of Sandpoint Idaho in the northern panhandle portion of
Idaho. LBTR supplies about 70 people from two wells and a spring source.
According to LBTR personnel the two wells do not produce significant
quantities of water and the development relies predominantly on the
spring collection system. It is our understanding that the spring
collection system produces minimal quantities of water during late
summer and greater production capabilities are needed to meet seasonal
demands. LBTR has requested assistance in evaluating potential water
well sites in the area to replace or supplement the spring collection
and water well systems and provide greater water system capacity.
LBTR Water System
The LBTR water system supplies their
service area with two water wells and a spring collection system. The
spring collection system consists of three pump stations and associated
collection devices. Currently only pump station #3 is utilized.
Estimates of spring discharge as measured at the three pump stations
range from 50 to 100 gallons per minute (gpm).
The two water wells (LBTR#6 and LBTR#15)
were completed in May of 1999. Both wells have been reported to have
flow rates at the time of drilling of between approximately 30 to 35
gpm. LBTR personnel report that the production rates of both wells have
significantly decreased with use and can no longer supply the LBTR
development with the necessary quantities of water.
GEOLOGIC SETTING
LBTR is located on the west shore
of Lake Pend Oreille in the northern panhandle area of Idaho. The LBTR
area is largely composed of three geologic units: 1) bedrock, 2) glacial
deposits, and 3) recent unconsolidated sediments. The predominant rock
type in the LBTR area is most likely derived by deposition of sediment
in a narrow roughly north-south oriented ocean basin (in an area known
as the Belt Basin). 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 sediments consisting
predominantly of silt, sand, and gravel. Recent unconsolidated sediments
are derived from erosion of the upland area surrounding LBTR. The
sediments are then transported downslope by gravity or within area
streams.
HYDROGEOLOGIC SETTING
The LBTR area consists predominantly
of metamorphic and intrusive igneous bedrock covered largely with
glacial deposits. Review of available information for the LBTR area
indicated the presence of two potential sources of ground water: 1)
bedrock and 2) glacial/alluvial deposits. The weak stratigraphy and poor
sorting within the glacial deposits most likely controls ground water
flow. The igneous and metamorphic basement rocks within the LBTR area
can be used as a limited water resource. Wells drilled into the basement
rocks produce small quantities of water that are suitable for domestic
purposes.
Bedrock is used as the primary
ground water source for wells in the vicinity of LBTR. The aquifer is
composed of fractures and/or faults in the bedrock. Wells completed in
the bedrock typically obtain water from a number of fractures that will
eventually contribute the desired quantity of water. Water well reports
in the area indicate that wells completed upto 160 feet bgs result in
yields ranging from 0 to 40 gallons per minute (gpm) averaging
approximately nine gpm. There is some evidence that pumping at rates
necessary for municipal supplies may result in a significant decrease in
well yields over a short period of time.
The glacial/alluvial deposits in the
LBTR vicinity represents a complex assemblage of sediments. The
glacial/alluvial aquifers appear to be used as a secondary water well
source and also produce springs that are used as a surface water source.
There appears to be two aquifers, an upper and lower separated by the
silt/clay unit. It appears based on the available water well reports
that the silt/clay unit is continuous within the Krieger Creek Drainage.
The water elevation in the upper
aquifer coincides very closely with the approximate water elevation of
the springs observed near Pump Station #3. A reconnaissance of the
spring collection system completed on April 20, 2003 showed significant
quantities of water seeping from the south side of Krieger Creek near
pump stations #2 and #3. At the time of the reconnaissance the combined
overflow at pump stations #2 and #3 were estimated to be in excess of
approximately 100 gpm. LBTR personnel indicated that the overflow ceases
flowing during the summer months.
Aquifer Test and Analysis
A constant rate aquifer test was
completed in Well #32 on April 22, 2003 using the existing submersible
pump. The test was conducted to determine approximate aquifer
properties. The well was pumped for about three minutes with discharge
measured at a "frost free" spigot located at the wellhead. A
maximum of 10.10 feet of drawdown was measured in the well with an
average pump rate of 16 gpm. The subsequent transmissivity was
calculated using the Cooper-Jacob Method. Based on the results of the
limited aquifer test the calculated transmissivity is approximately 69
ft2/day. The aquifer test of well #32 is very
limited in duration and pump rate and the transmissivity value and any
subsequent interpretation should be used cautiously.
CONCLUSION
The potential locations for water well siting
that may acquire yields greater than 60 gpm in the LBTR area are very
limited. Based on existing information, the bedrock aquifers in the LBTR
area do not appear adequate for municipal supply and are not considered
as a potential target aquifer. There are two potential drilling sites
located near LBTR with significant limitations for use as a municipal
water well source. We would recommend the following that: 1) LBTR should
continue to use the spring as their primary water supply, 2) the
development of a 60 gpm well with the potential for long-term
productivity in close proximity to the LBTR service area is unlikely.
There is a possibility of developing a well near well #32 that may be
used to supplement the spring collection system, 3) LBTR may also want
to investigate the cost and benefit of further developing the current
spring collection system as the primary municipal supply as opposed to
completing a water well.
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