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Pinehurst,
a small community of less than 10 families, lies in the deeply incised
canyon of the Little Salmon River, approximately 10 miles south of
Riggins (Figure 1). The community derives domestic water from a spring
located on a hill slope several hundred feet above town. The spring
water flows into a collector box, into a concrete storage tank, and
then down slope to the community. Bacterially contaminated water
enters the system by percolating downward from surface along the
outside of the spring box into the open bottom.
Studies completed by the IWRRI Community Water Project team
culminated in a recommendation to improve the existing spring system.
Figure
1, Location map of the Pinehurst project area.
Geologic
mapping of the project area shows that the spring issues from an
ancient landslide that fills a high-gradient stream channel. The
deeply dissected landslide consists of a chaotic mixture of boulders,
cobbles, sand, silt and clay. Bedrock beneath the slide consists of
granitic rock, which forms a poor aquifer. Annual precipitation soaks
into the slide material and because the underlying granitic bedrock is
relatively impermeable, the water flows down gradient through the
slide material along the bedrock surface. The Pinehurst spring occurs
where water is forced to surface by an elevated sill in the bedrock
(Figure 2).
Figure
2, Cross section through the Pinehurst landslide, looking North.
During
our evaluation, we considered the two other potential sources of water
that the community might use, a well into the granitic bedrock, or a
well into the stream gravels along the bottom of the Little Salmon
River canyon. Tests from
nearby wells that penetrate the granitic bedrock show extremely low
yields. A new well developed in the granite would most likely yield an
inadequate volume of water to supply the community. Gravels that occur
along the floor of the Little Salmon River could potentially provide
an adequate volume of water for the community. However, water in these
gravels routinely acquires contaminants from numerous nearby sources,
including septic tanks and fuel spills along US Highway 95. Pinehurst
could not possibly protect the source area for water in the Little
Salmon River gravel aquifer.
Therefore, the only viable
option for a clean water supply, based on these considerations, is to
properly redevelop the spring that the community presently uses. This
will require removing the present spring box, installing perforated collectors embedded in
gravel, capping the collectors with clay-rich material, and covering
the entire spring area by an impervious barrier (Figure 3). If
properly redeveloped, contaminated surface water will not mix with
water flowing into the collectors.
Figure
3, Plan diagram and cross section showing proposed development style
for the Pinehurst spring.
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