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The community of Lava Hot
Springs, located approximately 30 miles southeast of
Pocatello
, faces two problems with their domestic water supply. The community
requires more water than present sources provide, and naturally
occurring arsenic renders one of the cities wells useless. The
“Technical Assistance” team helped the community to find additional
clean cold water. We felt that, because geothermal water mixes with
ground water, understanding the thermal system and defining its
distribution and flow paths would enhance our ability to locate
uninfluenced cold water. We geologically mapped the surrounding area and
delineated structures that form the plumbing system of the geothermal
water (Figure 1). The distribution of rocks altered by thermal water led
us to two conclusions: the hydrothermal system was once much larger, and
it occurs along west-northwest-trending faults at cross fault
intersections.
The elongation of breccia
bodies formed by the volatile passage of hydrothermal fluids (Figure 1)
shows a WNW structural control to extinct parts of the geothermal
system. Aligned thermal springs and wells define the same trend in the
active system. The hydrothermal water issues from a WNW fault near its
intersection with a more northerly structure (Figure 1).
An extensive travertine
terrace, deposited from paleo hot springs, extends at least 2 kilometers up the
PortneufRiver valley from town. The terrace in combination with the wide distribution
of the breccia bodies show that the hydrothermal system was much larger
in prehistoric time. A Pleistocene-age lava flow, dated at approximately
530, 000 years, erupted from a fissure vent at Lava Hot Springs that
lies along the structure that controls the thermal water. Perhaps the
magma chamber that fed the lava flow also provides thermal energy for
the hot springs
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