Effects of well discharges on hydraulic heads in and spring discharges from the geothermal aquifer system in the Bruneau area, Owyhee County, southwestern Idaho

Demand for ground water in the 600-
square-mile Bruneau study area has increased
since 1954 because of agricultural development. Declining flow at Indian Bathtub Spring
is adversely affecting a unique species of snail
that inhabits the spring.
The Bruneau study area is underlain by
sedimentary and volcanic rocks that form a
regional geothermal aquifer. Sedimentary
rocks range in thickness from zero in the
southern part of the study area to more than
3,000 feet in the northeastern corner. Volcanic
rocks underlie the entire study area and extend
southward to the Jarbidge Mountains. In the
central part of the study area, the volcanic
rocks are probably 2,000 to 3,000 feet thick.
For purposes of study, the regional geothermal
aquifer system was divided into sedimentary-
and volcanic-rock aquifers.
Ground water flows northward through
the volcanic-rock aquifer to the sedimentary-
rock aquifer, from areas of recharge along the
Jarbidge and Owyhee Mountains into the study
area, where it is discharged as spring flow or
leaves the study area as underflow. Prior to
extensive ground-water development, about
10,100 acre-feet was discharged by springs.
Ground-water discharge from wells
began in the late 1890's. From the 1890's
through 1951, annual discharge was less than
10,000 acre-feet. From 1952 to 1978, annual
discharge increased to about 40,600 acre-feet.
During 1978-91, well discharge declined from
the maximum of 49,900 acre-feet in 1981 to
34,700 acre-feet in 1991. Through 1991,
nearly 1,400,000 acre-feet of ground water
discharged from wells; about 546,000 acre-feet
discharged from 1978 through 1991. Most
pumped water is from the volcanic-rock
aquifer.
Ground-water development since the
mid-1890's locally has modified the direction
of water movement in both the sedimentary-
and volcanic-rock aquifers. In 1989, ground
water moved toward four cones of depression
created by pumping two in the northern part
of the study area are in the sedimentary-rock
aquifer, two in the southern part are in the volcanic-rock aquifer. Pumping has caused
hydraulic heads in the volcanic-rock aquifer to
decline more than 30 feet in much of the area
and at least 70 feet in one well. About 1 mile
from Indian Bathtub Spring, the water level
in one well declined about 10 feet during
1979-92, or about 0.7 feet per year.
Within the past 25 years, discharge from
monitored springs along Hot Creek and the
Bruneau River has declined, most notably
from Indian Bathtub Spring. Discharge from
Indian Bathtub Spring in 1964 was about
2,400 gallons per minute, and by the summer
of 1989, discharge was zero. Discharge began
to decline in the mid-1960's when the rate of
increase in pumpage accelerated. In contrast,
discharge from Pence Hot Spring has ranged
from about 700 gallons per minute to about
1,100 gallons per minute.
Changes in discharge from monitored
springs corresponded with changes in hydraulic head, which fluctuates seasonally, and are
substantially less in late summer than in the
spring. A hydraulic head/spring discharge
relation was developed for two sites at Indian
Bathtub Spring and a nearby test hole. The
relation for Indian Bathtub Spring indicated
that a spring discharge of 2,400 gallons per
minute would relate to a hydraulic head of
about 2,708 feet at the spring, which is about
34 feet higher than the head at zero spring
discharge.