Solute geochemistry of the Snake River plain regional aquifer system, Idaho and eastern Oregon

Four geochemical approaches were used to determine chemical
reactions controlling solute concentrations in the Snake River Plain
regional aquifer system: (1) calculation of a solute balance within the
aquifer, (2) identification of weathered products in the aquifer frame-
work, (3) comparison of thermodynamic mineral saturation indices with
plausible solute reactions, and (4) comparison of stable-isotope ratios
of the solutes with those in the aquifer framework. Solutes in the geo-
thermal groundwater system underlying the main aquifer were examined
by calculating thermodynamic mineral saturation indices, stable-isotope
ratios, geothermometry, and radiocarbon dating.
Water budgets, hydrologic arguments, and isotopic analyses for the
eastern Snake River Plain aquifer system demonstrate that most, if not
all, water is of local meteoric and not juvenile or formation origin. Thus,
the solutes must also originate within the basin. Solute balance, isotopic,
mineralogic, and thermodynamic arguments suggest that about 20 per-
cent of the solutes leaving the basin are derived from reactions with
rocks forming the aquifer framework. Most of the remaining solutes
are introduced from tributary drainage basins.
Mass-balance calculations, thermodynamic arguments, and petro-
graphic observations indicate that calcite and silica are precipitated in
the aquifer. Petrographic evidence and thermodynamic arguments sug-
gest that olivine, pyroxene, plagioclase, pyrite, and anhydrite are being
weathered from the aquifer framework. Large amounts of sodium,
chloride, and sulfate, relative to their concentration in the igneous rock,
are being removed from the aquifer. Release of fluids from inclusions
in the igneous rocks and initial flushing of grain boundaries and pores
of detrital marine sediments in interbeds are believed to be a major
source of these solutes. Identification and quantification of reactions
controlling solute concentrations in ground water in the eastern plain
indicate that the aquifer is not a large mixing vessel that simply stores
and transmits water and solutes but is undergoing diagenesis and is
both a source and a sink for solutes.
Evaluation of solute concentrations and stable-isotope ratios of
hydrogen, oxygen, carbon, and sulfur along groundwater flowpaths that
transect irrigated areas suggests that irrigation water may have altered
solute concentrations and isotope ratios in the eastern Snake River Plain
aquifer system. The changes, however, have been small because of the
similarity of solute concentrations and ratios in applied irrigation water
and in native ground water, and because of rapid movement and large
dispersivity of the aquifer.
Reactions controlling solutes in the western Snake River basin are
believed to be similar to those in the eastern basin but, because of dif-
ferent hydrologic conditions, a definitive analysis could not be made.
The regional geothermal system that underlies the Snake River Plain
contains total dissolved solids similar to those in the overlying Snake
River Plain aquifer system but contains higher concentrations of sodium,
bicarbonate, silica, fluoride, sulfate, chloride, arsenic, boron, and lithium,
and lower concentrations of calcium, magnesium, and hydrogen. These
solutes are believed to be derived from reactions similar to those in the
Snake River Plain aquifer system, except that ion exchange and hydrol-
ysis play a role in controlling solute concentrations in the geothermal
system.
Geothermometry calculations of selected ground-water samples from
known geothermal areas throughout the basin suggest that the geother-
mal system is large in areal extent but has relatively low temperatures.
Approximately half of the silica-quartz calculated water temperatures
are greater than 90 °C. Radiocarbon dating of geothermal water in the
Salmon Falls and Bruneau-Grand View areas in the south central part
of the Snake River basin suggests that residence time of the geother-
mal water is about 17,700 years.