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GEOCHEMISTRY OF SPRING WATER FROM THE BLACKFOOT
RESERVOIR REGION, SOUTHEASTERN IDAHO:
APPLICATION TO GEOTHERMAL POTENTIAL
by
kny Uutsinpiller
A thesis submitted to the faculty of The . University of Utah in
partial fulfillment of the requirements
for the degree of
Master of Science
in
Geology
Department of Geology and Geophysics
The Un;versi ty of Utah
December 1979
4952 ·'50'
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I...1 5 1 5'
I-J
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THE UNIVERSITY OF UTAH GRADUATE SCHOOL
FINAL READING APPROVAL
To the Graduate Council of The University of Utah:
I have read the thesis of Amy Hilts; npi lJ er In Its final form
and have found that (I) its format, citations, and bibliographic
style are consistent and acceptable; (2) its illustrative materials
including figures, tables, and charts are in place; and (3) the
final manuscript is satisfactory to the Supervisory Committee and
is ready for submission to the Graduate School.
Parry Member. Supervisory Commillee
Approved for the Major
7 .)
Stanley H./ L Chairman; Dcan
Approved for the Graduate Council
James L. Cl ayton {?Dean of The Graduate Sehool
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The Blackfoot Reservoir region in southeastern Idaho is
recognized as a potential geothermal area because of the
presence of
several young rhyolite domes (50,000 years old), Quaternary
basalt
flows, and warm springs. North- to northwest-trending
high-angle
normal faults of Tertiary to Holocene age appear to be the
dominant
structural control of spring activity in the Blackfoot
Reservoir
region. Surface spring-water temperatures average 14°C except
for a
group of springs west of the Reservoir Mountains which average
33°C.
Chemica1 geothermometers, which indicate temperatures of
last
water-rock equilibrium, applied to fifty water samples give
temperatures less than 75°C except for eight springs along the
Corral
Creek drainage. The springs along Corral Creek have Na-K-Ca
temperatures that average 354°C, which are a direct result of
1arge
potassium concentrations in the water. A correction for
carbon
dioxide applied to the Na-K-Ca geothermometer lowers the
estimated
temperatures of the anomalous springs close to the measured
surface
temperatures. Mixing model calculations suggest that hot water
with a
maximum temperature of approximately 67°C may be mixing with
cooler,
more dilute water in the springs from the Corral Creek
drainage.
Stability relations of low-temperature phases in the systems
K20-A1203-Si02-H20-C02 and Na20-A1203-Si02-H20-C02 indicate that
the
large concentrations of potassium in the eight anomalous springs
are
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t derived from equilibrium reactions with the potassium-bearing
minerals
muscovite and microcline. Other springs in the Blackfoot
Reservoir
region do not appear to obtain their sodium and potassium
contents
from equilibrium reactions with feldspars. Carbon dioxide
and
hydrogen sulfide gasses may be derived through the oxidation
of
organic matter by the reduction of sulfate. Concentrations of
major
and minor elements, and gasses found in springs of the
Blackfoot
Reservoir region are due to water-rock reactions at temperatures
less
than 100°C. Meteoric water circulating along faults may
reach
te~peratures up to 100°C within a few kilometers of the
surface.
Travertine deposited by the springs is co~posed primarily of
calcite
and aragonite, with minor amounts of gypsum, aoatite, and
phosphate.
Based on spring geochemistry. a geothermal reservoir of less
than
100°C may exist at shallow (less than 2 km) depths in th~
Blackfoot
Reservoir region.
v
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t
{ { 'f
> ,,
I i
TABLE OF CONTENTS
Page
AB STRACT
•...••••...•••••••.•.•...•••••.....•••.•.•••.••••.••...••. 1 v
LIST OF FIGURES
•.•.••••••••••.••••..•.•..••••••••••••••••.•..•.••.vi i i
LIST OF TABLES • . • •• • • • • • • • • •• • • . • • • •• • • •
• • • • • • • • . • • • • • • . • • • . • • • •• x
ACKNOHLEDGMENTS
•••••••••••••••••••••.•••••.•.••••.•••••••••.•...•. xi
INTRODUCTION
••••••.••••••••••••.••.•.••••••••••••••••••••••••..••. 1
GEOLOG I C BACKGROUND 2
Physiography ...•.•••.•.••.•••••..•.•.••••••••.•.•......•••... 2
Stratigraphy and Igneous Geology •....••.•••••.••.....•.•...•• 2
Structur,: ..••.••.••••••..••••••..•.•.•••••••••.••.•..•.•..... 8
Se i smi city •.••••••••••••••••••..•.....•.•.•..•...••••.•••.•••
10 Gravity and f1agnet ics .•...•••••..•.•.••.•.••••..•.•••••••....
13 Heat Flow ••.••.•••.•••.••..••.•.••...•••..•••••.••••.•••...•.
14
PREVIOUS SPRING WATER STUDIES
•••••••••••••.••.•••••.••••.••.•.•.•. 22
i4ETHODS
••...•.•.......••••••.•••.••••••.•..••••••••••••..••••..... 24
Collection and Analysis 24 Water
••••••••••.••••••••.•••••••••••••••••.....••....•.• 24 Traverti ne
••••••.•.•....••••.•••••••••.•••••••••••••.••• 28
Thermodynamic Modeling of Ion Species in Solution •••••••...••
29 Geothermo:i1eters ••••••••••••••••••.•••••••••••••••••••••••••••
30
Bas i c assumptions •••••••••••••••..•••••••••••••••••••••. 30
SiOz geothermometer •••••••.•••.••.•.•••••••••••••••••••• 32
~a-K-Ca geothermometer ••••••.•••.••..•.•.••.••••.•••.••. 33
Na-K-Ca-COz geotherrnometer ••••..•.••••.•••.••••••••••.•• 34
Na-K-Ca-Mg geotherrnometer .••......••••..•.•.••••.•.••••• 35
Mixing models ••.••.•••••••.•..••.•.•.••••.....•...•••.•. 36
WATER CiiEiJ11 STRY 38
Chern i ca 1 Cons t itl!ents
..••..•••.••••.•.••..••••••.•.••.•.....• 38 ;'la j 0 r e1ernent s
.•.••••...•.••.................•.•........ 38 Mi nor e1ernent s
.•..•..•.•.......••.••••••...•••••••..•.•. 47
Discussion of Chemical Te:'1peratures 42 5i02 and Na-K-Ca
temperatures ..•..•.•..•.••.•••.•....... 43 rlixing models
•••..•...•.•..••....•....•....••.•.•....... 52
Chemical £q:.dlibria ..•.•...••...................••.••...•...•
55
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TABLE OF CONTENTS (CONTINUED)
Page
GASSES
•••.••.••••••••.••••••••••••••••••.••..••••.••.••••..••••..• 62
Source of CO 2 and H2S ••••••••••••••••••••.••.••.••••••..•••.•
62 Source of Sulfate •.•••.••..•.••••••........•..•.••...........
68
TRAVERTINE
••••••••••••••••••••••••••••••••.•...•••.••.•..•........ 70 -
Description ••.•••••••••••••••.••...........••••.•..••.••.•...
70 Composition ••.••••••••••••.•••••..•..••.••.•••.•••.•.........
72
HEAT FLOW, GEOTHERNS, AND WARM SPRINGS
.••••••••••••••••••••••.••.• 75
SUMMARY AND DISCUSSION
••.•••••••••.••••••••••••••••.••••••.••••.•• 78
CONCLUSIONS
•••••••••••••••••••••••••••••.••••••••••.•••••••••••••• 82
REFERENCES 83
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LIST OF FIGURES
Figure 1. Sketch map of major physiographic features of the
Snake River Plain and adjacent regions, adapted from King (1969}
3
2. Index map of southeastern Idaho, adapted from Mansf i e1d (1
927 ) . . . . . . . • . . . . . . . . . . . . • . . . • . . •• • .
. • • . . . . . .. 4
3. Generalized map of seismicity in southeastern Idaho and
western Wyoming based on a compilation of University of Utah
epicenters, ca. 1975-1977 11
4. Generalized geologic map of the Blackfoot Reservoir region,
southeastern Idaho, modified from Dion (1974) .... 18
4a. Simplified structural map of faults in the Blackfoot
Reservoir region, southeastern Idaho, adapted from Armstrong and
Cressman (1963). Overlay to figure 4 18
4b. Location map of water and travertine samples collected
during Ju1Yt 1978, in the Blackfoot Reservoir region, southeastern
Idaho. Overlay to figure 4 18
5. Spring and well numbering system used by the U.S. Geological
Survey in Idaho, adapted from Mitchell (1975).27
6. Histograms of sodium, potassium, chloride, silica, calcium,
magnesium, and bicarbonate concentrations in spring and well water
from the B1ackfoor Reservoir region, southeastern Idaho 43
7. A portion of the (Ca+Mg)-Na-K triangular diagranl showing
mole percentages of the major cations in spring and well water from
the Blackfoot Reservoir region, southeastern Idaho 45
8. A portion of the HC0 3-C1-S04 triangular diagram showing mole
percentages of the major anions in spring and well water from the
Blackfoot Reservoir region, southeastern Idaho
......•................................ 46
9. Mixing models for springs in the Corral Creek drainage,
Blackfoot Reservoir region, southeastern Idaho 53
http:Idaho......�http:1927).......�............�...�..���..��
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LIST OF FIGURES (CONTINUED)
Figure 10. Stability of phases in the system K20-A1 203-Si0
2H20-C0 2 at 25°C, Ptotal= 1 atm, = 0.26 atm 58PC02
11. Stability of phases in the system Na20-A1203-SiOzH20-C02 at
25°C, Ptotal= 1 atm, PCOz= 0.26 atm 60
12. Stability of phases in the system CaO-MgO-CO z-H20 at 25°C
and 1 atm pressure 61
13. Correlation between concentrations of bicarbonate and
sulfate in spring and well water from the Blackfoot Reservoir
region, southeastern Idaho 65
14. Comparison of water compositions from the Blackfoot
Reservoir region with the calcite equilibriumcomposition defined
by: CaC0 3 + H+.= Ca 2+ + HC0 3 - •.••••• 71
15. Generalized conductive temperature profiles for major
heat-flow provinces in the western United States, modified from
Lachenbruch and Sass (1978) 76
ix
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LIST OF TABLES
Table 1. K-Ar dates for rhyolites in the Blackfoot Reservoir
region, southeastern Idaho (S. Evans, University of Utah, personal
commun q 1979) 7
2. Locations of spring- and well-water samples collected from
the Blackfoot Reservoir region during July, 1978.... 25
3. Concentrations of chemical species in spring and well water
from the Blackfoot Reservoir region, southeastern Idaho 39
4. Chemical geothermometers indicating temperatures of last
water-rock equilibria for spring and well waters in the Blackfoot
Reservoir region, southeastern Idaho .... 49
5. Mixing model results for springs in the Corral Creek
drainage, Blackfoot Reservoir region, southeastern Idaho. 52
6. Reactions used to establish stability relations among mi
nera1s 56
7. Composition of travertine from the Blackfoot Reservoir
region, southeastern Idaho 73
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ACKNOWLEDGMENTS
Dr. William T. Parry supervised the project. Dr. William P.
Nash
provided guidance and reviewed the manuscript. Dr. Walter J.
Arabasz
reviewed the manuscript. M. Cleary assisted in water
sampling.
J. Ballantyne revised WATEQF for use on the UNIVAC 1108. The
University
of Utah Research Institute Earth Science Laboratory permitted
use of
the rcp for water analyses. Mr. Maurice Magee, of Scintrex,
Inc.,
performed the uranium analyses. The project was supported by
U.S.
Geological Survey Research Grant l4-08-0001-G-545.
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INTRODUCTIDrl
Geothermal potential has been recognized in the region south
and
west of the Blackfoot Reservoir in southeastern Idaho because of
the
presence of several very young rhyol ite dornes, associated
Quaternary
basalt flows, warm springs, and anomalously high chemical
temperatures
in several soring waters. Evaluation of the geothermal potential
of
an area logically proceeds from analysis of regional geologic
data to
careful evaluation of volcanic rock age and petrology,
spring
geochemistry, and thermal gradients (Ward and others, 1973).
The
purpose of this report is evaluation of spring-Hater
geochemistry in
the Blackfoot Reservoir region of southeastern Icaho and
detection ~f
a geothermal reservoir in the subsurface. High apparent
che~ical
temperatures and the presence of abundant gasses are suggestive
of a
high-temperature co~ponent in the cooler springs, which may be
better
cefined with detailed study of the spring-\-1ater
gecche:nistry.
Water samples from forty-five springs an1 four wells Here
collected and analyzed for major and minor constituents, and
the
mineralogy of six travertine samoles \'1-35 determined. ~Iater
chemistry
is interpreted in terms of mineral equi~ibria. Temperatures of
last
water-rock equilibria are estimated from tne silica c~ntent of
the
water ~~i the rla-K-Ca geothermorneter with suitable corrections
fJr
carbon dic,;dde. :"1agr.::sium, and f71ixing ;.lith cooler
s'Jfface \laters •
. Mecha~i~~s 3re oropcsed to explain the oresence of gasses and
the war~
teT.perct~res of t~e springs.
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1 GEOLOGIC BACKGROUND
Physiography
The Soda Springs-Blackfoot Reservoir region lies in a
transition
region between the Basin and Range and the Middle Rocky
Mountain
physiographic provinces, approxi~ately 75 k~ southeast of the
Snake
River Plain, Idaho (fig. 1). The boundary between the two
provinces
has been variously placed by different workers (Fenneman, 1917,
D. 82;
1931, footnote 2, p. 170; ~1ansfield, 1927, p. 11). In general,
the
eastern province is characterized by mountains formed by folding
and
thrust faulting with a high proportion of mountains to valleys.
The
western province is typified by block-faulted ~ountain
ranges
separated by wide, deeply-filled basins anc valleys. A
com~or.ly
accepted boundary is placed along the western edge of the Bear
River
Range and northward along the eastern edges of the Blackfoot
and
Willow Creek Lava Fields, although arguments ~ay be made for
placement
along the western edge of the Aspen Range.
Stratiqraohy and Iq~eou$ Geology
Published reports of the geology include those of ~ansfield
(1927,1929), Armstrong and Oriel (1955), Armstrong (1969), and
a
summary by f·1abey an'd Oriel (1970). In the '.lountain ranges
of the
western, or Basin and Range, portion of the area, tilted and
faulted
Paleozoic and lavler 1vlesozoic (Triassic) sedi:nentary units
are expos2d
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.'.f $'II! o o oo o
(0 NV
iddle Rocky
P~. 9 Mts.
Utah
Salt Lake City
, t-
EXPLANATION
~ Terrestrial volcanic rocks Quaternary (Snoke River Plain) and
Tertiary age
Wt;l'1 Thick deposits In Intermontane depressions Quaternary and
Tertiary age
o Undifferentiated Paleozoic and Mesozoic rocks
Figure 1. Sketch map of major physiographic features of the
Snake River Plain and adjacent regions. adapted from King
(1969).
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4
t i j
:i 1. !
i
43°00' I
I ~ I ~ Ul I U'l ....... 1 ~
~ ~ U'l I
~ ~ ~ Ig'
\) Soda ~
"Z-. 'Z G'l (~0")J ·Springs"t ~ I; ~ :-z.• G'l I ~ ~ I ~ \) ")J
I 42°30'0 C4.Ic z ~ o'getown '2 I ~ ~
V'l IV'l
z ")J I~ Vl l>Q:: Iz G) Q:: tTl I
C4.I I~ .... Q::
Q::
~ ~
Q) Idoh 42°00'Utah
0 5 10 15 milea I I I I 0 8 16 24 kilometers
••• Area of fioure 4
Figure 2. Index map of southeastern Idaho, adapted from
Mansfield (1927).
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5 -
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o
basalt flows, but Middle Cone contains inclusions of d still
older
basalt. China Hat and Middle Cone have been dated at less than
0.1
m.y. by K-Ar methods (Armstrong and others, 1975) and at about
0.05
m.y. by K-Ar, thermoluminescence and hydration rind methods
(Leeman
and Gettings, 1977). Two islands in the Blackfoot Reservoir
are
composed of slightly older rhyolite which has been dated by
Phillips
Petroleum Company. Rhyolitic tuff from Sheep Island, in sections
11
and 14, T. 6 S., R. 41E., gives a K-Ar aqe of 1.3 ~ 0.2 m.y.
Rhyolitic vitrophyre from the smaller island immediately south
of
Sheep Island in section 14, T. 6 S., R. 41E., gives a si~ilar
age of
1.5 ! 0.6 m.y. (D. Hayr.lOnd, personal cO~'TIun. to t4. P. Nash,
1979).
Recalculation of these dates using decay constants AS and \_
from
table 1, gives ages of 1.4 ± 0.2 m.y. and 1.5 : 0.6 m.y.,
respectively. Several exposures of rhyolite occur in the
northern
part of the study area. In the Cranes Flat 15-~inute
quadrangle,
three stubby flows or flat domes (Mansfield, 1927) of rhyolite
cover a
tota~ area of approximately 3 square miles. Two of the exposures
have
been dated by K-Ar methods at the University of Utah (see table
1).
Rhyolite from the S~~NW~ sec. 33, T. 4 S., R. 42 E. (sample
BFR-3A), .J,.
gives an age of 1.59 ~ 0.06 m.y. and rhyolite from the NE~~E~
sec. 25,
T. 4 S., R. 41 E. (sample 8FR-5A), gives an age of 1.41 ± 0.15
m.y.
(S. Evans, personal cornmun., 1979). A small exposure of
rhyolite tuff
at the north end of the Chesterfield Range in sec. 20, T. 5 S.,
R. 40
E., has been dated by Phillips Petroleu~ Com~Jny at 10.2 ! 0.5
~.y.
f (D. Haymond, personal commun. to \!. P. :lash, 1979) and
recalculated using the new decay ccnstants to an age of 10.7 ! 0.5
m.y. 1:
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7
Table 1. K-Ar dates for rhyolites in the Blackfoot Reservoir
region, southeastern Idaho (S. Evans, University of Utah, personal
commun., 1979).
Samol e BFR-3A BFR-5A
Location S\~!4NW;4 sec. 33, NP.d~E~ sec. 25, 1. 4 S., R. 42 E.
T.4S.,R.41E.
Rock type rhyolite rhyolite
:-1aterial dated sanidine sanidine
Wei oht (gm) 0.79201 0.92494
;; K 9. 15 7.81
Moles/gm Ar~~d (xl0 11 ) 2.469 1.913
:~ Ar 40 56 87-'--' rad
Age (m.y.) 1.59 =: 0.06 1.41 :: 0.15
Constants
A = 0.581 x 10-10/yrE:
K40/Ktota1 = 1.167 x 10-4 mole/mole
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8
Quaterna~ travertine is widespread and delineates areas of
past
and present spring activity. The exposures vary in size up to
about
12 square kilometers and occur primarily along the edges of the
lava
fields adjacent to upper Paleozoic and lower Mesozoic
calcareous
rocks. The character of the deposits is discussed in a later
section.
Structure
The structural evolution of southeastern Idaho has been
discussed
by Mansfield (1927), Armstrong and Cressman (1963), and
Ar~strong and
Oriel (1965). Westward-dipping thrust faults have long been
recognized and were originally described by Richards and
Mansfield
(1912) as a single large folded thrust fault which they namea
the
Bannock overthrust. Later work by the U.S. Geological Survey as
part
of a study of the \~estern (U.S.) phosphate field resuited in
the
recognition of several discontinuous, separate thrust fcults.
Current
interpretation of the Bannock thrust zone by Armstrong and
Cressman
(1963) stems from the recognition of a westward-dipping
imbricate
thrust zone extending from southwestern nontana to north-central
Utah,
and the presence of tear faults and younger block faults as
progressive stages in the tectonic development of the
region.
Tnrusting occurred fro~ Late Jurassic through Cretaceous
ti~e,
preceded and accompanied by the develop~ent of large
north-trending
folds. Individual thrust faults are younger to the east, as are
the
rocks exposed in the overriding ~Iestern plates (Armstrong
and
Cress~an, 1963, Armstrong and Oriel, 1965). In the Soda Springs
area,
two major thrust faults are documented. The Paris fault is
2xposed
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9
near Cavanaugh Siding approximately 17 km southeast of Soda
Springs
where Cambrian Brigham Quartzite and Triassic Thaynes Limestone
are
juxtaposed (Mabey and Oriel, 1970). The trace of the Paris fault
is
covered to the north. The Meade fault places Mississippian
limestone
over Jurassic and Cretaceous strata and is exposed to the east
of
Georgetown where it bends to the north and splays into several
slices
(!~abey and Oriel, 1970).
High-angle faults are abundant and more conspicuous than
thrust
faults in this portion of southeastern Idaho. Two principal sets
of
high-angle faults have been recognized. The east- to
northeast
trending faults are thought to be tear faults originating in
response
to differential eastward movement among the thrust plates
and
therefore contemporaneous with thrusting (Armstrong and
Cressman,
1963). Several miles of horizontal movement has been taken up
along
tear faults such as the Blackfoot fault in T.l 5., Rs. 43 and 44
u. and a set of faults south of Tenmile Pass in the Soda Springs
Hills.
The other set of high angle faults trend north to northwest and
are a
result of block faulting which began in the Tertiary and has
continued
to the present (Armstrong and Oriel, 1965). Gem Valley, Bear
River
Valley, and Slug Creek Valley are recognized as grabens.
Steep
northwest-trending normal faults cutting basalt and o~e of
the
rhyolite domes in the Blackfoot Lava Field, and the presence of
fault
scarps on modern alluvial fans, attest to the recency of
block
faulting in the Blackfoot Reservoir-Soda Springs area.
Travertine
deposits located along the east side of the Bear River Valley
and
similar deposits elsewhere in the region ~re associated with
the
-
10
north- to northwest-trending block faults and not with thrust
fault3.
Sei smi city
Southeastern Idaho is located within the Intermountain
seismic
belt (ISB), a northerly-trending zone of earthquake activity
which, in
this region, roughly parallels the boundary betl-/een the Basin
and
Range and Middle Rocky Montain physiographic provinces (fig.
3).
Smith and Sbar (1974) and Smith (1978) discuss the ISB in
relation tG
the tectonics of the western United States, and Bones (1978)
has
studied the ISB in southeastern Idaho. 80th have judged
seismic
activity in southeastern Idaho to be characterized by
earthquake
s\'Iarms, \·,hich are composed of r.1any events \-Ih;ch increase
in frequency
to a maximum and then gradually decrease, and which lack a
single
outstanding mainshock. Ward (1972) reviews the association
of
swarm-type earthquake activity with major geothermal ~reas in
Iceland,
the United States, Ne\~ Zeal and, Japan, and other countri es.
\Shere
detailed data are available, there is a close spatial
relationship
between microearthquakes and geothermal areas. ihe
microearthquakes
rarely had Richter magnitudes greater than 4.5, probably because
of
the small dimensions of most geothermal areas and the weakening
of the
crust due to hydrothermal alteration and the presence of
fluids.
Focal depths ranged from near surface to 6 km. Ward (1972) and
Smit~
and Sbar (1974) suggest :hat detailed seismic studies may be
useful ~n
lJeothermal areas to delineate active fault zones \"hich l7Iay
control the
movement of fluids in the geothermal system. A microearthquake
suney
in the Scda Springs region during May 31 to June 22, 1977
(Bones,
-
11
Figure 3. Generalized map of seismicity in southeastern Idaho
and western Wyoming based on a compilation of University of Utah
epicenters, ca. 1975-1977 (see Bones, 1973, and Smith and others,
1976). Known and suspected active faults from Witkind (1975).
-
•
v.
-
13
1978) indicated a pattern of diffuse seismicity with little
clustering
along the maDped active faults in the region. Focal depths
averaged
about 7 km. Earlier microearthquake surveys in southeastern
Idaho
have also shoYn shallow fecal depths, generally less than 15 km
(Smith
and Sbar, 1974). Even though the pattern of diffuse seismicity
does
not correlate with the surface expression of active faults in
the
region, fault plane solutions indicate general east-west
crustal
extension along normal faults with small co~ponents of
strike-skip
movement (Smith and Sbar, 1971, Bones, 1973). One exception is
a
fault plane solution derived from six events southeast of
the
Blackfoot Reservoir which shows approxi~ate1y equal amounts
of
strike-slip and reverse faulting related to a local cluster
of
slightly deeper earthquakes (Bones, 1978).
Gravitv and ~39netics
Gravity and aeromagnetic data in the Soda Springs region
have
been assembled and interpreted in an informative paper by Mabey
and
Oriel (1970), from which the following qenera1 izations are
made.
North\.,est-trendi ng gravi ty anomalies present ; n Gem Vall ey
and the
Bear River Valley are probably a result of low-density Tertiary
and
Quaternary sedimentary rocks filling the valleys. A compound
gravity
low of a more complex nature is found in the Blackfoot Lava
Field
south of Henry. Steep gravity gradients along the margins of
the
ano~a1y s~ggest the possibility of high-anqle faults. The
anomaly
could te produced by several thousand feet of low-density
material.
However, the irregular shape of the anomaly, the abundance of
volcanic
-
14
craters and cones, and the presence of rhyolite do~es sug~(S~
that the
gravity anomaly could be caused by a volcanic collapse structure
or
possibly a buried granitic intrusive body.
Magnetic anomalies are complex over areas of exposed
Cenozoic
volcanic rocks, and ~uch less complex when volcanic rocks are
absent.
The complexities can be interpreted as variations in the
thickness end
distribution of basalt flows and related basaltic intrusive
bodies.
Several negative magnetic anomalies east of the Blackfoot
Reservoir
are attributed to an older reversely-magnetized basalt flow,
indicating an age of greater than 0.7 m.y. In the valley of
Corral
Creek, magnetic and gravity data indicate that the valley
contains
low-density fill and little or no basalt except for an area of
thicker
basalt in the northern part of the valley. The high magnetic
intensity near China Hat may reflect thick basalt flows and
intrusives
related to the numerous vents in the immediate vicinity. This
high
magnetic intensity may also be attributed in part to an
underlying
granitic intrusive mass, as suggested by gravity
interpretations.
Leeman and Gettings (1977) have modeled a laccolithic
silicic
magma body of 330 km 3 to fit the observed gravity data. The
body is
estimated to extend from 0.5 km to a maximum of 6 km below the
surface
and to have a center of mass at 0.9 km. This remarkably large
and
shallow ~luton would, if present, constitute a generous
thermal
source.
Heat Flow
Brott and others (1976) have studied the regional heat flow
of
-
15
the Snake River Plain to identify zones of high heat flolv
related to
recent volcanism and to evaluate the Qeother~al potential of
the
Plain. Most heat-flow measurements were made in the western
Snake
River Plain, which is not complicated by the effects of the
Snake
Plain aquifer that underlies most of the eastern Snake River
Plain.
Ground water in the Snake Plain aquifer flows from the Island
Park
caldera near the Idaho-Wyoming-Montana border to the southwest
beneath
the Snake River Plain and discharges primarily at Thousand
Springs in
south-central Idaho. The water table is deepest near the
caldera
(approximately 1.5 km) and becomes shallower to the
southwest
(approximately 1.0 k~) (Mundorff and others, 1964). The rapid
flow of
water through the aquifer transfers heat laterally which results
in
low suriace he~t flow. High heat flow may exist beneath the
eastern
Snake River Plain; however, no wells were ~easured by Brott and
others
(1975) that penetrated the aquifer. In a subsequent paper (Brott
and
others, 1978), an eastward increase in heat-flow values is
proposed on
the basis of systematic increase in elevation eastward, and
west-to-east time-progression of silicic volcanism across the
Snake
River Plain.
Heat flow values are generally higher along the northern and
southern margins of the western Snake River Plain, compared to
values
within the Plain (Brott and others, 1976, 1978; Blackwell,
1978).
Heat-flow values along the southern margin vary from 2.2 to 4.1
HFU
(Heat flow units) (Brott and others, 1978). High heat flow may
also
extend along the southern margin of the eastern Snake River
Plain.
The Rexburg-St. Anthony area, which is approximately 120 km
north of
-
16
."" "j
"
'
ir
,
SOGa ~prlngs, is at the eastern boundary of tne Snake Plain
aquifer
and has average heat-flow values of 5.0 HFU (Brott and others,
1976).
Based on a time-progressive thermal model. Brott and others
(1978)
predict a regional heat flow of 2.5 to 3.0 HFU in the area fro~
Idaho
Falls to the Idaho-Wyoming border. The corresponding thermal
gradients would range from 85° to 100°C/km. The high heat
flow
predicted for the eastern Snake River Plain and its margins
imply that
there should be many active geothermal systems where heat f1o\1
is
sufficient for hydrothermal convection to become an
important
heat-transfer mechanism.
There is a large ga~ in heat-flow data in southeastern
Idaho,
which includes the Blackfoot Reservoir region. Although the
region is
approximately 50 to 75 km from the southern edge of the Snake
River
Plain, upper Cenozoic volcanic rocks that are present in the
Blackfoot
and Gem Valley Lava Fields join northward with similar rocks of
the
Snake River Plain. Evidence that volcanic rocks of the
8lackfoot
Reservoir region were extruded from local sources indicates that
heat
flow may be high in this area. Further heat-flow measurements
may be
complicated by aquifers in the basalt, uhere the major
water-bearing
zones occur in layers of scoriaceous ~ateria1 and cinders, and
at the
contacts between flows. The aquifer beneath the Blackfoot Lava
Field
is recharged by leakage fro~ the Blackfoot Reservoir and by
precipitation and irrigation water (Dion, 1974). Ground water
flo~s
to the south past the town of Soda Springs into the Bear River
and
Soda Point Reservoir. Wells drilled in the basalt aquifer have
yields
that vary from 300 gpm (gallons per minute) to 3500 gpm (Dian,
1974).
-
17
This is equivalent to about 19 and 220 liters per second,
respectively. The depth to the top of the aquifer is generally
less
than 100 meters (Dian, 1974); therefore, wells or holes used to
obtain
heat-flow measure~ents should be greater than 100 meters deep in
the
Blackfoot Lava Field.
-
18
Figure 4. Generalized geologic map of the Blackfoot Reservoir
region, southeastern Idaho, modified from Dion (1974).
Figure 4a. Simplified structural map of faults in the Blackfoot
Reservoir region, southeastern Idaho, adapted from Armstrong and
Cressman (1963). Overlay to figure 4.
Figure 4b. Location map of water and travertine samples
collected during July, 1978, in the Blackfoot Reservoir region,
southeastern Idaho (see table 2). Overlay to figure 4.
-
~..--:1~~r:d••.." Spt'lnV
FIQure 4b_
21
fXPLANATION
ot wll wotlr loapll
oTrovertlACI sampl'
EXPLANATION
~-.- ...... ..,... Block foull hotchurls on ~O~ thrown side
:. ~-r-r TronlYl"l fault hotJ:burel on do_· thrown 51delotrow
indiCltl. dlrlctloA 01 relative Inovement
-....--?T yTtlrUit fel ult querlld where doubtful i 10WltlU. in
upper plale and poinlinll down dill
fioure 40. Figure 4.
-
PREVIOUS SPRING WATER STUDIES
Springs in southeastern Idaho have been described by
Mansfield
(1927, 1929) as normal springs (non-saline and non-mineral
de~ositing)
\~hich grade into mineralized and thermal springs of four
types:
calcareous, iron-bearing, sulphureted, and saline. Springs in
the
Soda Springs-Blackfoot Reservoir area are various combinations
of the
first three types, all of which are carbonated. Most of the
sulfur
springs and vents are concentrated along the eastern edge of the
Bear
River Valley. Saline springs occur to the east in the Craw Cfeek
and
Freedom 7~-minute quadrangles. No chemical\analyses of springs
or
spring dep0sits were reported by Mansfield except from A~burn
Hot
Springs in western Wyoming and from a group of hot springs in
the Sear
La~e Valley.
Dion (1974) studied the hydrologic budget of the Blackfoot
Reservoir, Soda Creek, and the gear River, to dete~ine the
amount of
leakage from the reservoir under present conditions and the
effect of
raisi~g the water level of the reservoir. The Blackfoot River
basin
(Snake River drainage) is separated from the Bear River basin
(Great
Basin drainage) by a barely perceptible divide across the
Blackfeot
Lava Field between the reservoir and Soda Springs. Dion
deter~ined
that about 10 cubic feet of water per second was leaking i~to
the Sear
River basin fro~ the Blackfoot Reservoir and that the actual
yielas of
Soda Creek and the Bear River were greater th2n the expected
yields
-
23
due to contributions from numerous carbonated springs. He
speculated
that the source of the carbonated springs was deeply
circulating
meteoric water from the nearest higher drainage area, which is
the
upper Blackfoot River basin.
Young and Mitchell (1973), and Mitchell (1976) have
identified
geothermal potential in the Blackfoot Reservoir area based on
the
geologic setting and anamalous subsurface water temperatures
predicted
by the Na-K-Ca geochernical thermometer. Later work consisted
of
sampling and analyzing eleven springs in the region: Woodall
Springs,
Hooper Springs, Soda Springs, Sulfur Springs, four Corral
Creek
springs, two springs along the Blackfoot River below the dam,
and a
spring near Wilson Ridge. Mitchell (1976) concludes that there
is a
possibility of deep geothermal resources or shallow
low-temperature
~eothermal energy in the region.
-
~aj~r dete~ined a Pe~kin-Elmer
Model 603 atomic absorption spectrophoto~eter. Sodium,
potassi~rn,
METHODS
Collection and Analysis
Water
Fifty water samples were collected from an area of
approximately
900 km 2 surrounding the Blackfoot Reservoir in southeastern
Idaho.
Sample locations are given in table 2 and are shown in figure
4b.
Springs and wells are numbered according to the system used by
the
U.S. Geological Survey in Idaho (fig. 5). Sampling proc2dures
of
Brown, and others (1970) and Presser and Barnes (1974) were
followed.
The water was filtered through a·0.45-~icrometer filter and
four
500-ml samples were collected from each site in polyethylene
bottles
with polyseal caps. Two of the samples were acidified to below
pH 2
with hydrochloric and nitric acid. The third sample was diluted
10:1
to prevent polymerization of silica, although analysis later
showed
that the concentrations of silica were so low that the dilutions
were
unnecessary. Undiluted samples were actually used for silica
analysis
to eliminate any error introduced by dilution. Temperature, pH,
and
bicarbonate were measured in the field. The high CO 2 content of
the
water and the rapid re-equilibration of aqueous carbonate
species made
it necessary to perform the bicarbonate titration as quickly
as
possible at the sample site. Even so, the calculated
bicarbonate
contents are probably sl.ightly lower and the p~ measurements
slightly
higher than those of the actual springs.
dissolved cations \vere using
-
25
Table 2. Locations of spring- and well-water samples collected
from the Blackfoot Reservoir region, southeastern Idaho, during
July, 1973.
Sample
Woodall 1
Location
7S 42E 34baa5
Map Designation
A ~'Jooda 11 2 75 42E 27dbcS B ~~ooda 11 3 7S 42E 27acaS C
Woodall 4 75 42E 23badS o Hooper Spring 85 41E 36dddS Formation
Spring 85 42E 27cbbS
EF
Poison Creek 1 45 41E 32bbb5 G Poison Creek 2 45 41E 32cca5 H
Big Spring 85 41E 33aacS Corra1 Creek 1 65 41E 19baclS
IJ
Corra1 Creek 2 65 41 E 19bac2S J Cerra1 Creek 3 65 41E 19bac3S J
Corral Creek 4 6S 41E 19bac4 Carra1 Creek 6 6S 41 E 19bac65
JJ
Sulfur Spring 1 9S 42E 14aadS K Sulfur Spring 2 9S 42E 13bca5 SP
100 9S 42E 6badS SP 101 9S 42E 5bcd SP 102 9S 41 E 1cbcS
LMNo
SP 103 6S 40E 12dbbS P SP 104 65 40E 12bddS SP 105 6S 41E 6bac1S
SP 106 6S 41E 6bac25
QR S
SP 107 6S 41E 6dbc5 SP 108 7S 42E 16abc SP 109 5S 41E 30cddS SP
110 7S 40E 1aadS
TUV \~
SP 111 7S 40E 12adbS SP 112 7S 40E 23abaS SP 113 75 40E
23baa5
XYZ
SP 114 SP 115 SP 116 SP 117 SP 113 SP 119 SP 120 SP 121
5S 40E 15bdcS 55 40E 15abdS 5S 40E "' bbdS 5S 40E 14bcdS 5S 40E
25aabS 85 41E 23aaaS 8S 42E 15cccS 8S 40E 26dcc5
AA BB CC DO EE FF GG HH
"
-
26
Table 2. Locations of spring- and well-water samoles collected
from the Blackfoot Reservoir region t southeastern Idaho, during
July, 1978 (continued).
Sample Location Map Designation
Chubb Spring 5S 42E llcacS II Soda Spring 9S 41£ l2addS JJ Lone
Tree Spring 6S 41£ laddS KK Hopkin's Landing 6S 41£ 22add LL Henry
1 6S 42£ 10cbaS ~lM Henry 2 6S 42£ 9dbdS NN Henry 3 6S 42£ 9bccS 00
Henry 4 6S 42E 8addS pp Henry 5 6S 42£ 8dbaS QQRh Spring 1 4S 42£
30cbbS RR Rh Spring 2 4S 41£ 25adbS SS
-
Ff t---+--~.~ ~ t::l ~ EE
-lii:t-- ~/N
IW 801S£
IS 1£ 2£
R. 41 E.
6 5
7
13
21 22 23 24
30 29 28 27 26 25
3/ 32 33 34 35 36
T.
6
s.
IBAS: LIN I 40141£ 42£ 43£
S£CTlON 19
I b a I I ,,·:-;1 I
---1- .L - t - - -~ I - -C I d I I
I I - ~- -/9- -1-
I I I II ---~---l--- ~---
II I I I I I
6S-4IE-19bool
Figure 5. Spring- and well-numbering system used by the U.S.
Geological '\
Survey in Idaho. Example is well 6S 41E 19baal, which is located
in the NE~NE~NW~ sec. 19, T. 6 S., R. 41 E., and is the first well
sampled in that tract. Spring locations are followed by IISII and
travertine locations are followed by liP. Adapted from Mitchell
(1976).
-
1 1 t ! i
calcium, silica, magnesium, and strontium were determined by
direct
methods. Sulfate was determined indirectly by adding a known
concentration of barium-chloride solution, precipitating the
sulfate
as barium sulfate, and measuring the amount of bariu~ left
in
solution. Aluminum and iron were determined using the HGA
2000
graphite furnace. Chloride was measured with a specific ion
electrode
and by Mohr titration. An average value is reported, since the
two
methods were generally in agreement. Fluoride was also ~easured
with
a specific ion electrode. A phosphomo1ybdate method was used
to
dete~ine orthophosphate calorimetrically. Total orthophosphate
is
reported as P04 • Boron, lithiu~, barium, ~anganese, and
strontium
contents \iere determined with an ARl inductively coupled
plasma
spectrophotometer. Strontium values from both the AA and rcp
methods
are reported. At concentrations above 2 mg/l the results are
similar.
However, in samples which contain lesser amounts of strontiu~
the rcp
results are roughly twice those of ato~ic absorption. Seven
spring
samples were analysed for uranium using a Scintrex UA-3
uranium
anaiyser.
Travertine
Sixteen travertine sa~p1es were collected from active springs
and
from tufa mounds associated with extinct springs. After
initial
petrographic examination, six representative specimens were
chosen for
more detailed study. X-ray diffraction techniques were used
to
determine the mineralogy of the whole rock and of the
acid-insoluble
residu~. Several specimens were examined using the electron
-
29
microprobe and the scanning electron microscope for
se~i-quantit3tive
chemical analysis.
Ther~odynamic Modelinq of Ion Species in Solution
Activities, fugacities, activity ratios, and activity products
of
solution species were calculated using HATEQF (Plurnrner and
others,
1976), a Fortran IV version of \lATEQ (Truesdell and Jones,
1973). The
computer program calculates the equilibrium distribution of
inorganic
aqueous species and complexes for major and minor elements based
on
chemical analyses. Activity coefficients are first calculated
usinq
the extended Debye-Huckel equation for dilute aqueous
solutions.
Calculations are then made of the concentrations of weak acids
and ion
pairs by a series of mass action and mass balance equations.
The
calculated concentrations reduce the amount of free ions in
solution
and thus change the ionic strength and activity coefficients.
New
values for the activity coefficients and concentrations of
species are
re-entered in the equations, and the process is repeated until
the
sums of all species agree with the analytical values v/ithin
0.5
percent. The ratios of the ion activity products to the
sOlubility
products of minerals are compared. If the ratio is greate~ than
one,
the water is supersaturated with respect to that mineral. When
the
ratio is less than one, the wate~ is unde~saturated. Neither Eh
nor
dissolved oxygen were measured in the BlackfoJt Reservoir
springs,
therefore WATEQF was unable to calculate pE,
oxidation-reduction
reactions, or the partial pressures of 02 and CH 4 in the
water.
-
30
...• K!
Geothermometers
'
Basic assumptions
Temperatures of last water-rock equilibria were calculated
for
each water analysis using the basic tools provided by Fournier
and
Rowe (1966), Fournier and Truesdell (1973), Paces (1975), and
Fournier
and Potter (1979). The assumptions made in using chemical
constituents of hot spring waters to estimate the subsurface
temperature can be outlined, after Fournier and others (1974),
as
follows:
1) Chemical reactions which are temperature dependent
occur in the subsurface. The silica geothe~ometer
assumes that the amount of silica in the surface
spring water is controlled by the solubility of
quartz or chalcedony at depth. The Na-K-Ca
geother~ometer assumes that reaction betHeen
Na-fe1dspar and K-fe1dspar controls the con
centrations of these cations in solution and
also takes into account the influence of ca1cium,
which competes with sodium and potassium in the
silicate reactions. The solubilities of silica and
feldspars vary as a function of temperature and
to a lesser degree, of pressure.
2) The mineral phases are sufficiently abun1ant in
the subsurface so that supply is not the limiting
factor.
-
3) Equilibrium between rocks and water is approached
at the reservoir temperature. This assumption
is true primarily for high subsurface temperatures
and long reservoir residence times.
4) The water does not re-equilibrate or change
composition as it flov/s to the surface. This
depends upon factors such as the upward travel
time of the water, armoring of the channel rocks with
CaC03 or 5i02, dilution of the high-temperature
water with water from another source, and the
mechanism of cooling. Deeper water entering a
shallow reservoir may be diluted with chemically
different, cooler water and the spring water
emerging at the surface will be a mixture of the
two types. Hot water on its way to the surface
cools by different mechanisms. Deep water that
is at a higher temperature than the surface
boiling temperature will c~ol adiabatically (by
boiling) as it rises through l~wer pressure
regions. As the steam separates, non-volatile
components will be concentrated in the fluid
phase. Lower temperature w2ter cools conductive1y.
The amount of heat lost to the wa11rock through
conductive cooling depends upon the flow rate.
If the channel is sufficiently non-reactive
and the flow rate rapid, the water composition
-
32
should re~aln necrly tne sa~e. The task is to
distinguish between springs which cool adiabatically,
conductively, or by a combination of both mechanisms.
SiD? geothelinOmeter
Althouqh amorphous silica is the precipitated silica phase
under
surface conditions in hot spring pools, the a~ount of silica
in
solution is controlled by the solubility of quartz or other
silica
phases found at depth. The solubilities of amorphous silica,
cristobalite, chalcedony, and quartz at vario~s te~peratures
have been
studied by many workers (Fournier and Rowe, 1977, Morey and
others,
1962, 1964, Siever, 1962). Studies of active geothermal systems
show
that silica concentrations are relatively constant over long
time
periods, which implies that steady-state conditions have
Dersisted at
depth (Fournier and Rowe, 1965). Evidence also suggests that
little
~e-2Gu;librat;on occurs during the transport of water to the
surface
due to armoring of the channels with amorphous silica, rapid
co01in9,
and the sluggish deposition of quartz and other crystalline
silica
phases. Thus, the silica content of a surface hot spring can be
used
to estimate the temperature of last equilibriu~ with a silica
phase at
depth, provided the cooling takes place by conduction or that
a
correction is ~ade for steam separating from the solution
during
ascent. Adiabatic cooling by ste3m separation was not considered
for
springs in the Blackfoot Reservoir area because of l~w to
moderate
surface temperatures (well below 100°C) and the absenl.e of
stea~
fumaroles.
-
Silica temperatures were calculated fro~:
tOC (quartz, conductive cooling) = 5.205 1315
- log SiO z (mg/l) - 273.15
tOC (chalcedony, conductive cooling) = 4.655 1015. 1
- log SiO z(mg/l) "73 15
- L •
(Truesdell, 1976).
The quartz geothermometer works best at temperatures from
150°
225°C. At 10\ver ternperatures, the chalcedony geothermo:-:leter
'":lay give
more accurate results (Fournier, 1977).
Na-K-Ca oeother~o~eter "
The exchange reaction between sodium and potassium feldspars
is
the basis of a quantitative geothermometer from which
several
empirical curves relating atomic Na/K ratios in natural waters
to
temperature have been constructed (White, 1965; Ellis, 1970;
Mercado,
1970; and Fournier and Truesdell, 1970). Increasing evidence
that
calcium-rich waters did not yield reasonable Na/K
temperatures,
especially in low to moderate temperature systems, led to
the
development of the Na-K-Ca geother'llometer. Eve" though the
amount of
calcium in solution is controlled by the solubility of a
caicium
fr
carbo~ate at certain conditions of te~perar.ure, pH, and PCQ2,
Fournier
and Truesdell (1973) have empirically shown that aqueous
Na-K-Ca
relationships can be explained in terms of silicate reactions.
The ,
reactions are shown by tnree configurations:
(x + 2y) K+ + solid = x Na+ + y Ca2 + + solid
(2y - x) K+ + x ~a+ + solid = y Ca2 + + solid
-
34
(x - 2y) K+ + Y Ca 2+ + solid = x rJa+ + solid.
If one potassium ion is used in writing the reactions, then
a
generalized equilibrium constant, K*, can be written for all
configurations:
log K* = log Na + B 10gfCa K Na
where S depends on the stoichiometry of the reaction. By
plottin~
previously known hot spring compositions and temoeratures on a
graph
of log K* versus 10 3 /T (absolute), a best fit curve was
obtained when
s = 4/3 or 1/3, depending upon the temperature. Evaporative
concentrations do not affect the a1k~li ratios; however, there
is
evidence that significant water-rocx reaction occurs during
ascent
which alters the ratios and results in deceptive tla-K-Ca
ternperatL!res
(Fournier and Truesdell, 1973). tla-K-Ca geotherl1Cl71eter
temperatures
were calculated from:
1647 tOC = log (Na/K) + slog ()lCa/Na) + 2.24 - 273.15
...,here 8 = 4/3 for (;JCd/Na) > 1 and t < lOO°C,
a '" 1/3 for (fu/Na) < 1 or t4/3 > leQoC,
and Na, K, Ca are in mol es/l iter
(Fournier and Truesdell, 1973)
Na-K-Ca-C~2 geothermometer
Pace$ (1975) ~as e~pirical1y derived a correctic~ factor (I)
for
the Na-K-Ca ~eother~o~eter to be apolied at ground water
te~Deratures
of below 75 u C with the partial pressure of CO~ above 10-4
atmoscheres.
-
35
At these temperatures, an inf:~x of 2n 2c~ci~ying age~t such as
;~
may cause a steady-state situation in the silicate reactions to
exist
rather than an equilibrium state. I, the disequilibrium index,
is the
deviation of the water from its equilibriu~ co~position and is
shown
to be related to PC02 by a least squares method yielding the
equation
I = -1.36 - 0.253 log PC02 . Subsurface temperatures are
calculated using a modified form of the
Na-K-Ca geothermometer:
o 1647 t C = log (Na/K) + 4/3 log ()IC'a/Na) _ 1+ 2.24 -
273.15
where ~a. K, Ca are 1n moles/liter
(Paces. 1975).
Na-K-Ca-Mq geother~o'11eter
Observations of active geothermal syste~s and exoerimental
evidence by Ellis (1971) show that ~agnesium concentrations
are
generally low when temperatures are high (greater than a~out
175°C).
Cooler waters contain more r:1agnesium and may yield abnormally
high
~a-K-Ca temperatures. Fournier and Potter (1979) have devised
an
empirical method for correcting Na-K-Ca te~peratures for low
to
moderate amounts of magnesium. The correction depends on both
the
estimated Na-K-Ca temperature and the equivalent percent
magnesium
(O'lg/Ulg+K+Ca) x 100). Fournier and Potter recom::lend using
the
correction only when Na-K-Ca temperatures are above 70°C and
when R,
the equivalent percent magnesium, is less than 50. Waters that
have
R > sa do not fit the empirical curve as well, perna os
reflecting
-
36
equilibration at lower temperatures or a nonequilibrium
situation.
The correction factor can be obtained graohically or by the
equation:
ttMg = 10.66 - 4.7415(R) + 325.87 (log R)2 - 1.032 X 105
(log R)2/T
-1.968 x 107 (10g R)2/T2 + 1.605 X 107 (log R)3/T2,
where T = Na-K-Ca temperature (K),
and R = (Mg/(Mg + K + Ca)) x 100, with Mg, K, Ca in equivalent
units.
iltMg is then subtracted from the esti~ated fla-K-Ca
temperature.
i1ixing Models
Fournier and Truesdell (1974) have proposed a method to
deteJ.nine
the temperature of the hot water co~ponent and the fraction of
cold
water present in \'iarm spri ngs Vlhi ch are a mi xture of deep
hot water
and cold water of a shallower origin. The model used here is one
of
conservation of enthalpy and silica; that is, any steam
separated
during the ascent condenses in the cold water, and no solution
or
deposition of silica occurs after the hot water leaves the
reservoir.
Two equations aie written; the first relates entha1pies to
the
fraction of cold water present
(H cold) (X) + (H hot)(l-X) = H spring
where X is the fraction of cold water, and H is enthalpy, and
the
second equation relates silica contents ~o the fraction of cold
Hater
(Si cold)("X) + (Si hot) O-X) = Si spring
,. where Si is the silica content in ~g/l. The silica content cf
the hot
\·/ater is related to the te'l1perature by the solubility of
quartz.
-
;)/
Selected vJlues of enthalpy and silica for different hot
\'/ater
temperatures are given in Fournier and Truesdell (1974). A
graphical
approach can be used to solve the equations. The two equations
are
plotted on a graph of II temperature of hot \~ater" versus II
fract i on of
co~d water," by assuming a series of values for (H hot) and (5i
hot)
and knowing the temperatures and silica contents of the warm
springs
and the cold water component. The point of intersection of the
curves
gives the estimated hot water temperature and fraction of CQld
water.
-
H.lHER CHGlI STR \'
Chemical Constituents
Table 3 lists the surface te~peratures, pH, and concentration
of
major and minor elements present in water samples from the
Soda
Springs - Blackfoot Reservoir area. All of the elements are in
mg/l
except uranium, which is in pg/l. Most of the waters are neutral
to
very slightly acidic. Approximately 85% of the scmples are
between pH
6.2 and 7.0.
Major ele::lents
Figure 6 shows the distribution of major element
concentrations
and also indicates those samples which show consistently
anomalo~s
ccncentrations of several elements. Major cations present are
sodium,
potassium, calcium, silica, and ~agnesium. Silica concentrations
are
generally lJw, averaging 11 mg/l 5i. HooDer Springs contains 33
mg/l
5i, the largest amount measured in the region. The relatively
high
silica at Rh Spr. 1 and 2 was expected because these springs
flow
direct1y from glassy rhyolitic rocks. Hole percents of the
other
major cations are plotted on a portion of the (Ca+Mg)-Na-K
triangular
diagram in figure 7. Most of the springs contain very little
potassium relati>le to othel~ cations, are high in calcium
and
magnesiu~, and contain varyin1 a~our.ts af sodium. The most
noticeable
exceptions are SP 104, SP 114, SP 117, and Corral Creek 1,2,
3,'~,
-
" 39 .~
,f
Table 3. Concentrations of chemical species in spring and well
water from the Blackfoot Reservoir region, southeastern Idaho
(reported in mg/l; except uranium, which is in ~g/l).
-
,",~,~.",""i!o
-
.;.p, .i .""~'''J;,\~,_.~"." ,~~~:~~",."
Ca Si Mg Mn 5r Sr Ila Al Fe Il L1(1 F P04 504
I!COJu
5alllllle T: C vI! Na K (ICP) (ICP)(ICP) ocr) ( lCP)
1,2 3fJO SID 17 1011 19 a8 9 24 ~2 1'15 10 53
J7 1.24 0.00 890 35flOO.O,? 4.5 5.3 0.25 0.04 0.78 0.53 0.646.74
95 241 635 10 2~HSP 104 22 7.5 0.14 0.05 229
52 5
-
,_ ._~. _._ -,-,.~~.------""._",~_ '.fA. @:t ~.,' ---'"- ~un TS
Zt" . • ~ __ __ ...-.< _~ ... ""....... '.... 4-.' -,.,
--~".·ot_" ~"-"",'.,.,_ ~l_' •
Sample T:C pH Na K Ca SI Mg ·Mn (ICP)
Sr (lCP)
Sr 8a (ICP)
Al Fe 8 (lCP)
L1 (lCP)
Cl F .P04 S04 He03 u
SP 117 28 6.42 148 207 615 II 268
-
43
3S
30
25
20 ~ 20U Z LLI ::::I 0
15 LLI
• 15cr l&.()~ .... ...U ()~Z ....It)LLI 10 ..::::I 10 It)...
~ 0
--I
..IW .... .... ...a: ....~ ....l&. ....~ .....5 ~:::: ~
~
"" 0 50 100 150 200 60 100 150 200 250
Na,ppm K,ppm
30
25
20 20 ~ U Z W
15 ::::I 15 0
u ~ cr
w Z l&. W
10 10 0 ::::I
Wa: l&.
e
o 150 o 50 100 SiCa, ppm
Figure 6. Histograms of sodium, potassiunl, chloride, silica,
calcium, magnesium, and bicarbonate concentrations in spring and
well water from the Blackfoot Reservoir region, southeastern
Idaho.
-
•., 4 j. f
20 >u z IlJ " => 10a L&J a: u. 5
0
44
.... ~ ..:'.. Q.::: Ct).. ...
I~ l:I- "llCl. \.)
'0 Ct) \.) 0 \.) Ct)
\> r-/I 100 200 300 400 ~OO 600 700 800 900 1000
20
15
>U Z L&J :=I '0 0 L&J a: IJ.
5
0 200 2~0 300
15
.......... ~
't ....
'~......
Co, ppm
>U 10 Z \LI :=I a L&J a: 5 u.
4000350030002000I~1000500
't I)... Cl. Ct)
Fig~re 6 (continued). Histograms of sodium, potassium, chloride,
silica, calcium, magnesium, and bicarbonate concentrations in
spring and well water from the Blackfoot Reservoir region,
southeastern Idaho.
-
,, Co + Mg
f
• . • SP 103
ccr·~t .SP 104 .SP 1/4 -SP 1/7
.SP/oo
• HOPKINS LANDING
Rh Spr. I .R" $pr. 2
No K
Figure 7. A portion of the (Ca + Mg)-Na-K triangular diagram
showing mole percentages of the major cations is spring and well
waters from the Blackfoot Reservoir region, southeastern Idaho.
-
1 46
•
CI 50 4
Figure 8. A portion of the HC0 3 -Cl-S04 triangular diagram
showing mole percentages of the major anions in spring and well
waters from the Blackfoot Reservoir region, southeastern Idaho.
-
47
and 6, whic~ contain unusually large conce~trations 8f
potassi~m.
Major anions present are bicarbonate, sulfate, and chlorine.
The
eight springs that contain anomalous amounts of potassium also
contain
the largest amounts of sulfate and bicarbonate. Mole percents of
the
anions are plotted on a portion of the HC03-Cl-S04 triangular
diagram
in figure 8. Bicarbonate is the dominant anion in springs in
this
region. SP 100 contains anomalous concentrations of fluorine
and
phosphate, which is part of the reason for its unusual position
within
the triangle, as these elements are not considered when
calculating
the mole percentages. This spring is located less than one mile
from
the Monsanto Company·s elemental phosphorous plant and is
probably
contaminated with water draininq from the tailings and wastes.
The
contaminated spring water then enters Soda Creek, which flows
through
the town of Soda Springs.
Minor Elements
Minor elements that make up less than one weight percent of
the
total dissolved solids include strontiu~, barium, aluminum,
iron,
boron, lithium, fluorine, and phosphate. The largest
concentrations
of strontium, boron, lithium, and fluorine are found in waters
from
the Corral Creek springs, SP 104, SP 114, S~ 117. and Soda
Springs.
These elements are among the few whose concentration in
natural
ther~a1 waters is not governed by water-rock equilibria (Ellis
and
!1ahon, 1964). C1, B. Li, Sr, and F are not readily accomodated
in
secondary silicate structures and so tend to accumulate in the
liquid
Dhase, once liberated from their source mineral. Ellis and
~ahon
-
48
(1964, 1967) reported that chloride and boron are very
soluble
elements which tend to be concentrated on the surfaces of
minerals and
thus released easily from the rock. Lithium is more apt to
be
incorporated into the structure of minerals and thus its
concentration
in waters increases with temperature. The concentrations of
these
elements can be accounted for by water-rock interactions
without
calling upon addition of magmatic fluid. Their presence in
certain
springs in southeastern Idaho is probably due to water-rock
reactionsf f enhanced by high bicarbonate content and/or increased
temperature.
Iron and aluminum are present in barely detectable
concentrations in
most samples. The largest iron concentrations occ~r in springs
\~ith
noticeably dark brown to red, iron-stained travertine
deposits.
Discussion of Chemic~l Te~peratures
5i02 and Na-K-Ca te~peratures
Springs sampled in southeastern Idaho vary in surface
temperature
from 8°C to 40°C t with an average of 16°C. The war~est waters
occur
at a group of springs and wells drilled by FMC Corporation in
1966 to
1970 which are located on the west side of the Reservoir
r10untains in
the Corral Creek drainage. The relatively low temperatures
(as
compared to 100°C for boiling) and lack of evidence of steam
fumaroles
in the area indicate primarily conductive cooling.
Quartz and chalcedony saturation temperatures assuming no
stea~
loss during cooling were calculated from the measured silica
content
of the springs and are tabulated in table 4. Estimated
temperatures
vary from 43°C to 119uC for quartz saturation, and from SoC to
8goC
-
49
Table 4. Chemical geothermometers indicating temperatures of
last water-rock equilibria for spring and well waters in the
Blackfoot Reservoir region, southeastern Idaho.
Measured Quartz Chalcedony Temp. (cond.) (cond. ) Na-K-Ca
Na-K-Ca-CO Na - K-Ca -:~g
tOC tOC tee tOC 2 tOCt"'C
ilooda 11 1 12 43 8 -19 -59-.,-:)1"'Jooda i 1 2 14 53 19 -16
\'Jooda: 1 3 17 58 23 -6 -52 Woodall 4 8 53 19 -2 -42 Hooper
Spr. 10 119 89 58 -15 Formation Spr. 12 43 3 -19 -57 Poison Ck. 1
19 53 19 16 -23 Poison Ck. 2 14 58 23 18 -32 6ig Spr. 12 77 43 -6
-46 Carra1 Ck. 1 40 77 43 366 34 114 Corral Ck. 2 38 77 43 365 34
106 Corral Ck. 3 32 74 40 367 35 107 Carra 1 Ck. 4 28 82 49 368 32
111 Corral Ck. 6 25 62 28 347 23 101 Su1 f:Jr Spr. 1 21 95 63 '2
-116 Sulfur Spr. 2 11 58 23 -15 -58 S? 100 10 93 61 74 11 31 SP 101
21 103 71 49 6 SP 102 12 91 58 43 -18 SP 103 12 66 31 74 5 70 S?
104 22 66 31 370 36 108 SP 105 8 43 8 -22 -50 SP lD6 8 43 8 -20 -56
SP leo7 8 48 14 -24 -53 S? 103 13 53 19 -6 -.16 SP 109 10 58 23 -8
-49 SP 11 Q 2 48 14 -27 -61 SP 111 26 43 14 20 -23 SP 112 9 80 46 6
-30 S1> ~ 13 9 71 37 -9 -J5 SP 114 18 48 14 324 32 33 SP 115 11
53 19 3 -41 SP 116 25 58 23 31 -24 SP 117 28 71 37 325 33 81 S? 11B
3 62 22 -1 -43 S? 119 10 107 76 50 -13 S? 120 10 43 14 -11 -51 S;
121 16 77 43 34 -25 Chubb Sor. 13 43 14 10 -25
.~; Soda SOl'. 30 S~ 49 28 -27 Lone Tt'ee Spr. 24 62 28 47 -14
Hcokir,s Ldg. 9 80 46 25 -24
:; ...~enry 1 15 ..;) 19 1 -40 Henry 2 16 58 23 -1 -42 Henry 3
18 53 19 7 -39 ~enry c: 20 62 28 16 -34 Henry 5 24 71 37
-
1
r Wi:
J
J
..
50
for chalcedony saturation. The calculated ternperattires are
only
moderately high for the springs with the highest surface
temperatures.
In some cases. the chalcedony te~peratures are sirni1ar to the
measured
surface temperatures. Two cold springs flo~ing through rhyolite
to
the northeast of the Blackfoot Reservoir (Rh Sor. 1 and 2) give
high
(100°C) calculated subsurface temperatures probably due to
silica
dissolved from volcanic glass. The variability and
inconsistancies of
the silica saturation temperatures may be explqined by returning
to
the basic assumptions listed previously. Equilibrium with quartz
or
chalcedony may not have been reached due to low temperatures or
rapid
movement of water through the reservoir. The Si02 content may
be
controlled by aluminosilicates rather than a pure silica phase.
110st
of the warm springs have a rapid flow, which may mean little
re-equilibration has occurred; however, mixing with cooler,
more
dilute waters may be partly responsible for the low silica
content.
Na-K-Ca temperatures are also tabulated in table 4. The
estimates vary from -27°C to +74°C, except for eight springs
which
give anomalously high temperatures averaging 354°C. These
sprin~s
include Corral Creek 1,2, 3,4, and 6, SP 104, SP 114. and SP
117.
They are roughly aligned in a north-northwesterly direction
along the
valley of Corral Creek. The anomalous temperatures are a resuit
of
iarge potassium concentrations which drive the log (ila/K) value
used
in the calculation toward zero or a slightly negative number. As
can
be seen from figure 6 the anomalous springs have potassium
concentrations several times greater than any other springs in
the
area. Geother~omet2r temperatures below ooe are the result
of
-
, I 51
1
deviations from tne basic assumptions. There is most likely
an
abundance of feldspar minerals in the rocks; however, at
lower
temperatures the amount of Ma, K, and Ca in the water may be
controlled by reactions between minerals other than feldspars
such as
micas, clays, and carbonates.
An important characteristic of the springs in the Blackfoot
reservoir region, and one which is not taken into account in
the
silica or Na-K-Ca geothermometers, is the presence of large
amounts of
bicarbonate in the water. Concentrations range from less than
100
mg/l to over 3500 mg/l, with the highest amounts found in the
Corral
Creek springs, SP 104, SP 114, SP 117, and Soda Springs (see
fig. 6).
It is evident upon comparison (table 4) that a correction for
the C02
content in the water drastically lowers the Na-K-Ca
te~peratures. The
Corral Creek springs, SPI04, SP114, and SPl17 show
C02-corrected
Na-K-Ca temperatures that average 32°C, very close to their
~easured
surface temperatures (average 29°C). Most of the Blackfoot
Reservoir
area springs, other than those mentioned above, have N3-K-Ca-C0
2
temperatures below O°C, suqqesting that the steady-state exists,
not
between feldspars, but between other Na, K, and Ca-bearing
minerals.
Re-equilibration between the water and wall-recks may also
be
occurring, as is evidenced by widespread travertine deposition
in the
region.
A magnesium correction was applied to ten tta-K-Ca
temperatures
according to the recommendations of Fournier and Petter (1979).
The
correction lowered the eight anomalous Na-K-Ca te~Deratures
to
approximately 100°C, which ';s a more reasonable estimate.
The
-
52
i1a-K-Ca-Mg temperatures (table 4) are still three to four
times
greater than the measured surface !e~Deratures of the springs,
which
~ay or may not reflect high subsurface te~pera~ures.
In using any sort of correction to the rJa-K-Ca
geothermo'11eter,
the basic assumptions which were discussed previously are an
inherent
part of the correction procedure. !f there is substantial
deviation
from the assumptions, then neither the corrected te~peratures
nor the
original Na-K-Ca te~peratures will be of significance.
t'1ixing lTJodels
The springs near Corral Creek exhibit variations in surface
te~perature, silica content, and other constituents, thus
indicating
that they may be of a mixed origin. The cold water was assu~ed
to be
BOC and to contain 6 mgtl silica, which is similar to several of
the
coldest springs in the vicinity of Ca~ral Creek. Reasonable
mixinq
models were calculated for six springs (fig. 9, table 5).
The
estimated temperatur~ of the hot water comoonent ranges from
47°C to
67°C and the fraction of cold water varies from 0.30 to
0.77.
Table 5. Mixing model results for six springs in the Corral
Creek drainage, Blackfoot Res2rvoir region, southeastern Idaho
(from figure 9).
Spring Temperature of Hot ~~ater,OC Fraction Cold Hater
Corral Creek 53 ± 17 .30 +- . 13 +Corral Cre~k " c:: 56 13 .36
+- .22 ...Corral Creek 3 64 ± 15 .56 - .11
Carral Creek 6 47 t 10 .58 + .10 +
~p 104 67 t 30 .77 - .091....S? : / 60 + 20 .66 + .11
-
1 J.j
300r
250
A 200
150 r 100,
U 0
50 I-z
01w z 0 02 0 U
B0:: w I
-
54
41 ' 300r
I
::r 0 150
10C
. I
U 0
I Z W Z 0 0... ~ 0 U
0:: W I « 3
5 I
l.J... 0 w 0:: --...J ~ 0:: W 0... ~ W l
50
0
300\
250
200
150
I 100 L
50
0 1
3:)Cr
250~ 200'
I I
150,
loot
E
F
50
01 0
I I I
01 02 03 04 05
FRACT!ON OF COLD
I I
06 07
WATER
!
08 I
09 1
10
Figure 9 (continued). Mixino models for springs in the Corral
Creek drainage, Blackfoot Reservoir region, southeasterr. Idaho: 0,
Corral Creek 5; E, SP 104; F, SP 117.
-
, I I
I
Chemical Eguilibria
Activity diagrams are used to interpret so~e of the chemical
characteristics of the spring waters in relation to the
stability of
minerals which may be in contact with the water. Carbon dioxide
has
been shown to be an important component of the springs in the
area;
therefore, reactions between minerals have been written
involving C02
and H2 0 as reactants and bicarbonate (HC03-) as a product.
All
thermodynamic data is from Robie and others (1978). The
reactions
used to construct the activity diagrams are listed in table
6.
ihe relative stability fields of phases in the
plotted in figure 10. PC02 is assigned a value of 0.26
at~osDheres,
which is an average value for the 8lackfoot Reservoir springs.
Each
dot represents an individual spring. All of the springs have
silica
activities bet\teen those of quartz saturation and amorphous
Si02
saturation, as is typical of most ground waters (Garrels and
Christ,
1965; Stu~ and Morgan, 1970). Most of the waters plot in the
stability field of kaolinite, a mineral lacking potassiu~.
The
cluster of points near the musovite, microcline, kaolinite
junction
represent Corral Creek 1,2, 3, 4, and 5, SPI04, SP114, and
SP117.
ihe high concentrations of potassiu~ in these springs apoarently
are
due to equilibriu~ reactions bet~een the potassium-bearing
minerals
~uscQvite and feldspar, and kaolinite. A trend of increasing
potassiu~ and silica concentr1tions in the spring-water
3ctiviti2S
within the kaolinite stability field, ~ay be jue to
irreversible
-
5G
Table 6. Reactions used to establish stabilityrelations among
minerals.
2 KAl 3Si3010(OH)2 + 2 CO 2 + 5 H20 = 3 A1 2SizOs(OH)4 + 2 K+ +
2 HC0 3muscovite kaolinite
2 KA1Si 30s + 2 CO 2 + 11 H2O = A1 2Si 20s(OH)4 + 2 K+ + 2 HC0 3
+ 4H4SiO~ microcline kaolinite
3 KA1Si 309 + 2 CO 2 + 14 H2O = KA1 3Si 30 10 (OH)2 + 2 K+ + 2
HC0 3- + microcline muscovite 6 H4Si0 4
KA13Si3010(OH)z + CO 2 + 10 H20 = 3 Al (Orl)3 + K+ + HC0 3 + 3
HuSi0 4muscovite gibbsite
AlzSizOs(OH)4 + 5 H20 = 2 Al(OH)3 + 2 H4 Si04 kaolinite
gibbsite
NaA1Si30a + 3 H20 = NaA1Siz06'HZO + H4Si04 albite analcite
2 NaA1Si206"H20 + 2 C02 + 5 H20 = A1zSi20s(OH)4 + 2 Na+ + 2 HCO~
+ analcite kaolinite 2 H4Si0 4
NaA1Si20o"H20 + CO 2 + 5 H20 = Al (OH)3 + Na+ + HCO - + 2 H4SiO~
analcite gibbsite
A1 2SizOs(OH)4 + 5 H20 = 2 Al(OH)3 + 2 H4Si0 4 kaolinite
gibbsite
-
57
Table 6. Reactions used to establish stability relations among
minerals (continued).
CaO-MgO-H70-C~
Mg4(C03)3(OH)2·3H20 = 4 Mg(OH)2 + 3 CO 2 hydromagnesite
brucite
++ ++4 CaC0 3 + 4 Mg + 4 H20 = Mg4(C03)3{OH)2·3H20 + 4 Ca + CO 2
calcite hydromagnesite
2 CaMg(C0 3 )z + 2 14g++ + 4 H20 = Mg4(C03)3(OH)203H20 + 2 Ca++
+ CO 2 dolomite hydromagnesite
4 MgC0 3 + 4 H20 = Mg4(C0 3)3(OH)z·3HzO + CO 2 magnesite
hydromagnesite
++ ++CaC0 3 + Mg + HzO = Mg(OH)2 + CO 2 + Ca calcite brucite
2 CaC0 3 + Mg++ = CaMg(C0 3)2 + Ca++ calcite dolomite
CaMg(C0 3)z + Mg++ = 2 MgC03 + Ca++ dolomite magnesite
-
58
at 25°e, Ptotal= 1 atm, and P eoz = 0.26 atm.
.I
0
-I
-2
-3
....,I 0 u -4 :I:--• -~ C' 0
, '"
-9', 0
rl k
Figure 10.
MICROCllN£
I I I
MUSCOVITE I I~ I~ I~ I~ IN0I(j;
1(1) I§
Iz 1& 0I~
@ 1 ~==
I~ • IN
00
..I~
0
1:5 ~:,0 0•
o • I • 0 • 0KAOLINITE 0• o •0
0
I I . , 00 I I 0 I I II II II
! I I! -5 -4 -3 -2
log ( H4 Si04 )
I I I I
Stability of phases in the system K20-Al z03-SiO:-H:C-CO:
0
-
59
reactions of the water with muscovite and microcline.
Stability relations of analcite, albite, kaolinite, and
gibbsite
in the syste~ Na20-A1203-Si02-H20-C02 at 25°C and 1 atmosphere
are
sho;m in figure 11. Pca is fixed at 0.26 at~ospheres. The 2
activities of the springs fall well \~ithin the stability field
of
kaolinite, thus the amount of sodiu~ in the water does not
aPDear to
be controlled by the equilibrium reaction bet'tleen ;Ia-feldspar
and
kaolinite. The sodium could be the result of irreversible
reactions
of the water with tla-feldsoar or of reactions of other
low-te~perature
Na-bearing minerals such as s~ectites. The stabili~y field
of
~ontmorillonite is not shJwn i~ this dia0ra~ because of
variable
co~position and lack of thermodyna~ic data.
Fig,ure 12 sho~·ts the stable phases of ca1ciu:n and
;nagnesiuiTI
caroonates at 25°C and 1 atmosphere. Spring-water activities
fall
near the ca1cite-do1o~ite-hydro~agnesitein~ersectior.. The
abundance
of carbonate rocks throughout the Blackfoot Reservoir reqion
make it
nigh1y probably th3t the calcium and magnesium in solution is
derived
from reactions with limestone, dolc;nite, and other carbonate
rocks.
As can be seen fro~ figure 6, the springs containing high
amounts of
bicarbonate also contain the most ca1ciu~ and ~agnesium.
Bicarbonate
acts as Gn acidifying agent which expedites the reaction rate
between
rock and \/ater (E 11; sand j'lahon, 1964).
-
• •
60
:3
2
,.., -I I 0 U :I: -2
+ 0 Z
-3 ~ 0
'l -4
ir
J:.
, .....h .-5, .. ...~~l: ..#1
I : ••~l· -6 ~ . . ..,_1'
KAOLINITE I
I
.. .,.
ANALCITE
lAJ...-en II) II)-(.:>
Fig~re 11. Stability of phases in the system
Na20-A1203-SiOz-H20-C02
at 25°C, Ptota1= 1 atm, and = 0.26 atm. PC02
-
-----
......--,.,..."" •._".,,"' ... ,', ...:!':' .... ' -- ;r'~---~"
-.~:: m,tj, VHim:':::::Jr:::""" ....,,! II·M llil ~ 1-."' .t ..
5. \
4
3
2
+- +N N BRUCITEo 0\ U ~ - - 0
0\ o
-\
-2
-3"
-4l I I
I
CALCITE
~~ I 4, <
f~·
DOLOMITE
HYDROMAGNESI TE
MAWES/TE
I I I I I I I I I I,.
-14 -12 -10 -8 -6 -4 -2 0 2 4 6
log Peo 2
Figure 12. Stabil j ty of phases in the system
Ci:tO-~190-COrll::O at (J)25°C and 1 atlll pressure.
-
1,$ I
·,.1 -.11'-"''''''
,.;.-.~::':' :;
So~rce of CO~ era 4:S
~ll ~f the sa~pled sJrin~s in :~e Slc:kfoot ~eservoir area
are
characterized by lcr;e eGoun!s of carJon dioxice. There are ~lso
a
few areas of noticeable sulfur deoosition a~c H2S odor.
Sulf~r
Springs is located approximately four ~iles eas~ of Soda Sor;n~s
on
the western edge of the ~spen Ra~ge at the ~outh of Sulfur
Canyon.
Sulfur was produced here in the early nineteen-hundreds ~ut the
plant
was dis~antled before 192C. The sulf~r and assJc;ated s~a11
)YDSU~
crysta1s occur as the ce~ent of a fault breccia ccnoosed of
tuff,.
limestone, and quartzite frag7.ents (Richards and Bridges,
1911).
Hydrogen sulfide can also be detected in the Blackfoot Reservoir
where
it is associated with underwater springs and gas seeps.
Sulfur-cemented rhyolite fraguents ~ere found on the small
island
south of Shee~ Island. Gas bubbles can be seen extending fro~
this
island in a southeasterly direction, past the eas~ern ejge of
Cinder
Isl and to\/ard the southern end of the reservoi r. The eas:ern
side of
Cinder Island is a near-vertical cliff which appears to be a
fault
SCJrp. The northwest-southeast orientation of the islands and
gas
bubbles is si~ilar to that of other n~rmal faUlts to the south
and
west of the Blackfoot Reservoir.
There are several ways to account for the oresence of
hydrogen
sulfide and carbon dioxide gasses. Previous authcrs (Mansfield,
1927,
~;...,.....Richards 3nd 8rid~es. 1911) have suggested a volcanic
source for :....IC
gasses due to the proxi~ity cf volcanic centers. The oossibility
may
-
1 63 not be entirely ruled out; however, it is far more likely
that the
gasses, in addition to the aqueous species discussed previously,
are
derived from reactions of water with the enclosing rocks.
A simo1e model here proposed is the oxidation of organic
~atter
(general formula CH 20) by the reduction of sulfate. The two
half-cell
reactions are (thermodynamic data from Stu~m and Morgan
(1970)):
A reduction process will tend to oxidize equi~olar
concentrations of
a~other redox process which has a lower pEo value. Thus, S04 2 -
~an
oxidize CH20 to form CO 2 and H2S ~y the reaction:
'1/8 S042 - + 1/4 CH20 + 1/4 H+ = 1/8 H2S(g) + 1/4 CO2 (g) +
1/4 H2 0
log K(25°C) = 6.45.
To deter:nine \'Ihether the reaction is therllodynamically
possible under
actual c~nditi~ns, we can calculate the free-energy change for
the
reaction, ~G =2.303 RT log (Q/K). tG is the free-energy change
for the reaction, K is the equilibrium constant at 25°C, and Q is
the
reaction quotient at the same temperature. t·le ~'/i11 assu~e
that PC0 =
f.
2
0.435 atm., the activity of H20 = 1, the activity of SOu = 5.82
X 10- 4
and pH = 6.22 (from Sulfur Spr. 1, iJATEQF). Stur:m and ilorgan
(1970, p. 233) sug~est that the concentration c~ orqanic matter in
natural
waters ranges from 0.1 to 10 mg/1. This corresponds to CH 20
concentrations of about 10- 5 • 5 to 10- 3 • 5 ~o12s/l. In the
Blackfoot
-
64
Reservoir area, the actual concentration is probably somewhere
between
the extremes, due to the abundance of organic-rich
sedir.'lentary rocKs.
Organic matter averages 2.1 weight percent of the Phosphoria
For~ation
(Gulbrandsen, 1967), whose thickness ranges fro~ approxi~ately
75 to
150 meters in the Soda Springs area. Other black shale and
li~estone
units are scattered throughout the stratigraphic section. PHzS
can be
esti~ated between 10- 2 and 10- 8 atmosoheres for springs with
a
noticeable sulfur odor (Stu~m and Morgan, 1970, p. 364). For
the
reaction
\~e can calculate a range of valuEs for Q from the above
infor~ation: 11.l. 24
Q = 10· . to 10 •
10 51 6We know that K = • ; therefore,
Q/K = 10- 33 to 10- 28
and
LG = 2.303 RT log (Q/K) = -51 to -38 Kca1/~01e.
The oxidation of organic matter by su1fa:e is clearly
favored
thermcdynamical1y under the conditions given.
Figure 12 shows a stoichiometric correlation between
bicarbonate
and su1 fate concentrati ons in \/ater fro~ the 81 ackfoot
Reservoi r
region. A least-squares linear regression technique apolied to
the
data yi~lds a linea~ equation of the fG~:
Y = 2.8 X + 540
with a correlation coefficient of 0.91. In the redox reaction
above,
the reduction of one mole of sulfate to one mole of hydrogen
sulfi~e
-
65
10,OOO~--------------------/"""----
/ /
C" 1-4/SP 104 ~/'
SODA SPR. t/ SP 1/4C-:::< ··SP 1/7
/- "",0
/'
• .. /' /'
1000 I- ./ ........................ . .. _............... .-
-------- - --- .. ·S? 100 .. ........
0" 100 E Rh Spr.2 • Rh Spr. I
1:
-
66
is accompanied by the oxidation of two moles of organic matter.
The
hydrogen sulfide that is produced, is probably oxidized
under
near-surface conditions to sulfate. The slope (~HC03/~S04) of
2.8 in
the linear e~uation is correlative \Jith the stoichiometry of
the redox
reaction: one mole of H2S for two moles of CO z'
Depending upon the oxidation potential, pH, and the presence
of
biological mediators, the oxidation of organic matter to CO 2
may also
be accompanied by the reduction of oxygen, nitrogen, manganese,
and
iron species. The oxidation reaction most favored
thermodynamically
is:
log K(25°C)=87.8
(Stumm and t10rgan, 1970).
To determine whether the reaction could occur in the \Vaters
around the
Blackfoot Reservoir, ~G must be determined. The concentr~ti0ns
used
in the calculation are PC02 ; 0.26 atm and (CH 20) = 10- 5 • 5
(~inimu~
va 1ue) •
A rough estimate of P02 for use in the calculation can be
made
at Sulfur Springs, where the amount of sulfate and pH is known,
and an
estimate (10- 2 to 10- 5 atm) can be made for PH S' Using the
equatia~2
1/8 S042- + 5/4 H+ + e- = 1/8 H2S + 1/2 H20
\'/here
pEO(25°C) = 5.25 (Stu~~ and Morgan, 1970),
nE can be calculated,
pE = pEa + log ([ox] /[re~J)
= 5.25 + 1/3 leg [SO~:_] - 5/4 pH - li8 log PH2S
-
67
= -2.20 to -2.95.
From the relation
\-Jhere
pEe (25°C) = 20.75 (Stum.-n and t1organ, 1970),
the partial pressure of oxygen can be calculated from
log PO = -4(pEO) + 2 log [H 2 0] + 4 pH + 4 pEz = -83.0 + 4
(6.22) + 4 (-2.20 to -2.95)
= -67 to -70. iherefare, fer Sulfur Spring 1, Po is
approximately 10-67 to 10-70
2
atmospheres.
for the oxidation reaction
CH 20 + 02(g) = H20 + CO 2(g),
Q = 1064 ,
Q/K = 10-24 ,
and
LG = 2.303 RT log (Q/K) = -33 Kca1/ncle.
The oxidation reaction is favored ther~odynamica11y unGer
these
minimum conditions.
S;~ilar calculations could be made for other redox precesses
if
the concentraticns and oxidation states of other elements and
gasses
were kno\'ln. Mitchell (1976) reports nitrate (NO~) UP to 0.4
m~/l and, ~.
ammonid (NH3) up to 2 mg/1 in sprinq '..Jdters from this area,
\,hich
suggests that other reduction processes are also occurring.
211ddingtorite, an ammc:1iuiTI f?ldspar, hcs been ieDcrted to
occur in the
?hosohoria Formation in southeastern Idaho as an
