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Appendix A
USGS Report: Geochemical Investigation of the
Hydrothermal System on Akutan Island, Alaska, July 2012
U.S. Department of the Interior
U.S. Geological Survey
Scientific Investigations Report 2013–5231
Geochemical Investigation of the Hydrothermal System
on Akutan Island, Alaska, July 2012
Cover. View to the southeast of the main stem of Hot Springs Creek, showing the outflow from the group A hot springs (lower far
right edge of image) and steam from group B hot spring (near center). U.S. Geological Survey photograph by Deborah Bergfeld,
July 28, 2012.
Geochemical Investigation of the
Hydrothermal System on Akutan Island,
Alaska, July 2012
By Deborah Bergfeld, Jennifer L. Lewicki, William C. Evans, Andrew G. Hunt,
Kinga Revesz, and Mark Huebner
Scientific Investigations Report 2013–5231
U.S. Department of the Interior
U.S. Geological Survey
U.S. Department of the Interior
SALLY JEWELL, Secretary
U.S. Geological Survey
Suzette M. Kimball, Acting Director
U.S. Geological Survey, Reston, Virginia: 2014
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Suggested citation:
Bergfeld, D., Lewicki, J.L., Evans, W.C., Hunt, A.G., Revesz, K., and Huebner, M., 2014, Geochemical investigation
of the hydrothermal system on Akutan Island, Alaska, July 2012: U.S. Geological Survey Scientific Investigations
Report 2013–5231, 20 p., http://dx.doi.org/10.3133/sir20135231.
ISSN 2328-0328 (online)
iii
Contents
Abstract ...........................................................................................................................................................1
Introduction.....................................................................................................................................................1
Methods...........................................................................................................................................................3
Site Descriptions ............................................................................................................................................3
Hot Springs.............................................................................................................................................3
Other Waters .........................................................................................................................................8
Gas Vents ...............................................................................................................................................8
Results .............................................................................................................................................................9
Water and Gas Chemistry ....................................................................................................................9
Discharge ...............................................................................................................................................9
Discussion .......................................................................................................................................................9
Hot-Spring Water Geochemistry ........................................................................................................9
Hot-Spring Geothermometry .............................................................................................................12
Gases at Akutan ..................................................................................................................................12
Thermal Chloride Flux.........................................................................................................................15
Change in the Akutan Hydrothermal System .................................................................................16
Summary........................................................................................................................................................16
Acknowledgments .......................................................................................................................................18
References Cited..........................................................................................................................................18
Figures
1. Topographic map of northeastern part of Akutan Island, showing locations of
caldera and modern cinder cone of Akutan Volcano, flank fumarole field,
and Akutan village ........................................................................................................................2
2. Photos showing features in Hot Springs valley. Aerial photograph of Hot
Springs valley, with steam plume visible from group C hot spring. Vent-type
hot spring in group A. Pool-type hot spring with gas bubbles in group B ...........................8
3. Scatterplots showing positive correlation between Cl concentration and
other components in Akutan hot-spring waters and negative correlation
between Cl and Mg concentrations ........................................................................................13
4. Plots of isotopic composition and chloride concentrations for Akutan hot
springs and local cold creek water in 2012. δD versus δ18O values of hot springs
and cold creek water relative to World Meteoric Water Line. Cl concentration
versus δ18O values in hot-spring water, showing positive correlation ..............................14
5. Ternary N2-He-Ar diagram for gas samples collected at Akutan Volcano
in 1981, 1996, and 2012 ...............................................................................................................15
6. Scatterplots showing differences in water chemistry for Akutan hot springs
in 1980–81 and 2012 ....................................................................................................................17
7. Schematic cross section of hydrothermal system on Akutan Island ..........................................18
iv
Tables
1. Sample-collection parameters, chemical analyses and isotope values for
waters from Akutan Volcano, Alaska sampled during 1996 and 2012..................................4
2. Sample locations, gas chemistry and noble-gas data for degassing features
around Akutan Volcano, Alaska, sampled during 2012 ........................................................10
3. Chloride concentrations and stream discharges used to determine geothermal
flux and heat output from hot springs along sections of Hot Springs Creek
near Akutan Volcano, Alaska, in 1981 and 2012.....................................................................12
Conversion Factors
SI to Inch/Pound
Multiply By To obtain
Length
centimeter (cm)0.3937 inch (in.)
meter (m)3.281 foot (ft)
kilometer (km)0.6214 mile (mi)
Area
square meter (m2)0.0002471 acre
square meter (m2)10.76 square foot (ft2)
square kilometer (km2)0.3861 square mile (mi2)
Volume
liter (L)1.057 quart (qt)
liter (L)0.2642 gallon (gal)
Flow rate
liter per second (L/s)15.85 gallon per minute (gal/min)
Mass
gram (g)0.03527 ounce, avoirdupois (oz)
kilogram (kg)2.205 pound avoirdupois (lb)
metric tonnes per day (t/d)1.102 tons per day (ton/d)
Energy
joule (J)0.0000002 kilowatthour (kWh)
Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:
°F=(1.8×°C)+32
Vertical coordinate information is referenced to the World Geodetic System of 1984 (WGS 84).
Horizontal coordinate information is referenced to the World Geodetic System of 1984 (WGS 84).
Altitude, as used in this report, refers to distance above the vertical datum.
v
Abbreviations
ASMW Air-saturated meteoric water
DIC Dissolved inorganic carbon
EFHSC East fork of Hot Springs Creek
FF Flank fumarole field
Q Discharge
RC/RA 3He/4He corrected for atmospheric air
TI Thermal influx
VPBD Vienna Pee Dee Belemnite
VSMOW Vienna Standard Mean Ocean Water
WMWL World Meteoric Water Line
Chemical unit abbreviations
J/g Joules per gram
mS/cm millisiemens per centimeter
μS/cm microsiemens per centimeter
MW megawatts
Chemical symbols and chemical species abbreviations
Ar Argon
B Boron
Br Bromine; bromide (Br–)
Ca Calcium
CH4 Methane
Cl Chlorine; chloride (Cl–)
CO2 Carbon dioxide
δ13C 13C/12C in sample compared to that of a standard reference rock
δ18O 18O/16O in sample compared to that of standard mean ocean water
δD 2H/1H in sample compared to that of standard mean ocean water
H2 Molecular hydrogen
H2O Water
H2S Hydrogen sulfide
HCO3 Bicarbonate (HCO3
–)
He Helium
3He Helium-3 atom
K Potassium
Li Lithium
Mg Magnesium
N2 Molecular nitrogen
Na Sodium
Si Silicon
SiO2 Silica
SO4 Sulfate
vi
Geochemical Investigation of the Hydrothermal System on
Akutan Island, Alaska, July 2012
By Deborah Bergfeld, Jennifer L. Lewicki, William C. Evans, Andrew G. Hunt,
Kinga Revesz, and Mark Huebner
Abstract
We have studied the geochemistry of the hot springs on
Akutan Island in detail for the first time since the early 1980s.
Springs in four discrete groups (A-D) along Hot Springs Creek
showed generally higher temperatures and substantially higher
Na, Ca, and Cl concentrations than previously reported, and
total hot-spring discharge has also increased markedly. The
springs now account for a heat output of ~29 MW, about an
order of magnitude more than in 1981. Gas samples from the
hot springs and from a fumarolic area on the flank of Akutan
Volcano show high 3He/4He ratios (>6.4 RA) after correction
for air contamination and reveal a common magmatic heat
source. Hot-spring gases are unusually rich in N2, Ar, and
CH4, suggesting that the water has boiled and lost CO2 during
upflow beneath the flank fumarole field. Gas geothermom-
etry calculations applied to the flank fumarole field implies
temperatures of 200–240 °C for the reservoir, and Na-K-Ca
geothermometry implies temperatures near 180 °C for the out-
flow waters that feed the hot springs. The results of our study
confirm the existence of a substantial geothermal resource on
the island.
Introduction
Akutan Volcano is an active stratovolcano in the east-
central Aleutian Islands that has erupted at least 27 times since
the late 1700s (Finch, 1935; Byers and Barth, 1953; Waytho-
mas, 1999). The most recent eruption, in 1992, was followed
by a seismic crisis in March 1996 (McGimsey and others,
1995; Waythomas and others, 1998). The summit caldera, at
an elevation of 1,100 m, contains an active cone and ice-
covered lakes (Waythomas and others, 1998). During our site
visit in July 2012, we observed degassing from fumaroles and
steaming ground on the cone.
Surface expressions of the hydrothermal system on
Akutan Island include a ~5,000-m2 fumarole field (Motyka
and others, 1988) on the northeast flank of the volcano, at an
elevation of ~400 m, and a series of hot springs that dis-
charge at elevations close to sea level along lower parts of
the northeast-trending Hot Springs valley (fig. 1). Additional
warm water discharges from diffuse seeps at the mouth of Hot
Springs Bay.
Reconnaissance surveys of the hot springs on Akutan
Island began as early as 1953 (Byers and Barth, 1953). The
first detailed geochemical and geophysical investigations to
assess geothermal potential occurred during the early 1980s
(Motyka and Nye, 1988). Motyka and others (1988) catego-
rized the hot springs as belonging to one of five groups (A-E,
fig. 1) from southwest to northeast. Between 1980 and 1983,
Motyka and coworkers collected water and gas samples from
the springs and made discharge and load measurements of
the creek above and below the hot-spring inputs; they also
collected gas from the flank fumarole field. Additional inves-
tigations at Akutan took place in 1996, several months after
the seismic crisis (Symonds and others, 2003a, b). Studies at
Akutan beginning in 2009 were related to renewed interest in
geothermal development of the Akutan hydrothermal system
for use by the City of Akutan and other population centers
on the island. (Kolker and Mann, 2009; Kolker and others,
2012). That work included geochemical sampling, geophysical
studies, and the drilling of two small-diameter temperature-
gradient wells in Hot Springs valley (Kolker and others, 2012)
but no new investigation of the hot-spring chemistry.
We report here the results of a 5-day survey of the hydro-
thermal system on Akutan Island during July 2012. Samples of
gas and water were collected from the hot springs, and gas was
collected from fumaroles on the flank of the volcano and from
an area of acid-altered steaming ground on the cone. Fuma-
roles were also present on the cone but not readily accessible.
We made discharge measurements on the main stem of Hot
Springs Creek and two of its tributaries (fig. 1).
2 Geochemical Investigation of the Hydrothermal System on Akutan Island, Alaska, July 2012
3 MILES3 KILOMETERS001 MILE1 KILOMETER00ALASKAAkutan54° 10’166° 00’165° 50’FFRFFmodern cinder conecalderaHot Springs BayHot Springs BayHot Springs CreekHot Springs BayAkutan Village1,5001,0001,0002,0002,0002,5003,0003,5004,0003,0001,0005005003,0001,5001,5001,0001,5002,0002,500SGSDBACE414181311512152016T21317910500500Flank fumarole fieldLake IceLand-surface contour–Shows altitude of land surface in feet above NGVD 29. Contour interval 500 feetFault–Dashed where inferred, dotted where concealed Tributary gaging siteCreek sampling site location and numberGaged Not gaged Sample sites and 2012 sample numbers Hot spring group A Hot spring group B Hot spring group C Hot spring group DHot spring group E, tidal zone seepEXPLANATIONT18153512217500Figure 1. Topographic map of northeastern part of Akutan Island (after Richter and others, 1998), showing locations of caldera and modern cinder cone of Akutan Volcano, flank fumarole field, and Akutan village. A, B, C, D, and E, hot spring groups defined by Motyka and others (1988) and retained in this study. FF, flank fumarole field; S, snow sample location; R, rain sample location; SG, area of summit gas discharge.
Site Descriptions 3
Methods
General information on sample collection and analytical
techniques was presented by Bergfeld and others (2011). We
used digital meters to measure water temperature, specific
conductance and pH at each water-sampling site. Sampling-
site locations were determined by using a handheld Global
Positioning System device. Whenever possible, hot-spring
waters were collected from the spring orifice, but at springs
with deep pools we collected from the side of the pool. We
collected two samples of filtered (0.45 µm) water in plastic
bottles for bulk chemistry (including major ions and trace
metals), and two samples of raw water in glass bottles for
stable isotope (δD, δ18O) analysis and for determination of
alkalinity at all of the springs and most of the creeks. At five
springs we collected an additional sample of filtered water,
using a 0.1-µm filter specifically for determination of Al
concentrations. Samples for cation analysis were acidified
in the field to pH <2, using ultrapure nitric acid. Alkalinity
titrations were performed by using sulfuric acid and a digital
titrator. The titrations were typically performed on the day of
sample collection, but when this procedure was impossible,
the sample was stored in a refrigerator.
At five of the hot springs we collected a 60-mL water
sample for determination of dissolved inorganic carbon (DIC)
concentration. The water was collected in a syringe and
injected into a preweighed evacuated glass tube through a
rubber septum and acidified in the field by injecting 0.5 mL of
6N HCl.
No spring produced a vigorous gas upflow, but four
springs had sufficient gas discharge to allow for sample collec-
tion into an evacuated tube. The gas was collected by con-
necting the sample bottle to the neck of a funnel and placing
the mouth of the funnel over the bubble train. Narrow-gauge
tubing was threaded through the funnel into the neck of the
sample bottle, attached to a syringe, and used to pump away
trapped atmospheric gas. After the atmospheric components
were purged, the small tubing was removed, and the bottle was
opened and allowed to fill. At two of the group A hot springs
we collected a second aliquot of gas for analysis of noble-gas
ratios including 3He/4He. These samples were collected in a
copper tube sealed at both ends with refrigeration clamps.
Three types of gas samples were collected from fuma-
roles and steaming ground on the flanks and cone of Akutan
Volcano. At some sampling sites, the gas was collected into
empty evacuated bottles, using a funnel or metal tube to focus
the gas flow. Before opening the sample bottle, we flowed
gas through the collection apparatus to purge any trapped
atmospheric gas. Gas collected in evacuated bottles was used
for determination of bulk chemistry (for example, CO2, CH4,
CO, H2) and for determination of the δ13C composition of CO2
and CH4. When a sufficient quantity of steam condensed in
the bottles, the water was sent for δD and δ18O analysis. We
also collected gas in evacuated bottles containing 4N NaOH
solution. This method was used for determinations of bulk
chemistry, concentrations of trace-gas species, and H2S, which
might not be preserved in the evacuated bottles. A third set
of gas samples was collected in copper tubes for analysis of
noble-gas ratios.
Discharge was measured at four sites, using a standard
USGS wading rod and pygmy meter. The sites include Hot
Springs Creek above the group A hot-spring inputs and below
the group D inputs, a tributary containing the outflow from
group A hot springs, and a tributary flowing from the east side
of the valley that discharges into Hot Springs Creek (fig. 1).
Water samples collected at three of the discharge-measurement
sites were used to calculate the flux of dissolved constituents.
Chemical analyses of gas and water samples were deter-
mined at USGS laboratories in Menlo Park, Calif. Gases were
analyzed by using gas chromatographs equipped with thermal-
conductivity and flame-ionization detectors. Water samples
were analyzed for anions by ion chromatography, and for
cations by argon plasma optical-emission spectrometry. Stable
isotope analyses of waters, steam, DIC, CO2, and CH4 were
performed by mass spectrometry at the USGS Stable Isotope
Laboratory in Reston, Va. Noble-gas ratios were determined
by mass spectrometry at the USGS Noble Gas Laboratory in
Denver, Colo. Reported 3He/4He ratios are corrected for minor
amounts of air and are given as RC/RA values.
Site Descriptions
Hot Springs
On the basis of the map by Motyka and others (1988),
the layout of the hot springs in 2012 was apparently much like
that in the 1980s. In this report, we retain their nomenclature
for the hot-spring groups (fig. 1). Group E waters are thermal
seeps in the intertidal zone that discharge diffusely and mix
with seawater. We report the water chemistry for one group E
site in table 1 but omit additional discussion of those data.
Hot-spring groups A through D occur along a ~850-m
section of lower Hot Springs valley at elevations close to
sea level (fig. 1; table 1). Most of the hot springs discharge
from the northwest side of Hot Springs Creek (fig. 2A). Some
hot springs discharge from shallow discrete vents along the
margin of volcaniclastic deposits (fig. 2B), and others originate
on the valley floor and collect in pools that drain into the creek
(fig. 2C). Water temperatures ranged from 60 °C to ~100 °C,
with the higher temperatures at vent-type springs.
The uppermost hot springs (group A, fig. 1) consist
of two vents and a pool that are closely clustered and dis-
charge into a small tributary stream on the west side of Hot
Springs Creek. We collected water from all three hot springs
(AKU12-01, AKU12-02, AKU12-03, table 1), and gas from
the pool and the lowermost vent (AKU12-01 and AKU12-03,
respectively). Water in the pool was cool enough to support
aquatic plants, but the flow was sufficient that the surface of
the pool was clear.
4 Geochemical Investigation of the Hydrothermal System on Akutan Island, Alaska, July 2012
Table 1. Sample-collection parameters, chemical analyses and isotope values for waters from Akutan Volcano, Alaska,
sampled during 1996 and 2012.
[All analyses in milligrams per liter except as noted. Stable isotope values in per mil (‰) relative to Vienna Standard Mean Ocean Water-Standard
Light Antarctic Precipitation (VSMOW-SLAP) for δD and δ18O or relative to Vienna Pee Dee Belemnite-Space Vacuum Epitaxy Center (VPDB-SVEC) for δ13C;
---, no data. Water geothermometers: chalcedony (TCH) and quartz conductive (TQC) from Fournier (1981); Na-K-Ca (TNKC) from Fournier and Truesdell (1973).
Datum for Universal Transverse Mercator (UTM) coordinates is referenced to WGS84 zone 3U. Analytical uncertainties ±5 percent at 1σ level for major species;
0.2 and 2 per mil at 2σ level for δ18O and δD, respectively]
Sample
number Description Date Easting
(meters)
Northing
(meters)
Elevation
(meters)
Temp.
(°C)
Cond.
(µS/cm)pH δ18O
(‰)
δD
(‰)
Hot springs
AKU12-01 Group A pool 07/25/2012 444006 6000806 18 60.5 1,920 6.44 -9.2 -69
AKU12-02 Group A upper vent 07/25/2012 443989 6000805 16 84.3 3,200 6.33 -8.6 -69
AKU12-03 Group A lower vent 07/25/2012 444032 6000819 18 94.0 3,050 7.00 -8.7 -69
AKU12-04 Group B upper pool 07/25/2012 444132 6000890 19 76.8 2,300 6.67 -8.9 -69
AKU12-11 Group B lower pool 07/27/2012 444255 6001052 14 75.2 1,472 6.49 -9.3 -69
AKU12-05
Group C
geysering vent
07/25/2012 444349 6001222 18 100.8 3,240 6.69 -8.4 -67
AKU12-13
Group C
east bank vent
07/27/2012 444461 6001338 18 73.0 1,836 6.88 -9.2 -69
AKU12-14
Group C
west bank vent
07/27/2012 444339 6001217 18 99.5 2,670 6.65 -8.9 -69
AKU12-12 Group D vent spring 07/27/2012 444498 6001442 16 83.8 2,360 7.47 -8.9 -69
Cold waters
AKU12-15 Upper Hot
Springs Creek
07/28/2012 441925 5999860 73 5.3 55 7.49 -10.7 -74
AKU12-16
Hot Springs Creek,
lower gage
07/29/2012 444721 6001526 54 10.5 266 6.86 ------
AKU12-18
Upper Hot Springs
Creek at gage
07/29/2012 443357 6000037 31 5.8 60.2 7.02 ------
AKU12-19 Snow 07/30/2012 439151 5996100 505 ---33.4 ---------
AKU12-20 East Fork Hot
Springs Creek
07/30/2012 444634 6001377 16 5.7 115 6.71 ------
AKU12-21 Rain; Akutan Harbor 07/29/2012 449488 5998752 10 ---------------
Other
AKU12-17 Seep on beach 07/29/2012 444833 6001800 8 49.4 14,400 5.79 -6.3 -46
1996 Akutan waters
AK-01 Vig. bubbling spring 7/27/1996 ---------97.4 2,490 6.89 -9.2 -69
AK-08 Green pool 7/30/1996 ---------60.6 1,250 6.94 -9.6 -67
AK-11 Small green pool 7/30/1996 ---------64.2 2,470 6.49 -9.5 -65
AK-02 Acidic water in
fumarole field
7/28/1996 ---------90.7 2,290 2.50 -8.4 -57
Site Descriptions 5
Table 1. Sample-collection parameters, chemical analyses and isotope values for waters from Akutan Volcano, Alaska,
sampled during 1996 and 2012.—Continued
[All analyses in milligrams per liter except as noted. Stable isotope values in per mil (‰) relative to Vienna Standard Mean Ocean Water-Standard
Light Antarctic Precipitation (VSMOW-SLAP) for δD and δ18O or relative to Vienna Pee Dee Belemnite-Space Vacuum Epitaxy Center (VPDB-SVEC) for δ13C;
---, no data. Water geothermometers: chalcedony (TCH) and quartz conductive (TQC) from Fournier (1981); Na-K-Ca (TNKC) from Fournier and Truesdell (1973).
Datum for Universal Transverse Mercator (UTM) coordinates is referenced to WGS84 zone 3U. Analytical uncertainties ±5 percent at 1σ level for major species;
0.2 and 2 per mil at 2σ level for δ18O and δD, respectively]
Sample
number
δ13C-DIC
(‰)Al As B Ba Br Ca Cd Cl Co CO3 Cr Cu
Hot springs
AKU12-01 ---<0.03 0.76 14.0 0.04 1.6 37.4 0.01 530 <0.006 0 <0.006 <0.006
AKU12-02 ---<0.055 1.51 25.1 0.05 3 61.8 0.02 980 <0.011 0 <0.011 <0.011
AKU12-03 -10.7 <0.055 1.50 23.4 0.04 2.8 41.0 0.02 920 <0.011 0 <0.011 <0.011
AKU12-04 -11.6 <0.055 1.16 18.1 0.02 2.1 45.1 0.01 700 <0.011 0 <0.011 <0.011
AKU12-11 -11.7 <0.03 0.55 10.1 0.05 1.2 39.1 <0.006 410 <0.006 0 <0.006 <0.006
AKU12-05
-9.9 <0.055 1.69 25.2 0.04 3.3 73.2 0.02 1100 <0.011 0.1 <0.011 <0.011
AKU12-13
---<0.03 0.57 12.1 0.26 1.5 96.5 <0.006 510 <0.006 0.1 <0.006 <0.006
AKU12-14
---<0.055 1.22 17.9 0.04 2.7 52.1 0.01 880 <0.011 0 <0.011 <0.011
AKU12-12 -11.4 <0.055 0.63 16.2 <0.011 2.5 64.5 <0.011 800 <0.011 0 <0.011 <0.011
Cold waters
AKU12-15 ---0.01 <0.001 0.01 0.01 0.0039 5.3 <0.001 3.6 <0.001 0 <0.001 <0.001
AKU12-16
---0.01 0.05 1.20 0.01 0.16 11.7 <0.001 58 <0.001 0 <0.001 <0.001
AKU12-18
---0.01 <0.001 0.01 0.01 0.006 6.0 <0.001 4.3 <0.001 0 <0.001 <0.001
AKU12-19 ---0.05 <0.001 0.01 0.002 0.0014 0.57 <0.001 6.8 <0.001 ---<0.001 0.005
AKU12-20 ---<0.005 0.00 0.13 0.01 0.026 10.0 <0.001 13 <0.001 ---<0.001 <0.001
AKU12-21 ---------------0.057 ------12 ------------
Other
AKU12-17 <0.25 0.36 6.30 <0.05 23 209.5 <0.05 7,300 <0.05 0 <0.05 <0.05
1996 Akutan waters
AK-01 ---0.10 1.30 14.1 ---2.24 32.0 ---623 ---0 ------
AK-08 ---0.07 0.325 5.31 ---0.89 27.8 ---252 ---0 ------
AK-11 ---0.13 0.87 10.2 ---1.42 52.7 ---426 ---0 ------
AK-02 ---25.1 0.0051 0.059 ---<0.02 26.0 ---3.66 ---0 ------
6 Geochemical Investigation of the Hydrothermal System on Akutan Island, Alaska, July 2012
Table 1. Sample-collection parameters, chemical analyses and isotope values for waters from Akutan Volcano, Alaska,
sampled during 1996 and 2012.—Continued
[All analyses in milligrams per liter except as noted. Stable isotope values in per mil (‰) relative to Vienna Standard Mean Ocean Water-Standard
Light Antarctic Precipitation (VSMOW-SLAP) for δD and δ18O or relative to Vienna Pee Dee Belemnite-Space Vacuum Epitaxy Center (VPDB-SVEC) for δ13C;
---, no data. Water geothermometers: chalcedony (TCH) and quartz conductive (TQC) from Fournier (1981); Na-K-Ca (TNKC) from Fournier and Truesdell (1973).
Datum for Universal Transverse Mercator (UTM) coordinates is referenced to WGS84 zone 3U. Analytical uncertainties ±5 percent at 1σ level for major species;
0.2 and 2 per mil at 2σ level for δ18O and δD, respectively]
Sample
number F Fe HCO3 K Li Mg Mn Mo Na NH4 Ni NO3-N1 PO4-P1
Hot springs
AKU12-01 0.4 0.86 111 29.2 0.94 5.84 0.24 <0.006 325 <0.006 0.1 <0.1
AKU12-02 0.5 0.03 111 49.2 1.72 4.55 0.25 <0.011 578 ---<0.011 0.1 <0.1
AKU12-03 0.7 0.03 97 48.0 1.62 2.93 0.18 <0.011 557 ---<0.011 0.4 <0.1
AKU12-04 0.6 1.10 111 38.6 1.25 4.03 0.25 <0.011 434 ---<0.011 0.1 <0.1
AKU12-11 0.4 1.70 128 26.1 0.63 8.04 0.51 <0.006 246 ---<0.006 0.2 <0.1
AKU12-05 0.8 0.04 44 51.4 1.56 2.04 0.16 <0.011 636 ---<0.011 0.3 <0.1
AKU12-13 0.3 0.75 134 32.9 0.64 17.1 0.75 <0.006 228 ---<0.006 <0.1 <0.1
AKU12-14 0.6 0.35 42 40.5 1.27 3.89 0.26 <0.011 494 ---<0.011 0.1 <0.1
AKU12-12 0.5 <0.022 81 37.8 1.14 4.47 0.05 <0.011 452 ---<0.011 <0.1 <0.1
Cold waters
AKU12-15 0.03 0.04 16 0.34 <0.001 0.86 0.02 <0.001 3 ---<0.001 0.02 <0.01
AKU12-16 0.06 0.35 35 3.17 0.08 2.02 0.10 <0.001 33 ---<0.001 0.01 <0.01
AKU12-18 0.04 0.03 18 0.38 <0.001 1.00 0.02 <0.001 4 ---<0.001 0.02 <0.01
AKU12-19 0.01 0.03 ---0.82 <0.001 0.50 0.03 <0.001 4 ---<0.001 0.03 <0.01
AKU12-20 0.04 0.33 ---0.68 0.00 2.12 0.08 <0.001 9 ---<0.001 <0.01 <0.01
AKU12-21 0.074 ------------------------------0.89 <0.065
Other
AKU12-17 bdl <0.1 69 137 1.51 610 0.28 <0.05 3,578 <0.05 <1 <1
1996 Akutan waters
AK-01 0.81 0.08 176 39.2 1.40 2.74 0.15 ---433 0.99 ---<0.05 ---
AK-08 0.41 <0.01 117 15.0 0.48 2.90 <0.01 ---184 0.55 ---0.35 ---
AK-11 0.60 1.66 159 27.6 0.83 5.79 0.35 ---266 1.33 ---<0.02 ---
AK-02 0.22 41.0 0 2.59 <0.01 8.92 0.71 ---14.1 12.8 ---<0.02 ---
Site Descriptions 7
Table 1. Sample-collection parameters, chemical analyses and isotope values for waters from Akutan Volcano, Alaska,
sampled during 1996 and 2012.—Continued
[All analyses in milligrams per liter except as noted. Stable isotope values in per mil (‰) relative to Vienna Standard Mean Ocean Water-Standard
Light Antarctic Precipitation (VSMOW-SLAP) for δD and δ18O or relative to Vienna Pee Dee Belemnite-Space Vacuum Epitaxy Center (VPDB-SVEC) for δ13C;
---, no data. Water geothermometers: chalcedony (TCH) and quartz conductive (TQC) from Fournier (1981); Na-K-Ca (TNKC) from Fournier and Truesdell (1973).
Datum for Universal Transverse Mercator (UTM) coordinates is referenced to WGS84 zone 3U. Analytical uncertainties ±5 percent at 1σ level for major species;
0.2 and 2 per mil at 2σ level for δ18O and δD, respectively]
Sample
number Rb Se SiO2 SO4 Sr Zn TDS Cation Anion Balance TCH(°C)
TQC(°C)
TNKC(°C)
Hot springs
AKU12-01 0.15 <0.006 122 22 0.39 <0.012 1,202 17.4 17.3 0.9%123 148 181
AKU12-02 0.26 <0.011 155 31 0.69 <0.022 2,004 30.1 30.2 -0.3%140 163 184
AKU12-03 0.26 <0.011 158 37 0.56 0.025 1,891 27.9 28.4 -1.5%141 164 188
AKU12-04 0.19 <0.011 147 29 0.58 <0.022 1,534 22.7 22.2 2.0%146 160 184
AKU12-11 0.14 <0.006 161 17 0.47 <0.012 1,050 14.2 14.1 0.8%136 166 184
AKU12-05 0.30 <0.011 169 54 1.14 <0.022 2,165 33.0 33.0 0.1%129 169 181
AKU12-13 0.13 <0.006 134 28 0.95 <0.012 1,198 17.1 17.2 -0.4%142 154 192
AKU12-14 0.22 <0.011 140 43 0.83 <0.022 1,721 25.6 26.5 -3.2%133 157 180
AKU12-12 0.20 <0.011 141 41 0.85 <0.022 1,643 24.4 24.8 -1.7%133 157 178
Cold waters
AKU12-15 <0.01 <0.001 7 5.5 0.03 <0.002 42 0.5 0.5 3.6%---------
AKU12-16 0.01 <0.001 21 6.7 0.10 0.005 172 2.3 2.4 -2.8%---------
AKU12-18 <0.01 <0.001 8 5.3 0.04 0.002 47 0.6 0.5 8.1%---------
AKU12-19 <0.01 <0.005 0.28 1.1 0.01 0.064 14 0.3 ---------------
AKU12-20 <0.01 <0.001 15 3.2 0.05 0.008 53 1.1 ---------------
AKU12-21 ---------5.7 ---------------------------
Other
AKU12-17 <0.5 <0.05 116 970 4 <0.1 13,024 217.3 224.5 -3.3%---------
1996 Akutan waters
AK-01 ------175 49.0 0.42 ---1,552 21.9 22.2 -1.4%149 171 188
AK-08 ------88.6 33.0 0.31 ---729 10.1 10.0 1.2%103 131 168
AK-11 ------126 36.0 0.52 ---1,118 15.7 15.9 -1.4%126 151 182
AK-02 ------121 662 0.11 ---918 11.6 13.9 -18.0%122 148 34
1Nitrate is reported as milligrams per liter nitrogen; phosphate reported as milligrams per liter phosphorus.
8 Geochemical Investigation of the Hydrothermal System on Akutan Island, Alaska, July 2012
The hot springs in groups B, C, and D (fig. 1) discharge
into the main stem of Hot Springs Creek. We sampled gas
and water at two pools in group B ~200 m apart (AKU12-04,
AKU12-11, table 1). Some pools of thermal water along the
B-C traverse were either shallow and muddy or stagnant and
choked with mats of vegetation. Given time constraints, these
waters were not sampled.
No significant gas was discharging at any of the hot
springs below group B (fig. 1). All of the samples collected
downstream of the group B hot springs were from vents. We
sampled three group C hot springs along the edge of the creek
(AKU12-05, AKU12-13, AKU12-14) and a single group D hot
spring (AKU12-12) that discharged close to the creek. Two
of the group C hot springs on the west side of Hot Springs
Creek were boiling. At one site, the boiling water created a
small geyser that fountained to ~40 cm, with a highly visible
steam cloud. The water filled its basin and roiled violently for
more than a minute before draining away. The fill-drain cycle
repeated within 5 to 10 minutes.
Other Waters
Creek waters were collected concurrently with gaging
at Hot Springs Creek below the hot springs (AKU12-16,
table 1) at a tributary that flows in from the east (AKU12-20;
denoted as the east fork of Hot Spring Creek [EFHSC] by
Motyka and others, 1988) and at Hot Springs Creek above Hot
Springs valley (AKU12-18, fig. 1). We also collected creek
water above the upper gage site (AKU12-15), two samples of
precipitation that included “old” snow on the northwest flank
of an unnamed peak ~4 km southeast of the summit caldera,
and a rain sample along the coast in the village of Akutan
(AKU12-19 and AKU12-21, respectively, fig. 1).
Gas Vents
We observed gas discharging at several sites on the
modern cinder cone within the summit caldera, the most
accessible of which was a steeply sloping area of acid-altered
ground (SG, fig. 1). A few spots had a focused discharge, but
the overall discharge was weak across a broad area encrusted
with sulfur-bearing minerals. The ground surface was dry, but
the steam upflow created muddy conditions under the first few
centimeters of crust.
The area of degassing vents on the flank of the volcano
(FF, fig. 1) was described in earlier studies as containing “geo-
thermal fumaroles,” distinct from the “volcanic fumaroles” on
the summit cone (Motyka and others, 1993). This area is larger
and more active than the degassing sites on the cone. Large
plumes of steam discharge from fumaroles and roiling mud
pots, rivulets of acidic water flow through the area, and the
ground surface is acid-altered with little vegetation. In 2012,
we observed evidence of a recent mudflow from the lower part
of the active area down into a ravine. The 2012 sampling sites
included a vent fumarole with a strong focused discharge and
a drowned fumarole, herein called a frying pan.
A
B
C
Figure 2. Photos showing features in Hot Springs valley. A,
Aerial photograph of Hot Springs valley, with steam plume
visible from group C hot spring. View northwestward. B, Vent-
type hot spring (AKU12-02) in group A. C, Pool-type hot spring
with gas bubbles (AKU12-04) in group B.
Discussion 9
Results
Water and Gas Chemistry
The locations and chemical data for waters sampled in
2012 are listed in table 1, which includes four previously
unpublished analyses of Akutan waters collected by R.B.
Symonds in 1996. The 1996 data lack precise coordinates, and
so we cannot relate the results to a specific hot-spring group.
The hot springs are composed of neutral-chloride geothermal
waters (Goff and Janik, 2000) and have Cl concentrations
ranging from 410 to 1,100 mg/L, with Na as the main cation.
In comparison, the local meteoric waters show a mixed cation
composition and have Cl and SO4 concentrations as high
as 12.0 and 5.7 mg/L, respectively, greater than for typical
continental precipitation (Root and others, 2005) but much
lower than in the hot springs. The water in EFHSC contained
13 mg/L Cl, just slightly greater than in the rain sample. The
Cl concentration in Hot Springs Creek at the lower gage
(downstream of all the hot springs and the confluence with
EFHSC) was 58 mg/L, demonstrating a considerable flux of
geothermal chloride contributed from the springs.
The δD and δ18O values of the spring waters range from
-69 to -67 per mil and from -9.3 to -8.4 per mil, respectively,
relative to Vienna Standard Mean Ocean Water (VSMOW),
in comparison with the values of –74 and –10.7 per mil,
respectively, of creek water (table 1). DIC concentrations
of waters in the five sampled hot springs are relatively low
(0.5–3.7 µmol CO2/mg H2O), with δ13C-DIC values ranging
from -11.7 to -9.9 per mil relative to Vienna Pee Dee Belem-
nite (VPDB).
The chemical compositions of the collected gases are
diverse (table 2). Gas from the crater and the hot-spring
pools has high N2 and Ar concentrations. In general, these
samples are not air contaminated; only one sample (AK12-01,
table 2) contained significant oxygen. Gas from the pools is
distinguishable from crater gas by having high CH4 and low
H2S concentrations. Gas from the group A vent hot spring
(AKU12-03) is similar to that from the flank fumarole field in
its high CO2 and much lower N2 concentrations. He concen-
trations are generally low in all gas samples. He isotopes in
the gas from group A hot springs and the flank fumarole have
RC/RA ratios >6.4 that show strong inputs of 3He from a mag-
matic source.
Discharge
Discharge measurements on Hot Springs Creek above
and below the hot springs, on a small tributary to Hot Springs
Creek that receives the discharge of the group A hot springs,
and on EFHSC near the confluence with Hot Springs Creek
(fig. 1) are listed in table 3. The upstream discharge in Hot
Springs Creek was 890 L/s, and the downstream discharge was
1,348 L/s. The discharge of EFHSC was 245 L/s, suggesting
that the hot springs could add as much as ~200 L/s to the flow
of Hot Springs Creek.
Discussion
Hot-Spring Water Geochemistry
Chloride is a useful tracer of source fluids because it
behaves conservatively and Cl concentrations are low in
meteoric water and relatively high in the hot springs. Our
results indicate that large variations in Cl concentrations
occur within each hot-spring group and that pool waters typi-
cally have lower Cl concentrations than the waters in vent
hot springs (table 1). We observed no correlation between Cl
concentrations and the locations of springs along the flowpath
of Hot Spring Creek. In contrast, strong positive correlations
(R2 = 0.86–0.99) exist between Cl and Br, Na, B, Li, and
specific conductance (fig. 3). The strong correlations between
Cl and the conservative species Br and B suggest that all of
the hot-spring waters are varyingly diluted from a common
source that is not seawater. Concentrations of less conservative
species, such as SO4, Si, and Ca, correlate somewhat with Cl
but are more scattered.
Waters in high-temperature geothermal systems typically
have low Mg contents (Nicholson, 1993). The water from a
group C vent hot spring that discharges from the east side of
the creek is unusual in that it had relatively high Mg con-
centrations (AKU12-13, table 1). Excluding that site, a clear
negative correlation exists between Mg and Cl in the spring
waters (R2 = 0.80, fig. 3). Extrapolation of the Mg trendline to
zero suggests that the source fluid has a Cl concentration of
~1,300 mg/L.
Figure 3A shows that the δ18O values of hot-spring
waters are shifted to the right of the World Meteoric Water
Line (WMWL) by ~1.0 to 1.5 per mil. Although such a
shift could indicate a component of magmatic water, similar
shifts have been observed in numerous geothermal systems
with neutral-chloride waters and result from oxygen-isotope
exchange between the reservoir rocks and hydrothermal
fluids (Craig, 1963; Sheppard, 1986). Typically, no correla-
tive change occurs in δD values because rocks contain little
deuterium, and the hydrogen-isotope systematics are buffered
by the water (Craig, 1963; Sheppard, 1986).
Examination of the hot-spring waters on Akutan Island
shows slight differences in the isotopic composition of pools
and vents. Water in the geysering spring has the highest δD
and δ18O values and is an outlier relative to the other hot
springs. These high values could result from vigorous boil-
ing and loss of isotopically light steam. Water in the pools has
slightly lower δ18O values than that in most vent hot springs.
The shift cannot be a function of evaporation, which would
raise δD and δ18O values, and likely indicates that the pools
are mixed with slightly more meteoric water than the vent hot
springs.
We note that, except for the geysering spring, the
waters have a limited range of δD values, from –69.4 to
–68.6 per mil. We can extrapolate back to the WMWL and
derive an estimate of –9.95 to –9.80 per mil for the δ18O con-
tent of the cold groundwater that dilutes the hot springs and
pools (fig. 4A).
10 Geochemical Investigation of the Hydrothermal System on Akutan Island, Alaska, July 2012
Table 2. Sample locations, gas chemistry and noble-gas data for degassing features around Akutan Volcano, Alaska, sampled during 2012.[All gas compositions in volume percent; dashes, no data. Types: E, empty bottle; N, bottle containing NaOH solution. R/RA, 3He/4He ratio relative to air; RC/RA, 3He/4He ratio corrected for atmospheric components. Gas geothermometers: TD-P from D’Amore and Panichi (1980); TCO2-CH4 from Giggenbach (1996); TC1-C2 from Darling (1998). Datum for Universal Transverse Mercator (UTM) coordinates is referenced to WGS84 zone 3U. See Bergfeld and others (2011) for details on analytical uncertainties] Sample numberLocationDateEasting(meters)Northing(meters)Elevation(meters)TypeTemp(°C)CO2H2SHeH2CH4N2ArHot springsAKU12-01Group A pool07/25/12444006600080618E60.57.2<0.00050.00120.00022.486.41.19AKU12-03Group A lower vent07/25/12444032600081918E94.095.7<0.00050.00020.00160.613.450.11AKU12-04Group B upper pool07/25/12444132600089019E76.825.90.00560.00060.00549.463.61.04AKU12-11Group B lower pool07/27/12444255600105214E75.224.30.00120.00110.00106.767.81.11Akutan flankAKU12-09Flank fumarole07/26/124405506000347379E98.694.42.10.00090.670.222.570.005AKU12-09AFlank fumarole07/26/124405506000347379N98.694.61.70.00070.620.222.710.004AKU12-09BFlank fumarole07/26/124405506000347379N98.694.52.00.00080.650.232.600.004AKU12-10Flank frying pan07/26/124405476000367381E83.491.42.20.00080.590.214.870.034AKU12-10Flank frying pan07/26/124405476000367381N83.490.71.80.00080.590.226.560.059Akutan craterAKU12-07Crater site 207/26/1243631960002811070E95.014.82.10.00040.540.1781.30.89AKU12-08Crater site 307/26/1243632660002651082N95.013.30.60<0.0020.970.1383.81.06
Discussion 11
Table 2. Sample locations, gas chemistry and noble-gas data for degassing features around Akutan Volcano, Alaska, sampled during 2012. —Continued[All gas compositions in volume percent; dashes, no data. Types: E, empty bottle; N, bottle containing NaOH solution. R/RA, 3He/4He ratio relative to air; RC/RA, 3He/4He ratio corrected for atmospheric components. Gas geothermometers: TD-P from D’Amore and Panichi (1980); TCO2-CH4 from Giggenbach (1996); TC1-C2 from Darling (1998). Datum for Universal Transverse Mercator (UTM) coordinates is referenced to WGS84 zone 3U. See Bergfeld and others (2011) for details on analytical uncertainties] Sample numberO2COC2H6C3H8C4H10HClHFNH3R/RARc/RA40Ar/36ArT D-P(°C)T CO2-CH4(°C)T C1-C2(°C)Hot springsAKU12-012.8<0.0010.0083<0.0005<0.0005---------4.796.88297---------AKU12-030.177.8E-070.0086<0.0005<0.0005---------5.856.4929644163204AKU12-040.073.2E-070.02710.00090.0011---------------------------AKU12-110.05<0.0010.02690.00210.0011---------------------------Akutan flankAKU12-090.0623.6E-040.0015<0.0005<0.0005---------7.637.64297236199223AKU12-09A0.0023.7E-040.0017<0.0001<0.0020.00090.02530.026---------231199219AKU12-09B<0.033.9E-040.00160.0002<0.0020.00320.04970.001---------234198222AKU12-100.696.1E-040.0015<0.0005<0.0005---------0.967*0.835*295235200220AKU12-10<0.036.6E-040.0017<0.0001<0.0020.00490.0158<0.025---------232 199 218Akutan craterAKU12-070.14<0.001<0.0002<0.0005<0.0005---------1.412.09288---------AKU12-08<0.030.0011------0.03030.15820.001.803.80291---------*Air-contaminated sample.
12 Geochemical Investigation of the Hydrothermal System on Akutan Island, Alaska, July 2012
Table 3. Chloride concentrations and stream discharges used to determine geothermal flux and heat output from hot springs along
sections of Hot Springs Creek near Akutan Volcano, Alaska, in 1981 and 2012.
[Q, stream discharge; ---, no data]
Description
2012 1981
Sample
number
Cl
(mg/L)
Q
(L/s)
Cl
(t/d)
Sample
number
Cl
(mg/L)
Q
(L/s)
Cl
(t/d)
Upper Hot Springs Creek AKU12-18 4.3 890 0.33 ------------
Group A tributary ----1196 84 1.41 11 41 71 0.25
East fork Hot Springs Creek AKU12-20 13 245 0.3 ------------
Lower Hot Springs Creek AKU12-16 58 21,348 6.8 14 10 878 0.76
Geothermal Cl flux
(corrected for background)
---------6.15 ------------
Average Cl 2012 hot springs (mg/L)---759 ------------------
Discharge of spring water (L/s)---93.8 ------------------
Enthalpy change (J/g)---305 ------------------
Heat output (MW)---28.7 ------------------
1 Calculated from specific conductance.
2 Includes flow from the east fork of Hot Springs Creek, without which Q = 1,103 L/s. Enthalpy estimate is calculated by using the steam tables of
Keenan and others, (1969).
The strong correlation between Cl concentration and δ18O
value (R2 = 0.958, fig. 4B) can also be used to derive informa-
tion about the isotopic composition of thermal and nonthermal
endmembers. As noted earlier, Mg and Cl concentrations in
the springs suggest that the thermal end member has a Cl
concentration of ~1,300 mg/L. Extrapolation of the Cl-δ18O
trendline to a Cl concentration of 1,300 mg/L yields a δ18O
value for this fluid of about -8.25 per mil, whereas extrapolat-
ing the data to zero Cl gives a δ18O value of about -9.80 per
mil for the cold groundwater, in agreement with the value
derived from δD-δ18O relations (fig. 4A). These mixing lines
(fig. 4A, B) reflect the isotopic composition of the cold ground-
water more accurately than does the single cold creek sample
that was analyzed.
Hot-Spring Geothermometry
The average temperatures estimated from silica and alkali
geothermometers for the least diluted hot-spring waters, those
with Cl concentrations of ≥800 mg/L, range from 129 °C to
188 °C (table 1). Using the quartz conductive geothermom-
eter on the least diluted hot-spring water gives a maximum
estimated temperature of 169 °C. Temperatures estimated
from the Na-K-Ca thermometer (178–188 °C) of Fournier
and Truesdell (1973) are less likely to be affected by dilution
and closely agree with the measured downhole temperature of
182 °C at ~178 m in the thermal-gradient well TG-2, located
near the group B hot springs (Kolker and others, 2012). The
1996 waters from sampling sites along Hot Springs Creek
have lower Cl concentrations and Na-K-Ca temperatures of
168–188 °C.
Gases at Akutan
The composition of the gas on Akutan Island varies widely
(table 2), and much of the variation likely results from shallow
processes that greatly affect the final gas chemistry. He isotope
ratios in five samples indicate that the degassing features at all
sampling sites (summit crater, flank, lower Hot Springs valley)
have some input of magmatic gas that is varyingly diluted
with crustal gas before discharging. A sixth sample was badly
air contaminated.
The gas collected from the flank fumarole field on Akutan
Volcano has high CO2 and H2S concentrations and relatively
low concentrations of air-derived gas (table 2) and has more
geothermal characteristics than the gas from other sites on
Akutan Island. On a ternary He-Ar-N2 diagram (fig. 5), gas
from the flank fumarole falls along a trend defined by gas from
other Cascade Range and Aleutian Arc (CRAA) volcanoes
(fig. 2; Symonds and others, 2003b). Gas from the flank fry-
ing pan falls off the CRAA trend because of its higher N2 and
Ar concentrations and shows some influence of air-derived
components. Calculations using three gas geothermometers
suggest equilibration temperatures for gas in the flank fuma-
role field of ~200–240 °C (table 2).
Gas from two sampling sites in the summit crater had
elevated H2 and H2S concentrations similar to typical volcanic
gas (table 2), whereas high N2 and Ar concentrations indi-
cate that the deep-sourced component has been substantially
diluted by the addition of atmospheric air (fig. 5). The absence
of O2 indicates that the samples were not air contaminated
during collection. Similar trends are evident for the composi-
tion of summit and flank gases collected from fumaroles in
Discussion 13
Trend line for the dilution of seawater Linear regression through measured valuesGroup A springs Group B springs Group C springs Group D springsEXPLANATION2,5001,5005003,5002,0001,0003,0000200 4000600 800 1,000 1,200Cl, in milligrams per literSpecific conductance, in microsiemens per centimeterR2=0.96020406010030500 200 400 600 800 1,000 1,200SO4, in milligrams per literCl, in milligrams per liter1.40.21.00.61.81.200.80.41.6Li, in milligrams per liter0600 800 1,000 1,200200 400R2=0.861Cl, in milligrams per liter6040201008050301009070200 4000600 800 1,000 1,200Ca, in milligrams per literCl, in milligrams per liter1062181484016120 200 400 600 800 1,000 1,200Mg, in milligrams per literR2=0.802Cl, in milligrams per liter100602018014080400160120SiO2, in milligrams per liter0600 800 1,000 1,200200 400Cl, in milligrams per liter200 40070010050030060004002000600 800 1,000 1,200Na, in milligrams per literR2=0.961Cl, in milligrams per liter30020100 200 400 600 800 1,000 1,200B, in milligrams per literSeawaterR2=0.891Cl, in milligrams per liter3.50.52.51.53.002.01.00600 800 1,000 1,200200 400Br, in milligrams per literR2=0.997SeawaterCl, in milligrams per literFigure 3. Scatterplots showing positive correlation between Cl concentration and other components in Akutan hot-spring waters and negative correlation between Cl and Mg concentrations. Solid trend lines are shown for specific conductance and Br, Na, B, and Li concentrations where linear regression of data yield R2 ≥0.86. Trend line in Mg-Cl plot is calculated by excluding one sample with anomalously high Mg concentration.
14 Geochemical Investigation of the Hydrothermal System on Akutan Island, Alaska, July 2012
World Meteoric Water Linegeysering spring
predicted range for
cold groundwater
dilution by groundwater thermal end memberR2=0.958Linear regression through
measured values; dashed
red where extrapolated
Group A vents
Group A pool
Group B pools
Group C vents
Group D vent
Cold creek
EXPLANATION
-11.0 -10.5 -10.0 -9.5 -9.0 -8.5 -8.0-76
-74
-72
-70
-68
-66 A
B
Cl, in milligrams per literδ18O per mil (SMOW)δD per mil (SMOW)δ18O per mil (SMOW)
-11.0 -10.5 -10.0 -9.5 -9.0 -8.5 -8.0
0
200
400
600
800
1,000
1,200
100
300
500
700
900
1,100
1,300
Figure 4. Plots of isotopic composition and chloride concentrations for Akutan hot springs and local cold creek water in 2012.
A, δD versus δ18O values of hot springs and cold creek water relative to World Meteoric Water Line. Horizontal lines show range
of δD values in all hot-spring waters except that from geysering spring; vertical lines show range of δ18O values predicted for cold
groundwater that dilutes hot springs. B, Cl concentration versus δ18O values in hot-spring water, showing positive correlation.
Line of linear regression is extrapolated to 0 and 1,300 mg/L Cl to estimate δ18O values for groundwater diluting hot springs and for
undiluted thermal fluid, respectively. SMOW, Standard Mean Ocean Water.
Discussion 15
1996, although the amount of atmospheric dilution for the gas
from the summit crater was lower than in 2012. A sample of
gas from an acidic spring on the flank in 1981 is similar to the
2012 sample from the frying pan and plots along the atmo-
spheric-dilution trend (Motyka and others, 1988). Altogether,
it is reasonable to assume that the gases emitting from features
on the flank and summit share a common deep magmatic
source. The gas composition is consistent with a weak flow
of gas to the summit crater within a permeable volcanic cone
that acts as a chimney and allows atmospheric air to enter the
system. As the mixed gas rises to the surface, it is heated, and
atmospheric O2 is consumed in the process.
Gas discharge from the hot springs in 2012 was weak
and produced only small bubble trains that rose through the
water. The 3He/4He ratios in gas from two of these hot springs
demonstrate magmatic helium inputs, but the bulk gas com-
position of most samples had high N2 and low CO2 concentra-
tions (table 2) that are unlike most geothermal gas emissions
(for example, Goff and Janik, 2000). High N2 concentrations
have been a pervasive feature of the gas at the hot springs over
the years. Gas from a group A hot spring in 1981 consisted of
10.3 volume percent CO2 and 76.7 volume percent N2 (Motyka
and others, 1988). A vigorously degassing hot spring in 1996
contained 89.1 volume percent CO2 and 8.8 volume percent
N2 (Symonds and others, 2003a). This sample is similar to the
gas from the 2012 group A vent hot spring that had the highest
CO2 concentration of any sample from the hot-spring area, but
even this sample is low in gases (H2S, H2) that are typical of
high-temperature geothermal systems.
Thermal Chloride Flux
Using mass balance with the discharge measurements and
the Cl concentrations in waters from the creeks and springs,
we can calculate the thermal influx (TI) to Hot Springs Creek.
The flux of dissolved Cl (FCl) is calculated from stream-
discharge measurements (Q) and the collocated Cl concentra-
tions of the water [Cl], such that for each discharge point i:
(FCl)i = Qi*[Cl]i.
N2/100
10 x He Ar
Air
ASMW
(5.8)
(6.5)
(6.9)
(7.6)
(7.2)
(7.3)
(3.8)
(2.1)Cascade Range and Aleutian Arc trendYellowsto
n
e t
r
e
n
d
(6.0-6.3)
(7.1)The GeysersFUM
FP
2012 Flank
2012 Summit
2012 Hot Springs valley
1996 Flank
1996 Summit
1996 Hot Springs valley
1981 Flank
1981 Hot Springs valley
EXPLANATION
Figure 5. Ternary N2-He-Ar diagram for gas samples
collected at Akutan Volcano in 1981, 1996, and 2012.
Pre-2012 data are from Motyka and others (1988) and
Symonds and others (2003a). Numbers in parentheses,
RC/RA ratios. Tielines on 2012 flank fumarole samples
indicate that multiple samples were collected for
chemistry and only one He sample was collected for
isotopic analysis. Arrow indicates that flank and summit
samples fall along a mixing line between deep-sourced
gas and air. Trendlines are derived from data of Symonds
(2003b) for Cascade Range and Aleutian Arc (CRAA),
Bergfeld and others (2011) for Yellowstone National Park,
and Lowenstern and others (1999) for The Geysers, Calif.
ASMW, air-saturated meteoric water;
FUM, flank fumarole, FP, frying pan.
16 Geochemical Investigation of the Hydrothermal System on Akutan Island, Alaska, July 2012
We correct for the chloride in meteoric water above
the group A hot springs (U) and for the input from EFHSC.
Assuming that the remaining chloride in the downstream site
(D) is provided from the hot spring system,
TI = (FCl)D - ((FCl)U+ (FCl)EFHSC).
These calculations indicate that a thermal influx of
~6.15 metric tonnes of chloride is released by the hot-spring
system each day (t/d).
We can refine our understanding of individual hot-spring
inputs by using the discharge measurement for the tributary
below the group A hot springs. Although we did not collect
a water sample, the overall excellent correlation between
specific conductance and Cl concentration of the hot springs
(fig. 3) allows us to calculate a chloride flux for the group A
hot-spring waters. Using a linear regression of Cl concentra-
tion and specific conductance for all of the hot springs, we
estimate that the tributary water with a specific conductance
of 745 mS/cm had a Cl concentration of ~196 mg/L (table 3).
The discharge of 84 L/s yields a chloride flux of ~1.4 t/d. The
remaining 4.7 t/d of thermal influx is contributed below the
group A hot springs.
Change in the Akutan Hydrothermal System
Comparison of the hot-spring water chemistry from the
early 1980s through July 2012 shows large changes in the
hydrothermal system on Akutan Island (fig. 6). In general, the
concentrations of most dissolved components in each spring
group were higher in 2012 than in the 1980s, and we specifi-
cally note the high Ca, Cl, and Na concentrations in 2012.
Excluding the intertidal springs, the maximum Cl concentra-
tion was 424 mg/L in the early 1980s, 623 mg/L in 1996,
and 1,100 mg/L in 2012 (table 1; Motyka and others, 1988,
table 6-1). Only HCO3 and SO4 lack evidence of a consistent
increase from the 1980s to 2012 (fig. 6). A maximum tem-
perature of 84 °C was recorded for the hot springs in 1980, in
comparison with 97.4 °C at a vigorously bubbling hot spring
in 1996 and two hot springs with temperatures of >99 °C in
2012 (table 1).
The discharge data for Hot Springs Creek from 1981 and
2012 are comparable, even though gaging locations are not
identical. A direct comparison of discharge from the tributary
below the group A hot springs is possible because the stream is
short and places to make measurements are limited. Discharge
from the tributary in 1981 was 71 L/s, in comparison with
84 L/s in 2012 (table 3). The July 1981 discharge of 878 L/s in
lower Hot Springs Creek was obtained above the confluence
with EFHSC, whereas the 2012 measurement (1,348 L/s) was
obtained below. Subtracting the 2012 discharge of EFHSC
(245 L/s) results in a discharge of 1,103 L/s in Hot Springs
Creek above the confluence. Thus, the flow in both the group
A tributary and Hot Springs Creek appears to be 20–25 percent
higher in 2012 than in 1981.
This increase in flow could simply be attributed to
seasonal or interannual changes, but the large increase in
stream Cl concentrations (table 3) is clear evidence of a large
increase in the hydrothermal component in Hot Springs Creek.
The Cl concentration in the group A tributary increased by a
factor of ~5 between 1981 and 2012, and a similar increase
occurred in Hot Springs Creek below the group D hot springs,
from 10 mg/L in 1981 to 58 mg/L in 2012. Because of the
additional dilution from EFHSC, the measured fivefold
increase for lower Hot Springs Creek considerably underes-
timates the actual change. As described earlier, the Cl con-
centration in EFHSC was similar to precipitation, with little
evidence of thermal inputs to this stream. Thus, the Cl concen-
tration in the creek increased by a greater factor than in the hot
springs, a result that can be explained only by an increase in
hot-spring discharge rate.
We can estimate the heat output associated with the
chloride flux by using the average hot-spring temperature
of 83 °C and average Cl concentration of 759 mg/L for the
hot springs in 2012 (table 3). A thermal chloride flux of
6.15 t/d yields a hot-water discharge of 93.8 L/s. The enthalpy
anomaly of this hot water relative to the annual land-surface
temperature of 10 °C (Selkregg, 1976) is 305.5 J/g, yielding
a heat output of 29 MW. This value is approximately an order
of magnitude higher than the heat-discharge estimate of 2.2 to
4.1 MW from 1981 (Motyka and others, 1988).
Summary
The existing conceptual model of the geothermal system
on Akutan Island (Kolker and others, 2012) places the reser-
voir in the vicinity of the flank fumarole field, which caps an
upflow zone. The rising hot water loses gas and steam before
flowing laterally as an outflow plume northeastward, where it
discharges at the springs in Hot Springs valley after varying
dilution with groundwater (fig. 7). This model fits well with
our new results, particularly the gas data. The most robust gas
samples that were collected from the flank fumarole had a
3He/4He ratio (RC/RA=7.6) and N2-He-Ar concentrations typi-
cal of other CRAA volcanoes (Symonds and others, 2003a,
b). In contrast, the chemical compositions of the hot-spring
gases are strongly influenced by shallow processes. Low CO2
concentrations in most samples reflects the previous loss of
this gas in the flank fumarole field, allowing N2 and Ar from
the diluting groundwater and CH4 from organic sources to
dominate the gas composition. Although the gas compositions
are reset by low-temperature processes and are not representa-
tive of deep reservoir conditions, the similarity in the isotope
compositions of He in the gas and C in DIC to those in the gas
from the flank fumarole field, -11.2 to -10.8 per mil (Motyka
and others, 1988), support a connection between the two areas
through the envisioned outflow plume.
Samples collected since 1981 indicate that flank fuma-
roles provide the most reliable information on the deep reser-
voir temperature. Kolker and others (2012) used a gas geother-
mometer based on the ratios of CO2 and H2 to Ar (Powell and
Summary 17
Figure 6. Scatterplots showing differences in water chemistry for Akutan hot springs in 1980–81 and 2012. A through D, hot-spring
groups of Motyka and others (1988). Results for 1980s analyses with poor charge balance are omitted, and no comparisons of B
and Br concentrations are made because recent changes in analytical procedures have greatly improved precision and detection
limits. Diamonds, waters measured in this study; circles, compositions from Motyka and others (1988).
Cumming, 2010) to propose reservoir temperatures well above
200 °C. We obtain a temperature range of 200–240°C by using
three non-Ar-based gas geothermometers. Lower temperature
estimates are obtained by applying solute geothermometers to
the hot-spring waters, and these temperatures reflect the cool-
ing and mixing that occur on the outflow path. Temperatures
of 178–188 °C calculated from the Na-K-Ca geothermometer
are consistent with a measured temperature of 182 °C from
a thermal-gradient well in Hot Springs valley (Kolker and
others, 2012).
Results from our study document increasing
concentrations of hydrothermal components in the spring
waters and an increase in the volume of water discharging
from the hot spring system. Taken together, these findings
provide unequivocal evidence that large changes in the hydro-
thermal system on Akutan Island have occurred over the past
~30 years. We estimate the heat output of the current hydro-
thermal system at 29 MW, approximately an order of magni-
tude higher than in the early 1980s (Motyka and others, 1988).
We are unaware of any other neutral-chloride hydrothermal
Hot spring group Hot spring groupSpecific conductance, in microsiemens per centimeter500
2,500
1,500
3,500
3,000
0
2,000
1,000Ca, in milligrams per literSiO2, in milligrams per liter100
60
20
180
140
80
40
0
160
120
100
20
60
0
80
40
C DA B
C DA B
C DA B Mg, in milligrams per liter1,200
200
600
1,000
0
800
400
Cl, in milligrams per literHCO3, in milligrams per literC DA B
C DA B
C DA B
20
40
60
10
0
30
50
SO4, in milligrams per literLi, in milligrams per liter1.0
0.6
0.2
1.8
1.4
0.8
0.4
0
1.6
1.2Na, in milligrams per liter700
600
300
0
200
100
400
C DA B
C DA B
C DA B
500
10
6
2
18
14
8
4
0
16
12
100
60
20
180
140
80
40
0
160
120
Hot spring group
18 Geochemical Investigation of the Hydrothermal System on Akutan Island, Alaska, July 2012
Figure 7. Schematic cross section of hydrothermal system on Akutan Island. Cold groundwater infiltrates in summit region,
feeding reservoir situated somewhere near flank fumarole field. Heat and gases are supplied to reservoir by underlying
magmatic intrusion. Hot water rising from the reservoir boils, providing steam and gas to flank fumarole field. Degassed hot
water forms an outflow plume that discharges in Hot Springs valley after varying dilution by cold groundwater.
systems that have shown this kind of change. Many causes
could be invoked, but a permeability enhancement somewhere
in the system, such as in the outflow zone, would logically
explain the increase in hot-spring discharge. It is tempting
to link such a process to fault movement during the seismic
crisis of March 1996, given that a hot spring sampled by
Symonds later in that year had a higher Cl concentration than
that reported by Motyka and others (1988). The infrequency
of sampling may obscure the exact timing, but if the ten-
fold increase in heat output dates back to 1996, it can hardly
be considered a short-term transient, and so the increased
heat resource available for power generation over a 30-year
timespan may be much larger than indicated by the resource
data of Motyka and others (1988).
Acknowledgments
We thank Colin Williams (USGS, Menlo Park, Calif.)
and John Power (USGS, Alaska Volcano Observatory, Anchor-
age) for supporting this project, and Michelle Coombs, Kristi
Wallace, and Chris Waythomas (USGS, Alaska Volcano
Observatory, Anchorage) for logistical assistance in plan-
ning and executing the fieldwork. Additional thanks go to
Timothy Brabets (USGS, Anchorage) for the loan of some
gaging equipment. Helpful reviews of the manuscript were
provided by Fraser Goff (New Mexico Institute of Mining and
Technology, Socorro) and Cynthia Werner (USGS, Alaska
Volcano Observatory, Anchorage).
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20 Geochemical Investigation of the Hydrothermal System on Akutan Island, Alaska, July 2012
Menlo Park Publishing Service Center, California
Manuscript approved for publication December 5, 2013
Edited by George Havach and Claire Landowski
Design and layout by Ron Spencer
Bergfeld—Geochemical Investigation of the Hydrothermal System on Akutan Island, Alaska, July 2012—Scientific Investigations Report 2013–5231ISSN 2328–0328 (online)
http://dx.doi.org/10.3133/sir20135231