Using Stable Isotopes to Determine Hyporheic Zone Flow
Paths in Antarctic Streams
Michael Gooseff [email protected]
http://ucsu.colorado.edu/~gooseff
Diane McKnight
Bruce Vaughn
Overview
Dry Valleys Hydrology Introduction to Hyporheic Zone Introduction to Isotopes Methods and Field Work Results Conclusions
Winds strong enough to sculpt rock
Dry Valleys Hydrology
Polar desert “oasis” located at ~78o S “ice-free” Dry (<10 cm precip per year) Cold (average air temp = -20oC)
Barren landscape Glaciers, soils, streams and lakes No higher-order plants
Low anthropogenic disturbance
Dry Valley Stream Hydrology (cont’)
6-10 week flow season
large diel flow changes
streambeds = porous alluvium
driven by energy balance on the glaciers
Orange and Black benthic stream algal mats
The Hyporheic Zone The hyporheic zone is an area of saturated
alluvium under and adjacent to a stream Definition: subsurface mixing zone in which at
least 10% of the water has recently been in the stream and has a downstream direction of flow
Very important in Dry Valley stream hydrology Ecosystem processes Low flow years
active layeractive layer
permafrost
Modeling Equations Transient Storage model developed by
Bencala and Walters, 1983
0ˆ1,
1 ssss
CCCCA
A storagezone
advection
01
CCCx
CAD
xAx
C
A
Qs stream
dispersion
transientstorage
1o loss
storage lossexchange
Previous DV Tracer Studies 1994 – Huey Creek (Runkel et al., 1998)
Rapid hydrologic exchange between stream and hyporheic zone
as high as 1.62E-2 s-1, AS/A as high as 34.3
1995 – Green Creek (McKnight et al., in review) N uptake in-stream and in-hyporheic
1999 – Green Creek (Gooseff) agreement between observed and modeled
hyporheic zone concentrations
Green Creek Overview Photo
Green Creek, 1995
#
#
$T
#Y
#Y
#Y
#Y
#Y
Algal Transect
Sampling Transect
Stream Guage
Approximately 50 m
NLake Fryxell
Green Creek
GCT1GCT0
GCT2GCT3
GCT4
Topographic map of Green Creek, Antarctica
0
2
4
6
8
10
12
11 13 15 17 19
0
2
4
6
8
10
12
11 13 15 17 19
0
2
4
6
8
10
12
11 13 15 17 19
0
2
4
6
8
10
12
11 13 15 17 19
GC
Str
eam
Cl C
once
ntra
tion
s (m
g L
-1)
Hour of 06-Jan-99
GC59 GC161
GC257 GC357
Green Creek 1999
0
2
4
6
8
10
11 13 15 17 19
0
2
4
6
8
10
11 13 15 17 19
0
2
4
6
8
10
11 13 15 17 19
0
2
4
6
8
10
11 13 15 17 19
GC
Sto
rage
Cl C
once
ntra
tion
s (m
g L
-1) GC59 GC161
GC257 GC357
Hour of 06-Jan-99Green Creek 1999
permafrost
permafrost
saturated wetted zone
active layer
B
active layer
AFrozen infiltration from
previous season
permafrost
active layer
More active, faster exchanging hyporheic zone
Less active, slower exchanging wetted zone
Hypothesis: The wetted zones surrounding thestreams can be partitioned into 2 storage zones.
Tracer Approach Needs to be long term Chemical tracer experiments
Pro: transient characterization of hyporheic exchange
Con: has to be short term because extreme changes in flow over the long term logistically difficult in Dry Valleys pristine, protected ecosystem no long term
releases
Solution: stable isotopes !
Stable Isotopes of Water
“Isotopes are atoms of the same element that have different numbers of neutrons.”
Common isotopic tracers in hydrology: Deuterium (symbolized “D”, with 2 neutrons) 18O (Oxygen with 2 additional neutrons)
Expressed as a ratio of different to normal:
10001
STANDARD
SAMPLE
R
Rlight
heavy
isotopes
isotopesR
Stable Isotopes of Water (cont.’) Terminology:
– “permil”, symbolized units: ‰ “lighter”, “depleted” ratios have a more
negative value -180 ‰ is very “depleted” compared to –20 ‰
“heavier”, “enriched” ratios have a more positive value
20 ‰ is “heavier” than -2 ‰
Isobalance D and 18O values define a meteoric
water line, GMWL: D=(8*18O)+10
-300
-250
-200
-150
-100
-50
0
50
100
-40 -30 -20 -10 0 1018O (permill)
D (
per
mill
)
SMOW
GMWL
enriched, heavy
depleted, lighter
Fractionation
Fractionation is a change in the isotopic ratio
In water that can occur from: Evaporation: lighter isotopes evaporate,
remaining water gets enriched Freezing: 2 - 3‰ increase in 18O,
15 - 20 ‰ increase in D for ice relative to water
Modeling Equations Transient Storage model developed by
Bencala and Walters, 1983
t
RRRRR
A
A Ssss
s
ˆ storagezone
advection
t
RRRR
x
RAD
xAx
R
A
Qs
1
stream
dispersion
transientstorage
1o loss
storage lossexchange
Isotopic Ratios for Streamwater by Lake Basin (1993-94)
-350
-330
-310
-290
-270
-250
-230
-210
-190
-170
-45 -40 -35 -30 -25 -20
18O (permil)
D (
per
mil)
Fryxell Streams Hoare/Chad Streams Bonney Streams GMWL
Fryxell Basin Stream Isotope Data
-270
-260
-250
-240
-230
-220
-210
-200
-190
-34 -32 -30 -28 -26 -24 -22 -20
18O (permil)
D (
pe
rmil
)
Canada St. Huey Cr. Lost Seal St. McKnight Cr. Aiken Cr. Von Guerard St.
Harnish Cr. Crescent St. Delta St. Green Cr. Bowles Cr. Mariah Cr.
Canada GlacierStreams
Kukri HillsGlacial Streams
Commonwealth Glacier Streams
Delta Stream Synoptic Isotope Data (18-Jan-94)
-220
-218
-216
-214
-212
-210
-208
0 1000 2000 3000 4000 5000 6000 7000
Approximate Distance Downstream from Howard Glacier (m)
D r
ati
o (
pe
r m
il)
-27
-26.5
-26
-25.5
-25
-24.5
-24
18 O
ra
tio
(p
er
mil
)
D ratio 18O ratio
Delta Stream Synoptic Isotope Data (18-Jan-94)
-18.0
-16.0
-14.0
-12.0
-10.0
-8.0
-6.0
-4.0
-2.0
0.0
0 1000 2000 3000 4000 5000 6000 7000
Approximate Distance Downstream from Howard Glacier (m)
D E
xces
s (p
erm
il)
d = D – (8*18O)
1999-00 Sampling
Evaporation Pan experiment 6.5 hours Sampled hourly for isotopes and chemistry
Green Creek synoptics Sample stream and storage zones for
isotopes and chemistry
Evaporation Experiment
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4.0
9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00
Time on 11-Jan-00
Vo
lum
e o
f w
ater
in P
an (
L)
uncorrected corrected for evap.
Fractionation During Evaporation Experiment
-233.0
-232.5
-232.0
-231.5
-231.0
-230.5
-230.0
-229.5
-229.0
9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00
Time on 11-Jan-00
D (
per
mil)
-29.2
-29.0
-28.8
-28.6
-28.4
-28.2
-28.0
18O
(p
er m
il)
D 18O
D Excess Change During Evaporation Experiment
-6.0
-5.0
-4.0
-3.0
-2.0
-1.0
0.0
9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00
Time on 11-Jan-00
D E
xces
s (p
er m
il)
d = D – (8*18O)
Green Creek Synoptic #1 - Dec. 7, 1999
-253
-252
-251
-250
-249
-248
-247
-246
-245
-244
0 100 200 300 400 500 600
Distance Downstream (m)
D (
pe
r m
il)
-31.2
-31.0
-30.8
-30.6
-30.4
-30.2
-30.0
-29.8
-29.6
-29.4
18O
(p
er
mil)
D 18O
Instream Well Well A Well B
Left Hand Bank
2 m
4 m
Green Creek Synoptic #2 - Dec. 21, 1999 - D Isotopes
-258
-256
-254
-252
-250
-248
-246
-244
0 100 200 300 400 500
Distance Downstream (m)
D (
pe
r m
il)
stream D IS wells D Lat wells D
Green Creek Synoptic #3 - Jan. 7, 2000 - D Isotopes
-258
-256
-254
-252
-250
-248
-246
-244
0 100 200 300 400 500
Distance Downstream (m)
D (
pe
r m
il)
stream D IS wells D Lat wells D
Summary of SamplingTravel
Time (hr)D fractionation
rate (‰ hr-1)% fract.
evap.% fract. mixing
Evap. Exp. 6.5 +0.47 100 0Delta St. (18-Jan-94) 24 +0.38 100? 0?
Green Cr.
(07-Dec-99)2.46 +3.22 14.6 85.4
Green Cr.
(21-Dec-99)0.5 +3.95 11.9 88.1
Green Cr. (07-Jan-00) 2.3 +1.31 35.9 64.1
Green Creek Well Isotope Ratios
-258
-256
-254
-252
-250
-248
-246
-244
12/18/99 12/23/99 12/28/99 1/2/00 1/7/00 1/12/00 1/17/00 1/22/00 1/27/00
Date
D (
pe
r m
il)
GCT2WA GCT3WA GCT4WA GCT2WB GCT4WB
Conclusions of 1999-00 sampling Sub-surface water is generally more enriched Exchange of wetted zone water happens over
several weeks Sub-stream hyporheic zone seems to be a
mixing zone between old and new water Evidence from isotopes looks promising, but
how do we model this?!?
Considering the entire wetted zone area in cross sectionATOTAL = AS,1 + AS,2
storage 1
storage 2
AS,1 AS,2
2 1
Conceptually, we can then model a nested storage zone:
Modeling Approach Use Transient Storage model with
nested storage zone:
01
RRRx
RAD
xAx
R
A
QS
01,2,21,1,
1 SSSS
RRRRA
A storage zone 1
02,2,2,1,2,
1,2 SSSS
S
S RRRA
A storage
zone 2
stream
Acknowledgements
NSF Office of Polar Programs Ethan Chatfield, Jon Mason, and Harry
House – field work Antarctic Support Associates and PHI
Helicopters for logistical support
Gratuitous Penguin Photo