Using Stable Isotopes to Determine Hyporheic Zone Flow

Paths in Antarctic Streams

Michael Gooseff gooseff@colorado.edu

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