Using Stable Isotopes to Determine Hyporheic Zone Flow Paths in Antarctic Streams Michael 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 ~78 o S “ice-free” Dry (<10 cm precip per year) Cold (average air temp = -20 o C) 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 layer permafrost

Modeling Equations Transient Storage model developed by Bencala and Walters, 1983 storage zone advection stream dispersion transient storage 1 o 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, A S /A as high as – 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

Algal Transect Sampling Transect Stream Guage Approximately 50 m N Lake Fryxell Green Creek GCT1 GCT0 GCT2 GCT3 GCT4 Topographic map of Green Creek, Antarctica

GC Stream Cl Concentrations (mg L -1 ) Hour of 06-Jan-99 GC59GC161 GC257GC357 Green Creek 1999

GC Storage Cl Concentrations (mg L -1 ) GC59GC161 GC257GC357 Hour of 06-Jan-99 Green Creek 1999

permafrost saturated wetted zone active layer B A Frozen infiltration from previous season

permafrost active layer More active, faster exchanging hyporheic zone Less active, slower exchanging wetted zone Hypothesis: The wetted zones surrounding the streams 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) 18 O (Oxygen with 2 additional neutrons) Expressed as a ratio of different to normal:

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 18 O values define a meteoric water line, GMWL:  D=(8*  18 O)+10 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  18 O, ‰ increase in  D for ice relative to water

Modeling Equations Transient Storage model developed by Bencala and Walters, 1983 storage zone advection stream dispersion transient storage 1 o loss storage lossexchange

d =  D – (8*  18 O)

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

d =  D – (8*  18 O)

Instream WellWell AWell B Left Hand Bank 2 m 4 m

Summary of Sampling Travel Time (hr) D fractionation rate (‰ hr -1 ) % fract. evap. % fract. mixing Evap. Exp Delta St. (18-Jan-94) ?0? Green Cr. (07-Dec-99) Green Cr. (21-Dec-99) Green Cr. (07-Jan-00)

Conclusions of 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 section A TOTAL = A S,1 + A S,2 storage 1 storage 2 A S,1 A S,2 22 11 Conceptually, we can then model a nested storage zone:

Modeling Approach Use Transient Storage model with nested storage zone: storage zone 1 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