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| Titel |
Travel Times of Water Derived from Three Naturally Occurring Cosmogenic
Radioactive Isotopes |
| VerfasserIn |
Ate Visser, Melissa Thaw, Amanda Deinhart, Richard Bibby, Brad Esser |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250146756
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| Publikation (Nr.) |
 EGU/EGU2017-10796.pdf |
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| Zusammenfassung |
| Hydrological travel times are studied on scales that span six orders of magnitude, from daily
event water in stream flow to pre-Holocene groundwater in wells. Groundwater vulnerability
to contamination, groundwater surface water interactions and catchment response are often
focused on “modern” water that recharged after the introduction of anthropogenic tritium in
precipitation in 1953. Shorter residence times are expected in smaller catchments, resulting in
immediate vulnerability to contamination.
We studied a small (4.6 km2) alpine (1660-2117 m) catchment in a Mediterranean
climate (8 ˚ C, 1200 mm/yr) in the California Sierra Nevada to assess subsurface
storage and investigate the response to the recent California drought. We analyzed a
combination of three cosmogenic radioactive isotopes with half-lives varying from 87 days
(sulfur-35), 2.6 years (sodium-22) to 12.3 years (tritium) in precipitation and stream
samples.
Tritium samples (1 L) are analyzed by noble gas mass spectrometry after helium-3
accumulation. Samples for sulfur-35 and sodium-22 are collected by processing
20-1000 L of water through an anion and cation exchange column in-situ. Sulfur-35 is
analyzed by liquid scintillation counting after chemical purification and precipitation.
Sodium-22 is analyzed by gamma counting after eluting the cations into a 4L Marinelli
beaker.
Monthly collected precipitation samples show variability of deposition rate for tritium
and sulfur-35. Sodium-22 levels in cumulative yearly precipitation samples are consistent
with recent studies in the US and Japan. The observed variability of deposition rates
complicates direct estimation of stream water age fractions.
The level and variability of tritium in monthly stream samples indicate a mean residence
time on the order of 10 years and only small contributions of younger water during high
flow conditions. Estimates of subsurface storage are in agreement with estimates
from geophysical studies. Detections of sodium-22 confirm a small fraction of
younger (< 5 years) water. Low concentrations of sulfur-35 suggest very small
contributions of same-year snowmelt or precipitation. Results from two contrasting
years (severe drought in 2015 and near-normal conditions in 2016) illustrate travel
time responses to hydrological conditions and further characterize the catchment
properties.
Combined analysis of three cosmogenic tracers provides a unique insight into the
functioning of the catchment and constrains the volume of subsurface water storage.
Short-lived naturally occurring radioactive isotopes sulfur-35 and sodium-22 are especially
useful for vulnerability assessment of springs and karst systems where a contribution of very
young water is expected.
This work performed under the auspices of the U.S. Department of Energy by Lawrence
Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-717377 |
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