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| Titel |
Threshold groundwater ages and young water fractions estimated from 3H, 3He, and 14C |
| VerfasserIn |
James Kirchner, Scott Jasechko |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250130190
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| Publikation (Nr.) |
 EGU/EGU2016-10407.pdf |
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| Zusammenfassung |
| It is widely recognized that a water sample taken from a running stream is not described by a
single age, but rather by a distribution of ages. It is less widely recognized that the same
principle holds true for groundwaters, as indicated by the commonly observed discordances
between model ages obtained from different tracers (e.g., 3H vs 14C) in the same
sample.
Water age distributions are often characterized by their mean residence times (MRT’s).
However, MRT estimates are highly uncertain because they depend on the shape of the
assumed residence time distribution (in particular on the thickness of the long-time tail),
which is difficult or impossible to constrain with data. Furthermore, because MRT’s are
typically nonlinear functions of age tracer concentrations, they are subject to aggregation
bias. That is, MRT estimates derived from a mixture of waters with different ages (and thus
different tracer concentrations) will systematically underestimate the mixture’s true mean
age.
Here, building on recent work with stable isotope tracers in surface waters [1-3], we
present a new framework for using 3H, 3He and 14C to characterize groundwater age
distributions. Rather than describing groundwater age distributions by their MRT, we
characterize them by the fraction of the distribution that is younger or older than a threshold
age. The threshold age that separates "young" from "old" water depends on the characteristics
of the specific tracer, including its history of atmospheric inputs. Our approach depends only
on whether a given slice of the age distribution is younger or older than the threshold age, but
not on how much younger or older it is. Thus our approach is insensitive to the tails of the age
distribution, and is therefore relatively unaffected by uncertainty in the distribution’s
shape.
Here we show that concentrations of 3H, 3He, and 14C are almost linearly related to the
fractions of water that are younger or older than specified threshold ages. These
"young" and "old" water fractions are therefore immune to the aggregation bias
that afflicts MRT estimates. They are also relatively insensitive to the shape of
the assumed residence time distribution. We apply this approach to 3H and 14C
measurements from ∼5000 wells in ∼200 aquifers around the world. Our results
show that even very old groundwaters, with 14C ages of thousands of years, often
contain significant amounts of much younger water, with a substantial fraction
of their age distributions younger than ∼100 years old. Thus despite being very
old on average, these groundwaters may also be vulnerable to relatively recent
contamination.
[1] Kirchner J.W., Aggregation in environmental systems: Catchment mean transit times
and young water fractions under hydrologic nonstationarity, Hydrology and Earth System
Sciences, in press.
[2] Kirchner J.W., Aggregation in environmental systems: Seasonal tracer cycles quantify
young water fractions, but not mean transit times, in spatially heterogeneous catchments,
Hydrology and Earth System Sciences, in press.
[3] Jasechko S., Kirchner J.W., Welker J.M., and McDonnell J.J., Substantial young
streamflow in global rivers, Nature Geoscience, in press. |
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