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| Titel |
Groundwater travel time distributions of a drinking water well field characterized by 85Kr, 3H/3He and 39Ar |
| VerfasserIn |
Ate Visser, Hans Peter Broers, Roland Purtschert, Jürgen Sültenfuß |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250047113
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| Zusammenfassung |
| The travel time distribution of a drinking water well field largely determines the future
evolution of water quality. The goal of this study was to assess the value of groundwater age
tracers (85Kr, 3H/3He and 39Ar) to characterize the travel time distributions of the production
wells and the surrounding monitoring network for future water quality prognosis and well
vulnerability assessment.
The Holten well field produces drinking water from sandy ice-pushed ridges
and peri-glacial aeolian deposits in the east of the Netherlands. Seven production
wells were sampled for 85Kr, 3H/3He and 39Ar. Four wells are screened at 15-45 m
below surface (“shallow wells”), three at 45-70 m below surface (“deep wells”).
The stable noble gas samples were collected in copper tubes. The 85Kr and 39Ar
samples were collected by degassing 2-4m3 of well water at the site. A particle
tracking study using a numerical groundwater flow model provided initial estimates
of the travel time distributions and expected tracer concentrations for each of the
wells.
The travel time distributions were characterized by fitting various black box
models (exponential, dispersion) to the measured tracer concentrations. Because
these mathematical models can be too stringent, a discrete travel time distribution
model (DTTDM) was developed. The DTTDM consists of five discreet bins of
travel times: four 15-year wide bins (0-15, 15-30, 30-45, 45-60) and one for the
“old” groundwater component. The distribution of travel times within each bin is
assumed to be uniform. The contribution of each bin was varied by 5% increments,
resulting in 10626 possible travel time distributions. These travel time distributions are
“shape free”: the shape is not constrained by a mathematical function. The measured
tracer concentrations were compared to the 10626 predicted tracer concentrations
and the best fit was selected and accepted if the Chi-squared probability exceeded
5%.
The tracer concentrations predicted by the numerical model matched qualitatively with
the measured concentrations. The presence and age of old groundwater in the deep
production wells was indicated by the 39Ar measurements. Radiogenic helium was detected
in two of the three deep wells. The detection of 85Kr revealed a very young groundwater
component in one of the three deep wells that was not predicted by the numerical model. This
indicates a vulnerability to recent contaminations in the capture area of this production
well.
While 85Kr and 39Ar represent either the young and old component of groundwater ages,
3H/3He is sensitive to water components that recharged in the 1960s when the 3H
concentrations in precipitation were highest. The combination of 85Kr, 3H/3He and 39Ar
provides crucial information to determine accurate travel time distributions in the age range
up to 1000 years. The discrete travel time distributions calculated for these wells have been
used to assess the vulnerability to anthropogenic contamination and provide future water
quality prognoses.
LLNL-ABS-464629 Portions of this work performed under the auspices of the U.S.
Department of Energy by Lawrence Livermore National Laboratory under Contract
DE-AC52-07NA27344 |
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