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| Titel |
Evaluation of short-term tracer fluctuations in groundwater and soil air in a two year study |
| VerfasserIn |
Florian Jenner, Simon Mayer, Werner Aeschbach, Therese Weissbach |
| 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 |
250126954
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| Publikation (Nr.) |
 EGU/EGU2016-6750.pdf |
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| Zusammenfassung |
| The application of gas tracers like noble gases (NGs), SF6 or CFCs in groundwater studies
such as paleo temperature determination requires a detailed understanding of the dynamics of
reactive and inert gases in the soil air with which the infiltrating water equilibrates. Due to
microbial gas consumption and production, NG partial pressures in soil air can deviate from
atmospheric air, an effect that could bias noble gas temperatures estimates if not taken into
account. So far, such an impact on NG contents in groundwater has not been directly
demonstrated. We provide the first long-term study of the above mentioned gas tracers and
physical parameters in both the saturated and unsaturated soil zone, sampled continuously for
more than two years near Mannheim (Germany). NG partial pressures in soil air
correlate with soil moisture and the sum value of O2+CO2, with a maximal significant
enhancement of 3-6% with respect to atmospheric air during summer time. Observed
seasonal fluctuations result in a mass dependent fractionation of NGs in soil air.
Concentrations of SF6 and CFCs in soil air are determined by corresponding fluctuations in
local atmospheric air, caused by industrial emissions. Arising concentration peaks
are damped with increasing soil depth. Shallow groundwater shows short-term
NG fluctuations which are smoothed within a few meters below the water table. A
correlation between NG contents of soil air and of groundwater is observable during
strong recharge events. However, there is no evidence for a permanent influence of
seasonal variations of soil air composition on shallow groundwater. Fluctuating
NG contents in shallow groundwater are rather determined by variations of soil
temperature and water table level. Our data gives evidence for a further temperature driven
equilibration of groundwater with entrapped air bubbles within the topmost saturated
zone, which permanently occurs even some years after recharge. Local subsurface
temperature fluctuations may thus lead to subsequent variations of NG contents in
groundwater, independent of the former recharge temperature. This effect is of
major importance for gas tracer applications in recent and shallow groundwater. |
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