Regensburg 2016 – scientific programme

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UP: Fachverband Umweltphysik

UP 8: Hydrosphere, Soil and Agricultural Physics

UP 8.2: Talk

Wednesday, March 9, 2016, 14:45–15:00, H41

Evaluation of short-term noble gas fluctuations in groundwater and soil air in a two year study — •Simon Mayer, Florian Jenner, Therese Weißbach, and Werner Aeschbach — Institut für Umweltphysik, Heidelberg, Germany

The application of noble gases (NGs) in environmental tracer studies such as noble gas temperature (NGT) determination based on the temperature dependent dissolution of NGs in groundwater requires an exact knowledge of subsurface soil air composition. While deviations of soil air NG partial pressures from atmospheric values have already been found, an impact on NG contents of groundwater has not been investigated so far. We provide the first long-term study of NGs and physical parameters in both the saturated and unsaturated soil zone, sampled continuously for more than 2 years near Mannheim (Germany). Results show that NG partial pressures in soil air correlate with the sum value of O₂+CO₂ and thus differ seasonally from atmospheric values with an enhancement during summer time. Such soil air composition records allow to improve the reliability of NGTs in groundwater. There is evidence for a smoothing of short-term NG fluctuations in shallow groundwater within a few meters of increasing soil depth. Data indicates a further continuous equilibration of groundwater with entrapped air bubbles within the topmost saturated zone, 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, even though this effect is only relevant for shallow groundwater.