NOBLE GASES IN SOIL AIR AND GROUNDWATER

Noble gases in groundwater have successfully been used as paleotemperature proxies in many studies. The basic idea is that infiltrating water equilibrates with soil air near the groundwater table, such that dissolved noble gas concentrations reflect the ground temperature in the recharge area via their temperature-dependent solubility. However, the determination of reliable absolute noble gas recharge temperatures requires a correct accounting for two additional effects: (1) During groundwater table fluctuations, bubbles of soil air are entrapped and (partly) dissolved in the water, creating the so-called excess air component; (2) Due to biogenic gas consumption and production, the partial pressures of the noble gases in soil air can deviate from atmospheric air.

The effect of air entrapment is reasonably well described by the so-called closed system equilibration (CE) model for excess air. However, numerical parameter estimation based on fitting this model to measured noble gas data sets can be problematic for some samples. We have analysed the behaviour of the model in detail and propose new strategies to deal with problematic samples based on Monte Carlo simulations. A corresponding new evaluation software will soon be made available.

In order to account for an expected increase of noble gas partial pressures in soil air in response to a decrease of the sum of O₂ and CO₂ concentrations, the so-called oxygen depletion (OD) model has been proposed. Potentially this effect can lead to systematic offsets of noble gas temperatures if not corrected for, and it may be difficult to reliably determine the respective model parameter solely based on the observed noble gas concentrations. We have conducted several studies to investigate the noble gas composition in soil air and related groundwater in the field and confirmed a seasonal occurrence of the OD effect in summer. We have, however, also observed increases in the noble gas partial pressures that are not related to oxygen depletion. While the sum of O₂ and CO₂ concentrations is typically somewhat depleted, we also observed small enrichments. Finally, we find a mass-dependent response of the noble gases to changes of reactive gas pressures. At least qualitatively, these observations can be reproduced by reactive transport modelling of the soil gas.