Berlin 2012 – scientific programme

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UP: Fachverband Umweltphysik

UP 7: Cryosphere, Hydrosphere and Oceanography

UP 7.4: Talk

Wednesday, March 28, 2012, 16:15–16:30, HFT-FT 131

Plausibility and Stability of Models for Dissolved Noble Gases in Groundwater — •Werner Aeschbach-Hertig, Florian Freundt, Michael Jung, and Martin Wieser — Institut für Umweltphysik, Universität Heidelberg , 69120 Heidelberg

Several models describe dissolved noble gases in groundwater and the phenomena of excess air and degassing. The classical explanation for excess air is that air bubbles are entrapped during infiltration and subsequently completely dissolved. This "unfractionated air" (UA) model is hard to reconcile with the amount of entrapped air and the available pressures in soils. Models based on diffusive fractionation are in conflict with the absence of isotope fractionation. The "oxygen depletion" (OD) model postulates an increase of noble gas partial pressures in soil air as a result of oxygen consumption. Recent experiments have demonstrated this effect to be seasonally restricted and rather weak.

The "closed-system equilibration" (CE) model assumes that trapped air bubbles reach solubility equilibrium with groundwater at the local pressure. We find this model to provide the most physically realistic description of the gas - groundwater interaction. The flexible CE model includes both excess air and degassing and encompasses the UA model as well as a pressure effect similar to the OD model as limiting cases. The downside of this versatility is its tendency towards numerical instability. Occasionally solutions with clearly unrealistic parameter values occur, which produce a warm bias and large uncertainties of estimated temperatures. In some cases it appears possible to find corresponding well-behaved solutions of the model.