HR: 0800h
AN: H51C-0638    [Abstracts]
TI: A Field Test of Noble Gas Temperature Systematics from an Instrumented Monitoring Well: Implications for Gas Transport in the Capillary Fringe
AU: * Sun, T
EM: welsumm@umich.edu
AF: Dept. of Geological Sciences, U. of Michigan, Ann Arbor, MI 48109-1005, United States
AU: Hall, C M
EM: cmhall@umich.edu
AF: Dept. of Geological Sciences, U. of Michigan, Ann Arbor, MI 48109-1005, United States
AU: Castro, M C
EM: mccastro@umich.edu
AF: Dept. of Geological Sciences, U. of Michigan, Ann Arbor, MI 48109-1005, United States
AU: Lohmann, K C
EM: kacey@umich.edu
AF: Dept. of Geological Sciences, U. of Michigan, Ann Arbor, MI 48109-1005, United States
AB: Recent studies have found a significant bias of noble gas temperatures (NGTs) to values well below the average ground temperature at the water table (Hall et al., 2005; Castro et al., 2007). In order to test models that explain this bias, a new monitoring well has been drilled within ~30m of the well studied in Hall et al. (2005). The well is 5cm in diameter, has a total depth of 24.4m, is screened over the bottom 12.2m and has a typical depth to the water table of 13.1m. Preliminary measurements of CO₂ concentration in the screened region within 0.5 m above the water table plus dissolved O₂ in groundwater near the water table are consistent with total CO₂ and O₂ partial pressures of about 0.1 atm, in good agreement with the estimate made by Hall et al. (2005) with their oxygen depletion (OD) model for NGTs. An important feature of the OD model is that the extra noble gas partial pressures reduce the size and importance of the "excess air" component that forms the basis of all standard NGT models. Noble gases in water samples from eight different levels of the well were measured and the He concentrations were uniformly above those expected if groundwater were in equilibrium with the atmosphere. Indeed, the He concentration gradient is about 5 orders of magnitude lower than what would be expected if there was rapid gas diffusion through the unsaturated zone from air above a water table that is in equilibrium with groundwater. Standard models of gas transport suggest that gas diffusion can have extremely high tortuosity at the base of the unsaturated zone and this appears to be necessary to explain the apparently low upward helium flux. Given the potential for restricted transport within the gas phase and the relatively low apparent excess air needed with the OD model, it was decided to re-examine the Partial Re-equilibration (PR) model (Ballentine & Hall, 1999) in light of the possibility that the rate limiting step is diffusion within the gas phase rather than diffusion in water. A boundary layer model with diffusive re-equilibration of excess air at a rate proportional to D^2/3 gives an excellent fit to the data of Hall et al. (2005), yielding correct NGTs and an improvement of the fit over the OD model with unfractionated excess air. Because gas diffusion is less mass dependent than diffusion in water and because this revised NGT model needs comparatively low values of excess air, predicted isotope ratio anomalies are low and within measurement error of actual Ar isotope ratios. Therefore, it may not be strictly necessary to use NGT models that are continuously within equilibrium when measured isotopic ratios are close to their atmospheric values. Ballentine & Hall, 1999, Geochim. Cosmochim. Acta, 63, 2315-2336.
Castro et al., 2007, Earth Planet. Sci. Lett., v257, 170-187.
Hall et al., 2005, Geophys. Res. Lett., 32, L18404, doi:10.1029/2005GL023582
DE: 1694 Instruments and techniques
DE: 1833 Hydroclimatology
DE: 1875 Vadose zone
DE: 4914 Continental climate records
SC: Hydrology [H]
MN: 2007 Fall Meeting