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2.2. Physical processes and properties of soils 2 Theory The OD model resembles the UA model in formulation [Aeschbach-Hertig et al., 2008] and can be incorporated into the existing models: C OD i = Ci,eq(POD + A · Hi) (2.20) A modification of the OD model adding partial re-equilibration at the air-water boundary layer, the gas diffusion relaxation model, or GR model, was introduced by Sun et al. [2008] to describe a noble gas record from a Michigan aquifer: C GR i = Ci,eq � POD + A · Hi · exp � −FGRD β i �� (2.21) where FGR is a parameter that depends on the time taken for the gas transfer as well as on the length scale of the boundary layer. Comparing the different models Sun et al. [2010] showed in a recent comparison of the UA, the CE, the OD and the GR model that, while the absolute noble gas temperatures deviate between the different models, temperature differences are reproduced quite consistently by all models. While the accuracy of the fits achieved by the different models and the respective χ 2 values give an indication of which model might describe the given dataset best, the decisive criteria which model to employ should be the physical relevance of the model concepts and of the parameters resulting from the optimization process. Therefore a more detailed analysis of the processes involved in noble gas dissolution is called for, like the observation of excess air formation done by Holocher et al. [2002] and Klump et al. [2007, 2008]. This study’s intention is to test the physical foundation of the OD model’s assumption of increased noble gas partial pressures in soil atmospheres to better assess the significance of the good χ 2 values found by Hall et al. [2005] and Castro et al. [2007] using the OD model approach. 2.2 Physical processes and properties of soils Understanding the soil regime is essential, as it provides the environment in which the components of meteoric water relevant to noble gas paleoclimatology interact (ideally) for the last time with the atmosphere before the water enters the aquifer. Both soil temperature and soil atmosphere composition are related to conditions above the surface, but are not necessarily identical and therefore an understanding of these relations is required to be able to interpret the measured noble gas concentrations. The following summary of the soil regime’s properties and processes is largely based on Scheffer and Schachtschabel [2010]. 2.2.1 Subsurface thermal regime The main heat source dominating the upper soil’s temperature profile is the radiation of the sun. Geothermal heat flux can generally be neglected for depths relevant to ground water recharge 24
2 Theory 2.2. Physical processes and properties of soils Temperature [°C] 50 40 30 20 10 0 0 0 4 8 12 16 20 24 J F M A M J J A S O N D Time [h] (a) (b) 30 Temperature [°C] 35 25 20 15 10 5 Month Figure 2.5: Temperature variations in soil regimes. (a) Diurnal variations at different depths at a loam soil. (b) Annual variation. From Hillel [1980]. as it usually takes place too close to the soil surface to be affected by geothermal heat. Heat loss from the soil is caused by emission of infrared radiation from the soil and evaporation. The driving force of conductive and convective heat transfer within the soil are up- and downward temperature and pressure gradients. Convective heat transfer is usually mediated by the movement of liquid water and water vapor through the soil structure. The importance of water in the soil thermal regime is also visible in the fact that dry soils heat up more quickly and to a higher temperature than wet soils, as water acts damping on heat transfer due to its high heat capacity compared to air [Scheffer and Schachtschabel, 2010; Wieser, 2011]. Infiltrating meteoric water does, aside from convectively introducing heat, cause significant temperature changes depending on initial water content of the soil due to release of thermal energy during adsorption, called heat of wetting [Prunty and Bell, 2005]. However, as a rough approximation, the soil temperature at a certain depth can be described as oscillating sinusoidally around an average value, driven by atmospheric temperature changes (see Figure 2.5). Sinusoidal approximation of the soil temperature as a function of depth and time T (z, t) with the boundary conditions T (0, t) = ¯ T + A0 sin ωt and T (z = ∞, t) = ¯ T , with z = 0 being the soil surface and ¯ T the diurnal average surface temperature, leads to the introduction of a damping depth d and a phase shift −z/d [Hillel, 1980]: T (z, t) = ¯ � T + A0 · exp − z � � · sin ωt − d z � (2.22) d The damping depth d is related to the thermal properties of the soil and is therefore individual for any given soil regime. While the inclusion of the annual temperature fluctuations complicates the situation, the basic characterization of soil temperature at fixed depths following a harmonic oscillation around an average value as a crude description of soil temperature regimes remains viable. An detailed description of temperature modeling is given by Saito and ˇ Sim˚unek [2009]. 25
Page 1: Faculty of Physics and Astronomy He
Page 5: Measuring annual variation of soil
Page 8 and 9: CONTENTS CONTENTS 3.1 Setup of the
Page 11 and 12: Chapter 1 Introduction Understandin
Page 13: 1 Introduction Molins and Mayer [20
Page 16 and 17: 2.1. Noble gas temperatures 2 Theor
Page 26 and 27: 2.2. Physical processes and propert
Page 28 and 29: 2.3. Soil atmosphere composition 2
Page 35 and 36: Chapter 3 Sampling sites and method
Page 37 and 38: 3 Sampling sites and methods 3.1. S
Page 43 and 44: Chapter 4 Measuring methods 4.1 Mas
Page 45 and 46: 4 Measuring methods 4.1. Mass spect
Page 47: 4 Measuring methods 4.2. On site me
Page 50 and 51: 5.2. Temperature profiles 5 Results
Page 52 and 53: 5.3. O2 and CO2 profiles 5 Results
Page 54 and 55: 5.4. Borehole sealing 5 Results 5.4
Page 56 and 57: 5.4. Borehole sealing 5 Results Fig
Page 58 and 59: 5.5. Noble gases 5 Results 8 days,
Page 60 and 61: 5.5. Noble gases 5 Results 5.5.3 So
Page 62 and 63: 5.5. Noble gases 5 Results 5.5.4 So
Page 64 and 65: 6 Discussion This rather extreme am
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Page 68 and 69: 6 Discussion Table 6.1: Synthetic d
Page 71: Chapter 7 Summary The results of th
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8 Outlook Considering the manifold
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A.1. Gas sample size A Calculations
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A.3. Fractionation-Values A Calcula
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Table B.3: Noble gas volume mixing
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Table B.5: Calculated values of rel
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Table B.8: Precipitation record of
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Table B.10: Complete dataset of nob
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Appendix C Additional plots and fig
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C Additional plots and figures Figu
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C Additional plots and figures �
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C Additional plots and figures O 2
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C Additional plots and figures 4 He
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C Additional plots and figures 20 N
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C Additional plots and figures 10 1
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Appendix D Datasheets 107
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D Datasheets LogTag TREX-8 Technica
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Bibliography [Aeschbach-Hertig 1994
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BIBLIOGRAPHY BIBLIOGRAPHY [Friedric
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BIBLIOGRAPHY BIBLIOGRAPHY [Riveros-
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Acknowledegment At the end of this
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