Diploma thesis in Physics submitted by Florian Freundt born in ...

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5.4. Borehole sealing 5 Results 5.4 Borehole sealing Sealing of the sampling screens from atmospheric contamination as well as from each other is an important factor for the reliability of the depth information of the measured gas concentrations. While the estimated depth accuracy lies within the range of ±0.2 m, assuming homogeneous soil structure, the actual depth accuracy is highly dependent on vertical gas permeability. The weakest link of the sampling site setup is the refilling of the boreholes, possibly leading to large pore space and channels along the buried sampling tubes potentially allowing direct air exchange between the different sampling screens or even exchange with atmospheric air. Radon, O2 and CO2 concentrations were used as tracers for atmospheric air contamination as well as interconnectivity between the different screening depths. Combination of all three tracers allows for discrimination between the two kinds of sealing breaches. 5.4.1 Site A The small variations in 222 Rn measurements taken on Site A (see Figure 5.4) imply a fully contained sealing against atmospheric contamination spanning the entire sampling period. O2 and CO2 measurements, taken during the first 2 minutes as well as during the last 10 minutes of each 30 minute radon sampling showed no significant variation during each sampling, supporting the case of intact sealing as well. A first indication of sealing failure was observed only after the last noble gas sample (A23) was taken: A longer than usual O2 and CO2 measurement on May 21 st showed an increase of O2 and a decrease of CO2 after around 3 minutes of pumping, displayed in Figure 5.6. A 222 Rn measurement taken the day before did not show any signs of dilution with 222 Rn-free 2 atmospheric air. However, the analysis of the entire 222 Rn dataset of Site A led to the discovery of the slight upwards shift in 222 Rn concentrations at the 2 m depth starting in January 2011. This increase likely represents an interconnectivity of the 2 m and 4 m screening depths. This would not have been visible in the O2 and CO2 data before May 2011, since until then the concentrations of O2 and CO2 were of very similar magnitude in all three depths, thereby masking mixing effects. The assumption of an interconnectivity is supported by an increase in the concentration of the short-lived 220 Rn from the first to the second measuring cycle at each sampling of the 2 m depth, indicating the inflow of soil air from a lower depth rather than atmospheric air. Based on these observations, the samples taken until January 2011 (A01 to A09) contain air from the respective sampling depths, while samples taken afterwards until April 2011 (A10 to A16) at depths 2 m and 4 m likely contain mixed soil air. The samples collected in May 2011 (A17 to A23) were taken using the BM2000 instead of the RAD7, with less than 2 minutes of pumping time. Based on the measurement shown in Figure 5.6, these samples contain unmixed air from the respective sampling depths. 2 The atmospheric volume fraction of radon is about 6 × 10 −20 [Porcelli et al., 2002]. 54
5 Results 5.4. Borehole sealing �� Figure 5.4: Radon concentrations during sampling at Site A. The 2 m values show a slight upwards shift of about 10 % starting as of January 2011. Figure 5.5: Radon concentrations during sampling at Site B. Higher than usual 222 Rn values for atmospheric air are due to immediate previous use of the RAD7 with ground air on a different sampling site and are unlikely to be actual atmospheric fluctuations. 55
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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 24 and 25: 2.2. Physical processes and propert
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 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
Page 66 and 67: )! )! *+,-./-01234*-56-20301.7*38*9
Page 68 and 69: 6 Discussion Table 6.1: Synthetic d
Page 71: Chapter 7 Summary The results of th
Page 74 and 75: 8 Outlook Considering the manifold
Page 76 and 77: A.1. Gas sample size A Calculations
Page 78 and 79: A.3. Fractionation-Values A Calcula
Page 80 and 81: Table B.3: Noble gas volume mixing
Page 82 and 83: Table B.5: Calculated values of rel
Page 84 and 85: Table B.8: Precipitation record of
Page 86 and 87: Table B.10: Complete dataset of nob
Page 89 and 90: Appendix C Additional plots and fig
Page 91 and 92: C Additional plots and figures Figu
Page 93 and 94: C Additional plots and figures �
Page 97 and 98: C Additional plots and figures O 2
Page 99 and 100: C Additional plots and figures 4 He
Page 101 and 102: 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
Page 111 and 112:
Bibliography [Aeschbach-Hertig 1994
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BIBLIOGRAPHY BIBLIOGRAPHY [Friedric
Page 115 and 116:
BIBLIOGRAPHY BIBLIOGRAPHY [Riveros-
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Acknowledegment At the end of this
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Diploma thesis in Physics submitted by Florian Freundt born in ...

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