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

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3.2. Sampling methods 3 Sampling sites and methods sampling tube. Sampling was interrupted and could not be continued at this depth until the end of September 2010 (Sample ID C04). Additionally, several extractions of Site C samples within the mass spectrometer failed due to mechanical damage to the sample sealing and led to an even more incomplete record. Sporadic sampling during winter and limited access to the mass spectrometer led to the site finally being omitted from sampling. Furthermore, an O2 and CO2 measurement in May 2011 proved this site to suffer from atmospheric air contamination in all three depths as well. 3.2 Sampling methods Gas samples were extracted from the sites by attaching a Durridge RAD7 Radon Detector as a pump to the buried sampling tubes via the connector tube inside the equipment boxes. The perforated tips of the sampling tubes allow for soil air to be sampled from a screen of 10 cm height, however the actual ground layer being sampled was probably larger. The RAD7 -pump has a nominal maximum pump rate of 1 l/min, measurements of the pump rate resulted in an actual maximum pump rate of only 0.45 l/min though [T. Reichel, personal note]. Pressures during sample pumping were seldom more than 10 mbar below local atmospheric pressure, indicating low resistance allowing the pump to reach its actual maximum pump rate. Each sampling depth was pumped for 30 minutes before the actual noble gas sample was taken. The samples A17 to A23 were taken using the Geotech BM2000 Biogas Monitor pump instead of the RAD7 ’s, omitting accompanying Radon measurements but leading to reduced pump times per sample of approximately 2 min instead of 30 min 1 . The air within the largest occurring dead space (the 6 m sampling tube at Site A) is moved away within the first 1.5 minutes of pumping at a pump rate of 0.5 l/min. As Figure C.6 shows, O2 and CO2 measurements reached stable reading within the first 60 seconds of pumping with the BM2000. Scheffer and Schachtschabel [2010] gives a range of 65 – 35 % for pore space fraction of clay soils, assuming a worst case of 35 % leads to an upper estimate of evacuated soil volume (at 0.5 l/min pump rate and 30 min of pumping) of 0.043 m 3 per sampling depth. Assuming ideally homogenous soil structure leading to an isotropic, circular air inflow, this would correspond to an sampling radius of 22 cm. Therefore the depth accuracies for noble gas, O2 and CO2 measurements are given as ±0.2 m assuming intact sealing, while the depth uncertainties for the temperature data should not exceed ±0.1 m. 3.2.1 Noble gas samples The soil air was pumped through a 6x1 mm tube made of deoxidized copper (Wieland cuprofrio R○ Cu-DHP), see Figure C.4. After 30 minutes of pumping, the pump-ward end of the copper tube was squeezed shut airtight [Wieser, 2006] using a pneumatic plier. Pressure was measured inline after closing the pump-ward end of the copper tube: once the connection to the pump 1 Pump rates for the BM2000 were specified by the manufacturer as approximately 0.5 l/min. Comparison of the achieved pumping pressures of the RAD7 and the BM2000 during sampling led to the conclusion that the BM2000 ’s pump rate is likely higher than specified and lies somewhere in the range of 0.5 to 1.0 l/min. 40
3 Sampling sites and methods 3.2. Sampling methods Figure 3.4: Typical sampling setup using the RAD7 as a pump, picturing Site B. (a) Location of the refilled borehole. (b) Equipment box with access valves to the different sampling screens and containing the LogTag dataloggers. (c) Inline pressure measurement installed between borehole and sample tube. (d) O2 and CO2 measuring device installed between borehole and sample tube. (e) Copper sample tube and temperature sensor. (f) Drierite desiccant tube and RAD7 radon monitor. was separated the inline pressure rose from the slightly lower pumping pressure to a value corresponding to the current atmospheric pressure. Therefore all sampling tubes contain gas at atmospheric pressure. From there the copper tube was divided into three parts of roughly the same length, resulting in three samples per depth and sampling event. Sampling was always done from top to bottom, starting with the shallowest available depth. The copper tubes were marked by carving in the IDs rather than marking them with tape or a pen, to ensure minimal contamination when installed in the mass spectrometer. The sample ID consists of the site name (A, B or C), the two digit successive number of the sampling event and the depth given in roman numerals, ascending with depth (at Site A, I, II and III referred to 2 m, IV, V and VI to 4 m and VII, VIII and IX to 6 m). The ID A04-V therefore refers to a sample taken at Site B on the fourth sampling event of that site, at a depth of 4 m. The sample tube length was increased from ∼6 cm to ∼12 cm (corresponding to a volume of ∼0.7 cc and ∼1.5 cc) after the first batch of samples was analyzed in the mass spectrometer showing that the amount of 84 Kr in the small samples was technically impractical for the utilized mass spectrometer preparation line. Samples A01 – A08, B01 – B07 and C01 – C05 are of small size, all other samples are of the larger variant. 41
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 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 39: 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
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
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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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