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

5.5. Noble gases 5 Results
5.5.3 Soil atmosphere samples from Site A
A total of 23 noble gas depth profiles were sampled at Site A and measured, the entire dataset
can be found in Table B.9. A few readings (marked red in Table B.9) were omitted from
analysis as they scatter unreasonably strong. Except for the 36 Ar reading of sample A01-I, all
these outliers were connected to technical problems that occurred during the measurement in the
mass spectrometer. The 36 Ar reading of sample A01-I is the only isotope of that sample showing
such elevated readings and is therefore regarded as an artifact of an undiscovered problem during
that single measurement.
The sampling frequency was increased during the irrigation period. Samples were taken right
before irrigation (A21), within one hour after irrigation (A22) and days after irrigation events.
The noble gas sample that is accompanied by the lowest sum of O2 and CO2 (16.7 Vol%) is A19
2 m depth.
Figure 5.9 shows the measured concentrations of the four isotopes 20 Ne, 22 Ne, 36 Ar and 40 Ar
standardized to literature data on atmospheric composition, plotted over time. The plots for the
remaining isotopes can be found in Figure C.17. It is quite clear that most of the noble gas data
is located above or at 100 % relative to atmospheric air composition. A significant drop below
100 %, consistent in all isotopes, was never observed. Only the heavy noble gas isotopes 84 Kr
and 132 Xe showed such reduced noble gas concentrations, though suffering from low accuracy
and strong scattering their reliability is questionable.
The data can be separated into two different time frames based on the differing visible trends.
The first interval is August 2010 – April 2011, when all noble gases show a decline from initial
values well above 100 % at the beginning to values scattering closely around 100 % within the
data’s accuracy. This trend is visible at all three depths, however it is most pronounced at
6 m, where the initial values are between 104 – 106 %, depending on the isotope. The shallower
depths both start at 102 – 105 % and reach 100 % earlier than at 6 m. Between December 2010
and April 2011, the noble gas concentrations scattered around 100 % at all depths.
The second interval is May – June 2011, marking the artificial irrigation of Site A. The reaction
of the noble gases at a depth of 2 m is immediate and strong, concentrations jump from 100 %
to 104 – 107 % (depending on the noble gas isotope) at the last measured noble gas sample
(A23), taken eleven days after the first irrigation (see Figure 6.1). At depths 4 and 6 m noble
gas concentrations show no reaction at all, remaining at around 100 %.
The noble gas isotope ratios 3 He/ 4 He, 4 He/ 20 Ne, 20 Ne/ 22 Ne and 40 Ar/ 36 Ar were calculated,
however a specific evaluation of the mass spectrometer data focused on these ratios was not
done. The ratios displayed no irregularities or significant deviations from literature values (see
Table B.9). The change of O2 and CO2 concentrations had no effect on these ratios, as shown
exemplarily in Figures C.20 and C.21 for 4 He/ 20 Ne and 20 Ne/ 22 Ne.
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