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

5 Results 5.3. O2 and CO2 profiles
temperature changes is questionable, as the temperature record shows several spikes unrelated to
active irrigation and reminiscent of previously observed sensor failures due to moisture intrusion.
These spikes do indicate though that the water infiltrated the soil to a depth of 2.5 m within an
hour, and therefore likely reached the lower depths as well within a short time.
5.3 O2 and CO2 profiles
O2, CO2 and CH4 were measured at Site A and Site B starting in February 2011. Since
the sealing of Site B is questionable (see Section 5.4) only the measured gas concentrations at
Site A are discussed here in detail, the entire dataset is included in Table B.11. Furthermore,
CH4 was never measured in any significant amount: the rare and unsystematic occurrences of
maximum readings of 0.1 Vol% can be ignored with respect to the sensor’s accuracy of 0.5 Vol%.
During February to April 2011, O2 and CO2 were measured only when noble gas samples were
taken to minimize disturbance of the site’s soil. During that time, the depth profiles (see
Figure 5.2, A and B) of both O2 and CO2 showed uniform concentrations, indicating low CO2
production. CO2 concentrations at all depths fell during that time from an initial maximum of
(4.7 ± 0.5) Vol% to (3.4 ± 0.5) Vol% while O2 concentration remained constant at all depths
(within the measurement’s accuracy) at (18 ± 1) Vol% (see Figure C.13). While O2 levels
were depleted compared to atmospheric concentration at that time, the deficit was more than
compensated by the rise in CO2 concentrations.
Irrigation of Site A was started on May 9 th , 2011 as a reaction to the prolonged dry period,
accompanied by an increase of the frequency of O2 and CO2 measurements (see Figure 5.3).
Measurements were executed directly before each irrigation and hourly afterwards. The response
of the O2 and CO2 concentrations was immediate at all three depths. The most pronounced
reaction occurred at the shallowest depth of 2 m, while at 6 m only little change was observed
(see Figure 5.2, C, D and E).
The response of the CO2 concentrations is characterized by two phases: the initial reaction is
a steep drop in CO2 concentrations at all depths, the maximum at 2 m was a drop by 1.7 Vol%
while at 6 m the largest drop was 0.2 Vol%. During the following hours the CO2 levels recovered
to the preceding levels at 6 m, at 2 and 4 m depth they rose above them. The gain in CO2 was
most highest at 2 m, where the CO2 concentration peaked at (6.8 ± 1.0) Vol% about two weeks
after the first irrigation (see Figure 5.2, E). After the last irrigation was done on May 19 th ,
the CO2 concentration at 2 and 4 m started to decrease slowly, but steadily. At 6 m, the CO2
concentrations never peaked but rose slowly and steadily by about 1.0 Vol% during the first 50
days after the initial irrigation (see Figures 5.3 and 5.2, F).
O2 concentrations at 2 m rose slightly during the first few hours after each irrigation, the overlying
trend however was a depletion of O2 during the first two weeks, reaching a minimum
concentration of (10.9 ± 1.0) Vol% ten days after the first irrigation (see Figure 5.2, D). Once
irrigation stopped, the levels rose again. At 4 m the influence of irrigation was much less pronounced
but visible, leading to a minimum in O2 concentrations at (13.2 ± 1.0) Vol% 15 days
logging frequency was subsequently increased to 5 minutes to be able to resolve the water incursion.
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