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

Chapter 3
Sampling sites and methods
3.1 Setup of the sampling sites
The sampling sites were chosen based on preliminary data by Schneider [2010] suggesting high
probabilities of oxygen depletion, thereby being well suited to study whether oxygen depletion
has any influence on noble gas partial pressures. Additionally, all sampling sites are in a reasonably
close proximity to the Institute of Environmental Physics’ weather station as well as a
weather station owned by the company meteomedia.
3.1.1 Drilling and instrumentation
Drilling the sample sites was done using a pneumatic pile hammer driven by a gas compressor.
The pneumatic hammer was used to drive a hollow rod of 6 cm diameter and 1 m length into
the ground, allowing for 1 m long, partly compressed core samples to be extracted, from which
the soil layer structures were deduced. Using 1 m long solid extension rods the maximum depth
reachable with the rented equipment was 9 m, however the maximum achieved depth was 5.9 m
at Site A, 4.9 m at Site B and 2.9 m at Site C due to increasing friction at larger depths.
Ground water tables were not reached at any of the sites. For simplification all soil air sampling
depths are hereafter referred to by values rounded to the full meter.
Sampling equipment, consisting of three bundled TYGON R○ R-3603 Vacuum Tubing tubes of
different lengths and two LogTag TREX-8 temperature sensors, was lowered into the boreholes.
The tubes have an inner diameter of 8 mm and were closed at their tips and perforated over a
length of 10 cm by holes of 1 mm diameter (see Figure C.1). At 15 cm depth the tubes and sensor
cables ran horizontally into an underground box which was placed at a distance of about 20 cm
to the hole, with a footprint of 0.03 m 2 , minimally disturbing the immediate site surroundings.
The boxes were covered by removable grass sods and contained one LogTag TRIX-8 and two
LogTag TREX-8 temperature data loggers as well as connectors and valves to access the different
depth tubes. The sampling tubes were bundled into a single access tube, but remained isolated
from each other by valves, preventing contamination by backflow of atmospheric air.
35