Guidelines on remediation of contaminated sites
Appendix 4.4
Soil gas measurements
A soil gas measurement is measuring of the air in soil pores. Such measurements are carried out by pumping up a small amount of air through a hollow probe, which is either rammed or pressed into the soil. This air is then analysed.
The principle of measurement is outlined in figure 1. The driving down of the probe is carried out either manually, using e.g. a sledge hammer or a copper hammer, or mechanically, using e.g. an impact hammer, if necessary after pilot drilling. Alternatively, the probes can be pressed into the ground by hydraulic means. After being driven down, the probe is pulled slightly up. The resultant hollow space makes it possible to pump air out of the soil.
The depth of measurement is determined by the depth of the air intake, which usually corresponds to the ramming depth. Typical depths of measurement would be 0.5 – 5.0 m below ground level. With assessments of degasification to outdoor air, measurements should be made near the surface, at the top of the contamination; however, measurements should not be conducted at depths of less than 0.5 m below ground level.
Figure 1
Principle for execution of soil gas measurement
When sampling in connection with assessment of indoor air, the sample should be extracted immediately below floor level.
When sampling in compact soil, the measurement represents only a very small amount of soil, preventing the measurement from indicating the presence of any contamination located lower down.
All sampling equipment that comes into direct contact with the soil gas samples must be made of a material which neither absorbs nor emits chemical substances to the soil gas samples to such an extent that it can be detected in the samples.
For probes, it is possible to use anything from black plastic water pipes to specially made pipes of stainless steel. The probe must be able to withstand being driven down and must be made of a material which can be cleaned after use or which is sufficiently cheap to allow the probe to be thrown away after use.
The tubing used should be made of Teflon or polyethylene (PEL).
The air sample is pumped up using a vacuum pump. If sampling is done at a point after the pump, e.g. in a tedlar bag, an oil-free diaphragm pump is used.
The pump is connected to a manometer to measure the back-pressure during pumping. If the back-pressure is heavy, leaking may occur along the sides of the probe.
Before sampling is initiated, stagnant water is removed, where an amount of air approximately 5 times the size of the air volume of the hollow space in the soil, tubing, etc. is pumped up.
The actual soil gas sample can be extracted in several way, depending among other things on the analysis parameters of the analysis method selected, e.g.:
 | Extraction with a syringe through a probe in the tubing. The sample is then immediately injected into a portable gas chromatograph. |
| Collection in tedlar bags, after which the samples can either be analysed in the field using a portable gas chromatograph or be brought to a laboratory for analysis. This does, however, require a short transport period, as the bags are not completely airtight. |
| Collection in liquid or in adsorption tubes, e.g. charcoal sampling tubes, which are then sent to the laboratory for analysis. |
During sampling , sources of error should be avoided; for instance in-leakage of outdoor air due to leaks, including leaks along the probe.
When collecting samples in liquid or tubes, the amount of air that has been collected must also be registered. For this purpose, a gas meter or a flow meter can be used, combined with records of the collection time. Using a flow meter requires a constant flow. Both gas meters and flow meters must be placed after the pump.
When taking samples in charcoal sampling tubes, tubes must remain vertical during sampling to avoid the formation of air passages in the pipes. Furthermore, if the flow (pumping) is too forceful, desorption of the most volatile components may occur. However, this is normally registered in connection with analysis, where these substances are also found in the control zone of the pipe. Collection normally takes place at a pumping capacity of 250 - 1,000 ml/min.
When collecting in charcoal sampling tubes, sampling time is somewhat longer that with other methods, but in return lower limits of detection are obtained.
Substances from probes, tubing, syringes, bags, and pumps may contaminate samples. To avoid cross contamination from probes, a sufficient supply of probes is brought along to avoid using the same probe twice during the same day. The probes are cleaned after use, using either pneumatic flushing or steam.
Syringes are cleaned through heat treatment at 150° C for at least one hour. This cleansing process is subsequently checked by injecting ‘clean air’ into the gas chromatograph using one of the syringes. Tubing and pumps are cleaned through a longer period of pumping ‘clean air’, while mildly contaminated tedlar bags can be cleaned through repeated emptying. The cleaning process for tubing, pumps, and bags is checked through subsequent collection and analysis of ‘clean air’.