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Guidelines on remediation of contaminated sites

Appendix 4.6
Soil gas measurement of landfill gas

Soil gas measurements are measurements of the air in soil or waste. The air is pumped up from temporary probes or from fixed measuring points. The probes are driven down manually or mechanically, if necessary after pilot drilling.

The depth of measurement is determined by the depth of the probe or the air intake of the measuring point. Typical depths of measurement are found in the interval between 0.5 – 5.0 below ground level, depending on location of the fill and the purpose of the investigation. In cases where evaporation into the outdoor air is to be examined, the measurements should be conducted near surface level, approximately between 0.5-1.0 m below ground level. When taking samples in connection with indoor air assessments for existing buildings, the samples should be taken immediately below the floor.

Unlike soil gas measurements of organic and inorganic vapours, measurement of landfill gas calls for are no special requirements regarding which material probes, tubing, and fittings are made of.

The principle for driving down a probe is illustrated in Figure 1, whereas Figure 2 shows a design proposal for a fixed measuring point.

Look here!

Figure 1
Principle for driving down a probe.
  

Look here!

Figure 2
Design proposal for a fixed measuring point.

When measuring gas, the content of methane, carbon dioxide, and oxygen are measured using a portable metering instrument. It is recommended that methane and carbon dioxide contents are determined using infrared absorption, whereas oxygen concentrations are determined using an electrochemical cell. A description of other types of instruments can be found in Working Report No. 69 from the Environmental Protection Agency /1/.

If gas concentrations are continuously logged, continuous pumping of soil gas must be avoided, since this may disturb gas generation in the waste cell. Continuous pumping can be avoided in various ways, e.g. by setting the equipment to pump external air and soil gas at intervals. Gas measurement is recorded on a data logger when soil gas is being pumped.

If landfill gas is detected at a measuring point, soil gas pressure is measured. Such pressure measurements will indicate any pressure above that of the atmosphere caused by landfill gas contents. A liquid-charged U-pipe pressure gauge can be used for pressure measurement.

When monitoring landfill gas, gas concentrations should be seen in relation to meteorological conditions. Data might be obtained from the Danish Meteorological Institute (DMI) detailing observed barometric pressure (registered every three hours), recorded daily quantity of precipitation and recorded ground temperature from the monitoring station(s) nearest the site in question.

References

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