The International Information Center for Geotechnical Engineers

Landfill Gas Monitoring Systems - Near Surface Gas Monitoring

3.4 Near Surface Gas Monitoring

 Near surface gas monitoring is the measurement (usually by portable instruments) of gas concentrations within a few inches of the surface of the landfill. A common method of near surface gas monitoring is the use of a portable instrument such as a flame ionization detector (FID). An FID is a scientific instrument that measures the concentration of organic species in a gas stream. The operation of the FID is based on the detection of ions formed during combustion of organic compounds in a hydrogen flame. The generation of these ions is proportional to the concentration of organic species in the sample gas stream.


Figure 12

Figure 14. Flame ionization detector (Matthew Klee, GC Solutions)


The sampling technician walks over the surface of the landfill in either a random method or over a pre-defined grid. The sampling technician records the instrument readings, making careful note of the geographic location of each measurement and the surface conditions, height of the probe inlet from surface, and weather conditions such as wind speed. The measurements may be recorded as parts per million by volume, percent by volume, or percent of lower explosive limit.

Recent studies has shown that using FID as the monitoring device is unreliable. Sample bag or Summa canister can also be used to get a "grab" sample of near surface gas.

Near surface gas monitoring is useful for finding cracks in landfill cover, or the proper locations of installing the soil gas wells.


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