Municipal Solid Waste (MSW) Landfill: virtual
tour
How is Monitoring Performed?
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Monitoring
 Under the Clean Air Act, large landfills that are required to
install landfill gas collection and control systems by the EG/NSPS
rule must perform near surface methane monitoring quarterly to
show that the system is operating properly. Corrective action must
be taken if methane readings are more than 500 ppm above background.
Near surface gas monitoring is the measurement (usually by portable
instruments) of gas concentrations within a few inches of the surface
of the landfill.
How is near surface gas monitoring performed?
 A common method of near surface gas monitoring is the use of a
portable instrument such as an organic vapor analyzer-flame ionization
detector (OVA/FID). Normally, the instrument is calibrated for
methane but it can be calibrated for other gases commonly found
in landfills. The OVA may be fitted with a funnel over the monitoring
probe inlet. The probe inlet and funnel are then held within 2
to 3 inches of the ground surface and the measurement of gas is
recorded by the sampling technician.
 Using a method known as landfill gas sweeping or emissions screening,
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. The measurements may be recorded as parts per million,
percent by volume, or percent of lower explosive limit, depending
upon the type of portable instrument used.
A grab sample may also be taken using a sampling device fitted
with a Tedlar® bag or with a SUMMA®- polished canister.
In both cases the samples are taken to a laboratory for analysis.
The laboratory analysis may yield results for many more specifically
identified constituents of landfill gas than use of portable instruments.
  A combination of a portable instrument and Tedlar® bag sample
is sometimes used to provide a comprehensive analysis of gases
emitted through the landfill cover. The portable instrument is
used to locate “hot spots,” places in the landfill
surface where relatively high concentrations of methane are detected.
A sample is then taken using the Tedlar® bag and sent to a
laboratory for qualitative and quantitative analyses of several
contaminants composing landfill gas.
Results from near surface gas sampling should always be reviewed
in context of meterologic conditions at the time of sampling and
with knowledge of the height of the probe inlet from the surface
of the landfill. Even moderate surface winds of 5 to 10 mph will
greatly dilute the gas sample taken at 4 inches from the surface.
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