Waste Management Program fact sheet
Division of Environmental Quality Director: Kyra Moore


This document, prepared by the Missouri Department of Natural Resources’ Solid Waste Management Program (SWMP), provides guidance regarding the quarterly sampling of gas monitoring wells as required by 10 CSR 80-3.010(14) and 10 CSR 80-4.010(14). Gas monitoring wells are installed around landfills as a means to make sure methane gas does not migrate off-site and pose a threat to public safety.   

Sampling Equipment

Proper selection of sampling equipment is critical in obtaining accurate soil gas concentrations. Explosimeter-type instruments are not appropriate for measuring methane in monitoring wells because these instruments burn the sample to analyze it. The concentration of oxygen in a monitoring well may be insufficient for combustion of the sample. These instruments will typically give false low readings when high concentrations of methane are present.  Therefore, monitoring for methane and other landfill gases must be done using instruments designed by the manufacturer to provide accurate methane concentrations in zero oxygen environments. The department recommends instruments used to sample gas monitoring wells have automatic pumps with the ability to draw a sufficient volume of a representative sample of soil gas into the well. The instrument must be able to read oxygen, carbon dioxide, and methane concentrations.

In order to ascertain well competency, weather conditions and other information concerning the methane migration, the department asks landfill owners to employ instruments capable of reading the following parameters that are helpful in investigating and remediating methane gas migration:

  • Barometric (atmospheric) pressure
  • Differential pressure between the well and atmosphere
  • Hydrogen sulfide
  • Carbon monoxide

Sampling Procedure Steps

  1. Calibrate the instrument to methane per the manufacturer’s instructions.
  2. Prepare the instrument to collect a sample by allowing it to properly warm up per the manufacturer’s instructions.
  3. Connect the instrument to the well head and begin collecting a sample. If there is no sample port on the well, drop the hose from the methane monitor into the well and close off all other spaces to make a seal.  The department recommends a hydrophobic filter be placed between the well connection and the monitoring equipment to help prevent water from being drawn from the well into the sampling collection hose and into the instrument.
  4. Continue collecting the sample until the reading stabilizes. A stable reading is one that does not vary more than 0.5% by volume on the instrument’s scale.
  5. Record the stabilized reading including the methane concentration, oxygen concentration, carbon dioxide concentration, and other parameters, if available

Note: A proper reading should have two percent oxygen by volume or less. If levels of oxygen are higher, it may indicate that air is being drawn into the system giving a false reading of the true soil gas concentrations. Possible explanations for this problem are:

  • The gas monitoring well seal has failed.
  • Well head connectors are leaking.
  • A connection at the instrument is leaking.

When the problem is eliminated, repeat Steps 1-3. If the problem cannot be corrected, record those values and make sure the problem is well documented in the report sent to the department.

Obtaining accurate soil gas concentrations from gas monitoring wells is dependent upon using a consistently-implemented, proven method, using the monitoring equipment’s manufacturer’s recommendations. If you have problems using the sampling procedures described, contact the manufacturer of your monitoring equipment to determine what is preventing the collection of an accurate sample.

Sampling Times

The times chosen to monitor methane gas are almost as important as the procedures used to collect the sample. If possible, sample collection should occur when landfill gases are most likely to migrate. Scientific evidence indicates weather and soil conditions influence when and what distance the gas will migrate. For these reasons sampling should be conducted when:

  • Barometric pressure is low.
  • Soils are saturated.
  • The ground surface is frozen, snow- and/or ice-covered.


The Missouri Solid Waste Management Regulations require that gas sampling be conducted at least quarterly. The results must be submitted to SWMP electronically within one week of collection. The information to be included in the data submission and the format by which to submit the data is described in SWMP’s guidance document, “Guidance for Submitting Landfill Gas Monitoring Data.”


Missouri has stringent regulations governing methane gas migration. It is to everyone’s benefit to address the issue of migrating landfill gases before they present a threat to public health and safety or the environment.


Landtec Landfill Control Technologies, Landfill Gas System Engineering Design: A Practical
, course notes from Landfill Gas System Engineering Design Seminar, 1994.
Missouri Department of Natural Resources, Flood Grant Team, Landfill Gas Monitoring Protocol

Nothing in this document may be used to implement any enforcement action or levy any penalty unless promulgated by rule under chapter 536 or authorized by statute.

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