Solid Waste Management Program fact sheet
Division of Environmental Quality Acting Director: Steve Feeler

This document was prepared by the Missouri Department of Natural Resources’ Solid Waste Management Program (SWMP) to provide 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 ensure that 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 that are designed by the manufacturer to provide accurate methane concentrations in zero oxygen environments.  The department recommends that 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:

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 as 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 that 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 percent 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 2 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:

When the problem is eliminated repeat Steps 1-3. If the problem cannot be corrected, record those values and make sure that 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 that weather and soil conditions influence when and what distance the gas will migrate. For these reasons sampling should be conducted when:

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