Landfill Gas Screening Procedures

This document is released as guidance and is subject to review and revision as necessary. The information in this document should not be considered enforceable or regulatory in nature (i.e., this document does not have the force or effect of law). Mention of trade names or commercial products does not constitute CalRecycle endorsement or recommendation.  All references to regulations are to Title 27, California Code of Regulations unless otherwise noted.

I. Introduction

A. Purpose of this Document

The purpose of this document is to provide standardized procedures for inspectors to use when performing landfill gas (LFG) screening monitoring[1] during routine disposal site inspections. The standardization and simplification of the field gas monitoring procedures are intended to create an acceptable level of sureness in a site’s compliance status with respect to the State Minimum Standards (SMS) for LFG monitoring. The initial monitoring (screening monitoring) process will also provide a basis for determining whether it is necessary to conduct supplemental enhanced monitoring before making a determination of compliance with SMS. This document presumes that each inspector has already had some experience in conducting disposal site inspections including LFG screening monitoring.

Disposal site inspectors need to sample for LFG in the ground at the permitted facility boundary and in the structures within the permitted boundaries of a disposal site while conducting routine; closed, illegal, and abandoned (CIA) site; or pre-permit inspections.

§20921 requires all municipal solid waste landfill operators to ensure that the concentration of methane gas generated from their landfill does not exceed 1.25 percent (by volume in air) in all on-site enclosed structures, excluding LFG control structures, nor 5 percent (by volume in air) at the facility property boundary.  §20921 applies to: (1) active solid waste disposal sites; (2) solid waste disposal sites that did not commence complete closure prior to August 18, 1989, which was fully implemented by November 18, 1990, in accordance with all applicable requirements; and (3) new postclosure activities at any solid waste disposal site that may jeopardize the integrity of a previously closed site or pose a threat to public health and safety or the environment.

§20919 requires operators of disposal sites to conduct monitoring and implement control measures to limit LFG migration if either an enforcement agency (EA), CalRecycle, a local fire authority, or a local building authority notifies the operator that there is cause to believe that a hazard or nuisance may be created by LFG.  §20919 applies to all solid waste landfills.

[1] Please note that due to the variety of instruments in the field (Landtech, GMI, etc.), as well as different methods used by technicians and inspectors, detected gas levels may differ between instruments.

B. Screening Monitoring

Screening monitoring defined:

Screening monitoring defined: Routine expedient field monitoring to determine a LFG control violation or area of concern or if supplemental enhanced monitoring is required.

LFG screening is routinely conducted during facility inspections to determine if there is a LFG migration that could constitute a violation or area of concern or to determine if it is necessary to conduct further monitoring to verify that such violation exists. This monitoring is conducted whether or not a monitoring system is in place onsite.  A monitoring system usually consists of a series of in-ground LFG monitoring wells installed around the permitted facility boundary.  For optimum monitoring purposes, the wells and probes, to the extent feasible, should not be connected to or be in the vicinity of any significant negative pressure (vacuum) source[2].

An example of a basic multilevel well is shown in Diagram 1:

Multi-level Landfill Gas Monitoring Well

Diagram 1

Please Note: The bar hole punch with at least a 3-foot-long clean rod and sliding handle for driving the rod into the ground has been used by inspectors in the past but is no longer considered an appropriate standard tool for monitoring and screening except possibly in limited investigations. Therefore, use of the bar hole punch is discouraged, and the inspector should consult with Engineering Support Branch staff as to the appropriateness of such use on a case-by-case basis.

Diagram 1 shows a cross section of a typical multilevel gas monitoring probe. Details include well diameter, casing diameter and material, soil types at different levels, and backfill material. If this image is inaccessible to you and you need more detailed information, please contact Mike Wochnick.

[2] There may be instances where negative pressure influences may be unavoidable.  These instances should be minimized to the extent feasible and be temporary in duration.

II. Landfill Gas Screening Monitoring Procedures

A. Activities Performed in Screening Monitoring

Some or all of the following activities are performed during LFG screening monitoring:

  1.  Monitoring in-place wells/probes
  2.  Structure monitoring

B. Equipment to be used for Screening Monitoring

In order to conduct adequate screening monitoring for landfills with or without monitoring systems, the following equipment may be needed:

  • Properly calibrated infrared detector, combustible gas indicator (CGI), flame ionization detector (FID), or an equivalent instrument capable of detecting methane gas at concentrations of 0.5-100.0 percent by volume in air.
  • Plastic tubing and assorted connectors for ensuring airtight connections when hooking up the monitoring instrument to probes.
  • Auxiliary air pump (AP).
  • Tedlar bags or stainless steel summa canisters.
  • Evacuation chamber
  • Magnehelix.

Training for LFG screening monitoring and for using gas monitoring equipment is given on a periodic basis by the LFG Training Group (please contact the CalRecycle Training Unit via email at Individual training may also be available as resources permit.  As a matter of practice, new EA inspectors should be trained in the proper procedures for screening monitoring of LFG upon being hired and before evaluating a disposal site for compliance with the SMS for landfill gas by themselves. Additionally, CalRecycle and LEA staff should attend the periodic refresher training on how to conduct LFG screening monitoring. All staff training should be documented.   Maintenance and calibration is documented for each instrument as recommended by the manufacturer to satisfy quality assurance/quality control requirements.

C. Monitoring System Evaluation

1. Pre-inspection information gathering

Prior to the inspection the inspector should obtain and review copies of LFG monitoring reports submitted to the Local Enforcement Agency (LEA) by the operator.  The reports should cover at least a one-year period prior to the inspection.  For existing monitoring systems, the inspector should identify from the monitoring reports any particular wells and/or probes that should be sampled. Additionally, the inspector should obtain maps and as-built drawings for the monitoring well system that indicate location, depth, and number of perimeter wells/probes, as well as location(s) of any gas control system or energy extraction system wells. Any available gas control system drawings and specifications, well construction details, and boring logs would also be useful. This information can help identify wells and/or probes that should be sampled during the initial LFG monitoring. The inspector should also consult the LEA to determine (if applicable) what type of probe fitting/adapter may be used to connect the gas-monitoring instrument to the probes at a site.

If there is no existing and/or functional LFG monitoring system, the inspector should review any available documents containing technical information such as the report of facility information (RFI), joint technical document (JTD), and any closure documents that may provide the information needed to establish the best locations for limited bar hole punch monitoring as described below.

2. Criteria for field checking in-place monitoring systems[3]

Generally, the following should be verified:

  • Monitoring Well/Probe Design: Well/probe construction details should generally be included with the approved gas management plan for a site. A copy of the plan should be obtained from the LEA and placed in the facility file. When monitoring system plans are not available, consideration should be given to the types, quantity, and depth of buried wastes as well as to the proximity of sensitive receptors.[4]
  • Probe Functionality: Monitoring well probes may become plugged for a variety of reasons. Make sure probes are not damaged, partially or fully filled with water, covered with dirt or mud, or otherwise destroyed or contaminated. Additionally, probes should be clearly marked and include shut off valves and/or terminating caps securely attached to them. Probes in vaults are less likely to be damaged or plugged; however, they are more susceptible to flooding. An obstructed probe will result in the LFG monitor pump laboring noticeably or even shutting down completely.  Checking the latest monitoring records should help in determining what probes, if any, may not be fully functional.  Adequate care should be taken to ensure water or excessive water vapors are not drawn into the monitoring instrument.  A hydrophobic filter should always be used.  Additionally, a comparison with the existing gas monitoring records may show a change from previously recorded consistent levels of gas to a string of consecutive extremely low or zero readings.
  • Please note that not all of the LFG issues may be resolved employing the screening process during an initial inspection.  If necessary, upon return from the inspection, the inspector may consider consulting the appropriate technical staff at CalRecycle for further assistance in determining the functionality of plugged probes and to discuss the need for supplemental enhanced monitoring.
  •  Generally monitoring wells are adequately placed if they are at or near the facility property boundary if the landfill has no permit.  If the operator has chosen to install a well inside the permitted facility boundary closer to waste and the inspector finds more than 5 percent LFG in a probe, then the operator will have to either install another well closer to the permitted facility boundary or control LFG at the monitoring point to a level below 5 percent.  A point of compliance may be chosen at any monitoring well designated by the operator to meet §20921. If a methane concentration above 5 percent is found in a compliance probe located toward the interior of a disposal site, the need for further monitoring closer to the boundary is indicated. In such a case, a violation can also be given for an inadequate monitoring system.

Wells shall be placed every 1,000 feet or less around the facility permitted boundary or at least one for each linear side of the facility perimeter boundary (whichever can provide more coverage), for landfills that are subject to §20921. In some cases, exceptions from these guidelines may be allowed. (Additional probes may be required if there is a significant amount of LFG being generated within the site and there are no natural barriers to prevent off-site LFG migration or if significant receptors are nearby.)

Wells shall be placed adjacent to receptors (structures, orchards, crops, etc.) that are within 1,000 feet of the site’s regulatory boundary.

In some instances due to the distance from the refuse to the compliance boundary, probes may be under vacuum influence. Most sampling equipment is capable of overcoming a slight vacuum in a probe and collecting a viable gas sample. However, extraction wells placed directly adjacent to probes may invalidate gas readings from those probes. (See Section II.F.1.1. for more information concerning probes under vacuum influence.)

3. Results of field checking

Monitoring system evaluation will result in one of the following conclusions:

a.  Monitoring system is adequate

  1. Based upon determination by inspector.
  2. Based upon verification of in-place monitoring system constructed according to approved design.

b. There is no monitoring system in place

c. The monitoring system is inadequate

  1. Determined by inspector with or without approved plan.
  2. Determined by review of plans by appropriate technical staff prior to field inspection and verified by inspector.
  3. Based on key probes being partially or fully nonfunctional during screening monitoring.

[3] For additional guidance on monitoring well/probe design and probe functionality, see: Best Management Practices for Landfill Well/Probe Construction.  Additionally, CalRecycle has in-house equipment (borescope) and experience to conduct case-by-case specialized probe functionality tests (contact is Engineering Support Branch Manager).

[4] If possible, conduct a visual inspection to compare the system layout indicated in the available construction plans with the current field situation.

D.  Selection of which wells to monitor

The following sections show where and how screening monitoring should be conducted for facilities that have (1) an adequate monitoring system, (2) no monitoring system, or (3) an inadequate monitoring system after evaluating the monitoring system using the criteria in the previous section:

1. If the monitoring system is adequate:

Monitor selected in-place perimeter wells.
Wells that have one or more of the following characteristics should be selected for monitoring:

  • Well has a documented history of elevated readings.
  • Well at the perimeter of a waste unit without an active control system or that has a control system that is malfunctioning, disconnected, or has a recent history of unreliable operation.
  • Well is close to off-site receptors (within 1,000 ft.). Receptors include buildings, agricultural crops, and public gathering locations such as schools, playgrounds, parks, and golf courses. If multi-depth probes meet any one of the above criteria, all depths should be monitored.

Diagram 2

Wells in Diagram 2 above P1, P4, P5, P6, and P7 should definitely be screened. Either one of wells P2 and P3 are optional unless they have had a recent history of readings above 5 percent methane or they have not been monitored in the past.

2. If there is no monitoring system:

Consider conducting screening monitoring using a bar-hole punch if no other alternatives exist.  CalRecycle Engineering Support Branch staff should be consulted if this option is to be undertaken.

3. If monitoring system is inadequate:

A monitoring system is inadequate if it is significantly different from an approved monitoring plan or if it has not been approved by the LEA and/or CalRecycle. The operator should be required to submit a work plan to correct the inadequacies. A violation or area of concern can be given if a monitoring system is inadequate and differs from what was specified by the LEA, local fire control authority, local building authority, or CalRecycle.

An inspector can still use the guidance in 1 and 2 above to conduct Screening Monitoring. If a violation or area of concern for an inadequate monitoring system is given, the inspector should note all inconsistencies or system problems that led to the finding.

E. When to Monitor

The best time of the day to monitor is when the barometric pressure is lowest. Depending on the weather, this may be late in the morning or early in the afternoon. When the atmospheric pressure is at its lowest, gas can move out of the fill through convective forces caused by differences between the pressures inside the landfill and the atmospheric pressure above ground. The rate of LFG migration will be maximum when the difference in pressure between the fill and the ground surface is maximum. At depths greater than ten feet, the difference in pressure should have a negligible effect on the concentration of LFG present in the probe. Also, surface emissions are minimized during the winter when the landfill surface is wet from rainfall, resulting in increased subsurface gas migration due to the buildup of gas pressure.

F. How to Perform Screening Monitoring

1. At existing monitoring well probes:

  1. Check each probe’s condition and structural integrity and suitability for monitoring. Be sure each inspected probe is checked for negative pressure generated by nearby vacuum sources. A simple way to check for negative pressure is to hold a sheet of paper just above the opening of the probe and see if the paper is sucked to the opening. If the paper is sucked to the probe opening, the probe is more than likely influenced by negative pressure. A magnehelix, if available, should be used to determine whether or not a probe is under the influence of excessive negative pressure. The magnehelix is a device that measures pressure in terms of inches of water. If the probe is influenced by negative pressure in excess of 1 inch of water, and the instrument is unable to rapidly overcome the negative pressure, then it should not be sampled because the negative pressure could damage the instrument .[5] 
  2. Probes should also be checked for presence of water prior to monitoring since water vapor can damage the instrument.  If water is observed in any of the compliance probes, water traps should be used in order to not allow water to enter the instrument.
  3. Use a gas monitoring instrument that is properly calibrated and functional.[6] Open the petcock or otherwise ready the probe for sampling and connect the flexible intake tube assembly to the probe making sure that there is a tight seal.
  4. Direct connect the instrument to the probe as long as the probe is adequate to monitor from (i.e., no water in the probe). Monitor until a steady state reading is achieved for 30 seconds and record the reading. It is not necessary to purge one probe volume of gas.

[5] Probes under significant negative pressure (typically greater than 1 inch of water) by nearby vacuum sources, such as in-fill extraction wells or soil vapor extraction wells, should be evaluated on a site-specific basis in consultation with CalRecycle.  Negative pressure in probes influenced directly by vacuum sources may be necessary in the near term to achieve regulatory LFG compliance. However, the long-term goal should be to balance control of landfill gas at the source with minimizing the need for direct influence on the probe.  Furthermore, vacuum sources should not solely influence a small area represented by the impacted probe (e.g., “blinding” a probe), but should cover the equivalent lateral area between probes where similar migration is likely occurring.

[6] Prior to taking any reading, allow the CGI to warm up for at least five minutes. This will stabilize the instrument and any internal gas-measurement detectors.

G. Monitoring On-site Structures

All on-site structures, except for gas control facilities, should be checked for the presence of LFG. On-site structures may include a scale house (fee booth), maintenance shed, operator’s office, etc. Prior to entering a building to sample for LFG, get permission to do so from the site operator. Sample with combustible gas indicator (CGI) (i.e., Scout or GMI) in areas where cracks in the floor are apparent, as well as behind large objects and in corners. If gas exceeds 1.25 percent by volume at any point in any building within the permitted facility boundary, excluding actual gas control structures, then §20921 is being violated, and the operator must plan and institute controls to bring the level below 1.25 percent in the structure.

For the purposes of documenting the results of monitoring, a log or record should be kept to record the readings. Site location, well/probe number, sampling date, time, weather conditions, name of inspector, equipment model and serial numbers, calibration information, and readings taken should be recorded. A copy of the log or record should be placed in the facility file with the final approved inspection report.

H. When Screening Monitoring is Completed

Screening monitoring is completed once you have sampled:

  • Monitoring wells at or near the permitted boundary that are close to sensitive receptors.
  •  Monitoring wells at or near the permitted boundary where it is suspected that LFG migration may be occurring.

Once a violation has been detected, it is incumbent upon the inspector to decide if additional probes should be monitored in the course of the inspection.  Upon receiving a violation, the operator must conduct complete monitoring of the entire site as required by §20933, as well as remediate any violation of the gas control standards as required by §§20921 and 20937.

III. Instruments

Gas Monitors

The selection of effective portable field gas monitoring equipment for the screening monitoring will depend on whether the screening monitoring is being performed in ambient air or if the gas is being pulled from a migration probe. Screening for surface emissions from cracks in the soil surface near the boundary is performed under ambient oxygen concentrations, while sampling for methane migration is often done in oxygen-deficient monitoring well probes. In addition, selection of equipment will depend on the concentration range of the gas to be sampled. The volatile organic gases typically are in the parts per billion (PPB) or parts per million (PPM) range and will require more sensitive equipment and different sampling methodologies than methane and carbon dioxide which are found in much larger concentrations.

Combustible Gas Indicator (Catalytic Oxidation Method)

Combustible Gas Indicators (CGIs) were originally developed for the natural gas and mine industries and operate under two different principles: catalytic oxidation and thermal conductivity. Some CGIs operate by both methods, but the discussion on surface emission sampling will focus on the catalytic oxidation method, as the thermal conductivity detection method is used primarily for volume gas measurements in migration probes. By the catalytic oxidation method, the CGI measures the concentration of a combustible gas in air, indicating the results in parts per million or in percent of the lower explosive limit (LEL). Often these readings are taken in conjunction with oxygen readings. These instruments use a platinum filament that heats up during the combustion of the sampled gas. Any changes in the combustion temperature will affect the resistance of the filament, which results in an imbalance of the resistor circuit called the “Wheatstone Bridge.” This imbalance is measured via the analog or digital scale of the unit. Some CGIs have two scales, one measuring in parts per million by volume (ppmv) and the other in percent of the LEL.

Limitations of CGIs include:

  1.  The reaction is temperature-dependent and is therefore only as accurate as the incremental difference between calibration and ambient sampling temperatures;
  2. Sensitivity is a function of the physical and chemical properties of the calibration gas; therefore, methane should be used as the calibration standard;
  3. The unit will not work well in oxygen-deficient or oxygen-enriched atmospheres (it will give false negative readings in oxygen-deficient environments); and
  4. The filament can be damaged by certain compounds such as leaded gas, halogens, and sulfur compounds, and silicone will destroy the platinum filament. Since LFG contains some halogenated (chlorinated) hydrocarbons, the meter should be calibrated often to methane (the target constituent) and serviced yearly if it used on a routine basis to monitor methane surface emissions. In addition, if the meter contains an oxygen cell, this cell can be fouled by the carbon dioxide found in LFG, and replacement of the cell may be required frequently.

Advantages to the CGIs are that they are small and portable, self-contained for field use, have an internal battery, are easy to use, and typically are intrinsically safe.

Combustible Gas Indicator/Thermal Conductivity (TC) Method

High concentrations of methane (greater than 100 percent of the LEL or 5 percent methane by volume) are measured with a percent GAS instrument using a thermal conductivity (TC) sensor. This type of sensor is often used with a catalytic oxidation sensor in the same instrument. The catalytic sensor is used to detect concentrations less than 100 percent of the LEL.  At higher concentrations the TC sensor is used to measure up to 100 percent gas by volume.

The TC sensor is composed of two separate filaments heated to the same temperature. Combustible gases enter only the TC side of the filament; the other filament (compensating) maintains a steady heated temperature. Incoming gases cool the TC filament, and as the filament temperature decreases, the resistance across the Wheatstone Bridge also decreases, resulting in a meter reading. Instruments using a TC sensor do not require oxygen for a valid reading as burning of the gas is not involved.

Combustible gases vary in their ability to cool the TC filament. Methane absorbs heat well and efficiently cools the filament and is the calibrant gas of choice when using the instrument to measure methane in landfill gas. However, since landfill gas is comprised of a combination of different gases, readings on the meter will vary depending on the concentration of the other gases in the sample. Gases that cool the filament more effectively than methane (as the calibrant gas) will display a higher percent GAS reading than is actually present. The converse is also true (i.e., gases that are less effective in cooling the filament will display a lower percent GAS reading than is actually present.).

It is important to realize that certain gases can cool the filament and not be combustible. Carbon dioxide, typically found in landfill gas at high concentrations, absorbs heat readily and can produce a false positive reading. Meter sensitivity to carbon dioxide varies from manufacturer to manufacturer, and the inspector should be very familiar with the technical information supplied with the equipment. With certain meters, calibration with a methane/carbon dioxide mixture can minimize the interference of carbon dioxide with the monitoring of methane in landfill gas.

Flame Ionization Detector (FID)/Organic Vapor Analyzer (OVA)

Flame Ionization Detectors (FIDs) measure many organic gases and vapors. Some FIDs are commonly referred to as Organic Vapor Analyzers, or OVAs. FIDs operate by a sample being ionized in a detection chamber by a hydrogen flame. A current is produced in proportion to the number of carbon atoms present.

Since the sensitivity of the instrument depends on the compound, methane should be used as the calibration standard. These instruments are less rugged in the field than the CGIs and require hydrogen gas cylinders for use. The advantages to the FIDs are fast response in the survey mode and wide sensitivity (typically 1 to 100,000 ppm).  Also, some models offer a telescopic probe with cup intake that minimizes operator exposure to LFG and minimizes windy conditions at the site. The “cup” probe design can also serve to reduce the near surface dilution effects of the wind by providing a small sampling chamber when the probe is held normal to the surface.

Infrared (IR) Analyzer

Most IR analyzers are single beam spectrophotometers. Chemicals have a vibration energy that is specific to that chemical (gas). When the gas interacts with IR radiation, it absorbs a portion of the IR energy. The absorption spectrum for that gas is the pattern of vibrations from the atoms/functional groups, along with the overall molecular configuration. Specific gases will demonstrate optimal absorption within a small IR range. Since absorption ranges have been classified for different gases, it is possible to filter out all but a small part of the spectrum and measure the gas known to be present.

The instrument works by a sample being drawn into the sample cell.  IR radiation travels through the cell for a specific path length before reaching the instrument’s detector. IR absorbance by a gas over a given path length is proportional to its concentration. IR monitors used to analyze for landfill gas have fixed path lengths to detect methane and carbon dioxide in monitoring probes and gas extraction systems within landfills.

The advantage of IR analyzers is that the high carbon dioxide levels found in landfills will not affect methane readings. Portable IR meters available for the field are capable of measuring up to 100 percent by volume methane and carbon dioxide. The concentrations of these gases are detected by infrared absorption. Oxygen concentration gas is measured by an electrochemical cell. These meters are designed to measure large concentrations of methane and carbon dioxide and are not sensitive at concentrations less than 0.5 percent.

Field calibrant gas should be used to verify the accuracy of the monitoring results. A combination gas of 15 percent methane and 15 percent carbon dioxide is a common mixture when using the equipment to test migration probes. Higher concentrations of calibrant gases should be used if monitoring levels in gas extraction wells.