Landfill Gas

Screening Procedures, Part 3

E. When to Monitor

The best time of the day to monitor bar hole probes or in-place monitoring wells 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 landfill gas (LFG) migration will be maximum when the difference in pressure between the fill and the ground surface is maximum. At depths greater than 10 feet, the difference in pressure should have a negligible effect on the concentration of LFG present in the probe.

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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 not subject to excessive 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, then it should not be sampled because attempting to overcome the negative pressure could damage the instrument, and it may not detect gas at the correct concentration. 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 not to allow water from entering the instrument. Probes that are damaged or are under negative pressure are considered inadequate for use.
2. Use a gas monitoring instrument that is properly calibrated and warmed up. 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. (Note: Prior to taking any reading, allow the CGI to warm up at least 5 minutes. This will stabilize the instrument and any internal gas-measurement detectors.)
3. 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.

2. For bar hole probes:

1. Construct bar holes as follows:

1. Using the bar hole punch tool, drive the 1/2 inch diameter rod approximately 2.5-3 feet into the soil.
2. Use the bar hole punch only in soil in which the rod can be driven and extracted with a reasonable amount of effort so as to avoid injury to the individual doing the punching.

Notes to consider when constructing bar holes: You will be using the bar hole punch to drive a nominal 1/2 inch plastic PVC pipe cut in 12-18 inch lengths with one end cut off at a 45-degree angle into the soil. This pipe should prevent soil from falling into the hole. You will be inserting a cork or other gas-tight connection to the instrument sample tubing, which will seal the hole from ambient air. Gas can be then sampled from the probe anytime thereafter during the inspection. Gas is sampled with the instrument directly from the soil matrix that lies below the pipe.

Note: Soils conducive to bar hole punching include sands and sandy loams and some silty soils. Soils not conducive to bar hole punching are those with significant amounts (> 30 percent) of clay and hard rock.

Caution: Check with the operator for the location of any buried utility lines (gas, water, electrical) at, or near, the property boundary.

2. Withdraw the bar hole rod from the hole that was made and do either of the following:

1. Place the sampling tip of the gas monitoring instrument into the hole as soon as the rod is removed. Make a seal around the hole by slipping an appropriately sized rubber stopper over the gas sampling tip.

OR:

2. Slip a section of PVC pipe as described in the equipment list onto the bar hole rod, and carefully drive it with the punch into the hole, leaving three to six inches of the pipe exposed, and extract the rod, while leaving the PVC pipe in place. Place a solid rubber stopper or a connector with a closeable sampling port in the exposed end of the PVC pipe.

3. 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 out probe volume of landfill gas.

Note: When creating temporary probes with the bar hole punch do the following to limit the chances of injury:

• Let the weighted head of the bar hole tool drive the bar into the soil. Do not excessively force the bar into the soil. If the force of the weighted head cannot readily drive the bar into the soil, then bar hole monitoring should be attempted at a different point.
• Tuck the pelvis in, while bending the knees slightly, and use your legs and arms when extracting the bar hole tool from the soil to limit back strain and possible injury. The back should remain straight, while bending the knees slightly (do not lock the knees), when using the bar hole punch. 
• If more than one bar hole is made, alternate pounding and extracting the bar hole tool with your "buddy." A "buddy" could be an accompanying LEA inspector or an operator or a fellow Board inspector. Do not do all of the pounding and extracting yourself.

3.  Soil Gas Vapor Impact Probes (SGVIP) probes:

Instructions for placing SGVIP probes may be obtained from your supervisor or the Board LFG training group. The following circumstances may warrant the use of the SGVIP unit for purposes of conducting screening monitoring:

• When bar hole probe readings are between 0.5-5 percent LFG by volume.
• When soil is too hard for bar hole punching and a LFG problem is suspected
  Note: The SGVIP unit may also be used for specialized monitoring (details for specialized monitoring will be developed later by the Closure and Remediation Branch).

CAUTION: Consult with the operator and call Underground Service Alert (USA) at 1-800-227-2600 at least 2 days prior to installation of probes to assure no interference with any underground transmission systems while conducting ground penetration.

G. Monitoring On-Site Structures

All on-site structures, except for gas control facilities, should be checked for the presence of landfill gas. 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 the Scout or GMI CGI 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 27 CCR 20919.5 is being violated, and the operator must plan and institute controls to bring the level below 1.25 percent in the structure.

It is suggested that for the purposes of documenting the results of monitoring, log or record be kept to record the readings. Site location, 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.

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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 a problem.

Once a violation has been detected, it is up to 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 14 CCR 20919, as well as remediate any violation of the gas control standards as required by 14 CCR 20919 and 14 CCR 20919.5.

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I. 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/bar hole. 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 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 CGI 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 Method

High concentrations of methane (greater than 100 percent of the LEL or 5 percent methane) 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 and 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 one should be very familiar with the technical information supplied with the equipment. With some meters calibration with a methane/carbon dioxide mixture can help with 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) measures 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. There are two modes of operation, the survey mode and the gas chromatograph (GC) mode. For methane surface emissions, the survey mode is used if both are available on the instrument.

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, wide sensitivity (1 to 100,000 ppm), and 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.

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Last updated: September 05, 2008