N.C. Division of Waste Management - METHANE MONITORING GUIDANCE FOR LANDFILLS

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Methane Monitoring Guidance
Introduction
North Carolina Solid Waste Management Rules 15A NCAC 13B require quarterly monitoring of methane gas to insure that it does not exceed the lower explosive limit at the facility property boundary or 25 percent of the lower explosive limit in facility structures. If the concentration does exceed the specified limits then steps must be taken to ensure protection of public health and a remediation plan must be implemented. To make certain these performance standards are met, it is necessary to have a monitoring procedure that is technically sound.
This text has been drafted to provide guidance and technical assistance with methane gas detection monitoring at municipal solid waste landfills. The primary focus is to establish a standardized procedure for use when sampling methane probes.

There are pros and cons for various proposed sampling techniques. One of the biggest debates concerns purging the probes before measuring the methane concentration. Measuring without purging risks finding false elevated concentrations that have built up over time, whereas measuring after purging risks missing detection altogether. Since public safety is the issue, the Solid Waste Section will have a conservative approach.

Background
Organic matter, as found in landfills, begins to decompose almost immediately after being placed in a disposal site. Putrescible wastes such as food products and sewage sludges begin to break down by biological processes very rapidly whereas paper, cardboard or cellulose based materials are more slow to decompose. However, when conditions become right, most organic matter will decompose. Typically the decomposition goes through several stages that depend on several factors such as pH, temperature, and moisture content. The final stage results in the production of methane and although the rate of production may vary, almost all landfills produce methane.

Although methane is not toxic, it is explosive at concentrations between 5 and 15 percent when sufficient oxygen is present. Landfill gas typically has a methane concentration of 40 to 55 percent volume in air. Methane is odorless and slightly lighter than air and will accumulate in confined spaces. There are numerous records of deaths and injuries resulting form fires and explosions.

Methane moves from place to place by diffusion and pressure gradient. It usually takes the path of least resistance and small seams of porous soil can transmit large quantities. Underground off-site migration is common. Movement depends on soil type and moisture, and migration distances of 1500 feet have been observed. Barometric pressure also influences movement. Falling barometric pressure tends to force methane out of the landfill and into surrounding areas.

Instrumentation
Portable field instruments used to measure methane come in many shapes and sizes. It is very important that the operator understands the principles of operation and follows the manufacturer's instructions when using a particular instrument. Instruments differ in operational principles and may have certain limitations, depending on the circumstances. For example, some instruments may give false readings when methane concentrations are greater than the lower explosive limit and oxygen levels are low.

Please include on the gas monitoring data submitted to the Solid Waste Section the type of methane monitoring equipment used. Data collected with monitoring equipment that was not designed for methane analysis will not be accepted by the section.

Instrumentation Quality Assurance
All instruments must be used according to the manufacturer's instructions. This includes calibrating the instrument as directed. Measurements from non-calibrated or inaccurately calibrated instruments are not reliable.

Please include calibration procedure and results for each sampling event with sampling report.

Monitoring Probe Construction
All methane monitoring probes should be fitted with a cap that has a stopcock type valve that can be opened or closed to control flow. The valve should have a barb connection that will fit the gas meter's inlet probe tube. The valve should be closed between monitoring events. If the probe caps do not have stopcocks, they should be installed.

Data collected in any other fashion, unless pre-approved by the Section, will not be accepted.

Monitoring Procedure
As previously discussed, the Solid Waste Section's approach to measuring methane concentrations in monitoring probes is to be conservative. Any accumulation of methane in the probe is the result of methane migration. The possibility exists that, just as t can migrate and accumulate in monitoring probes, it can also do the same in buildings and structures. Because subsurface gas pressures are considered to be at a maximum during the afternoon measurements should be done in the afternoon. Consequently, the following procedure is recommended for sampling methane gas monitoring probes:

1. Calibrate the combustible gas meter according to the manufacturer's instructions.

2. Attach the gas analyzer probe tubing to the monitoring probe stopcock on the probe cap.

3. Open the valve and record the reading.

4. Record the methane gas concentration.

5. Turn the valve to the off position and disconnect the probe tubing.

6. Proceed to the next probe and repeat steps 2 through 5.