Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 9.4
METHANE

9.4 METHANE

Figure 9.1 Methane Mitigation Decision Matrix
Distance to structure Methane Concentration in Soil Gas
<1000 ppm (<0.1%, <2% LEL) 1000 - 5000 ppm (0.1% - 0.5% 2% LEL - 10% LEL) 5000 - 12,500 ppm (0.5% - 1.25% 10% LEL - 25% LEL) > 12,500 ppm (> 1.25% > 25% LEL)
0 ft (beneath structure) No Controls Recommended Methane Monitoring Conduit Seals and Utility Trench Dams Methane Monitoring Conduit Seals and Utility Trench Dams Vapor Mitigation Methane Monitoring Conduit Seals and Utility Trench Dams Vapor Mitigation
0 - 100ft No Controls Recommended Methane Monitoring Methane Monitoring Conduit Seals and Utility Trench Dams Methane Monitoring Conduit Seals and Utility Trench Dams Vapor Mitigation
100 - 300ft No Controls Recommended No Controls Recommended Methane Monitoring Methane Monitoring Conduit Seals and Utility Trench Dams
>300ft No Controls Recommended No Controls Recommended No Controls Recommended Methane Monitoring
1. Decision matrix modeled after Geonsyntec 2011.
2. Actions listed in this decision matrix assume soil gas pressure is < 2 in-H2O. If soil gas pressure is greater than 2 in-H2O, then the need for enhanced mitigation measures should be evaluated.
3. This mitigation matrix does not preclude site-specific evaluation of engineering controls. Engineering control requirements can be reduced if additional indoor/sub-slab monitoring is conducted following construction of building or if site conditions are in the more conservative end of the listed criteria (i.e., lower end of methane concentration and upper end of distance criteria). If reduced controls are utilized, a mitigation decision matrix for soil gas and indoor air data should be developed.
4. Methane Monitoring can include testing of exterior soil gas, sub-slab, and/or indoor air. A specific monitoring program should be proposed prior to building construction.

Methane is a colorless, odorless and highly flammable gas generated by the anaerobic biodegradation of organic material, including petroleum. Methane can pose explosion and fire hazards under some conditions. In order for methane to create hazardous conditions, three conditions must be met: 1) A sufficient concentration of methane; 2) A sufficient concentration of oxygen and 3) An ignition source. Potential safety risks should be assessed by considering concentrations of methane and oxygen in soil gas, significance of advective (i.e., under pressure) transport, and potential for methane attenuation between the soil gas and structure or enclosed space.

Advective flow of methane under pressure from a source area is primarily a concern at landfills. This creates a high risk for significant, offsite migration and potential intrusion into the lower floors of buildings or subsurface utility corridors. Methane can be present at high concentrations in vadose-zone soil at petroleum-contaminated sites but is rarely under significant pressure and typically migrates by diffusion rather than advection. While significant offsite migration is less likely, diffusion into subsurface utility corridors could pose localized flash explosion or fire concerns if the methane mixes with oxygenated air and is encountered during subsurface construction or utility work. Accumulation of methane in poorly ventilated rooms of buildings with cracked floors, gaps around utility penetrations in the floor or other vapor entry routes could also pose potential hazards.

Figure 9-1 presents a summary of recommended monitoring and mitigation actions for site where high levels of methane are detected in soil vapors (Geosyntec 2011).

Final monitoring and mitigation actions for potential methane hazards will necessarily be site specific, and depend in part on the estimated area and volume of the source area, planned remedial actions to address the source, the presence and use of existing buildings and the planned use or redevelopment of the site. Coordination with HDOH and submittal of a site-specific workplan for review is recommended. Additional methane guidance can be found in the following document: Advisory on Methane Assessment and Common Remedies at Schools Sites (CalEPA 2005b).

Be aware that high levels of light-end (C5-C12), petroleum vapors can cause false, elevated readings of methane in vapor samples using a standard, landfill gas analyzer. The use of a carbon trap is recommended when evaluating for methane when using field instruments at sites where high levels of petroleum may be present in soil gas. A carbon trap will remove the majority of petroleum aliphatic and aromatic compounds from the soil gas and allow for a more accurate reading of methane.