The City of Los Angeles requires methane testing before building in a Methane Zone or Methane Buffer Zone. These methane zones are areas of the city with high methane as a result of naturally occurring petroleum products. The risk is that methane could migrate into an unprotected structure and create an asphyxiation or explosion hazard.
Methane testing must be done by an engineering firm approved by the city and must be completed before site grading or at least 30 days after any site grading, if unavoidable, for all sites within the Methane and Methane Buffer Zones. The first step for the methane investigation is to complete Shallow Soil Gas Sampling. Usually, these soil gas samples are collected at approximately 5 feet below ground surface (bgs) and are used to identify the locations of the deeper soil gas samples.
Once the Shallow Soil Gas Samples are complete and sampled, nested soil gas probes sets are installed within the areas where the highest concentrations of methane were detected. A minimum of two sets of probes are installed with at least one soil gas probe set for every 20,000 square feet of site area. Each soil gas probe set consists of three nested probes at depths of 5 feet, 10 feet, and 20 feet below the elevation of the lowest building slab/footing. All probes must be installed at least 12 inches above groundwater, if encountered.
After completion of the methane testing, the approved firm must complete Form 1 – Certificate of Compliance for Methane Test Data and produce a site plan showing probe locations for submittal to the city. Any property within the Methane Buffer Zone with no detections of methane may proceed without any methane mitigation plan. However, any property within the Methane Zone is required to produce a minimum methane mitigation plan regardless of the concentrations of methane present.
Based on the concentrations detected, the site will be assigned a methane mitigation level, Level I being the lowest through Level V being the highest, which will determine the appropriate mitigation measures required by the city. Within the Methane Buffer Zone, Levels I and II (and Level III, with exceptions) are exempt from completing a methane mitigation plan as long as the Design Methane Pressure is less than 2 inches of water. Otherwise, the Minimum Methane Mitigation Design Requirements are provided in Table 71 of Los Angeles Municipal Code Ordinance Number 175790 (click here). As the Level increases, the requirements for mitigation also increase.
Feel free to contact me if you have any questions about your Los Angeles site and whether you might need Methane Testing, I’d be happy to help. 310-615-4500, firstname.lastname@example.org.