Plan Review Approach: Landfill Gas Monitoring Program

In those cases when an operator believes monitoring along all or a portion of the compliance boundary is not necessary, the operator shall demonstrate to the satisfaction of EA and CalRecycle that LFG migration could not occur due to geologic or hydrologic barriers and that no habitable structure or other property or land use, such as agricultural lands, within 1,000 feet of the disposal site permitted facility boundary is threatened by LFG migration [27 CCR 20925(a)(1)]. The demonstration shall be specific to the portion of the perimeter that the operator proposes not to monitor.

The submittal would need to demonstrate through geologic and/or hydrogeologic data that there is an impermeable barrier to LFG migration between the landfill footprint and the compliance boundary. For a hydraulic barrier, the operator would need to demonstrate that the barrier exists from ground level to the regional ground water table. An example of a location alternative would be those cases where a portion of the landfill permitted boundary is contiguous with a bay or ocean, which provides a hydraulic barrier to further LFG migration. Another example would be a stream that is in hydraulic continuity with the regional aquifer. Several partial alternatives have been approved especially for landfills located on bays where hydraulic barriers were demonstrated.

For a geologic barrier, the operator would need to demonstrate through geologic investigations that the barrier is impermeable and competent. Fractured bedrock or fine-grained soil interbedded with sand layers would not likely meet the alternative standard of an impermeable barrier. In their Technical Manual Solid Waste Disposal Facility Criteria (November 1993), the Federal Environmental Protection Agency stated that “…unless finer-grained soils are fully saturated, landfill gases also can migrate in a “semi-saturated’ zone.”

Well spacing and location alternatives are interlinked. Spacing between wells of greater than 1,000 feet is generally limited to those areas that qualify for a location alternative. Areas with structures or other sensitive uses within or near the facility permitted boundary would most likely require monitoring well spacing at less than 1,000 feet. Pursuant to 27 CCR 20925(b)(3) CalRecycle or EA may require closer well spacing to protect persons and structures threatened by landfill gas migration.

If an operator proposes an alternate system of equivalent well depths, the proposal must demonstrate to the satisfaction of EA and CalRecycle that site conditions limit the practicality or do not warrant the installation depth criteria and probes located at the proposed depths are sufficient to detect migrating LFG and provide protection to public health and safety and the environment consistent with 27 CCR 20925(c)(2). Pursuant to both State and Federal requirements, landfill operators must monitor LFG to ensure that the concentration of LFG does not exceed regulatory standards including gas at the property boundary at any depth. Wells that are not constructed to the required depth may not be able to meet the State and Federal standards for ensuring that gas migration does not exceed compliance levels. If the probes in the wells are not at adequate depths, the existing wells do not need to be abandoned but can be supplemented with additional wells or probes.

An example of where alternative depth wells may be appropriate is the case where at a canyon landfill a preferential LFG migration pathway (e.g., sand/gravel or decomposed granite layer) lies just above a bedrock plane that travels along the canyon wall toward the sides and head of the canyon. In this case, if an operator could demonstrate, with sufficient data, that any LFG migration would be through the permeable layer, the monitoring of this layer would provide the best monitoring of LFG migration. As long as this monitoring alternative indicates a low level of LFG migration, this monitoring would be considered sufficient. However, should this alternative monitoring indicate that LFG is migrating at significant levels, additional monitoring wells at deeper depths may be necessary to determine the extent of the LFG migration.

The preferential pathway alternative may also be applicable at non-canyon fill sites under similar circumstances (i.e., shallow bedrock with highly permeable preferential pathway). This alternative may also be applicable to the location alternative. An alternative of this type with appropriate documentation has been approved.

An operator who has installed LFG monitoring wells at an active landfill that were in compliance with the previous depth standard (i.e., depth of waste within 1,000 feet of the well location) may propose a depth alternative for these wells supported by geologic data indicating that the well depth is sufficient to detect migrating gas and will be protective of public health and safety and the environment.

For an operator to propose that an alternative depth is appropriate due to shallow ground water, the plan must demonstrate the historical low seasonal ground water level. The data must take into consideration seasonal and historical fluctuations and must consider the regional aquifer. Plans proposing shallower well depths based on ground water elevations, need to demonstrate the maximum seasonal low ground water level. Data based on limited or one-time data collection are insufficient to demonstrate historical ground water levels. In areas of tidal influences, larger screened intervals may need to be provided so that during the driest periods, the entire unsaturated zone is captured. Several landfills have received approval for both monitoring wells and cutoff trench systems based on historical ground water levels.

Also, LFG monitoring must be conducted both above and below perched ground water [27 CCR 20925(c)(1)(E)].

Monitoring wells deeper than 30 feet require a minimum of three (3) probes. To demonstrate that fewer than the minimum required number of probes is appropriate, the operator would need to demonstrate that there are fewer LFG pathways to be monitored than probes required. An example would be an area where the LFG migration barrier is incomplete due to defined zone(s) of LFG migration (e.g., preferential pathway). Substantial geologic data would need to be submitted to demonstrate this condition. Another example is where wells are less than approximately ten to 15 feet usually only one probe is needed.