AREA A/B ENGINEERING REPORT - Waste Management

Geosyntec Consultants
LFG migration monitoring (often termed explosive gas monitoring) at
MSW landfills is required under Subtitle D such that the concentration
of landfill gas in the unsaturated vadose zone does not exceed the
lower explosive level (LEL) for methane at the facility property
boundary and 25% of the LEL in facility structures. The vadose zone
gas migration monitoring network typically consists of a number of
appropriately-placed probes or wells along or near the perimeter of
the landfill property. Methane monitoring probes are screened in the
vadose zone above the groundwater table, and target preferential
gas flow pathways (e.g., layers of higher permeable soils such as sand and gravel that may exist
between layers of less permeable soils such as silts and clays). Facility structures that lie between
the landfill and the vadose zone monitoring probe network are also routinely monitored for
methane, and often include a combustible gas alarm device. Structures of concern include drain
culverts, vaults, buildings, shops, and sheds.
Vadose zone and facility structure monitoring provides additional confirmation that the liner
system, cover system, and GMS are performing effectively. If methane is detected in a facility
structure or vadose zone monitoring probe, prompt corrective measures are required.
5.2.4 Surface Emissions Monitoring
Surface emissions monitoring (SEM) is common at landfills located in areas of the
country with stringent air quality regulations, and is required by law at all
landfills with an active gas collection and control system (GCCS) operating under
a Title V Permit compliant with the USEPA’s New Source Performance Standards
(NSPS) for landfills. Landfills subject to these requirements are the larger
modern sites that have potential for high LFG generation and, thus, are federally mandated to
control atmospheric LFG emissions.
Methane is generally present at high levels in LFG (40
percent by volume or more), but is not typically
present in ambient air. Therefore, the presence of
methane in air directly above a landfill cover at levels
above a regulatory threshold is considered an
indication that LFG may be being emitted through a
breach in the cover system. Such breaches could
include small cracks and fissures in dry soil layers that
can be effectively repaired once detected.
There are several methods used in monitoring methane emissions, including:
• Surface monitoring, the most common method approved under landfill permit conditions,
which involves using an instrument to detect gases (such as an infrared gas analyzer fitted
MD10186.doc 72 29 March 2009