Guidelines for In Situ Remediation Following a Methane Gas Leak
Early this year the State of Michigan's Department of Environmental Quality published a new set of guidelines for environmental practitioners responsible for in situ remediation of spilled contaminants, which also apply to a methane gas leak. The guide provides recommendations for monitoring hazardous contaminants, including methane levels in various environments to minimize any potential impacts following a methane gas leak, as outlined below:
1. Monitoring a Methane Gas Leak in SoilsMethane levels in soils should be monitored with a combustible gas detector before, during and after an in situ discharge requiring remedial action, as a methane gas leak poses both a vapor and an explosion hazard. Methane can also potentially migrate to enclosed spaces both on- and off-site (see point 2 below).
According to the guide, monitoring should be conducted "...before, during, and after implementation of an in situ remedial discharge when the contaminants of concern or remedial reagents have the potential to lead to vapor or explosion hazards. This includes circumstances where a remedial discharge has the potential to generate, mobilize, or displace vapors or generate higher than normal concentrations of oxygen gas, and these vapors or gases have the potential to migrate into enclosed spaces. .... Soil gas monitoring is generally conducted for the purposes of sentinel monitoring to protect specific receptors; therefore, action levels should be specified for soil gas monitoring that will trigger specified response activity or corrective actions necessary to protect receptors.... The RRD Guidance Document for the Vapor Intrusion Pathway should be consulted for guidance on soil gas monitoring."
2. Monitoring Indoor Air and Enclosed SpacesEnclosed spaces such as man-holes, storm drains and basements, or any confined area where ventilation is limited, pose an explosion hazard if methane accumulates in these areas. Consequently, in cases where the explosion hazard potential is high following a methane gas leak, indoor air together with that in enclosed spaces should be monitored to reduce the risk of explosion resulting from methane accumulation in these areas.
According to the guide: "Air monitoring of enclosed spaces such as storm sewers, utility man-ways, etc. should be included in the monitoring program for any facility where vapor or explosion hazards are a concern..."
3. Monitoring Ambient Air Following a Methane Gas LeakIn ambient air, a methane gas leak can pose a fire or explosion hazard, and can also potentially pose an inhalation risk to people exposed to the gas, and thus requires monitoring.
According to the guide: "Monitoring is warranted whenever a discharge has a reasonable potential to generate concentrations of vapors in ambient air that either present unacceptable inhalation exposures to workers or non-workers, or that could present a risk of fire or explosion. In most applications, in situ discharges are applied at some depth beneath a cover material (i.e., soil and/or pavement), which usually inhibits the rapid diffusion of vapors to the surface, thereby minimizing the ability of gases or vapors to accumulate at hazardous concentrations in ambient air. However, this alone does not necessarily preclude the need for ambient air monitoring."According to the guide, in order to determine whether it is necessary to monitor ambient air during the course of in situ remedial action, the following points need to be taken into account:
- The concentrations of contaminants in soil or groundwater, especially where grossly contaminated media is present;
- The concentrations at which contaminants of concern or remedial constituents become toxic in air, especially if toxic at very low concentrations;
- The potential for explosive conditions to develop, in light of the chemical properties of the contaminants of concern and potential by-products from the discharge (e.g., generation of oxygen gas);
- The proximity of the treated media to the surface;
- The properties of the soil and/or cover above the treated media
- Whether engineering controls are implemented as part of the remedial process, such as soil vapor extraction, that will otherwise stop the migration of gases or vapors to the surface; and
- The presence of conduits to the surface for gasses and vapors, such as monitoring or treatment wells, that can result in the impact to the breathing zone air.