Free Methane: Because methane is colorless, odorless, and tasteless it cannot be readily detected without a combustible gas detector or meter. This is why an odorant that can be smelled at very low leakage is added to natural gas sold to the public. Using a meter, the presence of methane in air is commonly measured as a percent of the LEL. It is important to understand the units being used and how they relate to each other. For example, a meter reading of 10% LEL is equivalent to 0.5 % methane in air, or 5,000 ppm.
Dissolved Methane: Laboratory analysis of a water sample is required to determine the concentration of methane dissolved in water. The resulting laboratory report will provide the concentration of dissolved methane as mg/l, or micrograms per liter (µg/l). Again, it is important to understand the units being used: 1 mg/l is equal to 1,000 µg/l; the unit “µg/l” can also be expressed as parts per billion (ppb). The maximum concentration of methane dissolved in water varies with temperature and pressure. At 68oF and standard atmospheric pressure, the maximum dissolved concentration of methane in water is 28 mg/l; but when water is removed from a deep formation under higher pressure conditions, the concentrations can be higher.
WHEN SHOULD YOU BE CONCERNED?
Free Methane: The U.S. Department of the Interior, Office of Surface Mining Reclamation and Enforcement (OSMRE) suggests the following action levels for detections of free methane (based on percentage in air):
For unoccupiable spaces, such as a well head:
No Immediate Action – < 1%;
Warning: investigate/vent – >1% to <3%; and
Immediate Action: active venting/treatment – >3%.
For occupiable spaces of a home:
No Immediate Action – < 0.25%;
Monitor trends – >0.25% to <0.5%;
Warning: investigate/vent – >0.5% to <1%; and
Immediate Action: active venting/treatment – >1%.
State and local regulations should be consulted if free methane is detected in air at the warning levels indicated by the OSMRE guidelines. Likewise, the installation of an appropriate gas venting system is advisable if methane is present.
Dissolved Methane: Generally, when dissolved methane is present at concentrations of less than 10 mg/l, it isn’t necessary to take immediate action. State and local regulations should be consulted if dissolved methane is present in a water well at 10 mg/l or greater. However, even at lower concentrations, the installation of an appropriate gas venting well cap is an inexpensive precaution. When dissolved methane concentrations exceed 10 mg/l, or in accordance with state and local requirements, a properly qualified private water system contractor should be consulted to carefully select and install appropriate methane mitigation measures. This may include the use of a vented well cap; or the installation of a cistern, with or without an aeration device, to further reduce the concentrations of dissolved methane.
Methane is classified as a simple asphyxiant (impairs normal breathing) and explosion hazard. Methane is not known to be toxic and consuming water that contains methane does not present a health hazard. In addition, exposure to methane is not known to increase the chance of any type of cancer.
Methane is lighter than air and free methane gas may accumulate within an enclosed space such as a wellhead, subsurface well vault, or within a poorly ventilated basement or crawl space where it could present a risk of explosion if allowed to build up to concentrations above the Lower Explosion Limit (LEL). The LEL is the lowest concentration capable of producing a flash, fire or explosion. This concentration in air for methane is approximately 5.0% (50,000 parts per million (ppm)). When oxygen levels fall below 18% due to displacement by methane, methane can result in asphyxiation (i.e., impairs normal breathing). Methane dissolved in groundwater below its solubility (~28 milligrams per liter (mg/l)), is not flammable, however, at concentrations that exceed solubility, it can be ignited.
Methane (CH4) is a colorless, odorless, tasteless, and combustible gas that typically makes up 70 to 98 percent of the mixture known as natural gas. Methane is a naturally occurring hydrocarbon that can be found underground in both shallow and deep rock formations, including coal beds, and is also commonly associated with marshes and landfills. Methane can be created by thermogenic processes under heat and pressure typical of deep formations; or by methanogenic processes that include carbon dioxide (CO2) reduction (to produce sub-surface microbial gas), and fermentation (to produce near-surface microbial gas) that is typical of shallow formations, landfills, or marshes.
The presence of methane in water wells is relatively common. It may occur naturally, generated within an aquifer (i.e., groundwater) source rock formation, or as the result of migration through natural pathways. However, it may also be present when current or historical human activities such as landfills, coal mining and drilling for oil and gas create conditions for gas migration. Methane can be present in a water well as a gas dissolved within the groundwater (dissolved gas) or as free gas.