1. Occurrence and Significance
Methane (CH4) is a colorless, odorless, tasteless combustible gas occasionally found in groundwaters. Escape of this gas from water may cause an explosive atmosphere not only in a utility’s tanks, pumphouses, and other facilities, but also on the consumer’s property, particularly where water is sprayed through poorly ventilated spaces, such as public showers.
The explosive limits of CH4 in air are 5% to 15% by volume. At sea level, a 3.95% CH4 concentration in air theoretically could be reached in a poorly ventilated space sprayed with hot (68 °C) water having a CH4 concentration of only 0.7 mg/L. At higher water temperatures, the vapor pressure of water is so great that no explosive mixture can form. At lower barometric pressures, the theoretical hazardous concentration of methane in water will be reduced proportionately. In an atmosphere of N2 or other inert gas, at least 12.8% O2 must be present for there to be an explosion hazard.
Methane also is produced from wastewater and may be present in sewers and wastewater treatment plants (see Section 2720).
2. Selection of Method
The combustible-gas indicator method (B) offers the advantages of simplicity, speed, and great sensitivity. The volumetric method (C) can be made more accurate for concentrations of 4 to 5 mg/L and higher, but will not be satisfactory for very low concentrations. The volumetric method also can be applied to differentiate between CH4 and other gases, as when a water supply is contaminated by liquid petroleum gas or other volatile combustible materials.
Methane also may be determined with the gas chromatograph as described in Section 2720 C. This method permits differentiation between H2 and CH4, and/or its higher homologs.