There are a number of dissolved gases in water that may be a problem or concern. In general, these gases do not have a specific drinking water standard, but may cause an aesthetic or use problem, or be associated with a health concern. In this section we will discuss four gases: carbon dioxide, hydrogen sulfide, methane, and radon.
These gases are classified as No Standard, because the EPA and many states have not set a specific standard. In some cases, the secondary standards or the condition of the water, such as the presence of an odor (hydrogen sulfide), flammability hazard methane, potential radon air-exposure hazard, low pH (carbon dioxide), or other secondary condition may trigger an associated Primary or Secondary drinking water standard or a public health concern.
Methane (Well Water and Spring Water) - this gas is typically associated with wetlands, bogs, landfills, coal-producing formations, natural saline seeps, sources of natural gas like the Marcellus Shale, oil fields, some shallower glacial deposits, and gas storage areas. Methane is a colorless, odorless gas that is lighter than air, flammable, and, potentially, an explosion hazard.
Carbon Dioxide (City Water, Well Water, and Spring Water) - Levels of Carbon Dioxide may be high within a water source for a number of reasons which include, a change in temperature that causes the dissolved gas to just start coming out of solution or Bacterial regrowth in the distribution system. High levels of Carbon Dioxide may be associated with water that is potentially Corrosive (because of carbonic acid), water that may have bacterial contamination, or water with high levels of organic contaminants (natural or man-made).
Hydrogen Sulfide (Well Water and Spring Water but rare in City Water) - A Hydrogen Sulfide or rotten egg smell can be associated with the presence of hydrogen sulfide gas, high levels of sulfate, or the presence of Sulfate-Reducing Bacteria. If you can smell this gas in the air, the level is too high. It can irritate the throat, causing it, in extreme cases, to swell shut with resulting asphyxiation. Be aware that over time the nose because accustomed to the smell; you think it is gone but it is not.
Radon Gas (Well Water) - Radon is a radioactive gas produced by the radioactive decay of the element Uranium, Thorium, and Radium. It is a colorless, odorless, noble gas. Radon may be associated with elevated levels of Radiological Isotopes and other natural radionuclides or alpha /beta decay particles. There are regulatory standards for radon in air but only various recommended standards for radon in water.