Water safety has been in the news again in recent months. This time it is due to trihalomethanes, a water by-product thought to be a possible carcinogen. However, that doesn’t mean all homeowners who use a water well for their daily water supply are at risk.

Water well owners should always try to have a working knowledge about their well, its parts, and possible water contaminants. Below is information about trihalomethanes. However, if questions arise, the best solution is to contact a professional water well contractor and seek their advice.

Trihalomethanes are a group of four chemicals—chloroform, bromodichloromethane, dibromochloromethane, and bromoform—formed, along with other disinfection by-products, when chlorine or other disinfectants used to control microbial contaminants in drinking water react with naturally occurring methane, often derived from other forms of organic and inorganic matter in water, usually broken up by chlorine or bromine used to disinfect water. THMs are part of a larger class of halogenated organic compounds, including others of water supply health concern such as halogenated acetic acids (HAA). THM are the most common halogenated organics.

Chloroform—the trihalomethane often found in the highest concentration—is formed by a reaction of chlorine with certain compounds in water. Formation occurs during chlorination and can continue to occur as long as chlorine is available. The other trihalomethanes are formed by a reaction of bromine and iodine with the same certain compounds.

Depending on the characteristics of the water, the other three trihalomethanes may be formed at a higher concentration than chloroform.

Trihalomethanes are much more prevalent in public water supplies because most use chlorination as a disinfection technology. However, while trihalomethanes are more common in the public water systems, they are a threat to any water supply that uses chlorine—including private water wells.

High levels of trihalomethanes can be dangerous. In fact, in December 2000, the U.S. Environmental Protection Agency lowered the maximum allowable annual average level for large surface water public water systems from 100 parts per billion (ppb) to 80 ppb. The 80 ppb limit goes into effect for small surface water and all ground water systems in December 2003.

Some studies have suggested a small increase in the risk of bladder and colorectal cancers. Other investigations have found that chlorination by-products may be linked to heart, lung, kidney, liver, and central nervous system damage.

Of the different trihalomethanes, dibromochloromethane has been most closely associated with cancer, followed in order by bromoform, chloroform, and bromodichloromethane.

Pregnant women appear to be at the greatest risk, as some studies have linked trihalomethanes to reproductive problems, including miscarriage.

Yes. Dissolved chlorine gas and hypochlorites (liquid sodium hypochlorite or calcium hypochlorite salts) can form THMs. All of these form  a free chlorine residual in water, and any free residual can react with organic compounds to form trihalomethanes. Repeated concentrated dosage chlorination of wells to treat biofouling can leave THM residuals.

There are several methods that people can use in their homes to reduce the trihalomethanes. Water well owners should always discuss these methods with a water treatment professional or water chemist before deciding to use one (water filters in the retail market are often sold with erroneous claims). Among the methods are:

  • Filters
  • Aeration or boiling
  • Distillation
  • Activated carbon — normally in the form of a filter

For a private groundwater supply, continuous treatment with chlorine or bromine is usually not recommended, and indeed prohibited in some US states. However, if it is necessary or desirable for some purpose, a carbon filter to remove the halogen and THM downstream is recommended. These may be whole-house or at point of use. If a well has been shock chlorinated, a downstream sediment and carbon filter is also recommended. Public water supplies with THM issues often switch to alternative chlorine methods, or disinfection choices. These may be considered for private water supplies as well.

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