Trihalomethanes (THMs) | Santa Clara Valley Water
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Trihalomethanes (THMs)


Frequently asked questions about THMs

What are trihalomethanes? 
Trihalomethanes (THMs) are a byproduct of the water treatment process. They are formed when natural organic material, such as the decaying vegetation commonly found in lakes and reservoirs, reacts with chlorine used to treat the water. This reaction produces “disinfection by-products,” the most common of which are THMs. The water district conducts tests every month to monitor the levels of total THMs in treated water, and reports these levels to the State Water Board. The district also posts the results of our water quality analysis and issues a quarterly water quality report that is available to the public.  

What are the THM levels in the water the district provides?  
You can view the water district’s most recent lab data. First choose the monthly lab report you wish to view. THM levels are reported as "total trihalomethanes" in (mg/L) and are found in the second data grouping under Primary Standards - Mandatory Health-Related Standards. Since 1993, levels of THM measured in treated water range from .040-.100 mg/L. During the drought years of 1987-1992, levels tended to be higher (.080-.100 mg/L). This is because the drought condition increased levels of salt water intrusion and agricultural runoff in the source water from the Sacramento-San Joaquin Delta.  

Why do you use chlorine if it creates these byproducts?  
Chlorine has been used to disinfect water for almost a century due to its effectiveness at killing bacteria and viruses in water. There is no question that its use has been a huge public health benefit in largely eliminating plagues such as cholera and typhoid and reducing the incidence of intestinal illness and other health problems caused by waterborne germs. In 1974, drinking water professionals recognized the need to modify traditional chlorine treatment processes due to advances in knowledge about disinfection byproducts and their health impacts. In 1979, the U.S. Environmental Protection Agency adopted a regulated limit on the amount of THMs allowable in drinking water, of 100 parts per billion. The water district produces water with the least amount of THMs possible, by doing such things as delaying chlorination (which reduces the formation of byproducts by limiting the time that chlorine is in contact with organic materials), switching source water when levels of organic material are high, enhancing coagulation, which improves the removal of organic materials before the treatment process; and using chloramine instead of chlorine for residual disinfection.  

What can you do to get these out of the water?  
The water district is currently upgrading all three of its drinking water treatment plants in order to allow for new and improved methods of treatment that will further reduce the levels of THMs in treated water that we provide to the county. This will be accomplished by further delaying chlorination and enhancing coagulation, and eventually by switching to ozonation as the primary disinfection method instead of chlorination. This $150 million capital improvement project is scheduled to be completed in 2005. The district is examining ways that the project could be accelerated.

Another important factor in reducing the amount of THMs in the treated water we provide is the need to improve the quality of the source water. Water imported from the Sacramento-San Joaquin Delta currently experiences salt water contamination and has high levels of organic compounds from agricultural drainage. These salts and organic compounds form THMs during the treatment process. The delta serves as the primary source of drinking water for some 20 million Californians. The district is participating in the CALFED process, a state-federal program to restore the delta and improve the quality of the water that we import. Taxpayer support of CALFED initiatives will be critical to the success of this effort.  

I am pregnant. Should I stop drinking tap water? 
Pregnant women should talk to their physician for advice. It is important that pregnant women continue to drink sufficient water according to their doctor’s recommendations. Health officials are not recommending that pregnant women stop drinking their tap water. If you are concerned about the safety of your tap water, you have several options. These are explained in greater detail here.   

Are there additional exposures to THMs associated with showering or bathing?  
THMs can evaporate and be inhaled while showering, however, the EPA has determined that this exposure is minimal compared to that from consumption.  

Is the water safe for the general public to drink?  
Yes, the water is safe for the general population to drink. If you have a medical condition that suppresses your immune system, or if you are worried about the quality of your tap water, you can consult your doctor for recommendations or follow the advice given above.


Where can I get more information? 

Questions on tap water quality:

  • California Department of Health Services, (916) 323-4344 or (510) 450-3818 for questions about the recent study
  • US Environmental Protection Agency Safe Drinking Water Hotline 800-426-4791
  • Your water retail company
  • Santa Clara Valley Water District, (408) 265-2600
  • Environmental Protection Agency’s Safe Drinking Water Hotline, 800-426-4791

Questions on the health impacts relating to these issues:

  • Dr. Martin Fenstersheib, County Health Officer, (408) 885-4214

For copies of literature on water supply, home treatment systems, and other water quality issues:

  • Santa Clara Valley Water District Public Information Office at (408) 265-2607 ext. 2238

For information on home water treatment devices:

  • California Department of Health Services, (916) 323-6111
  • Environmental Protection Agency pamphlet, "Home Water Treatment Units: Filtering Fact from Fiction."
  • The National Sanitation Foundation, an organization that tests and certifies home water treatment units, at (800) 673-8010
  • The Water Quality Association, an organization that classifies units according to the contaminants they are designed to remove, at (800) 749-0234

For information on bottled water regulations and quality:

  • Wayland Ho, Food and Drug Investigator, California Department of Health Services, Food and Drug Branch, 100 Paseo de San Antonio, Room 304, San Jose, CA 95113

International Bottled Water Association, 113 North Henry Street, Alexandria, VA, 22314, (703) 683-5213