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

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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 byproducts,” the most common of which are THMs. Valley Water conducts tests every month to monitor the THM levels in treated water, and reports these to the State Water Board. Valley Water posts the water quality analysis results and issues a quarterly water quality report that is made available to the public.  

What are the THM levels in the water the district provides?  
Detected levels of THMs in our drinking water are well within limits established by the state and federal regulatory agencies.

You can view Valley Water’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 first data grouping under Primary Standards - Mandatory Health-Related Standards. https://www.valleywater.org/your-water/water-quality/water-quality-reports  

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.  

How are these compounds regulated? 
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 (ppb).

In 1989, the Surface Water Treatment Rule (SWTR) established minimum residual disinfectant levels at the entry points to distribution system, resulting in increased use of disinfectants and increased exposure to disinfection byproducts (DBPs).  In 1992, development of regulations for Disinfectants/Disinfection Byproducts began by assessing DBP occurrence and potential health effects.  

In 1998, Stage 1 DBP Rule was established to reduce drinking water exposure to DBPs. Stage 1 DBP rule established maximum residual disinfectant levels (MRDLs) for three disinfectants: chlorine, chloramines, and chlorine dioxide. Stage 1 DBP Rule also established MCLs for Total Trihalomethanes (TTHMs) at 80 ppb, Haloacetic Acids (HAA5s) at 60 ppb, Chlorite at 1,000 ppb, and Bromate at 10 ppb.

In 2006, Stage 2 DBP rule strengthened public health protection by requiring monitoring at sites with the highest DBP levels within the distribution system.

What can you do to get these out of the water?  
Valley water implemented major upgrades to add ozone as primary disinfectant at the Santa Teresa and Penitencia water treatment plants in 2006. Ozone disinfection is more effective than chlorine at inactivating microbial contaminants like Giardia and Cryptosporidium. Also, the use of ozone in place of chlorine can significantly reduce the formation of THMs in our drinking water.

The Rinconada water treatment plant, which was built in 1968, is currently undergoing major improvements to replace or upgrade all major plant components, add ozone as a primary disinfectant, and increase Rinconada’s treatment capacity to 100 million gallons of water a day.

Valley Water staff continuously optimizes treatment to minimize disinfection byproducts levels, such as THMs, in treated water provided to the county. Some examples of improvements and process changes include adjusting disinfectant dosage, delaying chlorination (which reduces byproduct formation by limiting the chlorine’s contact time 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.  

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 during pregnancy, please consult your doctor for recommendations.

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 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.

Where can I get more information? 

Questions on tap water quality:

  • State Water Resources Control Board - Division of Drinking Water, (916) 449-5577 or (916) 341-5455
  • US Environmental Protection Agency Safe Drinking Water Hotline, (800) 426-4791
  • Your water retail company
  • Valley Water, (408) 265-2600

For information on bottled water regulations and quality:

Visit California Department of Public Health, Food, and Drug Branch website at: https://www.cdph.ca.gov/Programs/CEH/DFDCS/Pages/FDBPrograms/FoodSafetyProgram/WaterFAQs.aspx Hotline, (800) 495-3232

For information on home water treatment devices:

Environmental Protection Agency pamphlet, "Home Water Treatment Units: Filtering Fact from Fiction.", (800) 426-4791 The National Sanitation Foundation, an organization that tests and certifies home water treatment units. Hotline, (800) 673-8010

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

Valley Water Public Information Office, (408) 265-2600 ext. 2881 Media Line, (408) 681-9265

Questions on the health impacts relating to these issues:

Rochelle Gaddi, County of Santa Clara Health Officer, (408) 918-3449; After Hours (5pm to 7am), (408) 299-2507