Public Notice: Important Information About Your Drinking Water

Total Trihalomethanes (TTHM) MCL Violation at the City of Chandler

Chandler’s water system recently exceeded a drinking water Maximum Contaminant Level (MCL) standard. Although this incident was not an emergency, as our customers, you have a right to know what happened and what we are doing to correct this situation.

Chandler’s Water Quality Division routinely monitors for the presence of over 90 drinking water contaminants regulated by the United States Environmental Protection Agency (EPA) under the Safe Drinking Water Act. One group of contaminants, Total Trihalomethanes (TTHMs), is monitored at twelve (12) monitoring locations throughout Chandler. The EPA defines the MCL for TTHMs at 80 parts per billion (ppb). Whether an MCL has been exceeded is determined by averaging all the samples collected quarterly at each sampling location for the past 12 months. Testing from Site 10-A between July 2018 and April 2019 showed an average TTHM of 85 ppb. Site 10-A is located in west Chandler near the intersection of Chandler Boulevard and 54th Street. The City’s eleven other TTHM sampling sites remain below the MCL.

This is not an emergency. If it had been an emergency, you would have been notified within 24 hours. TTHM are four volatile organic chemicals which form when disinfectants like chlorine react with natural organic matter in the water. People who drink water containing trihalomethanes in excess of the MCL over many years may experience problems with their liver, kidneys, or central nervous system, and may have an increased risk of getting cancer.


  • There is nothing you need to do. You can safely continue normal use of Chandler’s drinking water. You do not need to boil your water or take other corrective actions. If your water ever became unsafe to drink, you would be notified within 24 hours.
  • If you have a severely compromised immune system, have an infant, are pregnant, or are elderly, you may be at increased risk and should seek advice from your health care providers about drinking this water during the interim time period while the city is resolving the issue.

Chandler has implemented several immediate responses to ensure that TTHMs return to levels below the MCL. Measures include:

  • Using groundwater wells to blend TTHM-free water into the entire system
  • Operational changes at surrounding water reservoirs
  • Chemical adjustments at the surface water treatment plant
  • Additional monitoring for TTHMs at multiple locations in Chandler

Public Works and Utilities staff is working 24 hours a day, 7 days a week to ensure you have a safe drinking water supply. Noncompliance sampling taken on May 2, 2019 has shown that with the adjustments mentioned above, the TTHM levels are now well below the MCL. You will receive a notice in the mail when the compliance sampling shows the running annual average has returned to below 80 ppb. The next compliance sampling is scheduled for July 2019.   


They’re a group of chemicals that form when chlorine, used as a disinfectant, reacts with naturally occurring organic matter found in the source water.

Yes, the water provided by the City of Chandler is safe to drink, as well as for all other uses such as cooking, bathing and cleaning.

This information is posted because a recent sample result at one of our twelve sample locations showed an elevated 12 month average. Fluctuations in TTHMs are normal, which is why the regulation is based on an average, rather than a single sample.

The potential health effects from TTHMs are based on drinking water with elevated levels over a lifetime. The City has increased water quality monitoring and current results are well below the Maximum Contaminant Level (MCL) set by federal regulations.

The City has already made adjustments to the treatment process and non-compliance sample results verify that the TTHMs are currently well below the MCL. The City will take the next compliance samples in early July, based on the compliance schedule. Because the MCL is calculated based on an average, July will be the earliest we can achieve being below the MCL under the EPA rule.

Yes, a home reverse osmosis system (RO system) can reduce or eliminate TTHMs. Another way to reduce or eliminate THMs in drinking water is to use a water pitcher with a carbon filter or install a tap-mounted carbon filter. When using a filter, check to verify that it is certified to remove TTHMs and follow replacement instructions recommended by the manufacturer.

For more information, visit Water Research: Trihalomethanes Disinfection

The City has developed and implemented an action plan in order to lower TTHMs across the City including:

  • Operational changes at the water treatment plant and water reservoirs
  • Blending more TTHM-free water into the system
  • Optimization of disinfectant dosages
  • Increased water quality monitoring

Yes, for more information please contact our Water Quality Team at 480-782-3654.