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Water Quality Notice

Post Date:07/28/2017 8:00 AM


Gilbert exceeds limit on TTHMs in drinking water
This is not an emergency; Gilbert’s water is safe.

Stage 2 Disinfection  By-Products Rule TTHM MCL Violation
Total Trihalomethanes (TTHM)   MCL  Violation  at the Town  of Gilbert

Gilbert’s water system recently violated a drinking water 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.

Gilbert’s Water Quality Department routinely monitors for the presence of drinking water contaminants. We monitor for one group of contaminants, Total Trihalomethanes (TTHMs), at eight (8) monitoring stations spread across Gilbert.  The maximum contaminant level (MCL) for TTHMs is 80 parts per billion (ppb). The MCL is determined by averaging all the samples collected at each sampling location for the past 12 months. The level of TTHMs detected at one of our sampling locations in July was 82 ppb. The level of TTHMs averaged at Site 01-A for October 2016 to July 2017 was 84 parts per billion (ppb). The five other TTHM sampling sites in Zone 1 remain below the MCL.

Gilbert has four (4) water pressure zones. The site where the violation occurred (Site 01-A) is located in Zone 1 near the intersection of Ray Road and Gilbert Road. The four (4) water pressure zones are hydraulically isolated from each adjacent zone. This notice of violation only applies to customers within the Zone 1 boundary, displayed on the attached map. Customers who are located south of Baseline Road and West of SRP’s Eastern Canal who receive a utility bill from Gilbert are within the Zone 1 boundary.

What does this mean?

This is not an emergency. If it had been an emergency, you would have been notified within 24 hours. TTHMs are four volatile organic chemicals which form when disinfectants react with natural organic matter in the water. People who drink water containing TTHMs 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.

What should I do?

There is nothing you need to do, and you can safely continue normal use of Gilbert’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 Town is resolving the issue.

What is being done?

Gilbert has implemented several immediate responses to ensure that TTHMs return to target levels. Measures include:

  • Using groundwater wells to blend TTHM-free water in the entire system
  • Flushing at strategic locations within Gilbert’s  water system
  • Chemical  adjustments at water treatment plants
  • Operational changes at surrounding water reservoirs
  • Additional monitoring for TTHMs at multiple  locations in Gilbert
  • Utilizing  a consultant engineer to recommend TTHM mitigation strategies at Gilbert’s  North Water Treatment Plant
  • Procurement of equipment to allow for in-house testing of TTHMs in Gilbert’s water quality laboratory
  • Ongoing development of a water model to optimize system operations

Water Utilities staff is working 24 hours a day, 7 days a week to resolve the issue as quickly as possible. You will receive a notice in the mail when the running annual average returns to below 80 ppb.

This notice is being sent to you by the Town of Gilbert. State Water System ID#: 07-092. For more information, please contact Gilbert’s Water Division at (480) 503-6400 email us at or visit us online at


Water Quality Violation Map


Please share this information with all the other people who drink this water, especially those who may not have received this notice directly (for example, people in apartments, nursing homes, schools, and businesses). You can do this by posting this notice in a public place or distributing copies by hand or mail.

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