Important Information for Our Water Customers regarding Total Trihalomethanes (TTHM)

Important Information About Your Drinking Water

The City of San Luis water system registered levels of Disinfection Byproducts (DBPs) above drinking water standard. Although this incident was not an emergency, as our customers, you have a right to know what happened and what we did (are doing) to correct this situation.
We are required to monitor your drinking water for the presence of disinfection byproducts (DBPs) on a quarterly basis. Testing results for the second quarter of 2026 show that our system exceeded the standard, or maximum contaminant level (MCL), for Total Trihalomethanes (TTHM). The MCL standard for TTHM is 0.080 mg/L, and it is calculated based on the locational running annual averages (LRAA) of samples collected from the last four quarters. The level of TTHM averaged at one of our sampling locations was 0.089 mg/L. The level of TTHM averaged at another of our sampling locations was 0.088 mg/L.

What should I do?

• There is nothing you need to do. You do not need to boil your water or take other corrective actions. If a situation arises where the water is no longer safe to drink, you will be notified within 24 hours. However, if you have any specific health concerns, consult your doctor.
• 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.

What does this mean?
This is not an emergency. If it had been, you would have been notified immediately. TTHM are four volatile organic chemicals which form when disinfectants react with natural organic matter in the water. Some people who continuously drink water containing TTHM 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 is being done?
The City of San Luis immediately took additional samples in which results came back well below the MCL of 0.080 mg/L at all of our four sampling locations. As part of our mitigation measures, additional water main looping infrastructure will be built to improve water circulation. The City has completed the design drawings, purchased the materials, and will be installing the section of water main as soon as possible. The third quarter samples had been taken as of August 13th, 2026, and the City anticipates results to be within the compliance limits.

For more information, please contact Antonio Sandoval, Water System Manager at 928-341-8577 or asandoval@sanluisaz.gov

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.
This notice is being sent to you by the City of San Luis. Public Water System ID#: AZ04-14005
Date distributed: 08/27/2026

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Frequently Asked Questions

Important Information for Our Water Customers regarding Total Trihalomethanes (TTHM)
The City of San Luis is committed to providing safe, reliable drinking water to our residents and businesses. The City recently received water quality test results showing that the water system exceeded the drinking water standard for Total Trihalomethanes (TTHM) during the second quarter of 2026.

This is not an emergency, and residents do not need to boil their water or take other corrective action because of this TTHM exceedance.

The City has already taken additional water quality samples, which showed TTHM levels below the drinking water standard at all four of our sampling locations. The City is also implementing improvements intended to improve water circulation and reduce the formation of TTHMs.

This FAQ provides additional information to help residents understand what happened and what the City is doing about it.

1. What are TTHMs?
TTHM stands for Total Trihalomethanes. TTHMs are a group of four chemical compounds that can form when drinking water disinfectants react with naturally occurring organic matter in the water.
The disinfection process of potable water is an important and necessary part of providing safe drinking water because it helps control bacteria, viruses, and other microorganisms that can cause illness. However, under certain conditions, disinfectants contribute to TTHMs formation.

2. What are the potential health effects associated with TTHMs?
The drinking water standard for TTHMs is established to protect public health from potential risks associated with long term continuous exposure. The City takes this event seriously and is implementing measures to reduce TTHM levels. An important distinction is that the City’s current situation does not mean that residents are expected to experience health effects from the recent monitoring results. The concern addressed by the drinking water standard involves exposure over time (years).

3. Does this mean my drinking water is unsafe?
City water is safe to drink. The City is not issuing an unsafe drinking water advisory or a directive for residents to stop drinking the water because of this TTHM exceedance. This incident was not an emergency, and customers do not need to take other corrective actions.

4. Can I continue using my water for drinking and cooking?
Yes. Residents can continue using their water for drinking, cooking, brushing their teeth, bathing, or other normal household purposes.

5. Can I shower or bathe in the water?
Yes. The City has not issued any restriction on bathing or showering because of the TTHM exceedance.

6. Should I use bottled water instead?
The City is not directing residents to use bottled water because of this TTHM exceedance. Residents who independently choose to use bottled water may certainly do so, but bottled water is not being required as a corrective measure.

7. Are children, pregnant women, elderly residents, or people with compromised immune systems at greater risk?
The City’s public notice recommends that residents who are pregnant, have an infant, are elderly, or have a severely compromised immune system consult their healthcare provider if they have concerns about drinking the water. Residents with specific medical concerns should discuss their individual circumstances with their healthcare provider.

8. What is the City doing to correct the situation?
The City has already taken several actions.
First, the City collected additional samples. The additional sampling showed TTHM results below the regulatory levels.
Second, the City is implementing additional water main lines looping. Intended to improve circulation within the distribution system and help reduce conditions that can contribute to elevated TTHM levels. The City has completed the design drawings and purchased the required materials. Installation will begin in September 2026.
Third, the City is continuing its regulatory monitoring and continues working with the Arizona Department of Environmental Quality (ADEQ) regarding its monitoring and corrective-action requirements. Third-quarter samples were collected as of August 13, 2026. The City anticipates that these results will be within the applicable compliance limits.

A Message from the City of San Luis

The City understands that receiving a drinking-water notice can be concerning. We want our residents to know that the City takes this matter seriously.
The TTHM exceedance was identified through the City’s routine regulatory monitoring program. The City has already taken additional samples, and those samples were below the maximum contaminant level at all four monitoring locations.
The City is also implementing additional water main infrastructure to improve circulation and continuing to monitor the water system.
Our goal is to maintain the highest possible level of protection for our residents while ensuring that the City’s drinking water continues to receive the disinfection necessary to protect public health.

For questions or additional information, please contact:
City of San Luis
Public Works Department
928-341-8577
pwadministration@sanluisaz.gov