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Important Information About Your Drinking Water

The City of Boulder is in compliance with drinking water standards for haloacetic acids for the first quarter of 2018. 
Haloacetic acids are a byproduct of the drinking water disinfection process, and small amounts are always present in drinking water. As reported to water customers in July 2017 and November 2017, there was an increase in haloacetic acid levels in late May/early June that caused a drinking water violation in the second quarter of 2017. This same issue, which was corrected in June 2017, resulted in another violation in the third quarter, because of the way the drinking water standard (called the maximum contaminant level , or MCL) for haloacetic acids is calculated.

More information about how the standard is calculated, why the higher levels of haloacetic acids occurred and corrective actions can be found below, or you can read the notification mailed to all city of Boulder residents and businesses pdf. You can also review the violation notice from the state of Colorado. pdf

Haga clic aqui pdf para acceder a la notificacion en espanol. 

How the Drinking Water Standard for Haloacetic Acids Is Calculated

State and federal regulations require the city to collect water samples at eight locations throughout the city and test for haloacetic acids on a quarterly basis, or every three months. The four most recent sampling results at each location are averaged and reported to the state each quarter (this calculation is called the running annual average ).

The maximum contaminant level (MCL) for haloacetic acids is 60 micrograms/liter (µg/L). The running annual average of haloacetic acids at one sampling site for the third quarter of 2017 was 60.6 µg/L, or 0.6 µg/L above the MCL. Although levels of haloacetic acids have been below 60 µg/L at all eight sampling sites since late June, the increased levels in late May/early June caused the running annual average at one site to be above 60 µg/L.

The June 2017 sampling results will continue to be calculated into the running annual average through the first quarter of 2018. Therefore, it is possible that two additional violations will be reported, in January and April 2018.

Average haloacetic acid levels in Boulder typically range from 23 to 48 µg/L.

Why the Increased Levels of Haloacetic Acids Occurred

Haloacetic acids are formed when chlorine, a necessary part of the water disinfection process, reacts with natural organic matter in the water. Construction at the Betasso Treatment Plant required a change to the disinfection process to keep drinking water free of disease-causing bacteria and other pathogens. This change, combined with high levels of organic matter during spring snowmelt runoff, created higher levels of haloacetic acids in late May and early June 2017.

Corrective Actions

After the high levels of haloacetic acids were measured in June 2017, city staff adjusted water treatment operations, which lowered the level of haloacetic acids. Staff has also modified sampling routines and data analysis to identify potential problems earlier in the treatment process. The city has maintained levels below 60 µg/L at all sampling sites since late June. To view recent sampling results and additional information about the second quarter violation, please visit .

What This Means for Customers

  • You do not need to take any action related to your drinking water or seek an alternate supply of drinking water. If this had been an emergency, you would have been notified immediately. Staff estimate that high levels of haloacetic acids were present in the water system for less than thirty days in late May/early June and have not occurred since that time.
  • Health effects. Some people who drink water containing haloacetic acids in excess of the MCL over many years may have an increased risk of getting cancer. The level of haloacetic acids exceeded the MCL for less than 30 days and has been below the drinking water standard since June. Customers who wish to learn more about the potential health effects of exposure may visit for additional information or consult with their doctor if they have specific health concerns.
  • Who is affected. While only one of the city’s eight sampling sites indicated an exceedance of the byproduct, it is possible that levels exceeded the MCL throughout the city’s distribution system. Therefore, all customers may have consumed water with elevated levels of haloacetic acids during this time period. 

Additional Information and Links

City staff have provided the following external links for additional information about haloacetic acids and drinking water quality:

July 2017 Drinking Water Notification

The city notified customers in July 2017 of a similar water quality violation. The October 2017 violation resulted from the same issue that was resolved in June 2017. Read the July 2017 notification in English pdf and Spanish pdf. You can also view the notice of violation pdf from the Colorado Department of Public Health and the Environment.

Current Monitoring Results

City staff has been sampling for haloacetic acids on a weekly basis and has maintained levels below the water quality standard (60 ug/L) since June 2017.

You can view an updated haloacetic acids monitoring summary pdf  or the certified lab reports:


For more information, please contact Tom Settle at [email protected] or Michelle Wind at  [email protected], or by phone at 303-441-3200. You may also visit  to learn more about water quality monitoring in the city and to view the 2018 Drinking Water Quality Report.

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