Information and Frequently Asked Questions about HAA5
Information and Frequently Asked Questions about HAA5

Disinfection Byproducts Update – October 2021

During the week of October 4, 2021, the Commission is notifying its customers of an exceedance of the maximum contaminant level (MCL) for haloacetic acids (HAA5) in drinking water. Sample results taken on September 2, 2021, indicate that levels for HAA5 at 7 of the 8 sample locations exceeded the maximum contaminant level (MCL) established by the Massachusetts Department of Environmental Protection’s (MassDEP) Safe Drinking Water regulations.

HAA5 forms when chlorine reacts with dissolved natural organic matter (NOM) found in surface water supplies such as Cobble Mountain Reservoir, the main source of the drinking water supply. Quarterly sampling in 2020 and earlier in 2021 indicated that dissolved NOM levels had decreased from previously elevated levels and that the water treatment process had been effective in the reduction of HAA5. Higher-than-average rainfall across the region during summer 2021, and major weather events, including Hurricanes Henri and Ida, resulted in an increase in the amount of dissolved NOM in Cobble Mountain Reservoir. The increase in NOM in the raw water and necessary chlorine dosages contributed to elevated HAA5 levels in the distribution system as sampling was conducted in September 2021.

The exceedance was not an immediate health hazard and customers may continue consuming and using their water as normal. If this had been a public health emergency, customers would have been notified within 24 hours.

Solutions – October 2021 Update

The Commission has been actively working to more permanently prevent elevated HAA5 since 2015, when it initiated a comprehensive planning process to upgrade the West Parish Filters Water Treatment Plant. While the Commission has regularly optimized existing plant processes to meet regulatory changes over the years, the plant’s last comprehensive upgrade was in 1974. Regulations related to HAA5 were first adopted in 1998 and revised in 2012.

The planning process is already well advanced and will result in significant plant upgrades that will help ensure consistent water quality and regulatory compliance for HAA5. A pilot study was completed in the fall of 2020 to determine the most effective treatment process to remove more dissolved NOM and reduce HAA5. Results from the pilot study are currently being used to complete the West Parish Filters Facilities Plan and design permanent treatment plant upgrades necessary to reduce disinfection by-products, including HAA5. A panel of national experts convened by the Commission is guiding these activities.

The final design of the permanent treatment plant upgrades is scheduled to begin late in FY22.  After the design is approved by MassDEP, construction will begin in FY24 at an estimated cost of $168 million. The project is being financed with support from the U.S. Environmental Protection Agency’s (EPA) Water Infrastructure Finance and Innovation Act (WIFIA). The Commission recently announced it acquired the $250 million low-interest WIFIA loan to help finance over $500 million in critical water and wastewater infrastructure improvement projects. The unique and flexible terms of the WIFIA Program will allow the Commission to advance the water treatment facilities improvement projects on an accelerated schedule and address multiple pressing needs at once to reduce risk, improve water quality, strengthen climate resiliency, and ensure reliability well into the 21st century.

More information on the WIFIA Funding and the Water and Wastewater Infrastructure Renewal Program is available on the Commission’s website

What is HAA5 and how does it get into the water?

HAA5 is an organic compound that forms when chlorine, which is used for disinfection, reacts with dissolved natural organic matter (NOM) found in surface water supplies, such as Cobble Mountain Reservoir. Therefore HAA5 is considered a “disinfection byproduct.”

Dissolved NOM enters the reservoir from rain and snow melt from the surrounding forest. Both the amount and types of NOM in the reservoir impact the levels of HAA5 in the treated water. The issue of elevated HAA5 first arose in late 2018 when above-average rainfall in increased the amount of dissolved NOM in the reservoir water by approximately 50%.

The graphic below summarizes how HAA5 forms:

Why is HAA5 regulated?

Some studies have shown that long-term exposure to HAA5 at elevated levels above the regulatory limit over many years (i.e. decades or a lifetime) may increase the risk of developing health problems. The Maximum Contaminant Level (MCL) is set at a level intended to maintain a large margin of protection against health effects.

More information about the potential health risks of HAA5 is available from MassDEP, which regulates drinking water quality, at 617-292-5770 or at

Additional information on why HAA5 is regulated is discussed by UMass Professor Dr. David Reckow, a national expert on disinfection byproducts, on Connecting Point (Feb. 12, 2019):  See the video

Why is chlorine added to drinking water?

Chlorine has been used in drinking water since the early 1900s to prevent waterborne illness caused by pathogens (bacteria) such as cholera, typhoid, and E. coli. Such pathogens are considered the most widespread and immediate risk to public health associated with drinking water. Treatment with chlorine also eliminates viruses such as the coronavirus. Chlorine dosage levels are traditionally higher in summer months due to increased water temperatures that promote bacteria growth.

The regulatory limit for HAA5 is set at a level that balances the immediate health risk presented by waterborne pathogens, and the potential health risks presented by HAA5 after long-term (decades or a lifetime) exposure to elevated levels. While the Commission is committed to complying with all drinking water regulations, preventing the immediate health risk of waterborne illness through proper disinfection is its foremost priority.

Should I install a filter if I'm concerned about HAA5?

The Commission does not advocate the need to purchase a home water filter to remove HAA5, and does not recommend any particular models or brands.

For customers that still wish to install a home treatment device, it is advised to thoroughly research whether the filter they choose performs as advertised. According to the American Water Works Association, some home filters can be used to reduce some chemical compounds that form due to chlorination, but not all. Customers should inquire if the device they choose is certified by an independent third party. NSF International, the Water Quality Association, and Underwriters Laboratories all certify home filter products.

Questions About HAA5

Commission customers with questions about HAA5 should call 413-310-3501 or email