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

Disinfection Byproducts Update - April 2021

During the week of April 5, 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 March 2, 2021 indicate that levels for HAA5 at 1 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.

The MCL is 60 parts per billion (ppb) and is calculated as the average of the results from the past four quarters at an individual sample site. All individual sample-site results from March 2, 2021 were below 60 ppb. Since the annual average at one sample site exceeded 60 ppb, however, the results still qualify as an exceedance of the MCL. The annual average at the one location was 63 ppb. The Commission first experienced elevated HAA5 in September 2018, which led to the violation of the drinking water standard​ in successive quarters until December 2020.

Solutions – April 2021 Update

The Commission has been actively working to 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 ensure consistent water quality and regulatory compliance for HAA5. A pilot study was completed between fall 2019 and fall 2020 to determine the most effective treatment process to remove more dissolved NOM and reduce HAA5. Results from the pilot study will inform the design of permanent treatment plant upgrades necessary to reduce disinfection by-products, including HAA5. As a possible interim solution, and with approval from the Massachusetts Department of Environmental Protection (MassDEP), results of the pilot study were also utilized to conduct a half-plant trial of an alternative coagulant to remove more NOM within existing treatment processes. The trial did not yield sufficient improvements, underscoring the need for permanent treatment plant upgrades.

A panel of national experts convened by the Commission is guiding these activities. The design of the permanent treatment plant upgrades is scheduled to begin in FY22.  After the design is approved by MassDEP, construction will begin in FY24 at an estimated cost of $167.6 million. The Commission is accelerating this work as quickly as possible while committing significant resources to the process.

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