Water Quality

Total trihalomethanes (TTHM) are a group of disinfection byproducts that form when chlorine compounds that are used to disinfect water react with other naturally occurring chemicals in the water.  They are colorless, and will evaporate out of the water into the air.  There are four significant TTHM potentially found in disinfected drinking water and their combined concentration is referred to as total TTHM.

Levels of TTHM generally increase in the summer months due to the warmer temperatures, but can also be affected by seasonal changes in source water quality or by changing amounts of disinfection added.  Water systems often can experience temporary increases in TTHM due to short-term increases in chlorine disinfection.  Chlorine disinfection increases can occur when there is a water main break, when water systems are under repair, or when there is a potential microbial (example:  bacteria) problem or threat.

All water systems that use chlorine to disinfect the water are required by federal and state law to sample for TTHM on a regular basis (quarterly).

Total trihalomethanes are sampled quarterly at four locations throughout the town. Each site has its own Annual Locational Running Average and is simply the average of the previous four quarters. Those averages for the individual sites are then averaged for the system. However, each site must be below the maximum contaminant level (MCL) of 80 parts per billion to not be in violation. The water being pumped from the plant today is within the compliance threshold, but due to past high samples, our running average remains above the MCL and in violation.

Samples are collected every quarter – so four times per year. They must be 90 days apart. Although not required, the plant Chemist has implemented monthly in-process sampling to ensure operators stay on top of any changes in water quality and the treatment process; as of last month, water leaving the plant is 11 parts per billion. 

Per- and polyfluoroalkyl substances (PFAS) are a group of man-made chemicals that includes PFOA, PFOS, GenX, and many other chemicals. PFAS have been manufactured and used in a variety of industries around the globe, including in the United States since the 1940s. These chemicals are very persistent in the environment and in the human body – meaning they don’t break down and they can accumulate over time. There is evidence that exposure to PFAS can lead to adverse human health effects. MA DEP has now implemented a sampling program for PFAS, and Tewksbury regularly samples our finish drinking water. To date, our finish drinking water is well below the Maximum Contaminant Level (MCL) of 20 ng/L.

For more information, visit:

U.S. Environmental Protection Agency

Commonwealth of Massachusetts

Related Documents:

THM Averages Chart - February 2, 2022

PFAS Sampling Trends