BCWA: THM samples 'not a violation, an emergency or an immediate risk'


The Bristol County Water Authority issued a press release Thursday afternoon on recent trihalomethane readings, which reads as follows:

The Bristol County Water Authority, as an advanced precautionary measure, is notifying our customers that two of our eight sample sites for the last two quarters exceeded 80 parts per billion (ppb) for trihalomethanes.  Due to changes in the EPA regulations, any site that averages above 80 ppb for four quarters, will be in violation of the new regulation.


This is not a violation, an emergency or an immediate risk.

Surface water systems are required to treat the water with chlorine in order to inactivate bacteria that can cause disease.

Trihalomethanes (THMs) are formed when chlorine is added to water containing natural organics. Water from surface supplies, such as reservoirs, contain natural organics from the decay of leaves and other vegetation.

Total Trihalomethanes (TTHMs) include four compounds: chloroform, bromodichloromethane, dibromochloromethane, and bromoform, with chloroform as the major component. Long term exposure (over 70 years) to high levels of chloroform can affect liver and kidney function.  High doses of chloroform have been found to cause liver and kidney cancer in experimental animals.

The RI Department of Health  requires all surface water systems to maintain measurable levels of chlorine at every sample point in the distribution system.  The major concern is to protect the public health from waterborne disease caused by bacteria. The benefits of chlorine in preventing disease are considered to far outweigh the risks from cancer, if any, from low level exposures to THMs.


There are no actions that you need to take at this time.  You do not  need to use an alternate (i.e. bottled) water supply.  You do not need to boil your water or take other corrective action.  However, if you are have specific health concerns, you may want to contact your health professional.


In the warm weather months, BCWA boosts chlorine at the Barrington pump station and the Bay View storage tank to maintain adequate chlorine residuals in the system.  Chlorine is much more reactive in warm water.  It also reacts with the iron from the cast iron water mains installed prior to the 1950’s, requiring additional chlorine.

Trihalomethanes form gradually, over time.  The longer the chlorine is in contact with the water, the higher the potential for THMs.   BCWA purchases all of our supply from the Providence Water Supply Board’s Scituate Water Treatment Plant.  By the time the water reaches our connection in Barrington, the TTHM levels are 50-60 ppb.  The farthest reaches of the water system, and therefore the water in contact with chlorine the longest, is in southern Bristol.  The levels of two samples from this area are above 80 ppb.

The new regulation requires that no sample can exceed 80 parts per billion averaged over four quarters.  The previous regulation stated that ALL samples averaged together over four quarters could not exceed 80 ppb.


Only the first two quarters of the new monitoring program have been analyzed, and at the period of time that the chlorine concentrations were at the highest.  The addition of chlorine has been discontinued for the winter, and we do expect lower TTHM levels.

There are a number of methods to optimize the operation of the distribution system to decrease the amount of chlorine needed and the amount of time the water is in contact with the chlorine.  These projects are presently in our Capital Plan for 2013, but require a rate increase to institute:

Install instrumentation system to accurately measure and add chlorine based on flow and actual chlorine levels.

Institute microbial monitoring program to minimize chlorine use.

Install tank-mixing systems to reduce water age.

Perform unidirectional flushing to remove iron deposits (reduce reaction with chlorine)

Clean and cement line cast iron pipes in the distribution system to remove contact with iron.


If you are concerned about any of the health effects regarding TTHMs, please call your health care provider.

For general information on TTHMs, you may call the EPA Safe Drinking Water Hotline at (800) 426-4791 or visit the EPA website.

For more information, please contact Pamela Marchand, P.E. Executive Director at 245-2022.


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