New Tests Find High Levels of Hazardous Chlorination Byproducts in D.C. Tap Water

For Immediate Release: 
Thursday, May 24, 2007

WASHINGTON — In spite of the best efforts of the Washington Aqueduct to provide quality tap water to the District of Columbia, tests conducted by the Environmental Working Group (EWG) in May of 2007 revealed toxic by-products of the chemicals used to purify Potomac River water, at levels above annual federal health limits.

These results illustrate the tremendous difficulties that water utilities face when trying to provide tap water that is free of potentially deadly bacteria and pathogens, yet not contaminated with toxic by-products of the chemicals used to kill these same microbes. This problem is particularly acute when utilities draw water from poorly protected water sources like the Potomac River.

“These toxic byproducts will continue to persist until the Potomac River is adequately cleaned up. Until that happens residents of Washington and Northern Virginia should use carbon filters that can reduce these contaminants dramatically at one tenth the cost of bottled water.” said EWG Executive Director Richard Wiles.

EWG collected tap water samples from 18 locations across Washington D.C., including the U.S. Capitol, EPA headquarters, parks, schools, and residences of pregnant women and other groups susceptible to health problems from exposures to disinfection byproducts. Tests were commissioned from an accredited lab for two classes of disinfection byproducts — total trihalomethanes, or THMs, and haloacetic acids, or HAAs.

More than 40 percent of the tap water samples contained chemical byproducts of water treatment above annual federal health limits. The group of contaminants, known as haloacetic acids, or HAAs, were found at their highest levels since 2001, the last year before the Washington Aqueduct modified its treatment techniques in an attempt to reduce levels of related byproducts of tap water chlorination known as THMs.

EPA classifies HAAs as possible human carcinogens, and peer-reviewed studies have identified adverse reproductive and developmental effects, and the ability to damage DNA. The state of Oregon has warned that long term exposure to HAAs at levels equal to those found in DC tap water could cause injury to the brain, nervous system, the eyes, and the reproductive system.

The by-products of chlorination present a significant health issue that is not well addressed by current drinking water health standards. EPA scientists have identified a total of 600 disinfection byproducts in tap water but EPA has set legal limits in tap water for only 11. And these legal limits, such as those for HAAs and THMs, are established as a balance between health, treatment cost and feasibility.

“These results represent the tip of the iceberg for chemical contamination of DC tap water,” said Richard Wiles, Executive Director of EWG.


EWG is a nonprofit research organization based in Washington, DC that uses the power of information to protect human health and the environment. EWG's work on tap water contamination can be found at

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