WASHINGTON -- Unsafe levels of toxic chlorine pollutants were found in 40 percent of the District of Columbia tap water samples tested during a spring chlorine surge, according to a study released Thursday.

The Environmental Working Group, nonprofit organization, conducted the tests from May 1 to 4, at the end of a monthlong "chlorine burn" that the Washington Aqueduct conducts annually to remove sludge and sediment from pipes.

 The tests were done at private homes, a school, the U.S. Capitol and the Environmental Protection Agency.

The organization measured levels of toxins that could cause cancer, reproductive problems and developmental delays in children. The toxins are a byproduct of the reaction between chlorine and organic matter in Potomac River water. The group urged the U.S. Army Corps of Engineers, which operates the aqueduct, to switch to carbon filtration.

Washington Aqueduct General Manager Thomas P. Jacobus said the study's levels were probably temporary. He said D.C. water meets EPA safety standards because test results for the concentration of chlorination compounds are averaged over the year.

The EPA's regional office in Philadelphia is responsible for overseeing the city's water quality. A water official there said EPA regulations focus on limiting risks from long-term exposure, not short-term spikes.

"A couple of months of higher numbers -- we don't have any information that shows conclusively that that causes health risks," Rick Rogers said.

But the Environmental Working Group said the district's water issues reflect national water treatment problems.

"It's time to face up to the fact that it's impossible to take the Potomac River in its current polluted state, put it through a very old, rudimentary treatment system and get water that people can safely drink," said the group's executive director, Richard Wiles.

The Washington Aqueduct provides water to 1.1 million customers in the district, as well as parts of northern Virginia.

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