A report issued by an environmental group last week on the level of chlorine toxins found in city tap water supplies had District officials rushing to reassure local residents about the safety of their drinking water.

But according to the report’s findings, those who work on Capitol Hill would appear to have less to worry about than almost everyone else in Washington, D.C.

That’s because of the 18 water samples taken from locations from across Washington — including drinking fountains in local parks and kitchen taps in private homes — the water from a bathroom tap on Capitol Hill contained the lowest levels, by far, of toxic byproducts from the chemicals that are used to purify Potomac River water.

Meanwhile, 40 percent of the tested samples had chlorine concentrations higher than federal safety limits.

The D.C. Water and Sewer Authority distributes water to customers, including Congress, while the Washington Aqueduct, which is regulated by the Environmental Protection Agency, is responsible for treating city water.

The Architect of the Capitol does perform some additional filtration for water that is piped into certain Congressional buildings, but Eva Malecki, a spokeswoman for the AOC, declined to discuss the extent of those systems.

She said the AOC relies on the District itself “to provide water that meets expected quality standards and regulations.”

Richard Wiles, executive director of the nonprofit Environmental Working Group, which produced the report, said the low chlorine levels for Capitol Hill were surprising especially given some of Capitol Hill’s problems with lead in the water in recent years.

“Fortunately for Members of Congress and their staff and those that visit Capitol Hill, Congress can take measures into their own hands when determining what they want or do not want to do with the filtration of their water,” said Environmental Working Group spokesman Alex Formuzis, a former Congressional staffer. “For the rest of the country, it’s up to the EPA and over the last several years the EPA has been lax on a whole host of environmental issues that are important to the health of the American people. This study is just another example of that.”

According to the report issued Thursday by the Environmental Working Group, the tests were conducted to highlight the 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 byproducts of the chemicals used to kill those same microbes.

In particular, the Environmental Working Group had a private laboratory test 19 water samples, taken in early May, for the disinfection byproducts haloacetic acids (HAAs) and trihalomethanes (THMs), toxins that some studies have linked to certain cancers and even growth problems for the fetuses of pregnant women. The samples were taken when the Washington Aqueduct planned its annual chlorine flush, when higher levels of chlorine are added to the water system to remove bacteria and other hazardous residue from delivery pipes.

According to the findings, the water taken from a tap located in the Hart Senate Office Building had an HAA concentration of 24.1 parts per billion, or less than 50 percent of the allowable federal limit. Meanwhile, water from a bathroom tap at the Martin Luther King Jr. Memorial Library had an HAA concentration of almost 90 ppb, and water from a kitchen tap in a Northwest elementary school had an HAA concentration of 83 ppb.

The study found that while all of the locations’ THM concentrations were found to be under the EPA’s maximum containment levels, the water from Capitol Hill and the headquarters of the Environmental Protection Agency had by far the lowest levels of concentrations.

“The response here was positive in the fact that we were on the low end of chlorine this time,” said Kyle Anderson, press director for the House Administration Committee, in response to the report. “It’s certainly an issue we continue to monitor and will continue to develop policy around.”

Distrust of Congress’ drinking-water supply has been an issue that goes back decades on Capitol Hill.

In1991, tests conducted by aides to Rep. Henry Waxman (D-Calif.), then chairman of the Energy and Commerce Subcommittee on Health and the Environment, and Sen. Frank Lautenberg (D-N.J.) uncovered unsafe drinking water in locales across the Hill. The incident also sparked then-Architect of the Capitol George White to pledge to reduce the lead content to meet federal guidelines, while also prompting some House offices to move to bottled water.

More recently, the issue came to a head in 2004 when the Office of Compliance issued a citation against the AOC because lead was found to exceed safety levels in drinking water at all three Library of Congress buildings. Library unions were the first to raise those concerns with the OOC on Capitol Hill, but at that time there also was a city-wide outcry about lead levels in the drinking water.

Following the OOC intervention, the AOC shut down the drinking water system throughout the Adams Building for repairs. In addition, six drinking fountains in the Madison Building and three in the Jefferson Building were temporarily removed from service.

The AOC continues to provide bottled water to employees in the Adams Building. According to the OOC, which is monitoring the abatement of the lead hazard, the AOC will continue to do so at least through early next year. The AOC has informed the OOC that it will need until March 2008 to replace all of the affected plumbing fixtures and procure and install water filtration systems to ensure potable drinking water at all of the LOC buildings.

Today in Congressional offices it has become almost standard practice to buy bottled water using funding provided by Members Representational Allowances. But Nan Thompson Ernst, an employee in the Library’s Manuscript Division and a representative of the LOC Professional Guild, pointed out that Library of Congress employees outside the Adams Building have taken to forming “water clubs” whereby they pool together their own money to buy bottled water. She estimated that for a 10- to 12-person club, an employee could expect to contribute an average of $50 a year to buy their own bottled water.

Ernst said last week that the union is still concerned about the AOC’s efforts to address the lead problem at the Library.

“We’re so used to drinking bottled water that we forget that we have an obligation to provide safe drinking water for people who can’t afford it,” Ernst said.