Random tests performed in summer 2003 found unsafe amounts of a chemical toxin a few hundred yards from Washington's drinking water supply, a discovery that has set off a tense, largely behind-the-scenes debate over what steps are necessary to protect the water and the public.

The high concentrations of perchlorate, a toxin typically found in chemical weapons and explosives, were detected by chance in June and September 2003 as part of a long-term study and cleanup of chemical contamination in the surrounding Spring Valley neighborhood. A researcher with the U.S. Environmental Protection Agency found the chemical in groundwater near the Washington Aqueduct reservoir at levels 80 times the amount EPA considers a risk to humans.

For the past year, the results were known only to a relatively small group of scientists, government officials and neighborhood activists. Officials at the aqueduct said there is no cause for alarm because significant amounts of perchlorate have not been detected in the drinking water. The aqueduct has conducted tests several times this year that can detect perchlorate at concentrations as low as 4 parts per billion.

The U.S. Army Corps of Engineers, which is responsible both for operating the aqueduct and for leading the Spring Valley cleanup project, initially argued that aggressive steps to investigate the source of perchlorate or to monitor the flow of groundwater were not needed. Corps officials, at a meeting Oct. 7 with the EPA, proposed a series of tests to monitor groundwater contamination in Spring Valley, far uphill from the reservoir.

Amid more questions from the EPA and from a consultant hired by residents of Spring Valley, Corps officials said last week that they will also monitor groundwater on the banks of the reservoir, near a wooded area suspected of being a chemical weapons burial site from World War I.

"I don't like the fact that I can't answer the question of how groundwater moves around the reservoir," Thomas Jacobus, chief of the aqueduct, said in explaining the agency's new approach. "The question people will ask me is, {grv}'What are you doing to protect the water supply? Why haven't you stuck a stick in the ground?' That's a good point. So we're going to find out."

Ed Hughes, Corps project manager for hazardous and toxic waste at Spring Valley, said in a written statement that the team will reevaluate its plans in fall 2005 if information from its monitoring wells shows that contaminated groundwater is getting into the reservoir.

"We do not believe the [former munitions testing station] poses a serious current threat to the reservoir," he said. "If there was an indication of a problem, our approach would be different."

But experts on perchlorate contamination say immediate action should have been taken to determine the source of the chemical and whether it could seep into the reservoir. They say the latest response is still inadequate because the Corps is not trying to pinpoint the source of the perchlorate and has no plans until 2008 to investigate suspected weapon burials on the wooded aqueduct property.

"It's crucial to find the source in order to properly monitor the problem," said Henry Harrison, a national consultant on contamination cleanup. "It's basic."

The debate here echoes a contentious battle between the U.S. military and communities all over the country, including in Aberdeen, over how much danger perchlorate poses to the public and whether the military should be forced to clean it up.

The discovery of perchlorate near the reservoir, first reported last week in the Northwest Current, shows how this potential hazard is handled differently across the country. There is no federal standard for perchlorate, largely because the Department of Defense has fought it. In the absence of one, a handful of states have set their own public health and cleanup rules, with varying success.

Hughes said the Corps has a limited budget of $11 million for the site cleanup and must prioritize its work according to likely risk. The Corps will focus in the next year on determining whether groundwater is flowing toward the reservoir, which treats and supplies water to the District, Arlington County and the City of Falls Church.

"Only after we have a handle on answers to these two questions will we be concerned with determining the source of perchlorate," Hughes wrote.

Perchlorate, a chemical known to disrupt the thyroid gland and linked to hormonal dysfunction, developmental delays and infertility, is considered a health risk to humans at concentrations of 1 part per billion, according to a proposed EPA standard.

The level of 80 parts per billion was detected at a construction site at Sibley Memorial Hospital, across the street from the aqueduct property.

Tests also revealed perchlorate levels of 6 to 8 parts per billion in an underground drainage ditch that carries water leaking from the aqueduct's treatment basins.

Aqueduct officials said they still strongly believe that the perchlorate cannot get into the reservoir. Jacobus said the aqueduct will nevertheless conduct tests every three months for perchlorate levels as low as 1 part per billion.

Scott C. Heuer, a Spring Valley activist, said elevated levels of arsenic turned back yards into hazardous waste sites. He said the Army should now err on the side of caution and address any buried munitions and contamination left from the World War I testing station at American University.

"It can't be stalled any longer," Heuer said. "Is what went wrong in Spring Valley going to go wrong with the aqueduct or the Potomac River?"

In California, regulators have issued emergency orders to clean up sites where levels of perchlorate reach 6 parts per billion, even in cases where drinking water is not affected. Maryland told the Army more than a year ago to clean up contaminated soil and groundwater from the Aberdeen Proving Ground testing site after finding perchlorate in drinking wells in the town of Aberdeen. But the military is mulling over the proposal, and state officials said they cannot force the cleanup unless there is a proven health emergency.

The EPA has the legal authority to require the cleanup of perchlorate in contaminated soil or water if it reaches levels of 4 parts per billion and public health is at risk. But the Defense Department has successfully argued that perchlorate cleanup at all its bases is unnecessary, costly and could jeopardize the military's war on terror.

For local activists and environmentalists, however, the District's case stands out because of the number of people who could be affected.

"It's very troubling to us that you'd have known contamination 100 meters from the water supply for the nation's capital and known or likely weapon burial sites immediate adjacent to the reservoir, " said Erik Olson, a lead attorney at the Natural Resources Defense Council. "And because the Army is not willing to ask Congress for money -- and does not really want to find a problem -- they're still waiting for years before they aggressively take action."