The most extensive testing to date for groundwater problems in Northwest Washington neighborhoods contaminated by World War I munitions has found low levels of perchlorate and arsenic but no chemical warfare agents or explosives, officials announced yesterday.

A network of monitoring wells, installed this summer in the latest chapter of a contentious multimillion-dollar federal cleanup of the Spring Valley community, detected the two potentially harmful substances just east and south of the Dalecarlia Reservoir. The groundwater sampling is intended to determine any danger to the reservoir and the Washington Aqueduct, which supply drinking water to more than 1 million people in the District, Arlington County and Falls Church.

Several unexpected readings in 2003 and 2004 of perchlorate in the community's groundwater and in the aqueduct prompted accelerated and intensified testing. But officials with the government agencies involved said yesterday that the latest results were reassuring because they showed perchlorate and arsenic in relatively few wells and at levels below federal drinking water standards.

"We really are encouraged," said Gary Schilling of the U.S. Army Corps of Engineers, which is heading the massive effort to remedy the nearly century-old legacy of the Army's American University Experiment Station. The Corps of Engineers has long argued that the aqueduct is not at risk from compounds leaching from a wide area of buried ordnance or toxic materials, and it repeated its reassurances yesterday that the region's drinking water is safe.

Alma Gates, chairman of Advisory Neighborhood Commission 3D, is withholding judgment. She wants to know whether the groundwater sampling revealed the presence of any of 163 additional compounds or elements used at the wartime site, and she wants answers to the critical question of whether the area's groundwater permeates the reservoir as it flows south and west.

"I think there is still so much uncertainty," Gates said. "We just went through [the hazards of] lead in the drinking water, and didn't we hear the guys with the Army say, 'Oh, no, there's no danger there.' "

The results released yesterday said that the summer's highest reading for perchlorate, a chemical used as a stabilizer in weapons and explosives, was found in a sump on the grounds of Sibley Memorial Hospital. Although the quantity was less than half the 58 parts per billion that provoked such concern at the same location two years ago, it still measured 24 ppb.

A perchlorate level of 10.6 was detected in a monitoring well just northeast of the hospital, though other wells in the immediate vicinity showed levels a fraction of that or that could not be measured.

Only two wells indicated the presence of arsenic, a breakdown product of chemical warfare, with the highest concentration being 3.5 ppb, according to analyses.

At certain levels of exposure, both substances can cause significant health problems, including developmental delays and cancer. Arsenic has been the target of the extensive soil excavation that has removed the top layers of yards of more than 30 homes, with dozens more to go.

Schilling's optimism was echoed by the community co-chairman of the board that advises the Corps of Engineers on the project, though toxicologist Gregory Beumel wants to review the data of this and future testing.

"I think it's very good news," Beumel said yesterday afternoon. "I think it means the worst-case scenarios or the great fears of what was going on . . . are disproved."

The next set of results, on the chemicals and elements narrowed from a list of more than 600, could be available within weeks. By November, the Corps of Engineers expects to have ready nearly all of the 30 wells planned and to begin a second round of sampling late in the winter. The wells are from east and west of the reservoir to east of Dalecarlia Parkway and just north of Loughboro Road.

A community meeting to discuss the groundwater sampling and progress on the ground cleanup will be at 7 p.m. Oct. 18 at Metropolitan Memorial United Methodist Church, 3401 Nebraska Ave. NW.

Gary Schilling, left, and Ed Hughes of the Army Corps of Engineers at a water testing well in the Spring Valley section of Northwest Washington. A casings is sent deep into the ground to the water level to test for toxins. Perchlorate and arsenic were found at low levels in some wells.