An Army excavation of buried World War I chemical munitions has found widespread arsenic contamination in the back yard of the South Korean ambassador's home in Northwest Washington. Elevated levels of the poison have also been measured at two adjacent sites, a neighbor's yard and a small wooded area at American University.

The Army Corps of Engineers, which has uncovered more than a half-dozen artillery shells holding a toxic mustard agent during 10 months of digging up what was once a placid ornamental garden, plans to remove much of the ambassador's yard to get rid of the arsenic.

"This is a long-term mess," said Thomas Loughlin, who lives with his wife and two young children next door to the Korean residence on Glenbrook Road in Spring Valley, and on whose property arsenic also has been detected. "I'm not planning on having my kids out playing in the yard."

Officials with the Army and Environmental Protection Agency say the arsenic does not endanger other residents in Spring Valley, but they are testing nearby properties to make sure. They will hold an "availability session" from 3 to 8 p.m. today at their field headquarters near Sibley Memorial Hospital for anyone who has questions.

Arsenic, a naturally occurring substance that is dangerous at high levels, is a component of lewisite, one of several deadly chemical agents developed along with mustard at an experimental station that operated at the then remote, fledgling American University and the surrounding Spring Valley neighborhood during World War I.

The heaviest concentrations of arsenic have been found deep in a weapons burial pit, where levels of up to 1,000 parts per million (ppm) have been measured. Surface tests in the ambassador's yard show levels of between 100 and 250 ppm in some spots. EPA guidelines recommend soil removal for measurements of 43 ppm or greater.

Officials are awaiting the results of more tests before deciding what to do about the AU plot, where arsenic levels as high as 188 ppm have been measured. The land is on a wooded hill behind an academic building and has been cordoned off. Arsenic also has been found in the sediment of a small stream that receives run-off from the yard, but no arsenic has been detected in the water, according to the EPA.

"We haven't seen any level of health risk, except [the arsenic]," said Harry Harbold, the EPA official overseeing the project. "We don't want to sound the alarm. But I think it's prudent to do additional sampling."

A preliminary estimate is that about 75 percent of the approximately two-acre site will have to be dug down to a depth of two feet to remove the arsenic, said Maj. Brian Plaisted, the Army officer supervising the site. Part of the Loughlin yard also would be removed.

Removing the newly discovered arsenic-contaminated soil will likely not begin until April and last two to three months, according to preliminary plans. After that, the Army figures it will take an additional four to five months to restore the ornamental garden in the ambassador's back yard.

The estimated cost of the shell-removal project, which was supposed to be $4 million, has ballooned to $16 million. Removing the arsenic-contaminated soil probably will cost an additional $5.2 million, Plaisted said.

American University was chosen in 1917 as an ideal place for the Army to prepare for the chemical warfare unleashed in Europe. Clouds of poisonous gas were soon wafting near Massachusetts Avenue as tests were conducted. When the war ended in 1918, the tests were halted, and some munitions and chemicals were buried.

In 1993, a contractor digging a trench uncovered the chemical rounds. The Army evacuated homes in the area and launched an emergency cleanup. In 1995, it gave the area a clean bill of health.

But city officials were less sanguine, particularly after finding a 1918 photograph showing a soldier in a gas mask leaning over a hole. "The bottles are full of mustard, to be destroyed here," someone had written on the back.

Photographic analysis showed the pit was in what is now the Korean ambassador's back yard. Subsequent excavation shed light on this forgotten piece of Washington history, but for neighbors and the Army team, the novelty has long since worn off.

"It's 2001--a Spring Valley odyssey," Jeffrey Kraskin, a neighborhood leader, joked at a community meeting. Army workers wearing spacesuit-like protective gear have been painstakingly excavating for chemical weapons since March and don't expect to finish until next month.

At the ambassador's home, tons of gravel have been trucked in and about 25 trees have been cut down, leaving scattered stumps. A large steel hut sits atop one of two large holes, designed to protect the surrounding neighborhood should munitions unexpectedly explode.

"The Korean ambassador is a bit concerned," Plaisted said. "He'd like to get use of his garden back."

The Loughlin family was moved at Army expense to an apartment for seven months during the height of the digging.

From the start, the excavation has proved to be more than the Army anticipated. Ground-penetrating radar did not show the full extent of the dump, Plaisted said. The Army had to dig an 8,300-cubic-foot hole, 10 times as big as expected. About 30 people work at the site. Police officers on overtime sit in cruisers by the ambassador's residence and the holding facility, 24 hours a day. An ambulance is also on hand whenever digging is underway. The city will bill the Army more than $1 million for those costs, according to Plaisted.

"We want to make sure this is over and done with," Kraskin said. "We just want to have closure, so that you can safely have your children digging their forts and that you can grow a garden and not worry that what you're feeding your family could harm them."

CAPTION: Maj. Brian Plaisted, the Army officer supervising the project, shows off some of the containers being used by the Corps of Engineers to haul away dirt during an earlier excavation of the site.