Aberdeen Proving Ground's Old O-Field dump began exploding and spewing military poisons 50 years ago.
Now, the Army is designing a cap to cover the 4.5-acre munitions dump with sand and other material. Officials say the permeable cap and a sprinkler system, to be installed within a year, will guard against fires and explosions, and allow the Army to treat the dump's contents with chemical or biological solutions.
And, within days, Army engineers expect to start operating what they describe as one of the most advanced ground water treatment systems ever built in a new attempt to stop the dump from leaking pollutants into surrounding waterways.
The treatment system is designed to stop metals, solvents and warfare chemicals flowing from Old O-Field into Watson Creek, which empties into the Gunpowder River.
The engineers say a unique "bio-monitoring" component of the treatment system -- fish in tanks equipped with computer sensors to measure the animals' breathing and heart rates -- will be used to gauge continuously the toxicity of the treated ground water before it is piped into the Gunpowder.
An Army report prepared in October says one to three "explosive events" are expected to occur at Old O-Field every 10 years. The last was in the late 1980s, when a spontaneous white phosphorus fire burned vegetation at the dump.
The report says the possibility of a large fire or explosion is remote. But a resulting vapor cloud could endanger thousands of Army workers, boaters on the Chesapeake Bay and people living in nearby communities in Baltimore and Harford counties, the Army says.
Treatment of the dump may continue for 50 years, the Army says, perhaps pushing the total cost over $100 million.
Dealing with the toxic mess at Old O-Field is the most expensive and challenging cleanup project among dozens at the 72,000-acre Army weapons-testing and research installation. The dump, patrolled continuously by security forces, is one of several disposal pits and test areas on the base's Edgewood area that are designated by letters.
Excavation of the site is considered too dangerous. And it could cost billions of dollars, Cindy Powels, an Army environmental engineer supervising the cleanup, said before a recent visit to the site.
"They are proceeding in the best way" with the cleanup attempts, said Katherine Squibb, a University of Maryland toxicologist who is a technical adviser to a Joppa-based citizens' group monitoring the environmental cleanup at the proving ground.
"It is not going to make O-Field go away," Dr. Squibb said, "but it will take care of what is causing harm to the environment."
In a graphic written account of early cleanup attempts at Old O-Field, Lt. Col. Dean Dickey described spontaneous explosions and fires that burned for days.
He wrote that in September 1949, "We had just entered the field . . . when one of the pits exploded. We ducked behind piles of barrels and bombs until all the fragments had stopped falling. We then left the field and gave it a week or two to settle down."
Another spontaneous explosion occurred in December 1949. "On Saturday the pits really exploded and threw munitions everywhere . . . white phosphorus was all over the place and some of the woods were burning," Colonel Dickey wrote. "All we could do was to see which way the fire was heading and try to keep buildings from being burned down."
Poplars, sweet gums and other trees have grown up in Old O-Field since Colonel Dickey began his cleanup. Still, rusted bombs, piles of mortars and other munitions, laboratory glassware and other military junk is plainly visible from a narrow road bordering the dump.
As the late colonel described in his 10-page affidavit, "indiscriminate dumping of every conceivable item imaginable" occurred at Old O-Field.
That includes thousands of U.S., Japanese and German munitions containing mustard agent -- a blistering substance and a carcinogen -- and munitions containing white phosphorus, magnesium and napalm. More drums and munitions dumped there contained a "choking" agent called lewisite and a "vomiting" agent called adamsite. Smaller quantities of lethal nerve agents also were dumped.
Many of those munitions remain at the site.
Sheep, goat and horse carcasses from experiments, including some contaminated with radioactive material, were buried there but later removed.
Cleanup workers and bomb disposal experts had to clear explosives from areas around the dump before the ground water treatment system could be built. Tens of thousands of items were recovered, including hundreds of intact munitions.
"There was a very real cost to clearing every square foot of ground," said Lester Craig Maurer, an Army Corps of Engineers geologist. "We came up with tons of garbage that had to be detonated."
Mr. Maurer and his colleagues originally thought the ordnance clearing would take a month. It took two years.
"We've done our best to try and get it done as quickly as possible," Mr. Maurer said.