Encapsulated within special steel cases locked away in a remote storage igloo at this desert base are 784 reasons why the United States wants to destroy its aging chemical weapon stockpile.

Tooele Army Depot has 784 chemical arms that are leaking deadly nerve gas, Army officials say.Among them are a variety of bombs, land mines and projectiles made between the 1940s and the late 1960s. But most of the leaky weapons are old M55 rockets, said Tooele Public Affairs Officer Susan Voss.

Ironically the M55s could no longer be used in combat because the Army now lacks any device to fire them, Voss said. The M55s' only potential threat now is against friendly troops because of their leaking.

Each M55 contains 10.7 pounds of nerve agent GB or nerve agent VX.

Breathing air that has just 100 milligrams of either chemical per cubic meter is enough to cause death. The chemicals constrict the body's muscles - which prevents breathing unless a victim immediately receives antidotes.

As Army documents say, both chemicals are "very effective as a chemical warfare nerve agent because of the relatively small concentrations needed to produce casualties."

Worry that such deadly small concentrations could be caused by the leaks was voiced by Steve Erickson, spokesman for Downwinders, a defense watchdog group based 35 miles from Tooele in Salt Lake City.

"That's frightening. We knew some of the weapons out there were leaking. But that number (784) is rather frightening," he said.

Voss said, however, that number is a "very small" percentage of the total number of chemical arms stored at Tooele. The exact number of arms there is classified, but Tooele stores 42.3 percent of the nation's chemical stockpile.

Voss also said the Army is taking extra precautions to control the leaks and protect its workers and others.

And of note, those leaks plus similar ones at bases in Alabama, Arkansas, Colorado, Kentucky, Oregon and the Pacific Ocean are among the reasons why the Army plans to destroy all its older chemical weapons by 1997, said Marilyn Tischbin, public affairs officer for the program manager for chemical demilitarization at Aberdeen Proving Ground, Md.

The Army plans to replace the older weapons with newer "binary" weapons, which contain two chemicals that are harmless until they are mixed in-flight after the weapons are fired.

Critics say the new weapons may escalate a chemical weapons race. Supporters say the new weapons are safer to store and say that fear of them is why the Soviets began serious negotiations last year to ban chemical arms globally. Current treaties prevent use of the arms but not their storage or production.

Voss said storage of the leaky weapons at Tooele now poses little threat to workers and nearby residents.

She said when a weapon is found to be leaking, it is sealed in a special steel case and transported to an isolated storage igloo with other leaking arms. Monitors there would sound alarms if a large amount of agent escaped. Also, the igloo is checked daily with more sensitive devices that collect samples that are examined in a lab, which can detect even minute leaks.

She said no leaks from the special encapsulation cases have been detected. She also said the regular packing in which arms are stored and transported usually prevents any leaks from escaping to the atmosphere.

To check for leaks with other stored M55 rockets, Voss said every three months the Army looks at a random sample of the weapons. They are taken out of their protective packaging and examined closely for any amount of nerve agent.

Other monitoring without removing weapons from packaging is also routine _ and in the case of M55s, it is done daily.

Voss said most leaks are small. "Maybe `leak' isn't even a good word to use. When you hear of a leak, you think of a kitchen faucet. But what we are talking about isn't something dripping on the floor. Maybe it's a small residual amount of agent around the nose."

Voss said the leaks at Tooele have never caused an injury. "In fact, we have never had a lost work day due to an accident with chemical arms. We're very proud of that."

Tischbin said other bases have similar good safety records. "The only accident that I know of is that a couple of maintenance workers at Johnston Atoll (in the Pacific) were exposed, but were not injured, from a leak."

Tischbin added that seven of the nine bases that store chemical weapons have some arms that are leaking. "The only exceptions are Newport Army Ammunition Plant in Indiana and Aberdeen Proving Ground. Both facilities store only bulk containers of nerve agent, not other types of weapons."

She said Tooele likely has the most leaking weapons of any base not only because it has the most weapons overall, but also because other facilities used a portable "drill and drain" system that destroyed many of their most troublesome leaking weapons.

Tischbin said the troublesome M55 is among the first types of weapons scheduled to be destroyed when the Army completes arms destruction facilities at each base storing such arms. Such a facility at Tooele is scheduled to be completed by late 1991. A test facility that pioneered how to safely destroy those arms has been operating at Tooele's south area since 1979.

Tischbin said she is unsure whether weapons that are actually leaking will be given the highest priority for destruction. "They cause several problems in their handling. The safest thing for the time being is to leave them in their overpack."

Erickson of Downwinders said his group is scared by both the prospects of storing leaky chemical weapons and the dangers of destroying them.

For example, he said, a January 1987 accident at a test chemical arms destruction facility at Tooele released nerve agent GB to the atmosphere. The leak was discovered by monitors, and workers in the area were evacuated. The leak was called minor, but the Army did not again use real nerve agent at the facility for 15 months.

"Neither burning the weapons or not burning them is a very comforting prospect," Erickson said. "But this new information supports the decision that all the chemical arms should not be transported to Tooele for destruction. This leads us to support on-site destruction."