The Army Corps of Engineers said yesterday it will apply the controversial herbicide Diquat to the Potomac River in an experiment to stop the spread of hydrilla, a decision that alarmed some environmental groups.

"If hydrilla takes over, the experts say it eventually will cover the river out to a depth of 15 feet," said Rep. Stan Parris (R-Va.), as he applauded the engineers' decision to take action against the rapidly growing subaquatic weed.

Without preventative steps, Parris said, "We might as well kiss the Potomac goodbye. . . . "

Several environmental groups voiced objections to the Army's plans, which call for using the herbicide at two one-acre test sites along the Virginia side of the river.

"We are unalterably opposed to the use of herbicides," said Neal Fitzpatrick, enviromental education coordinator for the Audubon Naturalist Society.

"This is not something we're going into lightly," said Noel Beegle, a spokesman for the engineer's.

He noted that the Army had specifically decided not to use Diquat in the hydrilla-infested Dyke Marsh area south of Alexandria because it is a wildlife refuge. "We are considering it all in great depth."

Beegle said that Diquat -- a herbicide that interrupts photosynthesis in plants -- will be used on test plots at the Belle Haven Marina, south of Alexandria, and at the Old Town Yacht Club in Alexandria.

Beegle said that although Diquat has been approved for use in free-flowing waters by the U.S. Environmental Protection Agency and by Virginia and Maryland authorities, preliminary tests with a harmless red dye will be made at the two sites to measure the chemical's potential spread.

"[As a]. . . safety precaution . . . there will be no . . . fishing or swimming in the [test] areas for five days after the [Diquat] treatment," Beegle said. He described the precautions as helpful, but probably excessive.

But Ruth Shearer, a scientist who conducted a study of Diquat and other herbicides for Seattle in 1980, said "It doesn't seem like something that I'd want to put in the Potomac."

Shearer said, in a telephone interview, that as a result of her studies, Diquat was not approved for use in lakes around Seattle.

It poses threats to "the aquatic ecosystem and human health," she said.

"Why are they using herbicides? Everybody knows herbicides work," said Fitzpatrick of the Audubon Society. His comment was a sarcastic reference to environmentalists' fears that herbicides may be so effective that they eradicate more than just the unwanted hydrilla.

"Most of these pesticides were approved before long-term testing was required," said Ellen Rainer, a researcher for the Rachel Carson Council of Washington.

In addition to the two herbicide test sites, the Army plans to experiment with mechanical harvesting, diver-assisted dredging and a weed barrier as means of controlling hydrilla at sites in Broad and Swan creeks, two tributaries of the Potomac on the Maryland side.

Parris and Sen. John W. Warner (R-Va.), who jointly announced the engineers' latest anti-hydrilla strategies yesterday, praised the agency for redirecting $225,000 of research funds to a "crash program" to control the weed.

The amount of Potomac River bottom covered by the weed has more than tripled since June, according to a recent U.S. Geological Survey report. In all, about 600 acres of the river are infested by the weed, the experts have said.