Four environmental groups petitioned the federal government yesterday to stop the Army Corps of Engineers from testing the controversial herbicide Diquat in the Potomac River in an effort to control the rapid spread of hydrilla.

In a petition to the Interior Department, the National Park Service, the Army and the Corps of Engineers, the groups charged that the proposed use of Diquat fails to comply with the National Environmental Policy Act and said that they question its safety.

The National Coalition Against the Misuse of Pesticides, the Audubon Naturalist Society, the Friends of Dyke Marsh and the Rachel Carson Council cited concerns about the levels of a highly carcinogenic substance called ethylene dibromide (EDB) in the herbicide. The groups charged that the approved levels of EDB in Diquat are "inconsistent with recent regulatory actions severely curtailing EDB use and exposure."

Richard Mountfort, the U.S. Environmental Protection Agency official in charge of regulating Diquat and other chemicals, said that the planned use of Diquat, and its EDB levels, should pose no threat. He also said that the agency will likely require the Army corps to monitor the river during its planned tests of Diquat on hydrilla at two one-acre sites along the Virginia side of the river.

Douglas Campt, director of registration in the EPA's Office of Pesticide Programs, said that chemicals like Diquat, which have not yet been formally reevaluated by the agency as required by a 1972 law, legally can be used, if the manufacturer's instructions are followed.

Jay Feldman, coordinator for the national pesticide coalition, however, called Diquat "a casualty" of the reregistration program. Under the 12-year-old law, Feldman said, the EPA has been charged with reviewing "600 active ingredients." Thus far, he added, only four have been formally reregistered, and Diquat -- which was first registered in 1958 -- is not among of them.

A subaquatic weed, hydrilla has grown rapidly along the Potomac, more than tripling the amount of water it covers this year. A U.S. Geological Survey study recently noted that 600 acres of river is covered by it. The Army Engineers this week announced plans for the Diquat test to slow the weed's growth.