A committee of regional and federal environmental officials may require Arlington and Alexandria to change processing procedures at their sewage treatment plants that discharge water into the Potomac River. The proposal has alarmed local officials, who say the changes could cost the two localities at least $50 million.

A subcommittee of the State/EPA Management Committee, which oversees water quality in the Potomac, has proposed that the two Northern Virginia localities begin nitrification, a process that neutralizes oxygen-hungry ammonia and increases oxygen levels in the river.

Local public works and sanitation officials say they are not convinced the change, which would cost at least $50 million to institute and $1 million to $2 million a year to maintain, is necessary.

"We have not yet seen technical documentation that would lead us to conclude it is a prudent expense," said Jeff Harn, an Arlington public works planner. "Until we see the justification from the State/EPA Management Committee , it's hard for us to go along with it."

"There may be alternative measures that could be taken at a much lower cost," Arlington County Manager Larry J. Brown wrote in a memo to County Board Member Michael J. Brunner. "At this time it does not appear that such alternatives are being properly considered."

"We think there's sufficient doubt that it should be studied very carefully," said Samuel Shafer, engineer-director of the Alexandria Sanitation Authority.

Shafer said that the $50 million figure for initiating the procedure came from a 1982 study and was "very conservative."

The technical subcommittee of the State/EPA Management Committee proposed the change last month and is now preparing technical material to back it up, according to Charles App, chairman of the subcommittee and an environmental engineer.

Both the District's Blue Plains sewage treatment facility and Maryland's Piscataway facility have been using the nitrification process since 1981.

The State/EPA Management Committee includes environmental officials from Virginia, Maryland, the District and the EPA. The nitrification proposal would become legally binding only when it is made part of the discharge permits for the plants -- a process that requires public participation and can take up to a year.

Apps said the critical main stem of the river near the Woodrow Wilson Bridge should contain at least 5 milligrams of dissolved oxygen per liter of water. But in recent years, Apps said, the level has periodically dropped to 4 milligrams per liter, and sometimes as low as 2.

With a level of 4 mg/liter, "you don't have as healthy an aquatic population as you would at levels of 5 mg/liter and above," he said.

The proposal of Apps' subcommittee would require the waste water treatment plants to add oxygen to ammonia in the plant, a procedure that would require modification of the plants, he said.

The State/EPA Management Commmittee should make a decision on the nitrification requirement within a month, Apps said.