In an action signaling a new attention to costs in water pollution programs, the Prince William County Board of Supervisors - with the tacit approval of the staff of the metropolitan Washington Council of Governments - sharply revised yesterday its prior approval of a regional water quality plan.

The County Board's 11-part resolution does not address any specific costs but recommends that future decisions on quality standards and construction of facilities "be based upon cost effectiveness."

The Prince William action was colored by the county's recent fights with state, federal and area authorities on a wide number of issues, but still indicates a strong tide running for attempts to hold down the costs of government and environmental controls.

Kenneth Fujishiro, COG's chief of water pollution control who appeared before the board as a technical expert, said after the board's unanimous vote that it was his "personal agreement that you do meet a point of diminishing returns (in water pollution standards) . . . Cost is something that must be considered."

Fujishiro disagreed with many of the specifics of the extensive Prince William resolution, but he clearly supported the thrust of taking a closer look at the cost effectiveness of proposals.

He said that he expected other area governments to pass similar modifications to the "208 Plan," the major, federally mandated area water quality management plan. That plan must have local, state and federal approval.

Fujishiro said that the Prince William action "does not conflict" with the 208 Plan, and added that "I don't think the Virginia State Water Control Boards (SWCB) is in conflict with the sentiments of Prince William County."

Michael A. Bellance, deputy executive secretary for the SWCB, while saying that the Prince William County view is likely to have been strongly colored by recent water control board actions against the county, agreed that the cost effectiveness of programs was now of major interest to the state board.

The problem has been aggravated by the federal government's failure to appropriate the full $5 billion authorized for water cleanup at the same time that inflation cuts into the $88 million that Virginia has been allocated annually for pollution control.