The Fairfax County Board of Supervisors is pressuring the county water authority to lower rates because of what one county official called "unconscionable" surpluses in the agency.

Water authority officials estimate the 15 percent rate reduction proposed by some supervisors on Monday would shave about $12 off the average homeowner's annual $80 bill.

Board Chairman John F. Herrity said he proposed the rate reduction because of an anticipated $14.5 million surplus in the water authority budget for this year.

"It's unconscionable to let this kind of fund accumulate when we have areas of the county in dire need [of water service]," said Centreville Supervisor Martha V. Pennino.

Water authority spokesman James A. Warfield said the agency had planned to use the surplus to finance improvements in parts of the county's aging water system.

"We at least have to consider the rate reduction," said Warfield. "When the supervisors make a request, our board is not going to ignore it."

Warfield said that although the authority may have other priorities for using the surplus, "Politically, reducing retail rates is the most attractive."

Fairfax County officials, who said they have not increased water rates since 1977, attributed their $14.5 million surplus to investment income that exceeded expections, more new residential connections than anticipated and a lack of major cost overruns on construction projects.

The water authority's board of directors, whose members are appointed by the board of supervisors, could consider the proposals at a meeting tomorrow night.

The supervisors also have asked the water authority to take over the water service that the City of Falls Church provides to almost one-sixth of Fairfax County residents as well as to Falls Church and Vienna.

"The city has no interest in such an arrangement," said Joseph Livinski, Falls Church director of public utilities.

County residents served by the Falls Church water system pay an average of $1.15 per thousand gallons of water compared to the 70 cents per thousand gallons charged to county residents by the Fairfax County Water Authority.

Supervisors said they also were concerned about reports that Falls Church's wholesale water supplier, the U.S. Army Corps of Engineers, plans to increase the rates it charges the city by 29 percent.

Livinski said the city has received no official estimates on the proposed increases.