The governors of Virginia, Maryland and Pennsylvania and the mayor of the District of Columbia are to be congratulated for reaching tentative agreement on a joint program to clean up the Chesapeake Bay.

But before anyone in the Washington metropolitan area gets too caught up in the euphoria that will accompany next month's signing, he needs to understand the price (that is, higher sewage-treatment costs) that we will bear to adhere to an agreement to reduce pollution in the Chesapeake.

Most of us surely would subscribe to the need to clean up our nation's waters -- and certainly this especially applies to the protection of our nation's largest estuary, the Chesapeake Bay. We all enjoy the benefits of a healthy and clean bay, from recreational activities and scenic pursuits to the consumption of fish and shellfish.

However, the residents of the Washington area, who have already done more than our neighbors elsewhere in the Chesapeake Bay region to clean up our own waters of the Potomac River, will pay disproportionately more to meet the terms of the agreement.

Studies by both the Environmental Protection Agency and by the Environmental Programs Department of the Metropolitan Washington Council of Governments have determined that excess phosphorus is the primary cause of pollution in the Potomac River. In 1970, the main source of the pollution was the Washington area's municipal sewage treatment plants.

Although the total amount of waste-water discharges from Washington sewage treatment plants into the Potomac increased by 30 percent between 1970 and 1985, the phosphorus discharges have decreased by 97 percent. This is a record unmatched by any other community or metropolitan area lying along a tributary of the bay. Today, Washington is responsible for 5 percent of the phosphorus polluting the tidal estuary -- down from 54 percent in 1970. Most of the phosphorus now comes from agricultural runoff from lands upstream.

A look at other pollutants shows a similar dramatic cleanup since 1970 from Washington sewage-treatment plants. The discharge of solids into the river by our plants has decreased by some 95 percent, discharges of biochemical oxygen dropped 91 percent and total kjeldahl nitrogen 73 percent. The cost for such dramatic cleanup has not been cheap -- more than $1 billion over the past 15 years in capital investments alone, and $100 million per year in operating costs.

Now the much-heralded bay agreement comes along with an across-the-board 40 percent reduction (which is being questioned by technical experts) in all nutrients from all sources by the year 2000. This uniform requirement, which fails to acknowledge the conscientiousness already shown by some communities in meeting not only the spirit but the letter of the Federal Water Pollution Control Act of 1972, is simply unfair to the citizens of Washington. Sure we want a clean Potomac River and a clean and healthy Chesapeake Bay. But what have the citizens of other major metropolitan areas, such as Baltimore, Philadelphia and Richmond, done since 1970 to meet their commitments to clean water? Not nearly as much.

The fact that Washington enjoys a healthy and growing economy should be no justification for requiring it to spend more on cleanup when no one else in either of the bay's major drainage basins has yet come close to our record. And make no mistake: the costs will be passed on to individual homeowners both through increased local taxes and through higher monthly sewage treatment bills.

Washington has acknowledged its role in contributing to the bay's pollution and has done what it can to mitigate the problem. Now it wants Virginia and Maryland to acknowledge their ever-increasing contribution to the bay's pollution from farms and fields. Wouldn't you, as a local taxpayer, feel better doing your part if your rural and smaller community neighbors upstream were doing even a fraction of what you have already done to meet their own commitment to a cleaner bay? I certainly would. -- Gary D. Knight is chairman of the Water Resources Planning Board of the Washington Metropolitan Area Council of Governments.