A new agreement to limit use of the Potomac River's water supply during droughts needs only the signature of a U.S. Amry Corps of Engineers official to become effective.

The agreement, another interjurisdictional attempt at sharing the metropolitan area's major water supply, recently was signed by Maryland and Virginia officials.

The agreement would limit water withdrawal from the Potomac between Little Falls, near the D.C,-Maryland line, upstream to Seneca, Md., during dry spells when water flow in the river reaches extremely low levels.

The Potomac last approached critically low levels that threatened its use as a water supply during an extended drought in the summer of 1966. Environmentalists say the river is even more prone to water shortages now should a similar dry spell strike again because of increased population in the area.

The Virginia State Water Control Board, which is responsible for water quality and aupply in Virginia, the Washington Suburban Sanitary Commission, which provides water to Prince George's and Montgomery Counties, and the Maryland Water Resources Administration, which grants permits to withdraw Potomac River water, signed the agreement in December.

The Army Corps of Engineers, which supplies water to the District of Columbia, still needs to sign the agreement before it becomes legally effective. The agreement is under review by the Corps to "examine language and other small details," said Corps planning director Bill Treischman.

"The Corps is doing all it can to work out some details of the agreement and come up with a version that will be presentable to the states signing it," Treischman said. "I don't foresee any major problems in getting it finalized and we're hoping that will happen within several weeks, if not before."

The agreement already has been in the works two years. When finalized, it will be the area's first legally established arrangement to limit water withdrawal from the river in drought periods.

The agreement also satisfies a prerequisite for the Washington Suburban Sanitary Commission to build a long sought improved raw water intake structure near its River Road filtration plant in Montgomery County, and a diversion weir halfway out to Watkins Island, which lies adjacent to the C and O Canal. Commission officials say the weir is needed to divert more water to feed its intake system, which malfunctions when the Potomac is at low ebb.

The commission needs a permit from the Corps to build the structure, since the Corps controls the river as a navigable waterway. The regional agreement was one of the requisites recently established by Congress for the Corps to grant a permit for construction, said WSSC spokesman Arthur P. Brigham.

"Absolutely nothing can be cone until the Corps finds the agreement satisfactory and signs it," Brigham added. "Then several other things would have to be worked out as well. We could hope we would be in a position to go ahead with building the structures within the year."

Virginia's participation in the agreement covers the potential role of Fairfax County as a future supplier of water from the Potomac. Fairfax County currently has a permit from the Maryland Water Resources Administration to withdraw up to 24 million gallons a day from the river, said water authority director James Corbalis.

But the authority also needs permission from the Corps to build a proposed water withdrawal plant south of Rte. 7 on Stuart Road near the Fairfax-Loudoun county line. Corbalis said he does not know when the permit from the corps would be issued.

Both the proposed water intake facilities would be located upstream from the Corps' Dalecarlia plant, which supplies water to the District of Columbia.

"It's no wonder the Corps is treating the agreement with caution," said a spokesman for the Interstate Commission on the Potomac River Basin. "Those additional facilities would withdraw much more water from the river, possibly affecting the Corps' ability to withdraw water and supply the District. In a severe dry spell, too much withdrawal could have disastrous results on water supply. An agreement like this would make all the difference."

The agreement would establish three stages of low flow in the river when it would start regulating withdrawal.

Alert stage, when daily withdrawals from the river are equal to or greater than 50 per cent of the Potomac's total flow.

Restriction stage, when daily water withdrawal is equal to or greater than 80 per cent of total river flow.

Emergency stage, when daily withdrawals would probably exceed daily river flow anticipated durint the next five days.

River flow in the Potomac has ranged from about 300,000 million gallons a day (during the 1966 dry spell) to about 10 billion gallons a day during floods. About 400,000 million gallons of water is withdrawn daily from the river now. Proposed additional water withdrawal structures would withdrawal several more million gallons.

Under the agreement, allocation of water in dry spells would be based on a ratio of daily average water pumped to consumers by each supplier during the winter season. It would be the responsibility of each jurisdiction participating in the agreement to regulate water use.

Enforcement of the agreement's provisions would be vested in a moderator chosen by the Corps, Virginia and Maryland. The agreement also outlines operating procedures of water plants during dry spells.

A similar water sharing agreement, although one not legally enforceable, has been drawn up by the Metropolitan Washington Council of Governments. GOG's Water Conservation Coordination agreement calls for reduction of water use through voluntary cooperation of its participating member jurisdictions, 12 of which have signed a portion of the agreement.

It does not propose specific allocations of water in periods of low flow. It seeks instead to reduce consumer use of water through public education and research.