A preliminary study by two state groups of possible ways to solve Northern Virginia's water supply problems was attacked last night by a number of political and civic leaders from Fairfax and Loudoun counties.

Most of the criticism came from representatives from Loudoun County where, under three of the five special Virginia water study commission, alternatives offered by the Virginia State Water Control Board and the large areas of the county would be flooded for reservoirs.

George H.Yeager, vice chairman of the Loudoun Board of Supervisors, said, "There has to be some thing wrong with the policy that takes water from a rural area and diverts it through the leaky toilets of the metropolitian area."

The criticism came at the last of three public meetings the control board and the commission held on their study. Under a mandate of the Virginia General Assembly, the study commission must recommend by Dec.1 a solution for solving Northern Virginia's water problems.

In his remarks last night's meeting Loudoun's Yeager also read a statement adopted Monday by the county supervisors expressing "unalterable opposition" to new or enlarged reservoirs in Loudoun. The supervisors also criticized the preliminary study for failing to stress water conservation.

Fairfax COunty Supervisor Marie B.Travesky (R-Springfield) said the study's assumption that Northern Virginia would be served by a regional water authority - instead of the present variety of agencies - could "doom any solution to failure."

Many speakers were critical of the study's assumption that per capita water demands would increase by almost 50 per cent by the year 2020.

Laurence D.Bory, director of policy coordination for the Northern Virginia Planning District Commission, said the projection was based on simplistic methodology."

Brg said the much lower per capita projection for water consumption - 104 gallons per day, the same as the present rate - of the Fairfax County water supply committee would reduce the demand for new water supplies estimated in the study by 34 per cent by the year 2020.

The preliminary study of the control board and special commission estimated that Northern Virginia's demand for water would increase from the present total of 100 million gallons daily to 292 million gallons daily by the year 2020.

If demand is not as high as the study predicts, less money would have to be spent on supply projects than is estimated in the five alternatives that are listed. The five projects would range in cost, the study estimated from $153 million to $219 million.

Three of the proposals would involve building new reservoirs or expanding present ones in Loudoun County. The other two would involve connecting the present Occoquan Reservoir either with the potpmac River or the Shenandoah River.

All five alternatives assume the Fairfax County Water Authority would be able to take water from the Potomac River as a supplement to the Occoquan and other sources. However, the study says, the Potomac could supply a maxmum of only 57 million gallons a day.