WASHINGTON (AP) _ Uniform steel tests could prevent fractures in thousands of aging oil storage tanks from causing spills similar to one in Pennsylvania this year that contaminated the water supply in three states, a government report said.

''The absence of steel toughness standards until 1963 and the failure to comply with today's standards when rebuilding tanks means that thousands of tanks in existence today could be susceptible to a brittle fracture like this one,'' Rep. Doug Walgren, D-Pa., said Friday.

Walgren requested the investigation by the Commerce Department's National Bureau of Standards after the Jan. 2 collapse of an Ashland Petroleum Co. tank near Pittsburgh.

The Ashland tank split open as it was being filled with 3.8 million gallons of oil, pouring an estimated 713,000 gallons into the Monongahela and Ohio rivers. The spill forced communities in Pennsylvania, Ohio and West Virginia to close their drinking water intakes, leaving about 23,000 suburban Pittsburgh residents without tap water for almost a week.

The report confirmed Ashland's statements earlier this year that the spill resulted from a dime-sized flaw in the nearly 50-year-old steel wall of the 4- million-gallon container. The wall was assembled in 1940, dismantled and reconstructed at the company's Floreffe Terminal south of Pittsburgh in 1986, the company noted.

Walgren said Ashland Oil, by using old steel in reconstructing the tank, avoided industry standards in effect when the storage facility was erected in 1986.

''The report demonstrates that we must not only question the safety of most tanks in the United States, but we must also develop new testing techniques to assess the quality of existing tanks,'' he said.

If the steel is tough enough, the report said, ''normal spill-control measures will confine the spread of the contents, and the tank can be repaired - all without catastrophic consequences.''

The Ashland tank did not meet current, nationally recognized standards for fracture toughness and the welds in the tanks, while not contributing to the split, did not conform to current standards and only served to make the metal around the flaw more brittle, the report said.

Safety concerns regarding the older tanks currently in use can be alleviated if there is documentation indicating the steel meets ''sufficient toughness provisions,'' the report said.

For those tanks lacking ''sufficient toughness,'' the bureau suggested remedial actions, including installing special crack ''arresters,'' converting the tank to a higher temperature use or retiring the tank.

The report also stressed the need for standard testing and evaluation procedures to assess the fracture safety of tanks made from inadequately documented steel.