PITTSBURGH (AP) _ Emergency crews raced today to contain a 92-mile oil spill that has contaminated two of the city's three rivers, threatened to shut off drinking water for 750,000 suburban residents and halted shipping along the Monongahela.

The Western Pennsylvania Water Co. closed one of two Monongahela intakes to prevent the fuel from contaminating water supplies in towns south and west of here. No communities had lost their water by this morning, but 50,000 people were expected to lose their water today.

''They're going to go down,'' said utility spokesman Dennis Casey. ''The main tank ... is nearly empty.''

The spill began Saturday night when an Ashland Oil Co. fuel tank collapsed and about a million gallons of diesel fuel flowed into the Monongahela River.

Western Pennsylvania Water Co. implored its customers in two counties to continue conserving water because the largest of its two riverside treatment plants could not longer draw from the Monongahela. The public has apparently been heeding the calls, Casey said.

The towns of Greentree, Dormont, Carnegie, Crafton, Ingram, Mount Lebanon and Scott Township will probably lose their water supply today. Water trucks will be available to dispense potable water; residents would have to pick the water up in buckets. Gov. Robert P. Casey put the Pennsylvania National Guard on alert to deliver emergency water supplies.

Residents of East Liverpool, Ohio, 65 miles downstream from Pittsburgh, were also asked to cut back on water use because the town draws its water from the Ohio River and was preparing to switch to reserve supplies.

The city of Pittsburgh's water comes from the Allegheny River and was unaffected. The Allegheny and Monongahela converge near downtown Pittsburgh to form the Ohio.

Boat traffic on the heavily used Monongahela was halted for 25 miles, and adjacent rail lines and highways were temporarily closed. Hospitals were notified of the conservation plan and transferred their most serious patients to facilities unaffected by the crisis, water company spokesman Casey said.

The spill occurred when a storage tank collapsed at Ashland's Floreffe Terminal near West Elizabeth, officials said. Ashland had said the tank was new, but the Pittsburgh Press reported today that the tank was actually 40 years old and had recently been moved from Cleveland.

The cause of the collapse had not been determined, but one official speculated that the tank's foundation might have shifted.

About 7,000 of the approximately 1 million gallons of diesel fuel floating in the Monongahela and Ohio Rivers was recovered by late Sunday, officials said. In addition to drinking water, waterfowl and fish were threatened.

Speed is important because the fuel is slowly dissolving into the water, making it more difficult to recover, said Coast Guard Lt. John Farthing.

''It may become almost impossible to recover at all,'' Farthing said.

More equipment was heading to the area today to help, but officials said the cleanup could take weeks.

Booms stretched across the Monongahela, 2,000 feet at its widest, dammed much of the oil, but some of the smelly fuel escaped. The Coast Guard this morning said the pollution had reached Newell, W.Va., and East Liverpool, Ohio, 65 miles below the city and 92 miles downstream from the tank.

Specialized pumper trucks skimmed the oil from the surface, but downstream of the booms, greasy ribbons of oil meandered into the Ohio.

One million of the estimated 3.5 million gallons in the tank flowed into the river; the rest was contained near the tank, Farthing said. About 820,000 gallons have been recovered from the tank area.

Ashland spokesmen on Sunday had said the ruptured tank was new in August, but The Pittsburgh Press today quoted a Skinner Tank Co. official as saying his company moved the tank from Cleveland and estimated its age at 40 years.

The official, Larry Skinner, said the vessel was thoroughly tested and was sound. He speculated that the foundation of the tank may have shifted.

Ashland spokesman Roger Schrum said the tank's age was noted on construction applications filed with the U.S. Environmental Protection Agency.

Gov. Casey said he has invited the governors of Ohio, West Virginia, Kentucky, Indiana and Illinois and members of Pennsylvania's congressional delegation to join him in asking the federal Environmental Protection Agency for aid.

A spokesman for the Army Corps of Engineers said the cold weather should minimize the impact on wildlife. Most shore birds have gone south for the winter and fish were not spawning, said Michael Koryack a corps biologist.

Waterfowl and fish kills are expected, however. ''It will be a month before they can assess the damage to wildlife,'' said Samuel E. Lockerman, a supervisor for the state Game Commission.

Particularly vulnerable are surface-feeding fish like the white bass and gizzard shad, Koryak said. The birds most in danger are ducks that live near the shore, enticed there in many cases by people who feed them through the winter, he said.