A tanker truck loaded with 8,500 gallons of gasoline overturned on an embankment off the Capital Beltway in Fairfax County early yesterday, injuring the driver and causing major rush hour traffic delays, county police reported.

Police said the accident occurred about 5:20 a.m. on the Beltway between Telegraph Road and Van Dorn Street. Traffic was blocked on all four eastbound lanes for a short time as gasoline leaked from the truck, but police decided they could safely reopen two lanes.

"Under normal procedures we would block this thing completely," Deputy Chief Eugene R. Gray of the county police said at the scene."

He said, however, that the humid and wet weather conditions made it unlikely that the gas would be ignited by passing motorists.

Police identified the driver of the O'Boyle Tank Lines Co. truck as Ronald L. Oswald, 28, of 16636 Galt Ct., Woodbridge. Police said Oswald was admitted to Alexandria Hospital, where last night a hospital spokesman said his condition was satisfactory.

Police said that perhaps 1,000 gallons of gasoline had leaked from the overturned truck and entered a storm drain near the site of the accident. The gasoline then went into a creek and some reached the Potomac River.

Workmen dammed a section of the creek with dirt to prevent further gasoline from running into the river and a truck was brought in to drain the creek. A second tank truck drained the fuel still inside the tank of the overturned vehicle.