Swimming Banned Along 8 Miles of Beach After Spill : Environment: Diesel fuel may have come from a passing ship. Huntington Beach officials hope to reopen shoreline today.

HUNTINGTON BEACH —  Swimming was prohibited along eight miles of beach Friday afternoon after a small diesel fuel spill, probably from a passing ship, caused thin blotches of rainbow-colored sheen to spread across the ocean surface close to shore.

Huntington Beach city officials said they closed the ocean to swimmers from Golden West Street to the Santa Ana River because they feared the fuel could cause skin or intestinal ailments. An estimated 20,000 people were on the 8 1/2-mile stretch--which includes Huntington State Beach, the second most popular state beach in California--when the order to get out of the water came from lifeguards at 3:30 p.m.

Beach-goers were allowed to stay on the sand.

City officials said the stretch of ocean will be off-limits to swimmers and surfers for part of today, but they hoped to reopen it by this afternoon.

“When you have any kind of an oil spill, we are concerned about the impact,” said Brian Baird, the California Coastal Commission’s oil spill expert.

The fuel probably was bilge dumped by a passing vessel, officials near the scene said.

“It’s probably no more than one barrel (42 gallons), and that’s probably generous,” said Coast Guard Lt. (j.g.) Mike King. “It’s probably much less than that.”

Unlike crude oil, which is more viscous, a small quantity of diesel fuel can spread quickly and thinly over water, creating a shiny film. Even a gallon can spread over miles of ocean.

At 4 p.m., the light oily sheen was spread out in patches over an area 1 1/2 miles long and 200 yards wide, and was only about 50 yards offshore south of the Huntington Pier, King said. Some of the film washed up about a mile north of the pier onto beaches, he said.

The spill originally was spread over an area about five miles long, but it quickly evaporated.

The oily film was creeping Friday afternoon toward sensitive Huntington Beach wetlands off the mouths of the Santa Ana River and the adjacent Talbert Channel.

“I’m very worried because it only takes a little bit of oil to cause damage to birds and other wildlife,” said Gordon Smith, chairman of the Huntington Beach Wetlands Conservancy, which manages the 25-acre marsh that was restored as a sanctuary two years ago.

Haunted by memories of a 400,000-gallon oil spill in nearly the same place 18 months ago, Huntington Beach officials stressed that this spill was small. But officials added that they preferred to err on the side of caution.

Samples were taken for analysis although officials were virtually certain the substance was diesel fuel. Test results were not immediately available.

“I suppose that with the city’s memory of what happened before there, they felt it was the best thing to do,” said Mary Gale, a spokeswoman for the state’s Office of Oil Spill Response and Prevention.

Times staff writer Kevin Johnson contributed to this story.