CARLSBAD -- A truck headed north on Interstate 5 crashed early Friday and dumped as much as 19 tons of asphalt and 90 gallons of diesel fuel onto the marshy banks of the Batiquitos Lagoon.

Officials contained the spill with absorbent booms, and a low tide kept the pollutants from washing further out into the sensitive wetland, which straddles the border between Carlsbad and Encinitas.

Most of the pollutants were contained in the banks and did not make their way into the lagoon or the ocean, officials said. Crews ceased their cleanup efforts during the midday high tide, waiting for the water to recede in the early evening.

Officials said they expected little harm to the lagoon.

"I would say that the stuff that can float, the sheen, has stayed in the primary containment area," said Robin Lewis, a scientist with the California Department of Fish and Game, referring to an area of 100 feet by 30 feet that crews closed off with lengths of absorbent material called booms.

The nonfloating pollutants -- heavy oil and asphalt -- sunk into the sediment and stayed in place, Lewis said. He said he expected that crews would be able to dig out the area before the pollutants spread.

About 2 p.m. Friday, a large yellow excavator sat in the shoulder of the freeway and the dozen or so men on scene were standing around.

Lewis said crews were waiting for low tide at about 6 p.m. so they could reach the affected sediment. He expected the cleanup to take all night.

The trouble started about 1 a.m. Friday, when the left rear tire blew on a 2005 Peterbilt dump truck carrying asphalt to McClellan-Palomar Airport, CHP Officer Eric Newbury said.

The truck was in the right lane going about 55 mph at the time, and the driver struggled briefly to gain control of the vehicle before slamming on the brakes and crashing through the guardrail, Newbury said.

The truck traveled about 20 feet down the embankment and came to rest on its side on the bank of the lagoon.

The driver walked away and called for help, Newbury said. He was not injured.

On Friday, about 75 feet of missing rail and flattened saplings marked the truck's path.

The California Department of Transportation, County Environmental Health and the Coast Guard were among the many agencies whose vehicles lined the northbound side of I-5 just north of La Costa Avenue.

The heavy equipment and cleanup closed several lanes of the freeway until 2 p.m., and traffic backed up to Del Mar.

At least one lane was expected to close again Friday night when the cleanup resumed, officials said.

Eric Woynaroski, a pollution investigator with the Coast Guard, said the spill had been mostly contained while the tide was low, before rising water could carry pollutants into the lagoon.

By high tide, crews had been able to put booms around the spill and reinforcing booms behind the affected area, he said.

He anticipated the damage to the wetlands would be confined to the removed soil, the flattened trees and the shrubbery that had to be removed to haul away the dump truck.

Contact staff writer Sarah Gordon at (760) 740-3517 or sgordon@nctimes.com