The spill, which began Tuesday, prompted an emergency cleanup effort at Suisun Marsh, about 30 miles northeast of San Francisco. Several dead animals, mostly ducks, were found at the scene, said Coast Guard spokeswoman Clare Maranda.
State officials estimated that 40,000 gallons of fuel spilled. Initial worst-case estimates had put the spill 1 million gallons.
Kinder Morgan Energy Partners, the Houston firm that owns the pipeline, estimated that 500 to 1,000 barrels, containing 42 gallons of fuel each, leaked into the marsh, spokesman Jerry Engelhardt said.
The spill was mostly contained by Thursday and was limited to a diked area of roughly 600 acres, so the fuel couldn't easily escape to the rest of the marsh, officials said.
About 50 workers from state, county and federal agencies were using containment booms and absorbent pads to clean up the spill, which left a sheen atop the water.
The pipeline, which carries fuel from San Francisco Bay area refineries to Chico, Sacramento and Reno, Nev., ruptured sometime Tuesday. Kinder Morgan noticed a drop in pipeline pressure around 6 p.m. Tuesday night and shut down a section of the pipeline, Engelhardt said. Environmental officials were told about it Wednesday.
The broken section of the pipeline would be replaced and back in service by Saturday, but it could take several weeks or even months to completely clean up the spill, Engelhardt said.
The Suisun Marsh is considered California's second-largest natural marsh, according to Greg Green, a biologist for Memphis, Tenn.-based Ducks Unlimited, a wetlands conservation group. But it's also a highly managed area, with large sections diked off to control the flow of water.
"It's an important area for biological purposes," Green said. The marsh covers 57,000 acres and is frequented by about 700,000 birds, including migratory shorebirds and raptors.