While traffic on Interstate 580 is back to normal, agencies that will oversee cleanup of an oil spill south of Tracy still must learn the extent of contamination before they know how long it will take to clean up the entire mess.

Shell Oil Company workers and its contractors were at the spill Wednesday as they repaired a pipeline and dug up contaminated dirt from the side of I-580.

Shell spokeswoman Alison Chassin said the company was still looking into the cause of the Tuesday rupture of a 20-inch steel pipe. The spill sent about 4,200 gallons of crude oil across I-580 near Bird Road and forced the California Highway Patrol to shut down both eastbound lanes during Tuesday’s afternoon commute.

The oil gushed from the pipeline, ran down a hillside, flooded a ditch and spread across the two eastbound lanes of I-580. It also went into a culvert underneath the roadway and flowed to the other side of the freeway. The CHP reopened one lane Tuesday night and both lanes Wednesday afternoon.

Chassin expected the break would be repaired by midday Thursday but didn’t venture a guess as to how long it would take to clean up the site.

“Our priority is to make sure it’s cleaned up thoroughly,” she said, adding that the U.S. Environmental Protection Agency and the California Department of Fish and Game will let Shell know when the cleanup is done.

Mark Merchant, spokesman from the EPA’s San Francisco office, said the EPA is optimistic that cleanup would not take more than a few weeks, but it will depend on how deep the oil soaked into the ground.

“The cleanup will continue until we go out there, do more testing and find no more residual crude,” Merchant said.

Dana Michaels, information officer with Fish and Game’s spill prevention and response office, said that an agency field officer estimated it would be less than a week for the oil and saturated soil to be removed.

Though the cost of the cleanup is unknown, all of the agencies agreed that Shell would pay for it.

“One of the great things about California is that the polluter pays, always,” Michaels said. “They’ll have to reimburse our agency and the federal EPA’s costs.”

The pipeline break happened shortly after 4 p.m. Tuesday. Chassin said automated pressure sensors in the local section of pipe detected a pressure drop, shut down the pumps and immediately sealed off the broken section. She didn’t state the length of this particular section of pipe but noted that valves at regular intervals allow Shell to isolate sections of the line.

The pipeline runs 177 miles between oil fields near Coalinga and Shell’s refinery in Martinez. Chassin said this incident did not interrupt the refinery’s production.

Chassin said many details about operation of the pipeline are proprietary information and would not be released to the press. She would not say how much oil moves through the pipe on a daily basis, how much pressure the oil is under, or from what other sources crude oil goes to the Martinez refinery.

According to the California Energy Commission, Shell’s Martinez refinery processes about 154,900 barrels per day of crude oil. All of California’s refineries combined process about 2 million barrels per day.

To reach reporter Bob Brownne, call 830-4227 or e-mail brownne@tracypress.com.