The Pennsylvania Public Utility Commission on Wednesday ordered the immediate shutdown of Sunoco Pipeline's Mariner East 1 system after sinkholes exposed the bare pipeline in Chester County, which PUC investigators said "could have catastrophic results" if not repaired.

Gladys M. Brown, the PUC's chair, granted an emergency order to halt operations on the 8-inch-diameter pipeline, which went into service in 1931 originally to carry motor fuel. It now carries up to 70,000 barrels a day of high-pressure volatile natural gas liquids such as propane from the Marcellus Shale gas region to a Sunoco terminal in Marcus Hook.

Brown issued the order at the request of the PUC's Bureau of Investigation and Enforcement, which earlier Wednesday said sinkholes developed recently where the pipeline passes less than 50 feet from houses on Lisa Drive in West Whiteland Township, near Exton and near Amtrak's mainline corridor from Philadelphia to Harrisburg.

Brown said she agreed with investigators that if immediate steps are not taken, it "could have catastrophic results impacting the public." Though a shutdown will injure the pipeline's shippers, she said, "the risks to the general public outweigh the risks to the shippers."

The emergency order will require Sunoco to run an inspection tool through the pipeline to ensure it is undamaged, and to conduct geophysical testing to determine the extent of underground instability. Operations will not be allowed to resume until corrective actions are taken that satisfy the PUC's investigators.

Sunoco and the PUC estimate the pipeline will be shut down from 10 to 14 days to complete the investigations and corrective actions.

Sunoco, in a statement, said it conducted digs on Tuesday in coordination with federal and state pipeline regulators and "did not find evidence of any conditions that would impact the stability of the pipeline." The company is a subsidiary of pipeline giant Energy Transfer Partners LP,

"This period should allow us to share what our professional geologist has established to date — that the Mariner East 1 pipeline is stable, is located in suitably safe geology, and will continue to operate safely as it has done for decades," the company said in its statement.

The sinkholes are the latest complication to bedevil Sunoco's Mariner East system, which involves the construction of two additional pipelines alongside the 300-mile-long Mariner East 1 pipeline.

Construction has been plagued by problems associated with underground drilling for the new pipelines, which is suspected of contributing to the underground instability that caused the sinkholes to form. Sunoco last month agreed to pay a fine of $12.6 million to resume underground drilling on the project.

State Sen. Andy Dinniman (D. Chester) on Wednesday called on the state  to also halt construction of the Mariner East 2 pipeline. Dinniman said Chester County's underlying karst formation — a porous limestone that is eroded with fissures and sinkholes — is unsuitable for pipelines.

"This is just another example of how problematic karst formations can be and why they're no place for pipelines," Dinniman said.

Activists say the Chester County sinkholes are eerily similar to coal-mine subsidence in West Virginia that caused the ATEX Pipeline to rupture in 2015 in rural West Virginia, triggering an explosion that damaged the nearest house, 700 feet away. The ATEX pipeline transports the same type of gas liquids that are carried in the Mariner East system.

Though residents and activists on the pipeline have complained about sinkhole activity since November, the PUC's investigators said Sunoco did not notify the PUC or the federal Pipeline and Hazardous Materials Safety Administration of the sinkhole developments until March 3.

The PUC was closed Wednesday because of the winter storm, but an emergency order can be issued by any of the five commissioners, the commission's director of operations, or the commission's secretary. The full PUC can ratify, modify or rescind an emergency order at the commission's next meeting, on March 15.