The Pennsylvania Department of Environmental Protection on Friday ordered Sunoco to reroute a portion of its Mariner East 2 pipeline being dug in Upper Uwchlan Township, Chester County, after an August spill of 8,000 gallons of drilling fluid flowed into Marsh Creek Lake.

Marsh Creek State Park, with its 535-acre lake, is one of the most heavily visited parks in the state, with more than a million visitors a year. The spill enraged residents and environmental groups, and the DEP ordered drilling stopped at the time.

Though there is already a backup route as the result of a previous DEP order, rerouting the 20-inch pipeline could create a significant delay for Sunoco Pipeline LP.

“We are currently reviewing the DEP’s Administrative Order and will continue to work closely with the DEP on this issue as we have done throughout the duration of this project,” said Lisa Coleman, a spokeswoman for Energy Transfer, which owns Sunoco. “Our first priority remains the safe completion and then operation of this important infrastructure project.”

The spill occurred Aug. 10, when crews were drilling horizontally to install the pipeline that would transport natural gas liquids. The section of pipeline runs along Little Conestoga Road in Upper Uwchlan Township, passing from the northwest to the southeast in the Marsh Creek Watershed.

The drilling fluid typically contains a mixture of bentonite clay and water, and is classified as an industrial waste. The incident left a 15-foot wide by 8-foot deep sinkhole and impacted wetlands, two tributaries to the lake, and the lake itself. Officials closed off 33 acres of the lake from boating and other recreational uses.

“These incidents are yet another instance where Sunoco has blatantly disregarded the citizens and resources of Chester County with careless actions while installing the Mariner East 2 pipeline," DEP Secretary Patrick McDonnell said in a statement. “We will not stand for more of the same. An alternate route must be used.”

DEP spokesperson Virginia Cain said Sunoco has 30 days to file an appeal of the administrative order to reroute the section. If it does, the matter would go before the state Environmental Hearing Board.

Cain said Sunoco had to propose an alternate pipeline route in 2017 after a spill that year. She said Sunoco found at the time that the route was feasible. So, the new order states Sunoco must use that route.

The new route would run for a little over a mile, and be north of where the current pipeline drilling is taking place. The route would cross under the Pennsylvania Turnpike, then run parallel to the highway, cross Little Conestoga Road, then turn south, and cross back again under the turnpike.

It would still cross two waterways and forested wetlands, and would be closer to five homes.

Alex Bomstein, an attorney at Clean Air Council, said Friday afternoon that he had just learned of the DEP order and could not comment on the new route. But he commended the DEP for stopping the drilling at Marsh Creek after the August spill.

“That Sunoco cannot go ahead at this horizontal drilling site is correct and absolutely necessary given that its track record demonstrates that it’s not going to be able to build this safely through Marsh Creek State Park,” he said.

After the most recent spill, the DEP assessed a $355,636 fine against Sunoco for violations related to the construction of the pipeline in eight Pennsylvania counties. That penalty was for violations that took place between August 2018 and April 2019 involving leaks or spills of drilling fluid during horizontal drilling to open a space through which a steel pipeline is installed.

The incidents have led some environmental and residents' groups to call for a halt of the pipeline.

“Sunoco’s negligence has created a series of entirely predictable disasters, the most recent being the massive spill at Marsh Creek Lake," said Food and Water Action organizer Sam Rubin in a statement. “This dangerous, unnecessary pipeline does not need to be rerouted. It must be shut down entirely.”

Ginny Kerslake, with the West Whiteland Residents for Pipeline Safety, agreed.

“This action by the DEP is the least they could do,” Kerslake said. “Energy transfer has shown time and again that it is either unable or disinterested in complying with the permits. It is high time the DEP revoked the permits and put a halt to this ill-conceived project that threatens our families communities and water resources.”

But the Pennsylvania Energy Infrastructure Alliance, which advocates for energy projects, called the DEP’s action a blow that could extend the disruption residents are already experiencing, and also threaten jobs.

"Communities that thought this project was coming to an end now face potentially many more months of disruption, because this action has the potential of dramatically extending the construction life of a pipeline project that was nearly finished,” the group said in a statement.

Staff writer Andrew Maykuth contributed to this article.