The Pennsylvania Department of Environmental Protection has assessed a $355,636 fine against Sunoco Pipeline LP for violations related to the construction of the Mariner East 2 pipeline in eight Pennsylvania counties, the latest in a series of penalties connected to the contentious fossil fuel project.

The penalty was for violations that took place between August 2018 and April 2019 involving leaks or spills of drilling fluid comprised of bentonite clay and water during horizontal drilling to open a space through which a steel pipeline is installed.

Such leaks or “inadvertent returns” have become almost routine for Sunoco Pipeline, a subsidiary of Energy Transfer LP. Sunoco is building a system of three Mariner East pipelines across Pennsylvania to carry gas liquids such as propane from the Marcellus Shale region in western Pennsylvania to an export terminal in Marcus Hook. Just this month, DEP began investigating three similar leaks along the pipeline route in Chester County, including one that polluted Marsh Creek Lake.

The state on Thursday issued formal Notices of Violation against Sunoco for the August incidents, bringing the total number of formal violations for Mariner East 2 to 112 in the last four years, including 13 in 2020. The most serious violations resulted in a $12.6 million fine in 2018.

The $5.1 billion project is a key link in the state’s effort to promote Pennsylvania shale-gas development, and has been strongly supported by organized labor and the energy industry. But its construction has provoked sharp opposition from residents and aroused fears about pipelines transporting highly volatile liquid fuel near homes, schools, and nursing homes.

The Mariner East project, which initially had a completion date of 2019 and now is expected to be finished in 2021, has been delayed in part because of legal challenges of its construction permits and its public utility status. Also responsible for delays are complications that have arisen as Sunoco threaded the pipelines through densely populated Delaware and Chester Counties.

The project and Sunoco’s conduct have become targets of investigations by local prosecutors in Delaware and Chester Counties, and also Attorney General Josh Shapiro. Last year it was reported that the FBI is investigating how Gov. Tom Wolf’s administration issued construction permits for the project.

Sunoco announced the Mariner East project about 10 years ago, initially to repurpose an old fuel pipeline built in 1931 to carry highly volatile gas liquids to its terminal in Marcus Hook, where much of the product is loaded aboard ships and exported to Europe. The gas liquids -- propane, ethane, and butane -- are primarily used as raw material in plastics production.

Eventually Sunoco added two adjacent pipelines to its project, which altogether could carry up to 675,000 barrels a day. The two additional pipelines required Sunoco to acquire new rights of way, sometimes in neighborhoods with little spare room, and also conduct more obtrusive construction that either involved open trenches or horizontal drilling in stretches sometimes extending thousands of feet.

The latest fine relates to horizontal drilling leaks that affected Piney Creek in Blair County; tributaries and wetlands connected to Hinckston Run, Stewart Run, and Little Conemaugh Creek in Cambria County; Letort Run and wetlands and tributaries to the Yellow Breeches Creek in Cumberland County; a tributary to Chester Creek in Delaware County; Snitz Creek in Lebanon County; a tributary to Peters Creek in Washington County; and a tributary to the Conemaugh River in Westmoreland County.

“Protecting the waters of the Commonwealth is one of the top priorities of DEP and we will continue to hold polluters of those waters accountable,” DEP secretary Patrick McDonnell said in a statement.

A full list of the state agency’s actions can be found on DEP’s Mariner East 2 webpage.