Moving quickly, anti-pipeline activists have appealed the Pennsylvania Department of Environmental Protection's decision Monday to approve the fiercely contested Mariner East 2 pipeline and asked for an immediate halt to the $2.5 billion project.

The Clean Air Council, the Delaware Riverkeeper Network, and the Mountain Watershed Association filed the appeal with the Pennsylvania Environmental Hearing Board, alleging that the DEP's review process for the cross-state pipeline was flawed.

"Bowing to heavy political pressure, the Department of Environmental Protection greenlighted Sunoco's ill-conceived and dangerous construction plans for Mariner East 2 without having all the information it needed to make a reasoned and reasonable decision," the groups said in their 110-page plea for supersedeas, the legal term for suspending the permits.

The groups said they would suffer an "immediate and irreparable injury" if ground were broken on the pipeline and requested a construction halt until their appeal could be adjudicated. The pipeline, which is being built by Sunoco Logistics Partners LP of Newtown Square, would transport natural-gas liquids across Pennsylvania to Sunoco's terminal in Marcus Hook.

The appeals were filed late Monday, only hours after the DEP issued a series of permits allowing Sunoco to encroach on hundreds of waterways and wetlands during construction of the underground pipeline, which is also known as the Pennsylvania Pipeline Project.

The DEP conducted five hearings and spent more than 20,000 hours reviewing the permit applications and responding to 29,000 comments on the project, Acting DEP Secretary Patrick McDonnell said in a statement Monday. Approval was delayed last year after the DEP rejected the application to address "technical deficiencies."

In its appeal, the environmental groups criticized the DEP for failing to reopen the public comment period after Sunoco responded in December to more than 500 pages of technical deficiencies.

Sunoco Logistics, political leaders, and labor unions hailed the project, which they said would create an infrastructure system that is critical to the state's economic future.

The Mariner East 2 pipeline would run 350 miles from shale-gas producers in western Pennsylvania, West Virginia and Ohio to Sunoco's Marcus Hook Industrial Complex, a former refinery site on the Delaware River in Delaware County. Much of the material transported through the pipes will be exported by ships to petrochemical plants in Europe.

The $2.5 billion project was backed by the Corbett and Wolf administrations, along with labor and business interests. But it has aroused opposition from adjacent landowners and environmental groups.

The project's first phase, using an existing underground pipeline repurposed to deliver up to 70,000 barrels of natural-gas liquids a day, is already operating.

Sunoco wants to build one or two new adjacent pipelines as part of the Mariner East 2 project. Altogether, the pipelines could carry up to 675,000 barrels a day.

Construction of the western part of the pipeline, which runs from Houston, Pa., to West Virginia and Ohio, is already under construction after the DEP granted the necessary permits.

The Mariner East project provides a huge outlet for high-energy natural-gas liquids (NGLs) such as propane, ethane and butane that are contained in the rich "wet" gas produced from the Marcellus and Utica shale formations. NGLs are chiefly used as raw material in petrochemicals.

NGLs are a challenge because they burn hotter than methane, the main component of natural gas. If producers are unable to separate NGLs and sell them separately, they must limit production of natural gas to keep the energy levels within the allowable limits for power plants and home furnaces to burn.

So the Mariner East project not only would allow producers to sell the NGLs at higher prices than they could obtain if they were mixed with natural gas, it also frees up other pipeline capacity to allow drillers to produce more natural gas.

Officials in Delaware County also hope the new NGL highway to Marcus Hook will inspire investors to build petrochemical operations along the river there.

"We believe that this project will give Pennsylvania a great competitive advantage and continue to fuel regional revitalization," Delaware County Chamber of Commerce president Trish McFarland said in a statement.