FERC has defended its decision, arguing that it satisfied its National Environmental Policy Act obligations in reviewing the project and that its challengers are essentially asking the Second Circuit to “conduct a de novo review of project impacts.”

But the green groups shot back on Friday, rejecting FERC’s assertion that it had adequately considered the project’s indirect impacts on increased gas drilling and its cumulative effects on greenhouse gas emissions and climate change.

“Contrary to the commission’s assertions, there is no record evidence demonstrating that the project’s suppliers can fill the pipeline over its lifetime without drilling additional wells,” the groups said in a reply brief, arguing that the project would likely spur additional gas development.

In fact, FERC did an “unexplained about-face” when claiming that evaluating those impacts would be too speculative, despite the fact that information about the impact of natural gas production is “widely available,” the groups said.

“The commission’s analysis of the project’s climate change impacts fares no better under NEPA,” the groups continued. “FERC arbitrarily ignored significant volumes of the project’s emissions by inexplicably limiting its analysis to a single year of operations.”

Additionally, FERC rejected best available science and tools when gauging the consequences the project would have on climate change, the groups said, but it is not allowed to “eschew tools and methodologies that other expert agencies use and recommend.”

Further, FERC flouted the Clean Water Act by signing off on the project before the New York DEC considered it, and its failure to allow the state to act beforehand caused the “condemnation of property, irreversible removal of trees and contamination of water bodies,” the reply brief said.

As such, the groups, which also include Catskill Mountainkeeper Inc. and Delaware-Otsego Audubon Society Inc., pressed the appellate court to vacate FERC’s decision, pause construction of the project and remand the proceeding. Their appeal has been consolidated with another one challenging FERC’s approval, by Stop the Pipeline.

Meanwhile, in a parallel but separate appeal, Constitution has claimed that the New York DEC improperly leveraged a narrow permitting authority under the CWA to effectively veto FERC's approval for the project by refusing to issue a water quality permit.

However, the DEC has said its denial of the water quality permit falls within its discretion under the CWA and Natural Gas Act to protect the state's waters from pollution, and is well-supported by its administrative record.

The disputed pipeline would transport 650,000 dekatherms of gas daily from existing supplies in Susquehanna County, Pennsylvania, to an interconnection with the Iroquois Gas Transmission pipeline in Schoharie County, New York.

Representatives for FERC declined to comment Monday.

The environmental groups are represented by Moneen Nasmith and Deborah Goldberg of Earthjustice.

Stop the Pipeline is represented by Karl S. Coplan, Todd D. Ommen and Anne Marie Garti of Pace Environmental Litigation Clinic Inc.

FERC is represented by General Counsel Max Minzner, Solicitor Robert H. Solomon and senior attorney Holly E. Cafer.

Constitution Pipeline, which has intervened on behalf of FERC, is represented by Pamela S. Goodwin, John F. Stoviak and Elizabeth U. Witmer of Saul Ewing LLP.

Iroquois Pipeline, which has intervened on behalf of FERC, is represented by M. Lisanne Crowley and Kurt H. Jacobs of Troutman Sanders LLP.

The Natural Gas Supply Association, which has intervened on behalf of FERC, is represented by John Longstreth of K&L Gates LLP.

The consolidated cases are Catskill Mountainkeeper Inc. et al. v. Federal Energy Regulatory Commission and Stop the Pipeline v. Federal Energy Regulatory Commission, case numbers 16-345 and 16-361, respectively, in the U.S. Court of Appeals for the Second Circuit.

-- Additional reporting by Keith Goldberg and Stan Parker.  Editing by Aaron Pelc.