AUGUSTA — The Senate voted Thursday to require an independent study of how Central Maine Power’s proposal to build a high-voltage transmission line through western Maine would impact regional greenhouse gas emissions.

CMP is seeking regulatory approval for a 145-mile transmission line through western Maine that would carry electricity from Hydro-Quebec to Massachusetts. Among the many issues raised by opponents, some question whether the overall project will achieve the environmental benefits touted by the companies as they seek to supply Massachusetts with renewable energy from Hydro-Quebec’s massive network of dams.

The Senate voted 30-4 on Thursday to approve a bill that would require the Maine Department of Environmental Protection to commission an independent, third-party study of “total greenhouse gas emissions across all jurisdictions interconnected with electricity generation and distribution systems operated by the Hydro-Quebec company.”

The study would need to be completed by Aug. 15, well before the DEP expects to make a determination on CMP’s application for a permit for the New England Clean Energy Connect project. The bill would allow the DEP to accept contributions “from any source, public or private, to fund the costs of the study,” a provision aimed at avoiding a lengthy delay to secure state funds but that opponents said could open up the study to undue influence by groups with strong interests in the project.

An outside group examined greenhouse gas emissions impacts as part of a study for the Maine Public Utilities Commission process, estimating a 3 million metric ton reduction of carbon dioxide emissions per year. But that study was limited to New England.

Sen. Brownie Carson, a Harpswell Democrat who sponsored the bill, said many people “believe that the project must be questioned” as the state seeks to balance the overall costs and benefits of the proposal.

“We need to understand the full picture, not just the limited view of New England,” Carson said during a Senate floor speech. “As policymakers, we owe it to our constituents — the people of Maine — to provide this level of due diligence because we are being asked by Massachusetts and the developers of this project to accept a lot of impacts in Maine on areas that are special to Maine people.”

Critics of the bill, L.D. 640, regarded it as another attempt to slow or halt the project. The Maine PUC granted the project a key certificate last month after determining that the economic and grid reliability aspects of the transmission line outweighed the harder-to-gauge impacts on scenery and outdoor recreation in the western Maine mountains. The Maine DEP and the Land Use Planning Commission, meanwhile, were holding the final day of hearings on their respective permits Thursday in Bangor.

Sen. Nate Libby, D-Lewiston, pointed out that the project carries significant financial benefits to towns along the corridor route, including an estimated $3 million to $5 million in property tax revenue to his district. But while Libby said there are legitimate questions being raised about CMP’s project, he also questioned whether the DEP has the policy expertise needed to manage such a study.

“A lot is riding on this study. The study has to be done right,” said Libby, the Senate majority leader. “I don’t want us to get to a place in the fall where we question some of the assumptions that were baked into the model, where we question where the money came from and whether that produced some undue influence or conflicts of interest. I am concerned that the DEP is being asked to manage this project when they don’t work in this field of energy and utility law.”

The bill now goes to the House for consideration, potentially next week. Even if passed by that chamber, however, it still faces a potential challenge getting by Gov. Janet Mills. A supporter of the CMP proposal, Mills said last month that she believes the greenhouse gas analysis conducted as part of the Public Utilities Commission process was “sufficient.”

Nonetheless, project opponents celebrated their initial victory in the Senate on Thursday.

“The majority of Maine’s citizens oppose the CMP corridor and we are being bombarded with CMP’s propaganda with unsupported claims about it’s true impact on the environment, wildlife habitat, waterways, and climate impacts,” Sandi Howard, director of the organization Say NO to NECEC, said in a statement. “We are encouraged that our concerns are being heard and acted upon by representatives in Augusta. It’s clear that bipartisan legislators share our concerns, and they, too, want an independent climate analysis conducted at the DEP.”

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