SEARSPORT — The Federal Highway Administration has approved the public release of a supplemental environmental impact study for the Sears Island cargo port, clearing the way for a final round of debate and decision-making on the stalled project.

The highway administration is the lead agency for the federal permits for the half-built state-owned project, conceived in the late 1970s, twice backed by Maine voters in the early 1980s, but idled for the past six years through an injunction obtained by the Sierra Club.

The two-volume, 1,000-page document, in the works for four years, describes in exhaustive detail the ecology, geology, hydrology, flora and fauna of the 900-acre island, as well as the social and economic impacts of the proposed container cargo terminal and evaluations of off-island alternatives.

The study includes 12 different proposals for the design of a Sears Island terminal, which will occupy no more than 95 acres, and a conservation plan for the rest of the island.

Notice of the study’s availability will be published in the July 28 federal register. A public hearing will be held Sept. 12 at the Belfast Armory and written comments will be accepted by the Maine Department of Transportation until Sept. 29.

Although by law, the hearing and comment period could take place several weeks earlier, Brian Nutter, project manager for the DOT, said Monday the extra time was added “because of the enormous amount of information here and the great public interest in this. There is so much to absorb we felt it was important to give people more time.”

For those interested in Sears Island, but not 1,000 pages’ worth, the DOT is preparing a 40-page summary of the study, available at no charge. “It will cover every subject in the complete version, just without so much detail,” Nutter said. “It will give an excellent overview of the project and can be used as a guide to the full version.”

The price of the full version has not been determined. After its release, it can be inspected for free at several public libraries or at the DOT office in Augusta, which also has the 14 volumes of supporting documentation.

After the hearing and comment period, the DOT will incorporate its responses into a final version of the document, which then will be submitted to the highway administration and the U.S. Army Corps of Engineers. Acceptance by the two federal agencies and their issuance of the necessary permits to complete construction is not expected before early 1996.

Actual work probably would not recommence until late next year, as the state will have to go back to the voters for another bond issue to complete the project, which has an estimated cost now of at least $50 million. Bonds approved in 1981 and 1983 raised a total of $17.5 million.

Looming over the entire scenario is the Environmental Protection Agency, which has the power to veto any federal permits issued by other agencies and which has long opposed the development of Sears Island, preferring an expansion of the existing cargo facilities at Mack Point across Long Cove on the Searsport mainland.

Sears Island backers say Mack Point is an overcrowded, impractical and unnavigable option that would require more dredging and environmental upheaval than would Sears Island. After experiencing considerable pressure from Maine’s pro-Sears Island congressional delegation, the EPA early this year agreed to make a final determination on the suitability of Mack Point by Aug. 1.

In 1976, state economic and transportation planners picked Searsport to join Eastport and Portland in a three-port strategy to open the world market to Maine products, with Sears Island later chosen as the best deep water available in the midcoast region.

Work on a causeway and on dredging started after the bond issues of 1981 and 1983, but the Sierra Club filed the first two of its several lawsuits in 1984, challenging the causeway permit and protesting the start of the project without a full-blown environmental impact study.