If you want to get a sense of how contentious the decision is over whether the  Obama administration is going to block a planned copper and gold mine near Bristol Bay, consider this: the Environmental Protection Agency has just decided to allow the public another month to weigh in on a scientific review of the project they released a year ago.

Most people aren't aware of the fight over Bristol Bay, home to nearly half the world’s sockeye salmon. But it may be one of the most important environmental decisions the president faces in his second term.

Friday was supposed to be the last day the EPA would take comment on its draft final assessment of how a major mine in the area would affect the Nushagak and Kvichak watersheds, which are home to several Alaskan native tribes as well as a valuable commercial and recreational fishery. In 2010 six tribes petitioned the EPA to invoke its rarely-used authority under the Clean Water Act, known as Section 404(c), to block any mining in the area on the grounds it would have an “unacceptable adverse impact” on the region’s waterways, fish or wildlife.

The Pebble Limited Partnership, a joint venture of two mining firms, Northern Dynasty and Anglo American, has launched a major lobbying and public relations campaign aimed at deflecting any possible EPA intervention. On Thursday the group -- which had been pushing for an extension of the public comment period -- released an economic analysis they commissioned from the consulting firm IHS Global Insight estimating the project would generate 2,500 construction jobs during the five-year construction period. The report predicted the companies would spend approximately $1.2 billion per year on direct capital investment and wages during the construction phase, and the mine would ultimately generate between $136 million and $180 million in annual taxes and royalties.

“For perspective, the report indicates Pebble development alone would pay more in annual taxes to the state than the entire fishing industry combined,” said Pebble CEO John Shivley in a statement. “This clearly shows Pebble development could be an important economic driver for Alaska’s future.”

But a coalition of tribal, environmental and fishing groups question that analysis. “Pebble has a well-established track record of understating the costs and risks associated with a giant open-pit mine at Bristol Bay’s headwaters and exaggerating the benefits,” said Tim Bristol, director of Trout Unlimited’s Alaska program, adding that 14,000 jobs depend on a healthy salmon fishery in the region.

Opponents argue that a potential spill from the massive mine, which would rank as North America’s largest if constructed, would jeopardize a pristine ecosystem. In an earlier environmental assessment of the project, the EPA estimated a project on the scale of the Pebble Mine — which could ultimately produce 80 billion pounds of copper, 107 million ounces of gold and 5.6 billion pounds of molybdenum — would likely cause the loss of between 54 and 89 miles of streams and between 4 and nearly 7 square miles of wetlands.

Over the course of the mine’s operation, the draft assessment said, one or more accidents or failures could occur, “potentially resulting in immediate, severe impacts on salmon and detrimental, long-term impacts on salmon habitat.”

The lobbying on this issue is already intense. The Pebble Limited Partnership spent more than $500,000 on lobbying last year, according to federal election records, and has already spent at least $110,000 in 2013. Environmental and tribal groups also have put significant resources into the flight: the Bristol Bay Native Corporation spent $110,000 in 2012 and $20,000 in 2013, according to public records. Last year three environmental groups -- The Nature Conservancy, Natural Resources Defense Council and the League of Conservation Voters -- spent $270,000, $82,000 and $20,000, respectively.

In an e-mail Thursday night, EPA spokeswoman Alisha Johnson said the agency would extend the comment period to June 30. The agency is also getting feedback from 12 peer reviewers.

"An additional 30 days allows the public an opportunity to provide feedback on changes made to the assessment as a result of extensive input received in 2012," she wrote. "This extension is reasonable given the complexity and length of the revised draft assessment."

In the past, tribal groups in the region have sometime quarreled with commercial interests over salmon fishing. But now, according to Peter Andrew Jr., a board member of the Bristol Bay Native Corporation, they’ve formed an alliance.

“If we don’t protect this, we’ll have nothing to fight over in the future,” Andrew said in an interview during a lobbying visit to Washington this spring. “This is the last place on earth like this.”

The EPA is still a long way from making a final decision on whether to block the Pebble Mine: it aims to finalize its watershed assessment by the end of the year, and EPA Region 10 Administrator Dennis McLerran, who is overseeing the review, told reporters in late April the agency “has made no decision about if or how it might use our authorities under the Clean Water Act, or other laws, to protect Bristol Bay.”

Shivley has challenged several aspects of the EPA’s draft scientific assessment, arguing that key pieces of it are based on studies written by mining opponents such as the American advocacy group Earthworks.

One of the pivotal figures in the debate is likely to be Sen. Mark Begich (D-Alaska), who is up for re-election next year and has questioned whether the EPA should step in and issue what he calls “a pre-emptive veto.”

“While I remain opposed to a pre-emptive veto of this or any other project, an open, public process that answers Alaskans’ questions and puts better science on the table is a good thing,” Begich said when the EPA opened the comment period on the assessment in late April. “I hope the completed assessment will answer questions about whether this project can meet the high hurdle of developing a large-scale mine while protecting our renewable resources.”

In other words, stay tuned.

Alice Crites contributed to this article.