The clock is ticking for legal challenges to Gov. Andrew Cuomo's plans for a $5.9 billion new Tappan Zee Bridge, according to federal officials.

The announcement was published in the Federal Register by the Federal Highway Administration, where notices explained that a statute of limitations on lawsuits began on Oct. 31 and would run out within 150 days, or March 30, 2013.

While the project's most vocal critic has been the environmental group Riverkeeper, the Ossining-based organization said it is inappropriate to address this latest development now.

"We feel that while New York State is recovering from Hurricane Sandy this is not the time to comment on Tappan Zee," Riverkeeper president Paul Gallay wrote to Newsday in an email.

The organization was founded 46 years ago as an advocate for cleaning up the 315-mile Hudson River, the state's most famous waterway. It has grown to a staff of 26, including several full-time attorneys, and legal work by the Pace Environmental Litigation Clinic at Pace Law School in White Plains. Robert F. Kennedy Jr., who helped to establish the Pace clinic, has been Riverkeeper's chief prosecuting attorney since 1984.

On Sept. 25, the organization told Newsday that it would proceed with legal action if Tappan Zee Bridge project officials did not make changes to reduce the environmental hazards construction would impose on river marine life. Riverkeeper has charged that the Cuomo administration's bridge-building plan fails to take into full account the impact that dredging and pile driving would have on two endangered species, the shortnose sturgeon and Atlantic sturgeon.

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Officials with the state Thruway Authority, which is overseeing the new construction project, had no comment on the looming legal deadline.

Before it arrives, other critical deadlines are expected in the coming weeks. The governor is continuing efforts to find federal funding to pay for the massive construction project. State project officials are also expected to choose a designer-builder for the new bridge before the end of the year, with hopes of beginning construction in early 2013.