Turnpike officials have said taxpayers won't be saddled with the cost of fixing leaks in the $14.6 billion Big Dig (search) highway project — but a top federal watchdog isn't so sure.

Kenneth Mead, inspector general for the U.S. Department of Transportation, said the Massachusetts Turnpike Authority (search) faces huge hurdles in its effort to force contractors to pick up the tab for the leaks.

"While the authority has said its contractors, and not the taxpayers, will pay to fix all the leaks, we are not entirely confident of this," Mead told the House Government Reform Committee (search).

The turnpike authority oversees the Big Dig project, which has been plagued for years by delays and cost overruns, and in recent months by hundreds of leaks in the tunnels under Boston.

Mead said the biggest burden will be trying to assign blame for the leaks.

"The primary risk to the taxpayers involves establishing the responsible party," he said. "The problems could be caused by poor construction, design errors, poor oversight, or a combination of those factors."

Protecting taxpayers is all the more difficult, he said, because some contractors have already been paid in full.

Attorney General Thomas Reilly told the panel that if contractors try to duck their obligation to pay for leak repairs, he will take them to court.

"Cost recovery is going to go one of two ways. It is either going to be a negotiated settlement or it's going to litigation," said Reilly, whose office oversees Big Dig cost recovery efforts.

Turnpike Chairman Matthew Amorello said contractors responsible for the leaks will pay for repairs.