The city's Big Dig highway tunnels have about 500 leaks awaiting repair, and that number doesn't include leaks being handled by the project's contractors, according to state officials who warn that future leaks are inevitable. Leaks have vexed the $14.8 billion Big Dig, the most expensive public works construction in U.S. history, ever since it buried Boston's Central Artery under the city. One large leak into the Interstate 93 Thomas P. "Tip" O'Neill Jr. tunnel in 2004 backed up traffic for miles. Project manager Michael Lewis on Tuesday successfully lobbied the Massachusetts Turnpike Authority board to add another $2.3 million to an existing $5 million contract for repair crews.

Lewis maintained that the leak-repair program is "effective," and he said it should be viewed separately from new evidence indicating water continues to leak steadily into O'Neill tunnel. Inspectors found 2,000 to 3,000 leaks in roof-wall joints after the September 2004 incident, he said.

Unlike that 2004 leak, motorists likely would never notice the current leaks, which drain into holding areas below the surface, to pumps that discharge the water. The water volume from the roof-wall joints is "almost immeasurable," he said.

Lewis said the larger contributions of leaking water come from utility conduits.

"Those are somewhat difficult to identify, to find, but we are continuing to locate them and chase the water paths," he said.

The repairs to the existing leaks likely will continue until spring, Lewis said.

The board did not address an independent engineering firm's analysis that water continues to leak steadily into the O'Neill tunnel, beneath downtown Boston. The analysis contradicted Lewis' report to the board last month that said water discharge from the O'Neill tunnel had decreased.

The engineering firm of Wiss, Janney, Elstner Associates Inc., at Cohen's direction, found that 8 million gallons of water were discharged from the O'Neill tunnel pump stations in the first half of 2005, compared with 6.5 million gallons discharged in the first half of 2006 and 7.5 million gallons in the first half of this year.

Also Tuesday, Gov. Deval Patrick gained control of the MTA board when he appointed MetLife Inc. executive Thomas Stephens to fill a vacated seat. The appointment gives the Democratic governor three votes on the five-member board, which oversees the Big Dig and the state's turnpike system.