BOSTON (AP) – Nearly two million gallons of water have been leaking into the Big Dig’s Thomas P. O’Neill tunnel each month this year – an 18 percent jump over last year.

The increase comes three years after state managers pledged to plug thousands of small leaks in the nearly $14.798 billion Big Dig, the costliest public works project in US. history.

The leaks could also translate into heavier maintenance costs for the tunnel in the future.

“It’s no secret that leaks are an issue we will have to deal with for the foreseeable future,” said Jon Carlisle, spokesman for the Massachusetts Turnpike Authority, which oversees the downtown tunnel system told the Boston Sunday Globe. “It’s a problem that we will need to continue to monitor and remediate.”

Carlisle said the authority spends about $5 million a year to close the leaks by injecting grout into fissures in the concrete.

He said the actual number of leaks has dropped to 800 from the more than 3,500 acknowledged by tunnel manager in 2004. Part of the problem, he said is that new leaks open up where patches had been applied earlier.

Records from the Massachusetts Water Resources Authority showed the amount of water pumped out of the tunnel increased to 1.9 million gallons per month for the first three months of the year, compared to 1.6 million for the same period last year.

The leaks are a cause for concern because water can weaken concrete and deteriorate steel over time, leading to more frequent inspections and repairs than originally anticipated.

The leaks have been a problem ever since the roof of the tunnel was attached well before it opened to traffic in 2003. The bulk of the leaks are between the wall and roof where construction workers had trouble creating a waterproof seal in the mile and a half long tunnel.

In November 2004, the project’s overall design and construction management consultant Bechtel/Parsons Brinckerhoff said in a written statement that “the program to seal leaks will be completed within months, not years.”