SOMERVILLE IS NOW officially on the hook for contributing $50 million to the MBTA’s $2.3 billion Green Line extension project, a first-of-its kind donation by a municipality for a transit project that the state was required to do as environmental mitigation for the Big Dig. Cambridge is contributing $25 million to the project.

The $50 million authorization was greenlighted by the Somerville aldermen last Thursday and the MBTA approved a memo of understanding with the municipality on Monday. The city plans to borrow the $50 million and pay the money back over 35 years. If the interest rate is 5 percent, city officials say the total cost will be $103 million. At 4 percent, it would be $92 million.

I talked to Somerville Mayor Joseph Curtatone Monday morning about the city’s financial contribution to the Green Line extension project.

COMMONWEALTH: Walk me through where you are going to get the $50 million.

JOSEPH CURTATONE: We’re going to borrow like we would to build a park or a police station or any other capital project. The taxpayers of the city are going to pay. However, we are intending to negotiate with developers that come to the city to pay their fair share. We’re going to work with the developers in the area that will benefit directly from the Green Line extension, such as developers in Union Square. Our goal over time is that we’ll hopefully garner back approximately half of that $50 million from the private sector.

CW: Half of the $50 million or half of the total cost?

CURTATONE: It’s half of the $50 million, but we’ve built in escalators to tap more of the carrying costs. The timeline cost of the borrowing is unmitigated, so we’ll try to mitigate that with developer contributions, with permit fees, and other revenues over time.

CW: Many Somerville residents want the Green Line extension but they aren’t happy the town’s residents have to help foot the bill for a project that the state was required to do. How do you feel about it?

CURTATONE: This is a project we were not only promised but was legally required. Somerville, which has carried the burden of not just the Big Dig but I-93 and all the compounded negative impacts on our health and on our environment, has fought and advocated for this legal requirement. We did not mismanage the project; the Commonwealth did. Certainly we’re going to benefit directly. Our environment will benefit. But the environment in the region and the Commonwealth will also benefit. Transportation equity will be realized by helping some of the most underserved and vulnerable populations in the region. And the economy will benefit. Billions of dollars in net new revenue will come into the Commonwealth’s economy as a result of the Green Line extension and yet we’re going to be paying. This is an unprecedented move. That’s true.

CW: Did state officials put the screws to you to cough up the money?

CURTATONE: I was never eager to pay $50 million. It’s true Somerville and the region will see economic growth, but it’s not like we don’t have other demands. That’s $50 million we can’t use for some other capital projects. We’ve got hundreds of millions of deferred capital liabilities in the city. We have budgetary constraints. I don’t think we had much of a choice. We knew we had to pay something because it was pretty clear from the Fiscal Management and Control Board that this was not going to go forward without our contribution. Then we’d be left with years of litigation even though we felt strongly about our legal position. I made the best recommendation I could. I told the Board of Aldermen I didn’t want to come before them. I hated having to do this, but it was absolutely necessary. The upside was much greater than the liability of the $50 million.

CW: Can the city afford the $50 million?

CURTATONE: We’re not a rich city. More than three quarters of the children in our public schools are on reduced or free lunch. We’re a city that’s trying to stay affordable for everyone. We’re seeing great transformation now, but I think sometimes in the conversation it’s assumed or misrepresented that we’re flush with cash. We’re not flush with cash like Cambridge. Cambridge is basically paying cash. They can write a check. We’ve got to borrow it and we’ve got to shelve capital projects to pay for this.

CW: Are you pressuring T officials to finish the community path, the multiuse path that will run alongside the Green Line extension?

CURTATONE: We’re working with them. There are some harsh realities. T officials have used the term brutal design changes to describe how they had to rescope and value-engineer the project to keep within the projected budget. Remember, this project was mismanaged – hundreds of millions of dollars by the Commonwealth’s own admission. They used the wrong procurement process, which wasn’t even applied correctly. You had four lower level T employees managing 182 consultants and that’s just a recipe for disaster. There was going to be no path, it was pretty clear, after we went through this brutal redesign process. We worked collectively with the Commonwealth and the T to make sure we have a community path. We’ve been advocating to make sure that the next stage of the path from the projected terminus is not precluded so that it eventually can go all the way into North Point in Boston.