A scientist in Boston has been driving around the city measuring leaks in gas mains, and he's found a lot. Gas leaks are not uncommon, and gas companies spend a lot of time tracking them down and repairing them. But the scientific team says they are surprised at how many they've found, and what those leaks are doing to the health of the city's trees. NPR's Christopher Joyce has more.

CHRISTOPHER JOYCE, BYLINE: Biologist Nathan Phillips at Boston University was intrigued with what methane from underground gas mains might be doing to trees. On a stroll through Boston, he met a former gas line inspector who said Boston's gas system was, in fact, leaky. So Phillips obtained a methane detector, called a cavity ringdown spectrometer, and put it in a car.

NATHAN PHILLIPS: We just measure while we drive. It's a very fast-acting piece of equipment.

JOYCE: Together the team drove 785 miles of Boston and suburban roads. They found about 4,000 significant leaks.

PHILLIPS: Like many people, I really didn't know the scope of the problem, so I was very surprised.

JOYCE: In some cases, the levels were high.

PHILLIPS: The record level that we found for a leak, this is in the atmosphere that we breathe on the surface in Boston, was about 30 parts per million of methane, and that's over 15 times the normal background level.

JOYCE: Phillips notes that he's not a health expert and says he has no reason to believe these levels pose a risk to human health. But he does believe, as a plant physiologist, that the methane is probably harming trees.

PHILLIPS: Natural gas is largely methane. That displaces the oxygen. It's also dry gas, so it desiccates the soils as well. And roots need to have oxygen for the metabolism of the roots, for repair of the root membranes. If they're starved of oxygen, the tree will suffer.

JOYCE: A state advocacy group is suing utilities in the region for damages to trees and citing Phillips' research. The plaintiffs are communities that claim millions of dollars of damage has been done to trees in the Boston area.

Tom Kiley, head of the Northeast Gas Association, says it's true that methane can damage vegetation, but it's not common.

TOM KILEY: There certainly are a lot of potential causes to the damage to trees and vegetation. That can include insect infestation, vehicular damage, disease, storm damage, drought, salt, waste oil, gasoline.

JOYCE: The association represents gas companies in the region. Kiley says when there's damage, gas companies remove and replace the trees. That's confirmed by a spokesman for National Grid, the major supplier of natural gas to the Boston area.

As for the leaks, Kiley says gas companies do report how much they lose statewide to the state Department of Public Utilities. He doesn't know how many leaks there are in Boston but says leaks are a problem that the companies take seriously.

KILEY: It's an older system. It is being replaced. There are some cast-iron facilities and some bare steel - or unprotected steel – that are being replaced, so there are in fact leaks on the joints.

JOYCE: For his part, Nathan Phillips says he recognizes that the gas company is working to contain leaks. His aim, he says, is to make the information about the leaks' whereabouts and frequency easily available to the public.

PHILLIPS: The buried infrastructure, when it's out of sight, it's out of mind. And it's easy for us to just forget that it even exists.

JOYCE: Phillips notes methane is a very powerful greenhouse gas that warms the planet. And in fact, scientists at the company that makes the methane sniffer, Picarro in California, have begun mapping leaks in San Francisco, too.

Christopher Joyce, NPR News.


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