The U.S. natural gas boom is still going strong, thanks largely to fracking of the Marcellus Shale below Pennsylvania and West Virginia. Gas now represents nearly a third of all energy produced in the U.S., and it's expected to reach 40 percent in the 2040s.

Much of this gas moves around the country in a patchwork of pipelines totaling 2.47 million miles, or about 99 times the circumference of the Earth. Most are hidden underground, and while their safety record has improved in recent decades, lots of aging pipelines still secretly leak gas. This not only wastes money and pollutes the air with methane, but also raises the risk of dangerous ruptures and explosions. On average, troubled U.S. gas pipelines cause 17 deaths, 68 injuries and $133 million in property damage every year.

A new study spotlights this threat in the nation's capital, where researchers drove around last year in a car equipped with a spectrometer to detect methane and ethane. They found more than 5,800 pipeline leaks while covering all 1,500 miles of road in Washington, D.C., and occasionally lowered probes through manholes to investigate high readings.

There's no evidence any of the leaks posed a direct danger to public health or safety, but a dozen of the manhole tests did reveal methane levels as high as 500,000 parts per million of natural gas — roughly 10 times the concentration needed to fuel an explosion.

The researchers reported the worst leaks to Washington Gas, the local utility, after their initial tests last February. But upon returning four months later for follow-up tests, they were "surprised" to find nine of the 12 leaks still emitting unsafe levels of methane, according to lead author and Duke University environmental scientist Robert Jackson. (Washington Gas is in the midst of a $28 million, seven-year program to upgrade its pipeline connections and replace cast-iron components with more durable plastic.)

"Repairing these leaks will improve air quality, increase consumer health and safety, and save money," Jackson says in a statement about the study, which was published Jan. 16 in the journal Environmental Science & Technology. "Pipeline safety has been improving over the last two decades. Now is the time to make it even better."

The researchers reported similar issues last year in Boston, including more than 3,300 pipeline leaks below 785 miles of road. "The average density of leaks we mapped in the two cities is comparable, but average methane concentrations are higher in Washington," says Boston University professor Nathan Phillips, who worked on both studies.

Washington and Boston are hardly alone in their gas pains. While 95 percent of U.S. gas lines are low-pressure plastic pipes that handle local distribution — thus posing a lower explosion risk — large, metal transmission lines are a different story. Many U.S. cities still rely on decades-old gas infrastructure that may be prone to leaks or ruptures, especially if it's made from cast iron or predates the use of cathodic protection to fight corrosion. The gas pipeline that rocked a California neighborhood with a deadly fireball in 2010, for example, was more than 50 years old and had suffered lapses of inspection.

San Bruno pipeline explosion

Damage from a deadly 2010 gas main blast in San Bruno, Calif. (Photo: Justin Sullivan/Getty Images)

Such explosions are rare, however, and most leaked gas from pipelines disperses into the air before reaching dangerous levels. But that's still bad, since methane is a potent greenhouse gas, capable of trapping more heat than carbon dioxide, and can also react with sunlight and other gases to form toxic ground-level ozone. Pipeline leaks represent the largest manmade source of methane emissions in the U.S., and contribute to some $3 billion in lost natural gas each year. Up to 38,000 liters of methane wafted up daily from just four of the street-level leaks in Washington, the study's authors note, which is comparable to the amount of natural gas used by two to seven households.

There is some momentum in Congress to speed up the modernization of U.S. gas lines, including two bills introduced in November that would create federal incentives to soften the expense of repairing and replacing outdated pipelines. "We need to put the right financial incentives in place," Jackson says. "Companies and public utility commissions need help to fix leaks and replace old cast iron pipes more quickly."

In the meantime, you can learn more about pipelines in your state from data offered by the U.S. Pipeline and Hazardous Materials Safety Administration. And although local distribution lines are less likely to corrode and explode, plastic is still vulnerable to shovels, so it's wise to check pipeline locations by calling the national 811 hotline before you dig.

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Russell McLendon ( @russmclendon ) writes about humans and other wildlife.

Old, leaky gas lines lurk below U.S. cities
Researchers have found 10,000 natural gas leaks under just two U.S. cities, hinting at a broader problem.