Residents of Washington, DC are used to jokes about metaphorical hot air, humidity, and the swampy history of their city. But there’s something they may not know about the District: it’s overrun with methane, which sometimes makes manhole covers explode.
Natural gas is mostly methane, and it is carried through underground pipes to heat buildings and cook food. Those pipes are often old, and this led ecologist and chemical engineer Robert Jackson of Duke University to drive around DC over a period of two months, regularly measuring the air to take methane levels.
He and his research team found methane leaks everywhere, with thousands of places having significantly higher than normal methane concentrations, and some places reaching 50 times normal urban levels (100 ppm vs 2 ppm). A similar study in Boston last year found essentially the same results. In DC, the source wasn’t the swamp on which the city was built — it was fossil fuel. (The methane they measured had more carbon-13 rather than the normal, modern carbon-12.)
There are sources of methane all over the planet: landfills, swamps, rice paddies, gas wells, melting permafrost, and livestock all contribute. Jackson’s research found another major source: aging infrastructure. Methane isn’t immediately physically harmful, though it does lead to ground-level ozone which is known to harm tree growth and reduce lung function
But what about the exploding manholes? Science Now explains:
Even more disturbing than the thousands of large leaks on the street were the levels of methane in manholes. In some, the researchers found levels as high as 100,000 ppm. Natural gas companies typically consider 40000 ppm to be the threshold for a risk of explosion. D.C. manholes have a tendency to blow up — there are an average of 38 “manhole incidents” per year in the district, according to a report by Stone & Webster Consultants in Boston, including one yesterday on 33rd Street in Georgetown that forced the evacuation of a cupcake shop. Although Jackson cannot say for certain that leaking natural gas is the reason for these blasts, the leaks certainly don’t contribute to safety.
What usually happens is a spark from an exposed cable sets off an explosion of pressurized gas — what remains unclear is how much of this gas is methane. DC’s utility, Pepco, investigated a manhole explosion in 2000 and though their press release ruled out natural gas, their report to the DC Public Service Commission indicated otherwise, according to a February 26, 2000 article in the Washington Post.
Pepco issued a report to the D.C. Public Service Commission yesterday that strongly indicated a natural gas leak may have led to the fire and explosions last week that blew three manhole covers in Georgetown, an incident that shut down the 3100 block of M Street NW for 24 hours.
“We may never know what gas caused the fire — whether it’s sewer gas, gas emitted by burning rubber cable or natural gas,” said Nancy Moses, a spokeswoman for Potomac Electric Power Co. But the report said sewer gas “has historically not been a significant problem in [Pepco] manholes” and “the strength and location of the manhole explosions . . . raises questions as to whether the explosion could have been caused solely by gasses [caused] by burning of low voltage cable insulation.”
Tim Sargeant, a spokesman for Washington Gas, said yesterday that the utility was “disappointed that Pepco speculation and finger-pointing would continue. The report is more innuendo than investigation.”
A manhole cover explosion in Indianapolis last December happened as firefighters were responding to the smell of a natural gas leak, and the local natural gas company again denied the possibility gas caused the explosion:
Indianapolis Fire Department Capt. Rita Burris said fire crews had been dispatched to the area on reports of a strong gas odor.
Moments later, the first of five manhole covers exploded at Massachusetts Avenue and New Jersey Street, “with such significant force as to wake the other firefighters still at the station,” Burris said. “Flames were visible from the manhole for some time before they extinguished.”
…”From all indications, the explosion that occurred this morning at the intersection of Massachusetts, New Jersey and Michigan in Indianapolis, does not appear to be natural gas-related,” read a statement from Arish Rountree, corporate affairs coordinator for Citizens Energy Group.
DC and Indianapolis are not alone in experiencing manhole cover explosions. They have happened in cities all over the world, including Boston, Albany, Atlanta, San Francisco, New York City, Scranton, Indianapolis, Rio De Janeiro, Peterborough (UK), and of course Washington DC.
Dr. Jackson, the researcher who mapped the high levels of methane leaking in DC, hopes his research will inspire cities and natural gas companies to fix the leaks. As there are many cities with aging infrastructure that could potentially leak large amounts of methane into the atmosphere, these emissions could represent a large source of greenhouse gas emissions.
Methane is 100 times more efficient at trapping heat in the atmosphere than carbon dioxide over 20 years. NOAA has already confirmed that as much as 9% of the methane produced at natural gas wells leaks into the atmosphere. As the planet warms and permafrost melts, it starts the most dangerous feedback loop in the entire carbon cycle.
The Environmental Defense Fund is in the middle of a two-year effort to analyze methane leakage during the natural gas life cycle — from extraction to consumption. They have already commenced with studies at wellheads and on natural gas vehicles, while an examination of local distribution of natural gas is forthcoming.