Subterranean Rumblings

Especially in the winter months, first time visitors to New York City are often bemused by the sight of plumes of steam rising from manholes in the street (sometimes surmounted by jolly red-and-white “smokestacks”).  Where do they come from?  Are they byproducts of some subterranean “dark satanic mills”, or perhaps a covert entrance to Saruman’s workshops at Isengard?   Actually, they come from the Con Edison steam distribution system,which distributes steam from seven generating plants through underground pipes to most of Manhattan south of about 90th Street.  [Coverage map PDF]  The steam is used to provide heat, hot water, and other services; it serves around 100,000 commercial and residential establishments with more than 13.5 million tons of steam every year.  Since about 50% of the steam is produced as a by-product of electricity generation, the system as a whole is more efficient than individual building heating and hot water plants.  The system, the first parts of which went into service in 1882, works fairly well, though there have been some spectacular failures in the aging infrastructure, notably the 2007 explosion at 41st Street and Lexington Avenue, which left a crater in the street 35 feet wide and 15 feet deep.

Here in Washington DC, we have our own peculiar sub-surface activity: our manholes do not spout steam, but do occasionally explode.  Natural gas has been suspected as a cause of these explosions, but hard evidence for this has been scarce.  Now, the ScienceNOW news service of the AAAS reports that researchers have found high methane concentrations in the city, and very high concentrations in manholes.

Researchers who mapped methane concentrations on the streets of the nation’s capital found natural gas leaks everywhere, at concentrations of up to 50 times the normal background levels, they reported here last week at a meeting of the American Physical Society. The leaking gas wastes resources, enhances ozone production, and exacerbates global warming—not to mention powering the city’s infamous exploding manholes.

Methane [CH4] can come from a variety of sources, including landfills, swamps, cattle flatulence, and oil/gas production.  It is also the principal constituent of natural gas.  It is potentially an important source of global warming, since it is more than 20 times more potent as a greenhouse gas than carbon dioxide [CO2], the usual suspect.  The research team, headed by Robert Jackson of Duke University, suspected that natural gas leaks might be contributing to increased methane concentrations in the atmosphere; the results confirmed their suspicions:

… they drove along every street in the District of Columbia and regularly sampled the air, mapping the concentration of methane over a period of 2 months. They found thousands of places with air concentrations significantly above the 2 parts per million background level typically found in cities, with some areas as high as 100 ppm.

Methane itself is non-toxic and odorless (the gas company adds a chemical, typically a thiol like tert-butyl mercaptan, to make the gas stink); however, methane can promote the formation of ozone, which is a respiratory irritant.

The research team got more striking results when they sampled methane concentration in manholes.  In some locations, they detected methane concentrations of 100,000 ppm (or 10%).  My well-worn Handbook of Chemistry and Physics gives (approximate) explosive limits for methane / air mixtures at 5% and 15%, so the detected values are well within the danger zone.  The team suspects corroding iron gas distribution mains as the source of the leaks.  There are an average of 38 “manhole incidents” per year in Washington, so something is clearly amiss. (The same researchers have some similar data from Boston.)

It is of course sort of interesting to discover what may be causing incidents like exploding steam pipes or manholes, but these incidents reflect a larger issue.  There is a great deal of very old infrastructure in the US. much of which has not received very much in the way of maintenance.  (As another, unrelated, example, in August 2007, a highway bridge carrying Interstate 35-W across the Mississippi River in Minneapolis collapsed.)  For years, the American Society of Civil Engineers, has issued an annual report card on the parlous state of the nation’s infrastructure; it does not make for encouraging reading.

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