Mapping methane plumes on the streets of Boston and San Francisco paints a picture of "clean" streets with few natural gas leaks, and more common "dirty" streets where methane concentrations can be more than 15 times global background levels. The difference is largely down to pipe work. Cast iron pipes, some more than 100 years old, are the oldest and leakiest pipes under US cities. The pipes were originally sealed with jute, which dries out over time allowing gas to escape.
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