And you probably can't even smell it.

Sewage runoff into our waterways is both an environmental and a political problem when we can see and smell it. If you live in an older U.S. city with a combined sewer system – one where storm water and sewage share the same pipe network, overflowing through the same outlets – you’ve probably had the visceral experience after a big storm of approaching a river with a musty sheen.

We know, though, that city sewage in some form finds its way into our rivers and bays even in good weather (and without as much noxious evidence).

"We know that the sewers leak," says Marion Divers, a Ph.D. candidate in geology and planetary science at the University of Pittsburgh. "But that’s the thing – we really don’t know how much they leak. That was our big unknown."

Divers and coauthors Emily Elliott and Daniel Bain recently published research in the journal Environmental Science & Technology trying to answer this question. They studied water samples from Pittsburgh’s Nine Mile Run, one of two urban streams that still exist within the city limits (before we used such streams to dump our refuse, then piped them up and built over them, most cities were covered in small streams: "If you look at any maps with all the buildings and political boundaries taken off," Divers says, "you can see where the streams should be").

The researchers were particularly looking for a kind of nitrogen that can come from sewer systems, industrial sources, lawn fertilizer or any fossil fuels burned into the atmosphere eventually creating deposits on the landscape (fascinating side note: scientists can estimate runoff from lawn fertilizers by looking at the housing stock and financial stability of neighborhoods).

Until now, there’s been very little information about how much sewer systems in particular contribute to this mix simply through leaky pipes.

"It’s quite a bit of leakage," Divers says. She and her coauthors estimated that 10-20 tons of reactive nitrogen flow into Pittsburgh's Monongahela River from the area around this one two-mile long stream. That would be the equivalent of about 12 percent of the sewage produced by people living there. "That’s just from one tiny little watershed in the city of Pittsburgh," Divers adds. "The bigger unknown here is that it suggests that cities in general can have a significant impact on waterways."

This nitrogen matters because it contributes to the overall poor quality of these habitats for the creatures that live there, as well as to dead zones in downstream waterways, like the Gulf of Mexico or the Chesapeake Bay.

The thought of trying to solve infinite small leaks – let alone the big storm overflows – is a little overwhelming. "Think of digging up every sewer line everywhere," Divers says.

Cities like Pittsburgh have had a hard enough time dealing with overruns from combined sewer systems (for which many of them are currently under consent decrees with the EPA). Back in 2009, the American Society of Civil Engineers gave the country a D-minus for its handling of wastewater, citing aging systems that discharge billions of gallons of untreated wastewater into waterways each year. The latest edition of that report card is expected this week, and it’s hard to imagine that our sewer systems are doing any better four years later, especially as researchers learn that they may be causing even more (less visible) harm than we thought.

"Even fixing the combined sewer system," Divers says, "taking all the combined sewer overflow discharges out of the water here in the city will not solve the problem of sewage getting into our waterways."

Top image of a drain pipe emptying into Jamaica Bay in Brooklyn: Mike Segar/Reuters

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