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In Puget Sound, one of America’s great estuaries, killer whales, seals, and schools of salmon swim not far from more than 3 million people who live in the Seattle region. The presence of such impressive marine life, however, belies the fact that the sound is seriously polluted.

When it rains, storm water washes into the same system of underground pipes that carries the region’s sewage, and 1 billion gallons a year overflow into the sound when area sewer systems contain more water than can be treated. In addition, motor oil, lawn chemicals, PCBs, heavy metals, pet waste, and many other substances run unabated into the sound, both through the storm water pipes and from roads and other shoreline structures. “The biggest threat to Puget Sound is non-point sources [of pollution],” says Nancy Ahern, Seattle Public Utilities deputy director.

Blowhole samples taken from killer whales have revealed fungi, viruses and bacteria living in their respiratory tracts, some of them antibiotic-resistant and once found only on land. Health officials often have to shut down oyster beds because of fecal contamination. Salmon in streams are killed by torrents of dirty storm water.

To lessen this deluge of diffuse pollution – a problem faced by many regions worldwide – Seattle is looking not at new and expensive sewage treatment infrastructure. Instead it is embracing an innovative solution to storm water runoff called green infrastructure, which experts increasingly say is not only the most cost-effective way to deal with such a large-scale problem, but also offers a range of other benefits. A growing number of places, from New York City to Sweden, are investing in everything from rooftop gardens to pollution-filtering assemblages of trees to reduce tainted runoff.