Sewage overflows are the biggest source of water pollution in New York. During a rainstorm, the city's combined sewer system automatically discharges storm runoff from all of the city’s paved surfaces, mixed with raw sewage, directly into the nearest water body—everywhere from the mighty Hudson River, Jamaica Bay’s wildlife refuge, and Long Island Sound, to long-neglected (but on-the-comeback) places like the Bronx River and Newtown Creek. City-wide, it's a 30-billion-gallon-a-year problem.