"Many expect those changes to lead to increased rainfall in the Northeastern United States, which would make it harder for a city like Philadelphia to deal with the water that surges into its sewers after a heavy storm. More rain could make flash floods more common and increase pollution in the Delaware River, the main source of the city's drinking water.
As in many older cities, Philadelphia's underground sewer pipes collect both waste and rainwater and channel them together to treatment plants. But when it rains hard, facilities can't keep up and have to release untreated sewage into the city's rivers, city Water Commissioner Howard Neukrug told the symposium."
One strategy being used in Philadelphia is to reduce the city's impervious surfaces to better absorb water.