Impervious surfaces are an environmental concern because, with their construction, a chain of events is initiated that modifies urban air and water resources:
The pavement materials seal the soil surface, eliminating rainwater infiltration and natural groundwater recharge. From a recent article in the Seattle Times: "While urban areas cover only 3 percent of the U.S., it is estimated that their runoff is the primary source of pollution in 13 percent of rivers, 18 percent of lakes and 32 percent of estuaries.
Some of these pollutants include excess nutrients from fertilizers; pathogens pet waste; gasoline, motor oil, and heavy metals from vehicles; high sediment loads from stream bed erosion and construction sites, and waste such as cigarette butts, 6-pack holders and plastic bags carried by surges of stormwater. In some cities, the flood waters get into combined sewers, causing them to overflow, flushing their raw sewage into streams. Polluted runoff can have many negative effects on fish, animals, plants and people.