It is estimated that when rain falls on natural landscapes like forested areas, approximately half of the rainwater soaks into the ground, another 40% is lost into the atmosphere through evapotranspiration, and another 10% moves off-site through stormwater runoff. However, with urban development, increased impervious surfaces prevent much of the stormwater from naturally soaking into the ground. Instead, much larger volumes of runoff flow from rooftops, over paved areas, on saturated or compacted soil and across sloped lawns.
One impact of stormwater runoff is the collection and transport of soil, pet waste, pesticides, fertilizer, oil and grease, leaves, litter and other potential pollutants that are transported through storm drain systems. In a city, stormwater runoff entering a storm drain is discharged, untreated, into the waterbodies that our local communities use for swimming, fishing, and providing drinking water.
A second effect is the increased volume and speed that the stormwater generates as it is concentrated through the smooth, straight storm drain pipes. When this stormwater runoff reaches the drain outlet and empties into the receiving stream, the intense volume and power has a tremendous erosive effect on streambanks.
You don’t need a heavy rainstorm to carry pollutants into streams. Your home garden hose can supply enough water if you are washing your car in driveway, hosing stains or spills off the pavement or letting excess irrigation water spray into the street. Even if your home is not on a streambank, storm drains along your street gutter carry stormwater runoff from your neighborhood to the nearest body of water.
Stormwater is unavoidable, but you can minimize its effects by:
1) reducing the potential pollutants you might contribute before they are picked up in runoff
2) enhancing infiltration and reducing stormwater runoff