An impervious surface is a surface that does not allow water to pass through it. Most impervious surfaces are artificial or “man-made”, but some surfaces can become impervious naturally, like a herd path or a walking path from soil compaction.
Rooftops, parking lots, concrete, asphalt, and the list goes on and on, are all impervious surfaces. The city pictured above (courtesy wikipedia) is a huge area where water cannot infiltrate back into the ground. Where lots of impervious surfaces exist, polluted runoff tends to be found in much higher volumes. When it rains, the water runs off the roofs and parking lots picking up gases, oils, trash, etc. and deposits it into storm drains. These storm drains then carry the water to the nearest surface water, usually a river or the ocean.
The above photo (www.stormwaterok.net) shows an outfall of a storm drain loaded with trash and algae from excess nutrients.
Impervious surfaces go against the natural process of things. Rain water in pristine areas, with no urban development, soaks back into the ground where trees and grasses absorb some of it and the rest helps recharge our groundwater system. When that water cannot infiltrate back into the ground, it can cause some major issues in the form of polluted runoff and erosion.
Whenever building new infrastructure, always consider where the rain water is going to go. Will it runoff across the land into a body of water carrying harmful pollutants or will it seep into the ground and be filtered by the sediments before entering our groundwater system. Check out AWWA’s Youth Conservation Corps page for more information on water friendly conservation techniques. Below is a picture of infiltration steps, one of the many techniques you can use to stop erosion and prevent polluted runoff from entering waterbodies.