The U.S. Environmental Protection Agency recently announced that it would start testing a variety of permeable pavements, surrounded by rain gardens, at an Edison, New Jersey, parking lot to see which are the most efficient at keeping pollutants like motor oil and pesticides out of storm drains.
Permeable pavement acts like a water filter, allowing water to soak through the surface into sandy soils beneath, where the water feeds lawns or evaporates back into the air. It helps reduce erosion, as well as the amount of pollution that washes into rivers and streams.
Impermeable surfaces, on the other hand, can act like log flumes, overwhelming storm systems and water-treatment facilities to the point that they overflow, and sometimes even carry raw sewage into waterways. "With asphalt, all you ask is for it to carry a load and shed water," says William F. Hunt III, PhD, associate professor in the biological and agricultural engineering department at North Carolina State University. "That's why they're so efficient, and the reason impermeable pavement was developed and became the standard."
Porous concrete and other surfaces are so efficient at managing runoff that they're used on all the interstates in Georgia and Oregon.