During a heavy storm, sheets of rainwater pour down on roadways, sidewalks and parking lots, scouring up surface debris and pollutants and washing potentially toxic chemicals like gasoline directly into sewers and streams. The U.S. Environmental Protection Agency (EPA) identifies storm water runoff in paved urban areas as a major source of water pollution.
Nature has its own way of filtering out toxins from rainwater. Soil is a magnificent filter for metals and other inorganic materials. As rainwater passes down through soil levels, microorganisms and plant roots absorb excess chemicals [source: ESA]. Knowing this, engineers have created a new type of permeable concrete that allows rainwater to pass right through pavement and let nature do its work.
Permeable or pervious concrete is made with larger grains of rock and sand, leaving between 15 and 35 percent of open space in the pavement [source: EPA]. Slabs of permeable concrete are laid atop gravel or another porous base material that lets rainwater settle to the soil substrate beneath. Permeable concrete is an excellent replacement for asphalt in parking lots. Not only does it significantly decrease runoff, but also the lighter color of concrete reflects sunlight and stays cooler in the summer.