Permeable pavement is one of those great innovations about which everyone says, “Why didn’t they come up with this sooner?” In parking lots, its purpose is to allow water to infiltrate directly into the ground, instead of running off into the streets, storm drains and eventually into rivers and oceans, polluting them with oil and grease. EPA states that “used oil from one oil change can contaminate 1 million gallons of fresh water.” The main source of this runoff is the incredible amount of impervious surface area we have put on the land. As Joni Mitchell sang in a 1960s environmental lament, “They paved paradise and put up a parking lot.”
One of the main sources of ocean pollution is so-called non-point source (NPS) runoff from streets and parking lots, carrying oil and grease (and other contaminants) from parking lots into coastal waters. The most common NPS pollutants are sediment and nutrients, washing into water bodies from agricultural land, small and medium-sized animal feeding operations, construction sites and other areas of disturbance; other common NPS pollutants include pesticides, pathogens (bacteria and viruses), salts, oil, grease, toxic chemicals and heavy metals.
The trick in permeable paving is to design the parking lot right in the first place to allow for subsurface water infiltration. The Port of Portland installed a 35-acre porous asphalt pavement to allow stormwater to drain through a paved surface and recharge groundwater. Incorporating a series of bioswales (planted ditches) and natural vegetation allowed all of the stormwater from the expansion to infiltrate the ground.
There are good environmental reasons to let rainfall take its natural course and run oª into streams and lakes, but the increase in flooding from urban development and the polluted nature of the runoff make the argument stronger for keeping as much onsite as possible. In many larger metropolitan areas along the coasts, authorities are beginning to require that large developments design their landscaping and parking areas to hold runoff onsite, either in detention/retention ponds, bioswales or similar devices. In these cases, permeable pavement might be an excellent complementary technology that would allow a developer to reduce the size of other drainage elements.
Permeable paving can include a variety of techniques. Porous asphalt contains conventional asphalt with small aggregate omitted from the mixture. Placed under the porous asphalt surface is a base of further single-sized aggregate. Porous concrete, like porous asphalt, can bear frequent traffic and is universally accessible. Single-sized aggregate (usually seen in gravel parking lots) without any binder is the most permeable paving material in existence — and the least expensive. Although it can be used only in very low-traffic settings such as seldom-used parking stalls, its potential cumulative effect is great. Porous turf is sometimes used for occasional parking like that at churches and stadiums. Living turf transpires water, counteracts the heat-island effect with what appears to be a green, open lawn. Open-jointed blocks are concrete or stone units with open, permeable spaces between the units. They give an architectural appearance and can bear surprisingly heavy traffic.