The hard surfaces that comprise the majority of our driveways, sidewalks, and parking lots are impervious...they do not allow water to soak into the ground. Instead, stormwater will run off the surface and mobilize any pollutants that were there. Opting for pervious surfaces and using non-coal tar based products to seal parking lots and driveways can make a big difference in the quality and quantity of stormwater runoff.
In this section:
When rainfall hits hard surfaces such as conventional concrete and asphalt, the water runs off, collecting pollutants along the way and ends up in storm drains and waterways. Pervious pavements allow water to pass through the surface and infiltrate into the soil below rather than running off impervious surfaces and into surface water.
Benefits of using pervious pavement include:
- Increased infiltration and groundwater recharge
- Reduced runoff rates
- Decreased pollution
- Decreased need for sand and salt during winter months, as moisture seeps into the surface and ice formation is reduced.
Types of pervious pavement:
- Pervious asphalt consists of small and large stones bound together by an asphalt binder. This creates large spaces in the rocks, allowing water to move downward.
- Pervious concrete is a mixture of Portland cement, fly ash, washed gravel, and water. Unlike conventional concrete, pervious concrete usually contains 15 to 25 percent air space, which is achieved by the addition of a fine, washed gravel.
- Pervious interlocking concrete pavers form patterns that create openings through which rainfall can infiltrate. These openings are typically filled with pea gravel.
- Plastic grid systems, sometimes referred to as geocells, consist of flexible plastic interlocking units that allow for infiltration through large gaps filled with gravel or topsoil planted with turf grass.
Permeable pavement maintenance should include an annual vacuum sweeping. Additional vacuuming may be necessary if sediment is visibly accumulating and clogging the pores of the surface. Some restriction on the use of sand or anti-skid material might be needed if repeated use shows an accumulation is problematic.
Polycyclic aromatic hydrocarbons (PAHs) occur naturally in coal and gasoline and are present in products such as asphalt sealants. Some PAHs are listed as possible or probable human carcinogens and also cause a wide variety of problems in aquatic environments. In urban areas like Rochester, a common source of PAHs is from driveways and roads coated in coal-tar sealant. When PAHs degrade, they are transported from our driveways and roads into the environment by stormwater runoff. What can you do? Unsealed driveways and parking lots are the most water friendly option. Read labels and avoid coal-tar sealants as they release 65 times more PAH into the environment than uncoated parking lots. If you must seal-coat a surface, choose a product that is asphalt based.
More information on PAHs:
- USGS fact sheet on PAH: Parking Lot Sealcoat: A Major Source of Polycyclic Aromatic Hydrocarbons (PAHs) in Urban and Suburban Environments
- The Minnesota Pollution Control Agency: Seal Coating Without a Nasty Chemical