Impervious surfaces are mainly artificial structures, such as pavements, rooftops, sidewalks, roads, and parking lots - covered by impenetrable materials such as asphalt, concrete, brick, and stone. Soils compacted by urban development are also highly impervious. They are an environmental concern because, with their construction, a chain of events is initiated that modifies urban air and water resources:

• Impervious surfaces seal the soil surface, eliminating rainwater infiltration and natural groundwater recharge. Stream-flow in dry summers declines, leaving some cities with local water shortages. Stormwater runs directly across the impervious surfaces, raising flood peaks into destructive bursts. Stream channels erode; sediment loads are high. The shifting substrate eliminates aquatic habitats. Oil and heavy metals, that leak and corrode from automobiles, flush into streams without modification. In some cities, the flood waters get into combined sewers, causing them to overflow, flushing their raw sewage into streams.
• Impervious construction materials collect solar heat in their dense mass. When the heat is released, it raises air temperatures, producing urban "heat islands", and increasing energy consumption in buildings. The warm runoff from impervious surfaces reduces dissolved oxygen in stream water, making aquatic life still harder.
• Impervious pavements deprive tree roots of aeration, eliminating the "urban forest" and the canopy shade that would otherwise moderate urban climate. Because impervious surfaces displace living vegetation, they reduce ecological productivity, and interrupt atmospheric carbon cycling.

The total coverage by impervious surfaces in an area, such as a municipality or a watershed is usually expressed as a percentage of the total land area. The coverage increases with rising urbanization. In rural areas, impervious cover may only be one or two percent. In residential areas, coverage increases from about 10% in low-density subdivisions to over 50% in multi-family communities. In industrial and commercial areas, coverage rises over 70%. In regional shopping centers and dense downtown areas, it is over 90%. In the contiguous 48 states of the USA, urban impervious cover adds up to 43,000 square miles (110,000 km²) — an area nearly the size of the State of Ohio. Continuing development adds another quarter of a million acres (1,000 km²) each year. Typically two-thirds of the cover is pavements (streets, sidewalks, parking lots, driveways, etc.), and one-third is building roofs.

Impervious surface coverage can be limited by restricting land use density (such as number of homes per acre in a subdivision), but this approach causes land elsewhere (outside the subdivision) to be developed, to accommodate growing population. Alternatively, urban structures can be built differently to make them function more like naturally pervious soils; examples of such alternative structures are porous pavements and green roofs.

Rainwater from impervious surfaces can be collected in rainwater tanks and used in place of mains water.

Partly in response to recent criticism by local municipalities, a number of concrete manufacturers such as CEMEX and Quikrete have begun producing anti-impervious surfaces which partly mitigate the environmental impact of conventional impervious concrete. These new materials are composed of various combinations of naturally-derived solids including fine to coarse-grained rocks and minerals, organic matter (including living organisms), ice, weathered rock and precipitates, liquids primarily water solutions, and gases.

See also

• Bioswale
• Hardscape
• Hydraulic conductivity
• Hydrophobic soil
• Infiltration (hydrology)
• Permeability (fluid)
• Permeable paving
• Soil crust
• Surface runoff
• Sustainable urban drainage systems

References

Bibliography

• Ferguson, Bruce K., 2005, Porous Pavements, Boca Raton: CRC Press.
• Frazer, Lance, 2005, Paving Paradise: The Peril of Impervious Surfaces, Environmental Health Perspectives, Vol. 113, No. 7, pg. A457-A462.