Impervious Ground Cover

Increased acidity

Nitrogen oxides (NOx) and sulfur dioxide (SO²) are the primary air pollutants that result in acid rain and highly acidic stormwater. Acid rain occurs when SO², emitted primarily by electric utilities fired by coal, and nitrogen oxides (NOx), emitted primarily by transportation sources and utilities, are deposited in the form of wet or dry deposition.

Several aspects of urbanization tend to create local conditions that may make receiving waters susceptible to impacts from acidity. High levels of airborne SO² and NOx in large urbanized areas increase the acidity of the rainfall to levels above those typically found in the region. Runoff from paved surfaces and other impervious surfaces may have little or no opportunity to contact soils that could buffer the acidity of the rainfall. In urbanized areas with acidic rain, higher runoff volumes and rates associated with urban development can increase the acidity of receiving streams rapidly and lead to high peak acidity levels.

Higher water temperature

High volumes of runoff from hot paved surfaces and rooftops may cause a rapid increase in surface water temperatures. Discharges from stormwater management devices, which retain collected runoff in unshaded ponds, also may increase stream temperatures.

Increased temperature can harm fish and other aquatic life. Water holds less oxygen as it becomes warmer, which may affect habitat and make the water more susceptible to oxygen-demanding pollutants. Sustained water temperatures in excess of 70°F are considered stressful or lethal to many cold water fish species and stream insects. The availability of food, attendant life cycle chemistry, and water quality changes are all affected by water temperature.

^{Source: Environmental Protection Agency. “Our Built and Natural Environments: A Technical Review of the Interactions between Land Use, Transportation, and Environmental Quality.” January 2001.}