An impervious surface is a surface which cannot be penetrated by water. A classic example of an impervious surface is paving, utilized to make roads and parking lots all over the world. Roofing and other building materials are also classically impervious. Routine human use of land can also create impervious surfaces; for example, dirt paths can develop highly compacted soil which is effectively impervious, and mismanagement of farmlands can also create compacted soil conditions.
Impervious surfaces are closely associated with humans, with the percentage of impervious surface expressed as a percentage of total land mass rising dramatically in more heavily settled areas like cities. In a rural community, the coverage might be lower than 10%, while in some cities, it can approach 90%. The growth of impervious surfaces is a major environmental concern, for a number of reasons.
Under normal conditions, when it rains, the water is absorbed by the soil. From the soil, it slowly trickles into groundwater supplies, recharging them, and it reappears in surrounding rivers, lakes, and streams. While the soil may become swollen and moist, flooding is relatively rare, because the natural environment is designed to absorb water from even heavy storms. An impervious surface, however, does not allow liquid to reach the soil, which means that it stays on the surface of the Earth, and this causes an assortment of problems.
One of the most common issues associated with an impervious surface is flooding. If the water has nowhere to go, water levels can rise radically, even after a small storm. Impervious surfaces also inhibit groundwater recharge, generate a great deal of polluted runoff, and reduce the aeration of the soil. Furthermore, they collect heat, making the surrounding environment much hotter, and they inhibit the growth of trees and plants, which contributes to the development of heat by eliminating shade while also reducing air quality, as trees and plants normally act like giant scrubbers to pull impurities out of the air.
The conditions associated with impervious materials are often likened to those found in a desert. Many environmental agencies have advocated for changes in building policy to address this issue. For example, permeable and semi-permeable paving can be used to allow water to return to the Earth, or floodwaters can be collected in tanks and redispersed in a controlled way. Such measures would benefit the environment in addition to making communities safer by reducing flooding.