As land is urbanized with growth, the natural landscape is changing. Buildings, roads, housing additions, parking lots (impervious surfaces) and so on are replacing, forests, farm fields and grassy areas (natural areas). The natural areas play a part in the water cycle as it allows the water to seep into the ground (infiltration). With less natural areas, the water has fewer places to seep in. This creates stormwater runoff and flows over the impervious surfaces until it goes through the storm drain system.
An impervious surface is any surface that is a hardened or paved surface that do not allow water to pass through. Examples of impervious surfaces are: roads, rooftops, sidewalks, pools, patios, parking lots, basically anything that does not let water pass through.
With watersheds becoming more urbanized and vegetation is being reduced and impervious surfaces water is not able to infiltrate the ground. This causing more stormwater runoff. The stormwater runoff is then collected in the drainage systems that flow directly into local streams. With stormwater not seeping into the ground, this increases the possibility of flooding and erosion. According to the Chesapeake Bay Program, the impervious surfaces increase the amount and speed of the stormwater runoff and alter the natural stream flow & pollute aquatic habitats.
Pollution from the stormwater runoff is increased as it flows over the impervious surfaces. Pollutants can be leaked fluids from vehicles, litter, animal waste, yard debris, amongst other things. It is very important for the stormwater to remain as free from pollutants as possible. The stormwater flows into the storm drain system until it reaches a local body of water. Stormwater is not treated before it flows into the local waters.