Stormwater run-off is a natural part of the hydrological cycle. The hydrologic cycle is the distribution and movement of water between the earth's atmosphere, land, and water bodies. Stormwater run-off includes rainfall, snowmelt, and other forms of precipitation that falls to the earth's surface. When precipitation reaches the earth's surface it can either infiltrate into the natural landscape or run-off. Infiltration and run-off is heavily influenced by land use. Typically, run-off will be less from a forested landscape than that from an urbanized landscape.
Physical, chemical, and biological characteristics of a watershed are generally altered as land disturbance occurs on active construction sites and for post-construction run-off conditions. Urban stormwater run-off quantity and quality are significantly affected as the watershed undergoes development. The hydrology of the land is altered and the generation of pollutants that are unique to the land use becomes a threat to water quality. Land that is developed undergoes a significant change when impervious surfaces replace natural landscapes. The impact of impervious surfaces typically results in increased run-off volumes and pollutant loading.
According to a United States Environmental Protection Agency (U.S. EPA) fact sheet on stormwater, “When it rains or snows, the water that runs off of city streets, parking lots, and construction sites can wash sediment, oil, grease, toxics, pathogens, and other pollutants into nearby storm drains. Once this pollution has entered the sewer system, it is discharged-(usually) untreated-into local streams and waterways. Known as stormwater run-off, this pollution is a leading threat to public health and the environment today.” Phase I, promulgated by U.S. EPA in November 1990, set up the initial, basic stormwater program for states to adopt in the early 1990s. However, regulations, known on the federal level as stormwater Phase II, have been established in Indiana to reduce the impacts of stormwater run-off from construction sites, industrial facilities, municipal, governmental, and institutional sources.