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Stormwater is a term used to describe water that originates during precipitation events. It may also be used to apply to water that originates with snowmelt or runoff water from overwatering that enters the stormwater system. Stormwater that does not soak into the ground becomes surface runoff, which either flows into surface waterways or is channeled into storm sewers.

Stormwater is of concern for two main issues: one related to the volume and timing of runoff water (flood control and water supplies) and the other related to potential contaminants that the water is carrying, i.e. water pollution.

Because impervious surfaces (parking lots, roads, buildings, compacted soil) do not allow rain to infiltrate into the ground, more runoff is generated than in the undeveloped condition. This additional runoff can erode watercourses (streams and rivers) as well as cause flooding when the stormwater collection system is overwhelmed by the additional flow. Because the water is flushed out of the watershed during the storm event, little infiltrates the soil, replenishes groundwater, or supplies stream baseflow in dry weather.

Pollutants entering surface waters during precipitation events is termed polluted runoff. Daily human activities result in deposition of pollutants on roads, lawns, roofs, farm fields, etc. When it rains or there is irrigation, water runs off and ultimately makes its way to a river, lake, or the ocean. While there is some attenuation of these pollutants before entering the receiving waters, the quantity of human activity results in large enough quantities of pollutants to impair these receiving waters.

In the United States, the Environmental Protection Agency (EPA) is charged with regulating stormwater pursuant to the Clean Water Act (CWA).^[1] The goal of the CWA is to restore all "Waters of the United States" to their "fishable" and "swimmable" conditions. Point source discharges, which originate mostly from municipal wastewater (sewage) and industrial wastewater discharges, have been regulated since enactment of the CWA in 1972. Pollutant loadings from these sources are tightly controlled and limited. However, despite these controls, thousands of water bodies in the U.S. remain classified as "impaired," meaning that they contain pollutants at levels higher than is considered safe by EPA for the intended beneficial use of the water. Much of this impairment is due to polluted runoff.

Under the CWA, point source discharges to "Waters of the United States" require National Pollution Discharge Elimination System (NPDES) permits. To address the nationwide problem of stormwater pollution, in 1987 Congress broadened the CWA definition of "point source" to include industrial stormwater discharges and municipal separate storm sewer systems ("MS4").^[2] These facilities were required to obtain NPDES permits. This 1987 expansion was promulgated in two phases: Phase I and Phase II. Phase I required that all municipalities of 100,000 persons or more, industrial dischargers, and construction sites of 5 acres or more have NPDES permits for their stormwater discharges. Phase I permits were issued in much of the U.S. in 1991. Phase II required that all municipalities, industrial dischargers, construction sites of 1 acre or more, and other large property owners (such as school districts) have NPDES permits for their stormwater discharges. Phase II rules came into effect in 2003.

EPA has authorized 45 states to issue NPDES permits.^[3] In addition to implementing the NPDES requirements, many states and local governments have enacted their own stormwater management laws and ordinances, and some have published stormwater treatment design manuals. Some of these state and local requirements have expanded coverage beyond the federal requirements. For example, the State of Maryland requires erosion controls and sediment controls on construction sites of 5,000 ft² or more.^[4]

Agricultural runoff (except for concentrated animal feeding operations, or "CAFO") is considered by the CWA to be nonpoint source pollution. It is not included in the CWA definition of "point source" and therefore not subject to NPDES permit requirements. The 1987 CWA amendments established a non-regulatory program at EPA for nonpoint source pollution management consisting of research and demonstration projects. Related programs are conducted by the Natural Resources Conservation Service (NRCS) in the U.S. Department of Agriculture.

• Antecedent soil moisture
• Best Management Practice
• Flood
• Impervious surface
• Line source
• Permeable paving
• Sanitary sewer overflow
• Surface runoff
• Treatment wetlands
• Volumetric flow rate
• Water pollution
• Water quality

• Ferguson, Bruce K., 1998. Introduction to Stormwater. New York: John Wiley and Sons

• EPA Stormwater Permit Program
• EPA Nonpoint Source Management Program
• Center for Watershed Protection
• Stormwater Magazine - The Journal for Surface Water Professionals
• StormCon - World's Largest Stormwater Pollution Prevention Conference
• Toronto: Homeowner's Guide to Rainfall
• Onondaga Lake Improvement Project - How stormwater affects pollution of an urban stream and lake in Syracuse, NY.
• Charlotte-Mecklenburg, NC - Storm Water Services
• California Stormwater Quality Association