Stormwater

Stormwater, also spelled storm water, is water that originates during precipitation events and snow/ice melt. Stormwater can soak into the soil (infiltrate), be held on the surface and evaporate, or runoff and end up in nearby streams, rivers, or other water bodies (surface water).

In natural landscapes such as forests, the soil absorbs much of the stormwater and plants help hold stormwater close to where it falls. In developed environments, unmanaged stormwater can create two major issues: one related to the volume and timing of runoff water (flooding) and the other related to potential contaminants that the water is carrying (water pollution).

Stormwater is a potential water resource if managed and purified.

Stormwater pollution

Relationship between impervious surfaces and surface runoff

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 after 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.

A first flush is the initial runoff of a rainstorm. During this phase, polluted water entering storm drains in areas with high proportions of impervious surfaces is typically more concentrated compared to the remainder of the storm. Consequently, these high concentrations of urban runoff result in high levels of pollutants discharged from storm sewers to surface waters.

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.

• See also Urban runoff

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, and may also be labeled as nonpoint source pollution. 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.

Regulation in United States

In the United States, the Environmental Protection Agency (EPA) is charged with regulating stormwater pursuant to the Clean Water Act (CWA). 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, 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.

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 United States Congress]] broadened the CWA definition of "point source" to include industrial stormwater discharges and municipal separate storm sewer systems ("MS4"). 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.

Many States, such as Texas, California, South Carolina, New York etc., have created their own regulatory agencies to act as an intermediary between the EPA and the municipalities and industries. EPA has authorized 45 states to issue NPDES permits.

Nonpoint Source Pollution Management

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.

Mitigation

Permeable paving, as used, for example, in the Green Alley Initiative in Chicago may reduce stormwater runoff.

See also

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

References

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

External links

• EPA Stormwater Permit Program
• EPA Nonpoint Source Management Program
• Center for Watershed Protection
• Stormwater Magazine - The Journal for Surface Water Professionals
• StormwaterJobs.com - The Career Center Resource for the Stormwater Industry
• 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 Storm Water Services website
• Stormwater Systems - "Cleaning Our Waterways"
• California Stormwater Quality Association

Adapted from the Wikipedia article "Stormwater" http://en.wikipedia.org/w/index.php?title=Stormwater&oldid=168893611 released under the GNU Free Documentation License