Stormwater

Polluted runoff

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.

In history

Since the era that humans began living in concentrated village or urban settings, stormwater runoff has presented itself as an issue. Such dwelling styles can be generally related to the Bronze Age when considerable amounts of impervious surface emerged as a factor in the design of early human settlements. Some of the early incorporation of stormwater engineering is evidenced in ancient Greece.

An early specific example of stormwater runoff system design is found in the archaeological recovery at Minoan Phaistos on Crete.

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 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.

Stormwater runoff as a source of pollution

In addition to the pollutants carried in stormwater runoff research by Australian researchers is identifying urban runoff as a cause of pollution in its own right.

In natural catchments (watersheds) surface runoff entering waterways is a relatively rare event, occurring only a few times each year and generally after larger storm events. Before development occurred most rainfall soaked into the ground and contributed to groundwater recharge or was recycled into the atmosphere by trees as evapotranspiration.

Modern drainage systems which collect runoff from impervious surfaces (e.g., roofs and roads) ensure that water is efficiently conveyed to waterways through pipe networks, meaning that even small storm events result in increased flows in waterways.

In addition to delivering higher pollutants from the urban catchment increased stormwater flow can lead to stream erosion, encourage weed invasion and can alter natural flow regimes which native species rely on for a range for activities including spawning, juvenile development and migration.

Polluted runoff from roads and highways is the largest source of water pollution in coastal areas today .

Integrated water management

Rain garden designed to treat stormwater from adjacent parking lot

Integrated water management (IWM) of stormwater has the potential to address many of the issues affecting the health of waterways and water supply challenges facing the modern urban city.

Also known as low impact development in the United States, IWM has the potential to improve runoff quality, reduce the risk and impact of flooding and deliver an additional water resource to augment potable supply.

The development of the modern city often results in increased demands for water supply due to population growth, while at the same time altered runoff predicted by climate change has the potential to increase the volume of stormwater that can contribute to drainage and flooding problems. IWM offers several techniques including stormwater harvest (to reduce the amount of water that can cause flooding), infiltration (to restore the natural recharge of groundwater), biofiltration or bioretention (e.g., rain gardens) to store and treat runoff and release it at a controlled rate to reduce impact on streams and wetland treatments (to store and control runoff rates and provide habitat in urban areas).

IWM as a movement can be regarded as being in its infancy and brings together elements of drainage science, ecology and a realisation that traditional drainage solutions which utilise more efficient pipe networks transfer problems further downstream to the detriment of the health of our stressed environmental streams.

Regulation in the United States

Federal requirements

Retention basin for management of stormwater

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, 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"). 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 (20,000 m²) 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 (4,000 m²) 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.

On May 16, 2008 the United States Environmental Protection Agency announced the issuance of a new Construction General Permit (CGP) to replace the permit that expires on July 1, 2008. This new permit has no substantive changes to the current Construction General Permit save the extension of the current conditions through July 1, 2010.

State and local requirements

The EPA has authorized 45 states to issue NPDES permits. 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 sq ft (460 m²) or more.

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.

See also

• Antecedent soil moisture
• Best Management Practice (BMP)
• Certified Professional in Erosion and Sediment Control
• First flush
• Flood
• Line source
• Nationwide Urban Runoff Program
• Permeable paving
• Sanitary sewer overflow
• Treatment wetlands
• Volumetric flow rate
• Water quality
• Urban runoff

References

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

External links

• Template:Dmoz *Center for Watershed Protection *EPA Stormwater Permit Program *Homeowner's Guide to Rainfall - Toronto *Stormwater Best Management Practice Maintenance, Kansas State University *StormCon - World's Largest Stormwater Pollution Prevention Conference *Stormwater Magazine - a stormwater trade journal *Stormwater management Tools: **[1] - Smart Sponge **SNOUT