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Stormwater is pure rainfall plus anything the rain carries along with it.
In urban areas, rain that falls on the roof of your house or collects on paved areas, such as driveways, roads and footpaths, is carried away through a series of underground pipes and storm drains that are separate from the sewer system.
Unlike sewage, stormwater is not treated. In some cases, it is filtered through traps at the end of the system, but it still flows directly from the streets and gutters, through flood control and drainage facilities, into rivers and other waterways.
Stormwater pollution comes from many different sources, though the two main sources are point and non-point. Point source pollution occurs when high levels of pollution enter a water system, such as a wetland or river, from one source, such as a factory, mine, sewage plant or garbage dump. Point source pollution is easy to trace.
Non-point source pollution occurs when pollution enters a water system at various points and from various sources. This type of pollution is the most difficult to monitor and manage. The most common non-point source of stormwater pollution comes from local residents throughout a watershed.
Stormwater collects pollutants as water makes its way through the watershed. When stormwater passes over hard surfaces such as building sites and roads, it picks up pollutants with the flow the water. The faster the flow, the more pollutants the water is likely to pick up. Pollutants include silt, gasoline, oil, grass cuttings and leaves, paint, pesticides, detergents, fertilizer and litter. Stormwater is the primary way pollution enters a river or other waterway.
Stormwater pollution can be controlled if everyone plays a part in reducing or eliminating the various pollutants that may enter drains in the streets where they live and work. The most effective way to reduce stormwater pollution is to stop it from entering the drainage system in the first place.
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