Rain gardens are shallow, landscaped depressions that capture stormwater runoff from rooftops, sidewalks and streets. The runoff is filtered and allowed to infiltrate into the ground over a period of 24–48 hours. During this time, much of the pollutants found in the stormwater can be removed by physical, chemical and biological methods. Rain gardens:
- filter (physical)
- settle out pollutants (physical)
- remove nutrients through plant uptake (biological/chemical). Nutrients can lead to excess plant growth in lakes and streams
- remove some heavy metals as they are taken up by specific plants (biological/chemical)
- allow UV radiation to reach and have opportunity to kill bacteria (physical/biological)
- provide surface for pollutants to attach to soil and then be removed from the catch basin to stream cycle (chemical/physical)
The stormwater filters into the soil within 24–48 hours; specifically so that there is not sufficient time for mosquitoes to breed. This infiltration of stormwater does provide input to the groundwater. This also reduces the water entering the catch basin which travels to the nearest stream or waterbody, often causing flooding even during small rain events.