Written by: Tyler Padden
In the second of our series of articles on green storm water technologies, we will look at the use of bioretention to improve storm water quality. Bioretention is considered to be a best management practice (BMP) by the U.S. Environmental Protection Agency (USEPA) and was developed in the early 1990s by the Maryland Department of Environmental Resources. The USEPA defines bioretention as a process that utilizes soils and certain types of plant life to remove pollutants from storm water runoff. Various tests have shown that these systems can effectively reduce concentrations of heavy metals as well as nutrients such as phosphorous and nitrogen, which are key storm water contaminants that many industrial facilities struggle to deal with on a regular basis. To achieve this pollutant reduction, bioretention systems typically rely on one of three pollutant removal mechanisms: filtration, adsorption to soil particles, or biological uptake by plants. By using what is naturally available, a landscaping feature can be transformed into an unassuming treatment device through the use of bioretention.
Bioretention systems can be used in a variety of settings and land uses. A common use of these types of systems is to install a bioretention area or rain garden to treat incoming sheet flow from a nearby impervious surface such as a parking lot. Because these areas are typically customized to match the flow of storm water they need to treat, bioretention areas can be installed in a variety of configurations. A common design for a bioretention area is shown in the figure below:
Source: USEPA Storm Water Technology Fact Sheet: Bioretention, September 1999
The major components of most systems, including the one shown above, are a grass buffer, a sand bed filter, and a ponding area. The grass buffer slows the flow of storm water runoff into the system while the sand bed filter further retards the flow and helps to evenly distribute runoff into the ponding area. The ponding area provides the space for bioretention treatment. Excess flow, which may occur during heavy rainfall events, is diverted away from the bioretention area and into a nearby storm sewer system, similar to many other storm water treatment devices. Runoff that does enter the ponding area in the bioretention system will infiltrate into the soil and be treated. In areas that have more impervious subsurface soils, an underdrain can be installed to transport treated water away into the nearby storm sewer system.
When properly designed and installed, bioretention systems offer a beneficial, low-cost form of storm water treatment. These systems can be easily retrofitted into green spaces alongside the edges of parking lots, inside of roadway and parking lot medians, and along streetscapes. Bioretention areas provide a very high degree of treatment when compared with historical storm water treatment devices such as sand filtering. This is achieved through the use of multiple treatment mechanisms within each system. Another huge benefit of using a bioretention system is the reduction in required maintenance versus other treatment systems. The only required maintenance is periodic mulching, pruning, weeding, and if necessary, replacement of the plants within the ponded area.
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