Bioretention systems are designed based on soil types, site conditions and land uses. A bioretention area can be composed of a mix of functional components, each performing different functions in the removal of pollutants and attenuation of stormwater runoff.

Six components often found in bioretention cells:

1. Grass or ground cover buffer strips reduce runoff velocity and filter particulate matter.
2. Sand bed provides aeration and drainage of the planting soil and assists in the flushing of pollutants from soil materials.
3. Ponding area provides storage of excess runoff and facilitates the settling of particulates and evaporation of excess water.
   
4. Organic layer performs the function of decomposition of organic material, filters pollutants and prevents soil erosion.
   
5. Planting soil provides the area for stormwater storage and nutrient uptake by plants. The planting soils contain some clays which adsorb pollutants such as hydrocarbons, heavy metals and nutrients.
   
6. Vegetation (plants) functions in the removal of water through evapotranspiration and pollutant removal through nutrient cycling.

Bioretention facilities are less cost intensive than traditional structural stormwater conveyance systems. Additional savings can be realized in reduced construction costs for storm drainpipe. For example, bioretention practices reduced the amount of storm drain pipe at a Medical Office building in Prince George's County, Maryland from 800 to 230 feet, which resulted in a cost savings of 50% of the overall drainage cost for the site.

Annual maintenance is required for the overall success of bioretention systems. This includes maintenance of plant material, soil layer and the mulch layer. A maintenance schedule outlining methods, frequency and time of year for bioretention maintenance should be developed. Plants will provide enhanced environmental benefit over time as root systems and leaf canopies increase in size and pollutant uptake and removal efficiencies.

Next: Grass Swales