An introduction to bioretention

 

What is Bioretention?

Definition: An engineered process to manage stormwater runoff, using the chemical, biological and physical properties afforded by a natural, terrestrial-based community of plants, microbes and soil.  Bioretention provides two important functions: (i) water quantity (flood) controls; and (ii) improve water quality through removal of pollutants and nutrients associated with runoff.

Bioretention as a Best Management practice

The principle of utilizing biological properties for the retention and transformation of nutrients and pollutants is not new – it forms the basis of agricultural and wastewater treatment practices. However, the concept of bioretention described here has the advantage of being on site, minimizing the distance between the source of runoff (e.g. parking lots, roof tops) and the site of control (e.g. rain garden), unlike end-of-pipe storm water management practices. In effect, the strategic integration of bioretention facilities into the landscape can result in smaller, more manageable subwatersheds. 

How Does Bioretention Work? Structure and Function

Bioretention facilities (rain gardens) may range from simple shallow depressions to more complex designs, but all are structurally engineered to provide the following functions with respect to stormwater quantity control: interception/capture, infiltration, filtration, storage, and water uptake by vegetation. 

The nine major components of the bioretention facility are:

  • Pretreatment
  • Flow Entrance
  • Ponding Area
  • Plant Material
  • Organic Layer or Mulch
  • Planting Soil and Filter Media
  • Pea Gravel Diaphragm
  • Underdrain and Outlet
  • Surface Overflow

A recommended soil mixture of top soil (20-30%), leaf compost (20-30%) and coarse-grained sand (50%) produces an ideal filter media to maximize infiltration, filtration and storage (hydrologic loading) capacity. A key design aspect of a bioretention facility is its depressed bowl-shaped topography, creating a “ponding area”. This ponding area allows for surface storage of runoff when the soil storage is capacity; promotes evaporation; and allows sedimentation of particulate matter prior to infiltration. Further incorporation of an underdrain (or outlet) and surface overflow element allows the engineer to construct a bioretention facility that can handle the anticipated volume of storm water runoff in a given area. In fact, bioretention facilities can be designed to handle not only peak discharges (e.g. the “first flush” of spring thaw), but also the volumetric control of all storms by mimicking existing hydrologic conditions. 

Chemical, Physical and Biological Processes

Bioretention attempts to reproduce the physical, chemical and biological processes of the natural environment to create a more efficient, on site, water treatment facility. The incorporation of biomass (plants, mulch, soil) introduces biological processes and cycles, not seen in conventional physical/chemical systems, leading to the designation bioretention. With proper design, these biological processes and cycles will be self-perpetuating and low maintenance, adding new dimensions of not only water quality control, but also water quality improvement. 

The key processes are listed below: (Processes enhanced or contributed by the biomass are identified with an asterisk) 

  • Interception*
  • Infiltration*
  • Settling (sedimentation)
  • Evaporation
  • Filtration*
  • Absorption*
  • Transpiration*
  • Evapotranspiration*
  • Assimilation*
  • Adsorption*
  • Nitrification*
  • Denitrification*
  • Volatilization*
  • Thermal Attenuation*
  • Degradation*
  • Decomposition*

Water Treatment

Besides controlling water quantity, bioretention facilities can improve the quality of stormwater runoff prior to discharge to streams or recharging of ground water. 

Bioremediation

Bioremediation is a general term referring to a decontamination method utilizing the biological and biochemical processes associated with biota (bacteria, fungi, plants) to clean toxic contamination associated with pollution. Phytoremediation specifically refers to the use of plants for decontamination. Both bioremediation and phytoremediation have been regularly and successfully used in brownfields (commercial or industrial sites that have been abandoned due to environmental contamination). Particular plants have the ability and tolerance to take up high concentrations of toxic chemicals, and even processing some of these chemicals to less toxic derivatives.

Now the same principles can be applied to the treatment of the pollution load of stormwater arising from both “point” and “nonpoint” sources (e.g. sediments, nutrients, oil and grease, heavy metals, pesticides, temperature etc.). Indeed, it is not much of a stretch of the imagination to visualize stormwater runoff as a moving, fluid brownfield.   

Nutrient Assimilation: The Nitrogen and Phosphorus Cycles

Nitrogen and phosphorus are essential nutrients: abundant in terrestrial ecosystems, but limiting in aquatic ecosystems. Thus in excess, these nutrients are regarded as key pollutants, leading to eutrophication of streams and lakes with the creation of anoxic (no oxygen) conditions.  Since fertilizers (a rich source of nitrogen and phosphorus) are dominant component of storm water pollution, bioretention facilities have been specifically geared to tackling the clean up of these nutrients. This is achieved by promoting the establishment of natural and self-perpetuating nitrogen and phosphorus cycles within its biotic community.  Macroorganisms (e.g. plants) and microorganisms (e.g. bacteria and fungi) within the soil horizon efficiently recycle nitrogen and phosphorus, helping to regulate the mass balance of these nutrients.

For example, nitrogen-fixing and nitrifying bacteria (typically found in the root zone of plants) convert nitrogen gas and ammonia into nitrates, a form of nitrogen that is highly soluble and rapidly taken up and assimilated by plants. Conversely, denitrifying bacteria can convert excess nitrates to a volatile form, which diffuses into the atmosphere. Decomposers (including fungi) release nutrients from decaying material, making them available for plants. Also phosphorus readily absorbs to inorganic and organic compounds containing iron, aluminum and calcium, thereby reducing leaching into ground water.   

Heavy Metals and Organic Pollutants

Bacteria and fungi found in the planting soil mix or organic mulch component of bioretention facilities are very efficient at degrading organic pollutants, e.g. petroleum based solvents and polyphenolic compounds. The mulch itself has been shown to have extensive ability to absorb and thereby immobilize heavy metals. Also clay, a negatively charged (anionic) component of the soil mix, is also efficient in absorbing and immobilizing positively-charged (cationic) heavy metals, nutrients, hydrocarbons and other pollutants.

How efficient is bioretention at removing pollutants?

Laboratory and field studies performed by the University of Maryland have shown that bioretention is very efficient at removing heavy metals such as copper, lead and zinc, and organic compounds such as ammonia and phosphorus (decreased by 60-80%). Also there was a marked decrease in thermal pollution, a form of pollution often forgotten when dealing with run-off. Unfortunately, the removal rates for nitrates were lower than ideal.
 
However, it is important to note that the ability of bioretention to handle different types and degrees of pollutant loading is design-specific, and the different bio-chemical-physical processes described above can be modulated to achieve the desired result. For example, adding an anaerobic zone will promote the growth of denitrifying bacteria, which volatilize nitrates. The latter design feature can easily be incorporated into a site where excessive nitrate runoff is anticipated.

Author: Clinton Shane Boyd

References:
Prince George's County Bioretention Manual, Maryland

Images courtesy Prince George's County, Maryland