Bioretention areas function as soil and plant-based filtration devices that remove
pollutants through a variety of
physical, biological, and chemical treatment processes. The reduction of pollutant
loads to receiving waters is necessary for achieving regulatory water quality goals.
For example, several states, including Maryland, have agreed to work towards reducing
nutrient runoff to the Chesapeake Bay by 40%. A number of laboratory and field
experiments have been conducted by the University of Maryland in conjunction with
Prince George's County Department of Environmental Resources and the National Science
Foundation in order to quantify the effectiveness of bioretention cells in terms of
pollutant removal.1 A web site dedicated to this work can be found at
http://www.ence.umd.edu/~apdavis/Bioret.htm.
In general, the studies have found that properly designed and constructed bioretention
cells are able to achieve excellent removal of heavy metals. Users of this
technique can expect typical copper (Cu), zinc (Zn), and lead (Pb) reductions of
greater than 90%, with only small variations in results. Removal efficiencies as high
as 98% and 99% have been achieved for Pb and Zn. The mulch layer is credited with
playing the greatest role in this uptake, with nearly all of the metal removal
occurring within the top few inches of the bioretention system. Heavy metals
affiliate strongly with the organic matter in this layer. On the other hand,
phosphorus removal appears to increase linearly with depth and reach a maximum
of approximately 80% by about 2 to 3 feet depth. The likely mechanism for the removal
of the phosphorus is its sorption onto aluminum, iron, and clay minerals in the soil.
TKN (nitrogen) removal also appears to depend on depth but showed more
variability in removal efficiencies between studies. An average removal efficiency
for cell effluent is around 60%. Generally 70 to 80% reduction in ammonia was
achieved in the lower levels of sampled bioretention cells. Finally, nitrate
removal is quite variable, with the bioretention cells demonstrating a production of
nitrate in some cases due to nitrification reactions. Currently, the University of
Maryland research group is looking at the possibility of
incorporating into the bioretention cell design a fluctuating aerobic/anaerobic zone
below a raised underdrain pipe in order to facilitate denitrification and thus
nitrate removal.2
These studies indicate that in urban areas where heavy metals are the focal
pollutants, shallow bioretention facilities with a significant mulch layer may be
recommended. In residential areas, however, where the primary pollutants of concern
are nitrogen and phosphorus, the depth dependence will require deeper cells that
reach approximately 2 to 3 feet.
Other pollutants of concern are also addressed by the bioretention cells. For example,
sedimentation can occur in the ponding area as the velocity of the runoff slows and
solids fall out of suspension. Field studies at the University of Virginia have
indicated 86% removal for Total Suspended Solids (TSS), 97% for Chemical
Oxygen Demand (COD), and 67% for Oil and Grease.
3 Additional
work with laboratory media columns at the University of Maryland has demonstrated
potential bioretention cell removal efficiencies greater than 98% for total suspended
solids and oil/grease.4
One of the primary objectives of LID site design is to minimize,
detain, and retain post development runoff
uniformly throughout a site so as to mimic the site's predevelopment hydrologic
functions.5 Originally designed for providing an element of water quality
control, bioretention
cells can achieve quantity control as well. By
infiltrating and temporarily storing runoff water, bioretention cells reduce a site's
overall runoff volume and help to maintain the predevelopment peak discharge rate
and timing. The volume of runoff that needs to be controlled in order to replicate
natural watershed conditions changes with each site based on the development's impact
on the site's curve number (CN). The bioretention cell
sizing tool can be used to determine what cell
characteristics are necessary for effective volume control. Keep in mind that the use
of underdrains can make the bioretention cell act more like a filter that discharges
treated water to the storm drain system than an infiltration device.6
Regardless, the ponding capability of the cell will still reduce the immediate volume
load on the storm drain system and reduce the peak discharge rate. Where the
infiltration rate of in situ soils is high enough to preclude the use of
underdrains (at least 1"/hr), increased groundwater recharge also results
from the use of the bioretention cell. If used for this purpose, care should be taken
to consider the pollutant load entering the system, as well as the nature of the
recharge area. An additional hydrologic benefit of the bioretention cell is the
reduction of thermal pollution. Heated runoff from impervious surfaces is
filtered through the bioretention facility and cooled; one study observed a
temperature drop of 12°C between influent and effluent water.7 This
function of the bioretention cell is especially useful in areas such as the Pacific
Northwest where cold water fisheries are important.
Bioretention cells are dynamic, living, micro-ecological systems.8 They
demonstrate how the landscape can be used to protect ecosystem integrity. The design
of bioretention cells involves, among other things, the hydrologic cycle, nonpoint
pollutant treatment, resource conservation, habitat creation, nutrient cycles, soil
chemistry, horticulture, landscape architecture, and ecology8; the cell
thus necessarily demonstrates a multitude of benefits. Beyond its use for stormwater
control, the bioretention cell provides attractive landscaping and a natural habitat
for birds and butterflies. The increased soil moisture, evapotranspiration, and
vegetation coverage creates a more comfortable local climate. Bioretention cells can
also be used to reduce problems with on-site erosion and high levels of flow energy.
1 Davis, A.P., M. Shokouhian, H. Sharma and C. Minami, 2001: Laboratory study of
biological retention for urban stormwater management. Water Environment Research,
73(1), 5-14.
2 Kim, H., E.A. Seagren and A.P. Davis, 2000: Engineered bioretention for removal
of nitrate from stormwater runoff. WEFTEC 2000 Conference Proceedings on CDROM
Research Symposium, Nitrogen Removal, October, Anaheim, California.
3 Yu, S.L., X. Zhang, A. Earles and M. Sievers, 1999: Field testing of ultra-urban
BMPs. Proceedings of the 26th Annual Water Resources Planning and Management
Conference ASCE, 6-9 June, Tempe, Arizona.
4 Hsieh, C. and A.P. Davis, 2002: Engineering bioretention for treatment of urban
stormwater runoff. WEF Watershed 2002 Specialty Conference, 23-27 February, Ft.
Lauderdale, Florida.
5 Coffman, L.S., R. Goo and R. Frederick, 1999: Low impact development an
innovative alternative approach to stormwater management. Proceedings of the 26th
Annual Water Resources Planning and Management Conference ASCE, June 6-9, Tempe,
Arizona.
7 United States Environmental Protection Agency Office of Water, 2000:
Bioretention Applications - Inglewood Demonstration Project, Largo, Maryland, and
Florida Aquarium, Tampa, Florida. EPA-841-B-00-005A.
8 Winogradoff, D.A. and L.S. Coffman, 1999: Bioretention water quality performance
data and design modifications. Proceedings of the 26th Annual Water Resources
Planning and Management Conference ASCE, June 6-9, Tempe, Arizona.
il and plant-based filtration devices that remove
pollutants through a variety of
physical, biological, and chemical treatment processes. The reduction of pollutant
loads to receiving waters is necessary for achieving regulatory water quality goals.
For example, several states, including Maryland, have agreed to work towards reducing
nutrient runoff to the Chesapeake Bay by 40%. A number of laboratory and field
experiments have been conducted by the University of Maryland in conjunction with
Prince George's County Department of Environmental Resources and the National Science
Foundation in order to quantify the effectiveness of bioretention cells in terms of
pollutant removal.
ncies as high
as 98% and 99% have been achieved for Pb and Zn. The mulch layer is credited with
playing the greatest role in this uptake, with nearly all of the metal removal
occurring within the top few inches of the bioretention system. Heavy metals
affiliate strongly with the organic matter in this layer. On the other hand,
phosphorus removal appears to increase linearly with depth and reach a maximum
of approximately 80% by about 2 to 3 feet depth. The likely mechanism for the removal
of the phosphorus is its sorption onto aluminum, iron, and clay minerals in the soil.
TKN (nitrogen) removal also appears to depend on depth but showed more
variability in removal efficiencies between studies. An average removal efficiency
for cell effluent is around 60%. Generally 70 to 80% reduction in ammonia was
achieved in the lower levels of sampled bioretention cells. Finally, nitrate
removal is quite variable, with the bioretention cells demonstrating a production of
nitrate in some cases due to nitrification reactions. Currently, the University of
Maryland research group is looking at the possibility of
incorporating into the bioretention cell design a fluctuating aerobic/anaerobic zone
below a raised underdrain pipe in order to facilitate denitrification and thus
nitrate removal.
cells can achieve quantity control as well. By
infiltrating and temporarily storing runoff water, bioretention cells reduce a site's
overall runoff volume and help to maintain the predevelopment peak discharge rate
and timing. The volume of runoff that needs to be controlled in order to replicate
natural watershed conditions changes with each site based on the development's impact
on the site's curve number (CN). The bioretention cell