United States
                    Environmental Protection
                    Office of Water
                    Washington, D.C.
September 1999
Storm Water
Technology  Fact  Sheet

Bioretention is a best management practice (BMP)
developed in the early 1990's by the Prince George's
County, MD, Department of Environmental
Resources (PGDER).  Bioretention utilizes soils
and both woody and herbaceous plants to remove
pollutants from storm water runoff.  As shown in
Figure 1, runoff is conveyed as sheet flow to the
treatment area, which consists of a  grass buffer
                   strip, sand bed, ponding area, organic layer or
                   mulch layer, planting soil, and plants.  Runoff
                   passes first over or through a sand bed, which slows
                   the runoffs velocity, distributes it evenly along the
                   length of the ponding area, which consists of a
                   surface organic layer and/or ground cover and the
                   underlying planting soil.  The ponding area is
                   graded, its center depressed. Water is ponded to a
                   depth of 15 centimeters  (6 inches) and gradually
                   infiltrates  the  bioretention  area  or  is
                               EVAPO-TRANSPI RATION
 Source: PGDER, 1993.
                         FIGURE 1 BIORETENTION AREA

evapotranspired. The bioretention area is graded to
divert excess runoff away from itself.  Stored water
in the bioretention area planting soil exfiltrates over
a period of days into the underlying soils.

The basic bioretention design shown in Figure 1
can be modified  to accommodate more  specific
needs.  The City of Alexandria,  VA, has modified
the bioretention  BMP  design  to   include  an
underdrain within the  sand bed to collect the
infiltrated water and discharge it to a downstream
sewer system.   This  modification was required
because  impervious  subsoils and marine clays
prevented complete infiltration in the soil system.
This modified design makes the bioretention area
act more as a filter that discharges treated water
than as an infiltration device.  Design modifications
are also being reviewed that will potentially include
both aerobic and anaerobic zones in the treatment
area.     The   anaerobic  zone  will  promote
denitrifi cation.


Bioretention typically treats storm water  that has
run  over impervious   surfaces at   commercial,
residential, and industrial areas.  For example,
bioretention is an ideal  storm water  management
BMP for median strips, parking lot  islands,  and
swales. These areas can be designed or modified so
that  runoff  is  either diverted  directly into the
bioretention area or conveyed into the bioretention
area by  a  curb  and  gutter collection  system.
Bioretention is usually best used upland from inlets
that receive  sheet flow  from graded  areas and at
areas that will  be excavated.  The  site must be
graded  in  a  manner  that  minimizes  erosive
conditions  as  sheet  flow  is  conveyed  to  the
treatment area, maximizing treatment effectiveness.
Construction of bioretention areas is best suited to
sites where grading or excavation will occur in any
case so  that the bioretention area can be readily
incorporated  into the  site plan without  further
environmental damage. Bioretention should be used
in stabilized drainage areas to minimize sediment
loading in the treatment area. As with all BMPs, a
maintenance plan must be developed.

Bioretention has been used as a storm water BMP
since 1992.  In addition to Prince George's County
and  Alexandria,  bioretention  has  been  used
successfully  at  urban  and  suburban  areas  in
Montgomery County, MD; Baltimore County, MD;
Chesterfield County, VA; Prince William County,
VA; Smith Mountain Lake State Park, VA; and
Cary, NC.


Bioretention is not an appropriate BMP at locations
where the water table is within 1.8 meters (6 feet)
of the ground surface and where the surrounding
soil stratum is unstable. In cold climates the soil
may freeze, preventing runoff from infiltrating into
the  planting  soil.    The BMP  is also  not
recommended for areas with slopes greater than 20
percent, or where mature  tree removal would  be
required. Clogging may be a problem, particularly
if the BMP receives runoff with high sediment

Bioretention provides storm water treatment that
enhances the quality of downstream water bodies.
Runoff is temporarily stored in the  BMP and
released over a period of four days to the receiving
water.  The BMP is also able to provide shade and
wind breaks, absorb noise, and improve an area's


Design details have been  specified  by the Prince
George's County DER in  a  document entitled
Design Manual for the Use of Bioretention in Storm
Water  Management  (PGDER,  1993).    The
specifications  were developed  after extensive
research  on soil  adsorption capacities and rates,
water balance, plant pollutant removal potential,
plant   adsorption  capacities   and  rates,  and
maintenance  requirements.   A  case study was
performed  using  the  specifications  at  three
commercial sites and one residential site in Prince
George's County, Maryland.

Each of the components of the bioretention area is
designed to perform a specific function. The grass
buffer strip reduces incoming runoff velocity and
filters particulates from the runoff.  The sand bed
also reduces the velocity,  filters particulates, and
spreads flow over the  length of the bioretention

area. Aeration and drainage of the planting soil are
provided by the 0.5 meter (18 inch) deep sand bed.
The  ponding  area provides a temporary storage
location  for runoff prior  to  its evaporation  or
infiltration.  Some particulates not filtered out by
the grass filter strip or the sand bed settle within the
ponding area.

The  organic or mulch layer also filters pollutants
and  provides  an environment conducive  to the
growth  of   microorganisms,   which  degrade
petroleum-based  products  and  other   organic
material. This layer acts in a similar way to the leaf
litter in a forest and prevents the erosion and drying
of underlying  soils.  Planted ground cover reduces
the potential  for erosion as well,  slightly more
effectively than mulch.  The maximum sheet flow
velocity prior to erosive conditions is 0.3 meters per
second (1 foot per second) for planted ground cover
and 0.9 meters per second (3 feet per second) for

The  clay in the planting soil provides adsorption
sites for hydrocarbons, heavy metals, nutrients and
other pollutants.    Storm water storage is also
provided by the voids in the planting soil.  The
stored water and nutrients in the water and soil are
then available to the plants for uptake.

The  layout of the bioretention area is determined
after site constraints such as location of utilities,
underlying soils, existing vegetation, and drainage
are considered.  Sites with loamy sand  soils are
especially appropriate for bioretention because the
excavated soil can be backfilled and  used as the
planting soil, thus eliminating the cost of importing
planting soil. An unstable surrounding soil stratum
(e.g., Marlboro Clay) and soils with a clay content
greater than 25 percent may preclude the use of
bioretention, as would a site with slopes greater
than 20 percent or a site with mature trees that
would be removed during construction of the BMP.
Bioretention can be designed to be  off-line  or
on-line of the  existing drainage system. The "first
flush"  of runoff is diverted to the off-line system.
The first flush of runoff is the initial runoff volume
that   typically  contains  higher  pollutant
concentrations than those in the  extended runoff
period. On-line systems capture the first  flush but
that volume of water will likely be washed out by
subsequent runoff resulting  in  a release  of the
captured pollutants.  The size of the drainage area
for one bioretention area should be between 0.1 and
0.4 hectares  (0.25  and  1.0 acres).   Multiple
bioretention  areas  may be  required  for  larger
drainage areas.  The maximum  drainage area for
one bioretention area is determined by the amount
of sheet flow generated by a 10-year storm.  Flows
greater than 141 liters per second (5 cubic feet per
second) may  potentially erode stabilized areas.  In
Maryland,  such a flow generally occurs with a
10-year storm at one-acre commercial or residential
sites.  The designer should determine the potential
for erosive conditions at the site.

The size of the bioretention area is a function of the
drainage area and the runoff generated from the
area.  The size should be 5  to  7 percent  of the
drainage area multiplied by  the rational method
runoff coefficient, "c," determined for the site. The
5 percent specification applies to a bioretention area
that includes a sand bed; 7 percent to an  area
without one.   An example of sizing a facility is
shown in Figure 2.   For this discussion,  sizing
specifications are based on 1.3 to  1.8 centimeters
(0.5 to 0.7 inches) of precipitation over a 6-hour
period   (the  mean   storm   event   for   the
Baltimore-Washington area), infiltrating into the
bioretention area. Other areas with different mean
storm events will need to account for the difference
in  the  design  of the  BMP.    Recommended
minimum dimensions of the bioretention area are
4.6 meters (15 feet) wide by 12.2 meters (40 feet) in
length. The minimum width allows enough space
for a dense, randomly-distributed area of trees and
shrubs to  become  established  that replicates a
natural forest and creates  a  microclimate.   This
enables the bioretention area to tolerate the effects
of heat stress, acid rain, runoff pollutants, and insect
and disease infestations which landscaped areas in
urban settings typically are unable to tolerate.  The
preferred width is 7.6 meters (25 feet), with a length
of twice the width.   Any facilities wider than 6.1
meters (20 feet) should be twice as long as they are
wide.  This  length  requirement  promotes  the
distribution of flow and decreases the chances of
concentrated  flow.

The maximum recommended ponding depth of the
bioretention area is 15 centimeters (6 inches).  This

                        Area Locatio
                                          BIORETENTION AREA
                                          SIZING COMPUTATION
                      B OTENT ON AREA S ZE
                       1.  With Sand Bed (5% Sum of C x Area)
                           = 05 x 23,900 = 1,195 OR SAY 1.200 sq.ft.
                       2.  Without Sand Bed (7% Sum of C x Area)
                           = 07 x 23,900 = 1,1673 OR SAY 1.700 so. ft.
Source: PGDER, 1993.
                            FIGURE 2 BIORETENTION AREA SIZING
depth provides for adequate storage and prevents
water from standing for excessive periods of time.
Because of some plants' water intolerance, water
left to stand for longer than four days restricts the
type of plants that can be used. Further, mosquitoes
and  other insects  may start to  breed if water is
standing for longer than four days.

The  appropriate planting soil should be backfilled
into the excavated bioretention area. Planting soils
should be sandy loam, loamy sand, or loam texture
with a clay content ranging from 10 to 25 percent.
The soil should have infiltration rates greater than
1.25 centimeters (0.5 inches) per hour, which is
typical of sandy loams, loamy sands, or loams. Silt
loams and clay loams generally have rates of less
than 0.68  centimeters (0.27 inches) per hour.  The
pH of the soil  should be between 5.5 and 6.5.
Within this  pH  range,  pollutants (e.g., organic
nitrogen and phosphorus) can be adsorbed by the

soil and microbial  activity  can flourish.   Other
requirements for the planting soil are a 1.5 to 3
percent organic content and a maximum 500 ppm
concentration of soluble salts. In addition, criteria
for magnesium, phosphorus, and potassium are 39.2
kilograms per  acre  (35 pounds  per acre),  112
kilograms per acre (100 pounds per acre), and 95.2
kilograms  per  acre  (85  pounds  per   acre),
respectively.  Soil tests should be performed for
every  382 cubic meters  (500 cubic  yards)  of
planting soil, with the exception of pH and organic
content tests, which are required only  once per
bioretention area.

Planting soil should be  10.1 centimeters (4 inches)
deeper than the bottom of the largest root ball and
1.2 meters (4 feet) altogether.  This depth will
provide adequate soil for the plants' root systems to
become established and prevent plant damage due
to severe wind. A soil depth  of 1.2 meters (4 feet)
also provides adequate moisture capacity.   To
obtain the recommended  depth,  most sites will
require excavation.  Planting soil depths of greater
than 1.2 meters (4 feet) may require additional
construction  practices  (e.g., shoring measures).
Planting soil should be placed in 18 inches or
greater lifts and lightly compacted  until the desired
depth is reached.  The bioretention area should be
vegetated to resemble a terrestrial forest community
ecosystem, which is dominated by  understory trees
(high  canopy  trees may be  destroyed  during
maintenance) and has discrete soil  zones as well as
a mature  canopy and a distinct  sub-canopy of
understory trees,  a shrub layer,  and  herbaceous
ground covers. Three species each  of both trees and
shrubs are recommended to be planted at a rate of
2500 trees and shrubs per hectare  (1000 per acre).
For example, a 4.6 meter (15 foot) by 12.2 meter
(40 foot) bioretention area (55.75 square meters or
600 square feet) would require 14 trees and shrubs.
The shrub-to-tree ratio should be 2:1 to 3:1.  On
average, the trees should be spaced 3.65 meters (12
feet) apart and the  shrubs should be spaced 2.4
meters  (8  feet)  apart.    In  the metropolitan
Washington, D.C., area, trees and shrubs should be
planted from mid-March through the end of June or
from   mid-September  through   mid-November.
Planting periods in other areas of the U.S. will vary.
Vegetation should be watered at the end of each day
for fourteen days following its planting.
Native species that are tolerant to pollutant loads
and varying wet and dry conditions should be used
in the bioretention area.  These species  can be
determined  from  several  published  sources,
including Native Trees, Shrubs, and Vines for
Urban and Rural America (Hightshoe, 1988). The
designer should assess aesthetics, site layout, and
maintenance  requirements when selecting  plant
species.   Adjacent non-native  invasive  species
should be identified and the designer should take
measures (e.g., provide a soil breach) to eliminate
the threat of these species invading the bioretention
area.   Regional  landscaping manuals  should be
consulted to ensure  that the  planting  of the
bioretention  area   meets   the   landscaping
requirements established by the local authorities.

The  optimal placement of vegetation within the
bioretention area  should be  evaluated  by the
designers.  Plants should be placed at irregular
intervals to replicate a natural forest. Shade and
shelter from  the wind  will be provided to the
bioretention area if the designer places the trees on
the perimeter of the area. Trees and shrubs can be
sheltered from damaging flows if they are placed
away from the path of the incoming runoff. Species
that  are  more  tolerant  to cold  winds  (e.g.,
evergreens) should be placed in windier areas of the

After the trees and shrubs are placed, the  ground
cover and/or mulch should be established. Ground
cover such as grasses or legumes can be planted
during the  spring of the year.  Mulch should be
placed immediately  after trees and shrubs are
planted.   Five to 7.6  cm  (2  to  3 inches) of
commercially-available  fine shredded hardwood
mulch or shredded hardwood  chips should be
applied to the bioretention  area to protect  from
erosion.  Mulch  depths  should be kept below 7.6
centimeters (3 inches) because more would interfere
with the cycling of carbon dioxide and oxygen
between  the soil and the atmosphere. The mulch
should be aged for at least six months (one year is
optimal), and applied uniformly over the site.


Bioretention  removes  storm  water  pollutants
through  physical  and  biological  processes,

including adsorption,  filtration,  plant  uptake,
microbial activity, decomposition, sedimentation
and  volatilization.  Adsorption  is the  process
whereby particulate pollutants attach to soil (e.g.,
clay) or vegetation surfaces. Adequate contact time
between the surface and pollutant must be provided
for in the design of the system  for this removal
process to occur.  Therefore, the infiltration rate of
the soils must not exceed those specified in the
design criteria or pollutant removal may decrease.
Pollutants removed by  adsorption include metals,
phosphorus, and some hydrocarbons.  Filtration
occurs as runoff passes through the bioretention
area media, such as the sand bed, ground cover and
planting soil.  The media trap particulate matter and
allow  water  to  pass  through.    The  filtering
effectiveness  of the bioretention area may decrease
over time.  Common particulates  removed from
storm water  include particulate organic matter,
phosphorus,  and suspended  solids.   Biological
processes that occur in wetlands result in pollutant
uptake by plants and microorganisms in the soil.
Plant growth is sustained by the uptake of nutrients
from the soils, with woody plants locking up these
nutrients through the seasons. Microbial activity
within the soil also  contributes to the  removal of
nitrogen and organic matter. Nitrogen is removed
by nitrifying and denitrifying bacteria, while aerobic
bacteria are responsible for the decomposition of
the organic matter (e.g., petroleum).  Microbial
processes require oxygen and can result in depleted
oxygen levels  if the  bioretention area  is  not
adequately aerated.

Sedimentation occurs in the swale or ponding area
as the  velocity  slows and  solids  fall out  of

Volatilization also play s a role in pollutant removal.
Pollutants such as oils and hydrocarbons can be
removed from the wetland via evaporation or by
aerosol  formation under windy conditions.  The
removal effectiveness  of bioretention has been
studied  during   field   and  laboratory  studies
conducted by the University of Maryland (Davis et
al, 1998).  During  these  experiments,  synthetic
storm water runoff was pumped through several
laboratory and field bioretention areas to simulate
typical storm events in Prince George's County,
MD. Removal rates for heavy metals an nutrients
are shown in Table  1.   As shown,  the  BMP
removed between  93  and 98 percent of metals,
between 68 and 80 percent of TKN and between 70
and 83 percent of total phosphorus.  For all  of the
pollutants analyzed, results of the laboratory study
were  similar  to  those  of  field  experiments.
Doubling or halving the  influent pollutant  levels
had little effect  on the effluent pollutants  levels
(Davis et al, 1998). For other parameters, results
from the performance studies for infiltration BMPs,
which are similar to bioretention, can be used to
estimate  bioretention's  performance.     These
removal rates are also shown in Table 1. As shown,
the BMP could potentially achieve greater than 90
percent removal rates for total suspended solids,
organics, and bacteria. The microbial activity and
plant uptake occurring in the bioretention area will
likely result in higher removal rates than  those
determined for infiltration BMPs.

Total Phosphorus
Metals (Cu, Zn, Pb)
Total Suspended Solids
Removal Rate
70%-83% 1
93%-98% 1
68%-80% 1
90% 2
90% 2
90% 2
Source: 1Davis et al. (1998)

Recommended maintenance for a bioretention area
includes inspection and repair or replacement of the
treatment area components.   Trees and  shrubs
should be inspected twice per year to evaluate their
health  and remove any dead or severely diseased
vegetation. Diseased vegetation should be treated
as necessary  using  preventative and  low-toxic
measures to  the  extent  possible.   Pruning  and
weeding  may also be necessary to maintain the
treatment area's appearance. Mulch replacement is
recommended when erosion is evident or when the
site begins to look unattractive.  Spot mulching may

be adequate when there are random void areas;
however, once every two to three years the entire
area may require mulch replacement.  This should
be done during the spring. The old mulch should be
removed before the new mulch is distributed. Old
mulch should be disposed of properly.

The application  of an  alkaline product,  such as
limestone, is recommended one to two times per
year to  counteract  soil  acidity  resulting  from
slightly acidic precipitation and runoff. Before the
limestone is applied, the soils and organic layer
should be tested to determine the pH and therefore
the quantity of limestone required.  When levels of
pollutants reach toxic levels which impair plant
growth and  the  effectiveness  of  the  BMP, soil
replacement may be required (PGDER, 1993).


Construction cost estimates for a bioretention area
are slightly  greater than those for the required
landscaping  for  a new development.   Recently-
constructed 37.16 square meter (400 square foot)
bioretention areas in Prince George's County, MD
cost approximately $500.  These units are rather
small  and their  cost is low. The cost estimate
includes the cost for excavating 0.6 to 1 meters (2
to 3 feet) and vegetating the site with 1 to 2 trees
and 3  to 5 shrubs.  The estimate does not include
the cost for the planting soil, which increases the
cost for a bioretention area.   Retrofitting a site
typically   costs  more,  averaging  $6,500  per
bioretention area. The higher costs  are attributed to
the demolition of existing concrete, asphalt, and
existing  structures and the replacement of  fill
material with planting soil. The costs of retrofitting
a  commercial   site  in  Maryland  (Kettering
Development) with  15  bioretention  areas  were
estimated at $111,600.

The use of bioretention can decrease the  cost for
storm water  conveyance systems  at  a  site.  A
medical office building in Maryland  was able to
reduce the required amount of storm drain pipe
from 243.8 meters (800 feet) to 70.1 meters (230
feet) with the use  of bioretention.  The drainage
pipe costs were reduced by $24,000, or 50 percent
of the total  drainage cost for  the site (PGDER,
1993). Landscaping costs that would be required at
a development regardless of the installation of the
bioretention area should also be considered when
determining the net cost of the BMP.

The  operation and maintenance  costs  for  a
bioretention facility will be comparable to those of
typical landscaping required  for a site.  Costs
beyond the normal landscaping fees will include the
cost for testing the soils and may  include costs for
a sand bed and planting soil.


1.     Bitter,  S., and  J. Keith Bowers, 1994.
       Bioretention  as a  Water  Quality  Best
       Management   Practice.   Watershed
       Protection Techniques, Vol. l,No.3. Silver
       Spring, MD.

2.     Davis, A.P.,  Shokouhian,  M., Sharma, H.,
       and Minani, C.,  1998.   Optimization of
       Bioretention Design for Water Quality and
       Hydrologic Characteristics.

3.     Hightshoe,  G.L., 1988.   Native  Trees,
       Shrubs,  and Vines for Urban and Rural
       America.  Van  Nostrand Reinhold, New
       York, New York.

4.     Prince  George's  County  Department of
       Environmental Resources  (PGDER), 1993.
       Design Manual for Use of Bioretention in
       Storm water Management.  Division of
       Environmental  Management,  Watershed
       Protection Branch. Landover, MD.

5.     Prince  George's  County  Department of
       Environmental Resources  (PGDER), 1997.
       Bioretention Monitoring: Preliminary Data
       Analysis.  Division   of   Environmental
       Management, Watershed Protection Branch.
       Landover, MD.

6.     Reed, P.B.,  Jr,   1988.  National List of
       Species  That   Occur  in   Wetlands:
       Northeast. United States Fish and Wildlife
       Service, St. Petersburg, Florida.

7.     Schueler, T.R.,  1987.  Controlling Urban
       Runoff:  A Practical Manual for Planning

             and    Designing   Urban  Best
             Management  Practices.
             Metropolitan Washington Council
             of Governments.

8.      Schueler,  T.R.,   1992.    A  Current
       Assessment of Urban  Best  Management
       Practices.  Metropolitan   Washington
       Council of Governments.


The City of Alexandria, Virginia
Warren Bell
Department of Transportation  and Environmental
P.O.Box 178, City Hall
Alexandria, VA 22313
The Town of Gary, North Carolina
Tom Horstman
Department of Development Review
P.O. Box 8005
Cary,NC  27513

Center for Watershed Protection
Tom Schueler
8391 Main St.
Ellicott City, MD  21043

Northern Virginia Planning District Commission
David Bulova
7535 Little River Turnpike, Suite 100
Annandale, VA 22003

Prince Georges County, Maryland
Larry Coffman
Department of Environmental Resources               For more information contact:
9400 Peppercorn Place
Largo, MD 20774                                  Municipal Technology Branch
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