Storm Water Best Management Practice Categories and Pollutant Removal Efficiencies
Background:
The Urban Storm Water Workgroup developed a list of BMP categories with associated pollutant
removal efficiencies and hydrologic effects. The workgroup developed this information so that the
Chesapeake Bay Program can better model the urban pollutant load reductions of TN, TP, and TSS
from storm water BMPs in the watershed.
Phase 4.3 of the Chesapeake Bay watershed model does not account for differences in pollutant
removal efficiencies among different categories of urban storm water BMPs. Currently, all BMPs
are lumped into one category called "storm water management" and are given one efficiency for
TN, TP, and TSS. For example, a wet pond will have the same pollutant removal efficiency as a
dry pond, an infiltration trench, and an oil/grit separator. Additionally, Phase 4.3 does not account
for reductions in pollutant loads that may result from hydrologic effects of the urban storm water
BMPs. In reality, many urban storm water BMPs reduce peak runoff flows and volumes and
increase time of concentration. When peak runoff flows are reduced, stream flow velocities are
reduced, which may result in reduced stream bank erosion. Currently, the model does not account
for reductions in sediment loads from reduced stream bank erosion that may result from urban
storm water BMP implementation.
It is important to note that these pollutant removal efficiencies apply to reductions of loads to
surface waters only. Also, these efficiencies are meant for modeling purposes and not for the
design and construction of BMPs.
Approach:
The Urban Storm Water Workgroup compiled data on the pollutant removal efficiencies of
commonly employed urban storm water management BMPs. Based on the BMP pollutant removal
efficiencies and general hydrologic effects these BMPs were grouped into categories. Each
category contains a number of BMP types that have similar pollutant removal efficiencies and
hydrologic effects.
Confidence Limits
It's important to note the studies on BMP pollutant removal efficiencies are variable and
oftentimes scarce. Additionally, many factors affect performance of BMPs such as the design,
frequency of inspection and maintenance, seasonality, and the life span and age of the BMP. Given
these uncertainties, the Workgroup rounded its estimates to the nearest 5%.
Maintenance
The Workgroup did not fully account for changes in pollutant removal efficiencies based on the
level of BMP maintenance and the life span of the BMPs. Due to lack of data on storm water
maintenance programs in the watershed, the group was unable to use a "multiplier" to account for
reductions in efficiencies due to insufficient maintenance. However, the workgroup did not
neglect maintenance altogether. Many of the studies evaluated for this effort were focused on
BMPs that were not regularly maintained. Therefore, the efficiencies, in part, may reflect some
lower reduction of pollutant loads due to insufficient maintenance. However, the BMPs are fairly
"young" and, therefore, probably do not fully account for reductions in pollutant removal

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efficiencies due to aging BMPs.
Low Impact Development/Environmental Site Design
The Workgroup decided not to include Low Impact Development (LID) or Environmental Site
Design as a BMP Category because no jurisdiction is reporting the number of acres under LID.
Jurisdictions are reporting number of acres under certain BMP practices that can be considered a
component of LID, such as bioretention or rooftop disconnection. These practices are already
accounted for in the BMP categories. In the future, if more and more jurisdictions use LID and
start to report the number of acres under LID, then a separate category.
Treatment Trains
Treatment trains are a number of BMPs that are connected in series to treat the same volume of
runoff. The Workgroup has concluded that there is not enough hard data to account for pollutant
removal efficiencies for "treatment trains". Funding opportunities to obtain literature and field
data are currently being pursued.
The following table summarizes the BMP categories and the pollutant removal efficiencies. See
the Support Document for a complete list of BMP types, BMP definitions, pollutant removal
efficiencies, and references that were used in this analysis.

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Category
% Pollutant Removal Efficiency
Comments
TN
TP
TSS
Category A:
Wet Ponds and Wetlands
30
50
80
This category includes practices such as wet ponds,
wet extended detention ponds, retention ponds,
pond/wetland systems, shallow wetlands, and
constructed wetlands.
Category B:
Dry Detention Ponds and
Hydrodynamic Structures
5
10
10
Hydrodynamic structures are not considered a stand
alone BMP. It acts similar to a dry detention pond
and therefore it is included in this group.
Category C:
Dry Extended Detention Ponds
30
20
60
This category includes practices such as dry extended
detention ponds and extended detention basins.
Category D:
Infiltration Practices
50*
70*
90*
This category includes practices such as infiltration
trenches, infiltration basins, and porous pavement that
reduce or eliminate the runoff.
*These efficiencies are based on limited studies.
Category E:
Filtering Practices
40
60
85
This category includes swales (dry, wet, infiltration,
and water quality), open channel practices, and
bioretention that transmit runoff through a filter
medium. Grass swales were excluded because they
have minimal water quality benefits.

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Category
% Pollutant Removal Efficiency
Comments
TN
TP
TSS
Category F:
Roadway Systems
TBD
TBD
TBD
We acknowledge that roadways make up a large
portion of the urban acreage in the watershed and that
there are practices that are on the ground today that
result in some water quality benefit. Due to lack of
data, the workgroup has not assigned pollutant
removal efficiencies to this category. Your data will
help the workgroup to develop an approach for
crediting these BMPs
Category G:
Impervious Surface Reduction
Model
Generated
Model
Generated
Model
Generated
This category includes a number of practices that
essentially turn impervious surfaces into pervious
surfaces. Examples of these practices are green roofs,
disconnected roofs, rain barrels, removal of
impervious surfaces. Pollutant load reductions will
be modeled based on the conversion of impervious
surfaces to pervious urban surfaces.
Category H:
Street Sweeping and Catch
Basin Inserts
TBD
TBD
TBD
This category includes municipal efforts such as street
sweeping, catch basins cleaning that prevent pollutant
loads from entering the Bay. Pollutant load reduction
efficiencies will be determined based on the number
of pounds of TN, TP, and/or TSS removed through
these practices.

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Category
% Pollutant Removal Efficiency
Comments
TN
TP
TSS
Category I:
Stream Restoration
0.02
lb/linear ft
0.0035
lb/linear ft
2.55
lb/linear ft
These numbers are based on a study conducted on
Spring Branch Stream, an urban watershed in
Baltimore County. The Urban Storm Water
Workgroup will work with other stream restoration
experts to refine these efficiencies, as data become
available and to develop criteria for what constitutes
water quality-based stream restoration. Please
provide details on the types of stream restorations
activities you undertook.

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