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Removal of Pathogens in Stormwater
Controlling pathogens in runoff presents
a growing challenge for stormwater
managers and designers. This fact sheet
provides an overview of pathogens:
what they are, how they affect people,
and how they are regulated; describes
their association with stormwater
runoff; and investigates stormwater
management practices that may limit
pathogen presence in surface waters.
WHAT ARE PATHOGENS?
Microorganisms are common in the
natural environment, often performing
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V\PELRWLFUHODWLRQVKLSV7KHWHUPmicroorganismJHQHUDOO\UHIHUVWRPDQ\
different organisms including bacteria,
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water-related recreational activities, and
even through skin contact with contamiQDWHGZDWHUV86(3$0LFURRUJDQLVPVDQGYLUXVHVWKDWFDQFDXVH
illness are referred to as pathogens and
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Common examples of Pathogens we
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HOW DO WE KNOW IF A WATER
BODY IS CONTAMINATED BY
PATHOGENS?
Indicator species are used to test for the
presence of harmful pathogens in surIDFHZDWHUV$OWKRXJKWKHVHVSHFLHVDUH
QRUPDOO\QRWKDUPIXOWRKXPDQVWKHLU
presence in surface waters can indicate
contamination from the fecal matter
of warm-blooded animals, a source of
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degradation due to pathogens, including
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ml should not be exceeded in more than
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fecal coliform in waters designated for
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the targeted geometric mean being no
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1
A
B
Figure
1: (A) Electron micrograph of E. coli cluster (U.S. Department
of Agriculture – Agricultural
A
B
Research Service) (B) Immunofluorescence image of Giardia lamblia cysts (H.D.A Lindquist, U.S. EPA)
Table 1: Pathogen types, descriptions, and example
Type
Brief Description
Example Pathogens (disease)
Bacteria
Single-celled organism with no
nuclear membrane. Cell structure is
simple, containing few organelles.
Salmonella (Salmonellosis), Escherichia coli
0157:H7 (Gastroenteritis), Vibrio cholera (Cholera),
Salmonella typhi (Typhoid fever)
Protozoa
Single-celled organism, genetic
material enclosed in nuclear
membrane. Described as
microfauna. Often feed on bacteria,
algae, and other microorganisms.
Giardia lamblia (Giardiasis), Cryptosporidium
(Cryptosporidiosis), Entamoeba histolytica
(amoebic dysentery)
Virus
Infectious agent consisting
(structurally) of either DNA or RNA
covered in a protein coat.
Hepatitis A (infectious hepatitis), Rotavirus
(Gastroenteritis), Adenovirus (respiratory disease,
gastroenteritis)
Table 2: Description of various water classes in North Carolina (NCDWQ, Surface Waters and Wetlands
Standards, 2007)
Class of Water
C
SA
SB
SC
2
Description
Freshwater systems protected for secondary recreation (humans are only partially
immersed in water, as when kayaking or rafting), fishing, and the propagation and
survival of wildlife and aquatic life. All freshwater systems are classed at least as C to
protect these minimum uses.
Saltwater systems suitable for commercial fishing and all other tidal saltwater uses.
Saltwater systems protected for primary recreation (humans are completely immersed
in water, as when swimming and diving), fishing, and the propagation and survival of
wildlife and aquatic life.
Saltwater systems protected for fishing and the propagation and survival of wildlife and
aquatic life. All saltwater systems are classed at least as SC to protect these minimum
uses.
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7KH86(QYLURQPHQWDO3URWHFWLRQ$JHQF\
(3$LVFXUUHQWO\SURPRWLQJWKHXVHRIHLWKHUE. coli
or enterococcus, rather than fecal coliform, as a bacterial indicator in fresh water because a stronger correlation between these two indicators and illness
Table 3: Water quality standards for indicator
bacteria in North Carolina (NCDWQ, Surface Waters
and Wetlands Standards, 2007)
Class of Water
Indicator
Geometric
Mean
(per 100 ml)
C
Fecal coliform
200
SA
Fecal coliform
14
SA
Enterococcus
35
SB
Enterococcus
35
SC
Enterococcus
35
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target pathogenic indicator bacteria concentrations for
HDFKFODVVRIZDWHULQ1RUWK&DUROLQD)RUDIXUWKHU
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SIGNIFICANCE OF PATHOGENS AS
POLLUTANTS
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river miles designated as impaired—either unable or
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WHAT IS A GEOMETRIC MEAN?
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arithmetic mean of a set of values; however, the
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working with indicator bacteria because somewhat
dramatic spikes and dips in bacteria measurements
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mean helps reduce the skew that such spikes can
have on the calculated value for a given sample
set and is considered a better metric for bacterial
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n = number of samples in the data set
merous streams throughout North Carolina are on the
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in the mountains and piedmont (the Catawba, French
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VWUHDPPLOHVLPSDLUHGE\IHFDOFROLIRUP7KHVHVWXGies illustrate the importance of understanding and
treating pathogen pollution throughout North CaroOLQD
PATHOGEN SOURCES AND PRESENCE IN
STORMWATER RUNOFF
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including through sewer pipe and septic tank leaks,
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into water bodies, and through fecal matter that is
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indicated that as the amount of impervious area in a
watershed increases, the amount of indicator bacteria
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Pathogen transport from urbanized areas to
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Extensive research has examined the impact of bacte3
rial pollution on ocean and estuarine environments,
where pathogens transported in stormwater and polluted streams can persist long enough to be consumed
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CAN WE TREAT PATHOGENS IN
STORMWATER?
Pathogens can be removed from or inactivated in surface waters and stormwater through several treatment
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WKULYHDQGVXUYLYH,QWROHUDQFHWRFHUWDLQHQYLURQPHQtal conditions such as high or low temperature and
pH, or predation from other microbes, can remove
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Figure 2: Waterfowl can produce substantial
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amounts of waste around urban ponds.
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can be sources of
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attract wildlife including deer, waterfowl,
rodents, and domestic
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around the BMPs,
resulting in direct
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research is being conGXFWHGDW1&6WDWH
8QLYHUVLW\DQGHOVHwhere to determine if
BMP designs can be
manipulated to provide or enhance treatment mechanisms and
environmental conditions that will stimulate
Figure 3: (a) Closed shellfish waters in North Carolina (b) Posted warning sign at
pathogen inactivation
North Carolina beach.
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4
Table 4: Description of various stormwater BMPs and theoretical pathogen removal mechanisms
BMP Type
Description
Treatment Mechanisms
Relevant to Pathogen
Removal
Dry detention basin
Fills during storm events, retains runoff for 1 to 2 days,
and then slowly, but completely, drains. Remains dry
between precipitation events. Primarily used for peak
flow mitigation
Drying, sun exposure,
sedimentation
Wet pond
Influent runoff theoretically replaces runoff captured
from previous events (plug flow). Retains runoff for
1 or 2 days, and then slowly drains. Maintains water
pool. Used for peak flow mitigation and some water
quality improvement.
Sun exposure,
sedimentation
Stormwater wetland
Fills during storm events, retains runoff for 1 or 2 days
as it slowly drains. Maintains water pool. Has shallower Sun exposure,
water and more vegetation than wet pond. Normally
sedimentation, some
used for water quality improvement, but can be used drying
for peak flow mitigation.
Sand filter
Runoff first enters a sedimentation chamber before
flowing through a column of soil. Sand chamber is dry
between events.
Drying, sedimentation,
filtration
Bioretention
Similar to sand filter, runoff enters system and passes
through a soil media, where it is filtered. May pond 6
to 12 inches. Primarily a water quality BMP. System is
dry between events.
Drying, sun exposure,
sedimentation, filtration
Grassed swales
Runoff flows through an engineered, grassed channel
used to convey it from one location to another.
Sedimentation, sun
exposure, drying
Proprietary devices
Use baffles, settling chambers, filtration, and other
means to separate floatable solids and promote
sedimentation. Primarily intended for water quality.
Varies based on
manufacturer: normally
sedimentation and
sometimes filtration
5
HOW EFFECTIVE ARE STORMWATER BMPS
IN REMOVING PATHOGENS?
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studied a wet pond receiving residential stormwater
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performance of the wet pond, relative to the wetland,
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was observed between fecal coliform concentrations
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concentrations also include samples taken during base
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Most of the BMP data associated with pathogen removal is available in a database format through
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6
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more research is needed to help communities throughout the United States reach their pathogen reduction
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RECENT RESEARCH AT N.C. STATE
Charlotte-Mecklenburg Stormwater Services conductHGDVWXG\ZLWKWKH1&6WDWH8QLYHUVLW\%LRORJLFDO
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than six samples—were collected from nine of these
BMPs to begin to evaluate their performance for both
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degree for E. coli2QO\WKHWZRZHWODQGVDQGWKH
ELRUHWHQWLRQDUHDKDGVWDWLVWLFDOO\VLJQL¿FDQWUHGXFWLRQV'HVSLWHWKHVHREVHUYHGUHGXFWLRQVLQLQGLFDtor bacteria concentration, reducing urban runoff to
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the data collected from the BMPs in Charlotte, NC,
VKRZHGRQO\RQH%03SURGXFHGHIÀXHQWFRQFHQWUDtions of fecal coliform below a geometric mean of
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nutrients, and aids in oxidation-reduction reactions
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Carolina, with a number of stormwater BMPs being
HYDOXDWHGIRUSDWKRJHQLFLQGLFDWRUEDFWHULDUHPRYDO
Table 5: Fecal concentration reduction efficiency for BMPs in Charlotte, NC.
BMP Type
Geometric
Geometric
Efficiency
Mean Influent Mean Effluent Fecal Coliform
(per 100ml)
(per 100ml)
(%)
% of effluent
samples
under 200
(per 100 ml)
Dry Detention 1
1985
2873
-31
0
Dry Detention 2
1327
1590
-211
0
Wet Pond
9033
2703
57
7
Wetland 1
9560
184
992
56
Wetland 2
8724
3874
702
13
Bioretention
2420
258
692
74
Proprietary 1
667
277
77
43
Proprietary 2
235
368
-1691
50
Proprietary 3
1472
2379
-3811
0
1: Negative values indicate an increase in concentration
2: Significant reduction between the influent and the effluent
Table 6: Relative pathogen removal capabilities of various stormwater BMPs
Proposed Fecal Coliform
Removal Ability
BMP
Dry extended detention basin
Medium
Wet detention basin (wet pond)
Medium
Stormwater wetlands
Medium
Sand filter
High
Bioretention
High
Grassed swale
Low
Proprietary devices1
Likely varies based on design
(NCDENR – Stormwater Best Management Practices Manual, 2007)
1
Category added to the table by authors
7
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effective current stormwater BMP designs are for
pathogen removal, but also what design features can
be manipulated to enhance pathogen removal and
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RESOURCES
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Figure 4: Substantial sun exposure at Wetland 1
in Charlotte, NC, possibly leading to low effluent
concentrations of pathogen indicator species.
WHAT’S THE VERDICT?
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however, some performance assumptions can be
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microorganisms are sequestered and killed in the
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RI:DWHU4XDOLW\¶V2007 Stormwater BMP Manual
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and are presented for a selected group of BMPs in
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2007 Stormwater BMP Manual
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from animals due to their lack of exposed standing
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Prepared by
Jon M. Hathaway, E.I. - Graduate Research Assistant
William F. Hunt, Assistant Professor and Extension Specialist
Published by
North Carolina Cooperative Extension Service
NC STATE UNIVERSITY
AGW-588-16W
10/08—VB/S3
E09 51807
Distributed in furtherance of the acts of Congress of May 8 and June 30, 1914. North Carolina State University and North Carolina A&T State University commit themselves to positive
action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, veteran status or disability. In addition, the two Universities welcome all persons
without regard to sexual orientation. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
10