EPA 600/R-13/208 I September 2013 I www.epa.gov/ada
United States
Environmental Protection
              A Meta-Analysis of Phosphorous
              Attenuation in Best Management
              Practices (BMP) and Low Impact
              Development (LID) Practices in
              Urban and Agricultural Areas
Office of Research and Development
National Risk Management Research Laboratory, Ada, Oklahi


                                                               September 2013
               A                        of
                                 Council, US

                                                  Diwision,      OK

                                      Lab, Ground
                           Division,      OK

               US                Protection
Office of Research and Development
National Risk Management Research Laboratory, Ada, Oklahoma 74820

               The U.S. Environmental Protection Agency through its Office of Research and Development funded
               the research described here. Mention of trade names or commercial products does not constitute
               endorsement or recommendation for use.

               All research projects making  conclusions or recommendations based on environmentally related
               measurements and information and funded by the Environmental Protection Agency are required to
               comply with the requirements of the Agency Quality Assurance Program. This project was conducted
               under an approved Quality Assurance Project Plan.

               This document has been reviewed in accordance with U.S. Environmental Protection Agency policy
               and approved for publication.
           Contact information:

               Shannon P. Schechter*

                 National Research Council, National  Risk Management Research Lab,  Ground Water and
                 Ecosystems Restoration Division, Ada,  OK 74820 USA, shannonp.schechter@gmail.com

                 'current position and address: Postdoctoral Researcher, Forest Pathology and Mycology Laboratory,
                 University of California-Berkeley, Berkeley, CA 94720.
               Timothy J. Canfield*

                 US Environmental Protection Agency, National Risk Management Research Lab, Ground Water
                 and Ecosystems Restoration Division, Ada, OK 74820 USA.

                 * Corresponding author: canfield.tim@epa.gov
               Paul M. Mayer

                 US Environmental Protection Agency, National Health and Environmental Effects Research Lab,
                 Western Ecology Division, Corvallis, OR 97333 USA. email:  mayer.paul@epa.gov
           Front cover photo information:

               Photo 1:  http://water, epa. gov/infrastructure/greeninfrastructure/giwhat, cfm
               Photo 2: Photo by Paul Mayer

Notice	ii
Abstract	iv
Introduction and Objectives	 1
    Phosphorous as a pollutant	 1
    Phosphorus attenuation in the Environment	1
    Phosphorus Biogeochemistry	2
    Overview of water quality regulation in the United States	3
    Mini-Review of P attenuation in urban BMPs and LIDs	5
    Mcta-analysis of Phosphorus Attenuation in BMPs and LID practices	6
       Purpose of a Meta-analysis	6
Materials and Methods	7
    Quality Metrics Used to Evaluate Data Inclusion	7
Results	8
Discussion  	  15
Future Research	  18
Conclusions	  19
Literature Cited	21
Table 1.    Studies utilized as data sources for the Phosphorus Meta-Analysis  	9
Table 2.    P removal effectiveness (%) of Low Impact Development (LID) and Best Management
           Practices (BMPs) 	11
Table 3.    Comparison of P removal effectiveness (%) of Low Impact Development (LID) practices
           based on phosphorus type measured	12

Figure 1.   Comparison of phosphorus source	13
Figure 2.   Relationship of P removal effectiveness  	14

While all living organisms require phosphorous to live and grow, adding too much phosphorus
to the environment can cause unintended and undesirable effects, such as eutrophication of
surface waters and harmful algal blooms. Urban and agricultural best management practice
(BMP) and low impact development (LID) are often employed to improve water quality because
of their ability to process and remove excess anthropogenic phosphorous (P) from surface and
ground waters. Urban and agricultural BMPs and LIDs are land development approaches that
attempt to mimic natural systems.  The efficiency  at which BMPs and LIDs remove P is not
clearly understood because data that generalizes patterns of P removal across ecosystems and
environmental conditions are not well synthesized.  Here, we use existing scientific literature to
conduct a meta-analysis to examine the capacity of various BMPs and LIDs to attenuate P. We
identify patterns that are intended to inform resource managers about the most effective approaches
for managing P. We found that P removal varies greatly among BMPs and local conditions such
as soil type.  We show the range of P removal effectiveness of a wide variety of BMPs and LIDs
and identify processes that are contributing to P attenuation.  We also describe an overview of the
development of current federal water quality regulations contained in the Federal Code that have
set the stage  for implementing BMPs or LIDs in the context of managing water quality.

                                               introduction  and  Objectives
The goal of this report is to synthesize the
existing scientific literature on the effectiveness
of best management practices (BMP) and low
impact development (LID) to improve water
quality through their ability to process and
remove excess anthropogenic phosphorous
(P) from surface and ground waters. In urban
settings, BMPs and LIDs are land development
approaches that attempt to mimic natural
systems in order to provide green space or
to manage stormwater in urban or suburban
environments (Passeport et al. 2013). In
agricultural settings BMPs are primarily
focused on incorporating natural features such
as grass strips, riparian areas and wetlands
to intercept runoff from the cultivated or
agricultural managed areas as a means of
attenuating anthropogenically derived nutrients
and sediments. Specific techniques include,
but are not limited to,  constructing wetlands,
green roofs, bioretention cells, planting
riparian zones, restoring streams, and installing
permeable pavement systems. BMPs and LIDs
often are employed as nutrient management
tools by resource management agencies by
designing features that are intended to decrease
the volume of stormwater runoff to drainage
systems and streams by intercepting water,
increasing infiltration, and/or disconnecting
impervious surfaces from conventional
stormwater networks.  Despite significant
research effort toward understanding the
ecological functions of BMPs and LIDs, there
remains no consensus for what constitutes
optimal design to achieve maximum P removal
effectiveness.  The objective of this report is
to identify patterns and trends of P attenuation
reported in the published literature in order
to provide guidance that will aid managers
in making decisions about implementing
BMPs and LIDs to better manage P as part of
comprehensive watershed management plans.
Phosphorous as a pollutant
While all living organisms require P to live and
grow, adding too much P to the environment
can cause unintended and undesirable effects
(Carpenter etal. 1998).  Eutrophication occurs
when excess P and/or nitrogen (N) is added to
aquatic systems which stimulates the growth
of algae which then die and decay, creating an
over abundance of decomposing bacteria that
consume the algae and, with it, the oxygen in
the water, causing low-oxygen dead zones that
suffocate aquatic life. Some of the algae that
bloom during eutrophication are themselves
toxic, producing harmful algal blooms (HABs)
that can directly kill fish in the water or even
livestock that drink the water.
Phosphorus attenuation in the Environment
There are two forms of P in the environment,
organic and inorganic. Organic P (e.g.,
polyphosphate and organophosphate) is
found as plant and animal biomass, metabolic
waste (including  sewage), and in pesticides
(Carpenter et al. 1998).  The main form of
inorganic P is orthophosphate (PO4~3), a term
used interchangeably with "phosphate" and
with "reactive P", referring to the form of
phosphorous that can be used directly by
plants and microorganisms (USEPA2012).
Orthophosphate in the environment is derived
from phosphate minerals (e.g. apatite minerals),
fertilizers, detergents, and industrial chemicals.
Stormwater runoff and soil erosion are the main
factors driving P transport. Phosphorus in the
form of Orthophosphate dissolved in stormwater
(dissolved phosphate) and P associated
with soil and/or organic matter (particulate
phosphate) can be transported offsite and cause
excess P to accumulate in the environment.
However, under certain agricultural settings
and biogeochemical conditions, Orthophosphate
can also be transported through the subsurface

into groundwater (Domagalski and Johnson
2012). Particulate phosphate (PP) accounts
for 75 to 90% of phosphate transported
from cultivated land (Randall et al. 1998).
Particulate phosphate is not immediately
bioavailable but may become a source of
orthophosphate that can dissolve when soil
solution P levels are depleted.  Dissolved
phosphate (DP) is bioavailable and therefore,
has the most immediate impact on aquatic
systems. Phosphorus concentrations and loading
are often monitored as total phosphorus (TP),
a measure of both dissolved and particulate
phosphate (USEPA2012).
P attenuation refers to the reduction in P
concentration in water and soil through
chemical and biological processes. Soil
mineralogy and pH are important factors
determining P attenuation in soil.  Many soils
bind tightly to large quantities of P, exchanging
reactive soluble, forms for paniculate, less
bioavailable forms (Bonn et al.  1985). The
chemical binding of P to soil particles (P
sorption) occurs in the soil through fast
and slow soil chemical reactions:  1. Fast
reactions (about one day) include absorption
and substitution between P and  other anions
(negatively charged molecules) on mineral
surfaces; and 2. Slow reactions  (several weeks
or longer) include a complex combination of
mineral dissolution and precipitation reactions
between P and cations on the surface and
within the inner-sphere of soil particles (Bonn
et al. 1985).  Phosphate reacts with Ca and
Mg minerals as well as Al, Fe, and Mn oxide
compounds in soil depending on soil pH (Bohn
et al. 1985).  At low pH, P forms poorly soluble
Fe and Al compounds, at near neutral pH, P
forms more soluble Ca and Mg compounds,
and at higher pH, P forms poorly soluble Ca
compounds (Bohn et al. 1985).  Phosphate is
most soluble in slightly acid to neutral pH
soils.  Under reducing conditions, P-Fe oxide
compounds may dissolve, thereby releasing P
(Denver etal. 2010).
Once soil P has reached its sorption capacity,
excess P will dissolve, and then, can potentially
be exported in water (Domagalski and
Johnson 2012, Lucas and Greenway 2011).
Phosphorus retention in BMPs is regulated
by the equilibrium P concentration (EPC),
the concentration at which P sorption equals
desorption (Hoffmann et al. 2009). The
EPC can dictate the soluble P concentration
supported by soils (Indiati and Sharpley 1998).
For instance, if the P concentration of the water
entering is higher than EPC, soils will sorb P
and be a P-sink. However, P will be released
from soils if the P concentration entering
is lower than EPC (Hoffmann et al.  2009).
Mineralogy also influences EPC. EPC declines
as the ratio of Al-oxides to Fe-oxides increases
(Lucas and Greenway 2011).
Biological processes also play a role in
P attenuation and release. Phosphorus is
attenuated in soil through biological uptake.
Bacteria, fungi, algae, and plants incorporate
P into biomass (e.g., P is on average, 0.2%
of plant dry weight (Bohn et al. 1985).
However, plants vary in P demand and uptake
effectiveness.  Plants employ varied strategies
to obtain the amount of P that they  need to
grow and thrive.  Some of these strategies are:
expansion of root network or root type to reach
additional sources of P, chemical releases from
the roots that increase solubility of soil bound
P thus enhancing uptake, symbiotic interactions
with fungi, bacteria, or other plants  to provide
bioavailable P in their root structure, and/or
changing the way P is utilized (i.e. internal
recycling of P, reduction of P loss from plant
cells) to more efficiently recycle P when soil
P is limited (Shen et al. 2011).   Conversely,
P is released from  organic matter through
decomposition. Decomposition rates depend
on pH, litter quality (C:P:N), Ca content,
redox potential, soil moisture, and temperature
(Hoffmann et al. 2009).

Overview of       quality            In the

The United States Federal Government has
been developing water quality regulations for
the last 130 years. The first federal legislation
was enacted by Congress in 1886 with the
development of the River and Harbor Act and
was subsequently re-codified in the Rivers and
Harbors Act of 1899 (33USC407 1899)  and
represents the oldest Federal environmental law
in the United States. Under this Act, the Federal
Government gained authority to monitor,
manage and regulate actions on the nation's
rivers that would impact navigation. Under
this Act, it became a misdemeanor to discharge
refuse of any kind into navigable waters and
tributaries of the United States without a permit.
This section is also known as the Refuse
Act which focused primarily on regulating
impediments to navigation though the Act
served indirectly to reduce water pollution.
Over the next half century, over 100 bills
were brought forth in an attempt to address
water pollution, but none of these bills
were adopted. By 1948, urban growth and
expanding industrialization brought on by
World War II had created a situation where the
amount and effects of uncontrolled pollution
discharges were becoming problematic.
The first legislation enacted by Congress to
specifically empower the Federal Government
to regulate water quality was  passed in 1948
in the form of the Federal Water Pollution
Control Act (FWPCA) (33U.S.C.1251-1376
1948). Although a step forward, this Act did
not achieve the desired water quality goals
because many legislators held that pollution
control in water bodies was a responsibility
of the States. Congress stated that the Act's
purpose was to "provide a comprehensive
program for preventing, abating and controlling
water pollution". Congress reaffirmed that
this policy was "to recognize, preserve, and
protect the primary responsibilities and rights
of States in controlling water pollution". As
part of the compromise to get this bill passed,
the Act requires the federal government to work
cooperatively with states to develop plans to
address water pollution.  The Act relegated the
federal authority to prepare pollution abatement
plans and provide support to the states. The
law did not specifically limit new sources
of pollution, prohibit activities that caused
pollution, or set standards to regulate pollutants
from entering water bodies. This approach
limited the enforcement authority of the federal
Over the next 14 years the FWPCA was
amended six times. The  1956 amendment
strengthens the federal authority to regulate
pollution by no longer requiring States'
consensus in order for the federal government
to take actions to prevent and address pollution.
The 1961 amendment gave authority to the
Secretary of Health,  Education and Welfare
to develop research programs to evaluate the
effects of pollution, identify potential treatment
methods and evaluate water quality in the Great
Lakes. Also, at the request of the States, the
federal government was authorized to take steps
to prevent pollution in navigable or interstate
waters of the United States. In  1965, the
FWPCA was amended to expand the federal
role in pollution control by authorizing the
development and establishment of water quality
standards. The Clean Water Restoration Act of
1966 expanded on the previous amendments
by establishing the federal government's
authority to fine polluters for not filling out
required reports, thus putting some "teeth"
into the application of the FWPCA. With the
development of standards in place, The Water
Quality Improvement Act of 1970 expanded
the federal role once again by establishing a
State certification process to prevent water
degredation below water quality standards.
This Act requires that all States develop and
implement a certification process for water
quality.  The federal  government took on greater
authority and thus responsibility for overseeing
the Nation's streams and rivers.  Along with
this Act, the Environmental Protection Agency

(EPA) was created to become the lead federal
agency responsible for the oversight and
protection of the Nation's waters. With the
creation of the EPA, the Federal Water Quality
Administration, a part of the Department of
Interior, was dissolved and all its functions,
responsibilities and authorities were transferred
to the EPA.
The FWPCA was amended in 1972 to include
language stating that this Act was to "restore
and maintain the chemical, physical and
biological integrity of the nation's waters."
The goal of this amendment was to provide
water quality sufficient to protect fish,
shellfish, wildlife and recreation ("fishable
and swimmable"), eliminate discharge of large
amounts of toxic substances into water, and
eliminate additional pollutants into navigable
waters of the US by 1985.  This amendment
made it illegal to discharge pollutants  from
a point source into waters of the US without
a permit. The establishment of the National
Pollutant Discharge Elimination System
(NPDES) permit program made it mandatory
for every point source discharger to obtain
a discharge permit. It also required EPA to
develop and implement technology-based
effluent limitations into the NPDES permits.
These amendments expanded the emphasis on
water quality standards, made them applicable
to interstate waters and required the permits to
be consistent with state water quality standards.
Additionally these amendments assigned
authority to the Army Corps of Engineers to
issue permits to dredge and fill in navigable
waters.  With the 1972 amendments, the
FWPCA represented a significant expansion of
the 1948 Act and became known from that point
on as the Clean Water ACT (CWA).  Contained
within this amendment were the first efforts
to evaluate the extent of non-point  source
pollution although little actual efforts were
expended to implement any controls on non-
point source pollutants.
The Water Quality Act of 1987 resulted in
the adoption of new provisions for water
quality standards in the CWA. One of the
biggest concerns of Congress was that the
States were relying by and large on narrative
criteria to control toxics which left the actual
amounts of toxics that could be discharged
very non descript. To address this issue,
Congress included section 303(c) (2) (B) in the
amendments which required the development
of numeric criteria for the discharge of toxic
pollutants where they were likely to negatively
affect the designated uses of those water
bodies. These standards were still primarily for
point source discharges and the development
and adoption of these standards continues
to be a long and arduous process. Still, the
development of standards for non-point
discharges was not made a priority focus and
has languished far behind the development of
standards for point source discharges.
Nonpoint source (NPS) water pollution
regulations attempt to restrict and limit the
amount of pollution entering the Nation's water
bodies from diffuse effluent sources, primarily
overland runoff and sub surface  seepages from
contaminated sources. NPS pollution originates
primarily from urban/suburban or agricultural
sources. NPS pollution may contain heavy
metals, pathogens, nutrients, sediments, and
organic contaminants. Addressing NPS is
costly and difficult, as the origins of NPS
pollution is often difficult to identify. Congress
included section 208 of the CWA as a way to
address the NPS problem (Szalay 201.0).  This
section directed States and local governments
to create management plans that would identify
future waste treatment needs and also identify
and control NPS pollution of water. This effort
was focused at controlling both urban and
agricultural derived NPS pollution.  Currently
33 of the 50 States have water quality standards
for nitrate, but not phosphorus for potable or
drinking water sources as reported to EPA in
the  State Numeric Criteria Reports (USEPA
2013b).   Additionally, there are  ambient
water quality criteria recommendations in
place for lakes, reservoirs, rivers and streams

of the US, with these criteria developed and
applicable on a nutrient ecoregional distribution
(USEPA2013a). Section 208 did not provide
an enforcement provision and primarily relied
on federally funding these efforts in the hopes
of getting the States to develop the control of
NFS pollution.  Lack of funding curtailed the
section's objective.  As part of the 1987 Water
Quality Act, Congress included a section 319
directing States to identify waters that cannot
meet water quality standards without control of
NFS pollution.  States must then develop BMPs
designed to address these sources of impairment
and an implementation plan to execute these
practices (33U.S.C.1329 1948) (33U.S.C.1329
1948). States have the autonomy to choose
from among various pollution control practices
to remediate polluted waters. States have
primarily taken the approach of adopting BMPs
in both urban and agricultural settings.  Low
Impact Development (LID) often has been
incorporated as part of the efforts to address the
NFS pollution problem in the context of urban
and suburban growth.  It is these BMP and
LID efforts under the broader umbrella of an
approach called Green Infrastructure (GI) that
represents the bulk of the efforts to control NFS
Mini-Review of P attenuation in urban
       and LIDs
A goal of Low Impact Development and Best
Management Practice (LID/BMP) design
is to alter hydrology of a site in order to
reduce stormwater runoff (peak and volume),
increase infiltration, groundwater recharge,
protect streams, and/or remove pollutants to
enhance water quality (Ahiablame et al. 2012).
Examples of structural LID/BMP practices
include: bioretention (rain garden), infiltration
wells/trenches, stormwater wetlands, wet
ponds, level spreaders, permeable pavements,
swales, green roofs, vegetated filter/buffer
strips, sand filters, smaller culverts,  and
water harvesting systems such as rain barrels/
cisterns (Passeport et al. 2013). These practices
promote infiltration, water residence, and
increase subsequent pollutant biodegradation
(Ahiablame et al. 2012). In contrast,
conventional stormwater management systems
route water offsite as fast as possible through
conveyance structures that do not allow time for
attenuation of pollutants like P (e.g., pipes and
concrete channels, (Ahiablame et al. 2012).
Several LID/BMP practices promote processes
that may improve P attenuation. Designs
that sustain physical (settling and filtration),
physicochemical (adsorption, precipitation, and
ion exchange), and biological (plant and algal
uptake) processes have the highest potential
for P removal (Scholes et al. 2008). Key to the
effectiveness of these processes on P removal
is efficient contact ratios between stormwater
and substrate/vegetation (Scholes et al. 2008).
Scholes et al.  (2008) suggested several BMP
characteristics that influence P removal: dry/wet
area volumes, stormwater retention times, flow-
attenuation, vegetation,  presence of sorption
sites and pore sizes of substrates, infiltration
potential, and aerobic/anaerobic conditions.
Ahiablame et al. (2012) recently reviewed the
effectiveness of LID practices on nutrient (N
or P) removal, reviewing 250 published studies
on bioretention, permeable pavements, green
roofs and swale systems. However Ahiablame
et al. (2012) only reported P removal data
from 9 studies which showed a range of -3 to
99% P removal effectiveness in bioretention,
permeable pavement, and swale systems
(Ahiablame et al. 2012). Only one of these
studies recorded a negative value, indicating
P release. In contrast, Ahiablame et al. (2012)
reported that green roofs did not retain any
significant amount of P, but instead were a
significant source of P. Ahiablame et al. (2012)
did not provide information about the features,
factors, or indicate processes that contributed
to those P removal ranges.  There is critical
need to identify mechanisms of P removal and
transformation LID/BMPs in order to better
guide P management.

              of Phosphorus Attenuation  in
       and LID
Purpose of a Meta-analysis
Meta-analysis is a powerful statistical tool
to summarize, synthesize, and evaluate
independent research studies in order to reach
general conclusions. Meta-analysis allows data
from multiple studies to be combined within
a rigorous statistical framework that provides
range and magnitude of effects, predictive
relationships among factors, and measures of
variability. Mayer et al. (2007) successfully
used meta-analysis to determine riparian buffer
characteristics associated with removal of
nitrogen and identified  considerations for future
research. Our goal was to perform a meta-
analysis of P attenuation in LID/BMPs in order
to help identify the factors contributing to P
attenuation and to provide ranges of P removal
effectiveness among these practices.

                                                                      and  Methods
In order to perform our meta-analysis, we
obtained data about P removal in LID/BMPs
from published literature. We performed
an extensive literature search using Web of
Knowledge, ScienceDirect, Google, Google
Scholar, PubMed, and Cambridge Abstracts.
We searched terms singly or in combination
including: bioretention, buffer strips, filter,
green roofs, permeable pavement, rain garden,
riparian, BMP, green infrastructure,  stormwater,
P, phosphate, urban, removal, and LID. We
limited search results to peer-reviewed studies
with original data describing P removal
effectiveness of LID/BMP practices. Papers
that did not specifically measure P influent and
effluent concentrations from LID/BMPs were
excluded from the study.
We created an Access Database and extracted
meta-data from the papers including study
location, LID/BMP, source of P, vegetative
cover type, P flow path distance, soil texture,
P type, P inflow and outflow concentrations,
and percent removal effectiveness ((inflow
concentration - outflow concentration/inflow
concentration) x  100). These data were used to
identify ranges of P removal effectiveness. We
also analyzed P removal  effectiveness based on
LID/BMP type, source of P, vegetative cover
type, P flow path distance, soil texture, and P
type measured (orthophosphate/phosphate or
total P) using a non-parametric test (Kruskal-
Wallis (K-W) one-way analysis of variance on
ranks ) with IMP v. 5.0.1 a and model fitting
with SkmaPlot 12.0.
Quality              to
The data used in the meta-analysis was
secondary data from the literature. No new-
data was generated in this project. To help
ensure data quality, we used data that was
from published papers that were subjected to
a peer review process as part of the journal
requirements. In an effort to evaluate/control
data quality of literature sources used in this
project, the papers must have used standard
analytical methods (i.e., methods published
and used by more than one author in a peer-
reviewed journal); sampling methods and
designs must include and identify P-inflow
and outflow concentrations, P type, and P
removal efficiency within that BMP/LID.
Works that were included in this effort were
from the primary literature and clearly stated
the analytical methods used to produce the
data, included pertinent project, site, and BMP
LID characteristics (location, BMP LID type,
width, soil texture, vegetation cover, flow path,
etc.).  To be included in the list of data sources,
the data contained in the papers must be
amenable to calculations of relative changes in
phosphorus associated with the  selected BMPs
or LIDs as described in the papers.  Ultimately
the data used in this report is the product of
the works of other researchers.  It is strongly
encouraged that the reader look at these works
first hand before any subsequent analysis of the
data or use of the data is done.

 We found 154 papers that included search
 terms but only 44 had information on P
 influent and effluent concentrations (Table 1).
 These 44 studies included 348 data examples
 of P removal effectiveness in biofilter,
 bioretention (including rain gardens), buffer
 strips, filter strips, filter systems, green
 roofs, permeable pavement systems, riparian
 buffers, and wetland BMPs. We noted that
 there is ambiguity throughout the literature
 regarding the definitions of buffer strips vs.
 riparian buffers, biofilters vs. bioretention,
 and other similar terms. Because there is no
 nomenclature guidance and because we did
 not want to misclassify or inadvertently lump
 BMP's or LID's, we took the original authors'
 classification  at face value and retained the
 original categories as listed. Data came from
 eight countries: Australia (21% of data), Canada
 (18%), Denmark (3%), New Zealand (0.8%),
 The Netherlands (0.8%), United Kingdom
 (4%), and USA (52%). USA data came from
 eleven states: Midwest (12%), Northeast
 (16%), Northwest (0.5%), Southeast (70%),
 and the Southwest (1%).  Phosphorus removal
 effectiveness  ranged from -488% to  100%
 across all studies, however only 18% of all of
 the records showed P release from BMPs (i.e., a
 negative percentage of P removal).
 The ranges of P removal effectiveness differed
 by LID/BMP type (Table 2). Phosphorus
 removal effectiveness differed among the 9
 LID/BMP types (Kruskal-Wallis (K-W) one-
 way analysis of variance on ranks, P < 0.0001).
 Tukey HSD mean comparison showed that
 mean P removal effectiveness of Biofilter (76%)
 was significantly higher than Bioretention
 (18%), Buffer Strips (5%), Riparian Buffers
 (-2%), and Green Roofs (-20%).  In addition, P
 removal in Filter Strips (47%) was significantly
 higher than Buffer Strips (5%), but no
 differences were found between any other
 practice types.
The percentage of data showing P release also
varied by LID/BMP type: Biofilter (0% showed
P release), Bioretention (23%), Buffer Strips
(30%), Filter (0%), Filter Strips (12%), Green
Roofs (100%), Permeable Pavement (0%),
Riparian Buffer (30%), Wetland (38%).  The
variability was large across studies (Table 2).
Highly engineered/controlled practices showed
the lowest variability: Biofilter (CV=0.21),
Filter (CV=0.24), and Permeable Pavement
(CV=0.54); while practices that utilized and/or
established vegetated strips showed the highest
variability: Buffer Strips (CV=13.23), Riparian
Buffer (CV=64.48).
Studies presented P removal effectiveness
as either total phosphorus (DP + PP) or
Orthophosphate (DP).  There was no difference
in P removal effectiveness across all studies by
P type (K-W, P = 0.85). However, P type (TP
or DP) had a significant effect on P removal
effectiveness in Biofilters, Filter Strips, and
Permeable Pavement. For Biofilter (K-W, P =
0.01) and Filter Strips (K-W, P = 0.001), mean
P removal effectiveness was significantly higher
for TP, and for Permeable Pavement (K-W,
P<0.0001), mean P removal was significantly
higher for Orthophosphate (Ortho-P, Table 3).
We also analyzed the fate and removal of
P depending on source of P as either from
stormwater (including natural concentrations
from overland runoff) or from P concentrations
prepared in the laboratory (synthetic).   Across
all mean P, removal effectiveness was
significantly higher when P entered the BMP
as a synthetic solution than as stormwater
(52% versus 9% mean P removal effectiveness;
K-W, P O.OOOl). Among LID/BMP types,
bioretention was most effective at removing
P entering as synthetic solution (K-W, P =
0.0025), although student-t mean comparison
was not significant (Figure 1).  Phosphorus
removal in Filter Strips differed between

Table 1.   Studies utilized as data sources for the phosphorus meta-analysis
Abu-Zreig et al.
Barrett et al.
Beecham et al.
Berretta and
Bratieres et al.
Carpenter, et al.
Chapman and
Homer (20 10)
Chaubey, et al.
Chaubey, et al.
Davis et al.
Davis et al.
Davis (2007)
DeBusk and
Deletic and
Fletcher (2006)
Dietz and
Clausen (2005)
Dillaha et al.
Hathaway et al.
Hatt et al. (2007)
Hatt et al. (2009)
Heinen et al.
Hoffmann et al.
Hunt ct al. 2006)
Hunt et al. 2008)
Istenic et al.
























Kandasamy et al.
Kohler et al.
Lee and Dunton
Lowrance and
Sheridan (2005)
Lucas and
Greenway (2011)
Luell et al.
Mankin et al.
McKergow et al.
Mothersill et al.
O'Neill and
Davis (2012 a,b)
Parsons cl al.
Passeport et al.
Schellinger and
Clausen (1992)
Schmitt cl al.
Sheppard et al.
Srivastava et al.
Tota-Maharaj et
al. (2010)
Wilcock et al.
Winston et al.
Yorig et al.























Table 2.   P removal effectiveness (%) of Low Impact Development (LID) and Best Management Practices
Practice Type
Buffer Strips
Filter Strips
Green Roofs
Riparian Buffer






Table 3.   Comparison of P removal effectiveness (%) of Low Impact Development (LID) and best man-
          agement practices (BMPs) based on phosphorus type measured.
Practice Type


Buffer Strips


Filter Strips

Green Roofs


Riparian Buffer


Total Count

Ortho P
Total P

Ortho P
Total P

Ortho P
Total P

Ortho P
Total P

Ortho P
Total P

Total P

Ortho P
Total P

Ortho P
Total P

Total P








o *j




















































stormwater and synthetic sources (K-W,
P = 0.06, synthetic 53% versus stormwater
24%) (Figure 1).
The mean influent concentration entering
BMPs differed between those using synthetic
(4.59 mg/L) solutions and those using
stormwater (0.701 mg/L).  Phosphorus influent
concentration explained a small but significant
portion of the variance in BMP P removal
effectiveness (R2 = 0.03, P = 0.02, N = 168;
Model y = axb). That is, P removal effectiveness
tended to increase with increased P influent
concentration.  There was an even stronger
relationship between P influent concentration
and P removal effectiveness for Permeable
Pavement (R2=0.82, P <0.001, N = 33; Model
y = axb), and Filter Strips (R2=0.22, P =0.03,
N = 21; Model y = axb (Figure 2). The model
was chosen based on Mayer et al (2007).
We recorded the distance that influent P
flowed to the outlet as P path distance.  This
is height, length, or width between input
source and outlet collection port reported by
author.  Phosphorus path distance explained a
significant portion of the variance in Permeable
Pavement P removal effectiveness (R2 = 0.90,
P < 0.0001, N = 27; Model y = axb), but not for
any other practice. For Permeable Pavement, P
influent concentration and P path distance was
significantly correlated  (R2 = 0.78, P < 0.0001,
N = 27; Model y = y(} + ax).
Soil texture also influenced P removal
effectiveness. Across all practices, soil
texture had a significant effect on P removal
effectiveness (K-W, P = < 0.0001), but Tukey
HSD mean comparisons showed only silt
loam (52% mean P removal effectiveness)
as significantly higher than clay soil texture
   7o) and no other significant differences
among soil texture means. When looking at
individual practices separately only Filter Strips
P removal effectiveness showed a response
to soil texture (K-W, P = 0.012).  Tukey HSD
mean comparisons showed that silty clay loam
                         (63% mean P removal effectiveness) and silt
                         loam (53%),  had significantly higher P removal
                         effectiveness than loam soil texture (-39%) in
                         filter strips.
                         Vegetation cover type showed no effect on
                         P removal effectiveness across all LID/BMP
                         types.  (K-W, P = 0.115). Furthermore,
                         vegetation type had no effect within any LID/
                         BMP type.
                     100 -
                      o -
                       0 -
                      100 -
                       50 -
                     -100 -
                     -150 -

                                 0       20       40       80

Figure 2.    Relationship of P removal effectiveness (%) to P influent concentration in A. Permeable Pave-
            ment, and B. Filter Strips. Lines are fitted to model y = ax". Fit to model is significant (P < 0.05),

This report is the first meta-analysis of P
removal effectiveness of urban LIDs and
BMPs that identifies factors and processes
that affect P attenuation. We report P removal
effectiveness from a larger variety of LID and
BMPs and larger number of studies than recent
reviews (Ahiablame et al. 2012). However, the
limited sample size of some LID/BMP types,
large variability across studies, and limited
geographic distribution restricts  interpretation
to broad patterns and general processes. Our
goal was to show the range of P  removal
effectiveness of LID/BMPs and  identify
processes that are contributing to P attenuation
and/or P release.  We found that P type (TP
or DP), P source (synthetic or storm water), P
influent concentration, P path distance, and soil
texture influenced P removal effectiveness.
Unlike Ahiablame et al. (2012) who only
reported P ranges from nine papers, we found
that most LID and BMPs show both substantial
P attenuation and P release. The ranges of P
removal effectiveness were large with high
variability. Single practices (e.g., bioretention)
employed a wide range of designs with
different dimensions, media characteristics and
vegetation types, as well as different conditions
including P type, P source, and P influent
concentrations.  Practices that were specifically
designed to adsorb and filter pollutants
(Permeable Pavement, Filters, and Biofilters)
are the only practices in the meta-analysis that
attenuated P without showing any P release and
had lower removal variability among studies.
The P removal effectiveness differed depending
on the type of P measured. Total P removal was
higher in Biofilter and Filter Strips than DP.
Phosphorus removal in Permeable Pavements
was significantly higher for DP than TP,
suggesting the relative importance of different
processes in these practices such as filtration
and settling for Biofilter and Filter Strips and
absorption for Permeable Pavement (Scholes
et al. 2008).  Our analysis suggests that the
type of P measured may dramatically alter the
interpretation of the effectiveness of P removal
reported for BMP/LID practices.
Phosphorus source and influent P concentration
entering the BMP/LID practices affected overall
P removal effectiveness. Phosphorus entering
as stormwater resulted in lower P removal
effectiveness  than P entering as synthetic
solutions. The main difference between P
sources is the P influent concentration; the
mean P influent concentration of synthetic
P solutions was over 6 times larger than the
mean P concentrations added as stormwater
in these studies. Therefore, it is likely that the
effect of P source is a function of P influent
concentration. We found a significant positive
relationship between influent P concentration
and P removal effectiveness, a result consistent
with other studies. In a study of filter materials,
Cucarella and Renman (2009) also found
that P removal effectiveness increased with
higher initial  P concentrations, with maximum
P sorption occurring at the highest initial P
concentration. Rosenquist et al. (2010) found
that P removal is dependent on and directly
correlated with the concentration  gradient
present between solution and adsorbed P. They
suggested that P removal during a given event
is likely dependent on previous P loadings of
the media and concentration of P  influent. Their
research predicted several potential P removal
outcomes related to P concentration gradient:
1) Less P removal may occur for lower
influent concentration than for higher influent
concentrations; 2) For equivalent influent
P concentrations, P removal in a BMP will
likely decrease with fewer available sorption
sites; 3) BMP substrates will likely slowly

gain additional concentration gradient after P
diffuses into media micropores; 4) Phosphorus
removal effectiveness may be increased by
increasing influent concentration, through the
addition of more sorption sites, or by harvesting
P from substrate.
Phosphorus content of the media/soil may also
affect P removal effectiveness. Several studies
showed that the initial P content of the BMP/
LID practice media is critical to P removal
performance (Davis et al. 2009, Hunt et al.
2006, McKergow et al. 2006). These studies
looked at media P retention index (the ratio of
P adsorbed in the solution to the concentration
of P remaining in solution at equilibrium)
of soil to explain differential P removal
performance. Hunt et al. (2006)  stated that a
high P index in bioretention media (indicating
that the media was saturated with P) was the
reason that the BMP was unable to sorb P from
stormwater. However, out of the 44 papers that
we reviewed, only three included a measure of
P index.
Phosphorus path distance was not a clear
indicator of P removal effectiveness in this
meta-analysis. While hydraulic pathways
of P influent within BMP/LID practices are
important to increase contact time between
influent and substrate and therefore absorption,
settling, and filtration processes (Scholes et
al. 2008), the distance between influent source
and outlet sampling port (P path distance)
did  not influence P removal effectiveness.
The exception is in Permeable Pavement, but
in this case P path distance was correlated
with P influent concentration confounding
the  result. Phosphorus path distance alone
may not properly indicate dry and wet area/
volumes stormwater retention and drain down
times, or hydraulics/flow attenuation processes
important for P retention (Scholes et al. 2008).
Phosphorus retention may increase with riparian
buffer width due to longer transport pathways
that allow more time for retention or dilution
(Schmitt et al., 1999), however, sediment
removal efficiency (and, therefore, particulate
P removal) is dependent upon slope; slopes
greater than  10% result in decreased P retention
(Zhang et al. 2010).  In a review of riparian
buffer characteristics on P removal, Zhang et al.
(2010) found a positive curvilinear (asymptotic)
relationship of P removal  efficiency (%) with
riparian buffer width and that, about 35% of
the variance  in efficiency  depends on width
alone. Nearly 100% of P  is removed in buffers
>20 m wide  (Zhang  et al.  2010). Sheppard et
al. (2006) suggested vegetated buffer strips
be 10 to 90 m wide,  and Davis et al. (2009)
suggested that bioretention media depth be 0.75
m in order to optimize P removal.  Similarly,
wider buffers more efficiently remove nitrogen
(Mayer et al. 2007; Zhang et al. 2010), but
unlike P, ground water flow paths dictate
efficient nitrogen removal (Mayer et al. 2007).
Although P may be efficiently retained in buffer
zones, remobilization of dissolved reactive P
may occur thereby creating source zones for P
depending on the degree of P saturation, soil
type, and size of buffer area compared to the
source area  (Dillaha et al., 1989; Lee et al.
1989; Uusi-Kamppa, 2005;).
Soil texture affected P removal effectiveness.
For instance, loamy  sand, sandy loam, and loam
soil textures  are recommended in bioretention
specifications in order to allow high infiltration
rates and because  soils with clay content >30%
can lead to failure of the BMP (Davis et al.
2009).  Our meta-analysis also found that
media with a coarser texture had higher mean
P removal effectiveness compared to finer
clay materials.  Others have shown that clay
materials may provide more P-sorption sites but
coarser materials may provide better hydraulic
conditions to support absorption, settling, and
filtration processes (Hoffmann et al. 2009,
Scholes et al. 2008). In a review of P removal,
Zhang et al. (2010) did not find an effect of
soil type on P; however, evidence from others
showed higher retention of total P and dissolved
P in sandy soils than in silty clay soils (Magette
et al., 1989;  Schwer  and Clausen, 1989).
Vegetation cover type played no role

in P removal effectiveness. Our meta-
analysis showed no difference on P removal
effectiveness among BMP/LID practices that
had grass, forbs, trees or were bare. Hoffmann
et al. (2009) and Zhang et al.  (2010) reported
better P retention with trees or shrubs compared
to grass.  However, some authors suggest that
the benefit of vegetation may be in the increased
infiltration and sedimentation due to improved
soil structure and soil permeability related to
plant roots (Davis et al. 2009, Sheppard et al.
2006). Lucas and Greenway (2011) found that
P retention by barren media eventually becomes
exhausted due to long-term exposure of P, but
vegetation delays P saturation by extending P
sorption capacity.

 Phosphorus removal effectiveness is determined
 by design features that support effective
 absorption, filtration, settling, and biological
 processes (Scholes et al. 2008). However,
 most studies do not record the meta-data that
 will help identify which processes are at work
 and design parameters that can improve P
 removal effectiveness.  For instance, P removal
 effectiveness of a material is closely related to
 material Al, Fe, Ca content and pH (Cucarella
 and Renman 2009). However, only four authors
 included in our study reported any information
 about mineralogy and only six authors reported
 pH. Reporting essential information about
 mineralogy and pH will allow for better
estimates of P removal effectiveness. We
support the Davis et al. (2009) conclusion that
BMP/LID research needs to clearly identify fill
media composition, media depth and geometry
(perimeter area, surface area, media volume,
and perimeter area to surface area ratio);
drainage configuration, and vegetation rooting
types and depths. In addition, reporting local
hydrology,  such as magnitude and duration of
storms/flooding, residence times, and sediment
deposition rates (Hoffmann et al. 2009) are
critical to understand P removal processes in

BMP's and LID's show varying effectiveness
at removing P. Our data show that there
is no single best practice but rather a suite
of practices that may work better under
one circumstance or another.  Multiple
considerations need to be taken into account
prior to selection of BMP/LID approach such
as the form of P in the water stream, source
of P, soil texture, slope, and available area for
BMP/LID placement. The presence of other
stressors may impact the effectiveness of some
approaches.  For example, N removal and
P removal may at times be at odds with one
another because conditions (e.g. low dissolved
oxygen, reducing conditions) that are prime
for fostering denitrification, a natural microbial
process that consumes nitrate nitrogen, may
lead to conditions that cause an increase in P
flux. Another consideration is the potential
for a BMP/LID to provide stacked benefits
in addition to P removal such as flood and
sediment control, increased water infiltration, or
increased aesthetics. Resource managers may
need to weigh trade-offs in the efficiency of a
practice to remove P with the efficacy of that
practice to provide other benefits.
Costs associated with various BMPs/LIDs were
not considered here but, of course, drive many
resource management decisions.  A thorough
cost-benefit analysis would further improve the
decision making process for selecting effective
practices.  While engineered approaches may
be effective and demonstrate lower variability
in ranges of effectiveness, maintaining and
protecting existing natural buffers, riparian
zone, and wetlands may be far cheaper in the
long-run than constructing BMPs/LIDs that
may or may not emulate those natural features.
Furthermore, the longevity of engineered
practices has not been assessed. The costs of
practices must be amortized over the expected
functional life of the BMP/LID.
Improved understanding of the importance
of historic land use practices is emerging as a
key to quantifying current impacts from P and
identifying the most effective means to mitigate
P in water runoff.  For example, colonial-era
water mill construction affects current P loads
to streams in the mid-Atlantic because of the
vast deposits of P-laden legacy sediments now-
eroding from floodplains to downstream water
bodies and estuaries including the Chesapeake
Bay (Walter and Merritts 2008.  In such cases,
removal of sediments as a source of P may
be an effective management and restoration
practice (Hartranft et al., 2011; Merritts et al.,
Further research is necessary to explain the
considerable variability in the performance of
BMP/LID practices (see above) and to model
watershed-scale removal rates. Research
designed to specifically fill gaps about
effectiveness of various BMP/LID approaches
will help to facilitate better decisions on which
practices should be used and where. For
example, we know of no studies that have
examined the implementation of multiple BMPs
or LIDs in tandem to determine if there may be
positive synergistic effects of certain practices.
Also, we know of no studies that have
examined that possibility that, while continuing
to be effective at retaining P, some BMPs
may simultaneously become sources of other
pollutants of concern such as  heavy metals, e.g.,
bioretention ponds near roads accumulating
copper residue from automobiles.
While P reduction is an objective for restoring
many impaired waters, globally the supplies of
P are limited and acute shortages are predicted
for the future (Elser and Bennett 2011). It
eventually may be necessary (and conceivably
profitable) to implement certain BMPs/LIDs to
capture and recycle P.  Long term solutions to

controlling excess P where it causes negative
impacts while maintaining strategic reserves
of this necessary nutrient will likely require
a comprehensive approach including source
control, improved distribution systems, land use
management, appropriate BMP/LID practices,
and functional policy.

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