APPROVAL SHEET
Title of Thesis: The Effectiveness of Street Sweeping and Bioretention in Reducing
Pollutants in Stormwater Name of Candidate: Catherine J. DiBlasi Master of Science in Civil Engineering, 2008 Thesis and Abstract Approved: ________________________________________ Dr. Upal Ghosh Associate Professor Department of Civil and Environmental Engineering Date Approved: ___________
CURRICULUM VITAE
Name: Catherine J. DiBlasi
Permanent Address: 2301 Birch Drive, Baltimore, Maryland 21207
Degree and Date to be Conferred: M.S., 2008
Date of Birth: May 19, 1980.
Place of Birth: Annapolis, Maryland.
Secondary Education:
Severna Park Senior High School, Severna Park, Maryland, May 1998
Collegiate Institutions Attended:
Columbia University, New York, New York. September 1998 to May 2002, B.S. May 2002.
Major: Environmental Biology. University of Maryland, Baltimore County, Baltimore, Maryland. September 2005 to April 2008, M.S. May 2008. Major: Civil Engineering – Focus in Environmental Engineering. Professional Publications:
DiBlasi, C., Law, N., Ghosh, U. (2007). Implementing a Sustainable Stormwater Management Program in an Urban Center – Baltimore, MD. Proceedings of the 2nd International Conference on Sustainability Engineering and Science, Auckland, New Zealand, Feb 21-23, 2007.
Professional Positions Held:
Maryland Department of Natural Resources Natural Resources Bioligist
1919 Lincoln Drive, Annapolis, Maryland, 21401. May 2002 to August 2005. Army Corps of Engineers Civil Engineer 10 South Howard Street, Baltimore, Maryland, 21201. April 2008 to Present.
ABSTRACT
Title of Thesis: The Effectiveness of Street Sweeping and Bioretention in Reducing Pollutants in Stormwater
Catherine J. DiBlasi, Master of Science, 2008
Thesis Directed by: Upal Ghosh, Associate Professor, Department of Civil and Environmental Engineering
Research has shown that a great majority of pollutants in urban stormwater are
strongly associated with particulate matter. Therefore, the effectiveness of a stormwater
best management practice (BMP) is largely dependent on its ability to reduce suspended
solids in stormwater. Both street sweeping and bioretention have the potential to
decrease stormwater suspended solid loads. Field investigations were performed in this
research to evaluate the effectiveness of these two BMPs. A paired-catchment study in
an urban watershed of Baltimore, Maryland was performed to physically and chemically
characterize street particulate matter (
THE EFFECTIVENESS OF STREET SWEEPING AND BIORETENTION IN REDUCING POLLUTANTS IN STORMWATER
By
Catherine J. DiBlasi
Thesis submitted to the Faculty of the Graduate School
of the University of Maryland Baltimore County in partial fulfillment of the requirements
for the degree of Master of Science in Civil Engineering
2008
1454791
1454791 2008
Copyright 2008 byDiBlasi, Catherine J. All rights reserved
© Copyright by
Catherine J. DiBlasi 2008
ii
ACKNOWLEDGEMENTS
I would like to sincerely thank the following people for their support: Dr. Upal Ghosh for his support and guidance, for always providing an example of excellence, and inspiring me to work harder. Dr. Neely Law for her enthusiasm and assistance throughout the many stages of this research project. Dr. Brian Reed, Dr. Claire Welty and Dr. Nagaraj Neerchal for their advice as members of my thesis committee and during my time here as a student. U.S. EPA Chesapeake Bay Program for funding this project. Dr. Allen Davis and Houng Li from the University of Maryland, College Park. Yan Zhuang for her assistance with all data organization and statistical analysis. Baltimore City DPW staff, especially Bill Stack, Matt Cherigo, Robert McAulay and Norm Seldon. U.S. Forest Service scientists, especially Rich Pouyat, Ken Belt and Ian Yesilonis. Baltimore County DEPRM and DPW staff, especially Steve Stewart, Megan Brosh and John Burnett. Fellow CEE students for their friendship and advice in the classroom, laboratory, office, and lunchroom. My family, especially my Mom and Dad, Rich, Beth, Glenn, Chris and Isaac for their encouragement, and for always listening. My fiancé, Russell, for his invaluable support, patience, and confidence in me.
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TABLE OF CONTENTS
ABSTRACT ACKNOWLEDGEMENTS ii TABLE OF CONTENTS iii LIST OF TABLES vi LIST OF FIGURES vii LIST OF ABBREVIATIONS xi 1. INTRODUCTION 1.1 Stormwater Runoff 1 1.2 Chesapeake Bay 4 1.2.1 A Holistic Approach to Stormwater Management 6 1.2.2 Baltimore, MD – Watershed 263 8 1.3 Evaluating the Effectiveness of Stormwater BMPs 10 1.4 Organization of Thesis and My Contribution 11 1.5 References 13 2. BACKGROUND 2.1 Street Particulate Matter 15 2.1.1 Sources of Street Particulate Matter, Associated Pollutants 16 2.1.2 Characterization of Street Particulate Matter 19 2.2 Street Sweeping as a Stormwater BMP 2.2.1 Literature Review of Street Sweeping as a BMP 24 2.2.2 Conceptual Model 27 2.3 Bioretention as a Stormwater BMP 2.3.1 Bioretention Concept 30 2.3.2 Literature Review of Bioretention as a BMP 33 2.3.3 Polycyclic Aromatic Hydrocarbons (PAHs) 36 2.4 References 39
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3. STUDY OBJECTIVES
3.1 Street Sweeping as a BMP for Solids, Nutrients and Metals 45 3.2 Bioretention as a Stormwater BMP for PAHs 46 4. EXPERIMENTAL METHODS 4.1 Street Sweeping Study 4.1.1 Description of Study Site – Watershed 263 47
4.1.2 Paired-Catchment Study Setup 48 4.1.3 Street Particulate Matter Sampling 53
4.1.3.1 Particle Type Analysis 57 4.1.3.2 Particle Size Analysis 57
4.1.3.3 Chemical Analysis 58 4.1.4 Stormwater Sampling 59 4.1.4.1 Water Quality Analysis 60 4.1.4.2 First Flush Inlet Samplers 62 4.1.4.3 Statistical Analysis of Water Quality Data 62 4.1.5 Precipitation Data 66 4.1.6 Flow Data and Runoff Coefficients 67 4.1.7 Annual Pollutant Load Calculations 68 4.2 Bioretention Cell Study 4.2.1 Description of Bioretention Cell Study Site 69 4.2.2 Stormwater Sampling 70 4.2.3 PAH Analysis 72 4.2.4 Bioretention Media Core Collection and Analysis 73 4.2.5 PAH Source Investigation 75 4.3 References 75 5. RESULTS AND DISCUSSION OF STREET SWEEPING STUDY
5.1 Street Particulate Matter 77
5.2 Water Quality Data 95 5.3 References 132
6. BIORETENTION STUDY PAPER 137 7. CONCLUSIONS AND RECOMMENDATIONS
7.1 Conclusions 166
v
7.2 Recommendations 169
8. APPENDICES 8.1 APPENDIX A 172
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LIST OF TABLES Table No. Title Page No. 1.1 Typical pollutant concentrations in stormwater 1 1.2 Nutrient sources to the Chesapeake Bay 5 2.1 US EPA’s 16 priority pollutant PAHs 37 4.1 Characteristics of catchments O and F 50 4.2 Street sweeping treatment periods in paired catchment study 51 4.3 Analytical parameters and methods for street particulate matter 59 4.4 Water quality parameters for stormwater analysis 61 4.5 The effect of sample dilution on laboratory detection limits 61 4.6 Distances from water quality sampling stations to rain gages 67 5.1 Collection date and type of street particulate matter samples 77 5.2 One-way ANOVA results for street particulate matter loadings 79 5.3 Gross pollutant sample weights by sample type 80 5.4 Street particulate matter particle size distribution 85 5.5 Chemical composition of street particulate matter 87 5.6 T-test results for particulate matter chemical composition 88 5.7 Pollutant load contribution of fine street particulate matter 92 5.8 Number of baseflow and stormwater samples collected 96 5.9 T-test results of baseflow vs. stormwater comparison 100 5.10 Percent of censored values in each water quality dataset 105 5.11 Effect of substitution methods for censored data on statistics 107 5.12 Median values of water quality parameters 110 5.13 T-test results of water quality in baseline and treatment periods 113 5.14 Runoff coefficients estimated from stormwater flow data 117 5.15 Stormwater concentrations in part 1 and part 2 of storms 119 5.16 Median concentrations in first flush samples 120 5.17 Comparing stormwater and first flush pollutant concentrations 121 6.1 Total PAH removal efficiency, TSS, rainfall and runoff depth 147 6.2 Log Kd values for phenanthrene and pyrene 155 S1 EPA’s 16 priority pollutant PAHs 164 A-1 Elgin whirlwind 4 MV specifications 172 A-2 Street particulate matter particle size analysis data 173 A-3 Street particulate matter chemical analysis data 174 A-4 Baltimore Street (Catchment O) water quality data – original 176 A-5 Lanvale Street (Catchment O) water quality data – original 182 A-6 Results of Shapiro-Wilk normality test 188 A-7 Example of ROS method and a substituted dataset 191
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LIST OF FIGURES Figure No. Title Page No.
2.1 Particle size distribution of street particulate matter 21 2.2 Conceptual model - street particulate matter sources and fate 29 2.3 Typical cross section of a bioretention cell 32 2.4 Image of a curbed parking lot median bioretention cell 33 4.1 Map of Baltimore, Maryland, and Watershed 263 48 4.2 Locations of catchments O and F within Watershed 263 49 4.3 Lanvale St. water quality monitoring station in Watershed 263 50 4.4 Street sweeper used in study – Elgin Whirlwind MV 52 4.5 Vacuum configuration for street particulate matter sampling 53 4.6 Locations of street particulate matter sampling streets 54 4.7 Diagram and photo of street particulate sampling methods 56 4.8 Picture of a first flush sampler installed in Watershed 263 62 4.9 Relationship between runoff coefficient and impervious area 68 4.10 Image of bioretention cell monitored in College Park, Maryland 70 4.11 Layout of monitored bioretention cell and sampling locations 74 5.1 Average street particulate matter loadings 78 5.2 Street particulate matter loadings throughout sampling period 79 5.3 Images of street particulate matter type fractions 83 5.4 Particle size distribution of street particulate matter samples 84 5.5 Comparing particle size distribution with literature values 86 5.6 Average chemical concentrations in street particulate matter 87 5.7 Chemical composition of particulate matter vs. literature values 90 5.8 Percent contribution of particle size fractions to pollutant loads 91 5.9 Relationship – particulate matter load and time since sweeping 94 5.10 Relationship – particulate load and time since sweeping or rain 95 5.11 Total metals in baseflow and stormwater at Baltimore St. 98 5.12 Total metals in baseflow and stormwater at Lanvale St. 99 5.13 Dissolved Zinc at Baltimore St. in baseflow and stormwater 102 5.14 E. coli at Baltimore St. in baseflow and stormwater 103 5.15 NO2-NO3 at Baltimore St. in baseflow and stormwater 104 5.16 Histograms - Effect of substitution method on censored data 108 5.17 Particulate metal concentrations during parts 1 and 2 of storms 119 5.18 First flush and stormwater concentrations of metals and solids 122 5.19 Lead concentrations in particulate matter and suspended solids 124 5.20 Estimated total phosphorus annual loads from Watershed 263 127 5.21 Estimated total nitrogen annual loads from Watershed 263 128 6.1 Image of monitored bioretention cell and sampling locations 145 6.2 Total PAH concentrations in stormwater influent and effluent 149 6.3 16 PAH concentrations in dissolved phase of stormwater 151
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6.4 16 PAH concentrations in particulate phase of stormwater 152 6.5 Comparison of PAH and TSS removal for five storms 153 6.6 Total PAH media profile in material from bioretention cell 158 S1 16 PAH concentrations in media profile 165 S2 16 PAH distribution profile of media and suspended solids 165
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LIST OF ABBREVIATIONS A – Accumulation Sample BaP – Benzo(a)pyrene BES – Baltimore Ecosystem Study BMP – Best Management Practice BOD-5 – Biological Oxygen Demand – 5 day BWI – Baltimore Washington International Airport NOAA weather station C – Control Sample CFS – Cubic Feet per Second CP – College Park CWA – Clean Water Act CWP – Center for Watershed Protection Cu – Copper (DisCu = Dissolved Copper, TotCu = Total Copper) DEPRM – Department of Environmental Resources and Management DHM – Downtown Baltimore Maryland NOAA weather station DPW – Department of Public Works E – Extractable E. coli - Escherichia coli EMC – Event Mean Concentration EPA – Environmental Protection Agency FecCol – Fecal Coliforms IA – Impervious Area Kd – partition coefficient between aqueous and solid phase LID – Low Impact Development MDE – Maryland Department of the Environment MPN – Most Probable Number MS4 – Municipal Separate Storm Sewer Systems NO2 – Nitrite NO3 – Nitrate NPDES – National Pollutant Discharge Elimination System PAH – Polycyclic Aromatic Hydrocarbons Pb – Lead (DisPb = Dissolved lead, TotPb = Total lead) PO4 - Phosphate ROS – Regression on Ordered Statistics Rv – Runoff coefficient, ratio of event runoff to event rainfall S – After Street Sweeping Sample SS – Suspended Solids, same as TSS (Total Suspended Solids) SO4 - Sulfate TKN – Total Kjeldahl Nitrogen TMDL – Total Maximum Daily Load TP – Total Phosphorus UMBC – University of Maryland, Baltimore County UMCP – University of Maryland College Park Zn – Zinc (DisZn = Dissolved Zn, TotZn = Total Zn)
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1. INTRODUCTION
1.1 Stormwater Runoff
In the United States, the Environmental Protection Agency (EPA) estimates that
stormwater runoff is responsible for 21% of impaired lakes and 45% of impaired
estuaries (U.S. EPA 2007b). Stormwater runoff is defined as any rainwater that does not
evaporate or infiltrate into the ground (Chesapeake Bay Program 2006). As stormwater
flows over land it can pick up metals, nutrients, bacteria, pesticides and other
contaminants and transport them to receiving waters. Typical median pollutant
concentrations in urban stormwater runoff for residential, mixed, and commercial land
uses, compared to values in untreated municipal wastewater are shown in Table 1.1.
Table 1.1: Typical median event mean concentrations of pollutants in urban stormwater runoff for residential, mixed, and commercial land uses, compared to typical concentrations in untreated municipal wastewater.
Pollutant (units) Residentiala Mixeda Commerciala Municipal Wastewaterb
Biological Oxygen Demand (mg/L)
10 7.8 9.3 120-370
Chemical Oxygen Demand (mg/L)
73 65 57 260-900
Total Suspended Solids (mg/L)
101 67 69 120-370
Fecal coliform (MPN/100 ml)
103-104 (b)
105-107
Total Lead (�g/L) 144 114 104 Total Copper (�g/L) 33 27 29 0.1-10 (c,d) Total Zinc (�g/L) 135 154 226 Total Kjeldahl Nitrogen (mg/L)
1.9
1.3
1.2
20-45
Nitrate + Nitrite (mg/L) 0.73 0.56 0.57 0 Total Phosphorus (mg/L) 0.38 0.26 0.20 4-16 Sources: a Davis (2005), bMetcalf and Eddy (2003), cU.S. EPA (1981), dU.S. EPA (2003).
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While biological oxygen demand and pathogens are more than an order of
magnitude lower in stormwater compared to untreated municipal wastewater, other
constituents like suspended solid and heavy metals are present at comparable or greater
concentrations in stormwater (Davis 2005, Metcalf and Eddy 2003, U.S. EPA 2003). In
addition to its pollutant load, the sheer volume of stormwater runoff can also cause
negative impacts, such as flooding and erosion. As stormwater flows over land, it may
erode soil and then redeposit the soil in receiving waters, decreasing water clarity and
degrading aquatic habitats. To reduce the negative impacts of stormwater runoff, it is
important to manage both stormwater quantity and quality.
Stormwater runoff occurs over a diffuse area and is typically considered a
nonpoint pollution source. Nonpoint source pollution is challenging to both assess and
regulate. Unlike many point sources, nonpoint sources are not constant, and inputs do
not follow a regular pattern. Additionally, the characteristics and pollution loads of
watersheds where nonpoint source pollution originates are constantly changing over time
(Burton and Pitt 2002). Urban nonpoint source pollution is usually covered by nonpoint
source pollution management programs developed by individual states, particularly
common in coastal environments, but the majority of stormwater management
regulations are focused on point sources (U.S. EPA 1993).
Much of the runoff that originates as a nonpoint source may eventually enter a
storm drain network, be channelized and become a point source. This is particularly
common in urban environments, where runoff which enters and is discharged through
conveyances such as municipal storm drain pipes are treated as point sources and subject
to permit requirements of the Clean Water Act (U.S. EPA 1993). Specifically, these non-
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agricultural sources of stormwater discharges are regulated by the National Pollutant
Discharge Elimination System (NPDES) permit program, under section 402(p) of the
Clean Water Act.
The NPDES permit program is a cornerstone of the 1972 Federal Clean Water
Act (CWA), which requires that all pollutant discharges entering Unites States waters
from a point source must be authorized by a NPDES permit. A 1987 amendment to the
CWA greatly expanded the scope of the NPDES permitting program to include
stormwater discharges. Stormwater systems are currently one of the major categories of
point sources regulated under the NPDES program, and authorization is typically given
by state environmental agencies. There are two main categories in which NPDES
stormwater discharge permits are given, Phase I and Phase II. Phase I of the NPDES
program covers municipal separate storm sewer systems (MS4s) serving greater than
100,000 people and eleven categories of industrial activity including construction sites on
greater than five acres of land. Phase II of the program covers MS4s serving less than
100,000 people and construction activity affecting between 1 and 5 acres of land (U.S.
EPA 2006a). For regulated municipalities, the most important element of the NPDES
program is the requirement to develop and implement a stormwater management program.
Stormwater management programs typically include measures to: identify major
outfalls and pollutions sources, detect and eliminate non-stormwater discharges to the
system, reduce pollutants in runoff from all existing land uses and control runoff from
new development or redevelopment areas (U.S. EPA 2007c). A critical element of
stormwater management programs is the use of practices (called Best Management
Practices), which reduce or prevent the discharge of pollutants into receiving waters.
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Best Management Practices (BMPs) are essentially any structural or non-structural
practice that reduces the quantity and/or improves the quality of stormwater in a cost-
effective manner. A closer look at a specific stormwater management program in section
1.2 of this paper will provide better understanding of urban stormwater management in
the context of Baltimore City and the Chesapeake Bay. Examining urban runoff in the
Chesapeake Bay watershed can provide insight to issues facing much of the nation. As
population and development trends indicate that more than half of the United States will
live in coastal towns and cities by the year 2010, urban stormwater runoff will continue to
be an increasingly important issue (U.S. EPA 2006b).
1.2 Chesapeake Bay
Estuaries have immense commercial and recreational value, providing habitat for
more than 75% of the United State’s commercial fish catch and between 80 to 90 percent
of recreational fish catch (U.S. EPA 2007a). However, more than 60 percent of coastal
rivers and bays are categorized as moderately to severely degraded by nutrient pollution,
and this problem is particularly acute in the mid-Atlantic states (Clement et al. 2001). In
the Chesapeake Bay, eutrophication resulting from excess nutrient loading is the main
cause of poor water quality and aquatic habitat loss, and reducing nutrient inputs to the
Bay is a critical element of restoration efforts. The three major contributors of nutrient
pollution to the Chesapeake Bay include effluent from wastewater treatment plants,
agricultural runoff, and urban stormwater (Chesapeake Bay Program 2006). Major
sources of nutrients to the Chesapeake Bay are included in Table 1.2, along with potential
management options to reduce these contributions.
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Table 1.2: Common sources of nutrients to the Chesapeake Bay and potential management options to reduce their contributions. Nutrient Source Type Management Options Wastewater Treatment Plant Effluent
Point - Limit concentration in effluent - Recycle water - Public education to reduce water consumption
Agricultural Runoff
Nonpoint - Riparian buffers - Limit manure/fertilizer application - Regulate animal waste disposal practices
Urban Stormwater Runoff
Nonpoint - Reduce impervious cover - Practice low impact development - Source control - Best Management Practices (BMPs)
Atmospheric Deposition Nonpoint - Restrict discharges from fossil fuel burning facilities
Groundwater Discharge Nonpoint - Septic tank restrictions
To date, the majority of action has been focused on reducing inputs from
wastewater treatment plants by upgrading plants with technology to reduce nitrogen and
phosphorus concentrations in their effluent. However, it is estimated that point sources
contribute only about 20% of nitrogen delivered to Chesapeake Bay, while nonpoint
sources contribute the remaining 80% (U.S. Environmental Protection Agency 2002).
Clearly, to improve the health of the Bay, nonpoint sources like agricultural and urban
runoff must be aggressively addressed. Although agricultural runoff is considered the
single greatest source of nutrients to the Bay, contributing about 40% of nitrogen and
50% of phosphorous loads, it is particularly challenging to regulate (Chesapeake Bay
Foundation 2003). Therefore, many municipalities in the Chesapeake Bay watershed are
primarily focused on urban stormwater management.
Urbanized areas have higher impervious cover, such as streets, rooftops,
sidewalks and parking lots. Pollutants settle and accumulate on the impervious areas
until wash off by rain into the storm drain system and eventually into receiving waters.
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Common pollutants which build up on impervious surfaces in urban environments
include: sediment, litter, pesticides, fertilizers, oils, road salt, and other debris.
The Chesapeake Bay Program (2006) estimates that urban runoff is responsible
for 16% of the phosphorus, 11% of the nitrogen, and 9% of the sediment loads entering
the Chesapeake Bay. To reduce the negative effects urban landscapes have on water
quality, the Chesapeake Bay Program (2006) recommends reducing impervious cover
and its impact through low impact development practices, source reduction, and best
management practices (BMPs). Examples of traditional BMPs include detention basins,
grass swales and vegetated buffers. However, in an urban environment, implementation
of stormwater BMPs is often limited by space and budget constraints, and staffing
shortages. Nevertheless, there are alternative stormwater management options available
which urban municipalities can implement to both improve water quality and positively
impact public health and attitude. To achieve these multiple objectives, municipalities
must adopt a holistic approach and develop a sustainable stormwater management
program.
1.2.1 A Holistic Approach to Stormwater Management
A holistic approach to stormwater management implies focusing not only on the
immediate problem of polluted stormwater runoff but on all the potential sources and
their underlying causes in a specific watershed. For instance, municipalities should work
beyond simply implementing a few individual BMPs to fulfill permit requirements, and
instead select practices with an understanding of their interactions and how they will
perform in synergy to impact the watershed. History has shown that the traditional
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method of rapidly conveying stormwater to receiving waters leads to environmental
degradation and sustainable or holistic stormwater management is largely a response to
this traditional method. Sustainable management of urban stormwater reflects values of
water conservation, pollution prevention, and ecological restoration (Brown 2005) while
continuing to utilize BMPs.
In addition to realizing the interactions among practices, municipal operators
should acknowledge the interconnectedness of their local environment and the local
community. For example, an urban stormwater management program should address the
fact that nearly all street litter is intentionally left there by humans. Therefore, a
cornerstone of any effective urban stormwater management program should be public
outreach and education, along with public provisions that encourage alternate behavior
(i.e. using trash receptacles). Overall, gaining community support and adopting a holistic
approach to stormwater management will make it possible for municipalities to design
and implement a sustainable, long term program. A sustainable stormwater management
program incorporates environmental, social, and economic concerns in the decision
making process. Additionally, a sustainable stormwater management program should
control and reduce the impact of current runoff and plan for future challenges with
respect to population growth and landscape changes. In Baltimore, Maryland, a unique
and collaborative project is currently underway to develop a sustainable stormwater
management program which aims to ultimately restore an impoverished watershed.
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1.2.2 Baltimore, Maryland – Watershed 263
The project site is a 376 hectare (930 acre) area called Watershed 263 (outfall
number) located in southwest Baltimore City. Several unique characteristics of the
research site make it a challenging and exciting study location. First, impervious cover in
Watershed 263 averages close to 75%, which is significantly higher than the 40% city-
wide average in Baltimore (Richardson 2006). Also, Watershed 263 is absent of any
flowing surface waters - all area streams were piped and buried about 100 years ago,
creating 69 km (43 miles) of pipes which serve as the main components of the storm
drain system (Center for Watershed Protection 2006). Within the storm drain system,
there is a substantial dry-weather baseflow, which is likely due to sewage, drinking water,
and groundwater entering the system through leaky pipes (Richardson 2006). Nearly all
of the neighborhoods within Watershed 263 have suffered moderate to severe economic
decline due to suburbanization and the loss of industrial development. Economic decline
in Watershed 263 has led to a large concentration of vacant houses and lots, a high
unemployment rate and a significant portion of the population living below the poverty
level (Center for Watershed protection 2006).
Watershed 263 has received significant research attention in the past several years
from a number of partners including the federal government (U.S. Forest Service), the
state (University of Maryland, Baltimore County), Baltimore City (Department of Public
Works), and non-profits (Center for Watershed Protection, Parks & People Foundation).
However, what sets this urban environmental research project apart from many others is a
commitment to community involvement and participation. The key to community
involvement is a partnership between the Baltimore City Department of Public Works
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and the Parks & People foundation, an organization dedicated to improving quality of life
for Baltimore residents. The Parks & People foundation worked to raise public
awareness in the watershed by holding over 40 community meetings to explain the
restoration plan, along with separate community forums to hear resident feedback. Since
the initial meetings, an advisory group of about 20 concerned residents has been formed
to represent each geographic neighborhood in planning decisions (Richardson 2006).
There are a number of innovative stormwater management projects being
implemented in Watershed 263, and two of the most notable are the schoolyard greening
initiative and the Clean & Green program. The schoolyard greening project has removed
more than 1.5 hectares of unused asphalt from several public schools and replaced it with
green lawns and gardens. This program uses critical area and stormwater management
credits to reduce impervious area in the watershed and improve aesthetics, while also
offering environmental education opportunities for elementary and middle school
students. These schools involve the students in the redesign of their schoolyards and
incorporate the planting of trees and gardens into the school curriculum. The Clean &
Green program is a partnership between two community outreach organizations in
Baltimore City developed to improve the hundreds of vacant lots across southwest
Baltimore. To date, this program has converted over 330 vacant lots (about 3.3 hectares)
to green space by planting grass and more than 500 trees (Center for Watershed
Protection 2006).
In addition to these high profile beautification projects, Watershed 263 is also the
site of an intensive field study designed to determine the effectiveness of stormwater
management practices. Developed as a paired-catchment study, this research is
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examining the effects of various stormwater practices on the water quality of two urban
catchments within Watershed 263. Details about the paired-catchment study, which
constitutes a large portion of this thesis research will be discussed further in section 4.1 of
this paper. The diverse work taking place in Watershed 263 provides examples of
challenges that many urban municipalities are facing and illustrates the breadth and
complexity of urban stormwater management. While sustainable stormwater
management programs vary by site, all municipalities requires the same essential
scientific information - data on the effectiveness of stormwater BMPs.
1.3 Evaluating the Effectiveness of Stormwater BMPs
When municipal operators design and implement their stormwater management
programs, it is vital that they select the most appropriate BMPs for the sites within their
community. Ideally, when a study on BMP effectiveness is performed, it will provide
useful information for that individual site as well as for other similar and different types
of BMPs at other locations. However, due to variation in study methods and lack of
information about specific design and reporting protocols, the majority of BMP studies
result in data which is difficult to use in comparing effectiveness and selecting individual
BMP design types (Strecker et al. 2001). Strecker and others (2001) reported on a U.S.
EPA funded cooperative research program with the ASCE to develop a more useful set of
data on BMP effectiveness in reducing pollutants in stormwater. One of the major
recommendations of this study was that effluent quality is a much more robust measure
of BMP efficiency rather than “percent removal” which is typically reported.
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Monitoring the input and output of an individual BMP is the typical approach
used, but control watersheds and before/after studies on a watershed are also performed.
Larger watershed-scale studies, such as Watershed 263, are valuable as long as all other
potential factors that may be contributing to changes are identified and accounted for.
Overall, large field studies can provide excellent information on whether the
implementation of BMPs cause a significant difference across the watershed (Strecker et
al. 2001).
This thesis focuses on the effectiveness of two BMPs in reducing pollutants in
stormwater. The BMPs selected include one of the oldest and most commonly used
practices, street sweeping, and one more recently developed practice, bioretention. Both
of these practices have the potential to be of particular value to urban stormwater
management programs. The impacts of street sweeping and bioretention on stormwater
quality were investigated in two separate, intensive field studies. Before discussing the
details and objectives of these studies, it is important to provide background information
on these practices and their use as stormwater BMPs. Street sweeping and bioretention
are discussed in detail in sections 2.1 and 2.2, respectively, including an overview of the
concepts, a history of their uses, and a literature review of previous research on their
effectiveness.
1.4 Organization of Thesis and My Contribution
This thesis is organized into seven chapters. The chapters, described in more
detail below include: 1) Introduction, 2) Background, 3) Study Objectives, 4) Materials
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and Methods, 5) Street Sweeping Study Results and Discussion, 6) Bioretention Study
Paper, and 7) Conclusions and Recommendations.
Chapter 1 includes an introduction to stormwater runoff and stormwater
management, followed by a discussion of how stormwater impacts the Chesapeake Bay.
The need for a holistic approach to stormwater management is discussed, using Baltimore
City Watershed 263 as an example. Chapter 2 provides background or literature review
information on three general topics: street particulate matter as a pollution source
(physical and chemical characterization), street sweeping as a stormwater BMP, and
bioretention as a BMP with an overview of PAHs included. Chapter 3 summarizes the
study objectives of this research in two sections entitled: street sweeping as a stormwater
BMP for suspended solids, nutrients, and metals, and bioretention as a stormwater BMP
for PAHs. Chapter 4 includes detailed materials and methods for all aspects of this
research project. Chapter 5 is presented as a report of the results and discussion section
of the street sweeping study, divided into two main sections: the results of the street
particulate matter monitoring and the results of the stormwater quality monitoring.
Chapter 6 covers the results of the bioretention study, and is presented as a stand alone
paper suitable for submission to a journal for publication. Chapter 7 summarizes the
major results of this research and provides recommendations for future research on street
sweeping and bioretention as stormwater BMPs. An appendix follows with detailed
datasheets that could not be included in the main body of the thesis.
Both the street sweeping and bioretention studies were collaborative projects and
it is important to clarify my role in these research efforts. In the street sweeping study,
my personal contribution included: a 6 month literature review of street sweeping as a
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stormwater management practice, field coordination and collection of all street
particulate matter samples with Baltimore City DPW, particle type analysis of street
particulate matter, and analysis of all water quality and street particulate matter data. All
major laboratory analyses of street particulate matter and stormwater were performed by
either Baltimore County or Microbac Laboratories. All statistical analysis of data was
performed in partnership with two statistician consultants, Dr. Nagaraj Neerchal and Yan
Zhuang. For the bioretention study, my role included: laboratory analysis of stormwater
influent and effluent samples for polycyclic aromatic hydrocarbons (PAHs) for five storm
events. Stormwater sampling was performed by Houng Li and Dr. Allen Davis at
University of Maryland, College Park. Additionally, I performed bioretention media core
sampling, and analysis of core segments for PAHs.
1.5 References
Burton, G.A. Jr. and Pitt, R.E. (2002). Stormwater Effects Handbook, A Toolbox for Watershed Managers, Scientists, and Engineers. Boca Raton: Lewis Publishers of CRC Press LLC.
Center for Watershed Protection (2006). Monitoring Plan – Deriving Reliable Pollutant
Removal Rates for Municipal Street Sweeping and Storm Drain Cleanout Programs in the Chesapeake Bay Basin. Updated 01/23/2006 by Neely Law.
Chesapeake Bay Program (2006). Urban Storm Water Fact Sheet. Last modified
01/11/2006. http://www.chesapeakebay.net/stormwater/htm. Chesapeake Bay Foundation (2003). Water Pollution in the Chesapeake Bay Fact Sheet.
Last modified 07/2003. http://www.cbf.org/site/PageServer?pagename=resources_facts_water_pollution
Clement, C., Bricker, S.B., Pirhalla, D.E. (2001). Eutrophic Conditions in Estuarine
Waters. In: NOAA’s State of the Coast Report. Silver Spring, MD: NOAA. Davis, A. P. (2005). Green Engineering Principles Promote Low-Impact Development.
Environmental Science & Technology A Pages, Vol. 39, pp. 338A – 344A.
- 14 -
Metcalf and Eddy (2003). Wastewater Engineering: Treatment and Reuse. Fourth Edition, Revised by: Tchobanoglous, G., Burton, F. L., and Stensel, H. D. McGraw Hill, New York, NY.
Richardson, D.C. (2006). Watershed 263: A Resource Uncovered. Stormwater. Volume 7, No. 6., September 2006.
http://www.gradingandexcavating.com/sw_0609_watershed.html Strecker, E. W., Quiqley, M. M., Urbonas, B. R., Jones, J. E., and Clary, J. K. (2001).
Determining Urban Storm Water BMP Effectiveness. Journal of Water Resources Planning and Management, May/June 2001, pp. 144 – 149.
U.S. Environmental Protection Agency (1981). Process Design Manual - Land
Treatment of Municipal Wastewater. EPA 625/1-81-013. October 1981. U.S. Environmental Protection Agency (1993). Guidance Specifying Management
Measures for Sources of Nonpoint Pollution in Coastal Waters. EPA 840-B-92-001 January 1993.
U.S. Environmental Protection Agency (2002). The State of the Chesapeake Bay
Watershed: U.S. Environmental Protection Agency Report 903-R-02-002. U.S. Environmental Protection Agency (2003). Relative Risk Assessment of
Management Options for Treated Wastewater in South Florida. EPA 816-R-03-010.
U.S. Environmental Protection Agency (2006a). Phases of the NPDES Stormwater
Program. Last Updated 03/10/2006 http://cfpub.epa.gov/npdes/stormwater/swphases.cfm
U.S. Environmental Protection Agency (2006b). Managing Urban Runoff, Pointer No. 7.
EPA841-F-96-004G. http://www.epa.gov/owow/NPS/facts/point7.htm Last Updated November 2006.
U.S. Environmental Protection Agency (2007a). The National Estuary Program Coastal
Condition Report – Fact Sheet. EPA-842-F-06-001. http://www.epa.gov/owow/oceans/nepccr/
U.S. Environmental Protection Agency (2007b). Stormwater Overview in Mid-Atlantic
Stormwater Quick Finder. http://www.epa.gov/reg3wapd/stormwater/ Last updated October 10, 2007.
U.S. Environmental Protection Agency (2007c). Permit Application Requirements for Medium and Large MS4s. http://cfpub.epa.gov/npdes/stormwater/lgpermit.cfm Last updated April 09, 2007.
- 15 -
2. BACKGROUND
2.1 Street Particulate Matter
Street sweeping ranks among the oldest practices used to control storm water
pollution, yet very limited and sometimes conflicting data has been published in regard to
its performance in removing nutrients and other pollutants (Burton and Pitt 2002, U.S.
EPA 1983, Mineart and Singh 1994, Sutherland and Jelen 1997). Historically, street
sweeping programs focused on aesthetics and maintaining sanitary conditions rather than
pollution reduction and stormwater management. However, as environmental awareness
increased in the 1960s and 1970s, the goals and motivations of street sweeping programs
began to shift. The knowledge that stormwater runoff contains substantial quantities of
contaminants emerged, and interest in the sources of stormwater contaminants led to
investigations into the materials which commonly reside on street surfaces.
There are many pollution sources within a catchment that contribute to the street
surface material load, including runon from adjacent land areas, vehicle emissions and
wear, atmospheric deposition, deterioration of the street surface, and direct deposits such
as sanding and littering. The combined material generated from these sources may be
defined as the total street load, and often contains sediment, paper and plastic litter, glass,
and vegetation. The street surface material smaller than 5 mm in size, here called street
particulate matter, is the focus of this discussion. For the purpose of this study, street
particulate matter is defined as any sediment, soil, and other particulate matter as well as
any vegetation or debris smaller than 5 mm and any associated pollutants, that resides on
the street surface and can be washed off by a storm event of sufficient intensity (at least
- 16 -
2.54 mm (0.1 inches)). Although the amount that each source contributes to the total
street particulate matter load and the composition of street particulate matter can vary
greatly from site to site, it is established that street surfaces are significant sources of
urban stormwater pollutants (Pitt and Amy 1973, Sartor and Gaboury 1984, Pitt 1985,
Waschbusch et al. 1999).
Bannerman and others (1993) found that streets were the “single most important
source area” for pollutants in urban runoff among five urban source areas (lawns,
driveways, rooftops, parking lots, and streets). Streets contributed the largest
concentrations of suspended solids, bacteria, and metals; with street inputs measuring
about four to eight times more than the other sources. A more recent study by
Waschbusch and others (1999) also reported that streets are the major source of
suspended solids in urban runoff, contributing about 70-80% of the total. Streets are also
typically considered the second most important source of nutrients in stormwater, with
lawns being the largest source (Pitt 1985, Bannerman et al. 1993, Waschbusch et al.
1999). Both Pitt (1985) and Waschbusch (1999) found that streets contribute about 20-
30% of the nitrogen and phosphorus load, with streets and lawns together contributing
70-80%. Focusing specifically on streets as a stormwater pollutant source, it is important
to discuss the specific sources of street particulate matter and its associated pollutants, as
well as the physical and chemical characterization of street particulate matter.
2.1.1 Sources of Street Particulate Matter and Associated Pollutants
This section provides a discussion of five major sources of street particulate
matter and its associated pollutants: runon from adjacent land areas, vehicle emissions
- 17 -
and wear, deterioration of the street surface, atmospheric deposition, and anti-skid
compounds. Street particulate matter contributed from adjacent land areas includes both
runon from impervious areas, and the erosion of local soils from pervious areas. Pitt
(1985) found that for very small rains, impervious areas contributed the majority of
pollutants, but when rain increased (to more than 2.5 mm (0.098 inches)) the contribution
of pervious areas became more important. Runon from impervious areas, such as
rooftops, sidewalks, driveways, and parking lots, can transport particulate matter, as well
as oils, grease, and other toxic substances that have accumulated during the interim storm
period. Erosion of local soils can be a result of rain or wind, and is typically one of the
largest sources of street surface particulates (Sartor and Boyd 1972, Pitt 1979). In areas
where soil erosion is a major source of street particulate matter, the type of parent
material found within the drainage basin may impact the grain size of the street
particulate matter. Therefore, local geology can play a large role in the makeup of street
surface contaminants. For example, in Florida where sandy soil is common, much of the
street particulate matter is coarse grained (Brinkmann and Tobin 2001). However, in
cold weather climates, the use of sand as a de-icing material will contribute to the particle
size distribution of street particulate matter, which may mask effects of local geology.
Street particulate matter load originating directly from vehicle emissions and wear
may contribute only a small percentage (by weight) of the total load, but these small
amounts are often very toxic (Sartor and Boyd 1972, Pitt et al. 1997). Aside from spills
and leaks of vehicle fluids, the normal operation and wear of vehicles is responsible for
significant amounts of contaminants on the street, particularly metals. A study of
nonpoint sources in the San Francisco Bay area found that vehicles contribute more than
- 18 -
50% of the copper, cadmium and zinc entering the Bay (Santa Clara Valley NSCP 1992).
The majority of the cadmium and zinc load can be attributed specifically to automobile
tire wear, and copper is mainly due to the wear of brake pads. Vehicle emissions,
particularly diesel engines, are often considered the primary source of lead, and also a
significant source of chromium, silver, mercury, copper and zinc (Santa Clara Valley
NSCP 1992).
The contaminant contribution made by wear or deterioration of the street surface
is strongly dependent on the condition of the road. Pitt (1979) found that the
contributions from the wear of smooth streets in good condition are insignificant. In
general, the rougher the road surface, the more sediment will erode from the street
surface. Sediment on rough streets may also contain more large particles than the
sediment of a smooth street. During a 1983 Milwaukee, Wisconsin field project (WI
DNR 1983), a single asphalt site was divided into two sections, one smooth in good
condition, and one rough in poor condition. Street loads at the rough site were three to
four times greater than the smooth site, and the smooth section had a greater percentage
of particles smaller than 250 microns than the rough section (WI DNR 1983).
Another source of street particulate matter is atmospheric deposition, which can
deliver finer sized particles by either dry or wet deposition. Some of the sources
discussed above, such as vehicle emissions and soil erosion due to wind, are often
grouped into the larger source category of atmospheric deposition. Nutrients, metals and
other pollutants are often found in both wetfall and dryfall samples and may vary by land
use type (NC DNRCM 1983, WI DNR 1983).
- 19 -
In some regions of the United States, anti-skid compounds such as salts (NaCl and
CaCl2), sand, and ash are frequently applied to roadways to melt ice and increase traction
during cold weather. Aside from adding to the total street load, fluctuating
concentrations of NaCl entering receiving waters can have detrimental effects on the
local ecosystems (Hvitvet-Jacobsen and Yousef 1991, Kaushal et al. 2005). The
accumulation of chloride or other anti-skid materials on streets is likely traffic-dependent
because the application of these compounds is focused on well-traveled streets (Pitt 1979).
2.1.2 Physical and Chemical Characterization of Street Particulate Matter
The amount of material on the street surface varies greatly by land use, street
condition and material, and environmental conditions. A typical amount of street
particulate matter load ranges from 250-300 g/curb-meter (887 to 1064 lbs/curb-mile) as
reported by Sartor and Gaboury (1984). In general, the quantity of material on a street
surface depends largely on the length of time since the last cleaning, either by sweeping
or rainfall. However, past research has also revealed a few other trends in street
particulate matter loadings.
Higher loadings on uneven, cracked streets is due to both the degradation of the
street surface itself and the street particulate matter getting trapped in the cracks or
potholes, where it is less likely to be removed by wind, rain, or street sweeping (Sartor
and Boyd 1972, Pitt 1979, WI DNR 1983). Shaheen (1975) also found that loadings
increase with curb height and that at most sites there is a strong positive relationship
between traffic volume and street particulate matter loadings.
- 20 -
Another important trend is that street particulate matter is not distributed
uniformly across a street surface. Sartor and Boyd (1972) concluded that a typical cross-
section of a street will have a small amount of material at the center or crown, very little
in the traffic lanes, a substantial amount in the parking lane, and the largest amount in the
gutter area. This uneven distribution is due to a higher elevation at the crown and due to
wind and traffic blowing the street particulate matter out of the vehicle lanes and into the
gutter area where it is contained by the curbs and accumulates. Sartor and Boyd (1972)
estimated that about 70-80% of all street debris lies within six inches of the curb, and
90% lies within 12 inches. Less extreme across-street loadings were observed by Pitt
(1979) and the NC DNRCM (1983), but they still found that between 40-80 percent of
the total street particulate matter lies within one foot of the curb.
Particle size distribution of street particulate matter varies among individual sites,
but typically the sand fraction (250-1000 �m) contains the highest percentage of particles
by weight. Particle size distribution data from four previous studies on street sweeping
are shown in figure 2.1.
In three of the four studies, sand particles were the largest fraction, making up 30-
50% of the total street particulate matter load, by weight. The fine sand fraction (63-250
�m) fraction also contributed a significant amount of material, about 20-25% of the total
load. While the clay and silt fraction, particles less than 63 �m in size, typically makeup
less than 10% of the total mass, studies have shown that this fraction contains a
disproportionate amount of contaminants. Considerable quantities of metals, nutrients,
and organics are transported from the street surface to receiving waters as sediment-
bound contaminants. This brief discussion of the chemical characterization of street
- 21 -
particulate matter will focus on nutrients and heavy metals with respect to particle size
and source.
0
10
20
30
40
50
60
>1 0.25-1 0.063 - 0.25
- 22 -
Some forms of nutrients are more strongly associated with street surface solids
than other nutrients, and nitrogen is an excellent example. Terstriep and others (IL
DENR 1982) measured the dissolved fraction for a number of constituents in urban
runoff and found that nearly 100 percent of ammonia-N and nitrite-nitrate N is dissolved
in runoff, meaning they have no clear relationship with solids. Supporting this finding,
Sartor and Boyd (1972) also noted that relative to the other contaminants measured,
nitrates were found in very small amounts in street particulate matter. The dissolved
fraction of Kjeldahl nitrogen was found to be smaller than ammonia and nitrite-nitrate,
measured at 69%, and therefore this parameter is more strongly associated with street
particulate matter (IL DENR 1982).
Heavy metals found in street particulate matter can be largely attributed to
automobile emissions and wear and commonly include: Pb, Fe, Zn, Ca, Cd, Cr, Cu, Hg,
Ni, Mn and Fe (Sartor and Boyd 1972, Shaheen 1975, Wilbur and Hunter 1979,
Fergusson and Ryan 1984). Concentrations of heavy metals in street particulate matter
can vary widely from site to site, and also with respect to land use. Fergusson and Ryan
(1984) measured metals in street particulate matter in five cities in the U.S. and Europe
and found that lead concentrations ranged from a few hundred ppm to over 10,000 ppm.
When looking at the three traditional land use types (residential, industrial and
commercial), Sartor and Boyd (1972) found that commercial sites had the highest
concentrations of zinc, lead, copper, and mercury. This was likely due to the higher
traffic volume that often occurs at commercial shopping areas. It is widely accepted that
heavy metals are strongly associated with the fine particle size fractions in street
particulate matter, and concentrations generally increase with decreasing particle size
- 23 -
(Shaheen 1975, Fergusson and Ryan 1984, Hvitved-Jacobsen and Yousef 1991, Schorer
1997). Sartor and Boyd (1972) found that more than 50% of all heavy metals are
associated with sediment particles smaller than 246 microns. Similar distributions were
observed by Shaheen (1975) who found that about 54% of zinc and 62% of lead are
associated with particles smaller than 250 microns.
However, for any contaminant, the physical composition of the street particulate
matter at each site must be taken into consideration as well, and depending on the particle
size distribution at a site, the high concentrations associated with the smallest size
fractions may be negligible. Therefore, the concentration of each size class must be
multiplied by the total solids load contributed by that size class to calculate the percent of
the total pollutant load in each class (Concentration x Mass = total pollutant load). In
general, the particle size fraction that contributes the greatest portion of the mass often
also contributes the greatest portion of the contaminant load.
In conclusion, material on the street surface is highly contaminated and
considered a major pollution source to urban stormwater. This knowledge emerged in the
early 1970s through a number of EPA studies (Sartor and Boyd 1972, Pitt and Amy 1973,
Shaheen 1975), along with the suggestion that street sweeping could possibly reduce
street particulate matter as a pollutant source. As a response to these findings, a number
of intensive field monitoring studies on stormwater runoff and street sweeping as a
stormwater pollution control practice were initiated.
- 24 -
2.2 Street Sweeping as a Stormwater BMP
2.2.1 Literature Review of Street Sweeping as a Stormwater BMP
In the late 1970s and early 1980s, the U.S. EPA conducted the Nationwide Urban
Runoff Program (NURP) at 28 cities across the country. While the 28 individual NURP
projects were different, they all focused on characterizing pollutant types, their effect on
water quality, and evaluating various practices for the control of stormwater pollution.
Five of the 28 NURP studies (Bellevue, WA; Champaign-Urbana, IL; Castro Valley, CA;
Winston Salem, NC; Milwaukee, WI) studied the effectiveness of street sweeping in
reducing pollutants in stormwater runoff (U.S. EPA 1983). These five NURP studies
were designed using a paired basin approach, where two adjacent or nearby basins were
monitored during a control (often unswept) period to establish baseline data and then
street sweeping was implemented or intensified in one treatment basin, while the other
remained as a control. Stormwater quality monitoring was performed throughout the
baseline and treatment periods to determine any impacts of street sweeping on
stormwater quality.
All five of the NURP studies found that street sweeping was ineffective in
reducing mean concentrations of pollutants in urban storm runoff, and therefore
ineffective as a stormwater management practice (Pitt and Shawley 1981, IL DENR 1982,
WI DNR 1983, NC DNRCM 1983, Pitt 1985). These studies reported that while street
sweeping was effective at removing litter and larger particles, the NURP-era street
sweepers were unable to pick up the finer grained sediment fraction (
- 25 -
not translate to significant reductions in stormwater pollutants. In the final report of the
NURP results (U.S. EPA 1983), the issue of variation in stormwater Event Mean
Concentrations (EMCs) was also a major concern. For each study site, median EMC data
for five parameters (TSS, COD, TP, TKN and Pb) was based on between 10 to 60 storm
events, with 30 events typical. The U.S. EPA (1983) reported that no reduction of
contaminant EMCs observed was greater than 50% and concluded that any benefits of
street sweeping that did occur were masked by the large variability of the EMCs, and
therefore a larger database is required. The NURP results led many street sweeping
manufacturers to focus on improving technology to pickup fine particles.
Ten years after the NURP-era studies, in the mid 1990s, interest in street
sweeping as a best management practice was renewed. Improvements in sweeping
technology with the development of vacuum sweepers and regenerative air sweepers
were thought to have improved their ability to pick up fine particulates. Further, new
stormwater regulations created the need for best management practices that were
relatively inexpensive and easy to implement in urban watersheds and street sweeping
provided an option.
More recent street sweeping studies (completed in the past 15 years) have been
more narrowly focused in project and geographic scope (e.g., Brinkmann and Tobin 2001,
Kuhns et al. 2003). In general, these studies involved sampling street particulate matter
or stormwater that largely compared the effectiveness of different types of sweepers, and
had less extensive field monitoring components compared to the NURP-era studies.
Researchers have also utilized modeling as a tool to assess the impacts of street sweeping
on stormwater quality (Sutherland and Jelen 1996, Sutherland and Jelen 1997, Zariello et
- 26 -
al. 2002). Another current trend in street sweeping research is the evaluation of sweeping
for the removal of total suspended particles (TSP) and particles less than 10 micrometers
(PM10) to improve air quality (Chang et al. 2005, Kuhns et al. 2003). The U.S. EPA is
particularly concerned with inhalable particles 10 �m in diameter or smaller (PM10)
because they can enter the body where they can affect the heart and lungs and cause
serious health effects (U.S. EPA 2006b).
As street sweeping studies have continued to focus on quantifying the amount of
material removed little is known about how these practices affect water quality on a
catchment or watershed scale. As a consequence there remains limited understanding on
how street sweeping can be used as a best management practice to effectively reduce
pollutant loadings to improve and maintain water quality. Although street sweeper
pickup performance and variability in stormwater quality sampling are important, there
are many other factors that can influence the effectiveness of street sweeping in
improving stormwater quality.
To determine the stormwater pollutant load reduction that may be achieved by
street sweeping, one needs to understand the factors and processes that effect street
sweeping performance. Pollutant load reduction is defined as the loading rate (e.g.
deposition) of street particulate matter less the rate at which material is washed off, plus
material that may be considered permanent storage in road cracks, etc. (Pitt et al. 1997).
The deposition and removal of street particulate matter depends on multiple factors that
are location specific and include the physical characteristics of the catchment,
environmental conditions such as weather, the design and operation of the street
sweeping program and the particle size distribution of pollutants.
- 27 -
In addition to these factors, streets are only one source of stormwater pollutants.
As previously discussed, potential sources of stormwater pollutants include streets,
parking lots, sidewalks, rooftops, driveways, and pervious areas. Therefore, reducing the
street particulate matter load through street sweeping is only reducing one of several
sources. Street sweeping has the highest potential to reduce stormwater concentrations of
contaminants for which streets are the major source (such as suspended solids) and which
are largely associated with solids (such as lead).
To date, although there is limited evidence to show how reductions in street
particulate matter can affect stormwater quality, most studies and street sweeping
programs continue to operate under the premise that if street particulate matter is
removed, then an improvement in water quality should follow.
2.2.2 Conceptual Model
To better understand the impact of street particulate matter and street sweeping on
stormwater quality, it is valuable to look at a conceptual model (Figure 2.2) of the inputs,
outputs, and fate of street particulate matter in an urban catchment. The conceptual
model incorporates three major components, the street surface, the storm drain or inlet,
and the storm drain pipe network. As discussed previously, there are many inputs from
many source areas of street particulate matter to the street surface including: littering,
runon, degradation of the street surface, vehicle emissions and wear, sand/salt, wet
deposition and dry deposition. Once the material is on the street surface, there are four
major removal mechanisms or outputs: street sweeping, loss by wind and traffic, washoff
to the storm drain, and runoff (not to an inlet). It is important to remember that the entire
- 28 -
load of street particulate matter is not available to be removed by these mechanisms. For
example, street sweeping cannot remove materials underneath parked cars and washoff
may not remove materials trapped in cracks or potholes of the street surface. Street
particulate matter that remains on the street surface may then be deposited in the storm
drain inlet by a rain event (washoff) or by wind, traffic or direct littering. Some material
is typically trapped at the bottom of the inlet, and inlet trapping efficiency is a function of
the type and capacity of the inlet.
- 29 -
Figure 2.2: Conceptual model of the fate of street particulate matter in an urban catchment.
Sediment Pool Water
Curb Street Surface
Inputs
Littering Runon Degradation of street surface Vehicle emissions and wear Sanding/Salt Wet deposition Dry deposition
Outputs Street Sweeping Runoff (not to inlet) Loss by wind and traffic
Dry Inputs by wind, traffic littering Washoff
Inputs to Storm Drain
Municipal Storm Drain Cleanout
Outputs
Bedload (settled solids)
Storm Drain Network
Receiving Waters
Stormwater or Baseflow
Flush out by large storm
- 30 -
Inlets will store this material until it is removed by municipal storm drain cleanout
or flushed out into the storm drain network by a storm event of adequate size. Once
material is flushed inside the storm drain network of pipes, larger particles and debris will
settle to the bottom of the storm drain pipe (called bedload) while other material is
carried in the stormwater to local receiving waters. Bedload may remain on the bottom
of a storm drain pipe until a storm of sufficient intensity removes it and transports it to
receiving waters as well. This conceptual model illustrates the interconnectedness of
streets, storm drain inlets and storm drain systems and the many factors which impact
stormwater quality in an urban catchment.
While street sweeping has the potential to remove street particulate matter before
it is picked up by stormwater runoff, another practice, bioretention, has the potential to
remove suspended solids and associated pollutants in stormwater runoff.
2.3 Bioretention as a Stormwater BMP
2.3.1 Bioretention Concept
Bioretention is a relatively new practice, which gets its name from the concept of
using biomass to retain pollutants in stormwater runoff. Developed in the early 1990s in
Prince George’s County, Maryland, bioretention is a natural-based BMP system also
known as a rain garden (Glass and Bissouma 2005). The practice of bioretention is
considered part of the larger concept of low impact development (LID), which integrates
environmental concerns with land development. In contrast to traditional management of
urban stormwater, which attempts to convey runoff away from a developed area as
quickly as possible, LID aims to manage stormwater runoff at the source (Davis 2005).
- 31 -
LID is considered a philosophy for development that focuses on minimizing
adverse environmental impacts and practicing sustainable water management. One of the
ultimate goals of LID designs is to replicate pre-development hydrologic conditions as
closely as possible. The practice of LID begins with site design and includes practices
such as: leaving wooded areas on lots, minimizing impervious cover by narrowing streets,
and using vegetated swales and filter strips in place of traditional curb and gutter systems
(Davis 2005). Obviously, some impervious area cannot be avoided and excess runoff can
be managed with onsite vegetated infiltration practices such as bioretention.
Vegetated bioretention areas or rain gardens, are management practices designed
to interrupt the flow of runoff from impervious surfaces to storm drain systems or
receiving waters. Bioretention is typically designed as individual cells (sized at
approximately 4-5% of the drainage area), which include soil, sand, organic matter and
vegetation engineered to store, infiltrate and treat stormwater runoff (Davis et al. 2003,
Hsieh et al. 2007). Many mechanisms can occur within a bioretention cell to improve
water quality, including: adsorption, precipitation, filtration, evapotranspiration and bio-
transformation processes (Davis et al. 2003, Hsieh and Davis 2005a). Water which
infiltrates a bioretention facility is either allowed to continue for groundwater recharge or
collected through a subsurface perforated pipe (underdrain) and conveyed to receiving
waters or traditional storm drain systems (Davis et al. 2001). Although designs vary, a
typical bioretention cell cross-section is shown in figure 2.3. As runoff enters a
bioretention area, typically through a cutout in the curb length, it will flow through
several cell components including (from top to bottom): native vegetation, a 7-15 cm (3-6
inch) ponding area, a 5-8 cm (2-3 inch) mulch layer, a soil layer (composed of 20-30%
- 32 -
top soil, 20-30% leaf compost, and 50% sand), and an underdrain surrounded by gravel
(Prince George’s County 2002).
Figure 2.3: Typical cross section of a bioretention cell. Source: Modified from Prince George’s County (2002).
There are many different designs and ways to implement bioretention as a
stormwater best management practices in commercial, industrial and residential areas.
Some examples illustrated in a Bioretention Manual published by Prince George’s
County in 2002 include: parking lot (curbed or uncurbed) perimeter bioretention, parking
lot (curbed or uncurbed) island and median bioretention, parking swale bioretention,
rooftop bioretention, residential on-lot bioretention, and tree and shrub pit bioretention.
Ponding Area
Mulch Layer
Soil Medium
Vegetation
1.5-3 ft., typically: 20-30% topsoil 20-30% leaf compost 50+% sand
1.5-2” Stone
2-3” provides excellent solids and metals removal
3-6”
Underdrain and Gravel Filter
- 33 -
A picture of a typical curbed parking lot island and median bioretention area is shown in
figure 2.4. In this photograph, the flow entrance cutout in the curb surrounding the
bioretention cell can clearly be seen.
Figure 2.4: Typical curbed parking lot island and median bioretention. Source: Prince George’s County (2002). Although bioretention is a relatively new practice, there have been several studies
evaluating the effectiveness of bioretention in removing stormwater pollutants both in the
lab and in the field.
2.3.2 Literature Review of Bioretention as a Stormwater BMP
The majority of bioretention research to date has focused on removal of
suspended solids, oil and grease, metals and nutrients in stormwater runoff. Bioretention
studies both in the lab and in the field have shown removal efficiencies of 91-98% for
suspended solids (Hsieh and Davis 2005a, Hsieh and Davis 2005b, Glass and Bissouma
2005). A laboratory study by Hsieh and Davis (2005a) used a bioretention test column to
- 34 -
treat a synthetic runoff event once a week for twelve weeks. During this time, the authors
examined the removal of total suspended solids and the long term effects of these filtered
solids on stormwater infiltration rates. For all but one storm, more than 90% of the total
suspended solids (TSS) was removed by the bioretention media, and the authors observed
that most of the TSS was filtered by the top mulch layer. Additionally, the filtered solids
had no effect on runoff infiltration rate, which remained constant at 0.35 cm/min
throughout the study (Hsieh and Davis 2005a). When visually examining the influent
and effluent stormwater at field bioretention facilities, color differences indicated that
most of the suspended solids in the effluent are materials from the bioretention media
rather than part of the influent TSS (Hsieh and Davis 2005b).
Oil and grease are also removed with excellent efficiencies (greater than 96%) in
both laboratory column studies and field studies at existing bioretention sites (Hsieh and
Davis 2005a, Hsieh and Davis 2005b). The removal of heavy metals (copper, lead and
zinc) through bioretention is also typically high (greater than 90%) and is often correlated
to the removal of TSS (Davis et al. 2003, Hsieh and Davis 2005b). However, these
values were reported for laboratory scale studies or well maintained field sites using
synthetic runoff. A study of 15 storm events at an existing bioretention facility by Glass
and Bissouma (2005) showed lower removals for Cu (75%), Pb (71%) and Zn (81%) and
the authors attributed this to lack of maintenance of the bioretention mulch layer. While
the mulch layer acts as a filter for suspended solids, Davis and others (2001) determined
that metal removal also occurs via adsorption to the mulch layer and soil media as water
flows though the cell.
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Although data vary with regard to nutrients, bioretention cells are typically more
effective at removing phosphorus from stormwater than nitrogen. Total phosphorus (TP)
and TKN removal efficiencies are moderate and quite variable, ranging from about 50-
85% (Davis et al. 2006, Hsieh and Davis 2005a, Hsieh and Davis 2005b). Nitrate
reduction is consistently poor (less than 20%), and studies report that nitrate removal may
be improved by adding an engineered denitrification layer (Kim et al. 2003, Hsieh and
Davis 2005b).
A study comparing different types and configurations of bioretention media found
that a layered medium with a permeable sand/soil mixture layer provided the best overall
pollutant removal efficiencies for bioretention (Hsieh and Davis 2005b). However,
bioretention cells should be designed for individual sites depending on infiltration needs,
pollutant removal goals and any other site concerns such as groundwater contamination.
As with any stormwater BMP, it is important to consider both the maintenance
costs and long-term efficiency of the practice. At sites where pollutant loadings in urban
stormwater are average or lower, long bioretention lifetimes (up to 20 years) are possible
(Davis et al. 2003). During this time, plant matter should be somewhat maintained, and
replacing the mulch layer may also extend the lifetime of a cell (Davis et al. 2001).
However, at sites where pollutant loadings are high, pollutants will accumulate more
quickly in the bioretention cell and maintenance and eventually disposal of bioretention
cell media are more complicated. Davis and others (2001) suggest that another option is
to use media which captures metals or other pollutants and renders them biologically
unavailable.
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In addition to improving stormwater quality, bioretention cells, and particularly
the plant life they support, have several other benefits. A properly designed bioretention
cell planting scheme can improve aesthetics, provide shade, shield wind, support wildlife,
adsorb noise and increase the value of a site (Prince George’s County 2002).
In conclusion, bioretention is a promising stormwater BMP which deserves
further research attention, particularly in regard to other types of pollutants. Since
bioretention is very effective at removing suspended solids in stormwater runoff, this
practice has the potential to remove many contaminants which are strongly associated
with solids, such as hydrophobic organic compounds. Polycyclic aromatic hydrocarbons
(PAHs) are a growing concern in stormwater runoff, and the fate of PAHs in stormwater
treated by bioretention is the focus of this study. Before describing the study objectives,
it is helpful to describe PAHs, their characteristics, sources and inputs to stormwater
runoff.
2.3.3 Polycyclic Aromatic Hydrocarbons (PAHs)
Polycyclic aromatic hydrocarbons (PAHs) represent the largest class of suspected
carcinogens (Bjorseth and Ramhahl 1985) and they are widely distributed in the air,
water and sediments of urban environments (Van Metre et al. 2000, Larsen and Baker
2003, Stout et al. 2004, Stein et al. 2006). There are more than 100 different PAH
compounds and although many are naturally occurring, the majority of PAHs in the
environment are from anthropogenic sources (Agency for Toxic Substances and Disease
Registry 1996). Anthropogenic sources of PAHs include the release of petroleum
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products (petrogenic source) and the combustion of organic matter such as petroleum,
coal, wood and oil (pyrogenic source) (Stein et al. 2006).
PAH compounds generally have low solubilities (Table 2.1) and are typically
associated with the solids rather than liquids in the environment. It is also notable that
PAH compounds range from two rings to six rings, and the solubility of PAHs tends to
decrease as the number of rings increase (Bjorseth 1983).
Table 2.1. EPA’s 16 priority pollutant PAHs.
Source: Modified from Stein et al. (2006) and Peters et al. (1999).
In urban environments, studies have found that pyrogenic sources dominate (Hoffman
1985, Menzie et al. 2002, Stein et al. 2006). Specifically, urban sources may include:
vehicle exhaust, home heating through coal and wood burning stoves, trash burning,
power plants and other industrial processes, and the leaching of PAHs in sealant used to
coat parking lots and driveways. Because PAHs are ubiquitous in urban environments
and largely associated with solid particles, they are also a very accessible contaminant to
16 PAHs Mol. Wt.
No. Rings
Aqueous Solubility at 25ºC (mg/L)
Naphthalene 128 2 31 Acenaphthylene 152 3 3.9 Acenaphthene 154 2 3.8 Fluorene 166 3 1.9 Phenanthrene 178 3 1.1 Anthracene 178 3 0.05 Fluoranthene 202 4 0.26 Pyrene 202 4 0.13 Benz(a)anthracene 228 4 0.011 Chrysene 228 5 0.002 Benzo(b)fluoranthene 252 5 0.0015 Benzo(k)fluoranthene 252 5 0.0008 Benzo(a)pyrene 252 5 0.004 Indeno(1,2,3-cd)pyrene 276 6 0.062 Dibenz(a,h)anthracene 278 5 0.0005 Benzo(g,h,i)perylene 276 6 0.0003
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be picked up and transported by stormwater runoff. Urban stormwater runoff is
considered an important source of PAHs to aquatic environments. Studies have found the
pattern of PAHs observed in near shore sediments closely reflects the pattern of PAHs
found in urban stormwater runoff. It is estimated that urban runoff contributes about 14-
36% of the total PAH load to aquatic ecosystems (Hoffman et al. 1984, Menzie et al.
2002).
A 1985 study in Narragansett Bay (Hoffman 1985) and a study by Menzie and
others (2002) both found that fluoranthene, phenanthrene, pyrene and chrysene are the
dominant PAH compounds in stormwater. All four of these compounds are categorized
by the U.S. EPA as category D – not classifiable as to human carcinogenicity, based on
no human data and inadequate data from animal assays. However, the U.S. EPA
classifies the PAH, Benzo(a)pyrene (BaP) as a probable human carcinogen (category B2),
and states that PAHs similar to BaP can potentially cause adverse health affects due to
both acute and chronic exposure (U.S. EPA 2006a). Possible exposure pathways to
humans include the inhalation of contaminated air, smoking of cigarettes, and
consumption of contaminated food (particularly grilled and smoked foods) and water
(U.S. EPA 2006c). Stormwater is a source of BaP and other PAHs to receiving waters,
where they often accumulate in sediments and their toxicity can affect sediment and
aquatic life.
A recently discovered source of PAHs in urban environments, which is
particularly relevant to stormwater runoff, is sealers used to coat parking lots and
driveways. These sealers, used to improve the appearance of asphalt pavement are most
frequently made with a coal-tar base, however asphalt-emulsion based sealers are also
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used (Mahler et al. 2004). A 2003 study in Austin, Texas by Mahler and others (Mahler
et al. 2004) investigated the concentrations and loads of PAHs in simulated runoff from
various parking lot surfaces and determined to what degree these sealers are a source of
PAHs in the urban environment. The authors reported that stormwater suspended solids
from parking lots with coal-tar emulsion sealcoat had mean PAH concentrations of 3500
mg/kg, 65 times higher than the mean PAH concentration in runoff from unsealed asphalt
and cement parking lots (Mahler et al. 2005).
Although the fate of PAHs in stormwater is a significant concern, the concept of
treating PAHs in urban stormwater has received little attention from researchers and
municipal stormwater operators. Bioretention has the potential to reduce PAH loads
from urban stormwater runoff before it enters the storm drain network and enters
receiving waters. The following section describes the objectives for both research
components included in this thesis, the street sweeping study and the bioretention cell
study.
2.4 References
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Brinkman, R. and Tobin, G. A. (2001). Urban Sediment Removal: The Science, Policy,
and Management of Street Sweeping. Boston: Kluwer Academic Presses. Bjorseth, A., Ed. (1983) Handbook of Polycyclic Aromatic Hydrocarbons;
Marcel Decker: New York, 1983. Bjorseth, A., Ramdahl, T., Eds. (1985) Handbook of Polycyclic Aromatic Hydrocarbons;
Marcel Decker: New York, 1985.
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Burton, G.A. Jr. and Pitt, R.E. (2002). Stormwater Effects Handbook, A Toolbox for Watershed Managers, Scientists, and Engineers. Boca Raton: Lewis Publishers of CRC Press LLC.
Center for Watershed Protection (2006). Technical Memorandum 1 – Literature
Review: Research in Support of an Interim Pollutant Removal Rate for Street Sweeping and Storm Drain Cleanout Activities. Prepared for the U.S. Chesapeake Bay Program, October 2006.
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Quality Improvement through Bioretention: Lead, Copper, and Zinc Removal. Water Environment Research, Volume 75, Number 1, pp. 73 – 82.
Davis, A. P. (2005). Green Engineering Principles Promote Low-Impact Development.
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for Treatment of Urban Storm Water Runoff. Journal of Environmental Engineering, November 2005.
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Hsieh, C., Davis, A. P., Needelman, B. A. (2007). Bioretention Column Studies of Phosphorus Removal from Urban Stormwater Runoff. Water Environment Research, Vol. 79, No. 2, pp. 177 – 184.
Hoffman, E.J., Mills, G.L., Latimer, J.S., Quinn, J.G. (1984). Urban Runoff as a source
of polycyclic aromatic hydrocarbons to coastal waters. Environmental Science & Technology, Vol. 18, pp. 580 – 587.
Hoffman, E. J. (1985). Urban Runoff Pollutant Inputs to Narragansett Bay: Comparison
to Point Sources, p. 159 – 164. In Proceedings from the Conference on Perspectives on Nonpoint Source Pollution. Report No. 440/5-85-001. U.S. Environmental Protection Agency, Kansas City, Missouri.
Hvitvet-Jacobsen, T. and Yousef, Y.A. (1991). Road runoff quality, Environmental
Impacts and Control in Road Pollution. In: Hamilton, R.S. and Harrison, R.N. (eds.), Highway Pollution, p.165-209. London: Elsevier.
Illinois Department of Energy and Natural Resources (IL DENR) (1982). Nationwide Urban Runoff Project, Champaign, Illinois: Evaluation of the Effectiveness of Municipal Street Sweeping in the Control of Urban Storm Runoff Pollution. IL Environmental Protection Agency and U.S. Environmental Protection Agency, PB83-209890.
Kaushal, S. S., Groffman, P. M., Likens, G. E., Belt, K. T., Stack, W. P., Kelly, V. R., Band, L. E., and Fisher, G. T. (2005). Increased Salinizat