United States
Environmental Protection
Agency
Office of Water (4303)
Washington, DC 20460
EPA-821-R-99-012
August 1999
EPA Preliminary Data Summary of
Urban Storm Water
Best Management Practices
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Acknowledgments
This report was prepared by Eric Strassler, Project Manager, Jesse Pritts, Civil Engineer,
and Kristen Strellec, Economist, of the Engineering and Analysis Division, Office of Science and
Technology. Assistance was provided by Parsons Engineering Science, Inc., Limno-Tech, Inc.
and the Center for Watershed Protection under EPA Contract No. 68-C6-0001. EPA reviewers
were Eugene Bromley, Rod Frederick, John Kosco, Marjorie Pitts, Marvin Rubin, Steven
Sweeney and Kathy Zirbser. EPA thanks its external reviewers for this report:
George Aponte Clarke, Natural Resources Defense Council
Edward U. Graham, P.E., and John Galli, Metropolitan Washington Council of
Governments
Jonathan E. Jones, P.E., Wright Water Engineers, Inc.
Eric H. Livingston, Florida Department of Environmental Protection
Eric W. Strecker, P.E., URS Greiner Woodward-Clyde
Lori L. Sundstrom, City of Phoenix, AZ
Ben Urbonas, P.E., Urban Drainage and Flood Control District, Denver.
Disclaimer
Mention of trade names or commercial products does not constitute endorsement by EPA or
recommendation for use.
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Table of Contents
List of Tables iv
List of Figures v
1.0 Summary 1-1
2.0 Introduction and Scope 2-1
2.1 Effluent Guidelines Program and Consent Decree Requirements 2-1
2.2 Types of Discharges Addressed 2-2
2.3 Data Sources and Data Collection Techniques 2-3
3.0 Existing Storm Water Regulations and Permits 3-1
3.1 PhaselNPDES 3-1
3.2 Phase IINPDES 3-2
3.3 Coastal Zone Act Requirements 3-3
3.4 Regional, State and Local Programs 3-4
4.0 Environmental Assessment 4-1
4.1 Overview of Storm Water Discharges 4-3
4.2 Pollutants in Urban Storm Water 4-6
4.2.1 Solids, Sediment and Floatables 4-11
4.2.2 Oxygen-Demanding Substances and Dissolved Oxygen 4-12
4.2.3 Nitrogen and Phosphorus 4-13
4.2.4 Pathogens 4-13
4.2.5 Petroleum Hydrocarbons 4-15
4.2.6 Metals 4-16
4.2.7 Synthetic Organic Compounds 4-18
4.2.8 Temperature 4-19
4.2.9 pH 4-22
4.3 Reported Impacts of Urban Storm Water 4-23
4.3.1 Flow Impacts 4-23
4.3.2 Habitat Impacts 4-32
4.3.3 Public Health Impacts 4-44
4.3.4 Aesthetic Impacts 4-48
5.0 Description and Performance of Storm Water Best Management Practices 5-1
5.1 Goals of Storm Water Best Management Practices 5-1
5.1.1 Flow Control 5-1
5.1.2 Pollutant Removal 5-4
5.1.3 Pollutant Source Reductions 5-6
5.2 Types of Storm Water Best Management Practices 5-7
5.2.1 Structural BMPs 5-7
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5.2.2 Non-Structural BMPs 5-30
5.2.3 Low-Impact Development Practices 5-39
5.3 BMP Selection 5-41
5.4 Monitoring BMP Effectiveness 5-42
5.4.1 Water Quality Monitoring of BMPs 5-43
5.4.2 Receiving Stream Assessments 5-46
5.5 Effectiveness of BMPs in Managing Urban Runoff 5-46
5.5.1 Controlling Pollution Generation 5-48
5.5.2 Controlling Pollution Discharges 5-50
5.5.3 Controlling Flow Impacts 5-83
5.6 Conclusions 5-85
6.0 Costs and Benefits of Storm Water BMPs 6-1
6.1 Structural BMP Costs 6-1
6.1.1 Base Capital Costs 6-2
6.1.2 Design, Contingency and Permitting Costs 6-13
6.1.3 Land Costs 6-13
6.1.4 Operation and Maintenance Costs 6-14
6.1.5 Long-Term BMP Costs: Two Scenarios 6-16
6.1.6 Adjusting Costs Regionally 6-19
6.2 Non-Structural BMP Costs 6-21
6.2.1 Street Sweeping 6-21
6.2.2 Illicit Connection Identification and Elimination 6-22
6.2.3 Public Education and Outreach 6-22
6.2.4 Land Use Modifications 6-25
6.2.5 Oil and Hazardous Waste Collection 6-27
6.2.6 Proper Storage of Materials 6-27
6.3 Benefits of Storm Water BMPs 6-28
6.3.1 Storm Water Pollutant Reduction 6-28
6.3.2 Hydrological and Habitat Benefits 6-32
6.3.3 Human Health Benefits 6-37
6.3.4 Additional and Aesthetic Benefits 6-37
6.4 Review of Economic Analysis of the NPDES Phase II Storm Water Rule ... 6-38
6.4.1 Analyses of Potential Costs 6-39
6.4.2 Assessment of Potential Benefits 6-41
6.4.3 Comparison of Benefits and Costs 6-42
6.5 Financial Issues 6-42
6.5.1 Municipal Financing of Storm Water Programs 6-43
6.6 Summary 6-44
References R-l
Index 1-1
in
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List of Tables
4-1. Median Event Mean Concentrations for Urban Land Uses 4-8
4-2. Sources of Contaminants in Urban Storm Water Runoff 4-9
4-3. Typical Pollutant Loadings from Runoff by Urban Land Use (Ibs/acre-yr) 4-10
4-4. Comparison of Water Quality Parameters in Urban Runoff with Domestic Wastewater
4-11
4-5. Densities of Selected Pathogens and Indicator Microorganisms in Storm Water in Baltimore,
Maryland Area 4-15
4-6. Fecal Coliform Concentrations Collected in Sheetflow from Urban Land Uses 4-15
4-7. Most Frequently Detected Priority Pollutants in Nationwide Urban Runoff Program Samples
(1978-83) 4-17
4-8. Probability of Event Mean Concentration of Constituents in Wisconsin Storm Water
Exceeding Wisconsin Surface Water and Ground Water Quality Standards: Metals
4-18
4-9. Probability of Event Mean Concentration of Constituents in Wisconsin Storm Water
Exceeding Wisconsin Surface Water and Ground Water Quality Standards: Synthetic
Organic Compounds 4-19
4-10. Impacts from Increases in Impervious Surfaces 4-26
4-11. Comparison of Estimated Runoff Volume and Peak Discharge for Developed and
Undeveloped Areas 4-27
4-12. Percent Increase of Two-Year Flood, Bankfull Width, and Bankfull Depth from Pre-
Development Conditions to Urbanized Conditions (Based on Modeling Results) . . . 4-30
4-13. Average Percent Base Flow of Selected Streams on Long Island by Area 4-32
4-14. Water Quality Parameters Affecting Habitat 4-35
4-15. Relative Toxicities of Samples Using Microtoxฎ Measurement Method 4-37
4-16. Delaware Insect Population Abundance by Degree of Urbanization 4-40
4-17. Relative Abundance of Native and Introduced Fish in Urbanized and Non-Urbanized Areas
in Coyote Creek, California 4-42
4-18. Effects of Urbanization on the Fish Community of Tuckahoe Creek, Virginia 4-44
4-19. Comparative Health Outcomes for Swimming in Front of Drains in Santa Monica Bay
4-47
5-1. Percent Runoff Volumes Contributed by Source Area in Two Urbanized Areas of Wisconsin
5-34
5-2. Contaminant Load Percentages in Two Urbanized Areas of Wisconsin 5-35
5-3. Recommended BMP Maintenance Schedules 5-38
5-4. Sources of Storm Water Runoff and BMP Monitoring Data 5-47
5-5. Monitoring Studies for BMP Categories 5-51
5-6. Extent of Monitoring for Selected Pollutants in BMP Performance Studies 5-52
5-7. Structural BMP Expected Pollutant Removal Efficiency 5-54
5-8. Pollutant Removal Efficiency of Infiltration Practices 5-55
5-9. Pollutant Removal Efficiency of Retention Basins 5-57
5-10. Summary of Prince William Parkway Regional Wet Pond Sampling Data 5-62
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5-11. Pollutant Removal Efficiency of Constructed Wetland Systems 5-68
5-12. Summary of Crestwood Marsh Constructed Wetland Sampling Data 5-72
5-13. Pollutant Removal Efficiency of Storm Water Filtration Systems 5-75
5-14. Summary of Hollywood Branch Peat/Sand Filter Storm Event Sampling Data 5-80
5-15. Summary of Hollywood Branch Peat/Sand Filter Baseflow Sampling Data 5-81
5-16. Pollutant Removal Efficiency of Open Channel Vegetated Systems 5-82
6-1. Typical Base Capital Construction Costs for BMPs 6-3
6-2. Base Costs of Typical Applications of Storm Water BMPs 6-4
6-3. Regional Cost Adjustment Factors 6-5
6-4. Base Capital Costs for Storm Water Ponds and Wetlands 6-7
6-5. Base Capital Costs for Infiltration Practices 6-9
6-6. Construction Costs for Various Sand Filters 6-12
6-7. Base Capital Costs of Vegetative BMPs 6-13
6-8. Design, Contingency and Permitting Costs 6-13
6-9. Relative Land Consumption of Storm Water BMPs 6-14
6-10. Annual Maintenance Costs 6-15
6-11. Data for the Commercial Site Scenario 6-17
6-12. BMP Costs for a Five Acre Commercial Development 6-18
6-13. Data for the Residential Site Scenario 6-19
6-14. BMP Costs for a Thirty-Eight Acre Residential Development 6-20
6-15. Street Sweeper Cost Data 6-21
6-16. Annualized Sweeper Costs 6-22
6-17. Public Education Costs in Seattle, Washington 6-23
6-18. Unit Program Costs for Public Education Programs 6-24
6-19. Comparison of Capital Costs of Municipal Infrastructure for a Single Dwelling Unit
6-26
6-20. Impervious Cover Reduction and Cost Savings of Conservation Development 6-27
6-21. Non-Structural BMPs Suited to Controlling Various Pollutants 6-29
List of Figures
4-1. Effects of Imperviousness on Runoff and Infiltration 4-4
4-2. Effects of Siltation on Rivers and Streams 4-12
4-3. Relationship Between Increasing Imperviousness and Urban Stream Temperature .... 4-21
4-4. Relationship Between Watershed Imperviousness and Baseflow Water Temperature . . 4-22
4-5. Proportions of Impaired Water Bodies Attributed to Urban Runoff 4-23
4-6. Relationship of Watershed Imperviousness to Runoff Coefficient Levels 4-25
4-7. Effect of Urbanization on Stream Slope and Flooding 4-28
4-8. Hydrographs for Urban and Non-Urban Streams 4-29
4-9. Sediment Loadings on Small Streams in Wisconsin 4-31
4-10. Relationship Between Urban Storm Water and Aquatic Ecosystems 4-33
4-11. Relationship Between Impervious Cover and Stream Quality 4-34
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4-12. Low pH Tolerance by Different Species 4-36
4-13. Comparison of a Healthy Stream Bank and an Eroding Bank 4-38
4-14. Effects of Sediment Deposits on Macroinvertebrates in Juday Creek, Indiana 4-41
4-15. Average Densities of Fish Eggs and Larvae in New York 4-43
4-16. Health Effects Observed Relative to Distance from Santa Monica Bay Storm Drains
4-46
4-17. Sources Associated with Shellfish Harvesting Restrictions, in Percent 4-48
5-1. Infiltration Basin 5-10
5-2. Porous Pavement System 5-11
5-3. Infiltration Trench 5-12
5-4. Detention Basin 5-13
5-5. Retention Pond 5-15
5-6. Constructed Wetland System 5-17
5-7. Filter Media 5-18
5-8. Austin Full Sedimentation-Filtration System 5-19
5-9. Underground Vault Sand Filter 5-20
5-10. Delaware Sand Filter 5-21
5-11. Alexandria Compound Filter 5-22
5-12. Bioretention System 5-24
5-13. Grass Filter Strip 5-27
5-14. Prince William Parkway Regional Wet Pond 5-61
5-15. Crestwood Marsh Constructed Wetland 5-71
5-16. Hollywood Branch Peat/Sand Filter 5-78
6-1. Rainfall Zones of the United States 6-6
6-2. Retention Basin Construction Cost 6-8
6-3. Infiltration Trench Cost 6-10
6-4. Infiltration Basin Construction Cost 6-11
6-5. Changes in Pollutant Load Associated with a Public Education Program 6-25
6-6. Effects of Impervious Cover on Stream Quality 6-33
6-7. Stormwater Control Points Along the Rainfall Frequency Spectrum 6-34
VI
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1.0 Summary
The significance of storm water runoff in affecting water quality in the United States has
become an increasing concern in recent years, as further improvements are made in controlling
other point sources such as municipal sewage and industrial waste. EPA conducted a broad
analysis of storm water runoff characteristics in its Nationwide Urban Runoff Program between
1979 and 1983. During the 1980's the Agency made several attempts to promulgate regulatory
controls for storm water runoff under the statutory framework of the 1972 Clean Water Act.
Following enactment of the Water Quality Act of 1987, EPA began development of a more
comprehensive regulatory program. During the course of these actions, the use of best
management practices (BMPs) in addressing runoff problems was frequently identified, however it
was known that additional research on the performance of BMPs was also needed.
EPA's Engineering and Analysis Division conducted a study on storm water best
management practices during 1997 and 1998 as part of its series of preliminary studies in the
effluent guidelines program. This report summarizes existing information and data regarding the
effectiveness of BMPs to control and reduce pollutants in urban storm water. The report
provides a synopsis of what is currently known about the expected costs and environmental
benefits of BMPs, and identifies information gaps as well.
Detailed information about BMP design is beyond the scope of this report. Readers are
encouraged to consult the wide range of storm water BMP design manuals available from states
and localities and other organizations for detailed design guidelines. Information regarding BMP
performance and selection is also provided in other EPA documents, such as Guidance Specifying
Management Measure for Sources ofNonpoint Source Pollution in Coastal Water (US EPA,
1993 a); Urban Runoff Pollution Prevention and Control Planning (US EPA, 1993c); and
Municipal Wastewater Management Fact Sheets: Storm Water Best Management Practices (US
EPA, 1996e). In addition, readers are encouraged to consult the ASCE/WEF Manuals of
Practice, Design and Construction of Urban Stormwater Management Systems (ASCE/WEF,
1992) and Urban Runoff Quality Management (ASCE/WEF, 1998) for a more thorough
discussion of storm water management design.
Summary of Findings
1. Waterways and receiving waters near urban and suburban areas are often adversely affected by
urban storm water runoff. Impacts may be manifested in terms of:
alterations in hydraulic characteristics of streams receiving runoff such as higher peak
flow rates, increased frequency and duration of bankfull and sub-bankfull flows,
increased occurrences of downstream flooding, and reduced baseflow levels
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changes in receiving stream morphology such as increased rates of sediment transport
and deposition, increased shoreline erosion, stream channel widening, and increased
stream bed scouring
aquatic habitat impacts leading to changes in fish and macroinvertebrate populations
and loss of sensitive species
public health and recreation impacts such as increased risk of illness due to contact
with contaminated water bodies, contamination of drinking water supplies, beach
closures, restrictions on fishing, and shellfish bed closures.
2. A wide variety of BMPs, both structural and non-structural, are available to address urban
storm water runoff and discharges.
For various reasons (such as cost, suitability to site, etc.) some of these BMP types are
widely used, some infrequently; some are relatively new designs that are not widely in
use.
Many BMPs are used primarily for water quantity control (i.e. to prevent flooding),
although they may provide ancillary water quality benefits.
Some BMP types have been analyzed for performance in terms of site-specific
pollutant removal, although not extensively enough to allow for generalizations.
The pollutant removal performance of some BMP types is essentially undocumented.
Some BMP types, particularly non-structural and those that do not have discrete inflow
or outflow points, are difficult to monitor.
There is no widely-accepted definition of "efficiency" or "pollutant removal" for storm
water BMPs.
The role of chemical pollutant monitoring vs. receiving stream biological monitoring in
evaluating BMP performance is not well documented.
3. Only a few cost studies have been conducted for storm water BMPs.
Due to the limited cost data, a lack of clear definitions of performance, and limited
"performance" data, it is difficult at this time to develop cost-effectiveness comparisons
for various BMP types.
4. The benefits of individual BMPs are site-specific and depend on a number of factors including:
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the number, intensity and duration of wet weather events;
the pollutant removal efficiency of the BMP;
the water quality and physical conditions of the receiving waters;
the current and potential use of the receiving waters; and
the existence of nearby "substitute" sites of unimpaired waters.
Because these factors will vary substantially from site to site, data are not available with which to
develop estimates of benefits for individual BMP types.
5. A number of researchers are continuing to work on BMP performance monitoring, and there
are several attempts underway to develop comparison frameworks through the construction of
comprehensive databases on BMP design characteristics and performance.
Organization of Report
This report is divided into six chapters. Chapter 1 presents a summary of the major
findings of the report. Chapter 2 presents a general introduction of the purposes and goals of this
evaluation. Chapter 3 summarizes existing regulations and permits developed by EPA to address
urban storm water discharges, including regulations under the National Pollutant Discharge
Elimination System (NPDES) and the Coastal Zone Act Reauthorization Amendments (CZARA).
Chapter 4 presents an assessment of the environmental problems attributable to urban storm water
discharges and Chapter 5 identifies the best management practices that can be used to control the
quantity and improve the quality of storm water prior to discharge. Chapter 6 identifies the costs
and benefits of storm water BMPs.
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2.0 Introduction and Scope
2.1 Effluent Guidelines Program and Consent Decree Requirements
Effluent guidelines are national standards for categories of dischargers to surface waters.
The program was established in 1972 under Title III of the Clean Water Act (CWA). Since that
time EPA has developed effluent guideline regulations for over 50 categories, primarily industrial
dischargers. In these regulations the Agency typically establishes numeric "end-of-pipe" effluent
limitations for specific chemical pollutants and/or indicator parameters (e.g. BOD, oil and grease).
For some categories, EPA has also issued narrative requirements for best management practices
(BMPs) to address control of storm water runoff, plant maintenance schedules and training of
plant personnel. The effluent limitations are generally based on the performance of available or
demonstrated control and treatment technologies. Resulting effluent limitations are commonly
referred to as "technology-based" standards. The regulations are implemented in National
Pollutant Discharge Elimination System (NPDES) permits, which are issued by EPA and State
agencies under the authority of CWA Section 402.
The Water Quality Act of 1987 added section 304(m) to the CWA. This provision
requires EPA to publish a biennial Effluent Guidelines Plan and develop additional regulations.
EPA's effluent guidelines program is currently subject to a consent decree ("Decree") in Natural
Resources Defense Council et al v. Browner (D.D.C. 89-2980, January 31, 1992, as amended).
The Decree requires the Agency to propose effluent guideline regulations and take final action for
20 point source categories, according to a specified schedule. Additionally, the Decree requires
that the Agency conduct 11 preliminary studies to assist in selecting categories for regulation
development.
The 1987 amendments also added section 402(p) to the CWA, which requires
development of a national program for regulation of storm water discharges. This is discussed
further in Chapter 3 of this report.
In 1996, the Natural Resources Defense Council (NRDC) recommended that EPA
develop effluent guidelines for categories of storm water dischargers, to supplement the existing
NPDES permit regulations covering storm water discharges. Because municipal storm water
discharges present a range of complex phenomena that have not been extensively documented in
the professional literature, and because there is a lack of generally accepted methods for
evaluating storm water management practices, EPA determined that conducting a preliminary
study would be appropriate to satisfy one of the study obligations under the Decree. This
preliminary study is intended to assist decision making on initiating regulatory development
projects.
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2.2 Types of Discharges Addressed
This study is focused on BMPs designed to prevent, control or treat storm water
discharges, and the nature and measurement of storm water discharges. Storm water discharges
may flow directly into surface waters, into municipal separate storm sewer systems ("MS4s"),
and/or infiltrate into groundwater. The emphasis on BMPs is intended to support the national
NPDES storm water program. Some aspects of the BMPs described herein may also be relevant
for other types of wet weather pollution problems, such as combined sewer overflows (CSOs).
Storm water BMPs may be organized into two major groups with multiple subgroups:
Structural BMPs include:
> infiltration systems such as infiltration basins and porous pavement
> detention systems such as basins and underground vaults
> retention systems such as wet ponds
> constructed wetland systems
> filtration systems such as media filters and bioretention systems
> vegetated systems such as grass filter strips and vegetated swales
> minimizing directly-connected impervious surfaces
> miscellaneous and vendor-supplied systems such as oil/water separators and
hydrodynamic devices
Non-Structural BMPs include:
> automotive product and household hazardous material disposal
> commercial and retail space good housekeeping
> industrial good housekeeping
> modified use of fertilizers, pesticides and herbicides
> lawn debris management
> animal waste disposal
> maintenance practices such as catch basin cleaning, street and parking lot
sweeping, road and ditch maintenance
> illicit discharge detection and elimination
> educational and outreach programs
> storm drain inlet stenciling
> low-impact development and land use planning.
The impacts of storm water discharges are described in Chapter 4. Various BMP designs for
addressing storm water discharges are described in Chapter 5, and the costs and economic
impacts of BMP are described in Chapter 6.
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2.3 Data Sources and Data Collection Techniques
ASCE National Stormwater BMP Database
Since 1995, EPA and the American Society of Civil Engineers (ASCE) have operated
under a cooperative agreement to develop a database of storm water BMP design and
performance. The initial version of this database provides pollutant removal data and other
performance measures on approximately 75 BMPs based on published studies and reports. These
studies and reports were carefully selected from a comprehensive screening of virtually all
available published literature on BMP performance, amounting to about 800 bibliographic
references.
A significant objective of the database is to provide a design tool for local storm water
designers and planners. The database has the capacity to report extensive detail about the design
of BMPs, along with descriptive information about the adjacent watershed, hydrology and other
geographic data.
As of early 1999, the initial version of the database is being tested, and a public release
will be available in mid-1999. EPA and ASCE are continuing to develop the database and are
encouraging organizations that have conducted BMP monitoring to submit their findings to the
ASCE Database Clearinghouse for entry into the database. As new data are gathered, periodic
updates will be made available to the public through use of the Internet.
Center for Watershed Protection National Pollutant Removal Performance Database
In 1997, the Center for Watershed Protection developed a database for the Chesapeake
Research Consortium titled, "National Pollutant Removal Performance Database for Stormwater
BMPs" (Brown and Schueler, 1997a). This database focuses on the pollutant removal efficiency
of commonly used and innovative urban BMPs for storm water control. The database is derived
from 123 research studies developed between 1977 and 1996.
All of the studies in the database utilized data collected with automated sampling
equipment and had documented methods to compute pollutant removal efficiencies. More than
three-quarters of the studies were based on four or more storm samples, while the remaining
studies were either based on fewer than four storms or the sample size was not stated.
Literature Cited
EPA reports including the Nationwide Urban Runoff Program (NURP), National
Water Quality Inventory, Coastal Nonpoint Pollution Program Guidance, NPDES
Rules, guidance documents and fact sheets.
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Other Federal agency publications from U.S. Geological Survey and U.S. Department
of Agriculture.
Professional journals and manuals of practice such as those from ASCE and the Water
Environment Federation
Publications of research organizations such as the Center for Watershed Protection,
Terrene Institute, Metropolitan Washington Council of Governments and the
Watershed Management Institute
State and local government BMP design manuals.
BMP Performance Data Developed for this Preliminary Study
EPA conducted field performance evaluations at three structural BMP sites during 1998.
While these evaluations contribute to the literature on BMP performance, EPA also intended that
the field testing would serve as an experimental framework for refining evaluation methodology.
Three sites in the Washington, D.C. area were monitored: a constructed wetland, a peat-sand
filter, and a regional wet pond. Data summaries for these monitoring activities appear in Chapter
5. Additional findings will be provided in a supplement to this report.
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3.0 Existing Storm Water Regulations and Permits
Congress added Section 402(p) to the Clean Water Act in 1987 to require implementation
of a comprehensive approach for addressing storm water discharges in two phases. Section
402(p)(4) required EPA to develop permit application regulations under the National Pollutant
Discharge Elimination System (NPDES), submission of NPDES permit applications, issuance of
NPDES permits, and compliance with NPDES permit conditions. Section 402(p)(6) requires
EPA to designate storm water discharges to be regulated (within the statutory definitions
provided in section 402(p)(2)) and establish a comprehensive regulatory program, which may
include performance standards, guidelines, guidance, and management practices and treatment
requirements.
3.1 Phase I NPDES
EPA promulgated the first phase of NPDES storm water permit application regulations
("Phase I") on November 16, 1990 (US EPA, 1990). The provisions addressing MS4s cover
those systems serving a population of 100,000 or more. This includes 173 cities, 47 counties and
additional systems designated by EPA or states based on such system's interrelationship with or
proximity to the aforementioned systems, such as state highway departments. A total of 260
permits, covering approximately 880 operators (local governments, state highway departments,
etc.) have been identified as subject to Phase I permit application requirements. As of late 1998,
approximately 228 such permits have been issued in final form.
The CWA requires that MS4 permits effectively prohibit non-storm water discharges into
the storm sewers as well as reduce the discharge of pollutants to the maximum extent practicable
(including management practices, control techniques and system, design and engineering methods,
and other provisions appropriate for the control of such pollutants).
Phase IMS4 permittees were required to submit an application that included source
identification information, precipitation data, existing data on the volume and quality of storm
water discharges, a list of receiving water bodies and existing information on impacts on receiving
waters, a field screening analysis for illicit connections and illegal dumping, and other information.
Following this submission, MS4 permittees were to gather and provide additional
information including:
discharge characterization data based on quantitative data from 5 to 10 representative
locations in approved sampling plans; estimates of the annual pollutant load and event
mean concentration of system discharges for selected conventional pollutants and
heavy metals; a proposed schedule to provide estimates of seasonal pollutant loads; and
the mean concentration for certain detected constituents in a representative storm
event;
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a proposed management program including descriptions of: structural and source
control measures that are to be implemented to reduce pollutants in runoff from
commercial and residential areas; a program to detect and remove illicit discharges; and
a program to control pollutants in construction site runoff.
The Phase I rule also covers storm water discharges "associated with industrial activity."
This includes facilities covered by effluent guidelines and other designated classes of industrial and
commercial facilities, such as hazardous waste treatment, storage, or disposal; landfills; recycling;
vehicle maintenance and equipment cleaning; sewage sludge handling; construction activity (sites
with 5 or more acres of disturbed land); and facilities where materials are exposed to storm water.
Permittees must prepare a storm water pollution prevention plan which describes pollution
sources, measures and controls.
EPA and the states used several permit mechanisms for the many facilities receiving
NPDES permits for the first time. EPA issued "baseline" general permits to cover a wide range of
facilities with basic requirements, with the intent that more specific requirements would follow in
subsequent permit cycles. Industry-specific or "group" permits were issued based on applications
submitted by business associations, and other sites were issued individual permits.
The management and pollution prevention plans prepared by MS4s and industrial
permittees vary in their level of detail and specificity regarding design and implementation of best
management practices (BMPs). EPA and some states have issued guidance on preparation of
these plans (US EPA, 1992d; US EPA, 1992e). The Agency has not conducted a nationwide
review of these plans.
3.2 Phase II NPDES
EPA proposed the NPDES storm water regulations for the second phase of storm water
discharge control ("Phase II") on January 9, 1998 (US EPA, 1998c). EPA is required to
promulgate the Phase II rule in 1999 under a separate consent decree.
The proposal designates two classes of facilities for automatic coverage on a nationwide
basis under the NPDES program, (1) small municipal separate storm sewer systems located in
urbanized areas (about 3,500 municipalities would be included in the program); and (2)
construction activities (pollutants include sediments and erosion from these sites) that disturb
equal to or greater than one and less than five acres of land (about 110,000 sites per year will be
included in the program). Those facilities designated above would need to apply for NPDES
storm water permits by 2002. EPA is anticipating that most permittees would be covered under
general permits.
EPA is also proposing to conditionally exclude from the NPDES storm water program
Phase I facilities that have "no exposure" of industrial activities, such as industrial products,
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processes, or raw materials, to storm water, thereby reducing application of the program to many
industrial activities currently covered by the program that have no industrial storm water
discharges.
Some facilities that EPA is proposing to cover under the Phase II rule are currently subject
to state and/or local storm water management requirements.
3.3 Coastal Zone Act Requirements
Section 6217 of the Coastal Zone Act Reauthorization Amendments (CZARA) of 1990
provides that States with approved coastal zone management programs must develop and submit
coastal nonpoint pollution control programs to EPA and the National Oceanic and Atmospheric
Administration (NOAA) for approval. Failure to submit an approvable program would result in a
reduction of federal grants to such states under both the Coastal Zone Management Act and
section 319 of the CWA.
State coastal nonpoint pollution control programs under CZARA are to include
enforceable policies and mechanisms that ensure implementation of the management measures
throughout the coastal management area. Section 6217(g)(5) defines management measures as
"economically achievable measures for the control of the addition of pollutants from existing and
new categories and classes of nonpoint sources of pollution, which reflect the greatest degree of
pollutant reduction achievable through the application of the best available nonpoint pollution
control practices, technologies, processes, siting criteria, operating methods, or other
alternatives." The amendments provide for a technology-based approach based on technical and
economic achievability under the rationale that neither States nor EPA have the money, time, or
other resources to create and expeditiously implement a program that depends on establishing
cause and effect linkages between particular land use activities and specific water quality
problems. If this technology-based approach fails to achieve and maintain applicable water quality
standards and to protect designated uses, sec. 6217(b)(3) requires additional management
measures.
EPA issued Guidance Specifying Management Measures for Sources of Nonpoint
Pollution in Coastal Waters under sec. 6217(g) in January 1993 (US EPA, 1993a). The guidance
identifies management measures for five major categories of nonpoint source pollution:
agriculture; forestry; urban; marinas and recreational boating; and hydromodification. The
management measures reflect the greatest degree of pollutant reduction that is economically
achievable for each of the listed sources. These management measures provide reference
standards for the states to use in developing or refining their coastal nonpoint programs. In
general, the management measures were written to describe systems designed to reduce the
generation of pollutants. A few management measures, however, contain quantitative standards
that specify pollutant loading reductions. For example, the new development management
measure, which is applicable to storm water runoff associated with construction in urban areas,
3 -3
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requires (1) that by design or performance the average annual total suspended solid loadings be
reduced by 80 percent and (2) to the extent practicable, that the pre-development peak runoff rate
and average volume be maintained. The management measures approach was adopted to provide
state officials with flexibility in selecting strategies and management systems and practices that are
appropriate for regional or local conditions, provided that equivalent or higher levels of pollutant
control are achieved.
Storm water discharges regulated under the existing NPDES program, such as discharges
from municipal separate storm sewers serving a population of 100,000 or more and from
construction activities that disturb 5 or more acres, do not need to be addressed in Coastal
Nonpoint Pollution Control programs. However, potential new sources, such as urban
development adjacent to or surrounding municipal systems serving a population of 100,000 or
more, smaller urbanized areas, and construction sites that disturb less than 5 acres, that are
identified in management measures under section 6217 guidance need to be addressed in Coastal
Nonpoint Pollution Control Programs until such discharges are issued an NPDES permit. EPA
and NOAA have worked and continue to work together in their activities to ensure that
authorities between NPDES and CZARA do not overlap.
EPA and NOAA published Coastal Nonpoint Pollution Control Program: Program
Development and Approval Guidance (US EPA, 1993d), which addresses such issues as the basis
and process for EPA/NOAA approval of State Coastal Nonpoint Pollution Control programs,
how EPA and NOAA expect state programs to implement management measures in conformity
with EPA guidance, and procedures for reviewing and modifying state coastal boundaries to meet
program requirements. The document clarifies that states generally must implement management
measures for each source category identified in the EPA guidance developed under section
6217(g). The document also sets quantitative performance standards for some measures. Coastal
Nonpoint Pollution Control programs are not required to address sources that are clearly
regulated under the NPDES program as point source discharges. Specifically, such programs
would not need to address small municipal separate storm sewer systems and construction sites
covered under NPDES storm water permits (both general and individual). The guidance also
clarifies that regulatory and non-regulatory mechanisms may be used to meet the requirement for
enforceable policies and mechanisms, provided that non-regulatory approaches are backed by
enforceable state authority ensuring that the management measures will be implemented. Backup
authority may include sunset provisions for incentive programs. For example, a state may provide
additional incentives if too few owners or operators participate in a tax incentive program or
develop mandatory requirements to achieve the necessary implementation of management
measures.
3.4 Regional, State and Local Programs
In addition to the existing Federal storm water management programs, there are a variety
of State, local and regional storm water management programs in existence. Many of these
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programs pre-date the Federal programs and may include BMP design or performance standards,
site plan review and inspection programs, and technical assistance. A review of these programs is
outside the scope of this report.
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4.0 Environmental Assessment
Waterways and receiving waters near urban and suburban areas are often adversely
affected by urban storm water runoff. The degree and type of impact varies from location to
location, but it is often significant relative to other sources of pollution and environmental
degradation. Urban storm water runoff affects water quality, water quantity, habitat and
biological resources, public health, and the aesthetic appearance of urban waterways. As reported
in the National Water Quality Inventory 1996 Report to Congress (US EPA, 1998d), urban runoff
was the leading source of pollutants causing water quality impairment related to human activities
in ocean shoreline waters and the second leading cause in estuaries across the nation. Urban
runoff was also a significant source of impairment in rivers and lakes. The percent of total
impairment attributed to urban runoff is substantial. This impairment constitutes approximately
5,000 square miles of estuaries, 1.4 million acres of lakes, and 30,000 miles of rivers. Seven
states also reported in the Inventory that urban runoff contributes to wetland degradation.
Adverse impacts on receiving waters associated with storm water discharges have been
discussed by EPA (1995b) in terms of three general classes. These are:
Short-term changes in water quality during and after storm events including temporary
increases in the concentration of one or more pollutants, toxics or bacteria levels.
Long-term water quality impacts caused by the cumulative effects associated with
repeated storm water discharges from a number of sources.
Physical impacts due to erosion, scour, and deposition associated with increased
frequency and volume of runoff that alters aquatic habitat.
As described in the Terrene Institute's Fundamentals of Urban Runoff Management
(Horner et al, 1994), pollutants associated with urban runoff potentially harmful to receiving
waters fall into the categories listed below:
Solids
Oxygen-demanding substances
Nitrogen and phosphorus
Pathogens
Petroleum hydrocarbons
Metals
Synthetic organics.
These pollutants degrade water quality in receiving waters near urban areas, and often
contribute to the impairment of use and exceedences of criteria included in State water quality
standards. The quantity of these pollutants per unit area delivered to receiving waters tends to
increase with the degree of development in urban areas.
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While water quality impacts are often unobserved by the general public, other storm water
impacts are more visible. Stream channel erosion and channel bank scour provide direct evidence
of water quantity impacts caused by urban storm water. Urban runoff increases directly with
imperviousness and the degree of watershed development. As urban areas grow, urban streams
are forced to accommodate larger volumes of storm water runoff that recur on a more frequent
basis. This leads to stream channel instability. The change in watershed hydrology associated with
urban development also causes channel widening and scour, and the introduction of larger
amounts of sediment to urban streams. Visible impacts include eroded and exposed stream banks,
fallen trees, sedimentation, and recognizably turbid conditions. The increased frequency of
flooding in urban areas also poses a threat to public safety and property.
Both water quality and water quantity impacts associated with urban storm water combine
to impact aquatic and riparian habitat in urban streams. Higher levels of pollutants, increased flow
velocities and erosion, alteration of riparian corridors, and sedimentation associated with storm
water runoff negatively impact the integrity of aquatic ecosystems. These impacts include the
degradation and loss of aquatic habitat, and reduction in the numbers and diversity offish and
macroinvertebrates.
Public health impacts are for the most part related to bacteria and disease causing
organisms carried by urban storm water runoff into waters used for water supplies, fishing and
recreation. Water supplies can potentially be contaminated by urban runoff, posing a public health
threat. Bathers and others coming in contact with contaminated water at beaches and other
recreational sites can become seriously ill. Beach closures caused by urban runoff have a negative
impact on the quality of life, and can impede economic development as well. Similarly, the
bacterial contamination of shellfish beds poses a public health threat to consumers, and shellfish
bed closures negatively impact the fishing industry and local economies.
Aesthetic impacts in the form of debris and litter floating in urban waterways and
concentrated on stream banks and beaches are quite visible to the general public. Storm water is
a major source of floatables that include paper and plastic bags and packaging materials, bottles,
cans, and wood. The presence of floatables and other debris in receiving waters during and
following storm events reduces visual attractiveness of the waters and detracts from their
recreational value. Nuisance algal conditions including surface scum and odor problems can also
be attributed to urban storm water in many instances.
Based on available information and data, the following general statements can be made
about urban storm water impacts.
Impacts to water quality in terms of water column chemistry tend to be transient and
elusive, particularly in rivers.
Impacts to habitat and aquatic life are generally more profound, and are easier to see
and quantify than changes in water column chemistry.
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Impacts are typically complex because urban storm water is often one of several
sources including municipal discharges and diffuse runoff from agricultural and rural
areas that affect urban waterways.
Impacts are often interrelated and cumulative. For example, both degraded water
quality and increased water quantity join to impact habitat and biological resources.
The following sections describe the sources of urban storm water runoff, the pollutants
contained in urban runoff and the impacts attributable to urban storm water discharges. Examples
supported by field observation and data have been used extensively to show storm water impacts.
The impacts described include water quality impacts, water quantity impacts, public health
impacts, habitat impacts, and aesthetic impacts.
4.1 Overview of Storm Water Discharges
Storm water runoff from urbanized areas is generated from a number of sources including
residential areas, commercial and industrial areas, roads, highways and bridges. Essentially, any
surface which does not have the capability to pond and infiltrate water will produce runoff during
storm events. When a land area is altered from a natural forested ecosystem to an urbanized land
use consisting of rooftops, streets and parking lots, the hydrology of the system is significantly
altered. Water which was previously ponded on the forest floor, infiltrated into the soil and
converted to groundwater, utilized by plants and evaporated or transpired into the atmosphere is
now converted directly into surface runoff. An important measure of the degree of urbanization
in a watershed is the level of impervious surfaces. As the level of imperviousness increases in a
watershed, more rainfall is converted to runoff. Figure 4-1 illustrates this transformation.
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Figure 4-1. Effects of Imperviousness on Runoff and Infiltration
Natural Ground Cover
A
40% Evapotranspiratiofi
25% Shallow
Infiltration
25% Deep
Infiltration
30-50% Impervious
A
35% Evapotranspiration
20 % Shallow
Infiltration
15% Deep
Infiltration
Source: Adapted from Arnold and Gibbons, 1996
10-20% Impervious
38% Evapotranspiratian
21% Shallow
Infiltration
21% Deep
Infiltration
75-100% Impervious
30% Evapotranspiration
10% Shallow 5% Deep
Infiltration Infiltration
The traditional means of managing storm water runoff in urban areas has been to construct
a vast curb-and-gutter, catch basin, and storm drain network to transport this runoff volume
quickly and efficiently away from the urbanized area and discharge the water to receiving streams.
Two types of sewer systems are used to convey storm water runoff: separate storm sewers and
combined sewers.
Separate storm sewer systems convey only storm water runoff. Water conveyed in
separate storm sewers is frequently discharged directly to receiving streams without
receiving any intentional form of treatment. (In a municipality with a separate storm
sewer system, sanitary sewer flows are conveyed in a distinct sanitary sewer system to
municipal wastewater treatment plants.)
In a combined sewer system, storm water runoff is combined with sanitary sewer flows
for conveyance. Flows from combined sewers are treated by municipal wastewater
4-4
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treatment plants prior to discharge to receiving streams. During large rainfall events
however, the volume of water conveyed in combined sewers can exceed the storage
and treatment capacity of the wastewater treatment system. As a result, discharges of
untreated storm water and sanitary wastewater directly to receiving streams can
frequently occur in these systems. These types of discharges are known as combined
sewer overflows (CSOs).
Historically, as urbanization occurred and storm drainage infrastructure systems were
developed in this country, the primary concern was to limit nuisance and potentially damaging
flooding due to the large volumes of storm water runoff that are generated. Little, if any, thought
was given to the environmental impacts of such practices. As a result, streams that receive storm
water runoff frequently cannot convey the large volumes of water generated during runoff events
without significant degradation of the receiving stream. In addition to the problems associated
with excess water volume, the levels of toxic or otherwise harmful pollutants in storm water
runoff and CSOs can cause significant water quality problems in receiving streams.
In addition to point sources such as municipal separate storm sewers and combined sewer
overflows, storm water runoff can enter receiving streams as a non-point source. Storm water
runoff from a variety of sources such as parking lots, highways, open land, rangeland, residential
areas and commercial areas can enter waterways directly as sheet flow or as a series of diffuse,
discrete flows. Due to the diffuse nature of many storm water discharges, it is difficult to quantify
the range of pollutant loadings to receiving streams that are attributable to storm water
discharges. It is much easier, however, to measure the increased stream flows during rainfall
events that occur in urbanized areas and to document impacts to streams that receive storm water
runoff.
Awareness of the damaging effects storm water runoff is causing to the water quality and
aquatic life of receiving streams is a relatively recent development. Storm water management
traditionally was, and still is in many cases, a flood control rather than a quality control program.
Local governments intending to improve the quality of their runoff-impacted streams are
incorporating best management practices (BMPs) into their drainage programs. BMPs which
reduce the volume of runoff discharged to receiving streams, such as minimizing directly
connected impervious surfaces, providing on-site storage and infiltration and implementing stream
buffers and restoring riparian cover along urban streams can help to prevent further degradation
and even result in improvements of streams which receive storm water discharges. However, in
many existing urbanized areas, the cost of infrastructure changes necessary to retrofit existing
storm water drainage systems with structural BMPsto provide for storm water quality as well as
quantity control-can be prohibitively expensive. In these cases, non-structural BMPs can be
implemented to reduce pollutant sources and to reduce the transfer of urban pollutants to runoff,
before more expensive, structural controls are instituted.
The climate of a region can have a significant impact on the quantity and quality of storm
water runoff. Factors such as the length of the antecedent dry periods between storms, the
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average rainfall intensity, the storm duration and the amount of snowmelt present can have
significant impacts on the characteristics of runoff from an area. In areas where there is a
significant amount of atmospheric deposition of particulates, storm water runoff can contain high
concentrations of suspended solids, metals and nutrients. Areas that have infrequent rainfall such
as the southwest U.S. can have runoff with significant concentrations of pollutants, especially
from "hot spots" such as roads, parking lots and industrial areas. These areas, which typically
have high-intensity, short-duration rainfall events, can generate significant loadings of suspended
solids in storm water runoff. Many specific geographic factors can influence the nature and
constituents contained in storm water runoff. Factors such as the soil types, slopes, land use
patterns and the amount of imperviousness of a watershed can greatly affect the quality and
quantity of runoff that is produced from an area.
4.2 Pollutants in Urban Storm Water
Storm water runoff from urban areas can contain significant concentrations of harmful
pollutants that can contribute to adverse water quality impacts in receiving streams. Effects can
include such things as beach closures, shellfish bed closures, limits on fishing and limits on
recreational contact in waters that receive storm water discharges. Contaminants enter storm
water from a variety of sources in the urban landscape.
Urban storm water runoff has been the subject of intensive research since the inception of
the Water Quality Act of 1965. There have been numerous studies conducted to characterize the
nature of urban storm water runoff and the performance of storm water BMPs. Data sources
include the "208 Studies," the area-wide waste treatment management plans conducted by states
under section 208 of the 1972 CWA; EPA's Nationwide Urban Runoff Program (NURP); the
U.S. Geological Survey (USGS) Urban Stormwater Database; and the Federal Highway
Administration (FHWA) study of storm water runoff loadings from highways. In addition to
these federal sources, there is a great deal of information in the technical literature, as well as data
collected by states, counties and municipalities. A recent data source is storm water monitoring
data collected by municipalities regulated by the Phase INPDES storm water regulations. As part
of the Phase I permit application, regulated municipalities were required to collect data from five
representative sites during a minimum of three storm events.
The most comprehensive study of urban runoff was NURP, conducted by EPA between
1978 and 1983. NURP was conducted in order to examine the characteristics of urban runoff
and similarities or differences between urban land uses, the extent to which urban runoff is a
significant contributor to water quality problems nationwide, and the performance characteristics
and effectiveness of management practices to control pollution loads from urban runoff (US EPA,
1983). Sampling was conducted for 28 NURP projects which included 81 specific sites and more
than 2,300 separate storm events. NURP focused on the following ten constituents:
Total Suspended Solids (TSS)
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Biochemical Oxygen Demand (BOD)
Chemical Oxygen Demand (COD)
Total Phosphorus (TP)
Soluble Phosphorus (SP)
Total Kjeldahl Nitrogen (TKN)
Nitrate + Nitrite (N)
Total Copper (Cu)
Total Lead (Pb)
Total Zinc (Zn).
NURP examined both the soluble and the particulate fraction of pollutants, since the water
quality impacts can depend greatly on the form that the contaminant is present. NURP also
examined coliform bacteria and priority pollutants at a subset of sites. Median event mean
concentrations (EMCs) for the ten general NURP pollutants for various urban land use categories
are presented in Table 4-1.
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Table 4-1. Median Event Mean Concentrations for Urban Land Uses
Pollutant
BOD
COD
TSS
Total Lead
Total Copper
Total Zinc
Total Kjeldahl
Nitrogen
Nitrate +
Nitrite
Total
Phosphorus
Soluble
Phosphorus
Units
mg/1
mg/1
mg/1
Hg/1
Hg/1
Hg/1
Hg/1
Hg/1
Hg/1
MS/1
Residential
Median
10
73
101
144
33
135
1900
736
383
143
cov
0.41
0.55
0.96
0.75
0.99
0.84
0.73
0.83
0.69
0.46
Mixed
Median
7.8
65
67
114
27
154
1288
558
263
56
COV
0.52
0.58
1.14
1.35
1.32
0.78
0.50
0.67
0.75
0.75
Commercial
Median
9.3
57
69
104
29
226
1179
572
201
80
COV
0.31
0.39
0.85
0.68
0.81
1.07
0.43
0.48
0.67
0.71
Open/
Non-Urban
Median
40
70
30
195
965
543
121
26
COV
0.78
2.92
1.52
0.66
1.00
0.91
1.66
2.11
COV: Coefficient of variation
Source: Nationwide Urban Runoff Program (US EPA 1983)
Results from NURP indicate that there is not a significant difference in pollutant
concentrations in runoff from different urban land use categories. There is a significant difference,
however, in pollutant concentrations in runoff from urban sources than that produced from non-
urban areas.
The pollutants that are found in urban storm water runoff originate from a variety of
sources. The major sources include contaminants from residential and commercial areas, industrial
activities, construction, streets and parking lots, and atmospheric deposition. Contaminants
commonly found in storm water runoff and their likely sources are summarized in Table 4-2.
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Table 4-2. Sources of Contaminants in Urban Storm Water Runoff
Contaminant
Contaminant Sources
Sediment and Floatables
Streets, lawns, driveways, roads, construction
activities, atmospheric deposition, drainage
channel erosion
Pesticides and Herbicides
Residential lawns and gardens, roadsides,
utility right-of-ways, commercial and
industrial landscaped areas, soil wash-off
Organic Materials
Residential lawns and gardens, commercial
landscaping, animal wastes
Metals
Automobiles, bridges, atmospheric deposition,
industrial areas, soil erosion, corroding metal
surfaces, combustion processes
Oil and Grease/
Hydrocarbons
Roads, driveways, parking lots, vehicle
maintenance areas, gas stations, illicit
dumping to storm drains
Bacteria and Viruses
Lawns, roads, leaky sanitary sewer lines,
sanitary sewer cross-connections, animal
waste, septic systems
Nitrogen and Phosphorus
Lawn fertilizers, atmospheric deposition,
automobile exhaust, soil erosion, animal
waste, detergents
The concentrations of pollutants found in urban runoff are directly related to degree of
development within the watershed. This trend is shown in Table 4-3, a compilation of typical
pollutant loadings from different urban land uses.
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Table 4-3. Typical Pollutant Loadings from Runoff by Urban Land Use (Ibs/acre-yr)
Land Use
Commercial
Parking Lot
HDR
MDR
LDR
Freeway
Industrial
Park
Construction
TSS
1000
400
420
190
10
880
860
o
5
6000
TP
1.5
0.7
1
0.5
0.04
0.9
1.3
0.03
80
TKN
6.7
5.1
4.2
2.5
0.03
7.9
3.8
1.5
NA
NH3-N
1.9
2
0.8
0.5
0.02
1.5
0.2
NA
NA
NO2+NO3-N
3.1
2.9
2
1.4
0.1
4.2
1.3
0.3
NA
BOD
62
47
27
13
NA
NA
NA
NA
NA
COD
420
270
170
72
NA
NA
NA
2
NA
Pb
2.7
0.8
0.8
0.2
0.01
4.5
2.4
0
NA
Zn
2.1
0.8
0.7
0.2
0.04
2.1
7.3
NA
NA
Cu
0.4
0.04
0.03
0.14
0.01
0.37
0.5
NA
NA
HDR: High Density Residential, MDR: Medium Density Residential, LDR: Low Density Residential
NA: Not available; insufficient data to characterize loadings
Source: Horner et al, 1994
As indicated in Table 4-3, urban storm water runoff can contain significant concentrations
of solids, nutrients, organics and metals. A comparison of the concentration of water quality
parameters in urban runoff with the concentrations in domestic wastewater is shown in Table 4-4.
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Table 4-4. Comparison of Water Quality Parameters in Urban Runoff with Domestic
Wastewater (mg/1)
Constituent
COD
TSS
Total P
Total N
Lead
Copper
Zinc
Fecal Coliform
per 100ml
Urban Runoff
Separate Sewers
Range
200-275
20-2,890
0.02-4.30
0.4-20.0
0.01-1.20
0.01-0.40
0.01-2.90
400-50,000
Typical
75
150
0.36
2
0.18
0.05
0.02
Domestic Wastewater
Before Treatment
Range
250-1,000
100-350
4-15
20-85
0.02-0.94
0.03-1.19
0.02-7.68
106-108
Typical
500
200
8
40
0.10
0.22
0.28
After Secondary
Typical
80
20
2
30
0.05
0.03
0.08
200
Source: Bastian, 1997
As indicated in Table 4-4, the concentrations of select water quality parameters in urban
runoff is comparable to that found in untreated domestic wastewater. When untreated urban
runoff is discharged directly to receiving streams, the loadings of pollutants can be much higher
than the loadings attributable to treated domestic wastewater.
The following paragraphs summarize the major pollutants which are commonly found in
urban storm water runoff.
4.2.1 Solids. Sediment and Floatables
Solids are one of the most common contaminants found in urban storm water. Solids
originate from many sources including the erosion of pervious surfaces and dust, litter and other
particles deposited on impervious surfaces from human activities and the atmosphere. Stream
bank erosion and erosion at construction sites are also major sources of solids. Solids contribute
to many water quality, habitat and aesthetic problems in urban waterways. Elevated levels of
solids increase turbidity, reduce the penetration of light at depth within the water column, and
limit the growth of desirable aquatic plants. Solids that settle out as bottom deposits contribute to
sedimentation and can alter and eventually destroy habitat for fish and bottom-dwelling organisms
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(see Figure 4-2). Solids also provide a medium for the accumulation, transport and storage of
other pollutants including nutrients and metals. Sediment bound pollutants often have a long
history of interaction with the water column through cycles of deposition, re-suspension, and re-
deposition. Impaired navigation due to sedimentation represents another impact affecting
recreation and commerce. The relative contribution of TSS in urban storm water from different
land uses is presented in Table 4-3. As shown in Table 4-4, the typical concentration of TSS in
urban runoff is substantially higher than that in treated wastewater (Bastian, 1997). Construction
produces the highest loading of TSS over other urban land use categories evaluated.
Figure 4-2. Effects of Siltation on Rivers and Streams
Sediment
abrades gills
Sediment suffocates
fish eggs and bottom-
dwelling organisms
Sediment smothers cobbles
where fish lay eggs
Source: US EPA, 1998d.
4.2.2 Oxygen-Demanding Substances and Dissolved Oxygen
The oxygen-demanding substances found in urban storm water can be measured by
Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), and Total Organic
Carbon (TOC). Maintaining appropriate levels of dissolved oxygen in receiving waters is one of
the most important considerations for the protection offish and aquatic life. The amount of
dissolved oxygen in urban runoff is typically 5.0 mg/1 or greater, and it rarely poses a direct threat
to in-stream conditions. As shown in Table 4-4, the level of COD associated with urban runoff is
comparable to treated wastewater. The direct impact of urban storm water runoff on dissolved
oxygen conditions in receiving waters is not thought to be substantial. However, the secondary
impacts on the dissolved oxygen balance in receiving waters due to nutrient enrichment,
eutrophication, and resulting sediment oxygen demand may be important.
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4.2.3 Nitrogen and Phosphorus
Nitrogen and phosphorus are the principal nutrients of concern in urban storm water. The
major sources of nutrients in urban storm water are urban landscape runoff (fertilizers, detergents,
plant debris), atmospheric deposition, and improperly functioning septic systems (Terrene
Institute, 1996). Animal waste can also be an important source. There are a number of
parameters used to measure the various forms of nitrogen and phosphorus found in runoff.
Ammonia (NH3) nitrogen is the nitrogen form that is usually the most readily toxic to aquatic life.
Nitrate (NO3) and nitrite (NO2) are the inorganic fractions of nitrogen. Very little nitrite is usually
found in storm water. Total Kjeldahl nitrogen (TKN) measures the organic and ammonia
nitrogen forms. By subtraction, the organic fraction can be determined. Total phosphorus
measures the total amount of phosphorus in both the organic and inorganic forms. Ortho-
phosphate measures phosphorus that is most immediately biologically available. Most of the
soluble phosphorus in storm water is usually present in the ortho-phosphate form.
The degree to which nitrogen and phosphorus are present in a river, lake or estuary can
determine the trophic status and amount of algal biomass produced. Excess nutrients tend to
increase primary biological productivity. The major impact associated with nutrient over-
enrichment is excessive growth of algae that leads to nuisance algal blooms and eutrophic
conditions. A secondary impact is the residual negative effect of decomposing algae in the form
of sediment oxygen demand that depletes dissolved oxygen concentrations, particularly in bottom
waters. The NURP study reported that nutrient levels in urban runoff appear not to be high in
comparison with other possible discharges. However, more recent studies and programs have
recognized that the amount of nitrogen and phosphorus present in urban storm water can be
substantial, and becomes increasingly important as other point sources of nutrients are brought
under control. Walker (1987) reported that "cause-effect relationships linking urban development
to lake and reservoir eutrophication are well established," and that "urban watersheds typically
export 5 to 20 times as much phosphorus per unit per year, as compared to undeveloped
watersheds in a given region." The nutrient loadings from different urban and suburban land uses
are presented in Table 4-3. As shown in Table 4-4, the total phosphorus and total nitrogen
concentrations in urban runoff are substantially less than treated wastewater concentrations, but
storm water volumes can be greater during wet weather events.
4.2.4 Pathogens
Pathogens are disease-producing organisms that present a potential public health threat
when they are present in contact waters. Since storm water runoff typically does not come into
contact with domestic wastewaters, and direct exposure to runoff is usually limited, there is
generally little threat of pathogens in storm water runoff causing a public health risk. However,
where runoff is discharged to recreational waters such as beaches and lakes, or where runoff
comes into contact with shellfish beds, there is a potential public health risk associated with
pathogen contamination.
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There are a number of indicator organisms that have been used to evaluate the presence of
harmful pathogens in storm water runoff. Several strains of bacteria are present naturally in the
soil and can be transported by runoff. In addition, BMPs with standing water can be breeding
grounds for naturally occurring bacteria. Therefore, interpretation of bacteriological sampling
results can be difficult. Nevertheless, indicator organisms can provide useful insight into the
public health risk associated with runoff. Fecal coliform has been widely used as an indicator for
the presence of harmful pathogens in domestic wastewaters, and therefore studies characterizing
storm water runoff have frequently used this indicator as well. Other bacterial indicators that
have been used to evaluate the presence of harmful pathogens in storm water runoff include
Escherichia coll, streptococci and enterococci. The presence of enteric viruses has also been
evaluated in storm water runoff, as well as protozoans such as Giardia lamblia and
cryptosporidium.
Fecal coliform concentrations in urban runoff were evaluated by NURP at 17 sites for 156
storm events. NURP reported that coliform bacteria are present at high levels in urban runoff and
can be expected to exceed EPA water quality criteria during and immediately after storm events in
many surface waters, even those providing high degrees of dilution. Concentrations of fecal
coliform found by NURP exhibited a large degree of variability, and did not indicate any
distinctions based on land use. Data from different sites did show a dramatic seasonal effect on
coliform concentrations. Coliform counts in urban runoff during warmer periods of the year were
found to be approximately 20 times greater that those found during colder periods. Based on this
data, NURP concluded that coliform sources unrelated to those traditionally associated with
human health risk may be significant.
The Terrene Institute (1996) reported that the primary sources of pathogens in urban
storm water drains are animal wastes (including pets and birds), failing septic systems, illicit
sewage connections, and boats and marinas. Field et al (1993) reported pathogens levels from
storm water runoff and urban streams as shown in Table 4-5. Pathogens enumerated included
bacteria (total and fecal coliform, fecal streptococci, enterococci, Pseudomonas aeruginosa,
Staphylococcus aureus, and Salmonella) and enteroviruses (poliovirus, Coxsackie virus, and
Echovirus).
4- 14
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Table 4-5. Densities of Selected Pathogens and Indicator Microorganisms in Storm Water
in Baltimore, Maryland Area
Geometric Mean Densities
Sampling
Station
Bush St.
Northwood
Entero-
virus
PFU/
10L
6.9
170
Salmon
sp.
MPN/
10L
30
5.7
Pseudomon.
aeruginosa
MPN/
10L
2000
590
Staph.
aureus
MPN/
100 mL
120
12
Total
Coliform
MPN/
100 mL
(10A4)
38
3.8
Fecal
Coliform
MPN/
100 mL
(10A3)
83
6.9
Fecal
Strep.
No./
100 mL
(10A4)
56
5
Enterococci
No./lOO mL
(10A4)
12
2.1
PFU: Plaque-forming units
MPN: Most Probable Number
Source: Field et al, 1993
As shown earlier in Table 4-4, typical fecal coliform concentrations for separate urban
storm sewers varied widely, ranging between 400-50,000 mpn/100 ml. An example of fecal
coliform concentrations measured in sheet flow associated with different impervious surfaces is
presented in Table 4-6. The broad range in concentrations illustrates the highly variable nature of
fecal coliform concentrations in storm water.
Table 4-6. Fecal Coliform Concentrations Collected in Sheetflow from Urban Land Uses
Land Use
Unpaved driveways and storage areas
Roof runoff
Sidewalks
Paved parking and driveways
Paved roads
Median
(MPN/100 ml)
26
1.6
55
2.8
19
Range
(MPN/100 ml)
0.02-300
0.56-2.6
19-90
0.03-66
1.8-430
MPN: Most Probable Number
Source: Field et al, 1993.
4.2.5 Petroleum Hydrocarbons
Petroleum hydrocarbons include oil and grease; the "BTEX" compounds: benzene,
toluene, ethyl benzene, and xylene; and a variety of polynuclear aromatic hydrocarbons (PAHs).
Sources of petroleum hydrocarbons include parking lots and roadways, leaking storage tanks,
4- 15
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auto emissions, and improper disposal of waste oil. Petroleum hydrocarbons are typically
concentrated along transportation corridors.
Petroleum hydrocarbons are known for their acute toxicity at low concentrations
(Schueler, 1987). A study by Shepp (1996) measured the petroleum hydrocarbon concentrations
in urban runoff from a variety of impervious areas in the District of Columbia and suburban
Maryland. The amount of car traffic affects the concentration of hydrocarbons in runoff, with
median concentrations ranging from 0.7 to 6.6 mg/1. Concentrations at these levels exceed the
maximum concentrations recommended for the protection of drinking water supplies and fisheries
protection. As pointed out by Shepp, the maximum concentration of petroleum hydrocarbons for
protection of fisheries is 0.01 to 0.1 mg/1.
4.2.6 Metals
The primary sources of metals in urban storm water are industry and automobiles.
Atmospheric deposition (both wet and dry) can make a substantial contribution in some parts of
the country. A major finding of the NURP study is as follows:
Heavy metals (especially copper, lead and zinc) are by far the most prevalent priority pollutant
constituents found in urban runoff. End-of-pipe concentrations exceed EPA ambient water
quality criteria and drinking water standards in many instances. Some of the metals are present
often enough and in high enough concentrations to be potential threats to beneficial uses.
Metals in urban storm water have the potential to impact water supply and cause acute or
chronic toxic impacts for aquatic life. Typical pollutant loading rates and urban runoff
concentrations for lead, zinc and copper are presented in Tables 4-3 and 4-4. The frequency with
which metals were detected as priority pollutants in the NURP study is presented in Table 4-7.
4- 16
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Table 4-7. Most Frequently Detected Priority Pollutants in Nationwide Urban Runoff
Program Samples (1978-83)
Inorganics
Organics
Detected in 75% or more
94% Lead
94% Zinc
91% Copper
None
Detected in 50-74%
58% Chromium
52% Arsenic
None
Detected in 20-49%
48% Cadmium
43% Nickel
23% Cyanides
22% Bis(2-ethylhexyl)phthalate
20% a-Hexachloro-cyclohexane
Detected in 10-19%
13% Antimony
12% Beryllium
11% Selenium
19% a-Endosulfan
19% Pentachlorophenol*
17% Chlordane*
15%Lindane*
15%Pyrene**
14% Phenol
12%Phenanthrene**
1 1% Dichloromethane
10%4-Nitrophenol
10% Chrysene**
10%Fluoranthene**
* Chlorinated hydrocarbon
** Poly nuclear aromatic hydrocarbon
Source: US EPA, 1983
A major study of the quality of Wisconsin storm water (Bannerman et al, 1996) found that
the probability of event mean concentrations for some metals (particularly copper and zinc)
exceeding Wisconsin water quality criteria for cold water fish communities was high (Table 4-8).
A study in Coyote Creek, California reported lead and zinc levels from urban runoff of 100 to 500
times the concentration in the ambient water column (Pitt, 1995).
4- 17
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Table 4-8. Probability of Event Mean Concentration of Constituents in Wisconsin Storm
Water Exceeding Wisconsin Surface Water and Ground Water Quality Standards: Metals
Constituent
Cadmium, total recoverable
Copper, total recoverable
Lead, total recoverable
Silver, total recoverable
Zinc, total recoverable
Probability of exceeding
acute toxicity criteria for
cold water fish
communities (percent)
Storm Sewers
11
87
18
20
91
Streams
0
9
0
-
7
Source: Bannerman et al, 1996.
4.2.7 Synthetic Organic Compounds
Synthetic organic compounds include a variety of manufactured compounds covering
pesticides, solvents and household and industrial chemicals. The frequency that synthetic
inorganics were detected as priority pollutants in the NURP study is presented in Table 4-7. In
general, organic contaminants were found in less than 20 percent of samples. Nevertheless,
synthetic organics do represent a threat. Even low concentrations of some synthetic organics
over a long period of time have the potential to pose a severe health risks to humans and aquatic
life though direct ingestion or bioaccumulation in the food chain. There is also some evidence
that pesticides are found in higher concentrations in urban areas than agricultural areas (US EPA,
1995b). Further, Bannerman et al found that the probability for storm water and urban stream
samples to exceed human cancer criteria for public water supply, and toxicity criteria for
coldwater fish communities equaled or approached 100 percent for 10 compounds (Table 4-9).
4- 18
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Table 4-9. Probability of Event Mean Concentration of Constituents in Wisconsin Storm
Water Exceeding Wisconsin Surface Water and Ground Water Quality Standards:
Synthetic Organic Compounds
Constituent
(Human cancer criteria
for public water
supply/ coldwater fish
communities)
B enzo [a] anthracene
Benzo[a]pyrene
Benzo[b]fluoranthene
Benzo[ghi]perylene
B enzo [kjfluoranthene
Chrysene
Indeno pyrene
Phenanthrene
Pyrene
DDT
Probability of exceedance
(percent)
Storm Sewers
98
99
100
99
99
100
100
100
100
98
Streams
100
100
100
100
99
100
99
99
100
100
Source: Bannerman et al, 1996
4.2.8 Temperature
Water temperature is an important measure of water quality. As described by Malina
(1996), "the temperature of water affects some of the important physical properties and
characteristics of water, such as... specific conductivity and conductance, salinity, and the
solubility of dissolved gases (e.g., oxygen and carbon dioxide)." Specifically, water holds less
oxygen as it becomes warmer, resulting in less oxygen being available for respiration by aquatic
organisms. Furthermore, elevated temperatures increase the metabolism, respiration, and oxygen
demand offish and other aquatic life, approximately doubling the respiration for a 10ฐC (18ฐF)
temperature rise; hence the demand for oxygen is increased under conditions where supply is
lowered (California SWRCB, 1963).
Certain species offish, such as salmon and trout, are particularly sensitive and require
relatively low water temperatures. Even lower temperatures are required for spawning and egg
4- 19
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hatching (US EPA, 1976). If the temperature of a stream reach is raised by 5 to 10ฐC (9 to
18ฐF), it is probable that such cold-water game fish will avoid this reach and that they will be
replaced by "rougher," more tolerant fish (California SWRCB, 1963). Thus, even without direct
mortality, the character of the fish life will change. Sudden changes in temperature directly stress
the aquatic ecosystem. The states have adopted varying criteria to protect fisheries from such
stresses. Typically, states limit in-stream temperature rises above natural ambient temperatures to
2.8ฐC (5ฐF). Allowable temperature rises in streams that support cold water fisheries may be
lower, with some states adopting values as low as 1 ฐC (1.8ฐF) and 0.6ฐC (1 ฐF) (US EPA, 1988).
The temperature of urban waters is often affected directly by urban runoff. Urban runoff
can be heated as it flows over rooftops, parking lots and roadways. When it reaches urban
waterways it can cause a temporary fluctuation in the in-stream water temperature. Other factors
that tend to increase summer water temperature in urban waters include the removal of vegetation
from stream banks, reduced ground water baseflow, and discharges from storm water facilities
with elevated water temperature. Frequent fluctuations in stream temperature stress the aquatic
ecosystem, and make it difficult for temperature-sensitive species to survive.
Galli (1990a) undertook a major study of thermal impacts associated with urbanization
and storm water management in Maryland. Temperature observations were taken at stream
stations representing different levels of development, with impervious cover ranging from 1
percent to 60 percent. Results were compared with Maryland Class III standards for natural trout
waters (68 ฐF) and Class IV standards for recreational trout waters (75 ฐF). As shown in Figure
4-3, streams in developed watersheds (Lower Whiteoak and Tanglewood Stations) have
significantly higher spring and summer temperatures than streams in less developed watersheds.
Galli also found that "imperviousness together with local meteorological conditions had the
largest influence on urban stream temperatures." As shown in Figure 4-4, the rate of increase in
baseflow water temperature in this study was determined to be 0.14 ฐF for each one percent
increase in watershed imperviousness.
4-20
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Figure 4-3. Relationship Between Increasing Imperviousness and
Urban Stream Temperature
85
80
75
70
u.
0
ฃ 65
IS
. -
1 ปi **'
-/-?
vy
^L^'
... *
^ i
-
Sw,
'Jl
/ ^ฃ
^
0 20 40 60 80
Percent of Water Temperature < Indicated Values
Source: Galli, 1990a
'XI
^/l
^
^ J
Stations with Percent
Imperviousness
Lower Whiteoak (60%)
Tanglewood (30%)
Countryside (12%)
Lakemont (1%)
100
4-21
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Figure 4-4. Relationship Between Watershed Imperviousness and Baseflow Water
Temperature
74
72
ฃ. 70
ฃ
= 68
s.
g. 66
E
,2 64
62
60
Source: G
Mean Summer Water Temperature (T) =
60.4 + 0.1 4* (Watershed % Imperviousness)
-x-
j
^*-
^
J
9>^
I*-*
X-*
_^^
^
x-*
^
_^^
^
J
x1"
^
-x
^
<*
,x-
X1
x""
, '
x*
i
10 20 30 40 50 60 70 80 90 100
Watershed Imperviousness (%)
alii, 1990a
4.2.9
As pointed out by Novotny and Olem (1994), "most aquatic biota are sensitive to pH
variations," and "fish kills and reduction and change of other species result when the pH is altered
outside their tolerance limits." Most pH impacts in urban waters are caused by runoff of
rainwater with low pH levels (acid precipitation). In fact, urban areas tend to have more acidic
rainfall than less developed areas. Some buffering of low pH rainwater occurs during contact
with buildings, parking lots, roads and collection systems, and during overland flow. This is often
very site specific. The alkalinity and thus the capacity of receiving waters to neutralize acidic
storm water can also be important, and again is very site specific. Examples of pH impacts on fish
populations are difficult to identify due to the cumulative, overlapping impacts from other factors.
However, it is thought that the acidification problem in both the United States and Canada grows
in magnitude when "episodic acidification" (brief periods of low pH levels from snow melt or
heavy downpours) is taken into account (US EPA, 1992a). The spring snow melt can coincide
with fish spawning periods.
4-22
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4.3 Reported Impacts of Urban Storm Water
Urban runoff, which includes runoff from impervious surfaces such as streets, parking lots,
buildings, lawns and other paved areas is one of the leading causes of water quality impairment in
the United States. Based on the 1996 state Water Quality Inventory reports, siltation (sediment
discharged from urban runoff, as well as construction sites, agriculture, mining and forests) is the
leading cause of impaired water quality in rivers and streams. In the portion of the inventory
identifying sources, urban runoff was listed as the leading source of pollutants causing water
quality impairment related to human activities in ocean shoreline waters and the second leading
cause in estuaries across the nation. Urban runoff was also a significant source of impairment in
rivers and lakes. Urban runoff accounts for 47 percent of impaired miles of surveyed ocean
shoreline, 46 percent of the impaired square miles of surveyed estuaries, 22 percent of the
impaired acres of surveyed lakes and 14 percent of the impaired miles of surveyed rivers. Figure
4-5 illustrates the level of impairment attributable to urban storm water runoff based on states'
Water Quality Inventory assessment reports.
Figure 4-5. Proportions of Impaired Water Bodies Attributed to Urban Runoff
10
Percent of Impaired Waters
20 30
40
50
Estuaries and Ocean Shoreline
Lakes
Rivers and Streams
EH Major Impairment EEI Moderate / Minor H Not Specified
Source: EPA, 1998d.
4.3.1 Flow Impacts
The volume and flow rate of storm water discharges can have significant impacts on
receiving streams. In many cases, the impacts on receiving streams due to high storm water flow
4-23
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rates or volumes can be more significant than those attributable to the contaminants found in
storm water discharges. While studies linking increased storm water flows due to urbanization to
stream degradation are generally lacking in quantitative data, there are a number of studies that
support this hypothesis. EPA summarized studies which contain documented evidence of impacts
on steams due to urbanization (US EPA, 1997a). Impacts of urbanization and increased storm
water discharges to receiving streams documented in this evaluation include:
Increase in the number of bankfull events and increased peak flow rates
Sedimentation and increased sediment transport
Frequent flooding
Stream bed scouring and habitat degradation
Shoreline erosion and stream bank widening
Decreased baseflow
Loss of fish populations and loss of sensitive aquatic species
Aesthetic degradation
Changes in stream morphology
Increased temperatures.
The amount of runoff generated within a watershed increases steadily with development.
The presence of impervious areas such as roofs, parking lots and highways limits the volume of
rain water infiltrated into the soil, and increases the amount of runoff generated. Urbanized areas
also tend to have reduced storage capacities for runoff because of regrading, paving, and the
removal of vegetative cover. Decreases in infiltration and evapotranspiration and an increase in
runoff are the result of urbanization, with runoff volume linked to the percent of impervious area.
The relationship between runoff coefficient and percent impervious area is illustrated in Figure 4-
6.
4-24
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Figure 4-6. Relationship of Watershed Imperviousness to Runoff Coefficient Levels
1 -
0.8
| 0.6
"G
o
ฃ 0.4
OL
0.2
0
0
^*
*
ป^\
""-
-
.r^
^^ m
-
^^
-
^*
'
20 40 60 80 100
Watershed Imperviousness (%)
Source: Schueler, 1987
As shown in Table 4-10, the physical impacts to streams associated with increased
imperviousness are substantial (US EPA, 1997a).
4-25
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Table 4-10. Impacts from Increases in Impervious Surfaces
Increased
Imperviousness
Leads to:
Increased Volume
Increased Peak
Flow
Increased Peak
Duration
Increased Stream
Temp.
Decreased Base
Flow
Changes in
Sediment Loading
Resulting Impacts
Flooding
Habitat loss
Erosion
Channel
Widening
Stream bed
Alteration
Source: EPA, 1997
The Delaware Department of Natural Resources and Environmental Control also
identified a list of impacts on physical stream habitat attributed to urban storm water (DE
DNREC, 1997). This list is as follows:
Accelerated bank erosion
Accelerated bank undercutting
Increased siltation (burial of stable habitats)
Elimination of meanders (channelization)
Channel widening
Reduced depth
Reduced baseflow
Loss of shade
Increased temperature.
Specific impacts in the areas of flooding, stream bank erosion, and ground water recharge
are described in the following subsections.
4-26
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Flooding
Urbanization increases the frequency and severity of flooding due to increased runoff.
Because of the decreased availability of pervious, permeable surfaces, and the related decrease in
storage capacity, smaller more frequently occurring storms can create flooding problems.
Hydrographs in urban streams peak higher and faster than streams in undeveloped areas. A
comparison of estimated runoff volume and peak discharge for developed and undeveloped areas
is presented in Table 4-11. As shown, both runoff volume and peak discharge are substantially
increased under developed conditions.
Table 4-11. Comparison of Estimated Runoff Volume and Peak Discharge for Developed
and Undeveloped Areas
Storm
Frequency
(years)
2
10
100
Undeveloped Conditions
(Woods in good condition)
Estimated
Runoff (in)
0.14
0.52
1.40
Estimated Peak
Discharge (cfs)
1.00
5.60
19.7
Developed Conditions
(Half-Acre Residential)
Estimated
Runoff (in)
0.60
1.33
2.64
Estimated Peak
Discharge (cfs)
11.6
27.4
58.6
Source: Horner et al, 1994
The effects of urbanization on stream shape and the flood plain are illustrated in Figure 4-
7. Increased peak discharge raises the flood plain level, flooding areas which were previously not
at risk.
4-27
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Figure 4-7. Effect of Urbanization on Stream Slope and Flooding
Undeveloped
Flcodplain Limit
Summer Low Flow Level
Urbanized
Summer Low Flow Level
Source: Adapted from Scbueler, 1987
A comparison of hydrographs from an urbanized stream (Lincoln Creek) and a non-
urbanized stream (Jackson Creek) in Wisconsin are presented in Figure 4-8 (Masterson and
Bannerman, 1994). As illustrated, the hydrograph for the urbanized stream exhibits a much
higher peak flow rate that would correspond to a higher flood level.
4-28
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Figure 4-8. Hydrographs for Urban and Non-Urban Streams
1000
800 --
(A
o, 600
0)
>
ra
400 --
200 ..
06/17/93 06/17/93 06/18/93 06/19/93 06/20/93 06/20/93 06/21/93
^M Jackson Creek (non-urban)
Source: Masterson and Bannerman, 1994
Lincoln Creek (urban)
Stream Bank Erosion
Stream bank erosion is a natural phenomenon and source of both sediment and nutrients.
However, urbanization can greatly accelerate the process of stream bank erosion. As the amount
of impervious area increases, a greater volume of storm water is discharged directly to receiving
waters, often at a much higher velocity. The increased volume and velocity of the runoff can
overwhelm the natural carrying capacity of the stream network. In addition, streams in urbanized
areas can experience an increase in bankfull flows. Since bankfull flows are highly erosive,
substantial alterations in stream channel morphology can result.
Excessive bank erosion occurs as streams become wider and straighter to accommodate
greater flows and an excess number of erosion-causing events. Signs of stream bank erosion
attributable to increased storm water include undercut and fallen stream banks, felled bushes and
4-29
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trees along the banks, and exposed sewer and utility pipes. Sediments from eroding banks (and
upland construction) are deposited in areas where the water slows, causing buildup, destruction of
benthic habitat, and a decreased stream capacity for flood waters. This ultimately results in a
greater potential for further erosion.
Krug and Goddard (1986) documented these phenomena in their study of Pheasant
Branch, a developing watershed of 24.5 square miles near Middleton, Wisconsin. Local
population grew markedly between 1970 to 1980, from 8,246 to 11,851, and is projected to reach
18,000 by the year 2000. Problems of stream channel erosion and suspended sediment developed
in Pheasant Branch as a result of this growth. The increased erosion and sediment loadings have
decreased the mean stream bed elevation by almost 2 feet, and increased the mean channel width
by nearly 35 percent.
Table 4-12 shows the modeled percent increase at three sites for the volume of the 2-year
flood, bankfull width, and bankfull depth under two development scenarios. These are the
projected development levels in the year 2000 (projected urbanization), and complete urbanization
of the watershed. The projected results are shown relative to pre-development conditions.
Table 4-12. Percent Increase of Two-Year Flood, Bankfull Width, and Bankfull Depth
from Pre-Development Conditions to Urbanized Conditions (Based on Modeling Results)
Site
Site 1
Site 2
Site3
Projected Urbanization
2-year
Width
Depth
(Percent Increase from Pre-
urbanization)
99
324
32
40
110
10
30
80
10
Complete Urbanization
2-year
Width
Depth
(Percent Increase from Pre-
urbanization)
140
361
224
60
110
80
40
80
60
Source: EPA, 1997a
An example of the impact of urbanization on increased sediment loadings in several small
streams in Wisconsin before, during and after development is illustrated in Figure 4-9 (Krug and
Goddard, 1986). Sediment loads are greatest during construction, but remain elevated after
construction relative to pre-development conditions.
4-30
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Figure 4-9. Sediment Loadings on Small Streams in Wisconsin
Cumulative Monthly Sediment Load
10000
1978
Source: Krug and Goddard, 1986
1979 I 1980
Water Year
1981
Ground Water Recharge
Urbanization can have a major impact on ground water recharge. As shown earlier in
Figure 4-1, both shallow and deep infiltration decrease as watersheds undergo development and
urbanization. Ground water recharge is reduced along with a lowering of the water table. This
change in watershed hydrology alters the baseflow contribution to stream flow, and it is most
pronounced during dry periods. Ferguson (1990) points out that "base flows are of critical
environmental and economic concern for several reasons. Base flows must be capable of
absorbing pollution from sewage treatment plants and non-point sources, supporting aquatic life
dependent on stream flow, and replenishing water-supply reservoirs for municipal use in the
seasons when [water] levels tend to be lowest and water demands highest."
Base flows on Long Island, New York were substantially impacted by the construction of
storm water conveyance systems during the period of rapid development between the 1940s and
4-31
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1970s. As illustrated in Table 4-13, a steady decline in the average percent of baseflow was
observed for streams in urbanized sewered areas relative to streams in un-sewered or rural areas
(US EPA, 1997a).
Table 4-13. Average Percent Base Flow of Selected Streams on Long Island by Area
Years
1948-1953
1953-1964
1964-1970
Urbanized Sewered
Area (% Flow from
Base Flow)
Stream 1
(No data)
63
17
Stream 2
86
69
22
Urbanized Un-sewered
Area (% Flow from
Base Flow)
Stream 1
84
89
83
Stream 2
94
89
84
Rural Un-sewered Area
(% Flow from Base
Flow)
Stream 1
96
95
96
Stream 2
95
97
97
Source: US EPA, 1997a
4.3.2 Habitat Impacts
Natural ecosystems are a complex arrangement of interactions between the land, water,
plants, and animals. The relationship between storm water discharge and the biological integrity of
urban streams is illustrated in Figure 4-10 (Masterson and Bannerman, 1994). As shown, habitat
is impacted by changes in both water quality and quantity, and the volume and quality of
sediment. As reported by Schueler (1987), "no single factor is responsible for the progressive
degradation of urban stream ecosystems. Rather, it is probably the cumulative impacts of many
individual factors such as sedimentation, scouring, increased flooding, lower summer flows,
higher water temperatures, and pollution."
4-32
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Figure 4-10. Relationship Between Urban Storm Water and Aquatic Ecosystems
Biological Integrity of Urban Streams
n
Stormwater Discharges
n
Poor water
quality
Contaminated
Sediments
Excessively
high and low
flows
Increased
sediment
volume
Source: Adapted from Masterson and Bannerman, 1994
Schueler and Claytor (1995) also suggest a direct relationship between watershed
imperviousness and stream health (Figure 4-11), and found that stream health impacts tend to
begin in watersheds with only 10-20 percent imperviousness (the ten percent threshold). As
shown, sensitive streams can exist relatively unaffected by urban storm water with good levels of
stream quality where impervious cover is less than 10 percent although some sensitive streams
have been observed to experience water quality impacts at as low as 5 percent imperviousness.
Impacted streams are threatened and exhibit physical habitat changes (erosion and channel
widening) and decreasing water quality where impervious cover is in the range of 10 to 25
percent. Streams in watersheds where the impervious cover exceeds 25 percent are typically
degraded, have a low level of stream quality, and do not support a rich aquatic community.
4-33
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Figure 4-11. Relationship Between Impervious Cover and Stream Quality
mpervious Cover
j^ en
o o
? 20
0)
13
0
Source: Set
.^
^^^^
^^^^
Non supporting (>25%)
^^^
^s^
^^
^
^^^" Impacted (1 1to 2b%)
^^
^^
good fair low
Level of Stream Quality
melerand Claytor, 1995
A summary of water quality impacts on habitat is presented in Table 4-14. The alteration
of species distribution is the major impact, with pollutant tolerant and less sensitive species
replacing native species in storm water impacted receiving waters.
4-34
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Table 4-14. Water Quality Parameters Affecting Habitat
Water Quality
Parameter
Bacteria
Heavy metals
Toxic organics
Nutrients
Sediment
BOD
Temperature
PH
Habitat Effect
Contamination
Alteration of species distribution
Alteration of species distribution
Eutrophication, algal blooms
Decreased spawning areas
Reduced dissolved oxygen levels
Reduced dissolved oxygen levels
Alteration of species distribution
Figure 4-12 illustrates that the pH tolerance of various forms of aquatic life varies
substantially (US EPA, 1992b). The tolerance of aquatic life to changes in temperature, turbidity
and toxic substances is also very important. Contaminants like heavy metals, pesticides, and
hydrocarbons can alter the species distribution in receiving waters. Acute and chronic toxicity
impacts may also occur. The relative toxicity of storm water samples from a variety of loading
source areas is presented in Table 4-15. Some of the identified chronic toxicity effects are
decreased growth and respiration rates (US EPA, 1996a). Toxic loads can reduce the hatching
and survival rates of aquatic organisms, cause gross effects such as lesions or fin erosion in fish,
and can eventually destroy the entire population of some sensitive species (Novotny and Olem,
1994). Hydrocarbons can be especially detrimental to benthic organisms because they can
become bound to urban runoff sediments (Schueler, 1987).
4-35
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Figure 4-12. Low pH Tolerance by Different Species
Mayfly
7.0 6.5 6.0 5.5 5.0
Source: EPA, 1992b
4-36
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Table 4-15. Relative Toxicities of Samples Using Microtoxฎ Measurement Method
Local Source Areas
Roofs
Parking areas
Storage areas
Streets
Loading docks
Vehicle service areas
Landscaped areas
Urban creeks
Detention ponds
All source areas
Highly Toxic (%)
8
19
25
0
0
0
17
0
8
9
Moderately Toxic (%)
58
31
50
67
67
40
17
11
8
32
Not Toxic (%)
33
50
25
33
33
60
66
89
84
59
Note: Microtoxฎ results are primarily for comparison purposes.
Source: Pitt etal, 1995.
The physical impacts to streams due to urbanization and changes in watershed hydrology
also cause many habitat changes. As illustrated in the comparison of healthy and eroding stream
banks in Figure 4-13, loss of depth, sediment deposition, loss of shoreline vegetation, and higher
temperatures combine to impact habitat.
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Figure 4-13. Comparison of a Healthy Stream Bank and an Eroding Bank
shoreline vegetation
cooler temperatures
ample depth
Healthy Stream Bank
shoreline
erosion
channel widening
Eroding Stream Bank
Source: Adapted from Corish, 1995
insufficient depth
increased sedimentation
Schueler (1987) states that sediment pollution in the form of increased suspended solids
can cause the following harmful impacts to aquatic life:
Increased turbidity
Decreased light penetration
Reduced prey capture for sight feeding predators
Clogging of gills/filters of fish and aquatic invertebrates
Reduced spawning and juvenile fish survival.
Sediment is also a carrier of metals and other pollutants, and a source of bioaccumulating
pollutants for bottom feeding organisms. The rate of bioaccumulation is widely variable based
upon site specific conditions including species, concentration, pH, temperature, and other factors.
Barren (1995) reports that the bioaccumulation of organic contaminants results primarily from
direct exposure to water and sediment rather than through the food chain.
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Macroinvertebrate Impacts
The biological integrity of receiving waters impacted by urban storm water is typically
reduced from more pristine, undeveloped circumstances. Impacts include a reduction in total
numbers and diversity of macroinvertebrates, and the emergence of more pollutant-tolerant
species. In a study in Delaware, it was found that approximately 70 percent of the
macroinvertebrate community in streams in undeveloped, forested watersheds consisted of
pollution sensitive mayflies, stoneflies and caddisflies, as compared with 20 percent in urbanized
watersheds (Maxted and Shaver, 1997). As shown in Table 4-16, the relative abundance of
pollution tolerant organisms increased with urbanization, including worms, midges and beetles.
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Table 4-16. Delaware Insect Population Abundance by Degree of Urbanization
Population Description
Class/ Order
Insecta/Trichoptera
Insecta/Ephemeroptera
Insecta/Plecoptera
Insecta/Ephemeroptera
Insecta/Coleoptera
Insecta/Ephemeroptera
Insecta/Coleoptera
Insecta/Coleoptera
Insecta/Trichoptera
Insecta/Trichoptera
Insecta/Diptera
Insecta/Diptera
Oligochaeta
Genus species
Diplectrona modesta
Ephemerella spp.
Allocapnia spp.
Eurylophella spp.
Anchytarsus bicolor
Stenonema spp.
Optiservus spp.
Oulimnius latiusculus
Cheumatopsyche spp.
Hydropsyche betteni
Simulium vittatum
Parametriocnemus spp.
unidentified (Tubificidae)
Common
Name
caddisfly
mayfly
stonefly
mayfly
beetle
mayfly
beetle
beetle
caddisfly
caddisfly
blackfly
midge
worm
PT
0
1
3
1
4
4
4
2
5
6
7
5
10
Relative Abundance
by Degree of
Urbanization (%)
None
14
12
10
8
6
5
4
4
1
1
0
0
0
Low
2
1
18
1
3
3
2
3
10
4
8
0
0
High
1
0
3
2
0
1
8
5
8
5
1
4
4
Note: rare organisms (fewer than 4 per 100 organisms) not included. Relative abundance (%) and pollution
tolerance (PT) of macroinvertebrate species commonly found in Piedmont streams of Delaware for three levels of
urbanization; none (0-2% impervious cover), low (6-13%), and high (15-50%); PT range from 0 (low tolerance) to
10 (high tolerance).
Source: Maxted and Shaver, 1997.
A study by Kohlepp and Hellenthal (1992) quantified the effects of sediment deposits on
macroinvertebrates in Juday Creek, a tributary to the St. Joseph River in Indiana. The study
included data before and after upstream channel maintenance operations introduced a large
amount of sediment to the creek, similar to increased sediment yield from urban areas. A dramatic
change in the species distribution of macroinvertebrates in the river was observed, and this was
attributed to the changing sediment load and increased sedimentation. As shown in Figure 4-14,
"the result was a shift from a community dominated by filter-feeders in both numbers and
production rate in 1981-82, to a community in 1989-90 in which less desirable collector-gatherers
and shredders increased in importance in terms of relative contribution to both numbers and
production."
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Figure 4-14. Effects of Sediment Deposits on Macroinvertebrates in Juday Creek, Indiana
Proportion by Functional Feeding Group (percent)
A. 1981-82 Total Densities
Collector-gatherers (10)-
Shredders (1)
Filter-feeders (89)
B. 1989-90 Total Densities
Shredders (4)
Collector-gatherers (38)
Filter-feeders (58)
C. 1981-82 Secondary Production Rates
Collector-gatherers^ _ Shredders(4)
(2)
Filter-feeders
(94)
D. 1989-90 Secondary Production Rates
Collector-gatherers (21)
Shredders (23)
Filter-feeders
(56)
Source: Kohlepp and Hellenthal, 1992
Fish Impacts
The health of an ecosystem is often measured by the abundance and variety offish species
present, and the presence of native species. A case study in California compared fish populations
in urbanized and non-urbanized sections of Coyote Creek (Pitt, 1995). The relative abundance of
different fish species in the different reaches is presented in Table 4-17. As shown, the native fish
are generally replaced by introduced fish in the urbanized section.
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Table 4-17. Relative Abundance of Native and Introduced Fish in Urbanized and Non-
Urbanized Areas in Coyote Creek, California
Species
Relative Abundance (%)
Non-urbanized Reach
Urbanized Reach
Native Fish
Hitch
Threespine stickleback
Sacramento sucker
34.8
27.3
12.6
4.8
0.8
0.1
Introduced Fish
Mosquitofish
Fathead Minnow
Threadfm shad
5.6
0.6
-
66.9
20.6
2.4
Source: Pitt, 1995
An illustration of the abundance offish eggs and larvae associated with different levels of
urban land use in New York is presented in Figure 4-15 (Limburg and Schmidt, 1990). This
graph supports the "10 percent rule" reported by Schueler and Claytor (1995): stream impacts
tend to begin in watersheds with only 10 to 20 percent imperviousness.
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Figure 4-15. Average Densities of Fish Eggs and Larvae in New York
4
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5
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0)
ง
M
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0)
_o
S
1 n
]
z
1
10% Thresho
Id
1
1
0 20 40 60
Percent of Watershed in Urban Land Use
Source: Limburg and Schmidt, 1990
80
The change in the resident fish community due to urbanization in Tuckahoe Creek in
Virginia was quantified by Weaver and Garman (1994). With urbanization increasing the percent
of urban land from 7 percent to 28 percent between 1958 and 1990, a dramatic change in the fish
assemblage was observed. As shown in Table 4-18, the total number offish observed dropped
sharply along with the total number of species present and the number of common species
present.
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Table 4-18. Effects of Urbanization on the Fish Community of Tuckahoe Creek, Virginia
(Composite of 6 Sites)
Indicator
% Urban (by land area)
total abundance
# species - total
# species - common*
% bluegill/shiner
Fish Assemblage
Year
1958
7
2,056
31
21
28
1990
28
412
23
6
67
* more than 10 individuals
Source: Weaver and Garman, 1994
4.3.3 Public Health Impacts
Public health impacts associated with urban storm water occur when humans ingest or
come in contact with pathogens. While these impacts are not widely reported, they do occur, and
some impacts have been documented. Examples related to swimming and contact recreation
impacts and shellfish impacts are presented.
Contact Recreation Impacts
Beach closures are a common occurrence in many communities throughout the United
States. Beach closures are primarily due to high levels of bacteria in water samples. The
presence of medical waste and other dangerous floatable substances on beaches can also cause
beach closures to occur. Storm water runoff can be responsible for both bacteria and floatables.
Elevated levels of bacteria and viruses represent the most common threat to public health.
Diarrhea and infection of the ear, eye, nose, or throat are possible.
A study of epidemiological impacts associated with swimming in the vicinity of storm
water outfalls in Santa Monica Bay in California was conducted in 1995 (SMBRP, 1996). The
study focused on health effects, and not on possible sources of contamination to the storm drain
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system, such as illicit sewage connections and infiltration.1 While the effects observed may be
atypical of properly constructed and maintained storm drain outfalls, the findings indicate the
potential health risks associated with pathogens. Major findings of this study are as follows:
There is an increased risk of illness associated with swimming near flowing storm drain
outlets in Santa Monica Bay.
There is an increased risk of illness associated with swimming in areas with high
densities of bacterial indicators.
The total coliform to fecal coliform ratio was found to be one of the better indicators
for predicting health risks.
Illnesses were reported more often on days when the samples were positive for enteric
viruses.
High densities of bacterial indicators were measured on a significant number of survey
days, particularly in front of drains.
People who swim in areas adjacent to flowing storm drains were found to be 50 percent
more likely to get sick than people who swam in other areas. The sicknesses included fever,
nausea, gastroenteritis, and flu-like symptoms such as nasal congestion, sore throat, fever, or
coughing. As illustrated in Figure 4-16, swimmers who swam directly in front of storm drains
were much more likely to become ill than those who swam away from the storm drains at
distances of 100 to 400 meters. A comparative health outcome in terms of relative risk for
swimming in front of the storm drain vs. swimming 400 meters away is presented in Table 4-19.
1 Pilot studies conducted in the Bay prior to 1995 noted that some outfalls had regular dry
weather discharges; this is a common indicator of storm drain contamination (SMBRP, 1990;
SMBRP, 1992).
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Figure 4-16. Health Effects Observed Relative to Distance
from Santa Monica Bay Storm Drains
a
11
i a.
SRD-sianificant respiratory disease
HCGI-2 - vomitting and fever
-100
Upstream
100
Downstream
200
300
400
Distance from Storm Drain Outlet (meters)
Source: Santa Monica Bay Restoration Project, 1996
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Table 4-19. Comparative Health Outcomes for Swimming in Front of Drains in
Santa Monica Bay
Health Outcome
Fever
Chills
Ear Discharge
Vomiting
Coughing with phlegm
Any of the above symptoms
HCGI-2
SRD
HCGI-2 or SRD
Relative Risk
for Swimming
in Front of
Drains*
57%
58%
127%
61%
59%
44%
111%
66%
53%
Estimated No. Of Excess Cases
per 10,000 Persons
259
138
88
115
175
373
95
303
314
* Compared to swimming 400 meters or more away from drains
Source: Santa Monica Bay Restoration Project, 1996
Seafood Hazard
The consumption of contaminated seafood, particularly shellfish, is a major public health
problem. Shellfish are susceptible to bioaccumulating bacteria and viruses because they are filter
feeders. In waters polluted by urban runoff, bacteria and viruses can be concentrated in the
shellfish to much higher levels than those found in the surrounding waters. This becomes a public
health concern because many potentially harmful bacteria and viruses can be ingested when people
eat contaminated shellfish. As shown in Figure 4-17, the largest proportion of shellfish
harvesting restrictions are caused by urban runoff (US EPA, 1995a).
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Figure 4-17. Sources Associated with Shellfish Harvesting Restrictions, in Percent
Septic Tanks (1 )
CSOs (1 )
Marinas (3) -
Industrial Discharges (6)
Point Sources _/^k ^IDOS^ Urban Runoff/Storm Sewer (30 )
(general) (10)
Nonpoint Sources
(general) (20)
Municipal Discharges (29)
Source: US EPA, 1995a
Fish can also be contaminated for a number of reasons. Recent fish sampling surveys in
regions of the U.S. have shown widespread mercury contamination in streams, wetlands,
reservoirs, and lakes. Based on 1997 data, 33 states have issued fish consumption advisories
because of mercury contamination (US EPA, 1998a). Mercury is an urban/industrial pollutant
that is released into the air and ends up in urban runoff by atmospheric deposition (Krabbenhoft
and Rickert, 1995). The effects offish contamination go beyond health issues, and hurt the
recreational fishing industry as a whole.
4.3.4 Aesthetic Impacts
The aesthetic impacts associated with urban storm water are often difficult to quantify.
However, aesthetic impacts are often very visible to the general public. EPA reports that "people
have a strong emotional attachment to water, arising from its aesthetic qualities-tranquillity,
coolness, and beauty" (US EPA, 1995c). The presence of floatables within urban waters and
deposited along the banks of waterways represents a common aesthetic impact in most urban
settings. Floatable wastes originate from street litter and improper solid waste disposal practices.
The average total street debris loading rate in New York City was quantified at approximately
156 pounds per curb-mile per day, with a range from 3 to 2,700 pounds (HydroQual, 1995).
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Aesthetic impacts from the eutrophication of urban waterways is caused in part by
nutrients delivered in urban storm water. As reported by Schueler (1987), aesthetic impacts and
nuisance conditions associated with eutrophication can include:
Surface algal scum
Water discoloration
Strong odors
Release of toxins.
The visual damage to urban streams from accelerated rates of storm water runoff also
contribute to aesthetic impacts. These include eroded stream banks, fallen trees, and
sedimentation. In summary, aesthetic impacts are often very visible in public areas where
shoreline recreation occurs. Aesthetic impacts are therefore the storm water impacts most
familiar to the general public.
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5.0 Description and Performance of Storm Water Best Management
Practices
A storm water best management practice (BMP) is a technique, measure or structural
control that is used for a given set of conditions to manage the quantity and improve the quality of
storm water runoff in the most cost-effective manner. BMPs can be either engineered and
constructed systems ("structural BMPs") that improve the quality and/or control the quantity of
runoff such as detention ponds and constructed wetlands, or institutional, education or pollution
prevention practices designed to limit the generation of storm water runoff or reduce the amounts
of pollutants contained in the runoff ("non-structural BMPs"). No single BMP can address all
storm water problems. Each type has certain limitations based on drainage area served, available
land space, cost, pollutant removal efficiency, as well as a variety of site-specific factors such as
soil types, slopes, depth of groundwater table, etc. Careful consideration of these factors is
necessary in order to select the appropriate BMP or group of BMPs for a particular location.
5.1 Goals of Storm Water Best Management Practices
Storm water BMPs can be designed to meet a variety of goals, depending on the needs of
the practitioner. In existing urbanized areas, BMPs can be implemented to address a range of
water quantity and water quality considerations. For new urban development, BMPs should be
designed and implemented so that the post-development peak discharge rate, volume and
pollutant loadings to receiving waters are the same as pre-development values. In order to meet
these goals, BMPs can be implemented to address three main factors: flow control, pollutant
removal and pollutant source reductions.
5.1.1 Flow Control
Flow control involves managing both the volume and intensity of storm water discharges
to receiving waters. Urbanization significantly alters the hydrology of a watershed. Increasing
development leads to higher amounts of impervious surfaces. As a result, the response of an
urbanized watershed to precipitation is significantly different from the response of a natural
watershed. The most common effects are reduced infiltration and decreased travel time, which
significantly increase peak discharges and runoff volumes. Factors that influence the amount of
runoff produced include precipitation depth, infiltrative capacity of soils, soil moisture, antecedent
rainfall, cover type, the amount of impervious surfaces and surface retention. Travel time is
determined primarily by slope, length of flow path, depth of flow and roughness of flow surfaces.
Peak discharges are based on the relationship of these parameters, and on the total drainage area
of the watershed, the time distribution of rainfall, and the effects of any natural or manmade
storage (USDA/NRCS, 1986).
High flow rates of storm water discharges can cause a number of impacts to receiving
streams (see section 4.3), and may also increase the pollutant concentrations in storm water
runoff. High velocity runoff can detach and transport significant amounts of suspended solids and
5- 1
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associated pollutants such as nutrients and metals from the urban landscape. In addition, high
flow rates in drainage channels and receiving waters can erode stream banks and channels, further
increasing suspended solids concentrations in waters that receive storm water discharges. In
order to reduce the pollutant concentrations in runoff and receiving water impacts associated with
high storm water flow rates, BMPs that provide flow attenuation are frequently implemented.
In areas undergoing new development or redevelopment, the most effective method of
controlling impacts from storm water discharges is to limit the amount of rainfall that is converted
to runoff. By utilizing site design techniques that incorporate on-site storage and infiltration and
reduce the amounts of directly connected impervious surfaces, the amount of runoff generated
from a site can be significantly reduced. This can reduce the necessity for traditional structural
BMPs to manage runoff from newly developed areas. There are a number of practices that can be
used to promote on-site storage and infiltration and to limit the amount of impervious surfaces
that are generated. However, the use of on-site infiltration can be limited in certain areas due to
factors such as slope, depth to the water table, and geologic conditions.
Site design features such as providing rain barrels, dry wells or infiltration trenches to
capture rooftop and driveway runoff, maintaining open space, preserving stream
buffers and riparian corridors, using porous pavement systems for parking lots and
driveways, and using grassed filter strips and vegetated swales in place of traditional
curb-and-gutter type drainage systems can greatly reduce the amount of storm water
generated from a site and the associated impacts.
Street construction features such as placing sidewalks on only one side of the street,
limiting street widths, reducing frontage requirements and eliminating or reducing the
radius of cul-de-sacs also have the potential to significantly reduce the amount of
impervious surfaces and therefore the amount of rainfall that is converted to runoff.
Construction practices such as minimizing disturbance of soils and avoiding
compaction of lawns and greenways with construction equipment can help to maintain
the infiltrative capacity of soils.
There are several guides that contain useful information regarding development practices that can
limit the impacts associated with storm water runoff (Delaware DNREC, 1997; US EPA, 1996b;
Center for Watershed Protection, 1998).
In areas that are already developed, flow control can be more complicated. Since a
drainage infrastructure already exists, retrofitting these systems to provide flow control can be
prohibitively expensive. Regional storm water management systems can be used to manage runoff
in these areas, but space considerations and high capital costs can limit their usefulness.
Depending on site-specific constraints, however, there are a number of practices that can be
incorporated on-site to reduce runoff volumes from these areas. Down spouts can be
disconnected from the storm drain system and this rainfall can instead be collected and stored on a
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property in rain barrels to be used for watering lawns and landscaping during inter-event periods.
Infiltration and retention practices such as bioretention areas and infiltration trenches can be
constructed to capture runoff from rooftops, lawns and driveways and reduce the volume of
runoff discharged to storm sewers. Curb-and-gutter systems can be replaced with grassed swales
or wetland channels to provide temporary ponding of runoff. Storm water from commercial areas
and golf courses can be collected and stored in ponds and subsequently be used for irrigation.
Storm water reuse can help to maintain a more natural, pre-development hydrologic balance in the
watershed (Livingston et al, 1998). Parking lots can also be used as short-term storage areas for
ponded storm water, and bioretention facilities placed around the perimeter of parking lots can be
used to infiltrate this water volume.
Where the generation of runoff cannot be avoided, end-of-pipe structural BMPs may be
implemented to decrease the impacts of storm water discharges to receiving streams. However,
BMPs are limited in their ability to control impacts, and frequently cause secondary impacts such
as increased temperatures of discharges to receiving streams. BMPs that can be designed to
provide significant flow attenuation include grassed swales, vegetated filter strips, detention and
retention basins, wetland basins, and wetland channels and swales. These BMPs can also provide
the added benefit of removing pollutants such as suspended solids and associated nutrients and
metals from storm water runoff.
The environmental aspects of storm water quantity control must be carefully balanced
against the hazard and nuisance effects of flooding. Large or intense storm events or rapid
snowmelt can produce significant quantities of runoff from urban areas with high levels of
imperviousness. This runoff must be rapidly transported from urbanized areas in order to prevent
loss of life and property due to flooding of streets, residences and businesses. This is frequently
accomplished by replacing natural drainage paths in the watershed with paved gutters, storm
sewers or other artificial means of drainage. These drainage systems can convey runoff at a faster
rate than natural drainage paths, allowing rapid transport of runoff away from areas where
flooding is likely to occur. However, as large quantities of runoff are conveyed rapidly from the
urban landscape and discharged to receiving streams, downstream areas can flood. Following
urbanization, large volumes of runoff can be produced from even small storm events due to the
high amounts of impervious surfaces. As a result, flooding of streams that receive runoff can
occur much more frequently following urbanization due to this excessive amount of runoff
production. Therefore, design of storm water drainage systems must always balance flood
protection with ecological concerns.
In highly urbanized and densely populated cities, little opportunity exists for retrofitting
storm drainage systems with BMPs to provide water quantity control due to flooding
considerations. The large area of impervious surfaces in heavily urbanized areas produce large
quantities of runoff. Rapid conveyance by the storm drain system is frequently the only option
that exists in order to prevent flooding of yards, streets and basements. In these areas, the most
appropriate BMPs are those that limit the generation of pollutants or remove pollutants from the
urban landscape. With this principle in mind, a unique opportunity exists in newly developing
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areas or in more sparsely populated suburban areas to use BMPs that control runoff at the point
of generation, instead of trying to manage it at the point of discharge to the receiving stream.
When rainfall is managed as a resource instead of as a waste stream requiring treatment, future
problems with quantity control may be avoidable. When rainfall is managed at the site level by
promoting the concepts of conservation design, and by providing on-site storage, infiltration and
usage of rainfall for irrigation of the urban landscape, the need for traditional curb-and-gutter
storm drainage system can be reduced. As a result, the need for constructing and maintaining
capital-, land- and maintenance-intensive regional BMPs to manage large flows from developed
watersheds may be reduced. Nuisance flooding of downstream areas can also be limited by
reducing the overall volume of water draining from a watershed. Limiting the discharge of large
volumes of storm water to urban streams can help to prevent the degradation of these streams to
the point of being non-supporting of a designated use.
5.1.2 Pollutant Removal
Urbanized areas export large quantities of pollutants during storm events. The high
population of pollutant sources in urbanized areas contribute large quantities of pollutants that
accumulate on streets, rooftops and other surfaces. During rainfall or snowmelt, these pollutants
are mobilized and transported from the streets and rooftops into the storm drain system, where
they are conveyed and ultimately discharged to waterways. In order to reduce the impacts to
receiving waters from the high concentrations of pollutants contained in the runoff, BMPs can be
implemented to remove these pollutants.
Properly-designed, constructed and maintained structural BMPs can effectively remove a
wide range of pollutants from urban runoff. Pollutant removal in storm water BMPs can be
accomplished through a number of physical and biochemical processes. The efficiency of a given
BMP in removing pollutants is dependent upon a number of site-specific variables, including the
size, type and design of the BMP; the soil types and characteristics; the geology and topography
of the site; the intensity and duration of the rainfall; the length of antecedent dry periods;
climatological factors such as temperature, solar radiation, and wind; the size and characteristics
of the contributing watershed; and the properties and characteristics of the various pollutants.
Pollutant removal in urban storm water BMPs can occur through the following
mechanisms:
Sedimentation
Sedimentation is the removal of suspended particulates from the water column by
gravitational settling. The settling of discrete particles is dependent upon the particle velocity, the
fluid density, the fluid viscosity, and the particle diameter and shape. Sedimentation can be a
major mechanism of pollutant removal in BMPs such as ponds and constructed wetlands.
Sedimentation can remove a variety of pollutants from storm water runoff. Pollutants such as
metals, hydrocarbons, nutrients and oxygen demanding substances can become adsorbed or
attached to particulate matter, particularly clay soils. Removal of these particulates by
5-4
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sedimentation can therefore result in the removal of a large portion of these associated pollutants.
The main factor governing the efficiency of a BMP at removing suspended matter by
sedimentation is the time available for particles to undergo settling. Fine particulates such as clay
and silt can require detention times of days or even weeks to settle out of suspension. Therefore,
it is important to evaluate the settling characteristics of the particulates in runoff before BMP
design in order to determine the detention time necessary for adequate settling to occur. The
overall efficiency of a BMP in removing particulates by settling is also dependent upon the initial
concentration of suspended solids in the runoff. In general, runoff with higher initial
concentrations of suspended solids will have a greater removal efficiency. In addition, some
particles, such as fine clays, will not settle out of suspension without the aid of a coagulant. As a
result there is usually a minimum practical limit of approximately 10 mg/1 of TSS, below which
additional TSS removal can not be expected to occur (UDFCD, 1992).
Flotation
Flotation is the separation of particulates with a specific gravity less than that of water.
Trash such as paper, styrofoam "peanuts" used for packaging, and other low-density materials can
be removed from storm water by the mechanism of flotation. If the inlet area of the BMP is
designed to allow for the accumulation of floatable materials, then these accumulated materials
can periodically be manually removed from the BMP. Significant amounts of floatables can be
removed from storm water in properly designed BMPs in this manner. In addition, oils and
hydrocarbons will frequently rise to the surface in storm water BMPs. If the BMP is designed
with an area for these materials to accumulate, then significant removals of these pollutants can
occur. Many modular or drop-in filtration systems incorporate an oil and grease or hydrocarbon
trap with a submerged outlet pipe that allows these contaminants to accumulate and to be
periodically removed.
Filtration
Filtration is the removal of particulates from water by passing the water through a porous
media. Media commonly used in storm water BMPs include soil, sand, gravel, peat, compost, and
various combinations such as peat/sand, soil/sand and sand/gravel. Filtration is a complex process
dependent on a number of variables. These include the particle shape and size, the size of the
voids in the filter media, and the velocity at which the fluid moves through the media. Filtration
can be used to remove solids and attached pollutants such as metals and nutrients. Organic
filtration media such as peat or leaf compost can also be effective at removing soluble nutrients
from urban runoff.
Infiltration
Infiltration is the most effective means of controlling storm water runoff since it reduces
the volume of runoff that is discharged to receiving waters and the associated water quality and
quantity impacts that runoff can cause. Infiltration is also an important mechanism for pollutant
control. As runoff infiltrates into the ground, particulates and attached contaminants such as
metals and nutrients are removed by filtration, and dissolved constituents can be removed by
adsorption. However, infiltration is not appropriate in all areas.
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Adsorption
Adsorption, while not a common mechanism used in storm water BMPs, can occur in
infiltration systems where the underlying soils contain appreciable amounts of clay. Dissolved
metals that are contained in storm water runoff can be bound to the clay particles as storm water
runoff percolates through clay soils in infiltration systems.
Biological Uptake
Biological uptake of nutrients is an important mechanism of nutrient control in storm
water BMPs. Urban runoff typically contains significant concentrations of nutrients. Ponds and
wetlands can be useful for removing these nutrients through biological uptake. This occurs as
aquatic plants, algae, microorganisms and phytoplankton utilize these nutrients for growth.
Periodic harvesting of vegetation in BMPs allows for permanent removal of these nutrients. If
plants are not harvested, however, nutrients can be re-released to the water column from plant
tissue after the plants die.
Biological Conversion
Organic contaminants can be broken down by the action of aquatic microorganisms in
storm water BMPs. Bacteria present in BMPs can degrade complex and/or toxic organic
compounds into less harmful compounds that can reduce the toxicity of runoff to aquatic biota.
Degradation
BMPs such as ponds and wetlands can provide the conditions necessary for the
degradation of certain organic compounds, including certain pesticides and herbicides. Open pool
BMPs can provide the necessary conditions for volatilization, hydrolysis and photolysis of a
variety of organic compounds to take place.
5.1.3 Pollutant Source Reductions
Source reduction is an effective non-structural way of controlling the amounts of
pollutants entering storm water runoff. A wide range of pollutants are washed off of impervious
surfaces during runoff events. Removing these contaminants from the urban landscape prior to
precipitation can effectively limit the amounts of pollutants contained in the storm water runoff.
Source reduction can be accomplished by a number of different processes including: limiting
applications of fertilizers, pesticides and herbicides; periodic street sweeping to remove trash,
litter and particulates from streets; collection and disposal of lawn debris; periodic cleaning of
catch basins; elimination of improper dumping of used oil, antifreeze, household cleaners, paint,
etc. into storm drains; and identification and elimination of illicit cross-connections between
sanitary sewers and storm sewers.
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5.2 Types of Storm Water Best Management Practices
There are a variety of storm water BMPs available for managing urban runoff. Regardless
of the type, storm water BMPs are most effective when implemented as part of a comprehensive
storm water management program that includes proper selection, design, construction, inspection
and maintenance. Storm water BMPs can be grouped into two broad categories: structural and
non-structural. Structural BMPs are used to treat the storm water at either the point of
generation or the point of discharge to either the storm sewer system or to receiving waters.
Non-structural BMPs include a range of pollution prevention, education, institutional,
management and development practices designed to limit the conversion of rainfall to runoff and
to prevent pollutants from entering runoff at the source of runoff generation. The descriptions in
this section provide summary information on a variety of commonly used structural and
nonstructural storm water BMPs. Information provided includes a general description of the
technology or practice, important components and factors to incorporate into BMP design and
planning, and the positive and negative aspects of the technology or practice. In addition,
maintenance considerations for structural BMPs are discussed. Quantitative performance data for
BMPs are not included in this section. These data are included in section 5.5, "Effectiveness of
BMPs in Managing Urban Runoff."
5.2.1 Structural BMPs
There are a wide variety of structural BMPs in use for storm water management.
Structural BMPs include engineered and constructed systems that are designed to provide for
water quantity and/or water quality control of storm water runoff. Structural BMPs can be
grouped into several general categories. However, the distinction between BMP types and the
terminology used to group structural BMPs is an area that needs standardization. In particular,
the terms "retention" and "detention" are sometimes used interchangeably, although they do have
distinct meanings. Storm water detention is usually defined as providing temporary storage of a
runoff volume for subsequent release (WEF/ASCE, 1992). Examples include detention basins,
underground vaults, tanks or pipes, and deep tunnels, as well as temporary detention in parking
lots, rooftops, depressed grassy areas, etc. Retention is generally defined as providing storage of
storm water runoff without subsequent surface discharge (WEF/ASCE, 1992). With the strict
interpretation of this definition, retention practices would be limited to those practices that either
infiltrate or evaporate runoff, such as infiltration trenches, wells or basins. However, retention is
also commonly used to describe practices that retain a runoff volume (and hence have a
permanent pool) until it is displaced in part or in total by the runoff event from the next storm.
Examples include retention ponds, tanks, tunnels, and underground vaults or pipes, and wetland
basins. For purposes of this document, and in being consistent with the definitions and
terminology used in the ASCE National Stormwater BMP Database, structural BMPs have been
grouped and defined as follows:
Infiltration systems capture a volume of runoff and infiltrates it into the ground.
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Detention systems capture a volume of runoff and temporarily retain that volume for
subsequent release. Detention systems to not retain a significant permanent pool of
water between runoff events.
Retention systems capture a volume of runoff and retain that volume until it is
displaced in part or in total by the next runoff event. Retention systems therefore
maintain a significant permanent pool volume of water between runoff events.
Constructed wetland systems are similar to retention and detention systems, except that
a major portion of the BMP water surface area (in pond systems) or bottom (in
meadow-type systems) contains wetland vegetation. This group also includes wetland
channels.
Filtration systems use some combination of a granular filtration media such as sand,
soil, organic material, carbon or a membrane to remove constituents found in runoff.
Vegetated systems (biofilters) such as swales and filter strips are designed to convey
and treat either shallow flow (swales) or sheetflow (filter strips) runoff
Minimizing directly connected impervious surfaces describes a variety of practices that
can be used to reduce the amount of surface area directly connected to the storm
drainage system by minimizing or eliminating traditional curb and gutter. This is
considered by some to be a non-structural practice, but is has been included under the
structural heading in this report due to the need to design and construct alternative
conveyance and treatment options.
Miscellaneous and vendor-supplied systems include a variety of proprietary and
miscellaneous systems that do not fit under any of the above categories. These include
catch basin inserts, hydrodynamic devices, and filtration devices.
5.2.1.1 Infiltration Systems
Infiltration systems include infiltration basins, porous pavement systems, and infiltration
trenches or wells. An infiltration BMP is designed to capture a volume of storm water runoff,
retain it and infiltrate that volume into the ground. Infiltration of storm water has a number of
advantages and disadvantages. The advantages of infiltration include both water quantity control
and water quality control. Water quantity control can occur by taking surface runoff and
infiltrating this water into the underlying soil. This reduces the volume of water that is discharged
to receiving streams, thereby reducing some of the potential impacts caused by an excess flow as
well as increased pollutant concentrations in the receiving stream. Infiltration systems can be
designed to capture a volume of storm water and infiltrate this water into the ground over a
period of several hours or even days, thereby maximizing the infiltrative capacity of the BMP.
Infiltration can have many secondary benefits such as increasing recharge of underlying aquifers
5-
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and increasing baseflow levels of nearby streams. Infiltration BMPs can also provide water
quality treatment. Pollutant removal can occur as water percolates through the various soil
layers. As the water moves through the soil, particles can be filtered out. In addition,
microorganisms in the soil can degrade organic pollutants that are contained in the infiltrated
storm water.
Although infiltration of storm water has many benefits, it also has some drawbacks. First,
infiltration may not be appropriate in areas where groundwater is a primary source of drinking
water due to the potential for contaminant migration. This is especially true if the runoff is from a
commercial or industrial area where the potential for contamination by organics or metals is
present. Also, the performance of infiltration BMPs is limited in areas with poorly permeable
soils. In addition, infiltration BMPs can experience reduced infiltrative capacity and even
clogging due to excessive sediment accumulation. Frequent maintenance may be required to
restore the infiltrative capacity of the system. Care must also be taken during construction to limit
compaction of the soil layers underlying the BMP. Excessive compaction due to construction
equipment may cause a reduced infiltrative capacity of the system. Plus, excessive sediment
generation during construction and site grading/stabilization may cause premature clogging of the
system. Infiltration systems should not be placed into service until disturbed areas in the drainage
have been stabilized by dense vegetation or grasses.
Infiltration Basins
Infiltration basins are designed to capture a storm water runoff volume, hold this volume
and infiltrate it into the ground over a period of days. Infiltration basins are almost always placed
off-line, and are designed to only intercept a certain volume of runoff. Any excess volume will be
bypassed. The basin may or may not be lined with plants. Vegetated infiltration systems help to
prevent migration of pollutants and the roots of the vegetation can increase the permeability of the
soils, thereby increasing the efficiency of the basin. Infiltration basins are typically not designed to
retain a permanent pool volume. Their main purpose is to simply transform a surface water flow
into a ground water flow and to remove pollutants through mechanisms such as filtration,
adsorption and biological conversion as the water percolates through the underlying soil.
Infiltration basins should be designed to drain within 72 hours in order to prevent mosquito
breeding and potential odor problems due to standing water and to ensure that the basin is ready
to receive runoff from the next storm (US EPA, 1993a). In addition to removing pollutants,
infiltration basins are useful to help restore or maintain pre-development hydrology in a
watershed. Infiltration can increase the water table, increase baseflow and reduce the frequency
of bankfull flooding events. A diagram of a typical infiltration basin is shown below.
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Figure 5-1. Infiltration Basin
Riprap
Settling Basin
and Level
Spreader
Profile View
Inlet
Valved Outlet
Perforated Pipe Under dram Svitem in Case of Standing Water Problem;
Source: Adapted from Schueler et al, 1992
Porous Pavement Systems
Porous pavement is an infiltration system where storm water runoff is infiltrated into the
ground through a permeable layer of pavement or other stabilized permeable surface. These
systems can include porous asphalt, porous concrete, modular perforated concrete block, cobble
pavers with porous joints or gaps or reinforced/stabilized turf (Urbonas and Strecker, 1996).
Permeable pavement can be used in parking lots, roads and other paved areas and can greatly
reduce the amount of runoff and associated pollutants leaving the area. Porous pavement systems
are suitable for a limited number of applications. Typically, porous pavement can only be used in
areas that are not exposed to high volumes of traffic or heavy equipment. They are particularly
useful for driveways and streets and in residential areas, and in parking areas in commercial areas.
Porous pavement is not effective in areas that receive runoff with high amounts of sediment due
to the tendency of the pores to clog. Porous pavements require maintenance including periodic
vacuuming or jet-washing to remove sediment from the pores. Paved areas should be clearly
marked to indicate that a porous pavement system is in use and to prevent frequent use by
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equipment, to prevent excess traffic volume, to limit the use of de-icing chemicals and sand, and
to prevent resurfacing with non-porous pavement.
The performance of porous asphalt has been historically very poor in the mid-Atlantic
region. However, many of these failures can be attributed to lack of proper erosion and sediment
controls during construction or lack of contractor experience with installation of porous pavement
systems. Porous concrete systems in use in Florida have performed very well (Florida Concrete
and Products Assn., 1993). When properly designed and maintained, porous pavement systems
can be an effective means of managing urban storm water runoff. Porous pavement systems are
particularly useful for overflow parking areas that are not used on a daily basis. A diagram of a
porous asphalt pavement system is shown below.
Figure 5-2. Porous Pavement System
Filter Fabric Layer
(along bottom and
sides)
Undisturbed Soil
Source: Adapted from Schueler, 1987.
Infiltration Trenches and Wells
An infiltration trench or well is a gravel-filled trench or well designed to infiltrate storm
water into the ground. A volume of storm water runoff is diverted into the trench or well where it
infiltrates into the surrounding soil. Typically infiltration trenches and wells can only capture a
small amount of runoff and therefore may be designed to capture the first flush of a runoff event.
For this reason, they are frequently used in combination with another BMP such as a detention
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basin to control peak hydraulic flows. Infiltration trenches and wells can be used to remove
suspended solids, particulates, bacteria, organics and soluble metals and nutrients through the
mechanisms of filtration, absorption and microbial decomposition. They are also useful to provide
groundwater recharge and to increase base flow levels in nearby streams. As with all infiltration
practices, the possibility for groundwater contamination exists and must be considered where
groundwater is a source of drinking water. A diagram of an infiltration trench is shown below.
Figure 5-3. Infiltration Trench
Wellcap
Observation Well
Emergency
Overflow
Berm
Runoff filters through 20
ft wide grass buffer strip
Protective layer of filter fabric
Finer fabric mes sides to
prevent soil contamination
P'Ti^ ^ ' i"p*" ^ ^
Y y Y
Sand filter (6-12
inch deep) or
fabric equivalent
Runoff exfiltrates through undisturbed subsoils
Source: Schueler et al, 1992.
5.2.1.2 Detention Systems
Detention systems are BMPs that are designed to intercept a volume of storm water runoff
and temporarily impound the water for gradual release to the receiving stream or storm sewer
system. Detention systems are designed to completely empty out between runoff events, and
therefore provide mainly water quantity control as opposed to water quality control. Detention
basins can provide limited settling of particulate matter, but a large portion of this material can be
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re-suspended by subsequent runoff events. Detention facilities should be considered mainly as
practices used to reduce the peak discharge of storm water to receiving streams to limit
downstream flooding and to provide some degree of channel protection. There are several types
of detention facilities used to manage storm water runoff, including detention basins and
underground vaults, pipes and tanks.
Detention Basins
Detention basins are designed to intercept a volume of storm water, temporarily impound
the water and release it shortly after the storm event. The main purpose of a detention basin is
quantity control by reducing the peak flow rate of storm water discharges. They are designed to
not retain a permanent pool volume between runoff events, and most basins are designed to empty
in a time period of less than 24 hours. The treatment efficiency of detention basins is usually
limited to removal of suspended solids and associated contaminants due to gravity settling. The
efficiency can be increased by incorporating a forebay or pre-settling chamber for the
accumulation of coarse sediment, facilitating periodic cleaning in order to prevent washout by
subsequent runoff events. Detention basins can limit downstream scour and loss of aquatic
habitat by reducing the peak flow rate and energy of storm water discharges to the receiving
stream, but their removal of pollutant of potential water quality concern can be limited. A
diagram of a typical detention basin is shown below.
Figure 5-4. Detention Basin
100_year
Riprap
Sediment Forebay
Barrel
Source: NVPDC, 1992.
Underground Vaults. Pipes and Tanks
Underground detention facilities, such as vaults, pipes and tanks, are designed to provide
temporary storage of storm water runoff. Significant water quality improvements should not be
expected in underground detention facilities. They should mainly be used for providing storage to
limit downstream effects due to high peak flow rates. Like detention basins, underground
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detention systems are designed to empty out between runoff events so that storage capacity is
available for subsequent runoff events. In addition, studies are being conducted to evaluate the
usefulness of in-system detention (storing runoff temporarily in the storm drainage system through
the use of valves, gates, orifices, etc.), although these evaluations are in the preliminary stages and
are only useful in certain cases (Lake Barcroft Watershed Improvement District, 1998). This is a
potential alternative for retrofitting existing storm drains in the upper portions of the drainage
system to delay the peak discharge rate and provide a limited amount of additional temporary
storage volume. However, a careful analysis of the storm drainage system is necessary in order to
prevent flooding in the upper reaches of the drainage area.
5.2.1.3 Retenti on Sy stem s
Retention systems include wet ponds and other retention systems such as underground
pipes or tanks. Retention systems are designed to capture a volume of runoff and retain that
volume until it is displaced in part or in total by the next runoff event. Retention systems can
provide both water quantity and quality control. The volume available for storage, termed the
water quality volume, is provided above the permanent pool level of the system. The main
pollutant removal mechanisms in retention systems is sedimentation. By retaining a permanent
pool of water, retention systems can benefit from the added biological and biochemical pollutant
removal mechanisms provided by aquatic plants and microorganisms, mimicking a natural pond or
lake ecosystem. Also, sediments that accumulate in the pond are less likely to be re-suspended
and washed out due to the presence of a permanent pool of water. In addition to sedimentation,
other pollutant removal mechanisms in retention systems include filtration of suspended solids by
vegetation, infiltration, biological uptake of nutrients by aquatic plants and algae, volatilization of
organic compounds, uptake of metals by plant tissue, and biological conversion of organic
compounds.
Retention Ponds
Retention ponds (also known as wet ponds) are designed to intercept a volume of storm
water runoff and to provide storage and treatment of this runoff volume. Water in the pond
above the permanent pool level is displaced in part or completely by the runoff volume from
subsequent runoff events. Retention ponds, when properly designed and maintained, can be
extremely effective BMPs, providing both water quality improvements and quantity control, as
well as providing aesthetic value and aquatic and terrestrial habitat for a variety of plants and
animals.
Pollutant removal in retention ponds can occur through a number of mechanisms. The
main mechanism is the removal of suspended solids and associated pollutants through gravity
settling. Aquatic plants and microorganisms can also provide uptake of nutrients and degradation
of organic contaminants. Retention basins that incorporate an aquatic bench around the perimeter
of the basin that is lined with aquatic vegetation can have an added pollutant removal efficiency.
This littoral zone can aid in pollutant removal efficiency by incorporating mechanisms found in
wetland systems. These mechanisms include removal of sediment by filtration by aquatic plants,
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removal of metals and nutrients through biological uptake by aquatic vegetation and degradation
of organic contaminants. If the bottom of the pond is not lined, then infiltration can occur aiding
in the maintenance of local groundwater supplies. A diagram of a typical wet pond is shown
below.
Figure 5-5. Retention Pond
Riser with Trash Rack
Emergency
Spillway
Principal Release Pipe
Set on Negative Slope
to Prevent Clogging
Deep Water Zone for
Gravity Settling
Emergent Aquatic
Riprap
Riprap for Shoreline
Protection
Sediment Forebay
Cutoff Trench
Concrete
Base
Low Flow Drain for Pond Maintenance
(Should be designed to provide easy access
and to avoid clogging by trapped sediments)
Source: NVPDC, 1992.
Retention Tanks. Tunnels. Vaults and Pipes
Retention systems other than ponds include surface tanks and underground vaults, pipes
and tunnels. These systems are not as prevalent as typical wet ponds, and therefore little
information is contained in the literature about their design, applicability and usefulness.
5.2.1.4 Constructed Wetland Systems
Constructed wetland systems incorporate the natural functions of wetlands to aid in
pollutant removal from storm water. Constructed wetlands can also provide for quantity control
of storm water by providing a significant volume of ponded water above the permanent pool
elevation. Constructed wetland systems have limits to their application. A water balance must be
performed to determine the availability of water to sustain the aquatic vegetation between runoff
events and during dry periods. In addition, a sediment forebay or some other pretreatment
provision should be incorporated into the wetland system design to allow for the removal of
coarse sediments that can degrade the performance of the system. Also, construction sediment
should be prevented from entering constructed wetlands, as the resulting sediment loading can
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severely degrade the performance of the system. Constructed wetlands are particularly
appropriate where groundwater levels are close to the surface because groundwater can supply
the water necessary to sustain the wetland system.
Storm water runoff should not be intentionally routed to natural wetlands without
pretreatment due to the potentially damaging effects runoff can have on natural wetland systems.
In addition, natural wetlands that receive storm water runoff should be evaluated to determine if
the runoff is causing degradation of the wetland, and if so measures should be taken to protect the
wetland from further degradation and to repair any damage that has been done. In addition, local
permitting authorities should be consulted prior to designing and maintaining constructed wetland
systems in order to determine if any local regulations apply to their use or maintenance.
Wetland Basins and Wetland Channels
Wetland basins and channels are any of a number of systems that incorporate mechanisms
of natural wetland systems for water quality improvement and quantity control. A wetland
channel is designed to develop dense wetland vegetation and to convey runoff very slowly
(Urbonas and Strecker, 1996). Generally, this rate is less that 2 feet-per-second at the 2-year
peak flow. Wetland basins may be designed with or without an open water (permanent pool)
component. Wetland basins with open water are similar to retention ponds, except that a
significant portion (usually 50 percent or more) of the permanent pool volume is covered by
emergent wetland vegetation. Wetland basins without open water are inundated with water
during runoff events, but do not maintain a significant permanent pool. Wetland basins of this
type, also known as a wetland meadow, support a variety of wetland plants adapted to saturated
soil conditions and tolerant of periodic inundation by runoff.
Pollutant removal in wetlands can occur through a number of mechanisms including
sedimentation, filtration, volatilization, adsorption, absorption, microbial decomposition and plant
uptake. In addition, wetlands can provide for significant water storage during runoff events, thus
supplying water quantity control as well. A diagram of a typical storm water wetland system is
included below.
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Figure 5-6. Constructed Wetland System
5.2.1.5 Filtration Systems
A filtration system is a device that uses a media such as sand, gravel, peat or compost to
remove a fraction of the constituents found in storm water. There are a wide variety of filter
types in use. There are also a variety of proprietary designs that use specialized filter media made
from materials such as leaf compost. Filters are primarily a water quality control device designed
to remove particulate pollutants. Quantity control can be included by providing additional storage
volume in a pond or basin, by providing vertical storage volume above the filter bed, or by
allowing water to temporarily pond in parking lots or other areas before being discharged to the
filter. Media filters are commonly used to treat runoff from small sites such as parking lots and
small developments, in areas with high pollution potential such as industrial areas, or in highly
urbanized areas where land availability or costs preclude the use of other BMP types. Filters
should be placed off-line (i.e., a portion of the runoff volume, called the water quality volume, is
diverted to the BMP, while any flows in excess of this volume are bypassed) and are sometimes
designed to intercept and treat only the first half inch or inch of runoff and bypass larger storm
water flows. A benefit of using filters in highly urbanized areas is that the filter can be placed
under parking lots or in building basements, limiting or eliminating costly land requirements.
However, placing filters "out of sight" may have implications for continued maintenance and
performance. Media filters should use a forebay or pre-settling chamber to remove a portion of
the settleable solids prior to filtration. This helps to extend the life of the filter run and prevent
clogging of the filter media by removing a portion of the coarse sediment. Also, care must be
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taken to prevent construction site sediments and debris such as fines washed off of newly paved
areas from entering the filter, as these can cause premature clogging of the filter.
Filter types in common use include surface sand filters such as the "Austin" sand filter and
underground vault filters such as the "D.C." sand filter and the "Delaware" sand filter. There are
a number of variations of these basic designs in common use. In addition, there are a number of
proprietary filtering systems in use. There are also a number of variations in the types of filtration
media that are in use in media filters. Designs may incorporate features such as a layer of filter
cloth or a plastic screen, a gravel layer, a peat layer, a compost layer, a layer of peat or a
peat/sand mixture. Typical variations in filtration media are shown below.
Figure 5-7. Filter Media
Underground Sand
Standard Surface Surface Sand Filter w/Gravel
Sand Filter Filter/Grass Cover Pretreatment
Underground
Sand Filter w/
Plastic Screen
Gravel
Sand
Underdrain
Perimeter Sand Filter
(Sand Chamber)
Compost Filter
System
Peat Sand Filter
w/ Grass Cover
Grate
Peat
;i;i;i;i;i;i;i;i;i;i;i;i;i; 50/50 Peat & Sand
Sand
Outflow
Source: Claytorand Schueler, 1996
Surface Sand Filter
The surface sand filter was developed in Florida in 1981 for sites that could not infiltrate
runoff or were too small for effective use of detention systems. The city of Austin, Texas took
the development of filter technology further in the mid-1980's. The surface sand filter system
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usually incorporates two basins. Runoff first enters a sedimentation basin where coarse particles
are removed by gravity settling. This sedimentation basin can be either wet or dry. Water then
flows over a weir or through a riser into the filter basin. The filter bed consists of sand with a
gravel and perforated pipe under-drain system to capture the treated water. The surface of the
filter bed may be planted with grass. Additional storage volume is provided above the filter bed
to increase the volume of water that can be temporarily ponded in the system prior to filtration.
This two-basin configuration can help to limit premature clogging of the filter bed due to
excessive sediment loading. There are several design variations of the simple surface sand filter.
Austin uses two variations, termed the partial and the full sedimentation-filtration systems. A
diagram of the Austin surface sand filter is shown in Figure 5-8.
Figure 5-8. Austin Full Sedimentation-Filtration System
TO STORMWATER
DETENTION BASIN
PLAN VIEW
FILTRATION BASIN
FILTERED OUTFLOW
STONE
RIP RAP
CHANNEL SLOPED TO
FACILITATE SEDIMENT
TRANSPORT INTO SEDI-
MENTATION BASIN
PERFORATED RISER
with TRASH RACK
ELEVATION A-A
UNDERDRAIN PIP1NS SKSTEM
Source: Bell, 1998
Underground Vault Sand Filter
The underground vault sand filter was developed by the District of Columbia in the late
1980's. This filter design incorporates three chambers. The first chamber and the throat of the
second chamber contain a permanent pool of water and functions as a sedimentation chamber and
an oil and grease and floatables trap, as well as provides for temporary runoff storage. A
submerged opening or inverted elbow near the bottom of the dividing wall connects the two
chambers. This submerged opening provides a water seal that prevents the transfer of oil and
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floatables to the second chamber which contains the filter bed. During a storm event, water flows
through the opening into the second chamber and onto the filter bed. Additional runoff storage
volume is provided above the filter bed. Filtered water is collected by a gravel and perforated
pipe under-drain system and flows into the third chamber, which contains a clearwell and a
connection to the storm drain system. Overflow protection can be provided by placing the filter
off-line, or by providing a weir at the top of the wall connecting the filter chamber with the
clearwell chamber to serve as an overflow. A schematic of the "D.C. Sand Filter" is shown
below.
Figure 5-9. Underground Vault Sand Filter
Access Grates
Inlet pipe
Temp, ponding
Submerged
wall
Manhole
I
-Steps
t?
.^Cleanouts
rj Gravel Tpp
*^*':' '\W': '-'*' ^fp:
Sand
^
Overflow
weir
Outlet
pipe^
Profile
Underdrain
CJ
Wet pool chamber
Filter bed chamber
Overflow
chamber
Manhole
Plan
fSBSAtt.
(Variable)
temp, ponding
Debris screen (1")
24" sand
Filter cloth
11" pea gravel
Typical Section
Source: Claytor and Schueler, 1996.
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Another underground vault sand filter, also termed a "perimeter" sand filter because it is
particularly suited for use around the perimeter of parking lots, was developed in Delaware by
Shaver and Baldwin and is known as the "Delaware Sand Filter." This system contains two
chambers and a clearwell. Storm water runoff enters the first chamber, which serves as a
sedimentation chamber. Water then flows over a series of weirs and into the second chamber
which contains the filter media. Additional storage volume is provided by water temporarily
ponding in both chambers. Filtered water is collected by a series of gravel and perforated pipe
under-drains, and flows into a clearwell that contains a connection to the storm drain system. A
schematic of the Delaware Sand Filter is shown below.
Figure 5-10. Delaware Sand Filter
Clearwell
Parking Lot Sheetflow
I I I I
/^ In let grates
zz3/sm -
= i i;
Outlet pipe
collection system
Plan
Curb stops
Overflow weirs
Inlet
chamber
1 r
itation
W m
^ F
... 1;:
f
"." '. -^*
'-f-*-^-" ~"^T .
-
.
Sedimentation ^ Sand chamber
chamber
Sand filter
Outlet pipe
Profile
^- Temporary ponding
*- Outlet pipes
Sand filter
" Filter fabric
Typical Section
Source: Shaver and Baldwin, 1991
In addition to the three basic filtering systems (D.C., Austin, and Delaware), there are a
number of variations in use. The city of Alexandria, Virginia has developed a compound storm
water filtering system (Bell, 1998). This design incorporates an anoxic filtration zone in a
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permanently flooded gravel layer in the filter. This anoxic zone aids in nitrogen removal by anoxic
denitrification. Another configuration uses an upflow anaerobic filter upstream of the sand filter
to enhance phosphorus removal by precipitating more iron on the sand filter. A diagram of the
Alexandria Compound Filtration System is shown below.
Figure 5-11. Alexandria Compound Filter
Maintenance access
Dewatering drains with
gate valves
Gooseneck outflow
46 cm (18") sand filter (phosphorus
removal, nitrification)
Perforated collector pipes (4 required)
Bottom flooded gravel filter (denitrification)
' Upflow flooded gravel filter (anaerobic) (denitrification iron precipitation
TSS removal)
Hydrocarbon trap in sedimentation chamber
Source: Bell, 1998.
Filters that use an organic filtration media, such as peat or leaf compost, are useful in areas
where additional nutrient or metal control is desirable due to the adsorptive capacity, its ion-
exchange capability, and is ability to serve as a medium for the growth of a variety of
microorganisms. However, peat must be carefully selected (fibric and/or hemic peat should be
used, not sapric) and one must question the environmental consequences of destroying peat bogs
to obtain filtration media when other technologies are available.
There are a number of references available that contain information on the design and
selection of filtering systems for storm water treatment (Urbonas, 1999; Bell, 1998; Claytor and
Schueler, 1996; Galli, 1990b; MDE, 1998; NVPDC, 1996a).
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Biofiltration/Bioretention Systems
Bioretention systems are designed to mimic the functions of a natural forest ecosystem for
treating storm water runoff. Bioretention systems are a variation of a surface sand filter, where
the sand filtration media is replaced with a planted soil bed. Storm water flows into the
bioretention area, ponds on the surface, and gradually infiltrates into the soil bed. Pollutants are
removed by a number of processes including adsorption, filtration, volatilization, ion exchange
and decomposition (Prince George's County, MD, 1993). Treated water is allowed to infiltrate
into the surrounding soil, or is collected by an under-drain system and discharged to the storm
sewer system or directly to receiving waters. When water is allowed to infiltrate into the
surrounding soil, bioretention systems can be an excellent source of groundwater recharge. A
diagram of a typical bioretention area is shown below.
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Figure 5-12. Bioretention System
Traffic Island
Bioretention Area
Limit
Flow
Pavement
Flow
Curb
Flow
Plan View
Ground cover or
mulch layer
3:1 (typ.
Flow
Planting
soil
I 2'
min.
I Max. ponded
water depth (6")
Curb
Flow
1 f
V .JL,
V
Proposed
Grade .
k
4' min.
r
Bioretention area-
Section
Source: Prince George's County, 1993.
5-24
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The components of a bioretention system include:
Grass Buffer Strips - runoff enters the bioretention area as sheet flow through the grass
buffer strips. The buffers reduce the velocity of the runoff and filter particulates from
the runoff.
Ponding Area - The ponding area provides for surface storage of storm water runoff
before it filters through the soil bed. The ponding area also allows for evaporation of
ponded water as well as allows for settling of sediment in the runoff.
Organic Mulch Layer - The organic mulch layer has several functions. It protects the
soil bed from erosion, retains moisture in the plant root zone, provides a medium for
biological growth and decomposition of organic matter, and provides some filtration of
pollutants.
Planting Soil Bed - The planting soil bed provides water and nutrients to support plant
life in the bioretention system. Storm water filters though the planting soil bed where
pollutants are removed by the mechanisms of filtration, plant uptake, adsorption and
biological degradation.
Sand Bed - the sand bed underlies the planting soil bed and allows water to drain from
the planting soil bed through the sand bed and into the surrounding soil. The sand bed
also provides additional filtration and allows for aeration of the planting soil bed.
Plants - Plants are an important component of a bioretention system. Plants remove
water though evapotranspiration and remove pollutants and nutrient through uptake.
The plant species selected are designed to replicate a forested ecosystem and to survive
stresses such as frequent periods of inundation during runoff events and drying during
inter-event periods.
In addition to providing for treatment of storm water, bioretention facilities, when
properly maintained, can be aesthetically pleasing. Bioretention facilities can be placed in areas
such as parking lot islands, in landscaped areas around buildings, the perimeter of parking lots,
and in other open spaces. Since local regulations frequently require site plans to incorporate a
certain percentage of open landscaped area, additional land requirements for bioretention facilities
are often not required. The layout of bioretention facilities can be very flexible, and the selection
of plant species can provide for a wide variety of landscape designs. However, it is important that
a landscape architect with proper experience in designing bioretention areas be consulted prior to
construction to insure that the plants selected can tolerate the growing conditions present in
bioretention facilities. Bioretention facilities can be adapted easily for use on individual residential
lots. Prince George's County, MD has developed the concept of "rain gardens" which are small
bioretention systems for use in single or multi-lot residential areas. They provide an easily
maintainable, aesthetically pleasing, and effective means of controlling runoff from residential
5-25
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areas. By disconnecting down spouts and placing a series of bioretention areas throughout a
residential area, the volume of storm water runoff produced and requiring subsequent
management can be significantly reduced.
Additional design information on bioretention facilities can be found in Design Manual for
Use of Bioretention in Stormwater Management (Prince George's County, 1993) and in Design
of Stormwater Filtering Systems (Claytor and Schueler, 1996).
5.2.1.6 Vegetated Systems (Biofilters)
Vegetated systems such as grass filter strips and vegetated swales are used for conveying
and treating storm water flows. These BMPs are commonly referred to as biofilters., since the
grasses and vegetation "filter" the storm water as it flows. Open channel vegetated systems are
alternatives to traditional curb-and-gutter and storm sewer conveyance systems. By conveying
storm water runoff in vegetated systems, some degree of treatment, storage and infiltration can be
provided prior to discharge to the storm sewer system. This can help to reduce the overall
volume of storm water runoff that is generated from a particular drainage area.
Grass Filter Strips
Grass filter strips are densely vegetated, uniformly graded areas that intercept sheet runoff
from impervious surfaces such as parking lots, highways and rooftops. Grass filter strips are
frequently planted with turf grass, however alternatives that adopt any natural vegetated form
such as meadows or small forest may be used. Grass filter strips can either accept sheet flow
directly from impervious surfaces, or concentrated flow can be distributed along the width of the
strip using a gravel trench or other level spreader. Grass filter strips are designed to trap
sediments, to partially infiltrate this runoff and to reduce the velocity of the runoff. Grass filter
strips are frequently used as a "pretreatment" system prior to storm water being treated by BMPs
such as filters or bioretention systems. Grass filter strips can also be used in combination with
riparian buffers in treating sheet flows and in stabilizing drainage channel banks and stream banks.
In semi-arid climates, grass filter strips may need to be irrigated to maintain a dense stand of
vegetation and to prevent export of unstabilized soil. A diagram of a grass filter strip is shown
below.
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Figure 5-13. Grass Filter Strip
>
50' max.
low length
Lawn
)
k
r
,.mป J*" '
ta-
inted with grass tolerant
o frequent inundation
75' max.
flow length
>y Parking
i iffit*
Filter strip
25' min. ^ .'ป*
length maximum
js#ป ponding limit
f .ป>-
t"\ / IL& ||AT\ / f-]
J A Irr 1 v .A V ^ x
Curb stops
Pea gravel
diaphragm
Pervious
berm
Outlet
pipes, spaced
@ 25' centers
Plan
Not to Scale
Pervious berm
(sand/gravel mix)
12"x24"
pea gravel
diaphragm
Water Quality
Treatment Volume
Profile
Outlet pipes
12" max.
Source: Claytorand Schueler, 1996.
5-27
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Vegetated Swales
Vegetated swales are broad, shallow channels with a dense stand of vegetation covering
the side slopes and channel bottom. Vegetated swales are designed to slowly convey storm water
runoff, and in the process trap pollutants, promote infiltration and reduce flow velocities.
Vegetated swales can be either wet or dry. Dry swales are used in areas where standing water is
not desired, such as in residential areas. Wet swales can be used where standing water does not
create a nuisance problem and where the groundwater level is close enough to the surface to
maintain the permanent pool in inter-event periods. Wet swales provide the added benefit of
being able to include a range of wetland vegetation to aid in pollutant removal.
5.2.1.7 Minimizing Directly-Connected Impervious Surfaces
Minimizing directly-connected impervious surface areas involves a variety of practices
designed to limit the amount of storm water runoff that is directly connected to the storm
drainage system. Runoff is instead directed to landscaped areas, grass buffer strips, and grassed
swales to reduce the velocity of runoff, reduce runoff volumes, attenuate peak flows, and
encourage filtration and infiltration of runoff (UDFCD, 1992). By incorporating these principles
into site designs, the size and number of conventional BMPs such as ponds and constructed
wetland systems can be significantly reduced.
Minimizing directly connected impervious surfaces incorporates both non-structural and
structural control measures. Discussions in this section address the structural measures that can
be incorporated into existing urbanized or newly developed areas to minimize the amount of
runoff discharged to the storm drain system. Additional discussion on non-structural practices
that can be used to minimize runoff generation in new developments is included in Section 5.2.3
"Low Impact Development Practices."
The Denver Urban Storm Drainage Criteria Manual (UDFCD, 1992) identifies the
following three levels of minimizing directly connected impervious areas:
Level 1: Runoff generated from impervious surfaces such as rooftops, driveways and
parking lots is directed to flow over vegetated areas before flowing to a storm sewer
system. This increases the travel time of runoff and promotes the removal of
suspended solids by sedimentation and filtration.
Level 2: Street curb-and-gutter systems are replaced by grassed swales and pervious
street shoulders. Conveyance systems and storm sewer inlets are still used to collect
runoff at downstream intersections and crossings.
Level 3: In addition to incorporating Levels 1 and 2, swales are oversized and driveway
and street crossing culverts are configured to use the grassed swales as detention
basins having the capacity to capture runoff volume for a design storm (2-, 5-, 10- or
100-year runoff).
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Practices that reduce the amount of directly connected impervious surfaces can easily be
incorporated into site design plans during the planning stages of development projects. Using
these practices can result in significant development cost savings due to the decreased need for
drainage infrastructure and large end-of-pipe structural BMPs such as ponds and constructed
wetlands. These practices can also limit secondary impacts from structural BMPs, such as
temperature increases from retention ponds. Minimizing directly connected impervious areas can
also be applied to existing urbanized areas through retrofit. Practices that can be used in retrofit
instances include disconnecting rooftop downspouts from the storm drain system, use of on-site
retention and infiltration to limit the amount of runoff leaving the site and replacing traditional
curb-and-gutter systems with grassed swales and wetland channels. Additional discussion on
practices that minimize runoff generation is included in Section 5.2.3.
5.2.1.8 Miscellaneous and Vendor-Supplied Systems
There are a wide variety of miscellaneous and proprietary devices that are used for urban
storm water management. Many of these systems are "drop-in" systems, and incorporate some
combination of filtration media, hydrodynamic sediment removal, oil and grease removal, or
screening to remove pollutants from storm water. A few of the systems available include:
Bay Saver
CDS Technologies
Hydrasepฎ
Stormceptorฎ
StormFilter
StormTreat System
Vortechs.
A thorough evaluation of vendor-supplied systems was not conducted in this report.
Readers are encouraged to contact the product vendors to obtain information regarding these
systems.
One of the main problems facing the use of proprietary devices is the lack of peer-
reviewed performance data for these systems. Several vendor-supplied storm water treatment
systems are being evaluated through EPA's Environmental Technology Verification (ETV)
program. With financial assistance from the ETV program, the Civil Engineering Research
Foundation (CERF) established the Environmental Technology Evaluation Center, commonly
known as "EvTEC, " in 1998. EvTEC is a private sector program designed to utilize networks of
experts, testing facilities and stakeholders to evaluate technologies dealing with a variety of
environmental problems. One of the EvTEC projects is a collaborative effort with the
Washington Department of Transportation (WSDOT) to verify the performance of innovative
storm water BMPs under field operating conditions. These evaluations, which are scheduled to
begin in 1999, are expected to provide comparable, peer-reviewed data on the performance of
these systems (CERF, 1998).
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5.2.2 Non-Structural BMPs
Non-structural BMPs include institutional and pollution-prevention type practices
designed to prevent pollutants from entering storm water runoff or reduce the volume of storm
water requiring management. Non-structural BMPs can be very effective in controlling pollution
generation at the source, which in turn can reduce or eliminate the need for costly end-of-pipe
treatment by structural BMPs. Non-structural BMPs discussed in this report include education
and source controls, recycling and maintenance practices.
5.2.2.1 Education, Recycling and Source Controls
Public education can be an effective means of reducing the amounts of non-point source
pollutants entering receiving streams. The public is often unaware that the combined effects of
their actions can cause significant non-point source pollution problems. Proper education on day-
to-day activities such as recycling of used automotive fluids, household chemical and fertilizer
use, animal waste control and other activities can significantly reduce non-point source pollutant
loadings to urban streams. The main components of a public education program include:
Automotive Product Disposal
Discharge of automotive fluids such as motor oil and antifreeze to the land or storm drains
can cause significant water quality problems. "Do-it-yourself automobile mechanics often
incorrectly assume that materials that are dumped into storm drains will receive treatment at a
wastewater treatment plant prior to discharge. Education on appropriate recycling and disposal
techniques for these materials can help to reduce pollutant loadings to streams. Education
programs should identify the location of community automotive products recycling centers. In
addition to impacts associated with dumping used oil and antifreeze, potential runoff pollutant
sources from home automobile maintenance activities include dirt, cleaners, oils and solvents from
car washing, leaking fluids such as brake and transmission fluid and gasoline spills. To reduce
impacts from these activities, the following practices should be used:
all spills or leaks should be cleaned up using a dry absorbent such as cat litter or
commercially available absorbents and disposed of appropriately;
car washing should be done away from storm drains using biodegradable cleaners, or at
a commercial carwash;
all used fluids should be recycled or disposed of appropriately;
all fluid leaks should be repaired as soon as possible to reduce loss to the environment.
Commercial and Retail Space Good Housekeeping
Commercial and retail areas can contribute significant pollutant loadings to runoff. The
biggest contributor of pollutant is usually impervious surfaces used for vehicle parking, storage
and maintenance areas, which can contribute sediment, metals and hydrocarbons. Other sources
include raw material and finished product storage areas, pesticides and fertilizers from grounds
maintenance, and rooftop runoff. Good housekeeping practices include using porous pavement or
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modular paving systems for vehicle parking lots; limiting exposure of materials and equipment to
rainfall; spill cleanup, using dry cleanup techniques instead of wet techniques; and limiting direct
runoff of rooftops to storm drains.
General Community Outreach
A main problem associated with identifying and controlling nonpoint source pollution is
that the public is generally unaware of the sources and control measures for urban nonpoint
source pollutants. Information dissemination is a critical need of most local storm water
programs. Information that explains the sources of nonpoint source pollution, control measures
available and the steps homeowners and commercial owners can do to reduce impacts of their
activities can help to increase the public awareness of the need to control nonpoint source
pollution. A few of the techniques available for providing educational materials to the public
include television, radio and newspaper announcements, distribution of flyers, community
newsletters, workshops and seminars, conducting teacher training programs at schools, and
supporting citizen-based watershed stewardship groups and volunteer monitoring programs.
Industrial Good Housekeeping
Industrial areas can contribute significant loadings of toxic pollutants to storm water
runoff. Therefore, educational programs that inform industrial site owners and operators about
pollution prevention and source control programs to reduce nonpoint source pollutant can
significantly reduce the amounts of pollutants discharged from industrial areas. Pollution
prevention practices include minimizing or eliminating exposure of materials and products to
rainfall by storing inside or under cover, spill cleanup, minimizing pesticide/herbicide and fertilizer
use, and minimizing discharges of equipment wash water to storm drains.
Storm Drain Inlet Stenciling
Since storm drains frequently discharge runoff directly to water bodies without receiving
any type of treatment, storm drain stenciling programs that educate residents not to dump
materials into storm drains or onto sidewalks, streets, parking lots and gutters can be effective at
reducing nonpoint source pollution associated with illegal dumping. Residents are frequently
unaware that materials dumped down storm drains may be discharged to a local water body.
Therefore, stenciling the inlets can be a simple yet effective means of alerting residents of this fact.
The Northern Virginia Nomtructural Urban BMP Handbook (NVPDC, 1996b) contains a useful
discussion on developing a storm drain stenciling program.
Pesticide/Herbicide Use
Due to their high aquatic toxicity, pesticides and herbicides can be a significant source of
water quality impairment in urban streams. Pesticide usage in the United States was estimated at
more than 1.2 billion pounds of active ingredients in 1995 (US EPA, 1997b). Of this total,
agricultural usage constituted 939 million pounds (77 percent), commercial, industrial and
government usage accounted for 150 million pounds (13 percent) and home and garden usage
accounted for 133 million pounds (11 percent). A significant portion of these applications find
their way into storm water runoff and ultimately into receiving streams through spray drift,
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transport by soils, solubilization by runoff, and by spillage, dumping and improper disposal of
containers and residuals. Education on the proper methods of application, application rates and
alternatives to pesticides can help to reduce the amount of pesticides that are carried by urban
runoff. Alternatives to pesticides, such as in integrated pest management program and pesticide
alternatives such as insecticidal soap or natural bacteria, can also reduce the need for pesticides.
Fertilizer Use
A significant amount of nutrients in urban runoff results from misapplication of fertilizer to
the urban landscape. Residential lawn and garden maintenance and maintenance of landscape and
turfgrass at golf courses, schools and commercial areas uses significant amounts of fertilizers
containing nitrogen and phosphorus. Since most fertilizers are water soluble, over-application or
application before rainfall events can allow significant quantities to be carried away by storm
water runoff. Education on proper application of fertilizers can help to reduce the quantities of
nutrients reaching receiving waters.
Household Hazardous Material Disposal
A variety of hazardous and potentially harmful chemicals and materials are improperly
used and disposed of by residential homeowners. Materials such as paints and thinners, cleaning
products, wood preservatives, driveway sealants and a variety of other miscellaneous household
chemicals can find their way into storm water if improperly used, stored or disposed of.
Education on usage and holding an annual or semi-annual community household hazardous waste
collection program can help to reduce the amounts of these materials that enter storm water
runoff.
Lawn Debris Management
Lawn debris such as grass trimmings and leaves require proper management in order to
reduce impacts to urban streams. Grass trimmings and leaves can be carried away by runoff and
can find their way into streams where they rapidly decompose and release nutrients. Grass
trimmings and leaf litter can be controlled by composting or by community curbside collection
programs. Composted yard debris can be an excellent source of mulch for residential landscape
and gardens. Use of mulch can greatly reduce the need for inorganic fertilizers, which helps to
keep nutrient loadings to streams to a minimum.
Pet Waste Disposal
Pet waste can cause significant loadings of bacteria, nutrients and oxygen demanding
substances to urban runoff. Pet waste deposited on yards, sidewalks and streets can be carried by
runoff into storm drains. As an example, it is estimated that 11,445 pounds of dog waste are
generated in the Four Mile Run watershed in northern Virginia each day.2 378 pounds of BOD,
39 pounds of total phosphorus and 189 pounds of total nitrogen are washed off into Four Mile
Run and its tributaries annually as a result of this pollution load (NVPDC, 1996b). In many areas,
2 This estimate was calculated based on the total waste load generated by the dog
population in the area, not the waste load deposited on yards, sidewalks and streets.
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regulations exist prohibiting the deposit of pet waste on public property. However, it is often
very difficult to enforce these laws. Community education on the impacts associated with pet
waste and alternative disposal methods such as flushing and disposal in the trash can help to
reduce impacts associated with pest waste. A particularly useful method of controlling pet waste
is for communities to provide pet waste receptacles in parks and other public areas for pet owners
to deposit droppings from their pets.
Illicit Discharge Detection and Elimination
Illicit discharges to storm sewers can be a significant source of pollutants in urban storm
water. A study conducted in Sacramento, California indicated that slightly less than one-half of
the water discharged from a municipal separate storm sewer system was not directly attributable
to precipitation runoff (US EPA, 1993b). A major source of illicit discharges to storm drain
systems are direct connections of sanitary sewer piping to the storm drain system. In addition to
direct connections, seepage and sewage from leaking sanitary sewer lines can find their way into
storm drains, especially in areas where storm drains run parallel to the sanitary sewer lines. Spills
can also be collected by storm drain inlets.
Detection and elimination of illicit connections and discharges can significantly reduce the
concentrations of bacteria, nutrients and oxygen demanding substances contained in storm water
discharges. Several methods exist for detection and elimination of illicit cross-connections.
Useful indicators of the presence of cross connections include dry weather flows in storm sewer
lines and biological indicators that indicate the presence of human fecal matter in storm drain
outfalls. Once illicit connections are detected, excavation and correction of the illicit connections
are necessary. In addition to detection and elimination of existing cross-connections, plans for
new development should be carefully reviewed and inspections should be conducted during
construction in order to prevent future cross-connections from being placed. Storm drain
stenciling programs and a public spill reporting system can help to educate the public on proper
procedures for managing spills to prevent discharge to the storm sewer system.
5.2.2.2 Maintenance Practices
Maintenance programs are necessary in order to reduce the pollutant contribution from the
urban landscape and to ensure that storm water collection and treatment systems are operating as
designed. Major maintenance practices that can be used include:
Catch Basin Cleaning
Catch basins naturally accumulate sediment and debris such as trash and leaf litter. In
order to ensure their continued effectiveness, catch basins need to be periodically cleaned. This
can be done by manual means, or by using a vacuum truck.
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Street and Parking Lot Sweeping
Urban streets and parking lots can accumulate large amounts of pollutants that can be
washed off during storm events. Streets and parking lots comprise a significant portion of the
total impervious area within a developed watershed, and a large percentage, if not the entire area,
of streets and parking lots are usually directly connected to the storm drain system. In an
investigation conducted by Bannerman (Bannerman et al, 1993), data on runoff volumes from
streets and parking lots collected during 4 years from two urbanized areas in Wisconsin indicated
that 54 percent of the total runoff volume from residential areas was due to direct runoff from
streets and parking lots, and that 80 percent of the total runoff volume from commercial areas was
due to direct runoff from streets and parking lots. A breakdown of the runoff volumes based on
source area is shown in Table 5-1.
Table 5-1. Percent Runoff Volumes Contributed by Source Area in Two Urbanized Areas
of Wisconsin
Land Use
Residential
Commercial
Source Area Percent Runoff Contribution
Feeder
Streets
34
Collector
Streets
20
10
Arterial
Streets
21
Parking
Lots
49
Total % due
to roads and
parking lots
54
80
Total
Other %*
46
20
* Other land uses include lawns, driveways, rooftops and sidewalks
Source: Adapted from Bannerman et al, 1993
Furthermore, Bannerman found that runoff from streets and parking lots contributed a
significant portion of the total runoff pollutant loading. Table 5-2 summarizes the pollutant load
contributions based on land uses, and indicates the total contaminant contribution in the urbanized
area attributable to runoff from streets and parking lots.
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Table 5-2. Contaminant Load Percentages in Two Urbanized Areas of Wisconsin
Contaminant
Total Solids
Suspended
Solids
Total
Phosphorus
Dissolved
Phosphorus
Dissolved
Copper
Total Copper
Total Zinc
Fecal
Coliform
Percent Contribution by Source Area
Residential
Streets
76
80
58
46
73
78
80
78
Parking
Lots
Commercial
Streets
57
68
56
50
50
60
45
82
Parking
Lots
31
27
28
27
39
32
32
10
Industrial
Streets
20
25
19
18
16
22
9
10
Parking
Lots
60
55
29
11
73
67
30
19
Total
Contaminant
Contribution
by Streets
and
Parking Lots
78
80
54
39
82
85
49
71
Source: Adapted from Bannerman et al, 1993
Based on these data, streets and parking lots can contribute significant pollutant loadings
to urban runoff. Therefore, sweeping programs that can remove a portion of these materials from
streets and parking lots may significantly reduce the pollutant load contributions to urban runoff.
Road and Ditch Maintenance
Road and street surfaces undergo breakdown due to frictional action of traffic, fireeze-
thaw breakdown, frost heaving, and erosion of road subbase. Failure to correct deteriorating
pavement can allow exposure of unstabilized subbase material to erosive forces of water and
subsequent increases in suspended solids concentrations. The same process occurs in roadside
ditches where high runoff rates cause channelization and erosion. Roadside ditches also
accumulate sediment and debris from the road surface, which enters runoff during rainfall events.
Maintenance of roads and cleaning and stabilization of ditches can help to reduce pollutant
loadings from these sources. In roadside ditches, reducing the length and slope of ditch runs and
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reducing the velocity of runoff by using check dams can help to prevent excessive channelization
and erosion.
Road Salting and Sanding
Road salting and sanding can contribute large quantities of sediment and salts to runoff.
Highway maintenance programs in areas where road icing is a problem frequently apply large
quantities of sand, salt, and coal ash to prevent icy road surfaces. Snowmelt can carry a large
portion of these materials into the storm drainage system and ultimately to receiving streams.
High salt concentrations can have significant impacts on receiving streams. In addition, road salt
can contain cyanide, which may cause acute or chronic toxicity to aquatic organisms. Alternative
deicing products such as acetates, formates and agricultural residues can be used if impacts due to
traditional deicing products are significant.
Sediment and Floatables Removal from BMPs
Sediment and floatables removal is an important component of maintenance for BMPs that
are designed for sediment capture. Removal of accumulated sediment is important so that the
BMP continues to operate efficiently. Accumulation of excess sediment in pond and constructed
wetland systems can lead to reduced storage capacity, short-circuiting and re-suspension of
previously settled particles. All of these can lead to decreased efficiency of the BMP. Floatables
in BMPs can accumulate and block outlet structures leading to changes in BMP hydraulics.
Floatables can cause aesthetic impacts, and floating material such as algal scum and other debris
can lead to odor problems. Sediment removal is also needed periodically in filtration systems.
Sedimentation chambers require periodic cleanout of sediments and floatables (including
accumulated oil) and filter beds will accumulate a sediment layer on the surface that will decrease
the filtration rate of the system over time. Periodic removal of this sediment layer and a portion
of the filtration media is necessary in order to restore the filtration capacity of the system.
Sediments also accumulate in infiltration basins. The accumulation of sediments, particularly
sediments from construction activities and improperly stabilized soil, will lead to a rapid reduction
of the infiltrative capacity of infiltration basins, trenches and wells.
The frequency of sediment removal in BMP types can vary widely. Some BMPs require
sediment removal every two or three years, while others may not need maintenance for more than
20 years. The frequency that sediment must be removed depends greatly on the land use and
degree of soil stabilization in the contributing watershed. BMPs that receive runoff from a
watershed that has significant construction activities will accumulate sediment at a rate much
faster than a watershed with little or no construction activity. In addition, watersheds with dense,
well established vegetation will contribute less sediment than sparsely vegetated watersheds.
Also, watersheds in arid or semi-arid regions, which frequently are subject to high intensity rainfall
and highly erosive storm water flows will produce large quantities of solids requiring frequent
removal from BMPs. Table 5-3 summarizes maintenance requirements and frequency for
different structural BMP types.
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Vegetation Maintenance
Vegetative BMPs such as constructed wetlands, grassed filter strips, vegetated swales,
and bioretention facilities require periodic vegetation maintenance to enhance performance.
Grassed filter strips and vegetated swales require a dense stand of vegetation in order to function
properly and to prevent export of sediment from unstabilized planting areas. Several seasons of
planting and re-seeding of sparsely vegetated areas may be needed in order to reach optimum
performance. Constructed wetland systems frequently require re-planting of wetland vegetation
in areas where original plantings failed to become established. Once wetland systems are
functioning, periodic vegetation harvesting is necessary to remove excess vegetation and stored
nutrients. Invasive species also need to be periodically removed to promote growth of beneficial
wetland vegetation. Grassed filter strips and vegetated swales require periodic mowing to remove
excess vegetation and stored nutrients. Mowing of these systems should not be done too close to
the ground, as dense vegetation is needed for optimum performance.
General BMP Maintenance
BMPs require a variety of periodic maintenance activities in order to enhance
performance. In addition to sediment removal and vegetation maintenance, periodic maintenance
and repair of outlet structures is needed, filtration media need to be periodically replaced, and
eroded areas need to be repaired, to name a few. Table 5-3 summarizes general maintenance
activities and frequency for a few BMP types. The actual maintenance schedule varies
considerably based on site-specific conditions, and the values given should be used only as a
general guideline for established residential or commercial areas without significant inputs of
construction sediment or other sediment loadings.
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Table 5-3. Recommended BMP Maintenance Schedules
BMP
Activity
Schedule
Cleaning and removal of debris after major storm events
Harvest excess vegetation
Repair of embankment and side slopes
Repair of control structure
Retention Pond
/ Wetland1
Removal of accumulated sediment from forebays or sediment
storage areas
Removal of accumulated sediment from main cells of pond once
the original volume has been significantly reduced
Annual or
as needed
5-year
cycle, or as
needed
20-year
cycle
(although
can vary)
Detention Basin
Removal of accumulated sediment
Repair of control structure
Repair of embankment and side slopes
Annual or
as needed
Infiltration
Trench1
Cleaning and removal of debris after major storm events
Mowing4 and maintenance of upland vegetated areas
Maintenance of inlets and outlets
Annual or
as needed
Infiltration
Basin2
Cleaning and removal of debris after major storm events
Mowing4 and maintenance of upland vegetated areas
Annual or
as needed
Removal of accumulated sediment from forebays or sediment 3- to 5-
storage areas year cycle
Sand Filters3
Removal of trash and debris from control openings
Repair of leaks from the sedimentation chamber or deterioration
of structural components
Removal of the top few inches of sand and cultivation of the
surface when filter bed is clogged (only works for a few cycles)
Clean-out of accumulated sediment from filter bed chamber
Clean out of accumulated sediment from sedimentation chamber
Annual or
as needed
1. Modified from Livingston et al (1997)
2. Modified from Livingston et al (1997), based on infiltration trench requirements
3. Modified from Claytor and Schueler (1996)
4. Mowing may be required several times a year, depending on local conditions
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Table 5-3. Recommended BMP Maintenance Schedules (continued)
BMP
Bioretention1
Grass Swale2
Filter Strip3
Activity
Repair of eroded areas
Mulching of void areas
Removal and replacement of all dead and diseased vegetation
Watering of plant material
Removal of mulch and application of a new layer
Mowing4 and litter and debris removal
Stabilization of eroded side slopes and bottom
Nutrient and pesticide use management
De-thatching swale bottom and removal of thatching
Discing or aeration of swale bottom
Scraping swale bottom, and removal of sediment to restore
original cross section and infiltration rate
Seeding or sodding to restore ground cover (use proper erosion
and sediment control)
Mowing4 and litter and debris removal
Nutrient and pesticide use management
Aeration of soil in the filter strip
Repair of eroded or sparse grass areas
Schedule
Bi-Annual
or as
needed
Annual
Annual or
as needed
5 -year cycle
Annual or
as needed
1. Modified from Prince George's County (1993)
2. Modified from Livingston et al (1997)
3. Modified from Livingston et al (1997) based on grass swale recommendations
4. Mowing may be required several times a year, depending on local conditions
5.2.3 Low-Impact Development Practices
There are a number of low-impact development practices that can be used at the site level.
While these practices often do not produce direct removal of pollutants from runoff, they can
significantly reduce runoff volumes that are generated, reduce the impacts associated with runoff
and reduce the need for conventional structural BMPs. There are a number of practices that are
in use, and therefore an exhaustive summary has not been included in this document. However, a
few of the more common practices in use are presented briefly in the following sections.
Minimizing Impervious Areas
Minimizing the amount of impervious surfaces that are created in a new development can
greatly reduce the volume of storm water runoff that is generated. There are many opportunities
that exist for reducing impervious surfaces, including:
limiting the number, length and radius of cul-de-sacs;
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using porous pavement or modular block pavers in parking areas and low-traffic areas;
reducing the width of streets;
placing sidewalks on only one side of the street;
reducing frontage requirements to lessen paved surface areas.
Although the above practices can reduce the amounts of impervious surfaces that are
created, there will still be a great deal of impervious surfaces that must be included into a site plan
such as rooftops, streets, driveways and lawns. To limit the impacts associated with runoff from
these surfaces, it is important to limit the amount of areas that are directly connected to the storm
drainage system. This can be accomplished by providing on-site retention and infiltration to
collect rooftop and driveway runoff, and through the use of BMPs such as grassed swales,
vegetated filter strips and wetland channels in place of traditional curb-and-gutter systems.
Directed Growth
Directed growth involves placing controls on land use through mechanisms such as master
planning and zoning ordinances. Local governments may utilize these mechanisms in order to
protect sensitive areas from development and to target growth to areas that are more suitable for
development where it is easier to control the impacts associated with runoff. Directed growth can
be a complex process, and must balance a number of factors such as economic considerations,
local laws and ordinances, secondary impacts such as increased traffic and population in certain
areas, as well as the availability of public utilities such as sewage treatment and drinking water
service, and schools, hospitals and fire stations. Nevertheless, with careful planning and
consideration, directed growth can help to reduce impacts associated with development of an
area.
Sensitive Area Protection
Sensitive area protection is an important component of conservation design. Sensitive
areas include the areas adjacent to streams, wetlands and natural drainage channels, cold water
fisheries, shellfish beds, swimming beaches, recharge areas, and drinking water supplies. These
areas are particularly susceptible to degradation by storm water runoff. Preservation of these
areas and incorporation of stream and wetland buffers into site plans can help to preserve the
integrity of these areas.
Open Space Preservation
Preservation of open space such as forested areas and meadows can help to reduce the
impacts associated with development of an area. Open space preservation helps to reduce the
generation of runoff, and can reduce the overall impact that results from development of an area
by limiting the amount of impervious areas that are created. Open space allows the preservation
of buffers and natural drainage corridors, and retains the natural storm water filtering, retention
and infiltration effects of these areas. Open space can also increase the aesthetics of a
development, and make the area more desirable to potential home buyers.
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Minimizing Soil and Vegetation Disturbance
Soil and vegetation disturbance can significantly increase the amount of runoff that is
generated from a site and the concentrations of pollutants that are transported by the runoff.
Disturbed soil areas are particularly susceptible to erosion during storm events. Vegetation helps
to stabilize soil and prevent detachment and transport by flowing water. By minimizing the area
that is disturbed to only areas undergoing active construction (often termed "fingerprinting"),
erosion of soil can be minimized. In addition, disturbance of soil and vegetation should be limited
to only those areas that are necessary. Disturbing soil by excavation, grading and compaction
reduces the infiltrative capacity of the soil, creating additional runoff that must be managed.
Maintaining naturally vegetated areas minimizes the amount of increased runoff that is produced.
5.3 BMP Selection
BMP selection is a complex process. There are a number of competing factors that need
to be addressed when selecting the appropriate BMP or suite of BMPs for an area. It should be
stressed that BMPs should be incorporated into a comprehensive stormwater management
program. Without proper BMP selection, design, construction and maintenance, BMPs will not
be effective in managing urban runoff. BMP selection can be tailored to address the various
sources of runoff produced from urbanized areas. For example, a particular suite of BMPs may
be developed for use on construction sites and new land development, where opportunities exist
for incorporating BMPs that are focused on runoff prevention, reducing impervious surfaces and
maintaining natural drainage patterns. In established urban communities, a different suite of
BMPs may be more appropriate due to space constraints. In these areas, BMPs may be selected
to focus on pollution prevention practices along with retrofit of the established storm drain system
with regional BMPs. Site suitability for selecting a particular BMP strategy is key to successful
performance. Most BMPs have limitations for their applicability, and therefore cannot be applied
nationwide. A few considerations to incorporate into BMP selection are:
drainage area;
land uses;
average rainfall frequency, duration and intensity;
runoff volumes and flow rates;
soil types;
site slopes;
geology/topography;
availability of land;
future development/land use in watershed;
depth to groundwater table;
availability of supplemental water to support vegetative BMPs;
susceptibility to freezing;
safety and community acceptance;
maintenance accessability;
periodic and long-term maintenance/rehabilitation needs.
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In addition to site-specific applicability requirements, factors such as BMP cost, local
regulations or requirements, aesthetics, the experience of a developer or contractor with a
particular design, and competing receiving water considerations such as temperature and nutrient
levels should be addressed. The combination of these factors make selection of appropriate
BMPs a difficult task, and one that should be done only by an experienced storm water
practitioner. This is especially true in established urban areas, where knowledge of local factors
that affect design and performance is needed. BMP use in arid and semi-arid climates also
presents unique challenges. The availability of water to support vegetative and open pool BMPs
such as retention ponds and wetland systems is of primary concern in these areas. Without
adequate water sources, these systems may not function properly and may become public
nuisances. A designer with adequate experience in designing BMPs for arid climates should be
consulted in these instances. In addition to arid climates, BMP use in areas where freezing
conditions can be encountered presents design problems. In cold climates, design modifications
may be needed to adjust for freezing and spring snowmelt (Caraco and Claytor, 1997). Given the
variety of local considerations that exist, developing a matrix of BMP applicability is outside of
the scope of this report. There are several references that readers should consult to obtain
additional information on BMP selection, including Fundamentals of Urban Runoff Management
(Horner et al, 1994), Controlling Urban Runoff: A Practical Manual for Planning and
Designing Urban BMPs (Schueler, 1987), A Watershed Approach to Urban Runoff: Handbook
for Decisionmakers (Terrene Institute, 1996), Urban Targeting and BMP Selection (Terrene
Institute, 1990), Guidance Specifying Management Measures for Sources on Nonpoint Source
Pollution in Coastal Waters (US EPA, 1993 a), Handbook Urban Runoff Pollution Prevention
and Control Planning (US EPA, 1993c), Municipal Wastewater Management Fact Sheets: Storm
Water Best Management Practices (US EPA, 1996e), Design and Construction of Urban
Stormwater Management Systems (WEF and ASCE, 1992), and Urban Runoff Quality
Management (WEF and ASCE, 1998).
5.4 Monitoring BMP Effectiveness
Monitoring the effectiveness of BMPs can be done in a number of ways. Since urban
runoff frequently contains pollutants that can contribute to water quality impacts to receiving
streams, the ability of a BMP to remove pollutants from runoff is often of concern. The typical
method for measurement of the pollutant removal efficiency of a BMP system is to collect and
analyze water quality samples. This can be accomplished by measuring the concentration of a
target parameter or group of parameters in an inflow sample or set of samples and comparing
these values to samples collected from the outflow of the BMP. The reduction in concentrations
or loading across the BMP can be termed the pollutant removal efficiency.
In addition to monitoring the pollutant removal efficiency of BMPs, it is important to
monitor the hydraulic performance of the BMP. A major problem associated with urban runoff is
the total volume and flow rate of water that is discharged to the storm sewer system or the
receiving stream. To evaluate the effectiveness of BMPs in reducing these impacts, hydraulic
parameters such as the reduction in peak discharge rate, reduction in total volume discharged, and
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the time effects of discharges are frequently measured. To do this, measurement of flow rates and
water volumes into and out of the system are conducted by using flow monitoring equipment.
Since the ultimate goal of BMPs is to protect or improve the quality of receiving streams,
another method of evaluating the effectiveness of BMPs is to evaluate the quality of waters
receiving runoff. Measures of water quality such as pollutant levels, pH, dissolved oxygen and
other parameters can give an indication as to the effectiveness of a given BMP or group of BMPs.
Evaluation of the contaminant levels present in sediments of receiving waters is also an important
measure of BMP effectiveness. In addition, measures of aquatic habitat and stream channel
morphology can give an indication as to the effectiveness of BMPs in controlling impacts or
improving channel or habitat quality. Another measure to evaluate the effectiveness of BMPs is
to measure the organisms that live in the receiving stream. Biological indicators such as
macroinvertebrate counts, fish counts, and aquatic plant surveys can indicate the overall health of
the receiving stream and indicate, over time, the effectiveness of BMPs. A potential problem with
in-stream indicators is that it is sometimes difficult to isolate the impacts or improvements
attributable to one particular variable. Since there are potentially a number of different factors
that can influence a stream such as the amount of riparian cover, the existence of point source
discharges, seepage from on-site disposal systems, as well as urban runoff, in can be very difficult
to isolate impacts or improvements attributable to one particular stressor. Therefore, many years
of data, collected both before and after a BMP implementation, may be needed to indicate a
change. In spite of these shortcomings, in-stream monitoring and evaluation of the cumulative
effects in a watershed as a result of BMP implementation is a very important measure of BMP
effectiveness.
5.4.1 Water Quality Monitoring of BMPs
BMP monitoring can be conducted for a number of reasons, and the type of monitoring
conducted and the instrumentation or equipment used can vary greatly depending upon the
parameters of interest. BMP monitoring and data analysis is a complex process, and therefore a
thorough explanation of all of the available monitoring practices and procedures is not included
here. An important point to emphasize with respect to BMP monitoring is that consistent data
reporting is needed in order to compare data between studies. Consistent reporting of BMP
design parameters and watershed parameters as well as consistent monitoring methods and data
analysis protocols is key to conducting data comparisons. It is recommended that individuals
conducting BMP monitoring use the data reporting protocols developed by the American Society
of Civil Engineers (ASCE) for the National Stormwater BMP Database (Urbonas and Strecker,
1996). These protocols are included with the database software, and are also available from the
ASCE website.3
The following discussion includes a description of the most common methods used to
evaluate BMP performance. Readers are encouraged to consult various monitoring manuals that
3 The website address is http://www.asce.org/peta/tech/nsbd01.html
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are available and papers that are contained in the literature for more detailed information on BMP
monitoring and data analysis. Recommended references include Monitoring Guidance for
Determining the Effectiveness ofNonpoint Source Controls (US EPA, 1997c), NPDES Storm
Water Sampling Guidance Document (US EPA, 1992c), and Stormwater NPDES Related
Monitoring Needs (Torno, 1995).
BMPs are frequently evaluated by collecting inflow and outflow samples and comparing
concentrations of pollutants. Samples can be collected in a number of different ways. The most
common way is by collecting flow- or time-weighted composite samples from inflow and outflow
points and measuring the concentrations of a targeted group of parameters in these samples.
Composite samples can be collected by using automatic samplers, or by collecting a series of
discrete samples and manually compositing. Composite samples are useful for determining an
overall average or "event mean" concentration for a particular sampling point, and are commonly
used to evaluate BMP performance. However, composite samples cannot be used to evaluate any
trends in pollutant concentrations over time or varying flow rates. In order to conduct these types
of evaluations, it is necessary to collect a series of discrete grab samples either by an automatic
sampler or by collecting grabs manually. By collecting a series of discrete time-weighted or flow-
weighted samples, a "pollutograph" of concentration versus time or flow rate can be prepared,
which can give insight into the performance of the BMP under various hydraulic loadings. Sample
results can then be combined mathematically to determine representative event mean
concentrations. Manual grab samples are also used for collecting samples that are not amenable
to collection by automated equipment, such as microbiological samples, samples for oil and grease
evaluation, and samples for volatile organic compounds analysis.
BMP monitoring frequently incorporates measurements of water flow rates and volumes
into and out of the system. Flow rates are frequently determined by using a combination of a
primary control device (weir, flume or orifice) that is calibrated to discharge water according to a
known relationship based on the depth of the water flowing over or through the device, along
with a secondary control device (bubbler, pressure transducer, float, etc.) that is used to measure
the depth of water flowing through or over the primary control device. A digital recorder is
frequently used to record the depth of water measured by the secondary control device and to
calculate the flow rate through the primary control device based on a pre-determined relationship
between water depth and flow rate. The digital recorder can be used to log this flow data for
subsequent retrieval and analysis, and can activate automated sampling equipment to collect
samples at pre-determined flow rates or times. By using a configuration such as this, flow-
weighted samples or discrete samples can be collected automatically, reducing or eliminating the
need for personnel to be on-site during an event.
In addition to measuring surface runoff contributions to BMPs, measurement of the
contribution of groundwater and subsurface flow may be necessary for BMPs that have a
significant groundwater contribution. Constructed wetland systems that are close to or at
groundwater level are a good example of BMPs where measurement of groundwater flows may
be necessary.
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BMP monitoring programs also frequently incorporate measurements of rainfall depths,
intensities and duration by using a rain gauge. Additional meteorological monitoring equipment
can measure parameters such as air temperature, solar radiation, humidity, atmospheric pressure
and wind speed and direction, which can aid in interpreting BMP performance data. Other
instruments such as continuous pH, dissolved oxygen, and conductivity meters are also frequently
incorporated into BMP monitoring programs in order to measure parameters of interest.
BMP monitoring programs can also include measurements of the atmospheric deposition
rates of pollutants by using wet deposition and dry deposition sampling equipment. Atmospheric
deposition can contribute significant loadings of pollutants to storm water BMPs, especially to
BMPs that have a large surface area such as ponds or constructed wetlands.
Analysis of data collected from BMP monitoring programs can be conducted in a number
of ways. Some of the most common methods used to measure effectiveness are measures of
pollutant removal efficiency based on event mean concentrations (EMC). An event mean
concentration can be determined directly from a flow-weighted composite sample. Estimations of
pollutant removal efficiency in use include the efficiency ratio, the summation of loads, and the
regression of loads. These methods are defined as follows (from Martin and Smoot, 1986 and
reported by Strecker, 1995):
The efficiency ratio (ER) is defined in terms of the average event mean concentration
of pollutants from inflows and outflows:
PD _ , Average outlet EMC
Average inlet EMC
The summation of loads method is based on the loads of pollutants removed during
monitored storms:
_ i ฐf outlet loads
1 ~~
of inlet loads
The regression of loads method defines the efficiency ratio as the slope of a simple
linear regression of inlet loads and outlet loads of pollutants. The equation is:
Loads in = /3 -Loads out
where P equals the slope of the regression line, with the intercept constrained at zero.
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The above are only a few of the methods available for computing BMP pollutant removal
efficiency. The selection of method can have a large impact on the reported removal efficiency.
As a result, reported removal efficiency is not always comparable between studies due to
differences in the way that pollutant removal was calculated. Additional work is needed in this
area in order to standardize BMP data analysis and reporting.
5.4.2 Receiving Stream Assessments
Receiving stream assessments are an important means of determining the effectiveness of
BMPs. The health of the biological community and the quality of the habitat present in the stream
can be strong indicators of the effectiveness of BMPs. There are a number of biological
indicators that can be used to evaluate streams, and a discussion of these methods is not within
the scope of this document. Readers are encouraged to consult available documents for
additional information on this subject, and for recommendations on developing biological criteria
programs. Recommended readings include Biological Criteria Technical Guidance for Streams
and Small Rivers (US EPA, 1996c) and Restoring Life in Running Waters: Better Biological
Monitoring (Karr and Chu, 1998).
Physical habitat and fish and macroinvertebrate diversity indices have been identified as
suitable indicators to assess the effectiveness of storm water controls (Center for Watershed
Protection, 1996). EPA's Rapid Bioassessment Protocols for Use in Streams and Rivers (US
EPA, 1997f) can be used to survey biological communities. In addition, many local and state
environmental protection agencies have developed monitoring protocols for streams within their
geographic area. Readers are encouraged to contact county and state environmental agencies to
obtain more information regarding stream assessments. In addition to surveys of biological
communities, measures of stream habitat are also useful for determining the effectiveness of
BMPs. Some available methods for assessing habitat include:
Physical habitat assessment component of EPA's Rapid Bioassessment Protocols;
The Rapid Stream Assessment Technique (RSAT);
The Ohio EPA's Qualitative Habitat Evaluation Index (QHEI);
The Rosgen Stream Classification.
EPA used several receiving stream assessment methods in its 1998 field work at one BMP
site. Findings from these assessments will appear in a supplement to this report.
5.5 Effectiveness of BMPs in Managing Urban Runoff
There has been a great deal of storm water and BMP monitoring data collected by a
number of organizations. However, most of these data have focused on characterization of
pollutants in runoff, and not on the effectiveness of various control measures. Several nation-
wide monitoring programs have been conducted to characterize pollutants in urban storm water
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runoff and to evaluate the performance of storm water BMPs. The major federal monitoring
programs that have been conducted are listed in Table 5-4.
Table 5-4. Sources of Storm Water Runoff and BMP Monitoring Data
Data Source
"208 Studies" under FWPCA
Amendments of 1972
Nationwide Urban Runoff
Program (NURP)
Federal Highway
Administration (FHWA)
USGS Urban Storm Studies*
Phase I NPDES Municipalities
(260 permittees)
Year
late 1970's
1978-83
1970's - 80's
1970's - 90's
1990's
Type of Monitoring Conducted
Limited storm water quality data
Storm water quality data collected at 8 1
outfalls at 28 cities for a total of 2,300 storm
events as well as some BMP data
Storm water runoff loadings from highways at
3 1 sites in 1 1 states
Rainfall, runoff and water-quality data for
areas throughout the United States
Storm water and BMP monitoring data for 5
representative sites during a minimum of 3
storm events
* USGS prepared a database that includes rainfall, runoff and water-quality data for 717 storms from 99 stations
in 22 metropolitan areas throughout the United States, including much of the data collected during the NURP
program, in the mid-1980's (Driver et al, 1985)
The USGS has been collecting urban rainfall and runoff data for several decades. In the
1970's and early 1980's, monitoring programs were conducted to collect water quality data in
addition to rainfall and runoff data in order to characterize the pollutants present in storm water
runoff and to evaluate the impacts attributable to wet weather discharges. The major programs
included the Nationwide Urban Runoff Program (NURP) conducted by EPA and USGS and the
FHWA evaluation of runoff from highways. Data from these evaluations indicated that urban
storm water runoff was contributing significant levels of pollutants to the nations waters, and that
control of urban runoff was warranted. However, these investigations also indicated that there
was insufficient data available to quantify the degree of impacts attributable to urban runoff and to
evaluate the effectiveness of various runoff control practices.
In addition to the major federal investigations, some data has been published in the
professional literature. A number of bibliographies have been prepared that include storm water
BMP-related literature. These include the ASCE Urban BMP Effectiveness Bibliography, and the
National Highway Runoff Water-Quality Data and Methodology Synthesis Bibliography compiled
by USGS and FHWA. The Center for Watershed Protection (CWP) has prepared a database
containing BMP performance data for 123 structural BMPs (Brown and Schueler, 1997a). The
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FHWA and ASCE are currently developing databases of published highway and urban BMP
effectiveness data. In addition to data in the published literature, a large amount of data has been
collected by various cities and municipalities as part of the storm water permitting program under
the Phase INPDES program for storm water discharges. To date, EPA has not undertaken a
concerted effort to collect and evaluate this data. In addition to published data sources, a number
of states, counties and cities have collected a significant amount of monitoring data for their own
use. The extent of this data is not currently known, but several county and city storm water
programs have collected a great deal of potentially useful BMP monitoring data. An effort to
collect and evaluate these data may provide more useful information on the effectiveness of
various control measures.
The effectiveness of BMPs can be measured in various ways. Non-structural BMPs deal
mainly with pollution prevention and limiting the amounts of pollutants that are carried away by
runoff. Their effectiveness is best measured in terms of the degree of change in people's habits
following implementation of the management program or by the degree of reduction of various
pollutant sources. It is oftentimes very difficult to measure the success of non-structural BMPs in
terms of pollution reduction and receiving stream improvements. Structural BMPs can be
measured in terms in the reductions of pollutants discharged from the system and by the degree of
attenuation of storm water flow rates and volumes discharged to the environment. Various
physical, chemical and biological evaluation methods exist for determining the pollutant removal
efficiency of structural BMPs. The following sections summarize existing data on the pollutant
removal efficiency of a variety of BMPs.
5.5.1 Controlling Pollution Generation
The literature on the effectiveness of BMPs in controlling the generation of pollutants is
not very extensive. Pollution prevention type BMPs such as street sweeping, public education
and outreach, collection of lawn debris, etc., are conceptually very effective means of controlling
the generation of pollutants that can enter storm water runoff. However, it is often very difficult
to develop a representative means of monitoring or evaluating their effectiveness. Additional
work in this area is needed in order to measure the effectiveness of these controls. Effectiveness
data and information for pollution prevention BMPs that has been identified is presented in the
following sections.
Education and Outreach
Evaluating the performance of education and outreach programs is difficult. There is little
quantitative data in the literature that measures the effectiveness of these programs in improving
water quality. Information exists on how educational programs have been implemented and what
their success rate has been as far as changing the habits of a select group of people, but data
linking implementation with improvements in water quality are scarce. Nevertheless, educational
programs are a valuable component of a comprehensive storm water management program.
Surrogate measures of the effectiveness of education and outreach programs include:
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numbers of flyers distributed per given time period;
number of radio or television broadcasts;
number of public workshops held per year;
the percentage of storm drains that have been stenciled;
the number of volunteer monitoring and stewardship groups that have been formed.
A literature review by ASCE (Strecker and Quigley, 1998) did not identify any published
studies that contained quantitative information evaluating the effectiveness of Education and
Outreach BMPs in improving water quality.
Recycling and Source Controls
Evaluating the effectiveness of recycling and source control programs can be measured in
terms of the quantities of materials that are being recycled, but it is often difficult to determine
water quality improvements as a result of these programs. Measures of effectiveness include:
surveys that evaluate how many residents have changed habits such as picking up pet
waste and composting lawn debris;
volumes of materials such as used oil and antifreeze that are recycled;
the volume and types of materials collected during community household hazardous
waste collection days;
the number of illicit cross connections that have been detected and eliminated;
the total curb miles of streets that are swept annually and the quantity of materials
removed; and
reductions in pesticide and fertilizer usage.
Monitoring of storm water quality to evaluate the effectiveness of source control
programs is possible, however very few studies have been conducted. The difficulty stems from
isolating the impacts of a particular source control program on the overall water quality draining
from the watershed. The ASCE bibliography identified one study that potentially contains
quantitative information about the effectiveness of recycling automotive products as a BMP
(Horner et al, 1985). Additional data are needed in this area in order to evaluate the effectiveness
of recycling and source controls.
Maintenance Practices
Maintenance practices are a necessary part of any municipal storm water program. In
addition to maintenance of storm water management infrastructure and BMP maintenance, a
range of municipal maintenance activities impact the quality of storm water runoff. As with other
non-structural control practices, data evaluating the effectiveness of maintenance practices at
reducing the impacts associated with storm water discharges are scarce.
Studies conducted during the NURP project indicated that street sweeping was generally
not an effective BMP. This is mainly due to the fact that street sweepers remove only the coarse
particles on streets, and are not generally effective at removing the fine particles. It is the "fines"
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that frequently contain the highest fractions of pollutants, especially metals. In fact, the NURP
study report from Winston-Salem reported that street sweeping could actually increase the
concentrations of select pollutants by removing the surface "armoring" of coarse particles, which
during normal runoff events inhibit the removal of fine surface loads (Noel et al, 1987). Likewise,
NURP studies conducted in Long Island, New York, Champaign, Illinois and Bellevue,
Washington found little or no benefit of street sweeping programs. A study in Durham, New
Hampshire, which evaluated the effectiveness of pavement vacuum cleaning, indicated that this
technology was effective at removing BOD and fecal streptococci bacteria. It was thought that
these contaminants were mainly associated with the coarser sediments, which this technology was
able to effectively remove. Although the NURP data indicated that street sweeping was not an
effective BMP for improving water quality, the usefulness of street sweeping programs cannot be
discounted. Improvements in sweeper technology have occurred since the NURP studies were
conducted, and today's sweepers may be more efficient at removing fine particulates. Regardless,
sweeping programs can remove a significant amount of dirt and debris from streets and parking
lots. However, obtaining data linking sweeping programs to water quality improvements may be
difficult due to the variety of pollutant sources present in urban areas.
Data on other maintenance practices are likewise scarce. Practices such as catch basin
cleaning, street pavement repair, and ditch maintenance are all necessary components of a storm
water management program. However, data that indicate their effectiveness may be difficult to
obtain due to the lack of appropriate evaluation methodologies and the difficulty associated with
isolating water quality improvements attributable to these practices. The ASCE bibliography
identified two NURP studies that included evaluating the effectiveness of catch basin cleaning as a
storm water BMP (Lake Hills and Surrey Downs, Bellevue, Washington).
5.5.2 Controlling Pollution Discharges
There has been a great deal of published data documenting the efficiency of BMPs in
removing pollutants from storm water. Much of this data provides useful insights into the
performance of various types of storm water BMPs. For the purposes of this study, efficiency has
been used to describe the ability of the management practice to remove pollutants from runoff.
Effectiveness refers to the actual improvements in water quality, habitat or other parameters as a
result of implementing the management practice. Most of the data contained in the literature
reports efficiency of a BMP. Little of the available data can be used to evaluate actual
effectiveness.
Brown and Schueler (1997a) documented the pollutant removal efficiency of commonly
used and innovative urban storm water BMPs. The number of monitoring reports of various
BMP categories included in this study are summarized in Table 5-5.
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Table 5-5. Monitoring Studies for BMP Categories
BMP Type
Detention Basins
Retention Basins
Wetland Systems
Filtration Systems
Swales and Filter Strips
Other
Number of Studies
8
35
36
15
20
4
Evaluation of the existing BMP monitoring data gives an indication of the information
gaps that exist in BMP monitoring studies that have been performed to date. Commonly used
BMPs that are seldom monitored include infiltration trenches, infiltration basins, bioretention
practices and filter strips. The reason for the limited number of monitoring studies for these
practices is due to the difficulty involved in collecting inflow and outflow samples to calculate
pollutant removals. Bioretention practices and filter strips frequently accept runoff as sheet flow,
which must be concentrated in order to collect a representative sample. Infiltration practices and
bioretention practices can discharge water through a large surface area into surrounding soil
layers, and therefore collection of a representative "outflow" sample is problematic. There are
also a number of innovative and infrequently used BMPs that are seldom monitored. These
include sand filters, vegetated filter strips, filters with organic media, wetland channels and swales.
In addition to a general lack of monitoring data for certain types of BMPs, there is also a
lack of performance data for all BMP types for certain parameters. While BMP monitoring
studies typically monitor for parameters such as total phosphorus, total lead, and total suspended
solids, there is little monitoring data available for parameters such as bacteria, dissolved metals
and hydrocarbons. Table 5-6 summarizes the frequency with which selected parameters have
been monitored in BMP performance studies (Brown and Schueler, 1997a).
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Table 5-6. Extent of Monitoring for Selected Pollutants in BMP Performance Studies
Parameter
Total Phosphorus
Total Lead
Total Suspended Solids
Total Nitrogen
Soluble Nitrogen
Total Zinc
Soluble Phosphorus
Organic Carbon
Total Copper
Bacteria
Total Cadmium
Total Dissolved Solids
Dissolved Metals
Hydrocarbons
Percent Monitored
94
94
92
70
70
67
60
55
42
19
15
13
10
9
Review of the existing BMP monitoring data gives an indication of the pollutant removal
efficiency of various BMPs. Several efforts have been conducted to attempt to evaluate the range
of pollutant removals that can be expected to occur in various BMP designs. Evaluation of these
data can give an indication of the range of pollutant removals expected, however arriving at a
fixed numerical "percent removal" for each BMP type or category is a difficult task. The main
problem associated with comparing BMP performance data is the variety of techniques that are
used to compute performance, as well as the variation in the ways that samples are collected and
in the parameters that are measured in the samples. Performance calculations are further
complicated by the errors that result from measuring flow rates and volumes of storm water that
pass through the BMP. A study conducted by USGS evaluated 23 flow measurement techniques
in order to determine potential differences in reported flows. Average percent differences
between reported total storm volumes were in many cases greater than 25 percent over a range of
storms (Strecker, 1998). With errors of this magnitude, calculation of pollutant loadings and
loadings reductions can be complicated significantly.
Efficiency of a BMP can be related to the removal of individual pollutants on both an
event basis and on a long-term basis. Frequently, the statistical rigor with which BMP sampling
data are analyzed is poor or even nonexistent. Most BMP performance data are reported as event
mean concentrations (EMCs). An EMC can either be determined directly from a flow-weighted
composite sample, or calculated based on a series of discrete samples. While an EMC may be an
appropriate method for determining the reduction in pollutant concentrations for an individual
event, an EMC may not give an indication of the long-term performance of the BMP or the
performance for runoff events of varying intensity and volume. A more appropriate means of
determining the long-term performance of a BMP may be to do a statistical evaluation of inflow
5-52
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and outflow loadings over a range of storm event sizes and durations. Samples must also account
for the seasonality of performance that results with certain BMP types such as ponds and
constructed wetlands. The selection of the method used can have a significant impact on the
reported performance. Additional work to standardize BMP monitoring protocols and to
standardize calculations for performance is needed in order to make BMP monitoring data
comparable from site to site.
BMP performance can vary considerably based on differences in the design criteria and
performance standards for which the BMP was designed. Comparing pollutant removal efficiency
for similar BMP types with very different performance goals may result in widely disparate
efficiency estimations. In addition to differences in performance goals, variations in watershed
parameters can cause significant differences in performance among otherwise similar BMPs. In
most cases, parameters such as the size of the drainage area, the level of watershed
imperviousness, the duration and volume of runoff entering the BMP, and the land use of
contributing drainage areas are not easily comparable from study to study. In addition,
differences in BMP design parameters such as the ratio of the BMP volume to the contributing
drainage area, the retention time in the BMP, the physical dimensions and the construction of the
BMP further complicate direct comparisons between BMP monitoring data. Also, a great deal of
variability exists in the performance of each BMP due to event and seasonal variations.
Despite these shortcomings, some general ranges of expected BMP efficiency have been
compiled from the literature. Documents that summarize BMP efficiency information include the
CWP's National Pollutant Removal Performance Database (Brown and Schueler, 1997a), the
Terrene Institute's report The Use of Wetlands for Controlling Stormwater Pollution (Strecker et
al, 1992), as well as a variety of articles and documents contained in the professional and scientific
literature. In addition, the ASCE National Storm Water BMP Database is expected to provide
BMP monitoring studies in a format that will facilitate evaluation and comparison of BMP
performance data. Readers are encouraged to consult the variety of referenced information
resources for more detailed BMP performance data than is presented in this report. Table 5-7
presents expected pollutant removal efficiencies for various BMP types (US EPA, 1993c). The
values found in this table give an indication of the expected overall pollutant removal efficiency
for a properly sited, designed, sized, constructed and maintained BMP. The sections that follow
Table 5-7 summarize the actual performance data contained in the literature on pollutant removal
efficiencies for selected BMP types.
5-53
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Table 5-7. Structural BMP Expected Pollutant Removal Efficiency
BMP Type
Dry Detention Basins
Retention Basins
Constructed
Wetlands
Infiltration
Basins
Infiltration Trenches/
Dry Wells
Porous Pavement
Grassed Swales
Vegetated Filter
Strips
Surface Sand Filters
Other Media Filters
Typical Pollutant Removal (percent)
Suspended Solids
30-65
50-80
50-80
50-80
50-80
65 - 100
30-65
50-80
50-80
65 - 100
Nitrogen
15-45
30-65
<30
50-80
50-80
65 - 100
15-45
50-80
<30
15-45
Phosphorus
15-45
30-65
15-45
50-80
15-45
30-65
15-45
50-80
50-80
<30
Pathogens
<30
<30
<30
65 - 100
65 - 100
65 - 100
<30
<30
<30
<30
Metals
15-45
50-80
50-80
50-80
50-80
65 - 100
15-45
30-65
50-80
50-80
Source: Adapted from US EPA, 1993c.
Infiltration Systems
Infiltration systems can be considered 100 percent effective at removing pollutants in the
fraction of water that is infiltrated, since the pollutants found in this volume are not discharged
directly to surface waters. Quantifying the removal efficiency of infiltration systems, therefore,
can perhaps best be determined by calculating the percent of the average annual runoff volume
that is infiltrated, and assuming 100 percent removal of the pollutants found in that runoff volume.
Since collecting samples of runoff once it has been infiltrated can be very difficult, little field data
exist on the efficiency of infiltration for treatment of storm water. Since infiltrated water does not
leave the BMP as a discrete flow, there is no representative way of collecting a true outflow
sample. Infiltration systems can be monitored by installing a series of wells around the perimeter
of the BMP for collecting samples. However, this can add significant costs to any monitoring
effort. Table 5-8 summarizes the available field data on the efficiency of infiltration practices in
treating storm water. Reported removal efficiencies are based on the results of three studies that
evaluated the performance of infiltration trenches and two studies that evaluated the efficiency of
porous pavement systems.
5-54
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Table 5-8. Pollutant Removal Efficiency of Infiltration Practices
Parameter
Total Phosphorus
Ammonia-Nitrogen
Nitrate
Total Nitrogen
Suspended Solids
Organic Carbon
Lead
Zinc
Median or Average
Removal Efficiency
(percent)
65
83
82
83
89
82
98
99
Number of
Observations
5
3
3
2
2
1
1
1
Source: Brown and Schueler, 1997'a
Conceptually, infiltration should provide significant pollutant removal for a wide variety of
storm water pollutants. As water moves through the underlying soil layers, suspended
particulates and associated pollutants should be filtered out. In addition, pollutants can be
adsorbed by soil particles and microorganisms in the soil can degrade organic pollutants. There is
little data available, however, regarding the potential mobility of metals and hydrocarbons that
enter groundwater due to infiltration of storm water. This may be a particular problem in areas
with extremely high soil permeabilities (such as coastal areas), where pollutants can rapidly enter
underlying aquifers with insufficient contact time for breakdown or adsorption of contaminants.
Consequently, additional data gathering to target the behavior of these pollutants is warranted.
The success of infiltration systems has been mixed. In same areas, infiltration has been
applied successfully, while in others infiltration systems have clogged in a very short time. Many
failures can be attributed to contractor inexperience, to compaction of soil by construction
equipment and to excess sediment loading during construction activities, and to improper design
and siting. In order to apply infiltration successfully, the following guidelines should be applied:
Permeability of soils must be verified. A percolation rate of 0.5 inches per hour or
more, and an soil layer of 4 feet or more is essential (Cahill, 1994).
Construction site runoff must be kept from entering the recharge bed, and the
infiltration system should not be placed into service until all disturbed land that drains
to the system has been stabilized by vegetation. Strict erosion and sediment controls
during any construction or re-landscaping is a must to prevent clogging of the system.
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A sedimentation basin or chamber placed before the infiltration system to remove a
portion of the sediment can help to extend the life of the infiltration system.
Use of filter fabric between the recharge bed and soil interface (in porous pavement
and infiltration trench systems) can prevent the migration of soil into the recharge bed.
Construction traffic should be directed away from the infiltration bed before and during
construction to prevent compaction of underlying soil layers and loss of infiltrative
capacity.
Porous pavement systems should be clearly marked to prevent use by heavy vehicles
and resurfacing with non-porous pavement.
A basin drain should be provided so that the basin can be drained and maintenance
performed if the basin becomes clogged.
Readers are encouraged to consult the ASCE/WEF manual of practice (WEF and ASCE,
1992) for additional guidelines on using infiltration systems.
Retention Basins (wet ponds)
Retention basins can be very effective systems for removing pollutants from storm water.
Retention basins provide quiescent conditions with long retention times that allow a large fraction
of suspended solids and associated pollutants such as metals, nutrients and organics to be
removed by sedimentation. In addition, degradation of organic compounds by microorganisms
and uptake of nutrients by aquatic vegetation can provide additional water quality benefits.
Retention basins have been one of the most widely-monitored storm water BMP types, mainly
due to their prevalence and relative ease of monitoring in comparison to other BMP types. In arid
regions, artificial or decorative lakes can function as retention basins. However, as with all other
BMP types, the available monitoring data are not always comparable from study to study due to
variations in procedures, protocols and methods. Although the mechanisms taking place in
retention basins are fairly well known, additional data are needed in order to determine what the
important design parameters are and to determine what event, seasonal and long-term
performance variances exist. Table 5-9 summarizes the pollutant removal efficiency of retention
basins systems. Reported removal efficiencies are based on data contained in 35 studies
evaluating retention basins.
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Table 5-9. Pollutant Removal Efficiency of Retention Basins
Parameter
Soluble Phosphorus
Total Phosphorus
Ammonia-Nitrogen
Nitrate
Organic Nitrogen
Total Nitrogen
Suspended Solids
Bacteria
Organic Carbon
Cadmium
Chromium
Copper
Lead
Zinc
Median or Average
Removal Efficiency
(percent)
34
46
23
23
23
30
70
74
35
47
49
55
67
51
Range of Removals
(percent)
Low
-12
0
-107
-85
2
-12
-33
-6
-30
-25
25
10
-97
-38
High
90
91
83
97
34
85
99
99
90
54
62
90
95
96
Number of
Observations
20
44
14
27
6
24
43
10
29
5
5
18
34
32
Source: Brown and Schueler, 1997a
The wide range of variability in reported removal efficiencies of retention systems is due to
a number of factors. Watershed variables such as the area draining to the pond, the percent
impendousness and land use of the watershed, the design features of the basin such as surface
area and depth of permanent pool, and hydraulic and hydrologic parameters such as rainfall
intensity, rainfall volume, length of antecedent dry periods, time of concentration and peak inflow
rate can have a large impact on the efficiency of a particular retention system. Studies that
contain data on the efficiency of retention systems sometimes report only pollutant removal
statistics, but fail to report the relationship to the hydraulics of the system. A thorough evaluation
of the hydraulics of the system is needed in order to properly evaluate the efficiency of ponds.
This evaluation should also include a measure of the expected suspended solids settling
characteristics of the pond influent through a settling velocity column test or particle size
distribution analysis, which can shed light on the observed efficiency of the pond in removing
5-57
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sediments and associated pollutants. Greb and Bannerman (1997) reported that the influent
particle size distribution plays a significant role in the overall solids removal efficiency. Perhaps
the greatest parameter influencing pond efficiency is retention time. Studies indicate that
residence times on the order of 14 days may be necessary to allow for sufficient removal of
sediment and associated pollutants and to meet receiving water standards (Rushton and Dye,
1993). In fact, Florida requires that the permanent pool volume of ponds treating runoff from
new land use activities must provide a minimum residence time of 14 days.
While retention systems can be very effective at removing pollutants from storm water,
there are some potential problems associated with these systems. During periods of intense
runoff, the retention time in the pond can decrease, resulting in decreased efficiency. In addition,
previously removed sediments can be re-suspended, resulting in a net export of pollutants from
the pond. This is one of the reasons that negative removals are frequently reported for pond
systems for parameters such as suspended solids and associated contaminants such as nutrients
and metals. Also, changes in water chemistry such as increased or decreased pH, alkalinity and
hardness can occur in the pond, which can effect the solubility of metals that are present in pond
sediments and the behavior of various nutrient species. This can also affect the chemistry of the
receiving waters, since the aquatic toxicity of certain metal species is dependant on hardness.
There is also evidence that anaerobic bottom sediments promote more soluble forms of
phosphorus and some metals, which can increase their release to the water column (Rushton and
Dye, 1993).
Perhaps the greatest problem is the increased temperature of discharges that occur from
storm water retention systems. Retention ponds can have a significant surface area, and during
summer months elevation of the temperature of water in the pond can occur. When this warm
water is displaced during the next runoff event, the elevated temperature can cause detrimental
impacts to the receiving waters, including loss of sensitive species and downstream shift of trophic
status (Galli, 1988). Ponds can also fail to function properly in the winter time when the surface
of the pond freezes. Water entering the pond can flow over the ice surface directly to the outlet
structure. This short circuiting can limit the retention time of storm water entering the ponds and
reduce the sedimentation efficiency. Outlet structures are also prone to freezing in the winter
time, which can cause serious flooding problems. In order to prevent cold-weather problems with
wet ponds, several design features can be incorporated in ponds that are used in cold climates.
Readers are encouraged to consult the Stormwater BMP Design Supplement for Cold Climates
published by CWP (Caraco and Claytor, 1997) for additional information regarding BMP designs
for cold climates.
Retention systems also present a potential hazard to nearby residents and children, can
often become populated with large number of waterfowl, and can be breeding grounds for
mosquitoes and odor producers if not designed and maintained properly. Large ponds also can
present a danger of downstream flooding and risk of catastrophic loss of life and property in the
event of an embankment or outlet structure failure. Several pond failures have occurred that are
attributable to piping around outlet structures and eventual failure of embankments due to poor
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installation. Careful adherence to design and construction standards is necessary and inspections
during construction should be conducted to ensure that ponds are installed correctly.
In addition to relating performance to measures of pollutant reduction across the BMP,
evaluations that measure effluent from BMPs and compares these values to receiving water
criteria can provide useful data. One such study was conducted in Florida, and it was determined
that effluent from 22 wet detention facilities was in most cases in compliance with class III Florida
state water quality standards. The ponds evaluated in this study were permitted by Florida and
met the required state design criteria. Parameters analyzed in samples included eight metal
species, six nutrient species, turbidity, TSS, temperature, dissolved oxygen, pH and conductivity.
The constituents that were in compliance 100 percent of the time included un-ionized ammonia,
iron, manganese (class II standard) and nickel. All other analyzed parameters, with the exception
of dissolved oxygen, were in compliance greater than 65 percent of the time, with most in
compliance greater than 79 percent of the time. Dissolved oxygen was in noncompliance 64
percent of the time (Carr and Kehoe, 1997). The results of this study indicate that evaluation of
constituents in BMP effluent and comparison with water quality standards may be an effective
measure of BMP effectiveness. In many cases, data of this nature may be more useful than data
that indicates percent removal of a targeted group of constituents across the BMP. It is also
important to note that the Florida study found that concentrations of constituents (with the
exception of dissolved oxygen) in samples collected at these systems did not vary significantly
between samples collected immediately before the outflow weir and after the outflow weir.
Therefore, sampling before the weir, where more convenient, does not significantly alter sample
results. This may be useful where the BMP discharges through an outflow structure where
samples are not easily collected (such as in a manhole or other confined space).
In addition to treating runoff, retention systems can be adapted for storm water reuse.
Florida is actively seeking reuse of storm water runoff for reuse as irrigation water. Reuse of
storm water reduces the volume of water and the amount of pollutants discharged to receiving
streams. In addition, reuse of storm water as irrigation water can help to recharge aquifers and
restore pre-development hydrologic conditions. Also, significant financial incentives exist for
reuse as irrigation in areas where water rates are high. However, the health risks of storm water
reuse have not been thoroughly investigated. Additional research in this area is warranted to
determine if a risk of exposure to potentially harmful microorganisms or other health risks exist.
Livingston et al (1998) presented a discussion of storm water reuse opportunities and discussed
design considerations for sizing ponds for reuse. Readers are encouraged to consult this reference
for additional information on storm water reuse.
There are a number of design features that can be included in retention system designs to
increase their effectiveness, reduce maintenance burdens and reduce impacts to receiving waters.
These include:
A broad, flat aquatic bench around the perimeter of the pond planted with emergent
wetland vegetation;
5-59
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A permanent pool volume that provides a long residence time to promote maximum
removal of suspended solids;
An irregular pool shape that increases sinuosity of flow paths;
A sediment forebay for removal of coarse sediments and ease of maintenance;
A submerged reversed-slope pipe or other non-clogging low-flow orifice;
Concrete, rather than corrugated metal risers and outlet structures;
Preservation of riparian cover along drainage channels to limit temperature increases;
Maintenance access to forebays and inlet and outlet structures for removal of sediments
and repairs.
Prince William Parkway Regional Wet Pond
In 1998 EPA conducted sampling activities at a retention system in Prince William
County, Virginia. The Prince William Parkway Wet Pond is a regional wet pond located adjacent
to a major county road in Dale City. The pond has a surface area of 4 acres, and has a total
volume of approximately 25 acre-feet at the permanent pool level. The pond is approximately
1,000 feet in length, 260 feet wide at its widest point, and was constructed by placing an earthen
dam in what appears to previously have been a natural drainage channel. The discharge is to Cow
Branch, a tributary of Neabsco Creek. The contributing drainage area to the pond is
approximately 310 acres. The land use of the watershed is approximately 20 percent commercial,
30 percent forested, 40 percent open land, 5 percent residential (mostly lots less than 1 acre) and
5 percent from other sources. The pond is designed to control up to the 100-year storm event for
the fully developed watershed conditions. There are a total of 5 discrete inflow points to the wet
pond. Three of these points were natural drainage channels (identified as PI, P2 and P3), while
the 4th and 5th points were concrete channels. Points PI, P2 and P3, which represent a majority
of the contributing drainage area, were monitored during the course of the study period. The
contributing drainage area and percent imperviousness of these sub-basins are:
Sub-Basin
PI
P2
P3
Area (acres)
84
122
107
Imperviousness (%)
41
17
17
The other two inflow points, which conveyed runoff from a small segment of Prince William
Parkway, were not monitored and their contributions of both storm flow and pollutant loadings
were considered negligible due to the small drainage area in comparison to the overall watershed
area. The outflow of the pond occurs through a pair of 8 by 8-foot concrete box culverts. A
concrete V-notch weir is installed at the outflow of the pond. See Figure 5-14.
5-60
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Figure 5-14. Prince William Parkway Regional Wet Pond
Natural channel
Drainage area = 122 acres
Imperviousness = 17%
Natural channel
Drainage area = 107 acres
Imperviousness = 17%
Natural channel
Drainage area = 84 acres
Imperviousness = 41%
P1 Inflow
P2 Inflow
P3 Inflow
P4 Outflow
P5 Wetfall/dry fall sampler
Not to scale
(2) 8' x 8' concrete
box culverts
Weir
During May through October 1998, rainfall and hydrologic data were collected for 14
storm events and water quality samples were collected during 10 storm events at the pond. In
addition, samples of atmospheric deposition (dryfall) and precipitation (wetfall) were collected for
a number of storms. The following tables summarize a portion of the analytical data collected
during the study period and the corresponding flow volume at each of the sampling points.
Wetfall volumes were determined by multiplying the total storm rainfall depth by the surface area
of the pond. A detailed presentation of the sampling results and an analysis of the sampling data
will be included in a supplement to this report.
5-61
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Table 5-10. Summary of Prince William Parkway Regional Wet Pond Sampling Data
Sample Location >
Sample Dates and Analytes
6/01/98
Total Kjeldahl Nitrogen (mg/1 as N)
Ammonia (mg/1 as N)
Nitrate/Nitrite (mg/1 as N)
Biochemical Oxygen Demand (mg/1)
Chemical Oxygen Demand (mg/1)
Total Organic Carbon (mg/1)
Phosphorus (mg/1)
Total Orthophosphate (mg/1)
Total Suspended Solids (mg/1)
Total Dissolved Solids (mg/1)
Volatile Suspended Solids (mg/1)
Chloride (mg/1)
Alkalinity (mg/L)
Hardness (mg/1 as CaCO3)
Runoff Volume (gallons)
6/11/98
Total Kjeldahl Nitrogen (mg/1 as N)
Ammonia (mg/1 as N)
Nitrate/Nitrite (mg/1 as N)
Biochemical Oxygen Demand (mg/1)
Chemical Oxygen Demand (mg/1)
Total Organic Carbon (mg/1)
Phosphorus (mg/1)
Total Orthophosphate (mg/1)
Total Suspended Solids (mg/1)
Total Dissolved Solids (mg/1)
Volatile Suspended Solids (mg/1)
Alkalinity (mg/L)
Hardness (mg/1 as CaCO3)
Runoff Volume (gallons)
PI
201,800
244,500
PI
(dissolved)
P2
0.97
0.45
4
ND(20)
7
0.02
ND (0.01)
266
46
26
4
5
35
619,700
5.6
ND(1)
0.53
45.6
9.35
0.25
0.094
14
49
o
J
13.2
18
849,400
P2
(dissolved)
0.95
0.39
0.52
7
ND(10)
9.1
ND (0.01)
1.12
ND(1)
0.59
3.65
26
7.91
0.051
P3
152,500
242,200
P3
(dissolved)
P4
0.39
0.13
4
ND(10)
5.4
0.02
ND (0.01)
48
65
14
13
26
35
1,071,000
9.52
ND(1)
0.13
6.9
27.6
9.35
0.25
0.062
8
62
3
24
26
1,434,000
P4
(dissolved)
1.1
0.37
0.18
3
ND(10)
6.5
0.03
9.52
ND(1)
0.16
2.8
27.2
6.47
0.017
Wetfall
0.67
0.61
0.18
2.2
ND (0.01)
61,000
2.24
ND(1)
0.18
3.58
0.044
ND (0.01)
81,500
Dryfall
ND (0.1)
ND (0.1)
ND (0.01)
ND(1)
ND (0.01)
ND (0.01)
8.96
ND(1)
0.42
3.58
0.038
5-62
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Sample Location >
Sample Dates and Analytes
6/12/98
Total Kjeldahl Nitrogen (mg/1 as N)
Ammonia (mg/1 as N)
Nitrate/Nitrite (mg/1 as N)
Biochemical Oxygen Demand (mg/1)
Chemical Oxygen Demand (mg/1)
Total Organic Carbon (mg/1)
Phosphorus (mg/1)
Total Orthophosphate (mg/1)
Total Suspended Solids (mg/1)
Total Dissolved Solids (mg/1)
Volatile Suspended Solids (mg/1)
Alkalinity (mg/L)
Hardness (mg/1 as CaCO3)
Runoff Volume (gallons)
6/13/98
Total Kjeldahl Nitrogen (mg/1 as N)
Ammonia (mg/1 as N)
Nitrate/Nitrite (mg/1 as N)
Biochemical Oxygen Demand (mg/1)
Chemical Oxygen Demand (mg/1)
Total Organic Carbon (mg/1)
Phosphorus (mg/1)
Total Orthophosphate (mg/1)
Total Suspended Solids (mg/1)
Total Dissolved Solids (mg/1)
Volatile Suspended Solids (mg/1)
Alkalinity (mg/L)
Hardness (mg/1 as CaCO3)
Runoff Volume (gallons)
PI
8.96
1.12
1
4.3
36.4
5.83
0.1
0.038
18
90
6
13.9
28
162,400
166,700
PI
(dissolved)
11.8
ND(1)
0.88
47.2
10.2
0.049
P2
8.96
ND(1)
0.57
2.5
106
5.83
0.82
0.099
53
37
6
9.7
20
237,600
ND(1)
ND(1)
0.2
5.9
32.8
4.37
0.35
0.097
31
245
6
5.2
10
411,600
P2
(dissolved)
2.8
ND(1)
0.61
2
14
5.83
0.08
7.84
1.12
0.25
2.5
25.2
4.37
P3
191,700
136,000
P3
(dissolved)
P4
1.68
ND(1)
0.15
3.7
27.2
10.2
0.18
0.099
9
65
6
24
28
631,000
1.68
ND(1)
0.28
3.9
22.8
5.83
0.091
ND (0.1)
8
54
4
22
24
765,000
P4
(dissolved)
8.4
ND(1)
0.12
40.8
20.4
10.2
0.04
17.4
ND(1)
0.29
ND(2)
20.8
5.83
0.17
Wetfall
5.04
1.12
0.66
4.37
0.046
36,000
43,500
Dryfall
5-63
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Sample Location >
Sample Dates and Analytes
6/15/98
Total Kjeldahl Nitrogen (mg/1 as N)
Ammonia (mg/1 as N)
Nitrate/Nitrite (mg/1 as N)
Biochemical Oxygen Demand (mg/1)
Chemical Oxygen Demand (mg/1)
Total Organic Carbon (mg/1)
Phosphorus (mg/1)
Total Orthophosphate (mg/1)
Total Suspended Solids (mg/1)
Total Dissolved Solids (mg/1)
Volatile Suspended Solids (mg/1)
Alkalinity (mg/L)
Hardness (mg/1 as CaCO3)
Runoff Volume (gallons)
6/17/98
Total Kjeldahl Nitrogen (mg/1 as N)
Ammonia (mg/1 as N)
Nitrate/Nitrite (mg/1 as N)
Biochemical Oxygen Demand (mg/1)
Chemical Oxygen Demand (mg/1)
Total Organic Carbon (mg/1)
Phosphorus (mg/1)
Total Orthophosphate (mg/1)
Total Suspended Solids (mg/1)
Total Dissolved Solids (mg/1)
Volatile Suspended Solids (mg/1)
Alkalinity (mg/L)
Hardness (mg/1 as CaCO,)
Runoff Volume (gallons)
PI
3.92
0.23
4.25
18.4
7.11
0.072
0.017
ND(4)
61
ND(4)
10
18
841,600
1.05
0.32
0.33
4
ND(10)
8.8
0.12
ND (0.01)
ND(4)
142
ND(4)
26
24
129,400
PI
(dissolved)
2.24
2.8
0.21
3.8
24
19
1.06
0.26
0.33
4
ND(10)
9
P2
8.4
1.12
0.2
4.05
22
4.47
0.2
ND (0.01)
21
58
5
4.2
10
1,260,900
215,200
P2
(dissolved)
13.4
9.52
0.35
2.7
66.4
9.75
P3
719,600
0.55
ND (0.1)
0.12
ND(10)
7.5
0.11
ND (0.01)
19
121
ND(4)
13
162,500
P3
(dissolved)
P4
21.3
16.8
0.29
1.8
21.6
5.79
0.069
ND (0.01)
ND(4)
19
ND(4)
ND(20)
24
3,097,500
0.75
ND (0.1)
0.19
6
ND(10)
8
0.13
ND (0.01)
8
79
ND(4)
10
12
554,500
P4
(dissolved)
5.6
6.16
0.31
4
17.2
20.3
0.97
ND (0.1)
0.2
4
26
7.9
Wetfall
176,000
31,500
Dryfall
5-64
-------
Sample Location >
Sample Dates and Analytes
6/23/98
Total Kjeldahl Nitrogen (mg/1 as N)
Ammonia (mg/1 as N)
Nitrate/Nitrite (mg/1 as N)
Biochemical Oxygen Demand (mg/1)
Chemical Oxygen Demand (mg/1)
Total Organic Carbon (mg/1)
Phosphorus (mg/1)
Total Orthophosphate (mg/1)
Total Suspended Solids (mg/1)
Total Dissolved Solids (mg/1)
Volatile Suspended Solids (mg/1)
Alkalinity (mg/L)
Hardness (mg/1 as CaCO,)
Runoff Volume (gallons)
6/24/98
Total Kjeldahl Nitrogen (mg/1 as N)
Ammonia (mg/1 as N)
Nitrate/Nitrite (mg/1 as N)
Biochemical Oxygen Demand (mg/1)
Chemical Oxygen Demand (mg/1)
Total Organic Carbon (mg/1)
Phosphorus (mg/1)
Total Orthophosphate (mg/1)
Total Suspended Solids (mg/1)
Total Dissolved Solids (mg/1)
Volatile Suspended Solids (mg/1)
Alkalinity (mg/L)
Hardness (mg/1 as CaCO,)
Runoff Volume (gallons)
PI
0.22
0.12
0.23
ND(2)
ND(10)
3.1
0.06
0.03
19
47
9
6
8
1,207,300
0.73
0.22
0.4
3
ND(10)
3.8
0.05
ND (0.01)
ND(4)
45
4
6
8
293,200
PI
(dissolved)
P2
1.44
0.2
0.4
ND(2)
31
4.4
0.02
ND (0.01)
33
79
16
13
20
1,064,200
0.71
0.22
0.61
4
ND(10)
4
0.1
ND (0.01)
57
48
5
6
11
682,400
P2
(dissolved)
6.1
0.5
0.42
6
15
7.7
P3
1.15
0.14
0.15
ND(2)
34
4.7
ND (0.01)
ND (0.01)
29
64
13
7
10
756,800
0.7
0.16
0.27
o
J
14
5.6
0.05
ND (0.01)
54
69
6
12
17
290,400
P3
(dissolved)
0.3
0.13
0.14
23
ND(10)
7.4
P4
0.76
0.31
0.21
ND(2)
ND(10)
3.2
0.04
ND (0.01)
10
69
11
14
19
3,499,000
0.53
0.55
0.32
2
ND(10)
3.1
0.12
ND (0.01)
8
51
3
8
15
1,415,000
P4
(dissolved)
0.51
ND (0.1)
0.17
52
ND(10)
3.8
Wetfall
199,000
80,000
Dryfall
5-65
-------
Sample Location >
Sample Dates and Analytes
7/31/98
Total Kjeldahl Nitrogen
(mg/1 as N)
Ammonia (mg/1 as N)
Nitrate/Nitrite (mg/1 as N)
Biochemical Oxygen Demand (mg/1)
Chemical Oxygen Demand (mg/1)
Total Organic Carbon (mg/1)
Phosphorus (mg/1)
Total Orthophosphate (mg/1)
Total Suspended Solids (mg/1)
Total Dissolved Solids (mg/1)
Volatile Suspended Solids (mg/1)
Alkalinity (mg/L)
Hardness (mg/1 as CaCO,
Runoff Volume (gallons)
PI
1.3
1
1.51
6
16
11
0.05
0.03
9
92
9
17
40
79,400
PI
(dissolved)
1
1.1
1.51
26
ND(10)
11
P2
2.7
0.74
1.81
27
106
20
0.36
ND (0.1)
51
115
16
15
52
436,900
P2
(dissolved)
6
P3
32,000
P3
(dissolved)
P4
1.2
0.96
0.13
5
ND(10)
8.1
0.09
ND (0.01)
11
90
7
25
36
593,000
P4
(dissolved)
Wetfall
1.5
ND (0.01)
33,500
Dryfall
5-66
-------
Constructed Wetland Systems
Constructed wetlands can be effective BMPs for removing pollutants from urban storm
water. The main mechanism of pollutant removal in wetland systems is sedimentation (Strecker
et al, 1992). Other pollutant removal mechanisms include filtration by aquatic vegetation and by
underlying soil and gravel in systems where subsurface flow is present, biological conversion of
organic compounds by microorganisms, uptake of nutrients by aquatic plants and algae, uptake of
metals by plant tissue, adsorption of metals by clay soils, and volatilization of hydrocarbons and
volatile organics. While the literature contains hundreds of references to constructed wetlands
systems, very few quantitative studies have been conducted with sufficient rigor to provide good
estimates of performance. Strecker's evaluation of the literature on wetland treatment systems
identified only 17 reports that discussed the results of research on a functioning wetland system
(of 140 reviewed reports). This indicates that there is a general lack of thorough, scientifically-
defensible evaluations on the performance of wetland treatment systems. As a result, there is a
wide range of variability in reported efficiency data. Table 5-11 summarizes the pollutant removal
efficiency of constructed wetland systems based on Strecker's evaluation of published studies.
5-67
-------
Table 5-11. Pollutant Removal Efficiency of Constructed Wetland Systems
Parameter
Soluble Phosphorus
Ortho-Phosphate
Total Phosphorus
Ammonia-Nitrogen
Nitrate
Organic Nitrogen
Total Nitrogen
Suspended Solids
Bacteria
Organic Carbon
Cadmium
Chromium
Copper
Lead
Zinc
Median Removal
Efficiency
(percent)
23
28
46
33
46
7
24
76
78
28
69
73
39
63
54
Range of
Removals
(percent)
Low
-30
-109
-120
-86
4
-36
-20
-300
55
-31
-80
38
2
23
-74
High
78
93
97
62
95
39
83
98
97
93
80
98
84
94
90
Number of
Observations
12
7
37
15
18
7
11
26
3
15
6
3
10
17
16
Sources: Strecker at al (1992); Organic Carbon, Bacteria and Metals from Brown and
Schueler, 1997a
Evaluation of wetland performance is problematic because the basic mechanisms taking
place in wetland systems are not well understood. Wetlands are complex ecosystems, and
variations in design and watershed factors can have a significant impact on performance. As a
result, data collected from various sites are not always comparable.
Due to the limited amount of comparable data that is available on the performance of
storm water wetland systems, it is difficult to arrive at any meaningful relationships indicating the
important factors in wetland system design. Strecker indicated that perhaps the greatest factor
influencing performance of constructed wetlands is the hydrology of the watershed and the inflow
hydraulics. Other factors having a major influence on performance are wetland size and volume,
the design of the inlet and outlet structures, flow patterns through the system, vegetational
5-68
-------
community structure, seasonal productivity and decay of wetland plants, and changes in
evapotranspiration rates. In addition, the presence of subsurface flows can complicate wetland
performance evaluations.
Strecker recommends that an important evaluation step in determining wetland
performance is to compare runoff volumes with storage volumes and contact surface area of the
wetland. However, he was unable to conduct this evaluation due to the lack of consistent
reporting of rainfall statistics, watershed impendousness, land uses, flow volumes, capacity and
surface areas for contact. In order to ensure the comparability of future data reporting for
wetland systems, it is recommended that a standardized set of monitoring protocols be adopted
for all future monitoring efforts.
An important factor in the variation of reported efficiency is the wide range of designs that
are used in constructed wetland systems. A few design variations include:
ponds with an emergent wetland area on the pond perimeter;
shallow wetlands with subsurface flow;
wetland channels;
pond-wetland systems;
extended detention wetlands.
Although the design of a particular systems is dependent upon a number of site-specific
variables, there are some important design factors that should be incorporated in wetland system
designs including:
a pre-settling chamber for removal of heavy sediments and to limit disturbance of the
wetland to remove accumulated sediments;
adjustable level control at the outlet by means of an adjustable weir or orifice;
design the flow path to limit short circuiting and dead space and to maximize detention
time;
a broad, densely planted aquatic bench;
selection of planting species to produce a dense stand of vegetation for filtration and
nutrient uptake;
periodic harvesting of excess vegetation to prevent nutrient release and to remove
undesirable species.
Crestwood Marsh Constructed Wetland
In 1998 EPA conducted sampling activities at a constructed wetland in Manassas,
Virginia. The Crestwood Marsh is located in a residential area and was originally constructed as a
dry detention basin, but conditions at the site were such that a wetland system formed on its own.
The outlet structure was modified in 1995 to provide an extended detention time of 24 hours
within the wetland. As a result of this modification, the system has developed into a shallow
emergent marsh and contains a variety of wetland species. The wetland has a surface area of
5-69
-------
8,830 ft2. The water quality detention volume of the wetland is 2,524 ft3, and the flood control
volume above the water quality volume is 3,523 ft3. The area draining to the wetland is a 7-acre
townhouse community, with the land area consisting of approximately 60 percent townhouses, 30
percent forested and 10 percent open space. The drainage area is estimated to be approximately
40 percent impervious. The constructed wetland is located at the headwaters of a small unnamed
stream that drains to Bull Run, within the Occoquan River Watershed.
Flow enters the wetland at the eastern corner through an 18-inch concrete pipe situated
flush with the bottom grade (point Cl). From this point, water gradually spreads throughout the
wetland and drains to the northern corner of the pond and is discharged through a 6-inch PVC
outlet pipe (point C3). There is an additional inlet point located at the southwest corner of the
wetland, which consists of overland flow from an adjacent area of forested parkland. EPA
concentrated the flow at this point in order to allow estimation of flow rates and volumes, and to
allow for collection of water quality samples (point C2). See Figure 5-15.
5-70
-------
Figure 5-15. Crestwood Marsh Constructed Wetland
Imperviousness = 10%
C7 /nffow
C2 /nffow
C3 Outflow
C4 Wetfall/dryfall sampler
Not to scale
During the spring and summer of 1998, storm event sampling was conducted during nine
events. Sampling consisted of collecting flow-weighted composite samples as well as recording
rainfall depth, and runoff flow rates and volumes into and out of the wetland. In addition to water
quality monitoring, atmospheric deposition and wetfall deposition samples were collected during
the study period. The water quality sampling data collected during the study period are
summarized in the tables below. Additional data, as well as a detailed description of the sampling
program, wetland design, and an evaluation of the performance of the wetland will be included in
a supplement to this report.
5-71
-------
Table 5-12. Summary of Crestwood Marsh Constructed Wetland Sampling Data
Sample Date >
Analytes Location >
Runoff Volume (gal)
Total Suspended Solids (mg/1)
Chemical Oxygen Demand (mg/1)
Total Organic Carbon (mg/1)
Total Kjeldahl Nitrogen (mg/1 as N)
Total Inorganic Nitrogen (mg/1 as N)
Ammonia (mg/1 as N)
Total Phosphorus (mg/1)
Ortho-Phosphate (mg/1)
Alkalinity (mg/1 as CaCO,)
Hardness (mg/1 as CaCO,)
Lead
Copper
Zinc
Nickel
Aluminum
Chromium
06/01/98
Cl
7,625
41
75
13
2
0.67
0.75
0.12
<0.01
31
~
~
~
~
~
~
~
C2
No Flow
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
C3
1,376
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
06/11/98
Cl
9,439
18
28.4
5.02
12.3
0.33
<1
0.084
0.032
6.8
~
~
~
~
~
~
~
C2
No Flow
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
C3
3,237
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
06/12/98
Cl
11,621
70
37.2
5.83
5.04
0.47
1.12
0.22
0.18
4.8
14
6.9
10.1
64.9
6.6
2690
4.4
C2
No Flow
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
C3
3,903
4
~
~
~
~
~
~
0.021
4.3
~
~
~
~
~
~
~
Sample Date >
Analytes Location >
Runoff Volume (gal)
Total Suspended Solids (mg/1)
Chemical Oxygen Demand (mg/1)
Total Organic Carbon (mg/1)
Total Kjeldahl Nitrogen (mg/1 as N)
Total Inorganic Nitrogen (mg/1 as N)
Ammonia (mg/1 as N)
Total Phosphorus (mg/1)
Ortho-Phosphate (mg/1)
Alkalinity (mg/1 as CaCO,)
Hardness (mg/1 as CaCO,)
Lead
Copper
Zinc
06/14/98
Cl
7,624
44
64
14.3
3.36
0.4
1.12
0.17
0.058
6.5
~
6.6
10.3
68.5
C2
No Flow
~
~
~
~
~
~
~
~
~
~
~
~
~
C3
No Flow
~
~
~
~
~
~
~
~
~
~
~
~
~
06/16/98
Cl
53,457
<4
12.8
4.47
2.8
0.2
<1
0.078
0.05
5.2
~
<2
<1
<2
C2
6,985
19
45.6
9.75
7.28
0.15
<1
0.2
0.032
20
24
<2
<1
<2
C3
57,216
<4
15.2
5.79
17.4
0.16
<1
0.11
0.026
6.7
44
<2
<1
<2
06/23/98
Cl
81,327
5
<10
4.1
0.57
0.81
0.37
0.2
0.12
11
20
~
~
~
C2
6,334
34
24
7.4
0.44
0.27
0.27
0.24
0.05
8
10
~
~
~
C3
139,390
6
<10
3.1
0.8
0.53
0.32
0.05
0.05
<1
<1
~
~
~
5-72
-------
Sample Date >
Analytes Location >
Nickel
Aluminum
Chromium
06/14/98
Cl
<1
3450
<1
C2
~
~
~
C3
~
~
~
06/16/98
Cl
2.3
<54
2
C2
<1
3420
4.7
C3
<1
<54
<1
06/23/98
Cl
~
~
~
C2
~
~
~
C3
~
~
~
Sample Date >
Analytes Location >
Runoff Volume (gal)
Total Suspended Solids (mg/1)
Chemical Oxygen Demand (mg/1)
Total Organic Carbon (mg/1)
Total Kjeldahl Nitrogen (mg/1 as N)
Total Inorganic Nitrogen (mg/1 as N)
Ammonia (mg/1 as N)
Total Phosphorus (mg/1)
Ortho-Phosphate (mg/1)
Alkalinity (mg/1 as CaCO,)
Hardness (mg/1 as CaCO,)
Lead
Copper
Zinc
Nickel
Aluminum
Chromium
06/24/98
Cl
11,766
37
<10
3.6
0.52
0.52
0.18
0.11
0.03
7
6
~
~
~
~
~
~
C2
7,604
45
67
17
1.28
0.11
0.13
0.12
O.01
17
28
~
~
~
~
~
~
C3
38,353
<4
18
9.2
0.48
0.15
0.15
0.02
O.01
15
17
~
~
~
~
~
~
07/24/98
Cl
20,820
68
32
7.9
1.14
0.48
0.46
0.15
0.09
2
15
18.2
10.4
75.2
10
2430
7.7
C2
9,645
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
C3
4,579
5
<10
10.4
0.9
0.33
0.22
0.83
0.28
8
~
~
~
~
~
~
~
07/31/98
Cl
11,929
30
27
12
1
0.67
0.97
0.1
0.03
4
21
16.2
8
86.3
11.1
1370
4.8
C2
761
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
C3
No Flow
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
~
5-73
-------
Filtration and Bioretention Systems
Filtration systems are seeing increased usage, especially in ultra-urban environments where
space constraints prohibit the use of detention, retention and constructed wetland systems.
Filtration systems can provide significant water quality improvements, but only a small amount, if
any, water quantity control. It should also be stressed that filters must be placed off-line in order
to assure continued functioning, and therefore only provide treatment of a volume of water based
on a design storm. Any volume in excess of the design storm is bypassed without treatment.
Limited monitoring data are available on the efficiency of storm water filtering systems.
This is mainly due to storm water filters being a relatively new technology, as opposed to more
conventional BMPs such as wet ponds and constructed wetland systems. As a result, only a few
published monitoring studies are available to evaluate the efficiency of various filter designs. The
following Table 5-13 summarizes the pollutant removal efficiencies for storm water filtration
systems. Removal efficiencies are based on data collected from 13 monitoring studies.
5-74
-------
Table 5-13. Pollutant Removal Efficiency of Storm Water Filtration Systems
Parameter
Soluble Phosphorus
Total Phosphorus
Ammonia-Nitrogen
Nitrate
Organic Nitrogen
Total Nitrogen
Suspended Solids
Bacteria
Organic Carbon
Cadmium
Chromium
Copper
Lead
Zinc
Median or Average
Removal Efficiency
(percent)
-31
45
68
-13
28
32
81
37
57
26
54
34
71
69
Range of
Removals
(percent)
Low
-37
-25
43
-100
0
13
8
36
10
N/A
47
22
-16
33
High
-25
80
94
27
56
71
98
83
99
N/A
61
84
89
91
Number of
Observations
2
15
4
13
2
9
15
5
11
1
2
9
11
15
Source: Brown and Schueler, 1991 a
Storm water filtration systems can be highly effective at removing pollutants from storm
water runoff. They are particularly effective at removing TSS and total phosphorus, although
many filters export inorganic nitrogen due to nitrification of ammonia and organic nitrogen in the
filter (Bell, 1998). Bell's study reported that significant phosphorus removals can be attributed to
reaction and precipitation with sand that contains iron, calcium and aluminum. Although the
limited data that are available on storm water filters indicates that their overall performance is
good, additional data are needed to evaluate their efficiency, especially data that can be used to
evaluate their long-term hydraulic performance and maintenance requirements. For example,
Urbonas et al (1997) found that the hydraulic flow-through rate of a sand filter decreased from 3
feet-per-hour per square foot of filter area to less than 0.05 feet-per-hour after only several
storms. This rapid decrease in flow-through rate causes a marked decrease in efficiency, since
5-75
-------
more of the storm flow will be bypassed unless adequate detention storage volume is provided up-
stream of the filter. Therefore, overall TSS removal rates are significantly lower when this bypass
flow is accounted for (for example, Urbonas' evaluation of storms over the 1995 season resulted
in only a 15 percent overall TSS removal when bypass flows were taken into account). Due to
the potential decrease in efficiency of sand filtration systems, careful consideration of design
parameters is needed. Urbonas (1999) presents a thorough discussion of sand filtration design.
Readers are urged to consult this reference for information on sand filtration system design.
In order to provide adequate filter functioning, the following basic design and operation
guidelines should be followed:
The filter should be placed off-line;
A sedimentation chamber or basin should be provided upstream of the filter bed in
order to allow for the removal of sediments to extend the length of the filter run
between maintenance activities;
The filter should be sized adequately or else adequate detention facilities should be
provided upstream of the filter in order to capture expected storm water flows and to
minimize bypasses;
Care should be taken to limit excessive sediment loadings to the filter during
construction or landscaping activities;
Periodic maintenance to remove accumulated sediments and restore the filter flow-
through rate may be necessary in areas with high solids loadings.
As with filtration systems, the available data on the performance of bioretention facilities
are limited. Since bioretention facilities incorporate many of the same mechanisms as filtration
systems, their performance for removal of parameters such as TSS are expected to be similar. Due
to their biological nature, however, bioretention facilities are expected to also provide conditions
necessary for uptake of nutrients by vegetation, degradation of organic contaminants by soil
microorganisms, and biochemical reactions within the soil matrix and around the root zone of
plants. Available data on the efficiency of bioretention facilities (based on laboratory data and one
field study) indicates that bioretention can obtain removals on the order of 95-97 percent for
metals, 75 percent for total phosphorus, 69 percent for TKN, 79 percent for ammonia, 21 percent
for nitrate and 56 percent for total nitrogen (adapted from Bell (1998), average of all reported
values).
The following general guidelines should be followed when designing bioretention facilities:
Water should not be allowed to pond for more than four days in order to prevent
mosquito breeding and to prevent adverse effects on plants;
Plants selected for bioretention should be tolerant to stresses found in urban areas such
as pollutants, variable soil moisture, periodic inundation, and high temperatures;
5-76
-------
Native plant species should be used whenever possible (Prince George's County,
1993), and species diversity should be maintained in order to prevent loss of all plants
in the event of disease or infestation.
Plants should be placed with regard to the elevation and moisture level of the planting
bed (i.e., more water-tolerant species should be placed in lower areas where water is
likely to pond longer);
A mulch layer should be installed and maintained in order to prevent erosion of soils
and to retain soil moisture;
Where concentrated runoff enters the bioretention system, reinforcement (such as stone
stabilization or synthetic erosion protection materials) may be needed to reduce erosion
of the mulch layer and disturbance of the planting bed (Claytor and Schueler, 1996).
The clay content of soils used in bioretention facilities may need to be limited to
prevent clogging of the soil bed (Bell, 1998).
Readers are encouraged to consult Design Manual for Use of Bioretention in Stormwater
Management (Prince George's County, 1993) and Design of Stormwater Filtering Systems
(Claytor and Schueler, 1996) for additional information on the bioretention concept.
Hollywood Branch Peat/Sand Filter
In 1998 EPA conducted sampling activities at a peat/sand filter in Montgomery County,
Maryland. The Hollywood Branch filter is a surface sand filter with a peat/sand filtration media
that was designed based on the Galli paper (1990b). The filter is located in a county park in the
Colesville area of Montgomery County and discharges to Hollywood Branch, a first-order stream
that discharges into Paint Branch approximately 3,000 feet downstream of the filter. The
drainage area covers approximately 140 acres and consists of 73 percent residential, 13 percent
industrial and 14 percent other sources. The filter was one of several retrofit projects installed by
Montgomery County as part of a watershed restoration effort in the Paint Branch watershed.
The filter is located off-line of the storm drainage system, and is designed to capture the
first 0.1 watershed inches of runoff via a flow-splitter located in the storm sewer. This
corresponds to a runoff volume of approximately 50,280 ft3. Any runoff in excess of this amount
bypasses the filter and is discharged directly to Hollywood Branch. Runoff from the flow splitter
first enters a small stilling basin before being discharged to the filter. The stilling basin functions
as a pre-settling chamber to remove coarse sediments in order to prolong the life of the filter. The
stilling basin has a volume of 16,940 ft3 at the permanent pool level, a depth of 3 feet, and length-
to-width ratio of approximately 3:1. The edge of the stilling basin is planted with emergent
wetland vegetation. Water is discharged from the stilling basin and into the filter through a
submerged 18 inch pipe. The filter has dimensions of 265 feet by 63 feet. The filter is designed to
pond water to a maximum depth of 2 feet, which corresponds to a volume of 33,880 ft3. The
filter bed is designed to have a minimum infiltration rate of 1.0 inch/hour. The filter bed consists
of a 12-inch peat top layer, underlain by a 4-inch sand/peat mix, which is underlain by a 20-inch
layer of sand. Water entering the filter is distributed by a series of interconnected 6 inch PVC half-
pipes placed along the surface of the filter bed. The filter contains an under-drain system
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consisting of 6 inch perforated PVC pipes encased in a crushed gravel layer. Filtered water
collected in the under-drain system is discharged to Hollywood Branch through a 12-inch
concrete pipe. See Figure 5-16.
Figure 5-16. Hollywood Branch Peat/Sand Filter
H3 Hollywood Branch
Concrete
trapezoidal
channel
Peat Sand Filter
63 x 265 ft.
Sampling Locations
H1 Inflow
H2 Outflow
H3 In-stream
Not to scale
The monitoring program consisted of recording runoff flow rates and volumes and
collecting flow-weighted composite samples from the inflow and outflow of the filter using
automatic sampling equipment. A tipping bucket rain gauge was used to record precipitation
levels. Flow monitoring and water quality sampling was conducted for five events during the
spring and summer of 1998. Baseflow samples were collected from the filter on three occasions.
In addition, in-stream sediment samples were collected on one occasion, and bioassessment and
physical habitat measurements were also conducted.
The following tables summarize the chemical sampling data collected during this
evaluation. Additional information describing the sampling program, additional sampling and
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assessment data (including sediment, bioassessment and physical habitat assessment) and an
analysis of the performance of the filter will be included as a supplement to this report.
5-79
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Table 5-14. Summary of Hollywood Branch Peat/Sand Filter Storm Event Sampling Data
Sample Dates >
Analytes
Event Volume (gal)
Total Suspended Solids (mg/1)
Chemical Oxygen Demand (mg/1)
Total Organic Carbon (mg/1)
Total Kjeldahl Nitrogen (mg/1 as
N)
Total Inorganic Nitrogen (mg/1 as
N)
Ammonia (mg/1 as N)
Total Phosphorus (mg/1)
Ortho-Phosphate (mg/1)
Lead ((ig/1)
Copper (ng/1)
Zinc (ng/1)
Nickel ((jg/1)
Aluminum (ng/1)
Chromium (ng/1)
06/01/98
Inflow
7,597
~
16
10.4
2
1.11
0.56
0.14
~
2.1
6.8
17.1
2.4
<54
< 1
Outflow
No flow
~
~
~
~
~
~
~
~
~
~
~
~
~
~
06/12/98
Inflow
68,436
10
25.2
7.29
7.28
0.77
< 1
0.094
0.047
13.6
8.3
40.1
2.4
488
1.4
Outflow
28,470
<4
17.6
4.37
5.6
2.14
< 1
0.16
0.27
2.4
2
<2
22.3
1360
6.3
06/14/98
Inflow
308,705
7
24
4.37
2.24
0.66
1.68
0.19
0.23
2.3
4.7
<2
<1
899
< 1
Outflow
149,184
17
14.4
4.37
11.2
2.07
14.6
0.14
0.049
3.7
1.3
<2
< 1
2010
< 1
06/24/98
Inflow
131,744
12
< 10
5.5
0.91
1
0.28
0.19
<0.01
2.7
7
26.8
1.5
956
< 1
Outflow
75,003
8
11
6
0.78
1.86
0.16
0.2
<0.01
5.6
4
22.6
4.7
2810
4.8
07/31/98
Inflow
151,395
38
30
14
1.8
1.13
0.32
0.18
0.1
3.8
6.5
43
3.2
497
< 1
Outflow
72,537
31
34
10.4
0.68
2.21
<0.1
0.15
<0.01
8.9
11.3
41.8
7.4
3450
7.7
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Table 5-15. Summary of Hollywood Branch Peat/Sand Filter Baseflow Sampling Data
Sample Dates >
Analytes
Total Suspended Solids
(mg/1)
Chemical Oxygen
Demand (mg/1)
Total Organic Carbon
(mg/1)
Total Kjeldahl Nitrogen
(mg/1 as N)
Total Inorganic Nitrogen
(mg/1 as N)
Ammonia (mg/1 as N)
Total Phosphorus (mg/1)
Ortho-Phosphate (mg/1)
Lead ((ig/1)
Copper (ng/1)
Zinc (ng/1)
Nickel (ng/1)
Aluminum (ng/1)
Chromium (ng/1)
05/19/98
Inflow
<4
< 10
< 10
1.66
1.79
0.49
0.02
0.02
<2
< 1
<2
2.3
59.8
< 1
Outflow
<4
< 10
< 10
19.8
0.64
<0.1
<0.01
0.02
<2
< 1
<2
3.1
<54
~
06/23/98
Inflow
...
___
___
Outflow
<4
< 10
3.5
0.55
0.74
<0.1
0.14
<0.01
08/14/98
Inflow
...
___
___
Outflow
<4
<20
6.6
1.28
0.23
< 1
0.054
0.069
<2
1.2
30.5
2.7
1590
2.3
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Open Channel Vegetated Systems
Open channel vegetated systems are used widely for storm water quality control.
However, these systems can be difficult to monitor, especially systems that intercept runoff as
sheet flow such as grass filter strips. As a result, data on these types of systems are not as
prevalent as other more readily monitored BMP types such as ponds and constructed wetlands.
Table 5-16 summarizes the pollutant removal efficiency of open channel vegetated systems.
Removal efficiencies are based on data collected from 20 monitoring studies.
Table 5-16. Pollutant Removal Efficiency of Open Channel Vegetated Systems
Parameter
Soluble Phosphorus
Total Phosphorus
Ammonia-Nitrogen
Nitrate
Organic Nitrogen
Total Nitrogen
Suspended Solids
Bacteria
Organic Carbon
Cadmium
Chromium
Copper
Lead
Zinc
Average or Median
Removal Efficiency
(percent)
11
15
3
11
39
11
66
-25
23
49
47
41
50
49
Range of Removals
(percent)
Low
-45
-100
-19
-100
11
-100
-100
-100
-100
20
14
-35
-100
-100
High
72
99
78
99
86
99
99
0
99
80
88
89
99
99
Number of
Observations
8
18
4
13
3
10
18
5
11
6
5
15
19
19
Source: Brown and Schueler, 1997a
Evaluation of available data does not provide a good indication as to the actual
performance of these systems. The above data indicate that a wide range in pollutant removal
efficiency is reported in the literature for open channel vegetated systems. Since there are a
variety of system designs lumped into the above summary, arriving at efficiency estimates for a
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particular system type given available data is difficult. In general, these types of BMPs should be
effective at removing suspended solids and associated pollutants from runoff by sedimentation and
by filtration by vegetation, and are certainly effective at slowing the velocity of storm water runoff
and for providing detention of runoff if check dams or other structures are incorporated to
provide ponding of runoff. However, dense vegetation must be maintained in order to assure
proper functioning. In addition, negative removals are frequently reported for sediment and
nutrients. If open channel vegetated systems are not properly maintained, significant export of
sediments and associated pollutants such as metals and nutrients can occur from eroded soil. In
addition, standing water in these systems can be a significant source of bacteria and can provide
the conditions necessary for mosquito breeding. Additional data gathering is needed in order to
support these assumptions and to quantify the efficiency of these systems.
Open channel vegetated systems can be used as pretreatment devices for other BMPs, or
can be used in a "treatment train" approach. For example, grass filter strips are commonly used
to accept sheet flow from parking lots in order to pre-treat runoff prior to being treated by a
bioretention facility or a filter. Vegetated swales can be used to convey runoff to BMPs such as
ponds or constructed wetlands, providing pretreatment of the runoff volume. When used in
combination with other BMPs, the overall quality of the treated runoff can be improved and the
total runoff volume can be reduced due to infiltration that occurs in the open channel vegetated
systems.
Miscellaneous and Vendor-Supplied Systems
Little data exist in the published literature on the efficiency of vendor-supplied systems.
Data is frequently available from the vendors, and as more of these systems are installed it is
expected that more data will become available. An evaluation of the efficiency of these systems
has not been included in this report. The EvTEC program (see section 5.2.1.8) and other
evaluation programs should provide useful information that indicates the efficiency of these
systems in removing pollutants from runoff.
5.5.3 Controlling Flow Impacts
The removal of pollutants from storm water runoff is an important function of storm water
BMPs. However, in many cases receiving water problems are not due to the pollutants contained
in storm water, but rather can be attributed to the large flow rates that result in receiving streams
that receive storm water discharges. Therefore, in some cases, controlling the volume and flow
rate of storm water discharges is as important, if not more important, than removing pollutants
prior to discharge. Site-specific parameters will dictate the importance of flow control in
preventing degradation of receiving waters.
Evaluating the effectiveness of BMPs in controlling flow impacts is not an easy task. Site-
specific variations such as slope, soil types, ground cover, and watershed-imperviousness can
greatly impact the hydraulic response of a watershed to rainfall. In addition, receiving water
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parameters greatly influence the degree of flow control that is necessary in order to prevent
degradation. As a result, little information is contained in the literature describing the
performance of BMPs at controlling impacts in receiving streams due to excessive storm water
flows. The literature that does exist, however, indicates a direct correlation between urbanization
and receiving stream degradation. It is not difficult to infer, therefore, that storm water flow is a
major contributor to receiving stream degradation, and that control of storm water flow rates and
volumes is warranted in order to restore degraded receiving waters and to prevent degradation of
receiving waters in newly developing areas. Additional information on the hydrological benefits
of BMPs is presented in section 6.3.2 of this report.
Important measures of the effectiveness of BMPs at controlling storm water flows include:
reductions in peak flow rate across the BMP;
total storage volume provided in the BMP;
infiltrative capacity of the BMP;
retention time in the BMP;
relationship of post-development hydrologic conditions to pre-development
hydrology;
retention volume necessary for receiving stream channel protection.
Local conditions will dictate the BMP design parameters that are necessary to reduce
impacts due to flow. For example, the state of Maryland has developed unified BMP sizing
criteria that is designed to provide adequate control of pollutants, limit degradation of streams,
provide adequate groundwater recharge, and protect downstream areas from flooding. Additional
work is needed in other areas of the country to evaluate the effectiveness criteria necessary to
limit flow impacts and to provide adequate BMP sizing standards.
Flow control can be accomplished by using both structural and non-structural practices.
Structural BMPs that can provide flow control include retention basins, detention basins,
constructed wetlands, infiltration practices, grassed swales and minimizing directly connected
impervious surface areas. Filters and bioretention facilities can also be adapted to provide some
degree of quantity control if they are used in conjunction with detention basins or other means of
providing detention of storm water prior to treatment, such as providing temporary ponding in
overflow parking areas. Non-structural BMPs and land-use practices that can help to reduce the
volume of storm water runoff discharged to receiving streams should also be considered a vital
component of storm water management. Practices that can reduce the impact of storm water
runoff due to excessive flows include land use regulations such as zoning, natural area and stream
buffer preservation, limits on impervious surfaces, and cluster development. Practices that limit
the generation of storm water can be very effective in preventing degradation of streams, and can
limit the need for structural storm water controls. Information on development practices aimed at
reducing impacts due to site development practices can be found in Conservation Design for
Stormwater Management (Delaware DNREC, 1997) and in Green Development (US EPA,
1996b).
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5.6 Conclusions
There are a wide variety of BMPs available to manage storm water runoff. The efficiency
of various BMP types has been documented to some degree, but there is still a great need for
focused research in certain areas, particularly for newer and innovative structural BMP types, as
well as non-structural BMPs. However, due to the complexity involved in isolating the reaction
of a complex and highly variable system such as a watershed to one isolated input, evaluations of
non-structural BMPs are ambitious tasks. Still, where storm water management is largely driven
by the availability of scarce funding, data that indicate the cost-effectiveness of various control
strategies are badly needed.
Ultimately, receiving stream morphology, habitat and biological communities may turn out
the be the driving factors indicating the success of BMPs at controlling impacts due to storm
water flows and the pollutants that they contain. In order for such measures to work, however, it
is necessary to isolate the response of a receiving stream system to the implementation of BMPs.
Frequently, there are too many variables in a watershed and too many other potential sources of
degradation to isolate the improvements (or even to indicate potential negative impacts) of a
particular BMP or group of BMPs. For example, Maxted and Shaver (1997) did not observe a
significant difference in macroinvertebrate communities between 8 sites with storm water
retention ponds and 33 sites with no storm water controls. In addition, the BMPs did not prevent
the almost complete loss of sensitive aquatic species. Whether or not these impacts were caused
by storm water flows, pollutants or other non-storm water sources was not indicated, and the data
to be able to answer these questions may not be forthcoming in the foreseeable future. Therefore,
until data are available to indicate that specific BMPs can prevent impacts and prevent
degradation of receiving streams in urbanized areas, one should not assume that structural, "end-
of-pipe" BMPs are the only answer to the storm water problem.
Available data seem to indicate that urbanization and traditional urban development at
almost any level can cause degradation of streams, and that BMPs may be able to mitigate these
impacts to a certain level. Accordingly, storm water management should start at the point of
runoff generation, and incorporate site planning principles that prevent or minimize the generation
of runoff, prevent development in floodplains, preserve natural drainage systems, and avoid
disturbing sensitive areas such as wetlands and riparian areas. Where runoff generation cannot be
avoided, then properly sited, designed, constructed and operated BMPs can be implemented to
attempt to reduce the impacts associated with this runoff. There are data available on the
effectiveness of BMPs in reducing pollutant loads, but these data are not comprehensive enough
to either characterize the performance of all BMPs in use or to determine if they are actually
controlling impacts to receiving waters. Additional data gathering is necessary, but the
monitoring and data analysis protocols necessary to do so have not been fully developed.
Standardization of monitoring protocols for data transferability is a vital component of successful
data evaluation, and is an area that should be actively pursued in the near future.
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6.0 Costs and Benefits of Storm Water BMPs
Storm water best management practices (BMPs) are the primary tool to improve the
quality of urban streams and meet the requirements of NPDES permits. They include both the
structural and non-structural options reviewed in Section 5.2 of this report. Some BMPs can
represent a significant cost to communities, but these costs should be weighed against the various
benefits they provide. This chapter will focus on reviewing available data on the costs and
potential benefits of both structural and non-structural BMPs designed to improve the quality of
urban and urbanizing streams, and the larger water bodies to which they drain.
As described in previous chapters, storm water runoff can contribute loadings of nutrients,
metals, oil and grease, and litter that result in impairment of local water bodies. The extent to
which these impairments are eliminated by BMPs will depend on a number of factors, including
the number, intensity, and duration of wet weather events; BMP construction and maintenance
activities; and the site-specific water quality and physical conditions. Because these factors will
vary substantially from site to site, data and information are not available with which to develop
dollar estimates of costs and benefits for individual types of BMPs. However, EPA's national
estimates of costs and benefits associated with implementation of the NPDES Phase II rule are
discussed in Section 6.4.
6.1 Structural BMP Costs
The term structural BMPs, often referred to as "Treatment BMPs," refers to physical
structures designed to remove pollutants from storm water runoff, reduce downstream erosion,
provide flood control and promote groundwater recharge. In contrast with non-structural BMPs,
structural measures include some engineering design and construction.
Structural BMPs evaluated in this report include:
Retention Basins
Detention Basins
Constructed Wetlands
Infiltration Practices
Filters
Bioretention
Biofilters (swales and filter strips).
The two infiltration systems focused on in this report are infiltration trenches and
infiltration basins. Although bioretention can serve as a filtering system or infiltration practice, it
is discussed separately because it has separate cost data and design criteria. In this report, wet
swales are assumed to have the same cost as biofilters, because there are little cost data available
on this practice. Additional information about these structural BMPs, including descriptions,
applicability and performance data can be found in Chapter 5 of this report. Other BMPs include
6- 1
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experimental and proprietary products, as well as some conventional structures such as water
quality inlets. They are not included in this analysis because sufficient data are not available to
support either the performance or the cost of these practices.
6.1.1 Base Capital Costs
The base capital costs refer primarily to the cost of constructing the BMP. This may
include the cost of erosion and sediment control during construction. The costs of design,
geotechnical testing, legal fees, land costs, and other unexpected or additional costs are not
included in this estimate. The cost of constructing any BMP is variable and depends largely on
site conditions and drainage area. For example, if a BMP is constructed in very rocky soils, the
increased excavation costs may substantially increase the cost of construction. Also, land
acquisition costs vary greatly from site to site.4 In addition, designs vary slightly among BMP
types. A wet pond may be designed with or without various levels of landscaping, for example.
The data in Table 6-1 represent typical unit costs (dollars per cubic foot of treated water volume)
from various studies, and should be considered planning level. In the case of retention and
detention basins, ranges are used to reflect the economies of scale involved in designing these
BMPs.
4 Land cost is the largest variable influencing overall BMP cost.
6-2
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Table 6-1. Typical Base Capital Construction Costs for BMPs
BMP
Type
Retention and
Detention
Basins
Constructed
Wetland
Infiltration
Trench
Infiltration
Basin
Sand Filter
Bioretention
Grass
Swale
Filter Strip
Typical
Cost*
($/cf)
0.50-1.00
0.60-1.25
4.00
1.30
3.00-6.00
5.30
0.50
0.00-1.30
Notes
Cost range reflects economies of scale in designing
this BMP. The lowest unit cost represents approx.
150,000 cubic feet of storage, while the highest is
approx. 15,000 cubic feet. Typically, dry detention
basins are the least expensive design options among
retention and detention practices.
Although little data are available to assess the cost of
wetlands, it is assumed that they are approx. 25%
more expensive (because of plant selection and
sediment forebay requirements) than retention
basins..
Represents typical costs for a 100-foot long trench.
Represents typical costs for a 0.25-acre infiltration
basin.
The range in costs for sand filter construction is
largely due to the different sand filter designs. Of the
three most common options available, perimeter sand
filters are moderate cost whereas surface sand filters
and underground sand filters are the most expensive.
Bioretention is relatively constant in cost, because it
is usually designed as a constant fraction of the total
drainage area.
Based on cost per square foot, and assuming 6 inches
of storage in the filter.
Based on cost per square foot, and assuming 6 inches
of storage in the filter strip. The lowest cost assumes
that the buffer uses existing vegetation, and the
highest cost assumes that sod was used to establish
the filter strip.
Source
Adapted from
Brown and
Schueler (1997b)
Adapted from
Brown and
Schueler (1997b)
Adapted from
SWRPC (1991)
Adapted from
SWRPC (1991)
Adapted from
Brown and
Schueler (1997b)
Adapted from
Brown and
Schueler (1997b)
Adapted from
SWRPC (1991)
Adapted from
SWRPC (1991)
* Base year for all cost data: 1997
In some ways there is no such value as the "average" construction cost for some BMPs,
because many BMPs can be designed for widely varying drainage areas. However, there is some
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value in assessing the cost of a typical application of each BMP. The data in Table 6-2 reflect
base capital costs for typical applications of each category of BMP. It is important to note that,
since many BMPs have economies of scale, it is not practical to extrapolate these values to larger
or smaller drainage areas in many cases.
Table 6-2. Base Costs of Typical Applications of Storm Water BMPs1
BMP Type
Retention
Basin
Wetland
Infiltration
Trench
Infiltration
Basin
Sand Filter
Bioretention
Grass Swale
Filter Strip
Typical Cost
(S/BMP)
$100,000
$125,000
$45,000
$15,000
$35,000-
$70,0002'3
$60,000
$3,500
$0-$9,0003
Application
50- Acre Residential Site
(Impervious Cover =
35%)
50- Acre Residential Site
(Impervious Cover =
35%)
5-Acre Commercial Site
(Impervious Cover =
65%)
5-Acre Commercial Site
(Impervious Cover =
65%)
5-Acre Commercial Site
(Impervious Cover =
65%)
5-Acre Commercial Site
(Impervious Cover =
65%)
5-Acre Residential Site
(Impervious Cover =
35%)
5-Acre Residential Site
(Impervious Cover =
35%)
Data Source
Adapted from Brown and
Schueler (1997b)
Adapted from Brown and
Schueler (1997b)
Adapted from SWRPC
(1991)
Adapted from SWRPC
(1991)
Adapted from Brown and
Schueler (1997b)
Adapted from Brown and
Schueler (1997b)
Adapted from SWRPC
(1991)
Adapted from SWRPC
(1991)
1. Base costs do not include land costs.
2. Total capital costs can typically be determined by increasing these costs by approximately 30%.
3. A range is given to account for design variations.
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Although various manuals report construction cost estimates for storm water ponds, EPA
has identified only three studies that have systematically evaluated the construction costs
associated with structural BMPs since 1985. The three studies used slightly different estimation
procedures. Two of these studies were conducted in the Washington, DC region and used a
similar methodology (Wiegand et al, 1986; Brown and Schueler, 1997b). In both studies, the
costs were determined based on engineering estimates of construction costs from actual BMPs
throughout the region. In the third study, conducted in Southeastern Wisconsin, costs were
determined using standardized cost data for different elements of the BMP, and assumptions of
BMP design (SWRPC, 1991).
Any costs reported in the literature need to be adjusted for inflation and regional
differences. All costs reported in this report assume a 3 percent annual inflation rate. In addition,
studies are adjusted to the "twenty cities average" construction cost index, to adjust for regional
biases, based on a methodology followed by the American Public Works Association (APWA,
1992). Using EPA's rainfall zones (see Figure 6-1), a cost adjustment factor is assigned to each
zone (Table 6-3). For example, rainfall region 1 has a factor of 1.12. Thus, all studies in the
Northeastern United States are divided by 1.12 in order to adjust for this bias.
Table 6-3. Regional Cost Adjustment Factors
Rainfall Zone
Adjustment
Factor
1
1.12
2
0.90
3
0.67
4
0.92
5
0.67
6
1.24
7
1.04
8
1.04
9
0.76
Source: Modified from APWA, 1992
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Figure 6-1. Rainfall Zones of the United States
Not shown: Alaska (Zone 7); Hawaii (Zone 7); Northern Mariana Islands (Zone 7); Guam (Zone
7); American Samoa (Zone 7); Trust Territory of the Pacific Islands (Zone 7); Puerto Rico (Zone
3) Virgin Islands (Zone 3).
Source: NPDES Phase I regulations, 40 CFR Part 122, Appendix E (US EPA, 1990)
6.1.1.1 Retention/Detention Basins and Constructed Wetlands
The total volume of the basin is generally a strong predictor of cost (Table 6-4). There
are some economies of scale associated with constructing these systems, as evidenced by the
slope of the volume equations derived. This is largely because of the costs of inlet and outlet
design, and mobilization of heavy equipment that are relatively similar regardless of basin size.
Erosion and sediment control represents only about 5 percent of the construction cost of
basins and wetlands (Brown and Schueler, 1997b). Thus, the construction cost estimates
presented in Table 6-2 are comparable. The cost of building storm water retention and detention
systems has increased since 1986 (Figure 6-2), even after adjusting for inflation. Part of the
reason for this increase is thought to be attributable to the improved design of these systems to
enhance water quality driven by a more complex regulatory and review environment (Brown and
Schueler, 1997b). The cost estimations made by SWRPC (1991) were generally a mid-range
between the earlier and more recent studies.
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Table 6-4. Base Capital Costs for Storm Water Ponds and Wetlands
BMP
Type
Retention
Basins
and
Wetlands
Detention
Basins
Retention
Basins
Cost Equation or Estimate
7.75V0'75
18.5V0'70
7.47V0'78
1.06V: 0.25 acre retention basin
(23,300 cubic feet)
0.43V: 1.0 acre retention basin
(148,000 cubic feet)
0.33V: 3.0 acre retention basin
(547,000 cubic feet)
0.31V: 5.0 acre retention basin
(952,000 cubic feet)
Costs Included
Construc-
tion
"
"
E&S
Control
*S
*S
Source
Wiegandetal, 1986
Brown and Schueler,
1997b
Brown and Schueler,
1997b
SWRPC, 1991
Notes
V refers to the total basin volume in cubic feet
Costs presented from SWRPC (1991) are "moderate" costs reported in that study.
6-7
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Figure 6-2. Retention Basin Construction Cost
$1,000,000
$100,000
$10,000 [-
Wiegand et al, 1986
*- Brown and Schueler, 1997b
A SI/I/RPC, 799?
70,000
100,000 1,000,000
Volume (cubic feet)
10,000,000
6.1.12 Infiltration Practices
Costs for infiltration BMPs are highly variable from site to site, depending on soils and
other geotechnical information. Perhaps because of this variability, cost estimates for infiltration
trenches have been widely different (Table 6-5; Figure 6-3). Brown and Schueler (1997b)
concluded that the Wiegand (1986) equation underestimated cost, partially because of the lack of
pretreatment in earlier designs, although they were unable to develop a consistent equation due to
a small sample size.
It is difficult to estimate the cost of infiltration basins, mainly due to a lack of recent cost
data. The costs estimates for SWRPC are dramatically higher than those estimated by Schueler,
1987 (Figure 6-4). This is largely because the SWRPC document assumes that 50 percent
6-8
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additional volume is excavated for the spillway, while Schueler uses a retention basin cost
equation.
Table 6-5. Base Capital Costs for Infiltration Practices
BMP
Type
Infiltration
Trenches1
Infiltration
Basins2
Porous
Pavement3
Cost Equation or Estimate4
33.7V0'63
2V to 4V; average of 2.5V
$4,400: 3 -foot deep, 4-foot
wide, 100-foot long trench
$10,400: 6-foot deep, 10-foot
wide, 100-foot long trench
3.9V+2,900: 3-foot deep, 100-
foot long trench
13.2V0'69
1.3V: 0.25-acre infiltration
basin (15,000 cubic feet)
0.8V: 1.0-acre infiltration basin
(76,300 cubic feet)
50,OOOA
80,OOOA
Costs Included
Construc-
tion
E&S
Control
Source
Wiegand et al, 1986
Brown and Schueler, 1997b
SWRPC, 1991
Modified from SWRPC,
1991
Schueler, 1987; Modified
from Wiegand et al, 1 986
SWRPC, 1991
SWRPC, 1991
Schueler, 1987
1. V for infiltration trenches refers to the treatment volume (cubic feet) within the trench, assuming a
porosity of 32%.
2. V for infiltration basins refers to the total basin volume (cubic feet).
3. A is the surface area in acres of porous pavement.
4. Costs presented from SWRPC (1991) are "moderate" costs reported in that study.
6-9
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Figure 6-3. Infiltration Trench Cost
$100,000
$10,000
o
o
$1,000
$100
100
1,000
Volume (cubic feet)
10,000
3-foot Deep (SWRPC, 1991)
Brown and Schueler, 1997b (low) -
Wiegand et al., 1986
- - Brown and Schueler, 1997b (high)
6-10
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Figure 6-4. Infiltration Basin Construction Cost
120,000
100,000
80,000
8 60,000
40,000
20,000
Schueler, 1987
A SWRPC, 1991
0 100,000 200,000 300,000 400,000 500,000 600,000
Volume (cubic feet)
6.1.1.3 Sand Filters
Since sand filters have not been used as long as other BMPs, less information is available
on their cost than on most BMPs. In addition, the costs of sand filters vary significantly due to
the wide range of design criteria for sand filters (Table 6-6). Brown and Schueler (1997b) were
unable to derive a valid relationship between sand filter cost and water quality volume, with costs
ranging between $2 and $6 per cubic foot of water quality volume, with a mean cost of $2.50 per
cubic foot. The water quality volume includes the pore space in the sand filter, plus additional
storage in the pretreatment basin.
Because of the lack of cost data, no equation referencing the economies of scale has been
developed. However, it appears that economies of scale do exist. For example, data from Austin
indicates that the cost per acre decreased by over 80 percent for a design of a 20-acre drainage
area, when compared with a 1-acre drainage area. (Schueler, 1994a).
6- 11
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Table 6-6. Construction Costs for Various Sand Filters
Region (Design)
Delaware
Alexandria, VA (Delaware)
Austin, TX ( < 2 acres)
Austin, TX ( > 5 acres)
Washington, DC (underground)
Denver, CO
Cost/Impervious Acre
$10,000
$23,500
$16,000
$3,400
$14,000
$30,000-$50,000
Source: Schueler, 1994a
6.1.1.4 Bioretention
Little information is available on the costs of bioretention because it is a relatively new
practice. Brown and Schueler (1997b) found consistent construction costs of approximately
$5.30 per cubic foot of water quality volume for the construction cost. The water quality volume
includes 9 inches above the surface area of the bioretention structure.
6.1.1.5 Vegetative BMPs
The two major types of vegetative BMPs include filter strips and grassed swales (also
called "biofilters"). The costs for these BMPs vary, and largely depend on the method used to
establish vegetation (Table 6-7).
6- 12
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Table 6-7. Base Capital Costs of Vegetative BMPs
BMP
Type
Filter
Strips
Grassed
Channels
Cost Equation or Estimate1
Existing Vegetation: 0
Seed: $13,800/acre
Sod: $29,000/acre
250 per square foot
Costs Included
Construc-
tion
E&S
Control
Source
SWRPC, 1991
SWRPC, 1991
1. Costs presented from SWRPC (1991) are "moderate" costs reported in that study.
6.1.2 Design. Contingency and Permitting Costs
Most BMP cost studies assess only part of the cost of constructing a BMP, usually
excluding permitting fees, engineering design and contingency or unexpected costs. In general,
these costs are expressed as a fraction of the construction cost (Table 6-8). These costs are
generally only estimates, based on the experience of designers.
Table 6-8. Design, Contingency and Permitting Costs
Additional Costs Estimate
(Fraction of base construction costs)
25%
32%
Source
Wiegand et
al, 1986
Brown and
Schueler,
1997b
Comments
Includes design, contingencies and permitting
fees
Includes design, contingencies, permitting
process and erosion and sediment control
6.1.3 Land Costs
The cost of land is extremely variable both regionally and by surrounding land use. For
example, many suburban jurisdictions require open space allocations within the developed site,
reducing the effective cost of land for BMPs to zero (Schueler, 1987). On the other hand, the
cost of land may far outweigh construction and design costs in ultra-urban settings. For this
6- 13
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reason, some underground BMPs that are relatively expensive to construct may be attractive in
this "ultra-urban" setting if sub-surface conditions are suitable (Lundgren, 1996). The land
consumed per treatment volume depends largely on how much of the BMP's treatment is
underground, and varies considerably (Table 6-9).
Table 6-9. Relative Land Consumption of
Storm Water BMPs
BMP Type
Retention Basin
Constructed
Wetland
Infiltration Trench
Infiltration Basin
Porous Pavement
Sand Filters
Bioretention
Swales
Filter Strips
Land consumption
(% of Impervious Area)
2-3%
3-5%
2-3%
2-3%
0%
0%-3%
5%
10%-20%
100%
Note: Represents the amount of land needed as a percent
of the impervious area that drains to the practice to
achieve effective treatment.
Source: Claytorand Schueler, 1996
6.1.4 Operation and Maintenance Costs
Maintenance can be broken down into two primary categories: aesthetic/nuisance
maintenance and functional maintenance. Functional maintenance is important for performance
and safety reasons, while aesthetic maintenance is important primarily for public acceptance of
BMPs, and because it may also reduce needed functional maintenance. Aesthetic maintenance is
obviously more important for BMPs that are very visible, such as ponds and biofiltration facilities.
In most studies, operation and maintenance (O&M) costs have been estimated as a
percentage of base construction costs (Table 6-10). While some BMPs require infrequent, costly
6- 14
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maintenance, others need more frequent but less costly maintenance.5 Accordingly, selection of
appropriate structural BMPs must factor in maintenance cost (and a responsible party to carry out
maintenance) to ensure the necessary long-term performance. Typical maintenance activities are
included in Table 5-3.
Table 6-10. Annual Maintenance Costs
BMP
Retention Basins and
Constructed Wetlands
Detention Basins1
Constructed Wetlands1
Infiltration Trench
Infiltration Basin1
Sand Filters1
Swales
Bioretention
Filter strips
Annual Maintenance Cost
(% of Construction Cost)
3%-6%
<1%
2%
5%-20%
l%-3%
5%-10%
11%-13%
5%-7%
5%-7%
$320/acre (maintained)
Source(s)
Wiegandetal, 1986
Schueler, 1987
SWRPC, 1991
Livingston et al, 1997;
Brown and Schueler, 1997b
Livingston et al, 1997;
Brown and Schueler, 1997b
Schueler, 1987
SWRPC, 1991
Livingston et al, 1997;
SWRPC, 1991
Wiegandetal, 1986;
Schueler, 1987;
SWRPC, 1991
Livingston et al, 1997;
Brown and Schueler, 1997b
SWRPC, 1991
(Assumes the same as swales)
SWRPC, 1991
1. Livingston et al (1997) reported maintenance costs from the maintenance budgets of several cities,
and percentages were derived from costs in other studies
5 Maintenance costs can also vary significantly based on a variety of site- and region-
specific parameters, therefore the maintenance costs presented in Table 6-10 should be considered
only as general guidelines.
6- 15
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6.1.5 Long-Term BMP Costs: Two Scenarios
In order to compare various BMP options, costs were calculated for a 5-acre commercial
site and a 38-acre residential site.6 Construction costs were evaluated using the following steps:
1. Calculate the water quality volume (WQV).7
Using a water quality volume based on a 1-inch storm, the volume is equal to:
WQV = (.05+ .97) ,4/12
where: WQV = Water Quality Volume (Acre-Feet)
/ = Impervious Fraction in the Watershed
A = Watershed Area (Acres)
2. Calculate the detention storage volume.
Total detention storage was determined using standard peak flow methods (USDA/NRCS,
1986). Detention storage was calculated for a 5-inch storm.
3. Calculate total volume.
Many BMPs do not require any detention storage, but for BMPs that do provide flood
storage, the total volume is the sum of the water quality and detention volumes calculated
in steps 1 and 2.
4. Determine the construction cost.
The construction cost for each BMP is determined based on equations described in
Section 6.1.1.
6 Although these evaluations are useful for comparing potential costs of various structural
BMPs, they should not be applied for use in all areas of the country. In addition, the BMPs,
selected in these examples and the sizing criteria that the costs were based on should not be
considered as recommendations for actual BMP selection and design. They are presented solely
for illustrative purposes.
7 "Water quality volume" refers to the volume of water that the BMP is designed to treat.
For example, a BMP may be designed to capture the first inch of runoff from the drainage area.
Any volume of rainfall over the first inch would bypass the BMP. Therefore water quality volume
for this BMP would be one watershed inch.
6- 16
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6.1.5.1 5-Acre Commercial Development
The following data were used as the basis for the 5-acre commercial development.
Table 6-11. Data for the
Commercial Site Scenario
Area (A)
Impervious Cover (I)
Water Quality Volume
P Rv A/12
P= 1" of rainfall
Rv = 0.5 + 0.9 (I)
A = Drainage Area
Total Detention
Storage
(using TR-55 model)
Total Storage
5 acres
65%
0.26
0.74
1.00
ac-ft
ac-ft
ac-ft
These data were then used to compare various BMP options (Table 6-12). Grassed
swales and filter strips were not included in this analysis because, although they do improve water
quality, they are typically used only in combination with other BMPs in a new development area.
Again, it is important to note that the cost of land is not included in this calculation. Although
retention basins are the least expensive option on an annual basis, the cost of land may drive
designs to less space-consumptive BMPs, such as sand filters or bioretention systems.
6- 17
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Table 6-12. BMP Costs for a Five Acre Commercial Development
BMP
Type
Retention
Basin
Infiltration
Trench
Infiltration
Basin
Sand Filter
Bioretention
Construction
Cost
Equation
18.5V/'70
3.9WQV
+2,900
1.3WQV
4WQV
5.30WQV
Construction
Cost
$32,700
$47,100
$14,700
$45,200
$60,000
Typical Design,
Contingency & Other
Capital Costs (30% of
Construction Costs)
$9,810
$14,100
$4,410
$13,600
$18,000
Annual
Maintenance
Costs (% of
Construction, $)
5%; $1,640
12%; $5,650
8%; $1,180
12%; $5,420
6%; $3,600
Notes
Much of the cost associated with
this BMP is the extra storage to
provide flood control and channel
protection. Ponds are very reliable.
Although infiltration trenches are
designed to last a long time, they
need to be inspected and rebuilt
if they become clogged.
Infiltration basins require careful
siting and design to perform
effectively..
Sand filters require frequent
maintenance in order to function
long-term.
Bioretention is a relatively new
BMP. Little is known about its
long-term performance.
Sources
a, b, c,
d,e
c, d, e
b, c, d, e
a, e, f
a, d
1. WQv = Water Quality Volume, cu. ft. 2. Vt = Total Volume, cu. ft.
3. Sand filter volume was estimated at 4 WQv, which is slightly high, to account for the relatively small drainage area.
a. Brown and Schueler, 1997b b. Wiegand et at, 1986 c. Schueler, 1987 d. SWRPC, 1991 e. US EPA, 1993a f. Livingston etal, 1997
6- 18
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6.1.5.2 38-Acre Residential Development
The following data were used as the basis for the 38-acre residential development.
Table 6-13. Data for the
Residential Site Scenario
Area (A)
Impervious Cover (I)
Water Quality Volume
Total Detention Storage
(using TR-55 model)
Total Storage
38 acres
36%
1.1 ac-ft
2.8 ac-ft
3. 9 ac-ft
The same analysis conducted for the commercial site was repeated for the larger site
(Table 6-14). Bioretention and infiltration systems were not included in this analysis, because
these BMPs are best applied on smaller sites. The costs of swales and filter strips were also not
included, although they could be effectively used in combination with retention systems to provide
pretreatment.
6.1.6 Adjusting Costs Regionally
The cost data in these examples can be adjusted to specific zones of the country using the
regional cost adjustment factors in Table 6-3. For example, if costs for Rainfall Zone 1 were
needed, the data in Tables 6-12 or 6-14 would be multiplied by 1.12.
In addition, design variations in different regions of the country may cause prices to be
changed. For example, wetland and wet ponds may be restricted in arid regions of the country.
Furthermore, while retention basins are used in semi-arid regions, they usually incorporate design
variations to improve their performance (Saunders and Gilroy, 1997). In cold regions, BMPs may
need to be adapted to account for snowmelt treatment, deep freezes and road salt application
(Oberts, 1994; Caraco and Claytor, 1997), which will cause additional changes in BMP costs.
6- 19
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Table 6-14. BMP Costs for a Thirty-Eight Acre Residential Development
BMP
Type
Retention
Basin
Sand Filter
Construction
Cost Equation
18.5Vtฐ-70
2WQV
Construction
Cost
$84,800
$95,800
Design, Contingency
and other Capital
Costs (30% of
Construction)
$25,400
$28,700
Annual Maintenance
Costs (% of
Construction; $)
5%; $4,240
12%; $11,500
Notes
Pond systems are
relatively easy to apply to
large sites.
Although the sand filter is
used in this example,
some evidence suggests
that sand filters may be
subject to clogging if used
on a site that drains a
relatively pervious
drainage area such as this
one.
Sources
a, b, c, d, e
a, e, f
1. WQv = Water Quality Volume, cu. ft. 2. Vt = Total Volume, cu. ft.
3. Sand filter volume was estimated at 2V, which is slightly low, to account for the relatively large drainage area
a. Brown and Schueler, 1997b b. Wiegand et al, 1986 c. Schueler, 1987 d. SWRPC, 1991 e. US EPA, 1993a f. Livingston etal, 1997
6-20
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6.2 Non-Structural BMP Costs
Non-structural BMPs are management measures that prevent degradation of water
resources by preventing pollution at the source, rather than treating polluted runoff. Non-
structural practices include a variety of site-specific and regional practices, including: street
sweeping, illicit connection identification and elimination, public education and outreach, land use
modifications to minimize the amount of impervious surface area, waste collection and proper
materials storage. While non-structural practices play an invaluable role in protecting surface
waters, their costs are generally not as easily quantified as for structural BMPs. This is primarily
because there are no "design standards" for these practices. For example, the cost of a public
education program may vary due to staff size. However, it is possible to identify costs associated
with specific components of these programs based on past experience.
6.2.1 Street Sweeping
The costs of street sweeping include the capital costs of purchasing the equipment, plus
the maintenance and operational costs to operate the sweepers, as well as costs of disposing the
materials that are removed. Both equipment and operating costs vary depending on the type of
sweeper selected. There are several different options for sweepers, but the two basic choices are
mechanical sweepers versus vacuum-assisted sweepers. Mechanical sweepers use brushes to
remove particles from streets. Vacuum-assisted dry sweepers, on the other hand, use a
specialized brush and vacuum system in order to remove finer particles. While the equipment
costs of mechanical sweepers are significantly higher, the total operation and maintenance costs of
vacuum sweepers can be lower (Table 6-15).
Table 6-15. Street Sweeper Cost Data
Sweeper Type
Mechanical
Vacuum-assisted
Life
(Years)
5
8
Purchase
Price ($)
75,000
150,000
Operation and
Maintenance
Costs ($/curb
mile)
30
15
Sources
Finley, 1996; SWRPC, 1991
Satterfield, 1996; SWRPC,
1991
Using these data, the cost of operating street sweepers per curb mile were developed,
assuming various sweeping frequencies (Table 6-16). The following assumptions were made to
conduct this analysis:
One sweeper serves 8,160 curb miles during a year (SWRPC, 1991).
6-21
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The annual interest rate is 8 percent.
Table 6-16. Annualized Sweeper Costs ($/curb mile/year)
Sweeper
Type
Mechanical
Vacuum-
Assisted
Sweeping Frequency
Weekly
1,680
946
Bi-weekly
840
473
Monthly
388
218
Four times
per year
129
73
Twice
per year
65
36
Annual
32
18
Modified from Finley, 1996; SWRPC, 1991; and Satterfield, 1996
6.2.2 Illicit Connection Identification and Elimination
One source of pollutants is direct connections or infiltration to the storm drain system of
wastewaters other than storm water, such as industrial wastes. These pollutants are then
discharged through the storm drain system directly to streams without receiving treatment. These
illicit connections can be identified using visual inspection during dry weather or through the use
of smoke or dye tests. Using visual inspection techniques, illicit connections can be identified for
between $1,250 and $1,750 per square mile (Center for Watershed Protection, 1996).
6.2.3 Public Education and Outreach
Public education programs encompass many other more specific programs, such as
fertilizer and pesticide management, public involvement in stream restoration and monitoring
projects, storm drain stenciling, and overall awareness of aquatic resources. All public education
programs seek to reduce pollutant loads by changing people's behavior. They also make the
public aware of and gain support for programs in place to protect water resources. Most
municipalities have at least some educational component as a part of their program. A recent
survey found that 30 of the 32 municipal storm water programs surveyed (94 percent) incorporate
an education element and 11 programs (34 percent) mandated this element in law or regulation
(Livingston et al, 1997).
The City of Seattle, with a population of approximately 535,000, has a relatively
aggressive education program, including classroom and field involvement programs. The 1997
budget for some aspects of the program is included in Table 6-17. Although this does not
necessarily reflect typical effort or expenditures, it does provide information on some educational
expenditures. These data represent only a portion of the entire annual budget.
6-22
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Table 6-17. Public Education Costs in Seattle, Washington
Item
Supplies for Volunteers
Communications
Environmental Education
Education Services /
Field Trips
Teacher Training
Equipment
Water Interpretive
Specialist: Staff
Water Interpretive
Specialist: Equipment
Youth Conservation Corps
Description
Covers supplies for the Stewardship Through
Environmental Partnership Program
Communications strategy highlighting a newly
formed program within the city
Transportation costs from schools to field visits
(105 schools with four trips each)
Fees for student visits to various sites
Covers the cost of training classroom teachers
for the environmental education program
Equipment for classroom education, including
displays, handouts, etc.
Staff to provide public information at two
creeks
Materials and equipment to support interpretive
specialist program
Supports clean-up activities in creeks
1997 Budget
$17,500
$18,000
$46,500
$55,000
$3,400
$38,800
$79,300
$12,100
$210,900
Source: Washington DOE, 1997
Some unit costs for educational program components (based on two different programs) are
included in Table 6-18.
6-23
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Table 6-18. Unit Program Costs for Public Education Programs
Item
Public Attitude
Survey
Flyers
Soil Test Kit*
Paint
Safety Vests for
Volunteers
Cost
$1,250-$1,750 per 1,000
households
10-2507 flyer
$10
25-300/SD Stencil
$2
Source
Center for Watershed Protection,
1996
Ferguson et al, 1997
Ferguson et al, 1997
Ferguson et al, 1997
Ferguson et al, 1997
* Includes cost of testing, but not sampling.
Although public education has the intended benefit of raising public awareness, and
therefore creating support of environmental programs, it is difficult to quantify actual pollutant
reductions associated with education efforts. Public attitudes can be used as a gauge of how these
programs perform, however. In Prince George's County, Maryland a public survey was used in
combination with modeling to estimate pollutant load reductions associated with public education
(Smith et al, 1994; Claytor, 1996; Figure 6-5). An initial study was conducted to estimate
pollution from field application of fertilizers, and use of detergents, oil and antifreeze. Pollutant
reductions were then completed assuming that 70 percent of the population complied with
recommendations of the public education program. A follow-up survey was used to assess the
effectiveness of the program. Although insufficient data were available to support a second model
run, a follow-up survey indicated that educational programs influenced many citizen behaviors,
such as recycling. They were unsuccessful, however, at changing the rate at which citizens apply
lawn fertilizers.
6-24
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Figure 6-5. Changes in Pollutant Load Associated with a Public Education Program
Based on a Public Survey
15
ra
o
o
o
o
ra
o
o
Q.
10
Pre-Program
Post-Program
Phosphorous
Nitrogen
Antifreeze
Motor Oil
Detergent
Source: Claytor, 1996
6.2.4 Land Use Modifications
One of the most effective tools to reduce the impacts of urbanization on water resources is
to modify the way growth and development occurs across the landscape. At the jurisdictional or
regional level, growth can be managed to minimize the outward extension of development.
Jurisdictions can direct growth away from environmentally sensitive areas using such techniques
as open space preservation, re-zoning or the transfer of development rights. At the site level, the
nature of development can be modified to reduce the impacts of impervious cover at individual
development projects through techniques such as reduced street widths, clustered housing,
smaller parking lots, and incorporation of vegetative BMPs into site design. While there are legal
fees associated with changing both local and regional zoning codes, data suggest that
concentrating development and minimizing impervious cover at the site level can actually reduce
construction costs to both developers and local governments.
By concentrating development near urban areas, the capital costs of development can be
lowered substantially due to existing infrastructure and other public services. With conventional
development patterns, the cost of servicing residential developments exceeds the tax revenues
from these developments by approximately 15 percent (Pelley, 1997). By encouraging growth to
occur in a compact region, rather than over a large area, these capital costs can be reduced
substantially (Table 6-19).
6-25
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Table 6-19. Comparison of Capital Costs of Municipal
Infrastructure for a Single Dwelling Unit
Development Pattern
Compact Growth2
Low-Density Growth (3 units/acre)
Low-Density Growth, 10 Miles from
Existing Development3
Capital Costs1
(1987 Dollars)
$18,000
$35,000
$48,000
Notes
1. Costs include streets (full curb and gutter), central sewers and water
supply, storm drainage and school construction.
2. Assumes housing mix of 30% single family units and townhouses; 70%
apartments.
3. Assumes housing is located 10 miles from major concentration of
employment, drinking water plant and sewage treatment plant.
Source: Frank, 1989
Savings can also be realized at the site level by reducing the costs of clearing and grading,
paving and drainage infrastructure. A recent study compared conventional development plans
with alternative options designed to reduce the impacts of development on the quality of water
resources. The cost savings realized through these alternative options are summarized in Table 6-
20. In all site designs, the road width was reduced from 28 feet to 20 feet, lot sizes were reduced
or reconfigured to consume less open space, and on-site storm water treatment was provided.
6-26
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Table 6-20. Impervious Cover Reduction and Cost
Savings of Conservation Development
Location
Sussex
County,
DE
New
Castle
County,
DE
Kent
County,
DE
Techniques Used
1 . Reduced street widths
2. Smaller lots
3. Cluster development
4. Houses clustered into attached
units around courtyards
5. Reduced road and driveway
widths
6. Minimum disturbance boundary
Impervious Cover
Reduction
38%
6%
24%
Cost
Savings
52%
63%
39%
Source: Delaware DNREC, 1997
6.2.5 Oil and Hazardous Waste Collection
Providing a central location for the disposal of oil or hazardous wastes protects water
quality by offering citizens an alternative to disposing of these materials in the storm drain.
Disposal costs vary considerably depending on the size of the program, and what types of wastes
are collected. One study estimated the capital costs at approximately $30,000, with about
$12,000 maintenance for a used oil collection recycling program in a typical MS4 (US EPA,
1998b). This estimate was based on data from the Galveston Bay National Estuary Program.
Data from the City of Livonia, Michigan indicates that the cost of hazardous waste disposal
averages about $12 per gallon (Ferguson et al, 1997).
6.2.6 Proper Storage of Materials
Proper storage of materials can prevent accidental spills or runoff into the storm drain.
The design of storage structures varies depending on the needs of the facility. There are also
training costs associated with the proper storage of materials. Typical cost estimates, based on
standard construction data, are $6 to $11 per square foot for pre-engineered buildings and $3.40
to $5 per square foot for a 6-inch thick concrete slab (Ferguson et al, 1997).
6-27
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6.3 Benefits of Storm Water BMPs
Although it is possible to estimate the economic benefits of water quality improvement
(US EPA, 1983a), it is difficult to create a "balance sheet" of economic costs and benefits for
individual BMPs. Ideally, benefits analysis would specify and quantify a chain of events: pollutant
loading reductions achieved by the BMP; the physical-chemical properties of receiving streams
and consequent linkages to biologic/ecologic responses in the aquatic environment; and human
responses and values associated with these changes. However, the necessary data to conduct
such an analysis does not currently exist. Instead, the benefits can be outlined in terms of: 1)
effectiveness at reducing pollutant loads; 2) direct water quality impacts; and 3) economic benefits
or costs.
6.3.1 Storm Water Pollutant Reduction
A primary function of storm water BMPs is to prevent pollutants from reaching streams
and rivers. While all BMPs achieve this function to some extent, there is considerable variability
between different types of BMPs. The extent of benefits from non-structural BMPs may be more
speculative, partly because their ability to influence human behavior is difficult to predict.
A detailed discussion of pollution removal efficiencies for individual structural BMPs is
provided in Section 5.5 of this report, so only non-structural BMPs will be reviewed in this
section. Unlike structural BMPs, it is generally not possible to associate specific pollutant
removal rates with non-structural BMPs, with the exception of street sweeping (Satterfield,
1996). However, some non-structural BMPs are targeted at specific pollutants. Table 6-21
outlines non-structural BMPs believed by designers to be the most effective for removing specific
types of pollutants.
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Table 6-21. Non-Structural BMPs Suited to Controlling Various Pollutants
Pollutant
Solids
Oxygen-Demanding
Substances
Nitrogen and
Phosphorus
Pathogens
Petroleum
Hydrocarbons
Metals
Synthetic
Organics
Temperature
pH
Appropriate BMPs
Street Sweeping
Street Sweeping
Education: Storm Drain Stenciling
Land Use Modifications
Street Sweeping
Education: Pet Scoop Ordinance
Land Use Modifications
Proper Materials Handling
Illicit Connections Eliminated
Land Use Modifications
Street Sweeping
Education: Storm Drain Stenciling
Proper Materials Handling
Street Sweeping
Education: Storm Drain Stenciling
Proper Materials Handling
Illicit Connections Eliminated
Education: Storm Drain Stenciling
Proper Materials Handling
Land Use Modifications
Education: Pet Scoop Ordinance
Illicit Connections Eliminated
Illicit Connections Eliminated
Education: Lawn Care
Materials Storage and Recycling
Education: Pet Scoop Ordinance
Illicit Connections Eliminated
Materials Storage and Recycling
Land Use Modifications
Illicit Connections Eliminated
Materials Storage and Recycling
Land Use Modifications
Education: Lawn Care
Materials Storage and Recycling
Land Use Modifications
Land Use Modifications
Illicit Connections Eliminated
Proper Materials Handling
Materials Storage and Recycling
Land Use Modifications
6.3.1.1 Solids
Both highway runoff and soil erosion can be sources of solids in urban runoff. Street
sweeping can reduce solids in urban runoff by removing solids from roadways and parking lots
before they can be detached and transported by runoff. The benefits associated with street
sweeping depend largely on the climate. In arid regions, airborne pollutants are a serious concern,
and there is a long time between storms for pollutants to accumulate8. In humid regions, on the
other hand, frequent rainfall makes the use of sweepers between storms less practical. In colder
8 Therefore, regular sweeping programs in these areas can potentially remove large
amounts of solids from roadways.
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regions, sweeping is recommended twice per year: once in the fall after leaves fall and once in the
spring in anticipation of the spring snowmelt (MFC A, 1989).
Modifying land use to preserve open space and to limit the impervious cover can also
reduce solids loads. By preserving open space and maintaining vegetative cover, the amount of
land cleared is limited, thus reducing the erosion potential during construction. Natural vegetated
cover has less than one percent of the erosion potential of bare soil (Wischmeier and Smith,
1978).
6.3.1.2 Oxygen-Demanding Substances
Since the primary oxygen-demanding substances are organic materials (such as leaves and
yard waste), BMPs that target these substances are best suited to reducing the oxygen demand in
storm water. BMPs that reduce sediment loads often also reduce the loads of the organic material
associated with that sediment. Pet waste is also a significant source of organic pollutants, and its
control can reduce the loads of oxygen demanding substances in urban runoff. Finally, programs
geared at reducing illegal dumping and eliminating illicit connections and accidental spills of
materials can reduce the oxygen demand associated with these sources.
6.3.1.3 Nitrogen and Phosphorus
Nitrogen and phosphorus are prevalent in urban and suburban storm water. Nitrogen and
phosphorus are natural components of soil, and can enter runoff from storm-induced erosion.
Additional sources include the use of fertilizer on urban lawns and airborne deposition. Street
sweeping can reduce nutrient loads by removing deposited nutrients from the street surface.
Programs that focus on lawn chemical handling or replacing turf with natural vegetation also act
to reduce nutrient loading. Finally, programs that educate the public or industry about illegal
dumping to storm drains can result in reducing the nutrient loads associated with dumping
chemicals that have high nutrient content. Energy conservation and reduced automobile use can
reduce airborne nitrogen deposition.
6.3.1.4 Pathogens
Pathogens, including protozoa, viruses and bacteria, are prevalent in urban runoff.
Bacteria can be found naturally in soil, and the urban landscape can produce large loads of
bacteria that can be carried by runoff. Dogs in particular can be a significant source of pathogens.
Thus, pet scoop ordinances and associated education are effective tools at reducing bacteria in
urban runoff. Illicit connections of sewage may also be a source of pathogens, therefore
eliminating these sources can effectively reduce pathogens in runoff.
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6.3.1.5 Petroleum Hydrocarbons
Petroleum hydrocarbons are present in many chemicals used in the urban environment,
from gasoline to cleaning solvents. Since roadways are a major source of petroleum pollution,
scheduled street sweeping can be used to remove hydrocarbon build-up prior to storm water
runoff. Programs geared at preventing spills of chemicals to the storm drain, either through
deliberate or accidental dumping, are effective at reducing hydrocarbon loads. Modifying the way
land is developed can reduce hydrocarbon loads on both a site and a regional level by reducing the
use of the automobile and replacing impervious surfaces with natural vegetation.
6.3.1.6 Metals
Metals sources in urban runoff include automobiles and household chemicals, which can
contain trace metals. Street sweeping can reduce metals loads deposited on the road surface. In
addition, programs that focus on reducing dumping and proper material storage can reduce
accidental or purposeful spills of chemicals with trace metals to the storm drain system. Finally,
modifying land use can reduce metals loads by reducing impervious cover, thus reducing total
runoff containing metals, and reducing the roadway length, which is often a source of runoff
containing metals.
6.3.1.7 Synthetic Organics
Much of the source of synthetic organics in the urban landscape is household cleaners and
pesticides. Thus, education programs geared at reducing chemical and pesticide use, and proper
storage and handling of these chemicals, can reduce their concentrations in urban runoff. In
addition, land use modifications that replace turf with natural vegetation will reduce pesticide use.
6.3.1.8 Temperature
Most non-structural BMPs are not able to prevent the increase in temperature associated
with urban development. One exception is the use of site designs that more closely mimic the
natural hydrograph by reducing impervious cover and encouraging infiltration.
6.3.1.9 pH
The primary source of low pH in urban runoff is acid rain, and most non-structural BMPs
are not used to treat this problem. BMPs that focus on proper materials handling and disposal can
prevent dumping of chemicals with extremely high or low pH, but this is generally not a major
problem in urban watersheds.
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6.3.2 Hydrological and Habitat Benefits
As reviewed in Chapter 4, one major impact of urbanization is induced through the
conversion of farmland, forests, wetlands, and meadows to rooftops, roads, and lawns. This
process of urbanization has a profound influence on surface water hydrology, morphology, water
quality, and ecology (Horner et al, 1994). In this section, the hydrologic and related habitat
impacts are briefly discussed as well as the potential benefits that can be achieved by managing
storm water runoff using structural and non-structural BMPs.
Many of these impacts can be directly or indirectly related to the change in the hydrologic
cycle from a natural system to the urban system. Figure 4-1 illustrates the fundamental effects
that occur along with the development process. In the natural setting, very little annual rainfall is
converted to runoff and about half is infiltrated into the underlying soils and water table. This
water is filtered by the soils, supplies deep water aquifers, and helps support adjacent surface
waters with clean water during dry periods. In the urbanizing conditions, less and less annual
rainfall is infiltrated and more and more volume is converted to runoff. Not only is this runoff
volume greater, it also occurs more frequently and at higher magnitudes. The result is that less
water is available to streams and waterways during dry periods and more flow is occurring during
storms. A recent study in the Pacific Northwest found that the ratio of the two-year storm to the
baseflow discharge increased more than 20 percent in developed sub-watersheds (impervious
cover approximately 50 percent) versus undeveloped sub-watersheds (May et al, 1997).
As a result of urbanization, runoff from storm events increases and accelerates flows,
increases stream channel erosion, and causes accelerated channel widening and down cutting
(Booth, 1990). This accelerated erosion is a significant source of sediment delivery to receiving
waters and also can have a smothering effect on stream channel substrates, thereby eliminating
aquatic species habitat. As a result, aquatic habitat is often degraded or eliminated in many urban
streams. The results are that aquatic biological communities are among the first to be impacted
and/or simplified by land conversion and resulting stream channel modifications. Subsurface
drainage systems which frequently serve urbanized areas also contribute to the problem, by
bypassing any attenuation achieved through surface flows over vegetated areas.
A unifying theme in stream degradation is this direct link with impervious cover.
Impervious cover, or imperviousness, is defined as the sum of roads, parking lots, sidewalks,
rooftops, and other impermeable surfaces in the urban landscape. This unifying theme can be
used to guide the efforts of the many participants in watershed protection. Figure 6-6 visually
illustrates this trend in degradation for a series of small headwater streams in the Mid-Atlantic
Piedmont. Here, four stream segments, each with approximately the same drainage area, and
subjected to the same physiographic conditions, respond to the effects of increased impervious
cover. Similar results have been observed in the Southern United States with studies in Virginia,
North Carolina and Georgia evidencing this same decline in fish and macroinvertebrate
populations with increasing impervious cover (Crawford and Lenant, 1989; Weaver and Garman,
1994; Couch et al, 1996)
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Figure 6-6. Effects of Impervious Cover on Stream Quality
Sensitive Stream -4
(Impervious Cover <10%)
- Stable Channel
- Excellent Biodiversity
- Excellent Water Quality
Restorable Stream -ป
(Impervious Cover -40%)
- Highly Unstable Channel
- Poor Biodiversity
- Poor to Fair Water Quality
Non-Supporting Stream -4
(Impervious Cover -65%)
- Poor to No Biodiversity
- Poor Water Quality
Impacted Stream
(Impervious Cover 10-20%)
- Channel Becoming Unstable
- Fair to Good Biodiversity
- Fair to Good Water Quality
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To mitigate this impact, many local and state governments have required the installation of
storm water management detention basins to attenuate this increased runoff volume. It is
important to recognize that the change in hydrology caused by urbanization affects more than just
a single storm return interval (e.g., the two-year event). Urbanization shifts the entire "rainfall
frequency spectrum" (RFS) to a higher magnitude. As illustrated in Figure 6-7, the most
significant change is to the smallest, most frequent storms that occur several times per year. In
the undeveloped condition, most of the rainfall from these events is infiltrated into the underlying
soil. In the developed condition, much of this rainfall is runoff. As the storm return interval
increases, the difference between the undeveloped and developed condition narrows. Many
jurisdictions only require management of specific storms, usually the two, ten and sometimes, the
one hundred year events. The two-year storm is probably the most frequently used control point
along this frequency spectrum. Hence, while BMPs may do a fairly good job of managing these
specific control points, there have been very few locations across the country that have specific
criteria in place to manage storm water over a wide range of runoff events. Claytor and Schueler
(1996) describe the RFS as:
...classes of frequencies often broken down by return interval, such as the two year storm return
interval. Four principal classes are typically targeted for control by stormwater management
practices. The two smallest, most frequent classes [Zones 1 and 2] are often referred to as water
quality storms, where the control objectives are groundwater recharge, pollutant load reduction,
and to some extent control of channel erosion producing events. The two larger classes [Zones 3
and 4] are typically referred to as quantity storms, where the control objectives are channel
erosion control, overbank control, and flood control.
Figure 6-7. Stormwater Control Points Along the Rainfall
Frequency Spectrum
Rainfal
Volume
(inches
8
6
4
2
0
0
1
'
;
Zone*
Ground Wafer Re*
and Water Quality
Zone 2
Water Qu
Channel I
Protectto
charge
^
ality and
Erosion
n
/
/
Zone 3
Channel Erosion
and
Flood Control
/
/
/
/
Zone 4 *
Flood Contro^
/
/
.01 0.1 1 10 100
Rainfall Recurrence Interval (Years)
Source: Claytor and Schueler, 1996
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One recent study by MacRae (1997) concluded that stream channels below storm water
detention basins designed to manage the two year storm experienced accelerated erosion at three
times the pre-developed rate. His findings went on to suggest that the streams were eroding at
much the same rate as if no storm water controls existed.
Other jurisdictions have employed an additional level of detention storage above and
beyond that required for the two year storm. This concept is often called "extended detention"
(ED). McCuen and Moglen (1988) conducted a theoretical analysis of this design criteria based
on sediment transport capacity of the pre-developed channel versus that with ED control. This
study found ED could produce an 85 percent reduction in the pre-developed peak flow of the
two-year storm. What it did not analyze, however, was the erosion potential over a wide range of
storms. MacRae (1993) suggested a different storm water control criterion called "distributed
runoff control" (DRC). Here, channel erosion is minimized if the erosion potential along a
channel's perimeter is maintained constant with pre-developed levels. This is accomplished by
providing a non-uniform distribution of the storage-discharge relationship within a BMP, where
multiple control points are provided along the runoff frequency spectrum.
6.3.2.1 Benefits of BMPs to Control Hydrologic Impacts
Numerous prior studies have documented the degradation of aquatic ecosystems of urban
and suburban headwater streams. As stated above, in general, the studies point to a decrease in
stream quality with increasing urbanization. Unfortunately, the benefits of BMPs to protect
streams from hydrologic impacts have only recently been investigated and only for a few studies.
Maxted and Shaver (1997), Jones et al (1997), and Horner et al (1997) attempted to
isolate the potential beneficial influence of local storm water best management practices on the
impervious cover/stream quality relationship. Horner examined the possible influence of stream-
side management on stream quality as a function of urbanization. Coffman et al (1998) recently
presented data on the potential hydrologic benefits of alternative land development techniques.
Called the "Low Impact Development Approach," this methodology attempts to mimic pre-
developed hydrology by infiltrating more rainfall at the source, increasing the flow path and time
of concentration of the remaining runoff, and providing more detention storage throughout the
drainage network, as opposed to a one location at the end of the pipe.
The preliminary findings of Maxted and Shaver, and Jones et al, suggest that, for the
BMPs examined, stream quality (as measured by a limited group of environmental indicators)
cannot be sustained when compared to reference stream conditions. Jones assessed several BMPs
by conducting biomonitoring (fish and macroinvertebrate sampling) above and below BMPs and
comparing them to a reference watershed. He found that the biological community tended to be
degraded immediately below BMPs as compared to the reference watersheds. One major flaw in
the study was the lack of analysis in developed watersheds without BMPs. This would have
compared the influence of BMPs on the aquatic community as compared to no BMPs.
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Maxted and Shaver examined eight sub-watersheds with and without BMPs. Their study
also concluded that BMPs did not adequately mitigate the impacts of urbanization once watershed
impervious reached 20 percent cover. While this study was useful in defining the cumulative
impacts of BMPs on watersheds, several critical questions remain. First, since no sub-watersheds
with less than 22 percent impervious cover were analyzed, little is known about BMP ability to
protect the most sensitive species seen in less developed watersheds. Data for sub-watersheds
with BMPs was collected approximately three years after data for the sub-watersheds without
BMPs, so climatic/seasonal constraints may have affected the outcome as much, or more than the
BMPs themselves.
Horner et al (1997) evaluated several sub-watersheds, with varying levels of impervious
cover, but only tangentially related the effectiveness of BMPs to protecting stream quality.
Horner found that at relatively low levels of urbanization (approximately 4 percent impervious
area) the most sensitive aquatic biological communities (e.g., salmonids) were adversely affected,
and stream quality degradation (as measured by a several indicators) continued at a relatively
continuous rate with increasing impervious area. Horner's study demonstrates a link between
urbanization and stream quality in the Puget Sound region, but since the effects of BMPs were not
directly assessed, the question of whether BMPs could "raise" these thresholds could not be
answered.
Horner did find a positive relationship between stream quality and riparian buffer width
and quality. Here, the otherwise direct relationship of degrading stream quality with increasing
impervious cover was positively altered where good riparian cover existed. In other words,
increasing the buffer width and condition tended to keep the stream systems healthier.
Coffman demonstrated techniques for maintaining pre-developed hydrologic parameters
by replicating the curve number and time of concentration. The analysis indicated the amount of
storage required on-site to accommodate the change in site imperviousness. The benefits of this
type of development, while not yet fully monitored in a field study, are likely to include increased
groundwater recharge, reduced channel erosion potential, and decreased flood potential.
One major hydrologic benefit of storm water management structures is the ability to
mitigate for the potential flooding associated with medium to larger storms. Storm water
detention and retention facilities have been applied in many parts of the country since about 1970
(Ferguson and Debo, 1990). These facilities include wet and dry basins, as well as rooftop and
parking lot detention and underground storage vaults. These storage facilities attempt to reduce
flooding downstream from developments by reducing the rate of flow out of the particular
structure being used. Although the rate of flow is reduced, the volume of flow is generally not
reduced. Instead, this volume is delivered downstream at a slower rate, and stretched out over a
longer time. With the exception of properly design wet ponds, these structures do not provide
any water quality benefit beyond the hydrologic modifications. This technique has proved to be a
successful method of suppressing flood peaks when properly applied on a watershed-wide basis.
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6.3.3 Human Health Benefits
Storm water can impact human health through direct contact from swimming or through
contamination of seafood. Most human health problems are caused by pathogens, but metals and
synthetic organics may cause increased cancer risks if contaminated seafood are consumed.
Mercury, PCBs, and some pesticides have been linked to human birth defects, cancer,
neurological disorders and kidney ailments. The risks may be greater to sensitive populations
such as children or the elderly. BMPs that reduce pathogens, metals and synthetic organics will
help to limit these health risks.
Economic benefits of avoiding human health problems can include swimming and
recreation costs, as well as saved medical costs. One study in Saginaw, Michigan estimated that
the swimming and beach recreation benefits associated with a CSO retention project exceeded
seven million dollars (US EPA, 1998c). As another example, EPA initially estimated that
proposed Phase II storm water controls would reduce the cost of shellfish-related illnesses by
between $73,000 and $300,000 per year (US EPA, 1997d).
6.3.4 Additional and Aesthetic Benefits
Storm water BMPs can be perceived as assets or detriments to a community, depending
on their design. Some examples of benefits include: increased wildlife habitat, increased property
values, recreational opportunities, and supplemental uses. Detriments include: mosquito breeding,
reduced property values, less developable land and safety concerns. These detriments can be
mitigated through careful design.
6.3.4.1 Property Values and Public Perception
The impacts of BMPs on property values are site-specific. The presence of a structural
BMP can affect property values in one of three ways: increase the value, decrease the value, or
have no impact. BMPs that are visually aesthetic and safe for children can lead to increased
property values. A practice becoming more prevalent is to situate developments around man-
made ponds, lakes, or wetlands created to control flooding and reduce the impacts of urban
runoff. Buffer zones and open areas that control runoff also provide land for outdoor recreation
such as walking or hiking and for wildlife habitat. In many cases, developers are able to realize
additional profits and quicker sales from units that are adjacent to such areas. A survey of
residents in an Illinois subdivision indicates that residents are willing to pay between 5 percent and
25 percent more to be located next to a wet pond, but that being located next to a poorly-
designed dry detention basin can reduce home values (Emmerling-Dinovo, 1995).
Safety is also a concern among the public. A childless adult may perceive a wet pond as
an amenity, but a family might view it as a potential hazard to children. These concerns can be
alleviated using such design features as gently sloping edges, a safety "bench" (a flat area
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surrounding a pond) and the use of dense vegetation surrounding ponds and infiltration basins to
act as a barrier.
Aesthetic maintenance is also important when considering long term impacts on property
values. Poorly-maintained wet ponds or constructed wetlands may be unsightly due to excess
algal growth or public littering. Wet ponds and constructed wetlands can also become mosquito
breeding grounds. However, mosquito problems can usually be reduced or eliminated through
proper design and/or organic controls such as mosquito-eating fish. Successful designs avoid
shallow or stagnant water, and reduce large areas of periodic drying, as occur in a dry detention
basin (McLean, 1995). All BMPs need to have trash and debris removed periodically to prevent
odor and preserve aesthetic values.
6.3.4.2 Dual-Use Systems
Since BMPs can consume a large amount of space, communities may opt to use these
facilities for other purposes in addition to storm water management. Two examples are "water
reuse" ponds and dual use infiltration or detention basins. In one study, a storm water pond was
used to irrigate a golf course in Florida, decreasing the cost of irrigation by approximately 85
percent (Schueler, 1994b). In the southwestern United States, BMPs are often completely dry in
between rain events. In these regions, it is very common to design infiltration basins or detention
basins as parks that are maintained as a public open space (Livingston et al, 1997).
6.4 Review of Economic Analysis of the NPDES Phase II Storm Water Rule
The proposed storm water Phase II rule specifies that Phase II municipalities and
operators of construction sites disturbing between one and five acres of land must apply for and
receive a storm water permit. To meet this requirement, municipalities must develop a storm
water pollution prevention plan that addresses six minimum measures9. Operators of construction
sites are required to incorporate soil and erosion controls into their construction sites and
implement a water pollution prevention plan. The analysis presented here is a summary of the
most recent benefit-cost analysis prepared for the proposed Phase II storm water rule (Preliminary
draft number 3). In order to address the issues raised in the public comments and during internal
review, EPA gathered additional data and information to refine the analysis of potential benefits
and costs conducted for the proposed Phase II rule. These data, analyses, and results are
described in detail in the Preliminary Draft of the Economic Analysis of the Final Phase II Storm
9 The six minimum measures are:
Public Education and Outreach on Storm Water Impacts
Public Involvement/Participation
Illicit Discharge Detection and Elimination
Construction Site Storm Water Runoff Control
Post-Construction Storm Water Management in New Development and Redevelopment
Pollution Prevention/Good Housekeeping for Municipal Operations (US EPA, 1998c).
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Water Rule ("EA"), and are summarized in the sections that follow. All cost and benefit estimates
are presented in 1998 dollars.
The reader should note that the Agency continues to revise the analysis based on internal
review and new data and information. EPA envisions completing the economic analysis in
conjunction with the Storm Water Phase II Final Rule. Hence, all estimates are subject to future
refinement.
6.4.1 Analyses of Potential Costs
This section provides an overview of the methodology used to estimate costs and
pollutant loading reductions for both municipalities and construction sites subject to the final
Phase II rulemaking. The specific components of the analysis are discussed in detail in the Draft
Final EA. Current Agency estimates of national compliance costs, which are subject to change,
are also provided.
6.4.1.1 Municipal Costs
EPA estimated annual per household program cost for automatically designated
municipalities (MS4s) using actual expenditures reported by 35 Phase I municipalities. Based on
census data, EPA estimated the Phase II municipal universe to be 5,040 MS4s with a total
population of 85 million people and 32.5 million households. An average annual per household
administrative cost was estimated to address application, record keeping, and reporting
requirements, which was added to the program per household cost to derive a total average per
household cost. To obtain the national estimate of compliance costs, the Agency multiplied the
estimated total per household compliance cost ($9.09) by the expected number of households in
Phase II communities. EPA estimates the national Phase II municipal compliance costs to be
approximately $295 million (see Section 4.2.1.3 in the draft EA)10.
6.4.1.2 Construction Costs
In estimating incremental costs attributable to the final Phase II rule, EPA estimated a per
site cost for construction sites of one, three, and five acres and multiplied the cost by the total
number of Phase II construction starts in these size categories to obtain a national estimate of
compliance costs. The Agency used construction start data from eleven municipalities that record
construction start information to estimate the number of construction starts disturbing between
one and five acres of land (see Section 4.2.2.1 in the Draft Final EA).
10 Estimated annual per household cost of compliance ranged from $0.63 to $60.44. See
Section 4.2.1.2 in the Draft Final EA for a discussion of how EPA chose the mean value of $9.09
per household. Note that the estimated per household cost does not include municipal
expenditures for post-construction storm water controls.
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In estimating construction BMP costs, EPA used standard cost estimates from R.S. Means
(R.S. Means, 1997a and 1997b) and created 27 model sites of typical site conditions in the United
States. The model sites considered three different site sizes (1,3, and 5 acres), three slope
variations (3, 7, and 12 percent), and three soil erosivity conditions (low, medium, and high). The
Agency used a database compiled by the Water Environment Federation (1992) to develop and
apply BMP combinations appropriate to the model site conditions. For example, sites with
shallow slopes and a low erosivity require few BMPs, while larger, steeper, and more erosive sites
required more BMPs. Detailed site plans, assumptions, and BMPs that could be used are found in
Appendix B-3 of the Draft Final EA. Based on the assumption that any combination of site
factors are equally likely to occur on a given site, EPA averaged the matrix of estimated costs to
develop an average cost for one, three, and five acre starts for all soil erodibilities and slopes. The
average BMP cost was estimated to be $1,206 for a one-acre site, $4,598 for a three-acre site,
and $8,709 for a five-acre site.
Administrative costs for the following elements were estimated per construction site and
added to each BMP cost: submittal of a notice of intent (NOT) for permit coverage ($74);
notification to municipalities ($17); development of a storm water pollution prevention plan
($1,219); record retention ($2); and submittal of a notice of termination ($17) for a total cost of
$1,329 per site. From this analysis, EPA estimated total average compliance costs (BMP plus
administrative) for a Phase II construction site of $2,535 for sites disturbing between one and two
acres of land, $5,927 for sites disturbing between two and four acres, and $10,038 for sites
disturbing between four and five acres of land.
The total per site costs were then multiplied by the total number of Phase II construction
sites within each of those size categories to obtain the national compliance cost estimate. EPA
estimated construction costs for 15 climatic zones to reflect regional variations in rainfall
intensity and amount. Once the Phase II storm water rule is fully implemented, the total annual
compliance cost is expected to be approximately $512 million (assuming 109,652 construction
starts in 1998).
6.4.1.3 Pollutant Loading Reductions
To estimate municipal pollutant loading reductions for the final Phase II rulemaking, EPA
used the results from a 1997 EPA draft report that calculated national municipal loading
reductions for TSS based on the NURP study (US EPA, 1997d). To estimate pollutant loading
reductions from Phase II construction starts, the U.S. Army Corps of Engineers developed a
model based on EPA's 27 models sites to estimate sediment loads from construction starts with
and without Phase II controls (US ACE, 1998). Estimating the pollutant loading reduction for
TSS does not capture the full extent of potential loading reductions that result from implementing
storm water controls, but provides a minimum estimate of the reductions that may result from the
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Phase II rule11. EPA also anticipates that the rule will result in reductions in oil and grease,
nitrogen, phosphorus, pathogens, lead, copper, zinc and other metals. Estimated annual TSS
loading reductions range from 639,115 to approximately 4 million tons for municipalities and 2
million to 8 million tons for construction sites assuming BMP effectiveness of 20 to 80 percent.
6.4.2 Assessment of Potential Benefits
A number of potential problems are associated with assessing the benefits from the Phase
II rule, including identifying the regulated municipalities as sources of current impairment to
waters and determining the likely effectiveness of various measures; difficulties in water quality
modeling; difficulties in modeling construction site BMP effectiveness; and most importantly, the
inability to monetize some categories of benefits with currently available data.
The national benefits of Phase II controls will depend on a number of factors, including
the number, intensity, and duration of wet weather events; the success of municipal programs; the
effectiveness of the selected construction site BMPs; the site-specific water quality and physical
conditions of receiving waters; the current and potential use of receiving waters; and the existence
of nearby "substitute" sites of unimpaired waters. Because these factors will vary substantially
from site to site, data are not available with which to develop estimates of benefits for each site
and aggregate to obtain a national estimate. As a result, the Agency developed national level
estimates of benefits based largely on a benefits transfer approach. This approach allows
estimates of value developed for one site and level of environmental change to be applied in the
analysis of similar sites and environmental changes.
6.4.2.1 Anticipated Benefits of Municipal Measures
As part of an effort to quantify the value of the United States' waters impaired by storm
water discharges, EPA applied adjusted Carson and Mitchell (1993) estimates of willingness to
pay (WTP) for incremental water quality improvements to estimates of waters impaired by storm
water discharges as reported by states in their biennial Water Quality Inventory reports12.
Potential Phase II benefits are assumed to equal the WTP for the different water quality levels
multiplied by the water quality impairment associated with Phase II municipalities multiplied by
the relevant number of households (WTP x percent impaired x number of households).
The Carson and Mitchell estimates apply to all fresh water, however it is not clear how
these values would be apportioned among rivers, lakes and the Great Lakes. Lakes are the water
11 To date, there are no national studies that estimate pollutant loading reductions due to
the implementation of municipal storm water controls for the other pollutants found in storm
water runoff and discharges.
12EPA adjusted the WTP amounts to account for inflation growth in real per capita
income, inflation, and a 30 percent increase in attitudes towards pollution control.
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bodies most impaired by urban runoff and discharges, followed closely by the Great Lakes and
then rivers. Hence, EPA applied the WTP values to the categories separately and assumed that
the higher resulting value for lakes represents the high end of the range and the lower resulting
value for rivers represents the low end of a value range for all fresh waters (i.e. high end assumes
that lake impairment is more indicative of national fresh water impairment while low end assumes
that river impairment is more indicative).
The extent to which impairment will be eliminated by the municipal measures is uncertain;
hence, estimates are adjusted for a range of potential effectiveness of municipal measures. EPA
expects that municipal programs will achieve at least 80% effectiveness, resulting in estimated
annual benefits from fresh water use and passive use in the range of $67.2 to $241.2 million. The
potential value of improvements in marine waters and human health benefits have not been
quantified at this time.
6.4.2.2 Anticipated Benefits of Construction Site Controls
EPA estimates the benefits of construction site controls using a benefits transfer approach
applying WTP estimates for an erosion and sediment control plan from Paterson et al (1993)
contingent valuation (CV) survey of North Carolina residents. The adjusted WTP estimates are
intended to reflect potential benefits of erosion and sediment control programs that protect all
lakes, rivers, and streams. In order to transfer adjusted WTP results to estimate the potential
benefits of the Phase II rule, EPA calculated the percentage of Phase II construction starts that
are not covered by a state program or CZARA for each state. This percentage is multiplied by the
number of households in the state and the adjusted mean WTP of $25. The results were then
summed across all states and indicate that WTP for the erosion and sediment controls of the
Phase II rule may be as high as $624.2 million per year.
6.4.3 Comparison of Benefits and Costs
EPA estimates the total compliance costs of the rule to be $807.2 million. The largest
portion of the total cost, $512 million, is associated with erosion and sediment controls at
construction sites. EPA was able to develop a partial monetary estimate of expected benefits of
both the six minimum municipal measures and the construction components of the rule. The sum
of these benefits ranges from $700 to $865 million annually [assuming 80 percent effectiveness of
municipal programs and using the mean WTP ($25) from Paterson]. The largest portion of
benefits, $624 million, are associated with erosion and sediment controls for construction sites.
6.5 Financial Issues
Effective storm water programs require both the existence of well-performing, cost-
effective BMPs and sufficient funding. Financing issues are discussed extensively in other Agency
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reports and only briefly reviewed below.13 Section 6.5.1 focuses on financing options for
municipal storm water programs but does not discuss regulatory impacts on municipalities.
6.5.1 Municipal Financing of Storm Water Programs
Around the nation, local government general tax funds are the most commonly used
source of funding for storm water programs. However, this may be the least suitable source of
storm water program or maintenance funding. General tax revenues originate at a number of
sources and are used to finance an equally diverse number of public programs, including
education, police and fire protection, civil and criminal courts, and social and economic support
programs. Storm water programs and maintenance must compete against a large number of other
vital public programs for a very limited number of tax dollars. This problem has been
compounded in recent years by tax caps and the public's general opposition to new or higher
taxes.
The unreliability of general tax funds has led many communities around the country to
develop storm water utilities. Storm water utilities rely on dedicated user charges related to the
level of service provided. Charges are typically paid by property owners and managed in a
separate enterprise fund. A variety of methods are used to determine charges, but are usually
based on some estimate of the amount of storm water runoff contributed by the property, such as
the total impervious surface or a ratio of impervious surface to total property area. Generally a
flat rate is charged for residential properties.
There are several advantages of using utility fees to finance storm water programs. Unlike
general tax revenues, utility charges are a dedicated, stable, and predictable source of funds and
are not subject to state "tax cap" limitations. Also, because charges are based on the user's
contribution to storm water runoff, it is often seen as more equitable or fair. Finally, utility fees
provide a mechanism to incorporate economic incentives for implementation of on-site storm
water management through reduced charges. For example, credits or discounts are often
provided for on-site retention of storm water by nonresidential property owners. Providing such
incentives creates greater flexibility by allowing each user to choose the cheaper option - paying
the utility charge or implementing on-site controls. Storm water utilities are now well established
as an effective financing option. As of 1991, over 100 communities across the country had
instituted storm water utilities (US EPA, 1994a).
Similar to utility fees, the use of inspection or permit fees to help publicly finance storm
water programs represents a relatively new application of an established component of
government revenues. Often, these fees are associated with the issuance of a permit, such as a
13 EPA has prepared publications to assist local governments in planning for program
funding (US EPA, 1994b). More recently the Agency has established an internet site with current
information, the "Environmental Finance Information Network." The website address is
http://www.epa.gov/efmpage/efm.htm .
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building permit, clearing permit, storm water permit, or sewer connection permit. A permit
program based upon fees for annual inspections, such as a storm water discharge or storm water
operating permit, can provide a continuing source of funds. However, many permit or inspection
fees are a one time charge, typically when the facility is first constructed. These are generally not
a good funding source for continuing storm water system maintenance.
Finally, the use of dedicated contributions from land developers may be used to finance
public maintenance of storm water systems. Under this program, the local government assumes
the operation and maintenance of a storm water system constructed as part of a private
development. All or a portion of the estimated required funding for the O&M is obtained through
a one-time contribution by the land developer to a dedicated account which is controlled by the
local government. Often the developer is responsible for O&M during a "warranty period,"
frequently the first two years. Dedicated contributions provide a secure, dedicated funding source
that is not subject to state tax cap limits. A disadvantage is that dedicated contributions are only
applicable to new storm water systems.
6.6 Summary
The use of BMPs to control storm water runoff and discharges where none previously
existed will ultimately result in a change in pollutant loadings, and there are indications that in the
aggregate BMPs will improve water quality. The actual manner in which the loadings reductions
are achieved will depend on the BMPs selected, which will determine the associated costs. The
physical-chemical properties of receiving streams and consequent linkages to biologic/ecologic
responses in the aquatic environment, and human responses and values associated with these
changes will determine the benefits.
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Approval Guidance. EPA 841-B-93-003. Washington, DC.
US EPA. 1994a. Storm Water Utilities: Innovative Financing for Storm Water Management:
Final Report. Office of Policy, Planning and Evaluation. Washington, DC.
US EPA. 1994b. A State and Local Government Guide to Environmental Program Funding
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US EPA. 1995a. National Water Quality Inventory: 1994 Report to Congress. EPA 841-R-95-
005. Washington, DC.
US EPA. 1995b. Storm Water Discharges Potentially Addressed by Phase II of the National
Pollutant Discharge Elimination System Storm Water Program: Report To Congress. EPA 833-
K-94-002. Washington, DC.
R- 10
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US EPA. 1995c. Economic Benefits of Runoff Controls. EPA 841-S-95-002. Washington,
DC.
US EPA. 1996a. Protecting Natural Wetlands: A Guide to Stormwater Best Management
Practices. EPA-843-B-96-001. Washington, DC.
US EPA. 1996b. Green Development: Literature Summary and Benefits Associated with
Alternative Development Practices. EPA-841 -B-97-001. Washington, DC.
US EPA. 1996c. Biological Criteria: Technical Guidance for Streams and Small Rivers.
Revised Edition. EPA 822-B-96-001. Washington, DC.
US EPA. 1996e. Municipal Wastewater Management Fact Sheets: Storm Water Best
Management Practices. EPA 832-F-96-001. Washington, DC.
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009. Washington, DC.
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733-R-97-002. Washington, DC.
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Office of Wastewater Management. Washington, DC.
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Benthic, Macroinvertebrates, and Fish. Revision. EPA 841-D-97-002. Washington, DC.
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Index
adsorption, 5.5, 5.6, 5.9, 5.16, 5.23, 5.25, 5.55, 5.67
antimony, 4.17
arsenic, 4.17
ASCE (American Society of Civil Engineers), 1.1, 2.3, 2.4, 5.7, 5.42, 5.43, 5.47-5.50, 5.53, 5.56
automotive product, 2.2, 5.30
bacteria, 4.1, 4.2, 4.7, 4.9, 4.14, 4.35, 4.44, 4.47, 5.6, 5.12, 5.32, 5.33, 5.50-5.52, 5.57, 5.68,
5.75, 5.82, 5.83,6.30
basin, 2.2, 4.4, 5.8-5.10, 5.12-5.14, 5.17, 5.19, 5.33, 5.38, 5.50, 5.56, 5.57, 5.60, 5.69, 5.76,
5.77, 6.3, 6.4, 6.6-6.9, 6.11, 6.14, 6.18, 6.20, 6.37, 6.38
benefits, 1.1-1.3, 5.8, 5.9, 5.56, 5.84, 6.1, 6.28, 6.29, 6.32, 6.35-6.38, 6.41, 6.42, 6.44
beryllium, 4.17
biological monitoring, 1.2, 5.46
bioretention, 2.2, 5.3, 5.23-5.26, 5.37, 5.51, 5.74, 5.76, 5.77, 5.83, 5.84, 6.1, 6.3, 6.4, 6.12, 6.14,
6.15,6.17-6.19
BMP (best management practices), 1.1-1.3,2.2-2.4,3.5, 5.1, 5.4,5.5,5.7-5.9, 5.11,5.17, 5.31,
5.36-5.39, 5.41-5.54, 5.56, 5.58, 5.59, 5.82, 5.84, 5.85, 6.1-6.5, 6.7, 6.9,
6.13-6.21, 6.28, 6.35-6.37, 6.40, 6.41
BOD (biochemical oxygen demand), 2.1, 4.7, 4.8, 4.10, 4.12, 4.35, 5.32, 5.50
cadmium, 4.17, 4.18, 5.52, 5.57, 5.68, 5.75, 5.82
capital costs, 5.2, 6.2, 6.4, 6.7, 6.9, 6.13, 6.18, 6.20, 6.21, 6.25-6.27
catch basin, 2.2, 4.4, 5.8, 5.33, 5.50
channel erosion, 4.2, 4.9, 4.30, 6.32, 6.34-6.36
chemical pollutant monitoring, 1.2
chlordane, 4.17
chromium, 4.17, 5.57, 5.68, 5.72, 5.73, 5.75, 5.80-5.82
chrysene, 4.17, 4.19
Clean Water Act, 1.1,2.1,3.1
Coastal Zone, 1.3, 3.3
COD (chemical oxygen demand, 4.7, 4.8, 4.10-4.12
cold climates, 5.42, 5.58
combined sewer, 2.2, 4.4, 4.5
construction activity, 3.2, 5.36
construction cost, 6.3, 6.5, 6.6, 6.8, 6.11-6.13, 6.15, 6.16
construction site, 3.2, 5.18, 5.55, 6.38, 6.40-6.42
contamination, 1.2, 4.2, 4.13, 4.35, 4.44, 4.45, 4.48, 5.9, 5.12, 6.37
copper, 4.7, 4.8, 4.11, 4.16-4.18, 5.35, 5.52, 5.57, 5.68, 5.72, 5.73, 5.75, 5.80-5.82, 6.41
costs, 1.1, 1.3, 2.2, 5.2, 5.17, 5.54, 6.1-6.9, 6.11-6.16, 6.18-6.28, 6.37-6.40, 6.42, 6.44
Crestwood Marsh Constructed Wetland, 5.69, 5.71, 5.72
cyanide, 5.36
cyclohexane, 4.17
database, 2.3, 4.6, 5.7, 5.43, 5.47, 5.53, 6.40
I- 1
-------
degradation, 4.1, 4.2, 4.5, 4.24, 4.32, 5.4, 5.6, 5.14-5.16, 5.25, 5.40, 5.56, 5.76, 5.83-5.85, 6.21,
6.32,6.35,6.36
detention, 2.2, 4.37, 5.1, 5.3, 5.5, 5.7, 5.8, 5.11-5.14, 5.18, 5.28, 5.38, 5.51, 5.54, 5.59, 5.69,
5.70, 5.74, 5.76, 5.83, 5.84, 6.1-6.3, 6.6, 6.7, 6.15-6.17, 6.19, 6.34-6.38
dichloromethane, 4.17
dissolved oxygen, 4.12, 4.13, 4.35, 5.43, 5.45, 5.59
domestic wastewater, 4.10, 4.11
education, 5.1, 5.7, 5.30, 5.32, 5.33, 5.48, 5.49, 6.21-6.25, 6.29-6.31, 6.38, 6.43
effectiveness, 1.1, 1.2, 4.6, 5.33, 5.42-5.50, 5.59, 5.83-5.85, 6.24, 6.28, 6.36, 6.41, 6.42
efficiency, 1.2, 1.3, 2.3, 5.1, 5.4, 5.5, 5.9, 5.13, 5.14, 5.36, 5.42, 5.45, 5.46, 5.48, 5.50,
5.52-5.58, 5.67-5.69, 5.74-5.76, 5.82, 5.83, 5.85
efficiency ratio, 5.45
effluent guidelines, 1.1, 2.1, 3.2
EMC (event mean concentration), 5.45, 5.52
endosulfan, 4.17
EPA (Environmental Protection Agency), 1.1-1.1, 1.3, 2.1, 2.3, 2.4, 3.1-3.4, 4.1, 4.6, 4.8, 4.12,
4.14, 4.16-4.18, 4.20, 4.22-4.26, 4.30, 4.32, 4.35, 4.36, 4.47, 4.48, 5.2, 5.9, 5.31,
5.33, 5.42, 5.44, 5.46-5.48, 5.53, 5.54, 5.60, 5.69, 5.70, 5.77, 5.84, 6.5, 6.6, 6.18,
6.20, 6.27, 6.28, 6.37-6.43
erosion, 1.2, 3.2, 4.1, 4.2, 4.9, 4.11, 4.24, 4.26, 4.29, 4.30, 4.33, 4.35, 5.11, 5.25, 5.35, 5.36,
5.39, 5.41, 5.55, 5.77, 6.1, 6.2, 6.6, 6.13, 6.29, 6.30, 6.32, 6.34-6.36, 6.38, 6.42
erosion control, 6.34
ETV (Environmental Technology Verification) program, 5.29
event mean concentration, 3.1, 4.18, 4.19, 5.45
Federal Highway Administration (FHWA), 4.6, 5.47, 5.48
filter, 2.2, 2.4, 4.40, 4.47, 5.2, 5.3, 5.5, 5.8, 5.17-5.23, 5.25-5.27, 5.36-5.40, 5.51, 5.54, 5.56,
5.74-5.83, 6.1, 6.3, 6.4, 6.11-6.15, 6.17-6.20
filtration, 2.2, 5.5, 5.8, 5.9, 5.12, 5.14, 5.16-5.19, 5.21-5.23, 5.25, 5.28, 5.29, 5.36, 5.37, 5.51,
5.67, 5.69, 5.74-5.77, 5.83
financial issues, 6.42
fishing, 1.2,4.2,4.6,4.48
floatable, 4.44, 4.48, 5.5
flood control, l.i, 4.5, 5.70, 6.1, 6.18, 6.34
flooding, 1.1, 1.2, 4.2, 4.5, 4.24, 4.26-4.28, 4.32, 5.3, 5.4, 5.9, 5.13, 5.14, 5.58, 5.84, 6.36, 6.37
flotation, 5.5
flow control, 5.1, 5.2, 5.83, 5.84
flow monitoring, 5.43, 5.78
flow rate, 4.23, 4.28, 5.13, 5.42, 5.44, 5.83, 5.84
fluoranthene, 4.17
good housekeeping, 2.2, 5.30, 5.31, 6.38
grass filter, 2.2, 5.26, 5.27, 5.82, 5.83
ground water, 4.18-4.20, 4.26, 4.31, 5.9
1-2
-------
habitat, 1.2, 4.1-4.3, 4.11, 4.24, 4.26, 4.30, 4.32-4.35, 4.37, 5.13, 5.14, 5.43, 5.46, 5.50, 5.78,
5.79, 5.85, 6.32, 6.37
hazardous material, 2.2, 5.32
herbicide, 5.31
Hollywood Branch Peat/Sand Filter, 5.77, 5.78, 5.80, 5.81
human health, 4.14, 6.37, 6.42
hydraulic performance, 5.42, 5.75
hydrocarbons, 4.1, 4.9, 4.15, 4.16, 4.35, 5.4, 5.5, 5.30, 5.51, 5.52, 5.55, 5.67, 6.29, 6.31
hydrograph, 4.28, 6.31
illicit connection, 6.21, 6.22
illicit discharge, 2.2, 5.33, 6.38
impervious surface, 5.28, 5.84, 6.21, 6.43
infiltration, 2.2, 4.4, 4.5, 4.24, 4.31, 4.45, 5.1-5.12, 5.14, 5.15, 5.26, 5.28, 5.29, 5.36, 5.38-5.40,
5.51, 5.54-5.56, 5.77, 5.83, 5.84, 6.1, 6.3, 6.4, 6.8-6.11, 6.14, 6.15, 6.18, 6.19,
6.22,6.31,6.38
lawn debris, 2.2, 5.6, 5.32, 5.48, 5.49
lead, 4.7, 4.8, 4.11, 4.16-4.18, 5.36, 5.51, 5.52, 5.55, 5.57, 5.68, 5.72, 5.73, 5.75, 5.80-5.82,
6.37, 6.41
lindane, 4.17
macroinvertebrate, 1.2, 4.39, 4.40, 5.43, 5.46, 5.85, 6.32, 6.35
maintenance, 2.1, 2.2, 3.2, 4.9, 4.40, 5.4, 5.7, 5.9, 5.10, 5.15-5.17, 5.30, 5.32, 5.33, 5.35-5.39,
5.41, 5.49, 5.50, 5.56, 5.59, 5.60, 5.75, 5.76, 6.1, 6.14, 6.15, 6.18, 6.20, 6.21,
6.27, 6.38, 6.43, 6.44
maintenance costs, 6.14, 6.15, 6.18, 6.20, 6.21
metals, 3.1, 4.1, 4.6, 4.9, 4.10, 4.12, 4.16-4.18, 4.35, 4.38, 5.2-5.6, 5.9, 5.12, 5.14, 5.15, 5.30,
5.50-5.52, 5.54-5.56, 5.58, 5.67, 5.68, 5.76, 5.83, 6.1, 6.29, 6.31, 6.37, 6.41
monitoring, 1.2, 1.3, 2.3, 2.4, 4.6, 5.31, 5.42-5.54, 5.56, 5.69, 5.71, 5.74, 5.78, 5.82, 5.85, 6.22
municipal separate storm sewer system (MS4), 3.1, 6.27
municipal financing, 6.43
nickel, 4.17, 5.59, 5.72, 5.73, 5.80, 5.81
nitrogen, 4.1, 4.7-4.9, 4.13, 5.22, 5.32, 5.52, 5.54, 5.55, 5.57, 5.62-5.66, 5.68, 5.72, 5.73, 5.75,
5.76, 5.80-5.82, 6.29, 6.30, 6.41
nitrophenol, 4.17
NOAA (National Oceanographic and Atmospheric Administration), 3.3, 3.4
non-structural BMP, 6.21
NPDES (National Pollutant Discharge Elimination System), 1.3, 2.1-2.3, 3.1, 3.2, 3.4, 4.6, 5.44,
5.47, 5.48,6.1,6.6,6.38
NURP (Nationwide Urban Runoff Program), 2.3, 4.6-4.8, 4.13, 4.14, 4.16, 4.18, 5.47, 5.49,
5.50, 6.40
nutrient, 4.12, 4.13, 5.6, 5.22, 5.25, 5.32, 5.39, 5.42, 5.58, 5.59, 5.69, 6.30
oil and grease, 2.1, 4.9, 4.15, 5.5, 5.19, 5.29, 5.44, 6.1, 6.41
organic, 4.9, 4.12, 4.13, 4.18, 4.19, 4.38, 5.5, 5.6, 5.8, 5.9, 5.14, 5.15, 5.22, 5.25, 5.44, 5.51,
5.52, 5.55-5.57, 5.62-5.68, 5.72, 5.73, 5.75, 5.76, 5.80-5.82, 6.30, 6.38
1-3
-------
oxygen demand, 4.7, 4.12, 4.13, 4.19, 5.62-5.66, 5.72, 5.73, 5.80, 5.81, 6.30
parking lot, 2.2, 4.10, 5.25, 5.34, 6.36
pathogens, 4.1, 4.13-4.15, 4.44, 4.45, 5.54, 6.29, 6.30, 6.37, 6.41
peak flow, 1.1,4.24,4.26,4.28, 5.13,5.16, 5.84,6.16,6.35
pentachlorophenol, 4.17
performance, 1.1-1.3, 2.1, 2.3, 2.4, 3.1, 3.4, 3.5, 4.6, 5.1, 5.7, 5.9, 5.11, 5.15-5.17, 5.29, 5.37,
5.41-5.45, 5.47, 5.48, 5.50-5.54, 5.56, 5.59, 5.67-5.69, 5.71, 5.75, 5.76, 5.79,
5.82, 5.84, 5.85, 6.1, 6.2, 6.14, 6.15, 6.18, 6.19
pesticide, 5.31, 5.32, 5.39, 5.49, 6.22, 6.31
pet waste, 5.32, 5.33, 5.49, 6.30
PhaselNPDES storm water rule, 3.1, 3.2, 4.6, 5.47, 5.48, 6.6, 6.39
Phase IINPDES stormwater rule, 3.2, 3.3, 6.1, 6.37-6.42
phenanthrene, 4.17, 4.19
phenol, 4.17
phosphorus, 4.1, 4.7-4.9, 4.13, 5.22, 5.32, 5.35, 5.51, 5.52, 5.54, 5.55, 5.57, 5.58, 5.62-5.66,
5.68, 5.72, 5.73, 5.75, 5.76, 5.80-5.82, 6.29, 6.30, 6.41
phthalate, 4.17
pollutant removal, 1.2, 1.3, 2.3, 5.1, 5.4, 5.9, 5.14-5.16, 5.28, 5.42, 5.45, 5.46, 5.48, 5.50,
5.52-5.57, 5.67, 5.68, 5.74, 5.75, 5.82, 6.28
porous pavement, 2.2, 5.2,5.8, 5.10, 5.11,5.30, 5.40, 5.54, 5.56,6.9,6.14
Prince William Parkway Regional Wet Pond, 5.60-5.62
public health, 1.2, 4.1-4.3, 4.13, 4.14, 4.44, 4.47
pyrene, 4.17, 4.19
rainfall frequency spectrum, 6.34
rainfall zone, 6.5, 6.19
receiving stream assessment, 5.46
recreation, 1.2, 4.2, 4.12, 4.44, 4.49, 6.37
recycling, 3.2, 5.30, 5.49, 6.24, 6.27, 6.29
regression of loads method, 5.45
retention, 2.2, 5.1, 5.3, 5.7, 5.8, 5.14-5.16, 5.29, 5.38, 5.40, 5.42, 5.51, 5.53, 5.54, 5.56-5.60,
5.74, 5.84, 5.85, 6.1-6.4, 6.6-6.9, 6.14, 6.15, 6.17-6.20, 6.36, 6.37, 6.40, 6.43
retrofitting, 5.2, 5.3, 5.14
road salt, 5.36,6.19
runoff coefficient, 4.24, 4.25
samples, 2.3, 4.17, 4.18, 4.35, 4.37, 4.44, 4.45, 5.42, 5.44, 5.51-5.54, 5.59, 5.61, 5.70, 5.71, 5.78
sand filter, 2.4, 5.18-5.23, 5.75, 5.77, 5.78, 5.80, 5.81, 6.3, 6.4, 6.11, 6.18, 6.20
scour, 4.1, 4.2, 5.13
scouring, 1.2,4.24,4.32
sediment, 1.2, 4.2, 4.9, 4.11-4.13, 4.23, 4.24, 4.26, 4.29-4.32, 4.35, 4.37, 4.38, 4.40, 4.41,
5.9-5.11, 5.13-5.15, 5.17, 5.19, 5.25, 5.29, 5.30, 5.33, 5.35-5.39, 5.55, 5.56, 5.58,
5.60, 5.76, 5.78, 5.79, 5.83, 6.2, 6.3, 6.6, 6.13, 6.30, 6.32, 6.35, 6.40, 6.42
sedimentation, 4.2, 4.11, 4.12, 4.24, 4.32, 4.40, 4.49, 5.4, 5.5, 5.14, 5.16, 5.19, 5.21, 5.28, 5.36,
5.38, 5.56, 5.58, 5.67, 5.76, 5.83
1-4
-------
selenium, 4.17
separate storm sewer, 2.2, 3.2, 3.4, 4.4, 5.33
siltation, 4.12, 4.23, 4.26
silver, 4.18
site design, 5.2, 5.29, 6.25
solids, 4.1, 4.6, 4.10-4.12, 4.38, 5.1-5.3, 5.5, 5.12-5.14, 5.17, 5.28, 5.35, 5.36, 5.51, 5.52,
5.54-5.58, 5.60, 5.62-5.66, 5.68, 5.72, 5.73, 5.75, 5.76, 5.80-5.83, 6.29, 6.30
source control, 3.2, 5.31, 5.49
source reduction, 5.6
storm drain, 2.2, 4.4, 4.44, 4.45, 5.2-5.4, 5.20, 5.21, 5.28, 5.29, 5.31, 5.33, 5.34, 5.41, 6.22,
6.27,6.29,6.31
storm sewer, 2.2, 3.2, 3.4, 4.4, 5.7, 5.12, 5.23, 5.26, 5.28, 5.33, 5.42, 5.77
storm water, 1.1, 1.1-1.3, 2.1-2.3, 3.1-3.4, 4.1-4.6, 4.8-4.20, 4.22-4.24, 4.26, 4.29, 4.31-4.35,
4.39, 4.44, 4.48, 4.49, 5.1-5.17, 5.21-5.23, 5.25, 5.26, 5.28-5.33, 5.36, 5.39, 5.40,
5.42, 5.44-5.50, 5.52-5.56, 5.58, 5.59, 5.67, 5.68, 5.74-5.76, 5.82-5.85, 6.1,
6.4-6.7, 6.14, 6.22, 6.26, 6.28, 6.30-6.32, 6.34-6.44
street construction, 5.2
street sweeping, 5.6, 5.48-5.50, 6.21, 6.28-6.31
structural, 1.2, 2.2, 2.4, 3.2, 4.5, 5.1-5.4, 5.6-5.8, 5.28-5.30, 5.36, 5.38, 5.39, 5.47-5.49, 5.54,
5.84, 5.85, 6.1, 6.5, 6.15, 6.16, 6.21, 6.28, 6.29, 6.31, 6.32, 6.37
summation of loads method, 5.45
swale, 5.39, 6.3, 6.4
tax, 3.4, 6.25, 6.43, 6.44
temperature, 4.19-4.22, 4.26, 4.35, 4.38, 5.4, 5.29, 5.42, 5.45, 5.58-5.60, 6.29, 6.31
trench, 5.11, 5.12, 5.26, 5.38, 5.56, 6.3, 6.4, 6.9, 6.10, 6.14, 6.15, 6.18
TSS (total suspended solids), 4.6, 4.8, 4.10-4.12, 5.5, 5.59, 5.75, 5.76, 6.40, 6.41
underground vault, 5.18-5.21
urban runoff, 1.1, 2.3, 4.1-4.3, 4.6, 4.8-4.14, 4.16, 4.17, 4.20, 4.23, 4.35, 4.47, 4.48, 5.4-5.7,
5.32, 5.35, 5.41-5.43, 5.46, 5.47, 6.29-6.31, 6.37, 6.42
USAGE (US Army Corps of Engineers), 6.40
USDA (US Department of Agriculture), 5.1, 6.16
USGS (US Geological Survey), 4.6, 5.47, 5.52
Water Quality Act, 1.1, 2.1, 4.6
water quality monitoring, 5.43, 5.71
water quality standards, 3.3, 4.1, 4.18, 4.19, 5.59
water quantity, 1.2,4.1-4.3,5.1, 5.3, 5.7, 5.8, 5.12,5.14,5.16, 5.74
wet pond, 2.4, 5.15, 5.60-5.62, 6.2, 6.37
wetland, 2.2, 2.4, 4.1, 5.3, 5.7, 5.8, 5.14-5.17, 5.28, 5.29, 5.36, 5.37, 5.40, 5.42, 5.44, 5.51, 5.59,
5.67-5.72, 5.74, 5.77, 6.3, 6.4, 6.14, 6.19
zinc, 4.7, 4.8, 4.11, 4.16-4.18, 5.35, 5.52, 5.55, 5.57, 5.68, 5.72, 5.73, 5.75, 5.80-5.82, 6.41
1-5
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