UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

WASHINGTON, D.C. 20460

OFFICE OF WATER

November 17, 2008 

Dear Colleague: 

Today we are making available the draft technical document TMDLs to Stormwater Permits Handbook for your 

review and comment. Stormwater runoff is a significant contributor to water quality impairments across the 

country, particularly runoff from developing and urbanized areas. Currently there are thousands of Clean Water 

Act section 303(d) waters listed as impaired for stormwater-source pollutants such as pathogens, nutrients, 

sediments and metals. To effectively address these impairments it is important to strengthen connections 

between two key federal programs under the Clean Water Act – the TMDL program and the NPDES 

stormwater permitting program. 

This Handbook provides a reference for TMDL practitioners and permit writers on current methods being used 

to develop more detailed stormwater-source TMDL allocations, TMDL implementation plans including best 

management practices, and methods for translating TMDL allocations into NPDES stormwater permit 

requirements. The Handbook assumes that the reader has a working knowledge of both TMDLs and NPDES 

stormwater permits, and provides background information on the components of these programs. Please note 

that last month the National Research Council released a lengthy report, Urban Stormwater Management in the 

United States (NRC, the National Academies Press, October 15, 2008). We intend to review the publication for 

consideration in producing the final version of the Handbook. 

Please provide us feedback on the methods and approaches presented in this Draft Handbook, as well as 

additional TMDL-stormwater case studies, implementation plans, other valuable resources, or permits 

and/or permit language that would be useful to include. Comments on the document should be sent to 

Christine Ruf (ruf.christine@epa.gov), Dean Maraldo (maraldo.dean@epa.gov) and Jack Faulk 

(faulk.jack@epa.gov) by February 27, 2009. 

Thanks again for your interest. 

John Goodin /s/ 

Chief, Watershed Branch 

Jack Faulk, /s/ 

Acting Chief, Industrial Permits Branch 

Kevin Pierard /s/ 

Chief, Watersheds and Wetlands Branch, Region 5 

Peter Swenson /s/ 

Chief, NPDES Programs Branch, Region 5
2

For more information on this technical document, please refer to the TMDL or NPDES contacts, below: 

EPA Core Handbook Team: Christine Ruf (ruf.christine@epa.gov), Dean Maraldo 

(maraldo.dean@epa.gov), Menchu Martinez (martinez.menchu-c@epa.gov), Jack Faulk 

(faulk.jack@epa.gov), Bob Newport (newport.bob@epa.gov), and Jamie Fowler (fowler.jamie@epa.gov) 

TMDL Regional contacts

Region 1 – Steve Winnett (winnett.steven@epa.gov) 

Region 2 – Antony Tseng (tseng.antony@epa.gov) 

Region 3 – Jennifer Sincock (sincock.jennifer@epa.gov), Helene Drago (drago.helene@epa.gov) 

Region 4 – William Melville (melville.william@epa.gov), Jennifer Eason (eason.jennifer@epa.gov) 

Region 5 – Dean Maraldo (maraldo.dean@epa.gov) 

Region 6 – Curry Jones (jones.curry@epa.gov) 

Region 7 – Bruce Perkins (perkins.bruce@epa.gov), Glenn Fernandez (fernandez.glenn@epa.gov) 

Region 8 – James Ruppel (ruppel.james@epa.gov) 

Region 9 – Peter Kozelka (kozelka.peter@epa.gov), Cindy Lin (lin.cindy@epa.gov) 

Region 10 – Laurie Mann (mann.laurie@epa.gov), Dave Ragsdale (ragsdale.dave@epa.gov) 

NPDES Regional contacts 

Region 1 – Thelma Murphy (murphy.thelma@epa.gov) 

Region 2 – Stephen Venezia (venezia.stephen@epa.gov), Sergio Bosques (bosques.sergio@epa.gov) 

Region 3 – Andrew Dinsmore (dinsmore.andrew@epa.gov), Paula Estornell (estornell.paula@epa.gov) 

Region 4 – Michael Mitchell (mitchell.michael@epa.gov) 

Region 5 – Bob Newport (newport.bob@epa.gov), Brian Bell (bell.brianc@epa.gov) 

Region 6 – Brent Larsen (larsen.brent@epa.gov) 

Region 7 – Mark Matthews (matthews.mark@epa.gov) 

Region 8 – Gregory Davis (davis.gregory@epa.gov) 

Region 9 – Eugene Bromley (bromley.eugene@epa.gov) 

Region 10 – Misha Vakoc (vakoc.misha@epa.gov) 


TMDLS TO STORMWATER PERMITS HANDBOOK

DRAFT 

November 2008 

Assessment and Watershed Protection Division 

Office of Wetlands, Oceans and Watersheds 

U.S. Environmental Protection Agency 

1200 Pennsylvania Avenue NW 

Washington, DC 20460 

Water Permits Division 

Office of Wastewater Management 

U.S. Environmental Protection Agency 

1200 Pennsylvania Avenue NW 

Washington, DC 20460 

Water Division 

U.S. Environmental Protection Agency Region 5 

77 West Jackson Blvd. 

Chicago, IL 60604 
DISCLAIMER TMDLS TO STORMWATER PERMITS HANDBOOK

ii DRAFT November 2008 

DISCLAIMER

This document provides technical information to TMDL and NPDES practitioners who are familiar with 

the relevant technical approaches and legal requirements pertaining to developing TMDLs and NPDES 

stormwater permits, and refers to statutory and regulatory provisions that contain legally binding 

requirements. This document does not substitute for those provisions or regulations, nor is it a regulation 

itself. Thus, it does not impose legally binding requirements on EPA or States, who retain the discretion 

to adopt approaches on a case-by-case basis that differ from this information. Interested parties are free 

to raise questions about the appropriateness of the application of this information to a particular 

situation, and EPA will consider whether or not the technical approaches are appropriate in that 

situation. 
TMDLS TO STORMWATER PERMITS HANDBOOK CONTENTS

November 2008 DRAFT iii 

CONTENTS

Preface....................................................................................................................................................... ix

Acknowledgements ....................................................................................................................................x

Acronyms and Abbreviations ................................................................................................................. xi

Introduction................................................................................................................................................1

1. Understanding the Connections between TMDLs and Stormwater Permits.................................4

1.1. What Every Permit Writer Should Know about the TMDL Program .........................................................4

1.2. What Every TMDL Writer Should Know about the NPDES Stormwater Program....................................7

1.2.1. NPDES Program Framework........................................................................................................7

1.2.2. NPDES Stormwater Program........................................................................................................8

1.2.2.1. Types of NPDES Stormwater Permits...............................................................................9

1.2.2.2. Standards and Limits in Stormwater Permits ..................................................................10

1.2.2.3. Basic Stormwater Management Program and Pollution Prevention Plan Requirements 11

1.3. Key Challenges Associated with Connecting the TMDL and NPDES Stormwater Programs..................12

1.3.1. Challenge 1: Addressing Differences in Organizational Structure .............................................12

1.3.2. Challenge 2: Developing Consistent Stormwater Allocations in TMDLs ..................................13

1.3.3. Challenge 3: Translating Numeric TMDL WLAs into Implementation Strategies and 

Permit Requirements...................................................................................................................13

1.3.4. Challenge 4: Reconciling Spatial Boundaries of TMDLs with Boundaries of NPDES 

Stormwater Permits.....................................................................................................................13

1.3.5. Challenge 5: Incorporating Monitoring, Tracking, and Adaptive Management Elements into 

TMDL WLAs and Stormwater Permits ......................................................................................13

1.4. Resources...................................................................................................................................................14

1.4.1. TMDL Program...........................................................................................................................14

1.4.2. TMDLs and Stormwater .............................................................................................................15

1.4.3. NPDES Stormwater Program......................................................................................................15

1.4.3.1. MS4s................................................................................................................................16

1.4.3.2. Industrial Activities .........................................................................................................16

1.4.3.3. Construction Activities ....................................................................................................17

2. Identifying Opportunities to Coordinate TMDLs and Stormwater Permits ...............................20

2.1. Improving Internal Communication and Coordination..............................................................................20

2.2. Improving Stakeholder Communication and Coordination.......................................................................24

2.3. Resources...................................................................................................................................................25

2.3.1. TMDL and Stormwater Permitting .............................................................................................25

2.3.2. Identifying and Involving Stakeholders ......................................................................................26

3. Characterizing Impairments and Stormwater Sources .................................................................28

3.1. Understanding the Impairment ..................................................................................................................29

3.1.1. Stormwater Effects on Receiving Waterbodies ..........................................................................29

3.1.2. Data Analysis to Characterize Impairment .................................................................................30

3.1.2.1. Identifying Pollutants or Other Causes of Impairment....................................................31

3.1.2.2. Identifying Spatial Patterns..............................................................................................32

3.1.2.3. Identifying Temporal Trends...........................................................................................33

3.1.2.4. Evaluating Relationships among Parameters...................................................................36

3.1.2.5. Identifying Critical Conditions ........................................................................................36

3.2. Identifying TMDL Targets ........................................................................................................................37

3.3. Identifying and Assessing Potential Sources .............................................................................................39
CONTENTS TMDLS TO STORMWATER PERMITS HANDBOOK

iv DRAFT November 2008 

3.3.1. Identifying the Type and Location of Stormwater Sources ........................................................40

3.3.2. Delineating the Drainage Areas of Stormwater Sources.............................................................44

3.3.3. Characterizing Discharges from Stormwater Sources ................................................................45

3.3.3.1. MS4-Generated Data .......................................................................................................47

3.3.3.2. Industrial Discharger Generated Data..............................................................................48

3.3.3.3. Construction Project Generated Data ..............................................................................49

3.4. Resources...................................................................................................................................................50

3.4.1. Stormwater Effects on Receiving Waters ...................................................................................50

3.4.2. Understanding Flow in TMDL Development .............................................................................51

3.4.3. Chemical, Physical and Biological Data.....................................................................................52

3.4.4. Biological Assessment and Guidance .........................................................................................52

3.4.5. Permitted Stormwater Sources in a Watershed ...........................................................................53

3.4.6. Land Use and Impervious Surface Coverages ............................................................................53

3.4.7. Field Reconnaisance and Illicit Discharge Detection .................................................................53

4. Developing TMDLs with Stormwater Sources................................................................................56

4.1. Overview of Approaches for Developing TMDLs ....................................................................................56

4.1.1. Land-based Approaches..............................................................................................................57

4.1.2. Waterbody-based Approaches ....................................................................................................58

4.2. Selecting an Approach for Developing Stormwater-Source TMDLs........................................................59

4.2.1. How Are Water Quality Criteria or TMDL Targets Expressed? ................................................61

4.2.2. What Are the Stormwater Sources Affecting Impairment? ........................................................63

4.2.2.1. Type and Distribution of Sources ....................................................................................63

4.2.2.2. Pollutant Delivery............................................................................................................65

4.2.3. What Are the Critical Conditions?..............................................................................................66

4.3. Applying Approaches for Stormwater-Source TMDLs.............................................................................67

4.3.1. Land-based Approaches..............................................................................................................69

4.3.1.1. Watershed Models ...........................................................................................................69

4.3.1.2. Impervious Cover Method...............................................................................................77

4.3.1.3. Export Coefficients..........................................................................................................78

4.3.1.4. Simple Method.................................................................................................................78

4.3.2. Waterbody-based Approaches ....................................................................................................79

4.3.2.1. Receiving Water Models .................................................................................................80

4.3.2.2. Load Duration Approach .................................................................................................81

4.3.2.3. Percent Reduction Method ..............................................................................................82

4.3.2.4. Mass Balance or Steady-State Analysis...........................................................................83

4.4. Categorizing WLAs for Stormwater Sources ............................................................................................83

4.4.1. Single Aggregated WLA for All Stormwater Sources................................................................85

4.4.2. Separate Aggregated WLA for Each Type of Permitted Stormwater Source.............................86

4.4.3. Individual WLAs for Each Permitted Stormwater Source..........................................................88

4.4.4. Individual WLAs on an Outfall Basis.........................................................................................90

4.4.5. Other Elements in a TMDL.........................................................................................................91

4.5. Resources...................................................................................................................................................92

4.5.1. General TMDL Development .....................................................................................................92

4.5.2. Watershed Models.......................................................................................................................92

4.5.3. Simple Method............................................................................................................................93

4.5.4. Receiving Water Models.............................................................................................................93

4.5.5. Load Duration Curves.................................................................................................................94

4.5.6. WLA Expression Options ...........................................................................................................94

4.5.7. Required Elements of a TMDL...................................................................................................94

5. Promoting Effective Stormwater Management ..............................................................................98
TMDLS TO STORMWATER PERMITS HANDBOOK CONTENTS

November 2008 DRAFT v 

5.1. Implementation Roles and Responsibilities...............................................................................................98

5.2. Key Questions for Promoting Effective BMP Implementation...............................................................100

5.2.1. Establishing the Baseline Load and Accounting for Existing Load Reductions (Key 

Questions 1 and 2).....................................................................................................................100

5.2.1.1. Answering Key Question 1: Determining Current Pollutant Loading from Stormwater 

Source Discharge...........................................................................................................101

5.2.1.2. Answering Key Question 2: Determining Additional Load Reductions Necessary to 

Implement the WLA......................................................................................................105

5.2.2. Selection of Additional BMPs to Implement WLAs (Key Question 3)....................................105

5.2.2.1. Key Question 3, Activity A: Identifying List of Possible BMPs...................................106

5.2.2.2. Key Question 3, Activity B: Determining Expected BMP Performance.......................108

5.2.3. Monitoring and Assessing Implementation Progress (Key Questions 4-5) ..............................115

5.2.3.1. Answering Key Question 4: Monitoring and Assessing Implementation of Additional 

BMPs .............................................................................................................................115

5.2.3.2. Answering Key Question 5: Determining Whether Implementation Progress is Adequate

.......................................................................................................................................119

5.2.4. Adjusting Implementation for Continuous Improvements (Key Question 6)...........................122

5.2.4.1. Answer Key Question 6: Identifying Modifications to Implementation Strategy .........122

5.3. Resources.................................................................................................................................................122

5.3.1. BMP Inventory..........................................................................................................................122

5.3.2. BMP Selection ..........................................................................................................................123

5.3.3. Volume Control.........................................................................................................................124

5.3.4. Green Infrastructure and Low Impact Development.................................................................124

5.3.5. BMP Performance Literature Values ........................................................................................125

5.3.6. Model Applicability ..................................................................................................................126

5.3.7. Stormwater Program Evaluation ...............................................................................................127

5.3.8. BMP Monitoring .......................................................................................................................127

5.3.9. Outfall Monitoring ....................................................................................................................128

5.3.10.Receiving Water Monitoring...................................................................................................128

6. Coordinating TMDLs and Stormwater Permits...........................................................................132

6.1. Options for Implementing TMDLs in Permits.........................................................................................132

6.1.1. Requirements Related to Determining Applicability ................................................................133

6.1.2. Requirements Related to Identifying and Implementing Water Quality Controls....................135

6.1.3. Monitoring Requirements .........................................................................................................139

6.1.4. Compliance Considerations ......................................................................................................140

6.2. Options for Connecting Programmatic Documents.................................................................................141

6.2.1. Using the WLA to Connect to Permits .....................................................................................142

6.2.2. Using Implementation Plans to Connect to WLAs and Permits ...............................................143

6.3. Other Information to Consider.................................................................................................................144

6.3.1. No separate WLA for NDPES Stormwater source ...................................................................144

6.3.2. Impaired Waterbody with No Approved TMDL ......................................................................144

Appendix: TMDL and NPDES Stormwater Permit Language Excerpts.........................................147

Bibliography ...........................................................................................................................................181

Documents .......................................................................................................................................................181

Web Sites and Databases .................................................................................................................................186

Glossary ..................................................................................................................................................189
CONTENTS TMDLS TO STORMWATER PERMITS HANDBOOK

vi DRAFT November 2008 

TABLES

Table 1. Estimated number of stormwater permittees and permits by type of stormwater 

discharge ................................................................................................................................10 

Table 2. Potential opportunities for coordination based on status of TMDL and stormwater 

permit development ...............................................................................................................21 

Table 3. Examples of the effect of stormwater runoff on common designated uses ...........................30 

Table 4. Examples of sources within regulated stormwater areas associated with common 

pollutants and impairments ....................................................................................................40 

Table 5. Data and information generated by stormwater sources through the permitting process......45 

Table 6. Commonly used TMDL approaches......................................................................................57 

Table 7. Summary of technical considerations for selecting a TMDL development approach...........61 

Table 8. Summary of commonly used TMDL development approaches ............................................67 

Table 9. Commonly used watershed models and select capabilities for evaluating stormwater 

sources in TMDLs..................................................................................................................71 

Table 10. Example summary of allocations calculated for a TMDL using the load duration 

framework ..............................................................................................................................82 

Table 11. Options for assigning WLAs to stormwater sources .............................................................84 

Table 12. Description of modeling tools available for BMP selection, sizing, and siting decision 

making..................................................................................................................................111 

Table 13. Example of screening criteria for stormwater management practices .................................113 
TMDLS TO STORMWATER PERMITS HANDBOOK CONTENTS

November 2008 DRAFT vii 

FIGURES

Figure 1. Typical steps in the TMDL development process. ..................................................................6 

Figure 2. Number of facilities or sources permitted under various programs of the NPDES 

program over time....................................................................................................................9 

Figure 3. Illustration of the steps in the TMDL process, including the step of stakeholder 

involvement and public participation.....................................................................................20 

Figure 4. Illustration of the steps in the TMDL process, including the step of watershed 

characterization discussed in this chapter. .............................................................................28 

Figure 5. Analysis of upstream and downstream data to evaluate potential impact of an expected 

source. ....................................................................................................................................33 

Figure 6. Evaluation of spatial variations in turbidity data to identify locations of potential 

sources....................................................................................................................................34 

Figure 7. Examples of different data representations to evaluate the relationship between flow 

and fecal coliform. .................................................................................................................35 

Figure 8. Duration curve analysis indicating illicit discharges as stormwater source. Circled data 

points represent those exceeding the target for Escherichia coli...........................................36 

Figure 9. Options for identifying targets for TMDLs that include stormwater sources........................38 

Figure 10. Potential spatial complexities of regulated and unregulated stormwater sources..................43 

Figure 11. Required MS4 permit documents and associated information and data................................48 

Figure 12. Required industrial stormwater permit documents and associated information and data......49 

Figure 13. Required construction stormwater permit documents and associated information and 

data.........................................................................................................................................50 

Figure 14. Illustration of the steps in the TMDL process, including activities related to 

calculation and documentation of the TMDL and its allocations. .........................................56 

Figure 15. Considerations for selecting a TMDL development approach. .............................................60 

Figure 16. Typical watershed model elements for simulating runoff and pollutant loading from 

watershed land uses to receiving waterbodies. ......................................................................70 

Figure 17. Process for calculating MS4 allocation when the MS4 drainage area is included in the 

model as a discrete land unit..................................................................................................74 

Figure 18. Process for calculating MS4 allocation when the specific stormwater drainage 

boundary is not included in the model and MS4 load is calculated based on percent of 

area within each modeled land use. .......................................................................................76 

Figure 19. Example TMDL using duration curve framework.................................................................82 

Figure 20. Illustration of the steps in the TMDL process, including activities related to TMDL 

implementation. .....................................................................................................................98 

Figure 21. Approaches to evaluating storm water program effectiveness. (Source: CASQA 2007)....116 

Figure 22. Illustration of the steps in the TMDL process, including activities related to TMDL 

implementation. ...................................................................................................................132 

 
CONTENTS TMDLS TO STORMWATER PERMITS HANDBOOK

viii DRAFT November 2008 

This page intentionally left blank 
TMDLS TO STORMWATER PERMITS HANDBOOK PREFACE

November 2008 DRAFT ix 

PREFACE

Thousands of impaired waterbodies require total maximum daily loads (TMDLs) for pollutants, such as 

pathogens, nutrients, sediments, and metals – pollutants commonly associated with urban stormwater 

discharges. As a result, it is important for the TMDL program and the National Pollutant Discharge 

Elimination System (NPDES) Stormwater program to identify opportunities and approaches for 

promoting TMDL implementation through stormwater permits. The U.S. Environmental Protection 

Agency (EPA) developed this TMDLs to Stormwater Permits Handbook (Handbook) to address 

challenges that are unique to TMDL development and implementation involving permitted stormwater 

discharges from municipal separate storm sewer systems (MS4s), industrial facilities, and construction 

activities. 

The Handbook is intended for federal and state TMDL writers and NPDES stormwater permit writers 

responsible for addressing waterbodies impaired by discharges from stormwater sources. The Handbook 

assumes that the reader has a working knowledge of both the TMDL and the NPDES stormwater 

programs and provides limited background information on the basic regulatory and programmatic 

aspects of these programs. 

The Handbook contains information to give TMDL and stormwater permit writers a better 

understanding of (1) cross-program regulatory requirements and programmatic processes; (2) current 

efforts to establish better cross-program connections; and (3) opportunities to further improve how the 

TMDL and NPDES Stormwater programs interact to address stormwater-related water quality 

impairments. Real-world examples are the predominant mechanism to illustrate concepts and 

approaches for promoting improved implementation of TMDLs through stormwater permits. The 

information contained in this Handbook represents an initial step in identifying and comprehensively 

addressing these issues. The issues and challenges surrounding TMDL implementation through 

stormwater permits will continue to evolve as EPA and state TMDL practitioners and stormwater permit 

writers consider and test new approaches and strategies. 
ACKNOWLEDGEMENTS TMDLS TO STORMWATER PERMITS HANDBOOK

x DRAFT November 2008 

ACKNOWLEDGEMENTS

EPA would like to acknowledge the following individuals for their contribution to this Draft Handbook: 

EPA Core Team 

Christine Ruf, EPA Headquarters (Task Order Manager) 

Dean Maraldo, EPA Region 5 (Task Order Manager) 

Jack Faulk, EPA Headquarters 

Jamie Fowler, EPA Headquarters 

Menchu Martinez, EPA Headquarters 

Bob Newport, EPA Region 5 

Contributors and Reviewers 

Eugene Bromley, EPA Region 9 

Thomas Davenport, EPA Region 5 

Andrew Dinsmore, EPA Region 3 

Robert Goo, EPA Headquarters 

John Goodin, EPA Headquarters 

Curry Jones, EPA Region 6 

Maureen Krudner, EPA Region 2 

Evelyn MacKnight, EPA Region 3 

Jennifer Molloy, EPA Headquarters 

Chuck Schadel, EPA Region 3 

Stephen Silva, EPA Region 1 

This document was developed under EPA Region 5 Contract # EP075000 170 (GSA# GS-1 OF-0268K) 

with Tetra Tech (Cleveland, Ohio). 

Tetra Tech Handbook Team 

Kellie DuBay 

Jessica Koenig 

Kevin Kratt 

Bruce Cleland 

Christy Williams 

Andrew Parker 

Krista Carlson 

Regina Scheibner 

Kristin Shatmeyer 

Jeff Strong 
TMDLS TO STORMWATER PERMITS HANDBOOK ACRONYMS AND ABBREVIATIONS

November 2008 DRAFT xi 

ACRONYMS AND ABBREVIATIONS

303(d) Section 303(d) of the Clean Water Act 

AnnAGNPS Annualized Agricultural Non-Point Source model 

ASCE American Society of Civil Engineers 

BAT best available technology economically achievable 

BMP best management practice 

BOD5 five-day biochemical oxygen demand 

CADDIS Causal Analysis/Diagnosis Decision Information System 

CASQA California Stormwater Quality Association 

CBOD5 five-day carbonaceous biochemical oxygen demand 

CFB commercial fishing ban 

CFR Code of Federal Regulations

cfs cubic feet per second 

CGP Construction General Permit 

CWA Clean Water Act 

DA drainage area 

DMR discharge monitoring report 

DO dissolved oxygen 

DRSCW DuPage River Salt Creek Workgroup 

DURMM Delaware Urban Runoff Management Model 

EFDC Environmental Fluid Dynamics Code 

EMAP Environmental Monitoring and Assessment Program 

eNOI Electronic Notice of Intent 

EPA U.S. Environmental Protection Agency 

FCG fish consumption guidelines 

FEMA Federal Emergency Management Agency 

FHWA Federal Highway Administration 

GI green infrastructure 

GIS geographic information system 

GWLF Generalized Watershed Loading Functions 

HSPF Hydrologic Simulation Program in Fortran 
ACRONYMS AND ABBREVIATIONS TMDLS TO STORMWATER PERMITS HANDBOOK

xii DRAFT November 2008 

IC impervious cover 

ICIS Integrated Compliance Information System 

IDDE Illicit Discharge Detection & Elimination 

IDEAL Integrated Design and Evaluation Assessment of Loadings 

LA load allocation 

LID low impact development 

LSPC Loading Simulation Program in C++ 

MASTEP Massachusetts Stormwater Technology Evaluation Project 

MDAS Mining Data Analysis System 

MEP maximum extent practicable 

mg/L milligram per liter 

MOS margin of safety 

MRLC multi-resolution land characteristics 

MS4 municipal separate storm sewer system 

MSGP Multi-Sector General Permit 

MUSIC Model for Urban Stormwater Improvement Conceptualization 

NAWQA National Water-Quality Assessment (Program) 

NLCD National Land Cover Dataset 

NOI Notice of Intent 

NPDES National Pollutant Discharge Elimination System 

NRC National Research Council 

NRCS Natural Resources Conservation Service 

NSPS New Source Performance Standards 

NTU nephelometric turbidity units 

NURP Nationwide Urban Runoff Program 

NWIS National Water Information System 

O&M operation and maintenance 

P8-UCM Program for Predicting Polluting Particle Passage through Pits, Puddles, and Ponds—

Urban Catchment Model 

PCS Permit Compliance System 

PCB polychlorinated biphenyl 

PG BMP-DSS Prince George’s County Best Management Practice Decision Support System 

QAPP quality assurance project plan 
TMDLS TO STORMWATER PERMITS HANDBOOK ACRONYMS AND ABBREVIATIONS

November 2008 DRAFT xiii 

SB shellfishing ban 

SET Site Evaluation Tool 

SI stressor identification 

SIC standard industrial classification 

SLAMM Source Loading and Management Model 

STORET Storage and Retrieval (database) 

SWAT Soil and Water Assessment Tool 

SWMM Storm Water Management Model 

SWMP stormwater management program 

SWPPP stormwater pollution prevention plan 

TARP Technology Acceptance and Reciprocity Partnership 

TMDL total maximum daily load 

TSS total suspended solids 

TWR trophic weighted residue value of mercury in fish tissue 

UNRBA Upper Neuse River Basin Association 

UOD ultimate oxygen demand 

USDA U.S. Department of Agriculture 

USGS U.S. Geological Survey 

VAFSWM Virginia Field Scale Wetland Model 

VFSMOD Vegetative Filter Strip Model 

WLA wasteload allocation 

WQBEL water quality-based effluent limitations 

WQv water quality volume 
ACRONYMS AND ABBREVIATIONS TMDLS TO STORMWATER PERMITS HANDBOOK

xiv DRAFT November 2008 

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TMDLS TO STORMWATER PERMITS HANDBOOK INTRODUCTION

November 2008 DRAFT 1 

INTRODUCTION

Across the country, stormwater runoff is a significant contributor to water quality impairments, 

particularly in developing and urbanized areas. There are thousands of impaired waterbodies requiring 

total maximum daily loads (TMDLs) for pollutants associated with stormwater sources, such as 

pathogens, nutrients, sediments, and metals. Developing TMDLs that include stormwater sources can 

present unique challenges because of the variety and number of sources, the variability of the pollutants 

discharged by these sources, the limited availability of monitoring data, and the complexity of the 

mechanisms by which pollutants in stormwater discharges affect ambient water quality criteria. 

Implementing TMDLs via National Pollutant Discharge Elimination System (NPDES) stormwater 

permits can also present challenges because of difficulties in translating the numeric, water qualitybased TMDL wasteload allocations (WLAs) into permit requirements. 

The purpose of this Handbook is to provide information to TMDL practitioners and NPDES stormwater 

permit writers (referred to as TMDL writers and permit writers throughout this Handbook) on the 

following: 

• Current methods and other potential options for developing more precise WLAs for stormwater 

sources (referred to simply as sources throughout this Handbook) 

• TMDL implementation plans including best management practice (BMP) and other stormwater 

management strategy recommendations 

• Approaches for translating TMDL WLAs and implementation recommendations into NPDES 

stormwater permit requirements and implementation strategies 

Information contained in this Handbook specifically addresses the following: 

• TMDL WLAs for stormwater sources that clearly express and assign the targeted loading reductions 

necessary to attain and maintain water quality standards 

• TMDL implementation plans that connect WLAs and stormwater permits by either (1) including 

specific recommendations (e.g., performance standards, management measures) for implementing 

WLAs, or (2) providing technical information for permit writers and permittees on how to analyze, 

select, and implement provisions to implement the WLAs 

• Stormwater permits that are consistent with the WLAs by identifying specific elements, including 

management measures, that implement the WLA and, if available, TMDL implementation plan 

recommendations or specifying approaches for demonstrating that specific provisions will 

implement WLAs 

In addition, the goal is to provide TMDL and permit writers a better cross-program understanding of 

regulatory requirements and programmatic processes and a better understanding of opportunities to 

further improve how the TMDL and NPDES Stormwater programs should interact to address 

stormwater-related water quality impairments. The real-world examples provided in this Handbook are 

intended to provide a range of options for TMDL and permit writers to consider. Where real-world 

examples do not exist to illustrate a concept, the Handbook provides a hypothetical example for TMDL 

and permit writers to consider, and the writers hope that it provides insight into future directions to 

improve this linkage. 
INTRODUCTION TMDLS TO STORMWATER PERMITS HANDBOOK

2 DRAFT November 2008 

This Handbook is organized as follows: 

• Chapter One: Understanding the connections between TMDLs and stormwater permits. 

Provides overviews of TMDL and NPDES Stormwater programs and summarizes the challenges of 

developing TMDLs and implementing WLAs through stormwater permits. 

• Chapter Two: Identifying opportunities to coordinate TMDLs and stormwater permits. 

Identifies ideas and opportunities for TMDL and permit writers to coordinate at various points 

throughout the TMDL and stormwater permitting processes. 

• Chapter Three: Characterizing impairments and stormwater sources. Provides a detailed 

discussion of the type of information that TMDL and permit writers can use to generate a detailed 

stormwater source characterization, including data on water quality and watershed conditions, as 

well as data generated by stormwater permittees and where to obtain this information. 

• Chapter Four: Developing TMDLs with Stormwater Sources. Addresses key stormwater-specific 

issues that TMDL writers can consider when developing TMDLs using a specific technical 

approach. It also discusses options for categorizing stormwater WLAs to facilitate their 

implementation in permits. 

• Chapter Five: Promoting effective stormwater management. Presents an adaptive management 

framework for selecting, implementing, assessing, and modifying stormwater management strategies 

using information and tools that predict potential BMP performance 

• Chapter Six: Coordinating TMDLs and stormwater permit requirements. Provides a variety of 

options for effectively tying together TMDLs and associated permit requirements through the 

development of TMDL reports, stormwater permit language, and fact sheets, as well as TMDL 

implementation planning documents. 

• Appendix. Provides excerpts of TMDLs, implementation plans, and stormwater permit requirements 

to illustrate how states connect permitted stormwater source requirements among programmatic 

documents. 

• Bibliography. Provides a comprehensive list of documents, Web sites, and databases that are 

included in the Resources section of each chapter or cited in the Handbook. 

• Glossary. Defines key terms introduced throughout the Handbook. 
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1. UNDERSTANDING THE CONNECTIONS BETWEEN TMDLS AND 

STORMWATER PERMITS

Understanding the regulatory, programmatic, and technical issues associated with the TMDL and 

NPDES Stormwater programs can help TMDL and permit writers improve cross-program connections, 

leading to better TMDLs and stormwater permits. This chapter briefly summarizes the key statutory and 

regulatory elements of these two programs. 

1.1. What Every Permit Writer Should Know about the TMDL 

Program 

A TMDL reflects the total pollutant loading a waterbody can 

receive and still meet water quality standards. TMDLs are 

one of the many tools Congress authorized in the Clean 

Water Act (CWA) to “restore and maintain the chemical, 

physical, and biological integrity of the Nation’s waters.” 

(CWA section 101(a)). Section 303(d) of the CWA requires 

states, territories, and authorized tribes to identify and establish a priority ranking for waters for which 

technology-based effluent limitations required by section 301 are not stringent enough to implement 

applicable water quality standards, establish TMDLs for the pollutants causing impairment in those 

waters, and submit to the U.S. Environmental Protection Agency (EPA), from time to time, the list of 

waters in which water quality standards are not attained or maintained and for which TMDLs are 

required, as well as their associated TMDLs. EPA must review and approve or disapprove lists and 

TMDLs within 30 days. If EPA disapproves a list or a TMDL submitted by states, territories, and 

authorized tribes, EPA must establish the list or TMDL. In addition, some courts have interpreted the 

statute as requiring EPA to establish lists and TMDLs when a state fails to do so. EPA’s TMDL 

regulations can be found at Title 40 of the Code of Federal Regulations (CFR) sections 130.7 and 130.2. 

Listing impaired waters and establishing TMDLs for waters impaired by pollutants from point and 

nonpoint sources does not, by itself, create any new or additional implementation authorities to control 

point or nonpoint sources. Permitting authorities implement WLAs included in a TMDL through 

enforceable water quality-based discharge limits in NPDES permits authorized under section 402 of the 

CWA (see next section, “What every TMDL Writer Should Know about the NPDES Stormwater 

Program”). Mechanisms for implementing nonpoint source load allocations (LAs) within TMDLs 

include state section 319 nonpoint source management programs, coupled with a wide variety of other 

state, local, tribal, and federal programs—which may be regulatory, nonregulatory, or incentive-based, 

depending on the program—as well as voluntary action by committed citizens. To date, no tribe has 

sought or received CWA authority to establish TMDLs. 

A brief summary of the key aspects of the TMDL program is provided below. 

• Section 303(d) Lists of Impaired Waterbodies. Each state is required to identify “water quality 

limited segments,” or impaired waterbodies, for which federal technology-based controls, state, 

tribal, or local effluent limitations or other pollution control requirements (e.g., BMPs) required by 

local, state, tribal, or federal authority are not stringent enough to achieve water quality standards, 

including waters not meeting standards due to thermal discharges (40 CFR 130.7 (b)). The list that 

Resources: For more information on the 

TMDL program, refer to the Resources list 

at the end of this chapter in Section 1.4.1, 

including EPA’s TMDL Web site at 

www.epa.gov/owow/tmdl/. 
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identifies these water quality limited segments is 

known as the Section 303(d) list. States are 

required to submit Section 303(d) list updates 

every 2 years. The information that the Section 

303(d) list must contain (40 CFR 130.7) is as 

follows: 

9 The pollutant(s) causing (or expected to 

cause) the violation of water quality standards 

for each listed water 

9 A priority ranking of all listed waters and 

waters targeted for TMDL development 

within the next 2-year listing cycle 

9 Documentation to support listing decisions, including a description of the methodology used, 

data evaluated, rationale for not using any readily available data, and any other reasonable 

information requested by EPA to evaluate the listing decisions 

• Components of a TMDL. EPA’s regulations at 40 CFR 

130.2(i) define a TMDL as the sum of WLAs plus load 

LAs plus a margin of safety (MOS) to account for 

uncertainty between pollutant sources and resulting water 

quality. Allocations to pollutant sources vary on the basis 

of the type of pollutant sources—WLAs are loads allotted 

to existing and future point sources, and LAs are loads 

attributed to existing and future nonpoint sources, plus 

loads from natural background. Future growth allowances 

in TMDLs account for increased pollutant loadings and can 

be included as an allocation of pollutant loads from new 

sources expected in the future. For instance, in areas where 

land use changes are anticipated, TMDLs can include a 

reserve for future growth, which can be a separate 

component of the TMDL or included in WLAs or LAs. 

• TMDL implementation plans. Although not required by federal law or regulation, many states 

include (and some state regulations do require) TMDL implementation plans, which are typically 

developed in coordination with relevant stakeholders. A TMDL implementation plan typically 

identifies recommended management practices that major sources in the watershed are expected to 

implement, along with a general time frame and strategy for funding and monitoring. Factors such as 

waterbody ranking, data availability, court-ordered schedules, and anticipated management activities 

can affect the timing of TMDL development and implementation. 

As illustrated in Figure 1, the process for developing a TMDL typically includes the following steps: 

• Stakeholder involvement and public participation to engage affected parties and solicit input, 

feedback and buy-in for a successful TMDL. This process can occur throughout the TMDL 

development (and implementation) process. 

• Watershed characterization to identify the waterbody, watershed, and impairment conditions; TMDL 

targets; and potential sources. 

New Location for 303(d) Lists 

States are now integrating Section 303(d) lists of 

impaired waters and Section 305(b) water quality 

assessment reports to produce Integrated Reports. 

The Integrated Report groups a state’s waters into five 

categories, depending on a determination of whether a 

waterbody is impaired or is in attainment of water 

quality standards. Category 5 of the Integrated Report 

contains the list of waterbodies that do not meet water 

quality standards and require a TMDL. Therefore, 

Category 5 represents the Section 303(d) list of 

impaired waters. For more information on Integrated 

Reports, visit EPA’s Web site: 

www.epa.gov/owow/tmdl/2008_ir_memorandum.html

Characteristics of a TMDL 

• Addresses a variety of pollutants 

• Contains quantified targets 

• Considers all significant sources 

• Expresses pollutant loads in relationship 

to water quality standards 

• Provides allocations for known sources 

• Considers a MOS and future growth 

• Focuses on temporal nature of 

allocations to achieve standards 

• Requires sound data 

• Involves stakeholders 
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6 DRAFT November 2008 

• Linkage analysis to calculate the loading capacity. 

• Allocation analysis to evaluate and assign WLAs to point sources and LAs to nonpoint sources. 

• Developing the TMDL report and administrative record for submittal to EPA. 

• TMDL implementation to identify management activites to implement WLAs and LAs. 

Figure 1 also illustrates where to find a discussion of each activity in the Handbook. At the beginning of 

each chapter, you will find a modified version of Figure 1 that highlights the TMDL activities discussed 

in that chapter. 

Figure 1. Typical steps in the TMDL development process. 
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1.2. What Every TMDL Writer Should Know about the NPDES 

Stormwater Program 

To understand the NPDES Stormwater program, it is important to have an understanding of the NPDES 

program framework. This section provides a brief overview of the NPDES program to introduce basic 

permitting concepts and then provides details about the NPDES Stormwater program. 

1.2.1. NPDES Program Framework 

The CWA enacted in 1972 established the NPDES program and provides that either EPA or the state 

can administer (i.e., issue permits, assess compliance, take enforcement) the program. EPA, however, 

must first authorize a state to do so. Authorization requires that a state demonstrate to EPA’s satisfaction 

that it has the necessary legal authority, technical skills, and resources to administer the program. Once 

authorized, a state becomes the permitting authority, taking on the responsibility of administering the 

NPDES program, including issuance of NPDES stormwater permits. As of 2008, 45 states (excluding 

Alaska, Idaho, Massachusetts, New Hampshire, and New Mexico) and one territory (U.S. Virgin 

Islands) are authorized to administer the NPDES program.1

 Where a state is not authorized to administer 

the NPDES program, the EPA Regional office is the permitting authority. For the purpose of this 

Handbook, the term permit writer is intended to convey a state or EPA staff person responsible for 

acting on behalf of the state agency or EPA regional office serving as the NPDES permitting authority in 

that area. 

The NPDES regulations provide for two basic types of 

permits: individual and general. Dischargers requesting to be 

covered under an individual permit are required to submit an 

individual permit application, which the permit writer uses as 

the basis for developing site-specific permit requirements. 

The individual permit is then issued to that discharger for a 

period not to exceed 5 years, with a requirement to reapply 

before the expiration date. An individual permit is tailored 

specifically for an individual facility. 

When the permitting authority expects that many dischargers with similar types of activities will require 

coverage under an NPDES permit, the permitting authority may choose to issue a general permit in lieu 

of issuing individual permits to each of these dischargers. After a general permit is issued, dischargers 

wishing to be covered under the general permit submit a Notice of Intent (NOI) to the permitting 

authority. These dischargers, consistent with procedures specified in the general permit, are then 

authorized to discharge under the terms of that general permit. The CWA requires that NPDES permits, 

both individual and general, be made available to the public for at least 30 days for review and comment 

before final issuance. 

NPDES permits, with the exception of municipal separate storm sewer system (MS4) permits, must 

include technology-based effluent limitations based on best available technology economically 

achievable (BAT), or New Source Performance Standards (NSPS) for new sources, and any other more 

 

1

 EPA Regions retain permitting authority for most Indian lands, federal facilities in four states (Colorado, Deleware, 

Vermont, and Washington) certain oil and gas activities (Texas and Oklahoma) and agricultural activities (Oklahoma). 

Five Basic Components of General 

and Individual NPDES Permits 

• Cover page 

• Effluent limits (numeric or narrative; 

technology-based or water quality-based) 

• Monitoring and reporting requirements 

• Standard conditions 

• Special conditions 
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8 DRAFT November 2008 

stringent limitations as necessary to ensure that the discharge does not cause or contribute to in-stream 

exceedances of water quality standards. MS4s defined by the regulations as needing NPDES permit 

coverage are required to implement stormwater management programs (SWMPs) designed to control 

pollutants to the maximum extent practicable (MEP) to protect water quality rather than meeting a BAT 

requirement as for other NPDES permittees. NPDES-authorized states and territories may impose more 

stringent permit requirements than those set forth in federal regulations. 

1.2.2. NPDES Stormwater Program 

In 1987 Congress amended the CWA to require EPA to 

regulate certain stormwater discharges through the NPDES 

program. After promulgating stormwater rules in 1990 and 

1999 (known as the Phase I and Phase II stormwater rules, 

respectively), NPDES permitting authorities now issue permits to control stormwater discharges from 

(1) MS4s, (2) industrial activities, and (3) construction activities as follows: 

(1) Medium, large, and regulated small MS4s: MS4s, 

generally, are public storm sewer systems (including 

roads with drainage systems and municipal streets) 

that are owned or operated by a public body and not 

part of a combined sewer (i.e., storm and sanitary 

sewers combined). Incorporated places and counties 

meeting EPA’s definitions of medium and large 

MS4s are identified in the NPDES regulations in 40 

CFR Part 122, Appendices F through I. In general, 

these are separate storm sewer systems that serve 

populations over 100,000 people. Regulated small 

MS4s are identified according to the U.S. Census 

Bureau definition of urbanized area as established 

every 10 years in its decennial census. Populations 

served by these regulated small MS4s range from 

several hundred to tens of thousands of people, but in 

most instances these systems serve fewer than 

about 30,000–50,000 people. 

(2) Industrial facilities that fall under 11 categories of 

industrial activities that discharge to an MS4 or to 

waters of the United States (construction activity 

disturbing 5 acres or more is one of these 11 

categories, but because of the nature of its 

operations, it is addressed separately from the 

other 10 categories.). Industrial facilities (except 

construction) may certify to a condition of no 

exposure in lieu of obtaining NPDES permit 

coverage if their industrial materials and 

operations are not exposed to stormwater. 

Types of MS4 Infrastructure and 

Permittees 

The NPDES Stormwater program uses the term 

MS4 to describe the type of stormwater 

conveyance infrastructure, as well as the 

permittee required to obtain stormwater permit 

coverage. As a type of infrastructure, MS4s are 

not merely a system of pipes. As defined by 

regulations, an MS4 can also include drainage 

systems for roadways, gutters, and ditches. 

An MS4 permittee does not just include 

municipally owned storm sewer systems. The 

term MS4 can also apply to a variety of entities 

that own and operate MS4 infrastructure, such as 

departments of transportation, military bases, 

universities, hospitals, and prisons. 

Resources: For more information on the 

NPDES Stormwater program, refer to the 

Resources list at the end of this chapter in 

Section 1.4.3, including EPA’s NDPES Web 

site at www.epa.gov/npdes/stormwater. 

11 Categories of Industrial Activity 

Category One (i): Facilities subject to national effluent 

limitations 

Category Two (ii): Heavy Manufacturing 

Category Three (iii): Mining and Oil and Gas 

Category Four (iv): Hazardous Waste Storage, 

Treatment, or Disposal Facilities 

Category Five (v): Landfills 

Category Six (vi): Recycling Facilities 

Category Seven (vii): Steam Electric Plants 

Category Eight (viii): Transportation Facilities 

Category Nine (ix): Wastewater Treatment Works 

Category Ten (x): Construction Activity disturbing 5 

acres or more 

Category Eleven (xi): Light Industrial Activity 
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(3) Construction activity that disturbs one or more acre of land and less than one acre if the activity is 

part of a larger common plan of development or sale (USEPA 2004). The Phase I stormwater rule 

regulates construction activities 5 acres and above. The Phase II stormwater rule added sites between 

one and 5 acres. 

Figure 2 illustrates how the regulation of these stormwater sources affected the universe of regulated 

NPDES dischargers over time. From the beginning of the NPDES program, permit writers across the 

country developed and issued permits for approximately 60,000 facilities with wastewater discharges. 

The NPDES Phase I Stormwater program required stormwater discharges from large and medium MS4s, 

large construction activities, and industrial facilities to obtain NPDES permit coverage; bringing 

approximately 300,000 stormwater sources into the NPDES program. The NPDES Phase II Final Rule 

covering regulated small MS4s and construction sites between one and 5 acres added approximately 

200,000 additional stormwater sources to the NPDES program universe. 

Figure 2. Number of facilities or sources permitted under various programs of the NPDES program 

over time. 

A brief summary of key aspects of the NPDES Stormwater program is provided below. 

1.2.2.1. Types of NPDES Stormwater Permits 

To regulate the approximately 500,000 stormwater sources, the NPDES Stormwater program uses both 

individual and general permits. 

• Individual permits, issued to most medium and large MS4s and small MS4s in a few states, require 

the initial submission of a comprehensive permit application. However, because most MS4s have 

already been permitted, the content of future applications is expected to contain less information 

than the original submission. Applications will contain available data or a summary of that data and 

the permittee’s plans for future activities and controls to address any identified concerns. NPDES 

permitting authorities use the detailed permit application information to develop site-specific 

requirements. 
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• General permits, issued for most stormwater discharges associated with industrial and construction 

activities and small MS4s in most states, contain more widely applicable requirements. Stormwater 

dischargers submit an NOI to the permitting authority to obtain coverage under a general permit. The 

information necessary to complete an NOI for coverage under a general permit is usually (depending 

on state requirements) less burdensome than the information required for an individual permit. 

Coverage under a general permit is often relatively automatic when the discharger meets the 

eligibility requirements for coverage. Stormwater permittees that are unable to meet eligibility 

conditions for general permits must obtain coverage under an individual permit. Table 1 highlights 

the estimated number of stormwater permittees across the country by type of stormwater discharge 

and the number of general and individual permits issued to these permittees. 

Table 1. Estimated number of stormwater permittees and permits by type of stormwater discharge 

Type of stormwater discharge Permittees General permits Individual permits 

MS4 7,000 50 500 

Industrial 100,000 120 Unknown* 

Construction 250,000 60 Unknown* 

* These permits are not tracked separately under the stormwater program. EPA estimates that several hundred entities are 

permitted with individual permits each year. 

1.2.2.2. Standards and Limits in Stormwater Permits 

The CWA requires, with the exception of MS4s, 

that NPDES permits contain technology-based 

effluent limits and water quality-based effluent 

limits (WQBELs) when the technology-based 

limits alone do not adequately protect water 

quality. The CWA standard for MS4s is that the 

permit must require controls to reduce the 

discharge of pollutants to the MEP to protect 

water quality. Stormwater permits use a variety 

of approaches to incorporate these concepts into 

NPDES permit conditions. Generally, 

stormwater permits require implementation of 

BMPs, identified as narrative effluent limits, 

deemed by the permitting authority to be 

appropriate to meet the intent of the CWA. 

These narrative effluent limits include practices 

such as source control and pollution prevention 

BMPs. Stormwater permits typically stipulate 

general categories of controls, and charge the 

permittee to articulate the details in a stormwater 

plan. Occasionally, stormwater permits will 

establish numeric effluent limits that must be 

met at the discharge point. 

MEP Pollutant Reduction Standard for MS4s 

Operators of regulated MS4s must develop and implement 

SWMPs that reduce the discharge of pollutants to the MEP 

to protect water quality. While MEP is a pollutant reduction 

standard that applies to all permitted MS4s, the practical 

aspects of MEP varies from location to location depending 

on factors such as pollutant sources and local receiving 

water conditions and concerns. EPA’s Measureable Goals 

Guidance for Phase II MS4s

(http://cfpub.epa.gov/npdes/stormwater/measurablegoals/p

art1.cfm) states the following: 

The definition of “MEP” should adapt continually to 

both current conditions and BMP effectiveness, but 

ultimately, successive iterations of the mix of BMPs 

and measurable goals should be made to achieve 

the objective of meeting water quality standards. If, 

after implementing the minimum control measures, 

there is still water quality impairment associated with 

discharges from the MS4, you will need to expand or 

better tailor your BMPs (USEPA 2001). 

Stormwater permitting authorities must review MS4 

SWMPs to determine if implementation of the plan is likely 

to reduce the discharge of pollutants to the MEP. Where 

the permitting authority identifies deficiencies, the MS4 

must modify its SWMP. 
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1.2.2.3. Basic Stormwater Management Program and Pollution Prevention Plan 

Requirements 

The primary requirement of NPDES stormwater permits is the development and implementation of a 

plan that describes how the permittee will control the discharge of pollutants in stormwater to meet 

permit requirements. Operators of regulated MS4s develop and implement SWMPs that cover a variety 

of activities discharging to the MS4, while operators of industrial facilities and construction activities 

must develop and implement facility-specific stormwater pollution prevention plans (SWPPPs). A brief 

overview of the type of management activities required in MS4 (Phase I and Phase II), industrial, and 

construction permits is provided below. 

• Phase I MS4 Permits. Phase I MS4s, all of which 

were identified by incorporated place or county names 

in the 1990 Phase I Rule, were required to apply for 

permit coverage in the early 1990s with the application 

including the MS4s proposed SWMP to address 

programmatic, structural, and source control measures 

for stormwater discharges from commercial and 

residential areas, including discharges from areas of 

new development or redevelopment; illicit discharges; 

priority industrial facilities; and construction sites. 

Individual permits issued to Phase I MS4s specified the 

required SWMP activities and included other 

provisions to ensure effective implementation, such as 

monitoring and annual reporting. 

• Phase II MS4 Permits. Regulated, small MS4s are 

required to obtain individual or general permit 

coverage and implement an SWMP that addresses the 

six minimum control measures: (1) public education 

and outreach; (2) public participation and involvement; (3) illicit discharge detection and elimination 

(IDDE); (4) construction site stormwater runoff control; (5) post-construction stormwater 

management in new and redevelopment; (6) pollution prevention/good housekeeping for municipal 

operations. In addition, Phase II MS4s must also develop and specify measurable goals for each of 

the six minimum control measures in the SWMP. The Phase II Final Rule does not require 

monitoring data as part of the application or as a requirement of the permit, although permitting 

authorities may decide otherwise. Regulated small MS4s are required to submit periodic reports to 

the permitting authority ranging from annually to twice every 5-year permit cycle. 

Construction Stormwater Permits. NPDES permits for stormwater discharges associated with 

construction activities require control measures (i.e., BMPs) to address pollutants in stormwater 

discharges. These requirements, almost always addressed through the issuance of statewide general 

permits, include erosion and sediment control BMPs to prevent the discharge of sediment and 

measures to prevent the discharge of non-sediment materials such as construction debris, vehicle 

fluids, concrete washout, and trash. In addition, some permitting authorities require postconstruction stormwater management measures to minimize pollutant discharges after construction 

is complete. SWPPPs prepared by site operators describe activities to be performed and how the 

operator intends to comply with permit requirements (e.g., a description of controls to minimize 

•

Tip: Read SWMPs and SWPPPs—not 

just stormwater permits—to get 

implementation details. 

TMDL writers often go to NPDES stormwater 

permits looking for information on stormwater 

discharges that will assist in the TMDL 

development process. Reviewing stormwater 

permits will provide an understanding of what 

permittees must include in the required SWMPs 

and SWPPPs, but the permits rarely provide 

complete details on this information. TMDL 

writers might need to obtain and review SWMPs 

and SWPPPs to gather information to support 

TMDL activitites, such as pollutant source 

characterization and pollutant load allocation. 

Chapter 3 of this Handbook provides an indepth discussion of what information is available 

through SWMPs and SWPPPs and how TMDL 

writers can use this information to characterize 

stormwater sources. 
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12 DRAFT November 2008 

exposure of the materials to stormwater and spill prevention and response practices). NPDES 

construction permits also typically require the site operator to document in an SWPPP any interim 

and permanent stabilization practices for the site, including a schedule of when the practices will be 

implemented. 

• Industrial Stormwater Permits. Industrial stormwater permits typically focus on the 

implementation of BMPs to reduce stormwater pollutants. NPDES stormwater permits for industrial 

facilities typically require SWPPPs to document the facility’s pollution prevention team, describe the 

site, identify the receiving waters, and describe the BMPs that will be implemented to meet permit 

requirements. An SWPPP should also contain a summary of potential pollutant sources, including 

spills and leaks, and a summary of existing stormwater discharge sampling data. The focus of the 

SWPPP is the description of the existing and planned BMPs to reduce stormwater pollutants. BMPs 

may be procedural such as good housekeeping activities, spill prevention and response planning, 

preventative maintenance, routine facility inspections, and employee training or structural systems, 

such as containment systems or sediment basins. 

1.3. Key Challenges Associated with Connecting the TMDL and 

NPDES Stormwater Programs 

Improving the ways in which water quality programs work 

together and relate to one another often presents challenges 

because of programmatic, regulatory, and technical 

differences. Improving connections between the TMDL and 

NPDES Stormwater programs highlights challenges such as 

conflicting program priorities, and unsynchronized development schedules, lack of sufficient data, 

inadequate cross-program communication, and lack of adequate staff and resources. EPA and state 

TMDL and permit writers working to improve connections between the TMDL and NPDES Stormwater 

programs have identified some key challenges facing each program. EPA’s Total Maximum Daily Loads 

and National Pollutant Discharge Elimination System Storm Water Permits for Impaired Water Bodies: 

A Summary of State Practices (USEPA 2007) provides a detailed discussion of these key challenges and 

possible solutions. These key challenges are briefly summarized below and discussed in greater detail in 

subsequent chapters. 

1.3.1. Challenge 1: Addressing Differences in Organizational Structure 

Strengthening the connections between TMDLs and stormwater permits begins with communication and 

coordination among internal programmatic staff. Often this is challenging for a variety of reasons. In 

most instances, TMDL and NPDES permitting staff function not only in different programs, but in 

different organizational groups. In some states, the separation between programs is more drastic, with 

TMDL and NPDES staff functioning under different agencies. These organizational differences can 

create real and perceived obstacles for effective staff coordination. In addition, agency staff can face 

programmatic pressures (e.g., court ordered deadlines, permit reissuance schedules) that limit timely 

coordination. 

Resources: For more information on 

research and policy related to TMDLs and 

stormwater, refer to the Resources list at 

the end of this chapter in Section 1.4.2. 
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1.3.2. Challenge 2: Developing Consistent Stormwater Allocations in 

TMDLs 

Approaches for developing and expressing TMDLs that include stormwater sources can vary within and 

among states. For example, some TMDLs assign aggregated stormwater WLAs to all permitted 

stormwater sources within a watershed or to each type of permitted stormwater source. Collecting 

additional dataor information on the permitted stormwater sources within a watershed might allow 

TMDL writers to generate more detailed WLAs that could facilitate the development of permit 

requirements. 

1.3.3. Challenge 3: Translating Numeric TMDL WLAs into Implementation 

Strategies and Permit Requirements 

One way for TMDL and permit writers to facililitate the implementation of TMDLs is to provide 

permittees with information on specific types of management strategies that could be used to implement 

the WLAs. In most cases, this means facilitating efforts so that the WLA, the TMDL implementation 

plan (if applicable), and the stormwater permit requirements are developed to coordinate with each other 

in a meaningful way, particularly when stormwater sources are covered exclusively by general permits. 

1.3.4. Challenge 4: Reconciling Spatial Boundaries of TMDLs with 

Boundaries of NPDES Stormwater Permits 

The TMDL program often provides information on impaired waters by waterbody or watershed. The 

majority of actions under the NPDES Stormwater program focus on site-specific activities (e.g., 

construction sites and industrial facilities) and large stormwater conveyances systems (e.g., MS4s). The 

different spatial scales at which regulatory agencies provide information to, and require information 

from, permittees can create challenges as permittees attempt to identify applicable requirements and 

determine the appropriate locations to implement the requirements. 

1.3.5. Challenge 5: Incorporating Monitoring, Tracking, and Adaptive 

Management Elements into TMDL WLAs and Stormwater Permits 

Monitoring and evaluation are key components to determining if stormwater management efforts are 

producing the necessary pollutant load reductions identified in the TMDLs, thereby making progress 

toward attainment of water quality standards. Because it is not required, most TMDLs do not address 

monitoring or, if the need is mentioned, the type and frequency of monitoring necessary to demonstrate 

progress towards attaining and maintaining water quality standards. Stormwater permits require all 

stormwater permittees to evaluate the efficacy of their SWMP or SWPPP. However, the evaluation 

process does not usually involve end-of-pipe or in-stream analytical monitoring to directly evaluate 

stormwater discharge or ambient water quality. To more effectively connect TMDLs and stormwater 

permits, the TMDL could include recommendations regarding monitoring, tracking, and adaptive 

management activities, and the relevant stormwater permits could either reference the recommended 

activities or adopt these recommendations. Permit writers can consider permittees to identify milestones 

on the basis of criteria (water quality- or technology-based) that use the monitoring and tracking 

information to drive adaptive management efforts. 
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14 DRAFT November 2008 

1.4. Resources 

1.4.1. TMDL Program 

1. EPA’s TMDL Web site: www.epa.gov/owow/tmdl/ 

2. USEPA (U.S. Environmental Protection Agency). 2005. Guidance for 2006 Assessment, Listing, and 

Reporting Requirements Pursuant to Sections 303(d), 305(b), and 314 of the Clean Water Act [2006 

Integrated Report Guidance (IRG)]. U.S. Environmental Protection Agency, Office of Water, Office 

of Wetland, Oceans and Watersheds, Washington, DC. 

www.epa.gov/owow/tmdl/2006IRG/#documents

This document provides a recommended reporting format and suggested content to be used in 

developing a single document that integrates the reporting requirements of the CWA sections 

303(d), 305(b), and Part 314. The report also provides a comprehensive compilation of EPA’s 

previous guidance related to integrated reporting. 

3. USEPA (U.S. Environmental Protection Agency). 2006. Information Concerning 2008 Clean Water 

Act Sections 303(d), 305(b), and 314 Integrated Listing and Reporting Decisions. Memorandum 

from Diane Regas, Director, Office of Wetlands, Oceans and Watersheds to Regions 1-10 Water 

Division Directors. www.epa.gov/owow/tmdl/2008_ir_memorandum.html

This EPA memorandum dated October 12, 2006, provides information to assist in the 

preparation and review of 2008 integrated water quality reports to supplement the information 

provided in the 2006 Integrated Report Guidance. 

4. USEPA (U.S. Environmental Protection Agency). 1991. Guidance for Water-Quality-based 

Decisions: The TMDL Process. EPA 440/4-91-001. U.S. Environmental Protection Agency, Office 

of Water, Washington, DC. www.epa.gov/OWOW/tmdl/decisions/ 

This guidance document explains the programmatic elements and requirements of the TMDL 

process as established by CWA section 303(d) and by EPA’s Water Quality Planning and 

Management Regulations (40 CFR Part 130). It discusses the process for developing a TMDL, 

roles of EPA and the states in the process, and supporting or related water programs. 

5. Best-Wong, B. 2006. Clarification Regarding “Phased” Total Maximum Daily Loads. 

Memorandum from Benita Best-Wong, Director, Assessment and Watershed Protection Division, to 

Water Division Directors, Regions 1–10, August 2, 2006. 

www.epa.gov/owow/tmdl/tmdl_clarification_letter.html

This memorandum clarifies the Guidance for Water Quality-Based Decisions: The TMDL 

Process, issued in 1991, by explaining EPA’s interpretation of the term phased TMDL as used in 

EPA guidance and explaining the distinction between phased TMDLs, staged implementation, 

and adaptive implementation. 

6. EPA’s Guidelines for Reviewing TMDLs Under Existing Regulations Issued in 1992: 

www.epa.gov/owow/tmdl/guidance/final52002.html
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November 2008 DRAFT 15 

This Web document provides guidance on the required elements of a TMDL report for use in 

reviewing and approving TMDLs. 

1.4.2. TMDLs and Stormwater 

1. EPA’s TMDL and Stormwater Resources Web site: www.epa.gov/owow/tmdl/stormwater 

2. USEPA (U.S. Environmental Protection Agency). 2007. Total Maximum Daily Loads with 

Stormwater Sources: A Summary of 17 TMDLs. EPA 841-R-07-002. U.S. Environmental Protection 

Agency, Office of Wetlands, Oceans and Watersheds, Washington, DC. 

www.epa.gov/owow/tmdl/17_TMDLs_Stormwater_Sources.pdf 

This document summarizes 17 TMDLs that have been developed for stormwater-source 

pollutants in 16 states, representing a range of pollutants, models used, and different allocation 

and implementation methods. 

3. USEPA (U.S. Environmental Protection Agency). 2007. Total Maximum Daily Loads and National 

Pollutant Discharge Elimination System Storm Water Permits for Impaired Water Bodies: A 

Summary of State Practices. U.S. Environmental Protection Agency, Region 5, Chicago, IL. 

www.epa.gov/region5/water/wshednps/pdf/state_practices_report_final_09_07.pdf 

This report summarizes information on TMDL-stormwater practices in 10 states, provides 

specific TMDL and permit language, and identifies some specific technical and programmatic 

challenges. 

4. Wayland, R.H., and J.A. Hanlon. 2002. Establishing Total Maximum Daily Load (TMDL) Wasteload 

Allocations (WLAs) for Storm Water Sources and NPDES Permit Requirements Based on Those 

WLAs. Memorandum from Robert H. Wayland, III, Director, Office of Wetlands, Oceans and 

Watersheds, and James A. Hanlon, Director, Office of Wastewater Management, U.S. 

Environmental Protection Agency, Washington, DC. www.epa.gov/npdes/pubs/final-wwtmdl.pdf

This memo clarifies existing EPA regulatory requirements for, and provides guidance on, 

establishing WLAs for stormwater discharges in TMDLs approved or established by EPA. 

5. NRC (National Research Council). 2008. Urban Stormwater Management in the United States. 

Committee on Reducing Stormwater Discharge Contributions to Water Pollution, Water Science and 

Technology Board, Division on Earth and Life Studies, National Research Council of the National 

Academies. National Academies Press, Washington, D.C. 

The report provides a description of the history of stormwater management in the United States; 

an overview of stormwater regulations and the federal regulatory program; and information on a 

number of relevant scientific and technological issues such as hydrology, geomorphology, 

biology, monitoring and modeling. The report also provides a number of significant findings and 

recommendations on how stormwater management in the United States should be improved to 

achieve better environmental outcomes 

1.4.3. NPDES Stormwater Program 

1. EPA’s NPDES Stormwater program Web site: http://cfpub.epa.gov/npdes/home.cfm?program_id=6
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16 DRAFT November 2008 

2. EPA’s NPDES Stormwater Program Authorization Status: 

http://cfpub.epa.gov/npdes/stormwater/authorizationstatus.cfm

3. NRC (National Research Council). 2008. Urban Stormwater Management in the United States. 

Committee on Reducing Stormwater Discharge Contributions to Water Pollution, Water Science and 

Technology Board, Division on Earth and Life Studies, National Research Council of the National 

Academies. National Academies Press, Washington, D.C. 

The report provides a description of the history of stormwater management in the United States; 

an overview of stormwater regulations and the federal regulatory program; and information on a 

number of relevant scientific and technological issues such as hydrology, geomorphology, 

biology, monitoring and modeling. The report also provides a number of significant findings and 

recommendations on how stormwater management in the United States should be improved to 

achieve better environmental outcomes. 

1.4.3.1. MS4s 

1. EPA’s NPDES Stormwater Discharges from Municipal Separate Storm Sewer Systems Web site: 

http://cfpub.epa.gov/npdes/stormwater/munic.cfm

2. USEPA (U.S. Environmental Protection Agency). 2007. Municipal Separate Storm Sewer System 

(MS4) Program Evaluation Guidance. EPA-833-R-07-003. U.S. Environmental Protection Ageny, 

Office of Wastewater Management, Water Permits Division, Washington, DC. 

www.epa.gov/npdes/pubs/ms4guide_withappendixa.pdf and 

www.epa.gov/npdes/pubs/ms4guide_appendicesb-d.pdf

This guide is primarily for use by NPDES authorities to evaluate the quality of Phase I and Phase 

II MS4 programs for permit compliance, technical assistance, and other purposes. It can be used 

for comprehensive program evaluations or for certain components. MS4 program managers can 

also use it to evaluate their own programs. 

3. USEPA (U.S. Environmental Protection Agency). 2005. Stormwater Phase II Final Rule Fact Sheet: 

Small MS4 Stormwater Program Overview. EPA 833-F-00-002. U.S. Environmental Protection 

Agency, Office of Water. www.epa.gov/npdes/pubs/fact2-0.pdf

This three-page fact sheet provides the definition of regulated small MS4s and associated permit 

requirements for regulated small MS4s. 

1.4.3.2. Industrial Activities 

1. EPA’s NPDES Stormwater Discharges from Industrial Facilities Web site: 

http://cfpub.epa.gov/npdes/stormwater/indust.cfm

2. List of sectors of industrial activity that require permit coverage: 

http://cfpub.epa.gov/npdes/stormwater/swcats.cfm

3. USEPA (U.S. Environmental Protection Agency). 1992. Stormwater Management for Industrial 

Activities: Summary Guidance on Developing Pollution Prevention Plans and Best Management 
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Practices. EPA 833-R-92-002. U.S. Environmental Protection Agency, Office of Water, 

Washington, DC. www.epa.gov/npdes/pubs/owm0236a.pdf 

This document provides guidance on the SWPPP requirements and includes a set of worksheets, 

a checklist, and a sample SWPPP. 

1.4.3.3. Construction Activities 

1. EPA’s NPDES Stormwater Discharges from Construction Activities Web site: 

http://cfpub.epa.gov/npdes/stormwater/const.cfm

2. USEPA (U.S. Environmental Protection Agency). 2007. Developing Your Stormwater Pollution 

Prevention Plan: A Guide for Construction Sites. EPA 833-R-060-04. U.S. Environmental 

Protection Agency, Washington, DC. http://cfpub.epa.gov/npdes/stormwater/swppp.cfm

This guidance document is a reference for construction site operators who must comply with an 

NPDES stormwater permit. Through its description of the SWPPP development process, this 

guidance addresses the type of information required in an SWPPP and could help TMDL writers 

determine if SWPPPs will provide information and data useful to the TMDL development 

process. 
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2. IDENTIFYING OPPORTUNITIES TO COORDINATE TMDLS AND 

STORMWATER PERMITS 

Improving the connection between TMDLs and 

stormwater permits can start with TMDL and 

permit writers taking steps to improve 

communication through efforts such as (1) 

coordinating programmatic schedules and 

activities and (2) developing institutional and 

organizational communication mechanisms. 

Improved communication at the programmatic 

level can lead to improved coordination of 

technical activities, such as assessment and 

monitoring. Improving internal coordination and 

communication among TMDL and permit 

writers can foster good communication and 

information sharing with key stakeholders, 

including stormwater sources. As shown in 

Figure 3, stakeholder and public involvement is 

an important element of the overall TMDL 

process. Engaging and involving stakeholders is 

required under both the TMDL and NPDES 

Stormwater programs and provides stakeholders 

with the opportunity to share stormwater related data and information to strengthen and focus the overall 

TMDL analysis and related implementation efforts. This chapter addresses opportunities to improve 

coordination and communication both internally and with key stakeholders that affect stormwater 

management decisions and activities. 

2.1. Improving Internal Communication and Coordination 

Schedules and priorities for the TMDL program are driven by 

factors such as court-ordered deadlines, waterbody rankings under 

the section 303(d) list, rotating watershed basin planning 

approaches, as well as available staff and resources. Factors such 

as regulatory requirements, permit expiration dates, rotating 

watershed basin planning and assessment approaches, as well as 

available staff and resources, affect NPDES stormwater permit 

development schedules. Regardless of how internal program schedules and priorities are set, it could 

prove beneficial for TMDL and permit writers to participate in some type of internal planning to 

determine short-term and long-term schedules for TMDLs and stormwater permit development. 

Examining internal program schedules is a good starting point for identifying opportunities for 

coordination and collaboration between the programs. 

TMDL and permit writers should collectively examine each program’s schedule to determine the timing 

for (1) waterbodies and watersheds with ongoing TMDL development activities for impairments with 

known or suspected stormwater sources; (2) waterbodies and watersheds with planned TMDL 

Figure 3. Illustration of the steps in the TMDL 

process, including the step of stakeholder 

involvement and public participation. 

Resources: For more information on 

approaches states and EPA Regions 

use to improve coordination between 

the TMDL and Stormwater programs, 

refer to the Resources list at the end 

of this chapter in Section 2.3. 
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November 2008 DRAFT 21 

development activities for impairments with known or suspected stormwater sources; and (3) general 

and individual stormwater permits nearing expiration, expired, or administratively continued discharging 

to impaired waterbodies or within impaired watersheds that have ongoing or planned TMDL 

development activities. Ultimately, the goal is to identify stormwater source TMDLs and stormwater 

permits that fall into the following categories: impending, in progress, and developed. 

As illustrated in Table 2, there are a variety of ways that TMDL and permit writers can promote better 

connections between TMDLs and stormwater permits regardless of the development status. Significant 

opportunities for coordination include activities such as data collection and sharing, stakeholder 

involvement, permit and TMDL language development, and process administration. It is important to 

note that coordination opportunities might vary depending on the type of permit used to implement the 

TMDL. Table 2 highlights where differences in opportunities exist according to permit type. 

Planning and scheduling is only one factor affecting internal TMDL and NPDES Stormwater program 

coordination and collaboration. Organizational structures that affect how easily staff from the two 

programs can work together also influence TMDL and permit writer coordination and collaboration. 

Several EPA Regions and state agencies have reorganized to bring the TMDL and NPDES Stormwater 

programs under a common management unit (e.g., branch, division, group). Some have gone beyond 

bringing the programs together and have taken steps to ensure further integration either by developing 

TMDL-stormwater teams or specific positions tasked with promoting stormwater-source TMDL 

implementation. 

Table 2. Potential opportunities for coordination based on status of TMDL and stormwater permit 

development 

Status of 

TMDL 

Status of 

stormwater 

permit 

Potential opportunities for coordination 

Individual permit General permit 

Impending Impending 

(New or 

Anticipated 

Reissuance) 











Identify available data, data gaps, and 

develop integrated approach for collecting 

additional data 

Attempt to synchronize schedules for 

coordinated development 

Develop coordinated TMDL and permit 

stakeholder involvement process 

Conduct collaborative kick-off meeting 

among TMDL and permit writers and 

permittee(s) that integrates discussion of 

TMDL development issues and 

implementation considerations 

Determine how the permit can address 

future TMDLs 









Conduct internal collaborative kick-off 

meeting among TMDL and permit writers 

Determine if TMDL will recommend 

specific stormwater BMPs recommended 

to implement the WLA 

Coordinate development of TMDL and 

permit language to ensure consistency 

Determine how the permit can address 

future TMDLs 

In Progress 





Consider adjusting permit development 

schedule to track with TMDL 

development 

Share data and information collected 

through permit application process with 

TMDL writers to inform TMDL 

development 

Discuss potential TMDL data needs and 

incorporate permit requirements that 

focus on monitoring and other types of 

data collection that will inform TMDL 

development 

 Tailor permit language to acknowledge 

the need for permittees to review and 

modify SWMP/SWPPP to achieve 

consistency with WLAs upon approval of 

the TMDL 
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22 DRAFT November 2008 

Status of 

TMDL 

Status of 

stormwater 

permit 

Potential opportunities for coordination 

Individual permit General permit 

Developed 



Discuss options for modifying the permit 

to be consistent with TMDL WLA 

assumptions. 

Share data and information collected 

through permit application process with 

TMDL writers to inform TMDL 

development 







Determine if the permit will be consistent 

with TMDLs approved at the time of 

permit issuance or at the time of NOI 

submission 

Evaluate existing permit language 

regarding consistency with approved 

WLAs to determine if it is adequate or 

requires revision during permit reissuance 

process 

Consider developing a technical 

appendix/amendment to update existing 

stormwater permit, without triggering 

permit modification requirements, to 

compile and present TMDL requirements 

upon completion 

In-Progress Impending 

(New or 

Anticipated 

Reissuance) 





Discuss TMDL development process and 

options for WLA categorization to enable 

sources to implement them through 

planned permit requirements 

Develop specific requirements that 

directly relate to implementation of the 

proposed WLAs, such as monitoring and 

SWMP/SWPPP assessment and 

modification 





Consider including any implementation 

planning information from the TMDL into 

the permit either directly or by reference 

Identify data gaps discovered through the 

TMDL development process and 

determine what type of permit 

requirements are appropriate to include to 

facilitate filling data gaps for potential 

future revision of the TMDL or to support 

adaptive management activities 

In Progress 







Compare information collected to date 

under each program to ensure that each 

process is working with the same data 

and information and to determine if one 

program can fill any data gaps identified 

by the other program 

Attempt to streamline interaction with 

stakeholders by conducting joint meetings 

or consolidating data requests 

Identify activities under each process to 

determine if there is any overlap and any 

opportunity for integrating remaining 

activities 

Ensure that any language used in both 

the TMDL and the permit are consistent; 

provide updates on changes to language 

as necessary 





Identify activities under each process to 

determine if there is any overlap and any 

opportunity for integrating remaining 

activities 

Ensure that any language used in both 

the TMDL and the permit are consistent; 

provide updates on changes to language 

as necessary 

Developed 



Review existing permit requirements and 

required permit documents (e.g., SWMPs 

and SWPPPs) to determine if existing 

permittee-generated data and information 

can facilitate remaining TMDL 

development activities 

Discuss options for modifying the permit 

to be consistent with TMDL WLA 

assumptions and requirements 







Determine if the permit will be consistent 

with TMDLs approved at the time of 

permit issuance or at the time of NOI 

submission 

Evaluate existing permit language 

regarding compliance with proposed 

WLAs to determine if it is adequate or 

requires revision during permit reissuance 

process 

Consider developing a technical 

appendix/amendment to update existing 

stormwater permit, without triggering 

permit modification requirements, to 

compile and present TMDL requirements 

upon completion 
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November 2008 DRAFT 23 

Status of 

TMDL 

Status of 

stormwater 

permit 

Potential opportunities for coordination 

Individual permit General permit 

Developed Impending 

(New or 

Anticipated 

Reissuance) 





Review the approved WLA to determine 

how best to reflect the input factors and 

pollutant loads established in the WLA 

for implementation into new or reissued 

permit requirements 

Identify any TMDL implementation 

planning activities or information that the 

permit could incorporate or reference to 

help permitted stormwater sources 

implement the approved WLA 





Determine most feasible and appropriate 

requirements to include in the permit to 

demonstrate SWMP or SWPPP progress 

toward implementing the WLA 

Consider developing a technical 

appendix/amendment to compile and 

present TMDL information 

In Progress 



Review draft permit conditions to ensure 

consistency with the approved WLA and 

any TMDL implementation plan 

recommendations 

Develop and incorporate tailored 

monitoring requirements to assess 

progress toward implementing the WLA 





Review draft permit conditions to ensure 

requirements related to impaired 

waterbodies with approved TMDLs 

reflect WLAs and , if applicable TMDL 

implementation plan recommendations 

Ensure that draft permit conditions 

include meaningful monitoring and 

assessment requirements that will 

support adaptive management activities 

Developed 





Assess if current permit conditions are 

consistent with approved WLA 

Determine if mechanisms exist to assess 

progress toward implementing WLAs 

Plan potential changes to future versions 

of the permit 







Assess if current permit conditions are 

consistent with approved WLA 

Determine if mechanisms exist to assess 

progress toward implementing WLAs 

Plan potential changes to future versions 

of permit conditions 

In Practice: Efforts to Promote Improved Coordination Between TMDL and Permit Activities 

States and EPA are making strides to promote improved coordination between TMDL and permit writers. A few 

examples of internal efforts are provided below. 

Consolidating stormwater-source TMDL development with one group. The Vermont Department of 

Environmental Conservation has a Stormwater Management Section that focuses on administering the state’s 

stormwater program—both the federal NPDES Stormwater program and the state-authorized SWMP. Although 

staff in this section address stormwater permitting issues, they also tackle TMDL development if the impairment 

involves stormwater. TMDL staff hand off stormwater-related TMDL development and implementation to staff in 

the Stormwater Management Section that have expertise both in permitting issuance and stormwater BMP 

implementation. 

Assigning staff to coordinate and promote implementation. In Minnesota and Tennessee, the state agencies 

have recently created new positions intended to promote successful stormwater management implementation 

and connection to the TMDL program, as well as other related requirements. Oregon assigns one person to 

coordinate TMDL development and permitting within one watershed, although that one person might not 

necessarily be responsible for actually developing the TMDLs or crafting permit language. 

Bringing staff physically and organizationally together. Promoting more effective communication and data 

sharing often requires reorganizing staff to share physical space or space on an organizational chart. EPA 

Region 2 reorganized TMDL and permit staff to bring them together under one branch. EPA Region 10 

physically moved TMDL and permit staff to share a common area. EPA Region 4 is reorganizing to include 

TMDL development, permits, and nonpoint sources together in one branch 

Facilitating regular communication. EPA headquarters works with EPA Regional Offices to host monthly TMDL 

and Stormwater Workgroup conference calls. These internal calls provide EPA TMDL and permit staff from 

every region with the opportunity to exchange information and ideas related to TMDL development and 

implementation through stormwater permits. The group participates in developing technical resources and 
CHAPTER TWO: IDENTIFYING OPPORTUNITIES TO 

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24 DRAFT November 2008 

shares information on TMDL and stormwater projects. EPA Region 4 conducts bimonthly conference calls with 

state TMDL and stormwater permit staff to exchange information and ideas on issues affecting coordination 

between the two programs. 

2.2. Improving Stakeholder Communication and Coordination 

Another reason to promote coordination between TMDL and permit writers is to enable key 

stakeholders, including stormwater sources, to participate in the TMDL development process. Permit 

writers can help to encourage stormwater sources to participate in the TMDL development process and 

facilitate information sharing. Although TMDL writers are likely to work with permit writers to obtain 

permit-related information from stormwater sources, there might be instances when TMDL writers have 

to go directly to stormwater sources to obtain information and data. Stormwater sources might have an 

additional level of comfort and willingness to share information knowing their permit writer is involved 

in the process. 

Stakeholder involvement is an essential component of both the 

TMDL development and NPDES permitting process. Both the 

TMDL and the NPDES programs contain regulatory requirements 

for public participation that involve activities such as public 

review, public notice, public hearings, and public comment 

periods Ideally, TMDL and permit writers should identify and involve stakeholders early in both the 

TMDL development and NPDES permitting processes. The primary stakeholders in the TMDL and 

NPDES processes are parties directly affected by the TMDL allocations or the permit requirements. In 

the case of stormwater source TMDLs, the primary stakeholders are permitted stormwater sources that 

will receive a WLA under the TMDL and must implement the WLA through stormwater permits. In 

addition to permitted stormwater sources, other stakeholders include those affected by the TMDL and 

permitting decisions or have information to contribute to the process, such as local environmental 

organizations, homeowner associations, universities, local developers, and city planners. 

Providing data and information is one of the most significant contributions that stakeholders can make to 

the stormwater source TMDL development process. TMDL and permit writers can consider 

opportunities and approaches to facilitate information exchanges and technical participation in the 

TMDL development process. These opportunities could include face-to-face information exchange 

meetings, facility visits, focus group meetings for feedback on technical issues pertaining to stormwater 

source WLA development and associated permit requirements, or regular group conference calls to 

discuss ideas and progress. As mentioned in the previous section, permit writers might have access to 

much of the data and information generated by stakeholders through compliance with existing 

stormwater permit requirements. (Chapter 3 of this Handbook provides a discussion of the types of 

information sources generate through stormwater permit requirements and how TMDL writers can use 

this information in the stormwater source TMDL development process.) 

Resources: For more ideas and 

strategies to involve stakeholders, 

refer to the Resources list at the end 

of this chapter in Section 2.3.2.
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November 2008 DRAFT 25 

In Practice: Using Stakeholder Data and Information to Identify High-Priority Stormwater 

Sources 

The 2007 Charles River watershed (Massachusetts) pathogen TMDL relies on data from the Charles River Hot 

Spot monitoring effort to identify and prioritize bacterial sources of pollution. This monitoring effort, crucial to the 

TMDL development process, is the result of a single watershed stakeholder dedicated to voluntarily kayaking 

the Lower Charles River shoreline to conduct sampling after rain events. From 2002 through 2005, this 

dedicated watershed stakeholder followed sampling procedures detailed in the approved Charles River 

Watershed Association Quality Assurance Project Plan (QAPP) for fecal coliform. He was able to collect 

samples from several hundred storm drain outfalls during this period. Data from this stakeholder-based 

monitoring effort allowed the Massachusetts Department of Environmental Protection to identify and prioritize 

31 stormwater outfalls along the Lower Charles River as high-priority bacterial sources in the TMDL analysis. 

Ultimately, the data collected by this dedicated watershed stakeholder will help the Massachusetts Department 

of Environmental Protection and other key stakeholders target future bacterial source tracking efforts and 

implementation activities. 

In Practice: Locally Led TMDL Implementation 

The DuPage River Salt Creek Workgroup (DRSCW) is a collaborative stakeholder effort by sanitary districts, 

municipalities, counties, forest preserve districts, state and federal agencies, and private environmental 

organizations to address the water quality impairments identified in chloride and dissolved oxygen TMDLs for 

branches of the DuPage River and Salt Creek (Illinois). Although Illinois EPA held public meetings during the 

development of the TMDL reports, a watershed-based stakeholder group did not exist in the area when the 

TMDL reports were written. Stakeholders affected by the TMDL allocations wanted an opportunity to 

substantiate implementation strategies and determine whether there were other cost-effective options for 

achieving water quality standards (DRSCW 2004). Representatives from municipalities affected by the TMDL 

reports discussed forming the workgroup to collect data and carry out other technical activities to move forward 

with implementing the TMDLs. It was also envisioned that the DRSCW could help stakeholders establish a 

solid foundation for future TMDLs, contribute to developing nutrient criteria, and address other water quality or 

regulatory issues in the watersheds. A core group of municipalities generated support for the workgroup 

concept by emphasizing the importance of locally led decisions on where and how to spend local money to 

address water quality issues. 

2.3. Resources 

2.3.1. TMDL and Stormwater Permitting 

1. EPA’s TMDL and Stormwater Web site: www.epa.gov/owow/tmdl/stormwater/ 

2. USEPA (U.S. Environmental Protection Agency). 2007. Total Maximum Daily Loads and National 

Pollutant Discharge Elimination System Storm Water Permits for Impaired Water Bodies: A 

Summary of State Practices. U.S. Environmental Protection Agency, Region 5, Chicago, IL. 

www.epa.gov/region5/water/wshednps/pdf/state_practices_report_final_09_07.pdf 

This EPA report summarizes information on TMDL-stormwater practices in 10 states, provides 

specific TMDL and permit language, and identifies some specific technical and programmatic 

challenges. 
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26 DRAFT November 2008 

2.3.2. Identifying and Involving Stakeholders 

1. USEPA (U.S. Environmental Protection Agency). 2008. Handbook for Developing Watershed Plans 

to Restore and Protect Our Waters. EPA 841-B-08-002. U.S. Environmental Protection Agency, 

Office of Water, Nonpoint Source Control Branch, Washington, DC. 

www.epa.gov/nps/watershed_handbook/ 

Chapter Three: Building Partnerships (www.epa.gov/nps/watershed_handbook/pdf/ch03.pdf): 

This chapter provides guidance on initial activities to organize and involve interested parties in 

watershed-based water quality protection activities. Topics include identifying stakeholders, 

integrating other key programs, and conducting outreach. 

2. Conservation Technology and Information Center Web site, Know Your Watershed: Building Local 

Partnerships: www2.ctic.purdue.edu/KYW/Brochures/BuildingLocal.html
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November 2008 DRAFT 27 
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28 DRAFT November 2008 

3. CHARACTERIZING IMPAIRMENTS AND STORMWATER SOURCES 

The general process for developing a TMDL including stormwater sources is much the same as that for 

developing any TMDL, including the following typical steps: 

• Watershed characterization to identify the watershed, waterbody, and impairment conditions; TMDL 

targets; and potential sources 

• Linkage analysis to calculate the loading capacity 

• Allocation analysis to evaluate and assign WLAs to point sources and LAs to nonpoint sources 

• Development of the TMDL report and administrative record for submittal to EPA 

While these steps are common to all TMDL 

development projects, there are a number of 

considerations for each step when developing 

TMDLs that address stormwater sources. As 

shown in Figure 4, this chapter discusses the step 

of watershed characterization, and the remaining 

steps to calculate the loading capacity, establish 

allocations and document the TMDL report are 

discussed in Chapter 4. 

The TMDL process requires a thorough 

understanding of the waterbody and watershed 

characteristics, available data, causes of 

impairment, sources, water quality standards, 

and potential targets. Some of this information 

will be available through a state’s 303(d) list and 

waterbody assessment documentation, but much 

of the information will be gathered and 

summarized while completing the TMDL. 

Collectively, this is referred to here as the 

watershed characterization step of the TMDL. 

Watershed characterization serves as the foundation of the TMDL analysis, providing a basic 

understanding of the impairments of concern, the desired levels for restoration (e.g., water quality 

standards and TMDL targets) and the likely sources contributing to the impairment. Characterizing the 

waterbody and the associated impairments as well as the sources and other watershed characteristics 

provides the necessary background information to support decisions regarding the approach used for 

calculating the TMDL, the level of detail or focus of the analysis, and ultimately TMDL 

implementation. The following sections describe the major elements of the watershed characterization: 

• Understanding the impairment 

• Identifying TMDL targets 

• Identifying and assessing potential sources 

Figure 4. Illustration of the steps in the TMDL 

process, including the step of watershed 

characterization discussed in this chapter. 
CHAPTER THREE: COMPILING INFORMATION TO 

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November 2008 DRAFT 29 

At the end of the watershed characterization step for a TMDL addressing stormwater sources, the 

TMDL writer should understand how stormwater is affecting the impaired waterbody, what stormwater 

sources exist in the watershed, and what data and information are available to characterize the sources. 

3.1. Understanding the Impairment 

Understanding the impairment(s) being addressed by a TMDL is critical to establishing appropriate 

TMDL targets, identifying potential sources and eventually selecting a technical approach for 

calculating the loading capacity. The main objective of this step is to identify the nature of the 

impairment(s) being addressed by the TMDL, including location, timing, and magnitude of impairment. 

A state or tribe’s 303(d) list identifies the basic information regarding the impaired waterbody and the 

observed impairment, usually including the waterbody characteristics (e.g., name, location, size), the 

water quality standard that was violated, the pollutant of concern (if known), and the suspected causes 

and sources contributing to the impairment. It is usually necessary to analyze available monitoring data 

to further characterize and understand the impairments. This section first introduces the commonly 

observed impairments associated with stormwater and discusses how stormwater can affect waterbody 

conditions. The section then describes the types of data analyses that are typically used to support 

characterization of impairments for TMDL development, highlighting the issues unique to developing 

stormwater TMDLs. 

It is important to note that flow is a key component in 

characterizing, developing and implementing TMDLs for 

stormwater sources and is discussed throughout this Handbook. 

Quantity of flow and variation in flow regimes are important 

factors in transporting stormwater pollutants such as metals, 

pathogens or sediments that violate water quality standards. Flow 

is taken into account when developing loading analyses, and 

flow is specifically considered when calculating seasonal 

variation and critical conditions in a TMDL. The TMDL regulations specify that TMDLs can be 

expressed in terms of either mass per time, toxicity or other appropriate measure, and flow has been 

used as a surrogate for stormwater pollutants, as discussed in Section 3.2. Flow is also a critical 

component to consider when TMDLs are being implemented. Additonal information on this is 

presented in Chapter 5 and Chapter 6. 

3.1.1. Stormwater Effects on Receiving Waterbodies 

The purpose of this section is to briefly summarize the effects that stormwater has on streams and lakes 

and other receiving waterbodies. Stormwater can affect waterbodies in a number of ways depending on 

the type of stormwater source and the waterbody characteristics. While stormwater sources can include 

illicit discharges and dry-weather flows, most stormwater sources represent precipitation-driven runoff 

from impervious and pervious surfaces. The effects of stormwater runoff have been well documented in 

multiple journal aricles, books and other publications. Most recently, the National Research Council has 

issued a report, Urban Stormwater Management in the United States (NRC 2008), that describes in 

detail the hydrologic, geomorphic, and biological effects of urbanization on watersheds. The conclusions 

derived from these reports are that increased imperviousness and stormwateer discharges can lead to the 

following effects: 

Resources: Several EPA documents 

include discussion of flow in developing 

TMDLs and are included in the 

Resource section at the end of this 

chapter in Section 3.4.2. In addition, 

Resource sections at the end of other 

chapters include TMDL documents 

available for further information. 
CHAPTER THREE: COMPILING INFORMATION TO 

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30 DRAFT November 2008 

• Altered stream hydrology, including higher peak flows, higher peak flow duration, lower base flows, 

and decreased groundwater recharge 

• Increased pollutant loadings associated with higher runoff volumes (from increased imperviousness) 

These two processes—flow alterations and increased pollutant 

loading—are the primary causes of stormwater effects on 

receiving waters. These effects on both water quantity and water 

quality can in turn cause impairment to a number of designated 

uses, as shown in Table 3. For example, flow alterations can 

cause impairments, especially to aquatic life, by altering habitat, 

increasing channel instability, causing stream incision, increasing bank erosion, causing riparian 

degradation, and altering sediment supply and transport (Burton and Pitt 2001). Increases in pollutant 

loading from stormwater sources can create conditions that are harmful to human health, fish, and other 

aquatic life. Pollutants that are typically associated with stormwater runoff include pathogens, metals, 

sediment, nutrients, chlorides, pesticides, oil and grease, toxic organics, and polychlorinated biphenyls 

(PCBs) (Burton and Pitt 2001). The type of pollutant varies for each area depending on the specific 

sources within a watershed. In addition, permitted stormwater includes unique sources that are not 

necessarily precipitation-driven such as illicit discharges and dry-weather flows (e.g., from lawn 

watering or car washing). These types of stormwater sources are typically associated with effects due to 

water quality rather than water quantity. 

Table 3. Examples of the effect of stormwater runoff on common designated uses 

Designated use Water quantity effects Water quality effects 

Aquatic Life Change in stream hydrology resulting in 

habitat modification and degradation (e.g., 

change in riffle/pool ratio, streambed 

alteration, stream incision and streambank 

erosion, change in sediment transport) 

 Degradation of receiving water quality that 

can be detrimental to aquatic life (e.g., 

increased turbidity, increased temperature, 

eutrophic effects from increased nutrients) 

Recreation Alteration of stream channel or lake 

bathymetry impairing swimming or boating 

uses 





Increased pollutant levels that pose a risk to 

human health (e.g., bacteria, metals) 

Increased pollutants that degrade aesthetics 

(e.g., nutrients resulting in algal growth, oil 

and grease and litter causing odors or 

floatables) 

Drinking Water Less opportunities for infiltration to 

recharge groundwater supplies that serve 

as public drinking water 





Increased pollutant levels that pose a risk to 

human health (e.g., bacteria, metals) 

Increased pollutants that impede function of 

drinking water intakes (e.g., nutrients resulting 

in algal growth, blocked intakes from litter or 

increased sediment) 

3.1.2. Data Analysis to Characterize Impairment 

The analysis of waterbody monitoring data (e.g., flow, water 

quality, biological) supports an understanding of impairments by 

determining when, where, and under what conditions the 

problems are evident. These answers help to define many of the 

Resources: For more information on 

stormwater impacts to receiving waters, 

refer to the Resources list at the end of 

this chapter in Section 3.4.1. 

Resources: For more information on 

where to obtain water quality and flow 

data, refer to the Resources list at the 

end of this chapter in Section 3.4.3.
CHAPTER THREE: COMPILING INFORMATION TO 

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November 2008 DRAFT 31 

Tip: Using data analysis to support 

source identification 

Data analysis to understand the impairment is 

typically conducted in tandem with source 

identification activities (as discussed in Section 

3.3). Analysis of waterbody data along with 

review of watershed land uses, source 

locations, and other source information helps 

to determine what sources exist and whether 

they are affecting impairment. 

technical aspects of the TMDL, including what targets are 

appropriate, what sources are quantified, what approaches 

can be used, how allocations are determined, and on what 

time and spatial scale the analysis is conducted. Important 

aspects of the data analysis to understand the impairments 

for a TMDL include the following: 

• Data analysis to identify pollutant of concern or 

expected causes of impairment in cases of a listing 

based on general or biological impairment 

• Spatial analysis to identify spatial variations in waterbody and watershed conditions to identify 

environmental conditions or sources that affect impairment 

• Temporal analysis to evaluate the timing of impairment and potential source loading or other 

conditions contributing to impairment 

• Analysis of the relationships among multiple parameters or waterbody measures (e.g., pollutant 

concentration and flow) to understand impairment conditions and identify potential sources 

• Review of results of data analyses to identify critical conditions to support identification of TMDL 

targets and select appropriate TMDL development approach 

The following sections provide more detail on these common types of data analyses to support the 

characterization of impairments. These analyses also help to identify potential sources and evaluate their 

effect on impairment. Section 3.3 will discuss other activities and information that can help to identify 

sources and can be evaluated concurrently with the waterbody data to assess sources. 

3.1.2.1. Identifying Pollutants or Other Causes of Impairment 

Impaired waterbodies affected by stormwater sources are often listed as impaired due to such things as 

biological impairment or habitat alteration rather than for specific pollutants (e.g., metals, sediment). 

These listings are typically based on biological assessments or violations of biocriteria. Biological 

communities can show a response from multiple stressors or from a series of combined stressors such as 

water column pollutants, flow alterations, channel alterations, and other habitat alterations. Therefore, it 

might be difficult to identify the pollutant or suite of pollutants affecting the biological community. 

EPA developed the Stressor Identification Guidance Document (USEPA 2000c) to help practitioners 

determine which pollutants might be affecting biological communities. The stressor identification (SI) 

document covers the organization and analysis of available evidence to determine the cause of biological 

impairment. The general SI process entails critically reviewing available information, forming possible 

stressor scenarios that might explain the impairment, analyzing those scenarios, and producing 

conclusions about which stressor(s) are causing the impairment. The SI process is iterative, usually 

beginning with a retrospective analysis of available data, and the 

accuracy of the identification depends on the quality of data and 

other information used in the process. If the SI process identifies 

specific pollutant parameters (e.g., sediment, nutrients, temperature) 

that are causing the biological impairment, TMDL writers can 

establish targets for the pollutant(s) and use additional waterbody 

Resources: For more information 

on conducting biomonitoring, 

bioassessment, biocriteria or where 

to obtain biological data, refer to the 

Resources list at the end of this 

chapter in Section 3.4.4.
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32 DRAFT November 2008 

Tip: Use data analysis to 

support approach selection 

The conclustions drawn during data 

analyses will help to define the 

technical needs of the TMDL 

development approach. Chapter 4 

discusses selecting a TMDL approach. 

and watershed information (e.g., analysis of ambient in-stream data, field reconnaissance) to identify 

potential sources. 

However, because biological impairments often represent the cumulative effects of a number of 

stressors, it might not be possible to isolate a pollutant or even multiple pollutants as the primary cause 

of impairment. The impairment might be the result of hydrological and physical changes from the 

change in flow patterns often associated with impervious areas as well as the increase in pollutant 

loading. In these instances, the TMDL might be developed for a surrogate target that represents the 

combined effects from stormwater. Identifying TMDL targets, including the use of surrogates, is 

discussed in Section 3.2. 

3.1.2.2. Identifying Spatial Patterns 

Analyzing waterbody data to identify spatial variations in 

waterbody conditions and impairment can help to identify 

sources or waterbody or environmental conditions that are 

contributing to impairment. For instance, evaluating the data to 

identify spatial variations in water quality can identify hot spots

where sources are affecting a greater impact on water quality. A 

hot spot downstream of an urban area might indicate that 

stormwater is a potentially significant source. Spatial evaluation 

can also be useful in evaluating the effects of different land uses or stormwater source types. For 

example, comparing monitoring data from sites representative of heavily developed commercial or 

industrial areas to data representative of residential or undeveloped areas can help to evaluate the 

relative significance of the different land use types discharging to an MS4. 

Although the data might not always be available to support it, evaluation of conditions upstream and 

downstream of a suspected source can help to determine whether it has an effect on water quality. Figure 

5 presents paired (i.e., collected at the same time) total suspended solids (TSS) readings from stations 

upstream and downstream of a landfill that discharges runoff into a small stream. It was expected that 

the landfill might be a source of nutrients and sediment to the stream. Data were plotted together to 

evaluate the corresponding conditions upstream and downstream. Also plotted in Figure 6 is a line 

representing a 1:1 linear relationship, where the values upstream would be equal to those downstream. 

As shown in the figure, the downstream measurements are typically higher than those upstream of the 

landfill, suggesting that the landfill could be a significant source of TSS to the stream. 
CHAPTER THREE: COMPILING INFORMATION TO 

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November 2008 DRAFT 33 

250

)L/g200 (mSS T - )150 maetrsnw100 o (d 4noit

50

Sta

0

0.0 50.0 100.0 150.0 200.0 250.0

Station 3 (upstream) - TSS (mg/L)

1:1

Figure 5. Analysis of upstream and downstream data 

to evaluate potential impact of an expected source. 

3.1.2.3. Identifying Temporal Trends 

TMDL writers can also assess data for temporal trends to better understand the impairment and identify 

potential sources. Temporal variations in water quality, whether from month to month or year to year, 

can be the result of trends in environmental conditions, such as weather and resulting runoff and flows, 

or from variations in loading because of schedules or variations in source activities. For example, open 

areas or parks that drain to MS4s can experience increased wildlife activity or dog walking during 

summer months, potentially increasing pathogen loads. Similarly, increased loads of sediment or 

chlorides can occur during winter months from use of sand and deicers on roadways during winter 

weather. 

Longer-term, temporal variations such as trends over a decade rather than across seasons can also 

provide clues about watershed sources. Figure 6 illustrates a data analysis that evaluates both spatial and 

temporal variations using data from two stations on the same stream and collected over a 4-year period. 

The graph of time-series turbidity data shows that a significant increase in turbidity occurred in 2001 at 

the downstream station; since then, the levels have been consistently higher than upstream. Because 

upstream levels were measured at comparable levels before and after 2001, the data at the downstream 

station might suggest the introduction of a new source discharging between the two stations and 

contributing to the turbidity levels in the stream. 

While source activity can affect temporal variations in water quality, they are more often related to 

environmental conditions such as flow. Evaluating the relationship among water quality, flow, and 

seasonality can be done using a variety of techniques including simple visual comparison of graphed 

time-series data, regression analyses, or the use of flow duration curves. Figure 7 includes examples of 
CHAPTER THREE: COMPILING INFORMATION TO 

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34 DRAFT November 2008 

each of these types of data representation using the same data set. As shown in the figure, all the figures 

can be used to show the relationship between bacteria and flow. While the regression plot does not show 

a strong correlation between flow and bacteria, the chronological and flow duration graphs show that 

they do tend to follow similar patterns, with elevated bacteria typically occurring during higher flows. 

Because discharges from certain types of stormwater sources are typically observed during particular 

flow conditions, evaluations of flow and corresponding water quality can be a helpful tool in identifying 

potential sources of impairment. Many stormwater sources are related to increased runoff that can carry 

pollutants from impervious surfaces, such as parking lots and rooftops, and also lead to streambank or 

surface erosion, especially in areas of land-disturbing activities such as construction. Therefore, a 

waterbody influenced by stormwater would likely have observed water quality problems occurring at 

higher flows. However, waterbodies exhibiting the inverse relationship, with higher pollutant 

concentrations at lower flows, can also indicate MS4 sources such as illicit discharges to the storm 

sewer. Figure 8 depicts an example of pollutant loadings observed during low-flow conditions, possibly 

indicating illicit discharges entering the MS4 through either direct connections (e.g., sanitary sewer 

piping connected to storm drains, failing septic drain fields connected to ditches that are part of an MS4 

conveyance system) or indirect connections (e.g., infiltration into the MS4 pipes from cracked sanitary 

systems). 

10000

1000

)utn (y

100

biditTur

10

1

Jan-00 Jul-00 Jan-01 Jul-01 Jan-02 Jul-02 Jan-03 Jul-03 Jan-04 Jul-04

Upstream Downstream

The increase in downstream turbidity measurements from spring 

2001 to fall 2001 sampling events indicates the introduction of a new 

source of sediment discharging to the stream between the stations. 

Figure 6. Evaluation of spatial variations in turbidity data to identify locations of potential sources. 
CHAPTER THREE: COMPILING INFORMATION TO 

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November 2008 DRAFT 35 

Flow Duration Curve and Corresponding Fecal Coliform

1,200 6,000

1,000 5,000

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400 2,000

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0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

FC Flow Flow Duration Interval

Flow duration curve for continuous flow 

dataset shown above. Instantaneous 

fecal coliform measurements are 

plotted based on the corresponding flow 

duration interval of the flow measured 

on the sample date. Plot indicates that 

higher measurements of fecal coliform

generally correspond to higher flows.

Time-series of Flow and Fecal Coliform Data

10,000

1,000

100

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8

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Fecal coliform (#/100 mL) Flow (cfs)

Chronological graph showing 

continuous flow data and instantaneous 

fecal coliform measurements. Plot 

indicates that higher measurements of 

fecal coliform generally correspond to 

times of higher flows.

Regression - Flow vs. Fecal Coliform

1200

R2

 = 0.6796

1000

)Lm0

800

0/1#( mro

600

filol Cac

400

eF

200

0

0 100 200 300 400 500 600 700

Flow (cfs)

Regression of matching measurements of 

fecal coliform and flow. R2 of 0.7 might not 

indicate a strong correlation, but plot does 

show that the highest fecal coliform

measurements occurred during high flows. 

Figure 7. Examples of different data representations to evaluate the relationship between flow and 

fecal coliform. 
CHAPTER THREE: COMPILING INFORMATION TO 

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36 DRAFT November 2008 

Figure 8. Duration curve analysis indicating illicit discharges as stormwater source. Circled data 

points represent those exceeding the target for Escherichia coli. 

3.1.2.4. Evaluating Relationships among Parameters 

Evaluating the relationship among pollutants can also help TMDL writers to understand observed 

impairments and identify the types of sources in the watershed. Many pollutants causing impairments 

can originate from common watershed sources. If a waterbody is impaired by multiple pollutants, 

evaluating trends or patterns in all pollutants can investigate the potential of common sources among 

pollutants. For example, sediment, chlorides, and litter are often associated with road maintenance for 

snow and ice removal. Observed impairments by these parameters in the same waterbody segment might 

indicate snow removal activities as a source. Similarly, pathogens and nutrients often share common 

sources that could be contributing to stormwater loads, such as landfills, sanitary sewer breaks, and 

wildlife or domestic pet waste. 

In addition, some pollutants might be associated with other pollutants. For example, some pollutants 

(e.g., nutrients, metals) can be delivered to receiving waters adsorbed to sediment particles. Fertilizer or 

pesticide application in residential, commercial, or industrial areas can experience an accumulation of 

contaminants (e.g., nutrients, pesticides) that have adsorbed to sediment. Soil erosion and washoff in 

these areas can result in the delivery of loads of sediment and associated contaminants. Identifying a 

relationship between increased sediment concentrations and other pollutants can help to identify these 

situations to understand the nature of the impairment and identify potential sources. 

3.1.2.5. Identifying Critical Conditions 

EPA regulations require that the TMDL writer consider critical conditions while developing a TMDL. 

Evaluating the critical conditions builds on the previous analyses of spatial and temporal trends and 
CHAPTER THREE: COMPILING INFORMATION TO 

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November 2008 DRAFT 37 

relationships among pollutants and processes and identifies the combination of environmental conditions 

(physical, chemical, and biological) under which impairment occurs. When addressing stormwater 

sources, understanding the critical conditions can be crucial when identifying a TMDL target. Especially 

without an applicable numeric water quality criterion or when dealing with a biological impairment, 

evaluation of the critical conditions will help determine the causes and conditions associated with the 

impairment, such as times of elevated pollutant concentrations or high flows. As with all the other 

analyses discussed, understanding critical conditions can provide clues about the location, timing, and 

type of sources affecting impairment and guides selection of an appropriate TMDL development 

approach (as discussed in Section 4.2.3). 

3.2. Identifying TMDL Targets 

Impaired waterbodies requiring TMDLs are included on state 303(d) lists because of violations of water 

quality standards. Water quality standards include the designated use of a waterbody, the water quality 

criteria established to protect that use, and an antidegradation policy. Water quality criteria can be 

expressed as numeric or narrative criteria, affecting both the nature of the listing and developing the 

resulting TMDL. All TMDLs must have a numeric target for which to calculate a loading capacity. 

Figure 9 illustrates the potential steps or options for developing targets for TMDLs that include 

stormwater sources. When developing a TMDL for a waterbody listed for a specific pollutant that has an 

associated numeric criterion, the criterion serves as the target for the TMDL. However, many impaired 

waterbodies affected by stormwater sources are listed as impaired by pollutants with narrative criteria 

(e.g., sediment, nutrients) or due to biological impairments (e.g., biological assessments indicate poor 

benthic communities, increase in tolerant species, or decrease in fish populations). In such cases, it is 

necessary for the TMDL writer to identify a numeric TMDL target that can be used for calculating the 

loading capacity. When waters are listed for biological impairments, sometimes a TMDL writer will use 

data analysis and SI to identify a specific pollutant(s) (e.g., sediment) contributing to the impairment (as 

discussed in Section 3.1). If a specific pollutant is identified relating to a biological impairment, the 

TMDL writer can identify a numeric target based on data analysis (e.g., reference conditions, historical 

conditions) or appropriate site-specific or regional literature values. Similarly, a numeric target can be 

identified for a pollutant that does not have associated numeric criteria. 
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38 DRAFT November 2008 

Figure 9. Options for identifying targets for TMDLs that include stormwater sources. 

However, when dealing with stormwater effects on an impaired waterbody, it might be difficult to 

identify all the specific pollutants that are related to the impairment. The underlying problem might be 

due to hydrologic changes such as quantity of flow and variation in flow regimes that are important 

factors in transporting pollutants (e.g., metals, pathogens, sediment) that can violate water quality 

standards. For instance, the impairment might be the result of pollutant loads from flow-related instream scouring and and also increased pollutant loads being transported from specific activities within 

the stormwater source’s drainage area (e.g., road sanding, pesticide treatments to lawns) or more 

generally because of the increased runoff from impervious surfaces. Therefore, when developing a 

stormwater TMDL, a TMDL writer might use a surrogate measure (e.g., flow) to represent the 

impairment and establish a numeric target for the surrogate to represent attainment of water quality 

standards. (The TMDL regulations specify that TMDLs can be expressed in terms of mass per time, 

toxicity or other appropriate measure). Rather than representing a specific pollutant, the surrogate would 

represent the suite of pollutants contributing to the impairment. Examples of surrogates for use in 

stormwater TMDLs based on biological listings include the percent of impervious cover (IC) in the 

watershed of the impaired water and the flow volume in the impaired stream. The TMDL developed for 

Eagleville Brook, Connecticut, provides an example of a TMDL using IC as a surrogate and provides 
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November 2008 DRAFT 39 

information on how the target represents the impairment and how an appropriate target was established. 

The TMDL developed for Potash Brook, Vermont, illustrates the use of flow volume as a surrogate for 

stormwater-related impairments. The TMDL establishes a target for high flow in the brook on the basis 

of hydrologic conditions of two reference streams where aquatic life criteria are met. 

In Practice: Using Impervious Cover as a Surrogate for Water Quality Standards in Eagleville 

Brook, Connecticut 

Eagleville Brook does not meet water quality criteria and designated uses for aquatic life. The relevant criteria 

are based on distribution and abundance metrics for benthic invertebrates that inhabit lotic waters and are 

described in the state’s narrative water quality standards. An SI analysis concluded that the biological 

impairments are most likely due to a combination of pollutants related to stormwater runoff from developed 

areas and other related stressors (such as the physical impacts of stormwater flows). Because the major 

source of stormwater is runoff from the impervious surfaces in the watershed, the Connecticut Department of 

Environmental Protection selected iIC as a surrogate to represent attainment of aquatic life criteria and to 

establish the loading capacity. The IC target is set at 12 percent IC. This threshold is based on Rapid 

Bioassessment Protocol data from 125 small (< 50 square mile) watersheds indicating that no stream 

monitoring location with more than 12 percent IC in its watershed met criteria for full support of aquatic life use. 

The TMDL loading capacity of 12 percent was reduced 1 percent to provide for an MOS, yielding an overall 

allocation target of 11 percent. The TMDL applies the 11 percent IC target to all stormwater drainage areas and 

affects all sources subject to LAs and WLAs in the watershed. The percent IC TMDL and WLA/LA targets apply 

at all times (instantaneously, daily, monthly, seasonal, and annual). The final TMDL (2007) is at: 

www.ct.gov/dep/lib/dep/water/tmdl/tmdl_final/eaglevillefinal.pdf. 

3.3. Identifying and Assessing Potential Sources 

The step of identifying sources for TMDL development should be an extension of the data analyses 

conducted to understand the impairment and serves to further characterize the important sources and 

better define their location, behavior, magnitude, and influence. The source assessment should result in 

an understanding of what major sources are contributing to impairment and how (e.g., pollutants, 

delivery pathways). This can affect what approach is selected and how it is applied for TMDL 

development and helps to focus the allocation analysis as well as future implementation. 

While the pollutant loads originating with each source are typically quantified during the linkage 

analysis (Chapter 4), the information necessary to understand their location and discharge behavior and 

characteristics is compiled and reviewed during this step. In general, the methods that are used to 

complete a source assessment do not differ between a TMDL addressing stormwater sources and any 

other TMDLs, and they involve identification and characterization of point sources (e.g., stormwater, 

wastewater treatment plants, industrial facilities) and nonpoint sources (e.g., grazing, timber harvest, 

septic systems). The methods for completing a source assessment vary with the type of watershed, 

pollutants, and sources but typically rely on information from state or national databases, literature 

reviews, and local knowledge from state or local contacts. It is important to correlate the assessment of 

both point and nonpoint sources with the data analysis to characterize source impacts and behavior. For 

example, land use, locations of stormwater outfalls or facility discharges, and other source information 

should be evaluated along with water quality data analyses (e.g., spatial analysis) to understand potential 

effects from the various sources or explore unknown sources. The following are examples of 

information typically reviewed to identify sources for TMDL development: 
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40 DRAFT November 2008 

Tip: Maximize resources when 

characterizing stormwater sources 

A thorough characterization of stormwater 

sources can require a significant amount of 

time and resources. Therefore, it is 

important for TMDL and permit writers to 

work together to determine the significance 

of stormwater sources to the TMDL and, if 

it is determined that further characterization 

of stormwater sources is necessary, what 

type and amount of data are necessary to 

complete the characterization. 

• Query EPA’s NPDES databases to identify permitted facilities or sources discharging to the 

impaired water or its tributaries 

• Coordinating with state permitting staff to identify the number, type, and location of NPDES 

permitted point sources in a watershed (including stormwater sources) 

• Review geographic information system (GIS) coverages (e.g., land use, soils), satellite images, and 

aerial photos to identify potential nonpoint sources (e.g., agriculture, silviculture) 

• Conduct field or windshield surveys of the watershed to identify potential nonpoint sources (e.g., 

livestock operations, illegal dumping, failing septic systems) 

• Review previous watershed, local, or regional studies or reports to identify potential nonpoint 

sources 

Identifying and understanding the effect of stormwater 

sources might require additional investigation or information 

to support TMDL development. This section discusses the 

information available to support the following activities to 

further define and characterize stormwater sources: 

• Identify type and general location of stormwater sources 

• Delineate drainage area for stormwater sources 

• Characterize discharge from stormwater sources 

3.3.1. Identifying the Type and Location of Stormwater Sources 

Stormwater sources will fall within one of three categories—MS4, construction or industrial—but there 

can be a number of sources or activities within those permitted areas that might contribute to an 

impairment. Common stormwater-related pollutants of concern that cause water quality impairment 

include pathogens, metals (other than mercury), sediment, nutrients, and chlorides. Table 4 illustrates the 

range of permitted stormwater sources and related activities typically associated with certain pollutants 

or impairments. 

Table 4. Examples of sources within regulated stormwater areas associated with common 

pollutants and impairments 

Pollutant/stressor 

of concern 

Potential source(s) within permitted area 

MS4 Construction Industrial 

Pathogens 







Sanitary sewer breaks and crossconnections 

Restaurant trash areas and mat 

washing 

Landfills and transfer stations 

 Pet and wildlife waste 











Sanitary sewer breaks and 

cross-connections 

Food trash areas 

Landfills and transfer stations 

Improper disposal of sanitary 

waste (e.g., dumping from 

boats at marinas) 

Wildlife waste (sea gulls) 

Metals Vehicular emissions build up on 

impervious surfaces 

 Roadways 

 Driveways 

 Parking lots 





Vehicle and equipment use 

on-site 

 Materials storage/handling 

Varies with industry type∗

 Materials storage and handling

 Outdoor processing 

 Legacy pollutants in soil 
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Pollutant/stressor 

of concern 

Potential source(s) within permitted area 

MS4 Construction Industrial 

Industrial activities and materials 

storage 

 NPDES permitted industrial 

facilities 

 Corporation yards (i.e., municipal)

 Unpermitted industrial facilities 

Commercial activities and materials 

storage 

 Automotive repair facilities 

 Gas stations 

 Car washes 

 Auto dealerships 

Sediment 







 Active construction 

 NPDES permitted active 

construction 

 Non-permitted active 

construction (<1 acre) 

 Hillside development 

Roads and highways 

 Snow/ice management 





Erosion on-site causing offsite discharge 

 Road tracking 







Debris from materials storage 

Runoff from parking lots 

 Snow/ice management 

Nutrients 







Residential fertilizer application 

 Industrial/commercial fertilizer 

application 

Municipal fertilizer application 

 Pet and wildlife waste 

 Fertilizer application 

Chlorides  Snow/ice removal activities and 

storage 

 Snow/ice management 

Habitat Alteration 





New development in greenspaces

Building roads and highways 

Development in sensitive areas 

(i.e., near waterways, on hillsides)

 Sediment discharge from 

site 

Flow Alteration 





 Increased, unmitigated 

imperviousness due to new 

development or redevelopment 

 Increased, unmitigated 

imperviousness due to new roads 

and highways 

Increased connection of existing 

imperviousness 

 Change in drainage patterns 

due to removal of vegetation 

and changing grade 

Pesticides 







Residential pesticide application 

 Industrial/commercial pesticide 

application 

Municipal pesticide application 

 Fuel stations 

 

Oil and grease  Vehicle leaks on parking lots and 

roadways 

 Spills 

 Illegal dumping 

 Spills 

 Equipment maintenance 

and fueling 

Varies with industry type∗

 Materials storage and handling

 Outdoor processing 

 Legacy pollutants in soil 

Toxic Organics Varies with industry type∗

 Materials storage and handling

 Outdoor processing 

 Legacy pollutants in soil 

PCBs  Landfills Varies with industry type∗

 Materials storage and handling

 Outdoor processing 

 Legacy pollutants in soil 
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42 DRAFT November 2008 

∗ The federal Multi-Sector General Permit for Industrial Activities (MSGP 2000) covers 30 industrial sectors that fall into one or more 

of the 10 categories of stormwater discharges associated with industrial activity (and construction activities) described in 40 CFR 

122.26(b)(14)(i)-(xi). The 30 sectors are defined by either the facility’s Standard Industrial Classification (SIC) code or a general 

description of the facility’s industrial activities. MSGP 2000 expired at midnight on October 30, 2005. A new permit has not been 

issued. 

Compiling a list of all stormwater sources in the watershed of an impaired water should be, in theory, 

relatively easy because stormwater sources are covered under NPDES general and individual stormwater 

permits, and coverage under these permits is tracked by state and EPA NPDES permitting authorities. 

Each state has a stormwater permit program coordinator or 

multiple coordinators that TMDL writers can contact for 

information. NPDES-authorized states issue NPDES permits 

for MS4s and stormwater discharges associated with 

construction and industrial activities. The amount of 

information available online varies from state to state, but 

generally, it is a good idea to speak with the permitting 

authority directly to assess the adequacy of online resources. 

TMDL writers should be aware that in many states stormwater permittees are tracked in different data 

systems than other NPDES permittees. For example, EPA’s Permit Compliance System (PCS) and 

Integrated Compliance Information System (ICIS) data systems include permit information for a very 

small percentage of stormwater permittees regulated under the NPDES program. 

In nonauthorized states, TMDL writers should contact the individual EPA Regional office for 

information about dischargers within a watershed boundary and use the Electronic Notice of Intent 

(eNOI) system EPA has developed to track construction sites and industrial facilities that need to apply 

for coverage under EPA’s Construction General Permit (CGP) or Multi-Sector General Permit (MSGP). 

This system can be used to search, sort, and view NOIs and can be searched by city, county, or ZIP 

Code. The NOIs include location and receiving water name and for construction sites, the size of the 

disturbed area. 

It is important to work with the permitting authority to identify stormwater sources in the watershed of 

an impaired water. Regulatory definitions of permitted stormwater sources can result in challenges in 

identifying the boundaries of regulated (i.e., permitted) stormwater sources versus and unregulated 

stormwater sources. This is particularly true for MS4s. Figure 10 illustrates the complexities of 

identifying regulated versus unregulated MS4s within a watershed boundary. As Figure 10 shows, it is 

possible to have both regulated and unregulated MS4s within the watershed boundary of an impaired 

waterbody. Phase I MS4s are defined as large and medium MS4s on the basis of population served, 

while Phase II regulated small MS4s are defined as small MS4s (i.e., any MS4 not regulated under 

Phase I) within an urbanized area. Any portion of a small MS4 within the urbanized area is considered a 

Phase II regulated small MS4 and is subject to Phase II MS4 permit requirements. Small MS4s outside 

the urbanized area are not subject to Phase II MS4 permit requirements; however, TMDL writers should 

take into account pollutant contributions from these stormwater discharges in a manner similar to 

regulated, small MS4s. 

Resources: For links to online resources for 

identifying permitted stormwater sources in 

your watershed, refer to the Resources list 

at the end of this chapter in Section 3.4.5. 

Resources include links to state and 

regional stormwater contacts and EPA’s 

PCS/ICIS and eNOI systems. 
CHAPTER THREE: COMPILING INFORMATION TO 

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Figure 10. Potential spatial complexities of regulated and unregulated stormwater sources. 

While land use is often an indicator for the existence of MS4s within the watershed of an impaired 

waterbody, it is important to remember that not all MS4s are municipal entities (e.g., cities, counties, or 

towns). Other entities such as departments of transportation, irrigation districts, sanitary districts, 

universities, hospitals, federal facilities and other entities that own and operate separate storm sewer 

systems can also fall into the category of regulated MS4s. These nontraditional MS4s have the potential 

to add another layer of MS4 boundaries for TMDL writers to consider. In addition, TMDL writers might 

focus on MS4s as the primary stormwater sources, but it is important to remember that regulated 

construction activities and industrial facilities both inside and outside the boundaries of regulated MS4s 

can also contribute stormwater discharges. Permit information on all types of regulated stormwater 

sources should be readily available in PCS. Identifying unregulated stormwater sources, such as other 

small MS4s, will require an understanding of local land uses and communication with the entities that 

own and operate these systems. 

Field reconnaissance can also be used to identify any potential 

unknown stormwater sources as well as nonpoint sources. Field 

reconnaissance involves visiting the watershed and can range 

from a windshield survey while driving to a more comprehensive 

survey such as walking the stream to identify and geo-locate 

potential sources or identify potential monitoring sites to fill 

information gaps. Field reconnaissance is also useful to groundtruth available information used in characterizing the waterbody and its surrounding watershed, 

including areas within an MS4 boundary. For example, land use and land cover data are sometimes out 

of date and might include open space areas that have since been converted to residential, commercial, or 

industrial uses. Field reconnaissance is also helpful in identifying or better characterizing certain types 

of sources that might not be identifiable in the typical watershed coverages or information; for example, 

Resources: For more information on 

obtaining land use and coverage 

information and conducting field 

reconnaissance and visual surveys, 

refer to the Resources list at the end of 

this chapter in Sections 3.4.6 and 3.4.7. 
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44 DRAFT November 2008 

field investigations are an essential tool for MS4 communities to detect illicit discharges and 

connections, which can be significant sources of contaminants. Field reconnaissance can also help 

identify areas where stormwater volumes and velocities are affecting stream morphology and 

contributing to impairment by noting streambank, channel and habitat conditions (e.g., embeddedness, 

number of pools and riffles). 

3.3.2. Delineating the Drainage Areas of Stormwater Sources 

To include a stormwater source in the TMDL analysis, it is necessary to identify its regulated area and if 

possible isolate the specific area drained by its sewer system and discharged to the receiving waterbody. 

Differences in spatial scale between the geographic focus of a TMDL (e.g., waterbody or watershed) and 

stormwater sources (e.g., regulated MS4s, construction sites, and industrial facilities) can create 

unexpected challenges for TMDL writers at this stage of TMDL development. To identify stormwater 

source areas, it is important that the TMDL writers fully understand the types of sources and how they 

are regulated. For example, it might be assumed that identifying jurisdictional boundaries is a 

straightforward way to identify regulated MS4s. However, the jurisdictional boundary is not necessarily 

the same as the regulated MS4 boundary. For example, combined sewer system portions of storm sewer 

infrastructure are not regulated by the NPDES MS4 program. As another example, regulated small 

MS4s typically are only the portion of the MS4 system that is actually within the urbanized area 

boundary (Figure 10). These distinctions can affect how the stormwater sources are included in the 

analysis and how their respective WLAs are subsequently developed and assigned. 

It is also important to understand the stormwater conveyance methods for each stormwater source in a 

watershed to determine whether the source is discharging to or affecting the impaired waterbody and to 

delineate the boundary and drainage area. Stormwater can be conveyed to a waterbody through direct 

surface flow or through a pipe, ditch, or other conveyance. In addition, stormwater might be recharged 

to groundwater, which might or might not affect a stream. The type and location of these conveyance 

methods will help a TMDL writer to assess the potential effects that a stormwater source is having on a 

stream. For example, a concrete-lined channel that discharges stormwater directly to a receiving stream 

will likely have a greater effect than a discharge from a well-maintained wet pond designed to capture 

and treat 80 percent of the received water quality volume. Because of these issues, the outfall location 

and conveyance method for each stormwater source should be documented and summarized before 

completing a TMDL. This could be as simple as plotting the outfall locations in a GIS and comparing 

those locations to observed in-stream impacts, or it might require a more comprehensive analysis. 

TMDL writers should work with the permitting authority to identify the information available to 

delineate the areas and drainage boundaries for stormwater sources. Individual NPDES permit 

application requirements for Phase I MS4s [40 CFR 122.26(d)(1)(iii)(B)] include the submittal of a map 

showing the service boundaries of the MS4 covered by the application as well as the location of all 

outfalls that discharge into a water of the United States. Phase II MS4s applying for individual permit 

coverage are required to submit the same information. Phase II MS4s regulated under general permits 

submit NOIs that might have a map, a description of the regulated MS4 boundary, or both and should be 

available from the permitting authority. However, those MS4s are required to develop such maps as part 

as permit implementation, not at the time of application. Industrial and construction SWPPPs should 

include site descriptions or maps to identify the facility or site location and locations of outfalls or 

surface water discharges. 
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If this information is not readily available or not sufficient to determine the area draining a stormwater 

source, a TMDL writer should work with the permitting authority to determine if additional information 

is necessary to locate and delineate all the stormwater sources in the watershed of the impaired 

waterbody. City and county planning departments often have information on the location and extent of 

stormwater infrastructure and controls, and in some cases, detailed sewer and boundary maps or 

stormwater facility plans might be available. This information should be obtained, where available, 

because it is often the most detailed and accurate information about stormwater sources. Similarly, 

industrial facilities and construction sites will have, and might submit to the city, county, or permitting 

authority, plans that document stormwater sources and management practices. Stormwater facilities are 

often required to submit a facility plan that documents the characteristics of the facility. However, 

additional investigations might still be needed to locate all potential stormwater sources. 

3.3.3. Characterizing Discharges from Stormwater Sources 

To better understand the quality and quantity of discharge being delivered to an impaired waterbody 

from a stormwater source, it is useful to evaluate data and information generated by the source to satisfy 

permit requirements. Table 5 outlines the types of data and information generated by the three types of 

stormwater sources—MS4s, industrial facilities, and construction activities—and the following sections 

briefly describe the information. 

Table 5. Data and information generated by stormwater sources through the permitting process 

Stormwater permit document or 

activity Specific type of data generated by permittee 

MS4 discharger generated data 

Phase I MS4 permit application 





Description of land use and 10-year growth projections 

Outfall characterization sampling data 

 Receiving waters 

Phase II MS4 general permit NOI 



MS4 location and boundaries 

 Receiving waters 

Industrial facility inventory and  Location of industrial facility to determine the watershed 

inspections (Phase I MS4s) 







Activities, materials, and physical features of the industrial facility that might be 

sources of pollutants of concern during dry or wet weather 

 Prioritization based on location, pollutants of concern, etc. 

Compliance history of industrial facility 

Location and pollutants of concern from nonregulated industrial (and perhaps 

commercial) facilities 

Construction activities inventory and 

inspections 













Location of construction activity 

Size of disturbed area 

 Receiving water/watershed 

Prioritization based on size, location, compliance history, etc. 

Compliance history of project 

Number and location of nonregulated (less than one acre) construction projects 

IDDE and tracking 





Outfall map with receiving waters 

 Dry weather screening 

Tracking of citizen complaints, dumping, spills, restaurant inspections, etc. 

Post-construction BMP  Types of BMPs required 

implementation and tracking 





Locations of BMPs 

Operation and maintenance (O&M) records/agreements 

Inspection results 
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46 DRAFT November 2008 

Stormwater permit document or 

activity Specific type of data generated by permittee 

Outfall and ambient water quality 

monitoring data 







Characterization of discharges from particular land use types, subwatersheds, etc. 

Ambient data could provide baseline information before installing BMPs 

Habitat assessments might be part of monitoring program 

Annual reports 









Location and type of identified illicit discharges 

Location of approved erosion and sediment control plans 

Compiled post-construction BMP inspection results 

Compiled monitoring results 

Planned SWMP changes 

Industrial discharger generated data 

Industrial individual permit 

application 











Location of outfalls 

 Site drainage map 

Impervious area calculation 

Description of proposed activities, spills and leaks, on-site materials 

Sampling data (if available) 

Industrial general permit NOI 





Location of facility 

 Receiving water/MS4 

Applicable industrial sector 

Industrial SWPPP 





Location of industrial facility to determine the receiving water(s) and if the facility 

within an MS4 boundary 

Activities, materials, and physical features of the industrial facility that might be 

sources of pollutants of concern during dry or wet weather 

Map that shows outfalls into receiving waters 

is 

Monitoring data  Loading from facility for benchmark discharge monitoring parameters 

Industrial compliance evaluations 

and inspections 

 Assess any compliance or BMP implementation issues on-site which may contribute 

to loading 

Industrial sampling data 





Loading from particular facilities 

Assess general loading from types of industrial facilities 

Assess industrial loading from an MS4 

Construction project generated data 

Construction individual permit 

application 

 Location of construction activity 

 Total area and total disturbed area 

 Proposed BMPs 

 Runoff coefficient 

 Imperviousness created 

 Receiving water 

CGP NOI  Location of construction activity 

 Start/end dates 

 Total disturbed area 

 Receiving water 

Construction activity SWPPP 





Location and size of disturbance as well as a location with associated surface water 

discharges. 

A description of any discharge associated with industrial activity other than 

construction and the location of that activity on the construction site. 

Type and location of any post-construction BMPs to be implemented on-site 

Monitoring data  Loading from project 

The list of data types in Table 5 is based on the minimum federal permit requirements and assumed 

implementation strategies. Permitting authorities can choose to develop more stringent permit 

requirements; therefore, TMDL writers might want to take some time to carefully review individual and 

general permits issued by state permitting authorities to identify state or regional specific data and 
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information requirements. If a TMDL writer identifies potentially useful data generated by stormwater 

sources, the TMDL writer can work through the permitting authority to obtain this information. The 

permitting authority will have some information readily available on file (e.g., applications, NOIs, 

annual reports) or can make official requests to permittees for other types of information that the 

permitting authority might not have on file. It is important for TMDL writers to coordinate with 

permitting authorities to obtain this information, rather than directly contacting stormwater sources 

without the permitting authority’s knowledge. This will ensure efficient use of time and resources and 

maintain clear communication with stormwater sources. 

3.3.3.1. MS4-Generated Data 

Both individual and general MS4 permits require Phase I and 

Phase II MS4s to develop specific types of documents that 

contain information and data potentially useful to the TMDL 

development process. Figure 11 illustrates the type of 

documents that MS4s must develop to comply with either 

individual permit requirements or general permit 

requirements. Under individual permits, Phase I MS4s must 

complete a two-part application, develop an SWPPP, 

conduct monitoring, and generate an annual report that 

compiles information and data on from SWPPP 

implementation from the previous year. As shown in Figure 

11, Phase II MS4s have the option of obtaining permit 

coverage under either an individual permit or a general permit, depending on which type of permit the 

permitting authority makes available. Under an individual permit, a Phase II MS4 would likely have the 

same requirements to generate an SWPPP and prepare annual reports as a Phase I MS4. Although Phase 

II MS4s covered under a general permit must prepare the same type of documents, the information 

required in each document might vary. For example, the permit application for a general permit, the 

NOI, does not have the same extensive information requirements as the two-part application for an 

individual permit. In addition, not all general permits require monitoring. The Phase II MS4 general 

permit does require development of an SWPPP and annual reports, although the content might vary from 

those developed under an individual permit. 

Tip: Understanding the potential types 

of data not required by permits 

Municipal MS4s might have data valuable to 

the TMDL development process that is not 

required by NPDES permits. For example, 

comprehensive planning or zoning 

documents could provide information 

regarding land use distribution. In addition, 

municipalities might coordinate or support 

volunteering monitoring or watch groups that 

could provide valuable input (e.g., stream 

monitoring, cleanups). 
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Figure 11. Required MS4 permit documents and associated information and data. 

3.3.3.2. Industrial Discharger Generated Data 

Most stormwater discharges associated with industrial activity have permit coverage under an NPDES 

general permit rather than an individual permit. However, significant industrial dischargers might have 

stormwater discharges covered under the facility’s overall NPDES permit, which covers process 

wastewater, non-process wastewater, and stormwater. Figure 12 shows the type of documents required 

under both individual and general permits for industrial facilities. 

Where an EPA Region is the permitting authority, rather than the state, regulated industrial facilities 

obtain permit coverage under EPA’s MSGP. State permitting authorities often use EPA’s MSGP as a 

template for developing and issuing their state-specific industrial stormwater general permit, with 

modifications necessary to address state-specific issues. All industrial stormwater general permits 

require industrial facilities to submit NOIs, and most require these permittees to develop and implement 

SWPPPs and conduct comprehensive site compliance evaluations and periodic inspections. Also, most 

of these industrial stormwater general permits require at least some sectors to conduct analytical 

monitoring of stormwater runoff. Figure 12 illustrates the types of data and information generated by an 

industrial discharger as a result of complying with permit requirements. This information can assist 

TMDL writers in characterizing stormwater discharges from industrial stormwater sources during the 

TMDL development process. . 
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Figure 12. Required industrial stormwater permit documents and associated information and data. 

3.3.3.3. Construction Project Generated Data 

Construction projects disturbing one acre or more are required to obtain NPDES permit coverage for 

stormwater discharges. Figure 13 illustrates the data generated by construction stormwater sources as a 

result of complying with individual and general construction permit requirements. Construction 

stormwater discharges are most often permitted under an NPDES CGP, issued either by the state 

permitting authority or by an EPA Regional office. To obtain general permit coverage, the construction 

site operator must complete the required NOI and provide information such as project location and 

receiving waters. Construction stormwater general permits require operators of construction projects to 

develop and implement SWPPPs and conduct routine self-inspections. SWPPPs contain the majority of 

site-specific documentation of stormwater management activities performed. In limited instances, the 

state or EPA permitting authority may require certain construction projects (e.g., due to impaired 

receiving waters or to protect high-quality streams) to obtain coverage under an individual permit. When 

required, individual permits for construction activity typically contain requirements similar to those 

found in general permits; however, it is likely that the permittee will have specific monitoring 

requirements. Some construction stormwater permits (i.e., both individual and general) include effluent 

limits or action levels and associated effluent or receiving stream monitoring. 
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Figure 13. Required construction stormwater permit documents and associated information and 

data. 

3.4. Resources 

3.4.1. Stormwater Effects on Receiving Waters 

1. USEPA (U.S. Environmental Protection Agency). 1983. Results of the Nationwide Urban Runoff 

Program – Volume I Final Report. U.S. Environmental Protection Agency, Water Planning 

Division, Washington, DC. www.epa.gov/npdes/pubs/sw_nurp_vol_1_finalreport.pdf

The 5-year Nationwide Urban Runoff Program (NURP) was designed to examine the quality of 

urban runoff at different urban locations; whether urban runoff is a significant contributor to 

water quality problems in the United States; and the performance, effectiveness, and utility of 

management practices for controlling pollutant loads from urban runoff. This report presents the 

findings of the program. 

2. USEPA. Undated. Urbanization and Streams: Studies of Hydrologic Impacts. U.S. Environmental 

Protection Agency, Office of Wetlands, Oceans and Watersheds, Washington, DC. 

http://epa.gov/owow/nps/urbanize/report.html. Accessed July 2008. 

This Web-based report summarizes the findings of a literature search to document physical 

impacts and indications of water quality problems associated with stormwater runoff. The report 

summarizes the review documents, articles and reports, and provides citations for further 

information. 
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3. Burton, Allen, and Robert Pitt. 2001. Stormwater Effects Handbook: A Toolbox for Watershed 

Managers, Scientists, and Engineers. Lewis Publishers, Boca Raton, Florida. 

www.epa.gov/ednnrmrl/publications/books/handbook/index.htm

This handbook is intended to be a working document that assists scientists, engineers, 

consultants, regulators, citizen groups, and environmental managers in determining if stormwater 

runoff is causing adverse effects and beneficial use impairments in local receiving waters. The 

handbook provides an extensive discussion of the effects of stormwater (based on information 

documented in a number of other studies and documents) and focuses on providing information 

to support the design of a sampling program to assess stormwater impacts. 

4. Center for Watershed Protection. 2003. Impacts of Impervious Cover on Aquatic Systems. Center for 

Watershed Protection, Ellicott City, MD. www.cwp.org/Store/guidance.htm

This report examines more than 225 multidisciplinary studies documenting the hydrological, 

physical, water quality, and biological effects of urbanization and its accompanying IC. 

5. Center for Watershed Protection’s Stormwater Manager’s Resource Center Web site: 

www.stormwatercenter.net/

3.4.2. Understanding Flow in TMDL Development 

1. USEPA (U.S. Environmental Protection Agency). 2007. An Approach for Using Load Duration 

Curves in the Development of TMDLs. EPA 841-B-07-006. U.S. Environmental Protection Agency, 

Office of Water, Office of Wetlands, Oceans, and Watersheds, Washington, DC. 

www.epa.gov/OWOW/tmdl/duration_curve_guide_aug2007.pdf

This document provides an overview on the use of duration curves for TMDLs, describing the 

basic steps needed to develop duration curves and subsequently identify loading capacities, LAs, 

WLAs, MOS, and seasonal variations. The guide also discusses some considerations and 

limitations in using the approach and includes several case examples. 

2. USEPA (U.S. Environmental Protection Agency). 1999. Protocol for Developing Sediment TMDLs. 

EPA 841-B-99-004. U.S. Environmental Protection Agency, Office of Water, Washington, DC. 

www.epa.gov/owow/tmdl/sediment/pdf/sediment.pdf

This technical guidance document provides information to support TMDL writers in developing 

TMDLs for sediment. The document includes information on how to complete each step of the 

TMDL process, including problem identification, source assessment, linkage of water quality 

targets and sources, allocation analysis, and monitoring. 

3. USEPA. 2005. TMDL Model Evaluation and Research Needs. EPA/600/R-05/149. U.S. 

Environmental Protection Agency, Office of Research and Development, National Risk 

Management Research Laboratory, Cincinnati, OH. 

www.epa.gov/nrmrl/pubs/600r05149/600r05149.htm

This report documents the review of more than 60 available watershed and receiving water 

models for their applicability to TMDL development and implementation. It discusses model 

selection on the basis of model capabilities and provides a series of tables rating the capabilities 
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52 DRAFT November 2008 

or applicability the models using the categories of TMDL endpoints, general land and water 

features, special land processes, special water processes, and application considerations. The 

document also provides individual fact sheets for each reviewed model. 

4. USEPA (U.S. Environmental Protection Agency). 2008. Incorporating Green Infrastructure 

Concepts into Total Maximum Daily Loads. U.S. Environmental Protection Agency, Washington, 

DC. www.epa.gov/owow/tmdl/stormwater/

This 11 page fact sheet provides recommendations for incorporating GI and LID concepts into 

various elements of a TMDL and provides two TMDL case studies. 

3.4.3. Chemical, Physical and Biological Data 

1. EPA’s Storage and Retrieval Database (STORET): www.epa.gov/storet/

2. USGS’s National Water Information System Web site (NWISWeb): http://waterdata.usgs.gov/nwis 

3. EPA’s Environmental Monitoring and Assessment Program (EMAP) database: 

www.epa.gov/emap/index.html 

4. USGS’s National Water-Quality Assessment (NAWQA) Program: http://water.usgs.gov/nawqa/ 

5. USDA’s PACFISH/INFISH Biological Opinion Program: http://fsgeodata.fs.fed.us/pibo/

3.4.4. Biological Assessment and Guidance 

1. EPA’s Biocriteria Web site: www.epa.gov/waterscience/biocriteria/ 

2. USEPA (U.S. Environmental Protection Agency). 2000. Stressor Identification Guidance Document. 

EPA 822-B-00-025. U.S. Environmental Protection Agency, Office of Water and Office of Research 

and Development, Washington, DC. www.epa.gov/waterscience/biocriteria/stressors/ 

This document describes EPA’s process for identifying any type of stressor or combination of 

stressors that cause biological impairment. The SI Guidance is intended to lead water resource 

managers through a formal and rigorous process that identifies stressors causing biological 

impairment in aquatic ecosystems and provides a structure for organizing the scientific evidence 

supporting the conclusions. 

3. EPA’s CADDIS Web site: http://cfpub.epa.gov/caddis/

The Causal Analysis/Diagnosis Decision Information System (CADDIS) is an online application 

based on the process developed in EPA’s SI Guidance Document. It uses a step-by-step guide, 

worksheets, and examples to help scientists and engineers find, access, organize, share, and use 

environmental information to evaluate causes of biological effects observed in aquatic systems 

such as streams, lakes, and estuaries. 

4. EPA’s list of bioassessment publications from EPA and other federal agencies (e.g., U.S. Geological 

Survey [USGS], U.S. Department of Agriculture [USDA]): 

www.epa.gov/bioindicators/html/publications.html
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3.4.5. Permitted Stormwater Sources in a Watershed 

1. EPA’s list of state stormwater contacts: 

http://cfpub.epa.gov/npdes/contacts.cfm?program_id=6&type=STATE 

2. EPA’s list of Regional stormwater contacts: 

http://cfpub.epa.gov/npdes/contacts.cfm?program_id=6&type=REGION 

3. EPA’s PCS: www.epa.gov/enviro/html/pcs/index.html 

4. EPA’s eNOI system: http://cfpub.epa.gov/npdes/stormwater/enoi.cfm

5. Authorization Status for EPA’s Stormwater Construction and Industrial Programs by State: 

http://cfpub.epa.gov/npdes/stormwater/authorizationstatus.cfm 

6. USEPA (U.S. Environmental Protection Ageny). 2007. MS4 Program Evaluation Guidance. EPA833-R-07-003. U.S. Environmental Protection Ageny, Office of Wastewater Management, Water 

Permits Division, Washington, DC. 

http://cfpub.epa.gov/npdes/docs.cfm?program_id=6&view=allprog&sort=name#ms4_guidance

The MS4 Evaluation Guide is primarily for use by NPDES authorities to evaluate the quality of 

Phase I and Phase II MS4 programs, for permit compliance, technical assistance, and other 

purposes. It can be used for comprehensive program evaluations or for certain components. MS4 

program managers can also use it to evaluate their own programs. 

3.4.6. Land Use and Impervious Surface Coverages 

1. Multi-Resolution Land Characteristics (MRLC) Consortium’s National Land Cover Database 

(NLCD): www.epa.gov/mrlc/

2. National Agriculture Imagery Program (NAIP) Aerial Photos: http://165.221.201.14/NAIP.html

3. USGS’s Earth Explorer: http://edcsns17.cr.usgs.gov/EarthExplorer/ 

4. USGS’s Land Cover Institute: http://landcover.usgs.gov/ 

5. Google Earth: http://earth.google.com/

3.4.7. Field Reconnaisance and Illicit Discharge Detection 

1. Kitchell, A., and T. Schueler. 2005. Unified Stream Assessment: A User’s Manual. Version 2.0. 

Center for Watershed Protection, Ellicott City, MD. www.cwp.org/Store/usrm.htm#10 

This document outlines how to perform a Unified Stream Assessment, a rapid technique to locate 

and evaluate problems and restoration opportunities within the urban stream corridor. It also 

describes how to interpret the data collected to determine the stream corridor restoration 

potential. 
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2. Wright, T., C. Swann, K. Cappiella, and T. Schueler. 2005. Unified Subwatershed and Site 

Reconnaissance: A User’s Manual. Version 2.0. Ellicott City, MD. 

www.cwp.org/Store/usrm.htm#11 

The manual provides detailed guidance on how to perform each of its four components: the 

Neighborhood Source Assessment, Hotspot Site Investigation, Pervious Area Assessment, and 

the analysis of Streets and Storm Drains. Together, these rapid surveys help identify upland 

restoration projects and source control to consider when devising subwatershed restoration plans. 

3. Brown, E., D. Caraco, and R. Pitt. 2004. Illicit Discharge Detection and Elimination: A Guidance 

Manual for Program Development and Technical Assessments. Prepared for the U.S. Environmental 

Protection Agency, Office of Water and Wastewater, Water Permits Division, Washington, DC, by 

Center for Watershed Protection and the University of Alabama. 

www.epa.gov/npdes/pubs/idde_manualwithappendices.pdf

Chapter 7 of this document describes the use of watershed reconnaissance to search for illicit 

discharge problems in the field, with the process consisting of rapid screening of priority outfalls 

in priority subwatersheds followed by indicator monitoring at suspect outfalls to characterize 

flow types and trace sources. 
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4. DEVELOPING TMDLS WITH STORMWATER SOURCES

The TMDL analysis establishes a quantitative 

link between pollutant sources and receiving 

water response to identify the loading capacity 

of an impaired waterbody that will result in 

meeting water quality standards. There are a 

number of approaches to support this analysis 

and selecting which to use for any given 

TMDL is often guided by a number of 

technical and practical factors. Developing 

stormwater-source TMDLs can present some 

unique technical considerations that affect 

what approaches can be used and how they are 

applied. As shown in Figure 4, this chapter 

discusses the activities related to calculating 

the TMDL and its associated allocations. To 

address these activities as related to 

stormwater-source TMDLs, the chapter first 

introduces a number of approaches for 

developing stormwater-source TMDLs and 

then discusses the following: 

• Selecting an approach for developing the 

stormwater-source TMDL 

• Applying that approach to develop the 

TMDL 

• Expressing stormwater WLAs 

4.1. Overview of Approaches 

for Developing TMDLs 

There are a handful of approaches that are commonly used for developing TMDLs. This section briefly 

introduces these approaches, and Sections 4.2 and 4.3 describe the considerations for their selection and 

application when dealing with stormwater sources. 

The types of TMDL approaches discussed fall into to two major categories—modeling approaches and 

non-modeling approaches. The term model describes the set of equations or algorithms that are used to 

simulate a physical system. In this report, model refers to the available software tools that automate the 

calculation of equations or groups of equations representing the system. These can include watershed 

models that simulate the processes related to surface runoff and 

receiving water models that simulate a range of waterbody 

conditions and processes. Non-modeling approaches include 

those approaches that are not based on a standardized, automated 

software that simulates watershed or waterbody processes. They 

Figure 14. Illustration of the steps in the TMDL 

process, including activities related to calculation 

and documentation of the TMDL and its allocations. 

Resources: For more information the 

general approach for calculating 

TMDLs, refer to the Resources list at 

the end of this chapter in Section 4.5.1. 
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include those approaches that are based on monitoring data, empirical equations or other statistical or 

site-specific calculations. Examples include load duration curves developed using observed flow and 

water quality data and simple mass-balance calculations. 

Within those categories, there are various types of approaches, all of which can be further characterized 

according to the type of simulation or calculation they perform. The approaches either calculate landbased loads or the resulting waterbody loads. Table 6 presents the types of approaches discussed in this 

section and their respective categories of modeling vs. non-modeling and land-based vs. waterbodybased. The land-based approaches calculate loading from land-based runoff processes assuming some 

measure of precipitation and characteristics representative of the watershed (e.g., soils, imperviousness). 

The waterbody-based approaches calculate the delivered load in the waterbody on the basis of in-stream 

conditions, either using observed monitoring data (i.e., concentration and flow) or assuming some userdefined load inputs and outputs. Many of these approaches are applied in combination to represent both 

source loading and waterbody response to establish a loading capacity and associated WLAs and LAs to 

meet water quality standards. 

Table 6. Commonly used TMDL approaches 

Calculation process 

Type of TMDL approach 

Modeling Non-modeling 

Land-based  Watershed models (simple to complex)  Export coefficients 

 IC method 

 Simple Method 

Waterbody-based  Receiving water models (simple to complex,  Load duration method 

hydrodynamic and water quality)  Percent reduction method 

 Mass balance or steady-state analysis 

The following discussion provides a brief description of these common TMDL approaches. More detail 

on their application in stormwater-source TMDLs is then provided, with examples, in Section 4.3. 

4.1.1. Land-based Approaches 

The following are brief descriptions of several land-based approaches used for TMDL development: 

• Watershed Model. Many TMDLs use watershed models to evaluate existing and allowable 

pollutant loads to identify allocations, load reductions, and management scenarios. This group of 

models emphasizes description of watershed hydrology and water quality, including runoff, erosion, 

and washoff of sediment and pollutants. Some models simulate only the land-based processes, while 

some can also include linked river segments and simulate in-stream transport and water quality 

processes. Watershed models vary in the level of detail, including what processes they simulate and 

the simulation timestep (e.g., daily vs. monthly). The complexity of watershed models can range 

from the use of loading functions—empirically based estimates of load based on generalized 

meteorologic factors (e.g., precipitation, temperature)—to physically based simulations—

scientifically based equations to represent the physical, chemical, and biological processes 

associated with runoff, pollutant accumulation and washoff, and sediment detachment and transport. 

An example of a loading function model is Generalized Watershed Loading Functions (GWLF), 

which simulates basic watershed processes related to weather, erosion, and runoff and pollutant 

washoff and does not include waterbody response or in-stream fate and transport. GWLF provides 

watershed load and flow estimates on a monthly basis. Alternatively, Hydrologic Simulation 
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Program – Fortran (HSPF) combines watershed processes with in-stream fate and transport to 

simulate watershed hydrology, land and soil contaminant runoff, and sediment-chemical 

interactions. HSPF can generate time series results of any of the simulated processes on a daily or 

even sub-daily timestep. 

• Export Coefficients/Pollutant Budgets. This category encompasses a number of approaches built 

on empirical relationships among watershed processes and pollutant loading as well as the use of 

literature values of typical watershed loading rates. Examples include using monthly load rates from 

various land uses to calculate allowable loading from an impaired watershed. Another example is 

using an empirical relationship that allows a user to calculate an allowable load depending on 

desirable conditions (e.g., target runoff/waterbody concentration or indicator levels). This approach 

would typically be used to calculate existing loads and would often be combined with a 

supplementary approach that calculates an allowable load on the basis of in-stream targets (e.g., 

percent reduction). Another approach that could fall within this category is a Vollenweider type 

approach where an empirical equation relating reservoir tropic status to allowable phosphorus 

loading is used to identify an allowable load corresponding to the desired reservoir condition. 

• IC Method. This emerging method calculates a target of percent IC in a watershed to represent 

attainment of water quality standards. Available data are used to relate the percent IC with the 

resulting runoff, pollutant loading, and waterbody response to calculate the loading capacpity. 

• Simple Method. The Simple Method is an empirical equation used to calculate pollutant loading on 

the basis of drainage area, pollutant concentrations, a runoff coefficient, and precipitation. In the 

Simple Method, the amount of rainfall runoff is assumed to be a function of the imperviousness of 

the contributing drainage area. When using the Simple Method, the TMDL loading capacity would 

typically be calculated using a combination approach with a waterbody-based approach such as the 

percent reduction method. 

4.1.2. Waterbody-based Approaches 

The following are brief descriptions of several waterbody-based approaches used for TMDL 

development: 

• Receiving Water Model. Receiving water models simulate conditions within a receiving waterbody 

(e.g., lake, stream, estuary) on the basis of a representation of physical, chemical and biological 

processes. Inputs to the waterbody are often defined as user-defined boundary conditions or using 

linked dynamic output from a watershed model. Receiving water models are typically either steadystate or dynamic models. Steady-state models operate under a single, nonvariable flow condition 

with constant inputs, typically used to evaluate conditions for a design or critical flow. Dynamic 

models allow for variations in both flow and meteorologic conditions on a small timestep, typically 

shorter than daily. Level of complexity in receiving water models is also determined by spatial detail 

described as one, two, or three dimensions. 

• Load Duration. The load duration methodology relies on using observed flows and water quality 

criteria to establish a curve of loading capacities for various flow conditions. This builds on using 

flow duration curves, which use hydrologic data from stream gages to evaluate the cumulative 

frequency of historic flow data over a specified period. A water quality criterion or other target 

concentration can then be multiplied by the observed flows to create a curve representing the 

distribution of allowable loads as a function of daily flow, representing the loading capacity of the 

stream. The entire curve can be used to represent flow-variable loading capacities, or allowable loads 
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Tip: Evaluating Stormwater Issues 

when Selecting a TMDL Approach 

Stormwater-source TMDLs can present unique 

considerations affecting selection and 

application of a TMDL approach. TMDL writers 

should evaluate stormwater-specific 

considerations within the context of all the 

issues and characteristics of the TMDL, 

waterbody, and associated watershed. If 

stormwater is a significant source affecting 

water quality, these considerations might carry 

more weight in the decision-making process; 

however, they should be evaluated with all 

watershed-specific issues when selecting the 

most appropriate approach. 

can be identified for specified flow intervals, which can be used as a general indicator of hydrologic 

condition (e.g., wet versus dry). 

• Percent Reduction. This method assumes a 1:1 relationship between surface water concentrations 

and pollutant loading. The existing pollutant concentrations are compared to applicable water quality 

criteria to calculate a necessary reduction. This reduction is then applied to an estimate of existing

loading to calculate the loading capacity to meet water quality standards. Existing loads are often 

calculated using ambient monitoring data (e.g., concentration and flow) or some estimation of landbased loading (e.g., export coefficients). 

• Mass Balance or Steady-State. These approaches rely on the assumption of conservation of mass 

into a waterbody. The analysis might calculate loads entering a waterbody using export coefficients 

or observed data and calculate the resulting waterbody concentrations on the basis of estimated 

losses (e.g., settling, decay) and inputs. The approach relies on identifying the necessary loads 

entering a waterbody that will meet the desired waterbody target after considering all inputs and 

losses. These approaches can be applied for a steady-state critical condition or longer time periods, 

such as average monthly loading rates. 

4.2. Selecting an Approach for Developing Stormwater-Source 

TMDLs 

TMDL writers often consider a number of factors when deciding which approach to use to calculate the 

loading capacity and associated LAs and WLAs for TMDLs. As shown in Figure 15, these can include 

user needs or requirements, programmatic considerations, and technical needs. While user needs and 

programmatic considerations often guide the general type of approach (e.g., simple vs. complex, 

modeling vs. non-modeling), the technical considerations often guide the selection of a specific 

approach or methodology. The technical considerations define the following needs for the TMDL 

analysis: 

• Spatial scale/resolution 

• Temporal resolution/time scale 

• Processes or features that need to be included (e.g., pollutant type, surface runoff, in-stream 

transport) 

The watershed characterization step of TMDL 

development (Chapter 3) should generate the necessary 

information to define these needs by providing an 

understanding of the impaired waterbodies, the 

surrounding watershed and the associated impairments. 

Specifically, the major considerations or questions that 

were addressed during the watershed characterization that 

can support selection of an appropriate approach for 

TMDL development include the following: 

• What are the applicable water quality criteria? 

• What are the sources? 
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• What are the impairments and associated critical conditions? 

Table 7 summarizes the considerations related to each of the three technical needs for these defining 

topics of water quality standards, impairment, and sources. The answers to the questions outlined in 

Figure 15 and more specifically in Table 7 will guide approach selection for TMDL development. While 

these questions and considerations will not be much different for a stormwater-source TMDL than for 

any other TMDL, there might be some unique issues related to stormwater that will affect the selection 

of an appropriate approach for TMDL development. The following section discusses these stormwaterspecific issues to be considered when selecting an approach for TMDL development. 

Figure 15. Considerations for selecting a TMDL development approach. 
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Table 7. Summary of technical considerations for selecting a TMDL development approach 

Technical needs 

of approach 

Technical considerations for approach selection 

Water quality criteria and 

TMDL targets 

Impairments and critical 

conditions Sources 

Spatial Needs  Are different criteria or TMDL 

targets applicable in different 

locations within the 

watershed? 





How many impaired segments 

are being addressed? 

What are the location and 

distribution of impaired 

segments? 







What type of sources/land uses 

exist in the watershed? 

What are the location and 

distribution of sources? 

At what level do the sources 

need to be isolated (e.g., gross 

loading vs. land use specific 

loading)? 

Time-scale Needs  What are the duration and 

frequency of applicable 

criteria or targets? 





What is the timing associated with 

impairment (e.g., instantaneous 

vs. chronic or cumulative effects)?

Are there any temporal trends to 

capture (e.g., seasonality in 

waterbody conditions)? 







Are the effects due to 

cumulative or acute loading 

conditions? 

Are there temporal variations in 

source loading (e.g., due to 

weather patterns, seasonal 

activities)? 

At what temporal scale do the 

sources need to be estimated? 

Processes to 

Include 





Is criterion based on pollutant 

level (e.g., concentration) or 

a measure of response or 

condition (e.g., flow, habitat 

quality, eutrophication)? 

What are the pollutants? 









Is impairment based on a specific 

pollutant (e.g., sediment, metals) 

or based on cumulative effects of 

stressors (e.g., flow, habitat 

quality, pollutants)? 

Is meeting the target dependent 

on or affected by multiple 

waterbody measures (e.g., 

nutrient levels, temperature, pH)? 

What are the waterbody critical 

conditions for loading response 

(e.g., dynamic, flow variable vs. 

steady-state)? 

If dealing with multiple pollutants, 

how are they related? 







What is the source loading 

behavior (e.g., precipitationdriven, direct discharge)? 

Do sources impact multiple 

impaired segments (i.e., need 

for in-stream routing and 

transport)? 

Does the analysis need to 

evaluate individual and/or 

cumulative impact of sources? 

4.2.1. How Are Water Quality Criteria or TMDL Targets Expressed? 

The ultimate goal for any TMDL is to restore the impaired waterbody to meet water quality standards. 

As discussed in Section 3.2, identifying a TMDL target is typically based on using a numeric water 

quality criterion or interpreting narrative criteria. The expression of the resulting TMDL target can 

influence what types of TMDL development approaches are appropriate and how a TMDL analysis 

considers and represents water quality standards attainment. In many respects, considering the TMDL 

target for selecting an approach for a stormwater-source TMDL is no different than for any other 

TMDL. For example, numeric water quality criteria (or a numeric target representing narrative criteria) 

have an associated magnitude, duration, and frequency. The applicability of an approach can be affected 

by its ability to simulate at a time-scale necessary for comparison to the water quality target’s 

magnitude, duration and frequency. Targets designed to address acute (short-term) impairments are 

typically based on instantaneous maximums or daily averages while chronic (long-term) problems (e.g., 

eutrophication, sediment loading and deposition) can be represented by targets with longer durations 

(e.g., monthly average concentration, annual loading). 
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As discussed in Section 3.2, an issue unique to 

stormwater-source TMDLs is the use of surrogate 

targets (e.g., flow volume, percent IC) to represent the 

combined effects of stormwater quantity and quality 

on maintenance of water quality standards. In these 

situations, the use of a surrogate target will have a 

direct bearing on the choice of the technical approach. 

For example, if using an IC target, the approach 

chosen must be able to calculate flows from varying 

levels of imperviousness. Similarly, an analysis that 

uses peak flow as a target and identification of TMDL 

allocations and subsequent controls will depend on controlling flow volumes and peak flows, it will be 

necessary to use an approach that can either simulate the hydrology and hydraulics of a system in 

response to watershed characteristics or one that can otherwise relate in-stream flow conditions to 

watershed characteristics (e.g., imperviousness). 

In Practice: Integrating Stormwater Runoff Volume as a Surrogate for Water Quality 

Standards into a TMDL Approach in Potash Brook, Vermont 

Vermont’s water quality standards provide for the use of numeric biological indices to determine the condition 

of fish and aquatic life based on reference conditions for different waterbody types. Potash Brook, near 

Burlington, is on Vermont’s section 303(d) list as a result of biological impairments, including loss of sensitive 

taxa, and a compositional shift toward more tolerant species in macroinvertebrate communities. Data suggests 

that degraded habitat and increased sedimentation are the highest concerns, most likely caused by changes in 

water flow and increases in sediment erosion and deposition. To effectively address biological impairments in 

Potash Brook, the Vermont Department of Environmental Conservation selected stormwater runoff volume as a 

surrogate TMDL measure to represent issues associated with the following: 

• Sediment delivered through erosion processes 

• Physical effects on the stream channel such as sediment release from channel erosion and scour from 

increased flows 

• Diminished base flow (e.g., decreased groundwater recharge) 

• Amount of other potential pollutants (e.g., nutrients, toxics) delivered to Potash Brook is a function of 

the amount of storm water runoff generated within the watershed. 

A high-flow reduction target was established on the basis of the hydrologic conditions of two reference 

watersheds where aquatic life criteria are attained. Flow duration curves were used to evaluate existing and 

target conditions for the stream and establish the TMDL. Flow duration curves were developed for both Potash 

Brook and the reference watersheds, and the difference between the 0.3 percent duration point on the curves 

was used to derive the reduction target. The 0.3-percentile flow represents the flow that is exceeded by only 

0.3 percent of the measured flows and was selected because it approximately equals the one-year flow and the 

channel forming flow for this stream. The vast majority of reductions are assigned to the WLA, which applies to 

runoff from urban and developed portions of the watershed, and includes an allocation for future growth. The 

LA applies to the limited agricultural and open space portions of the watershed. 

More information on the Potash Brook TMDL (2006), as well as other hydrology-based TMDLs approved in 

Vermont, is available at www.anr.state.vt.us/dec//waterq/stormwater/htm/sw_impairedwaters.htm. Other 

examples of hydrology-based TMDLs include Maine’s Barberry Creek (2007) available at 

www.maine.gov/dep/blwq/docmonitoring/impairedwaters/TMDL/2007/barberry_ck_rep.pdf and Connecticut’s 

Eagleville Brook (2007) available at www.ct.gov/dep/lib/dep/water/tmdl/tmdl_final/eaglevillefinal.pdf. 

Tip: Select an approach consistent with 

water quality standards 

Regardless of the significant pollutant sources, the 

TMDL writer should select a technical approach 

that is suitable for the pollutant of concern. For 

example, some models do not simulate certain 

pollutants and, therefore, are not appropriate for 

certain types of TMDLs. The approach should also 

provide the necessary output to allow for a direct 

comparison to the temporal duration specified in 

water quality criteria. 
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4.2.2. What Are the Stormwater Sources Affecting Impairment? 

Analysis during the previous activity of characterizing the 

impairment and pollutant sources (Chapter 3) identified the 

regulated stormwater sources in the watershed that 

contribute to the impairment of concern. The TMDL writer 

should also have a general understanding of the relative 

importance of each source, which can guide decisions 

regarding technical approaches for TMDL development. It 

is important that a TMDL approach be able to represent 

important sources in a way that captures the sources’ effect 

on water quality to calculate appropriate allocations. This is 

no different for stormwater sources than for other types of point or nonpoint sources. Therefore, when 

stormwater is a major source affecting impairment, it is important to consider its characteristics when 

selecting an approach. As discussed in the followings sections, which type of approach is selected for 

stormwater-source TMDLs will be influenced by the type of sources, size of the source area, and how 

the source generates and delivers pollutants of concern. 

4.2.2.1. Type and Distribution of Sources 

If regulated stormwater sources are within the watershed of the impaired waterbody and discharge the 

pollutant of concern, they will be included in the TMDL analysis. However, there are varying levels of 

detail at which the sources can be characterized and quantified in the analysis, affecting both the 

selection and application of a TMDL approach. Decisions regarding the level of detail for including 

stormwater sources can include not only the spatial resolution at which sources are represented within 

the analysis, but also the types of spatial units (e.g., subwatershed, land use types) that can be included 

and how they are spatially related and represented in the analysis. These decisions will depend primarily 

on the significance of each source in their effect on the impairment of concern. For example, perhaps an 

MS4 in an impaired watershed is expected to be a minimal source of the pollutant of concern, whether 

because it covers a small portion of the watershed, it has already installed a number of effective controls 

minimizing its effect, or other active sources in the watershed (e.g., agriculture, mining, forestry) 

contribute significantly more pollutant load. In such a case, it might be appropriate to choose an 

approach that does not specifically include or calculate loads originating with individual stormwater 

sources but rather evaluates a cumulative load from all sources (e.g., load duration) and then distributes 

the total load among sources. Alternatively, if there are multiple regulated stormwater sources in the 

watershed, including MS4, construction, and industrial, that all represent major contributions of 

pollutant loading to the system, it will likely be necessary to choose an approach (e.g., watershed model) 

that can evaluate the watershed at a finer scale, both temporally and spatially, to isolate the sourcespecific loadings and evaluate their effect on water quality. 

One of the first considerations regarding sources is how well they are understood. The analysis of 

impairment conditions and sources discussed in Chapter 3 helps the TMDL writer understand what and 

how sources are affecting impairment. This can help to identify the type of information that the technical 

approach will need to include and also produce, thereby narrowing the range of approach options. If 

sources and their impacts are well understood based on available data and local knowledge, it might not 

be necessary to use an approach that evaluates individual sources or provides the ability to predict 

effects from existing and future source inputs. Some approaches, such as receiving water modeling, can 

Characteristics of Stormwater Sources 

that Can Affect Approach Selection 

• Type and distribution of sources 

o Pollutant generating activities 

o Spatial coverage 

• Pollutant Delivery 

o Timing of loading 

o Delivery mechanism 
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potentially provide a great deal of information on how known sources will affect receiving water quality 

but will not provide much information on unknown sources. Land-based watershed modeling, on the 

other hand, can help to quantify the relative significance of various sources such as urban runoff 

compared to point source discharges. Data-driven approaches, such as load duration curves, that rely on 

evaluation of in-stream loads based on monitoring data are very useful when significant observed data 

are available and the impairments predominantly occur during certain flow conditions. However, they 

do not typically support direct calculation of loads originating from individual sources and require a 

supplementary analysis to do so. 

Also affecting approach selection is the decision of the spatial resolution for representing the individual 

stormwater sources in the approach. Stormwater sources can have a variety of activities within their 

boundaries that contribute to pollutant loads. For example, within an MS4 different land uses (e.g., 

residential vs. commercial) and areas can contribute varying magnitudes of loads. It might be important 

to capture those variations in pollutant loading and evaluate subareas within the MS4 boundary, such as 

areas that drain to certain outfalls or different land uses, to more effectively target BMP implementation. 

Therefore, it would be appropriate to use an approach that can isolate and calculate pollutant loading 

from specified sub-sources. For example, using a watershed model allows for the inclusion of multiple 

types of land areas that are represented by characteristics (e.g., soils, pollutant accumulation, 

imperviousness) that can vary by land use or type, allowing a TMDL writer to not only isolate loads 

generated within a stormwater source boundary but also by other characteristics such as land use. 

However, approaches that do not specifically calculate loads from individual sub-sources can still 

accommodate and capture these variations depending on how they are applied. For example, while a 

load duration approach calculates a total allowable in-stream load without direct calculation of sourcebased loads, it can be applied at multiple strategic locations to capture the variability in loading from 

different sources. 

Alternatively, an area within a source’s boundary might be small or fairly homogenous, without much 

variation in pollutant generation. For example, regulated construction sites are relatively small in area 

and would not likely require further division within their boundaries. As another example, depending on 

the overlap of an MS4 and the watershed of an impaired water, the portion of the MS4 within the 

watershed might represent similar land uses and activities (e.g., all residential areas). In these cases, it is 

likely appropriate to use an approach that evaluates loading on a coarser level, possibly evaluating the 

cumulative load to an impaired waterbody rather than area-specific inputs. 

If regulated construction stormwater sources are important in your watershed, they might present a 

different set of considerations and challenges for the TMDL analysis than MS4s or industrial sites. 

Again, there is the question of spatial coverage but also one of temporal effects. MS4s and industrial are 

static with a definable boundary, while regulated construction sites represent a moving target in that they 

have definable boundaries, but their existence and effects are more intermittent. The temporal aspect of 

the locations, magnitudes, and activities related to stormwater from construction sites makes it difficult 

to include in a TMDL analysis. Through the stormwater source characterization and the impairment 

analysis, the TMDL writer should determine whether construction is an important potential source given 

the characteristics of the watershed and the impairment. For example, if the watershed is experiencing 

rapid growth and expansion, construction could be a significant current or future source and might 

require specific representation in the TMDL analysis. Or perhaps the watershed had a number of 

construction sites that were active during the time the waterbody was identified as impaired, but the sites 

are no longer active or contributing to the impairment. In such cases, it might be necessary to select an 
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approach that can evaluate the temporal variations in waterbody conditions on the basis of time-variable 

source inputs and other watershed factors (e.g., weather). This type of evaluation is most directly 

accomplished using a time-variable watershed model. However, other non-modeling approaches can be 

applied to represent different time frames and source inputs to more generally evaluate the variable 

effects over time. This can allow the TMDL writer to evaluate past, current, and future conditions 

related to construction impacts. 

Another level of detail in spatial resolution when representing stormwater sources is the potential for 

including loadings from individual outfalls. While the location of outfalls should be known, it is not 

always feasible to include or analyze loadings from individual outfalls in a TMDL analysis. To evaluate 

outfall-specific loads and effects, it is necessary to have data to characterize the outfall discharge (e.g., 

flow, pollutant concentrations) for at least the critical conditions (e.g., high flows) but preferably under a 

range of conditions. Data to support this level of detail are often not available. However, if sufficient 

data are available, it is important to consider the added benefit as well as the added effort in conducting 

the analysis. The TMDL writer along with the permit writer should identify the benefits of evaluating 

loading at the outfall level and whether those same benefits could be achieved by conducting the 

analysis at a broader scale (e.g., subwatershed level) and likely with less expended effort and resources. 

4.2.2.2. Pollutant Delivery 

How and when a pollutant is delivered to a receiving waterbody is an important consideration in 

selecting an appropriate TMDL approach. For stormwater-related sources, the typical pathway for 

pollutants is accumulation and washoff, where overland runoff transports pollutants deposited on land 

surfaces to stormwater conveyances and eventually receiving waterbodies. However, depending on the 

type of pollutant, the timing of delivery or mode of transport can present unique needs for a TMDL 

development approach. For example, many pollutants are delivered in dissolved form in stormwater 

runoff, while others can be transported adsorbed to sediment particles. Waterbodies that are impaired by 

sediment-associated pollutants such as phosphorus or organics can therefore require an approach that 

simulates the processes of sediment erosion and transport to fully evaluate the pollutant loading and 

impacts. 

In addition to the delivery pathway, the timing of pollutant loading is another consideration for the 

evaluation of stormwater. Stormwater is typically driven by precipitation events and subsequent runoff 

and discharge. However, dry-weather flows delivered through MS4 infrastructure caused by such things 

as automobile washing and lawn watering also might be a source of loading to a waterbody, especially 

in arid regions. This situation might require a TMDL approach that can evaluate both precipitationdriven sources during wet weather and also direct inputs occurring during dry weather. For example, 

load duration curves and watershed models can be applied to evaluate loads for different flow 

conditions. 

Regardless of the conditions of delivery, stormwater source activity can also occur during different 

times of year. For example, pollutants associated with deicing and winter road maintenance are more 

likely to be deposited and delivered during winter months while pollutant runoff from residential car 

washing might be more frequent during warmer summer months. These situations might warrant a 

TMDL approach that can evaluate and capture the variations in pollutant loading across months and 

seasons. For example, many watershed models can produce either daily continous loads or monthly 

loads that would capture the variations in loading during different seasons. 
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66 DRAFT November 2008 

4.2.3. What Are the Critical Conditions? 

As discussed in Chapter 3, the critical conditions for a TMDL represents the combination of 

environmental conditions (physical, chemical and biological) under which impairment occurs. This can 

include such things as the environmental processes that affect impairment (e.g., nutrient dynamics 

affecting dissolved oxygen levels, flow modification due to increased development affecting in-stream 

habitat) and the timing of impairment (e.g., certain months or times of day). From the characterization 

step discussed in Chapter 3, the TMDL writer should understand the impairment and the effect of 

stormwater sources on receiving water quality and can now evaluate how to capture that in the selected 

approach. 

For example, some stormwater-related impairments are not associated with specific pollutants and 

therefore require the analysis of a variety of pollutants and processes to capture the impairment 

conditions. Evaluating the effects of stormwater and identifying necessary allocations might require the 

evaluation of such things as sediment delivery and deposition, peak flows, and nutrient loading and 

resulting eutrophication dynamics. Another example is critical conditions related to low dissolved 

oxygen that can be affected by the timing and availability of nutrient loads and also on seasonal factors 

such as temperature, resulting algal growth, and flow. The TMDL writer should evaluate the processes 

that need to be included to sufficiently represent the critical conditions when selecting an approach for 

developing the TMDL. 

In Practice: Selecting a TMDL Approach to Capture Critical Conditions in Ballona Creek, 

California (2005) 

Segments of Ballona Creek and Sepulveda Canyon Channel were included on 1996, 1998, and 2002 303(d) 

lists for cadmium, copper, lead, selenium, silver, zinc and toxicity. Data analysis indicates differences in 

waterbody conditions (e.g., water quality, flow) and sources between dry and wet weather. To capture the 

varying critical conditions and source loading, two distinct approaches were used to develop the TMDL. 

Because the metals criteria are based on hardness, separate wet- and dry-weather targets were developed to 

reflect the different hardness values and flow conditions in the creek and its tributaries. For the purpose of this 

TMDL, wet weather is defined in terms of flow rather than rainfall. Wet weather is defined as any day in which 

the maximum daily flow is equal to or greater than 40 cubic feet per second (cfs) based on the 90th percentile 

of flows measured over a 10-year period. 

The dry-weather loading capacity for each metal was derived by multiplying the hardness-adjusted, dryweather numeric targets by the critical flow assigned to these two waterbodies. The wet-weather TMDL 

calculation was based on the simulation of the hydrologic processes and watershed metals loading using a 

watershed model (HSPF). Using simulated flows and metals concentrations, a load duration curve approach 

was used to establish the wet-weather loading capacity. Loading capacities were calculated by multiplying the 

daily storm volume by the appropriate numeric water quality target. 

A grouped mass-based WLA was developed for stormwater permittees (Los Angeles County MS4, Caltrans, 

General Industrial and General Construction) for both dry weather and wet weather. Because there are no 

identified areas in the watershed that discharge directly to Ballona Creek or a tributary rather than through the 

storm drain system, the WLA was equal to the calculated loading capacity minus the estimated loads from 

atmospheric deposition. The grouped stormwater WLA was partitioned among the MS4 permittees (77,546 

acres) and Caltrans (1,080 acres) using an estimate of the percentage of land area covered under each permit. 

The Ballona Creek metals TMDL report is at 

www.waterboards.ca.gov/losangeles/board_decisions/basin_plan_amendments/technical_documents/2005-

007/05_0831/StaffReport.pdf. 
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November 2008 DRAFT 67 

4.3. Applying Approaches for Stormwater-Source TMDLs 

Once an approach is selected for TMDL development, the TMDL writer will apply the approach to 

calculate the loading capacity and associated allocation for sources. While all sources present challenges 

in deciding how to accurately represent their inputs and effects, stormwater sources can require some 

unique considerations when applying a TMDL approach. The two key issues to address when 

developing TMDLs with stormwater sources are 

1. How to represent stormwater source characteristics (e.g., discharge flows and concentrations) 

2. How to isolate and estimate the loads transported and discharged through the stormwater system 

Several commonly used TMDL approaches were introduced previously in Section 4.1. This section 

discusses the considerations for developing TMDLs with stormwater sources using these different 

methods and is organized according to whether the approach is land-based or waterbody-based, as 

identified in Table 6. Table 8 summarizes the advantages and disadvantages of each method, and the 

following sections provide more detail on their application for stormwater-source TMDLs. 

Table 8. Summary of commonly used TMDL development approaches 

Approach 

Can be combined 

with... Advantages Disadvantages 

Land-based Approaches 

Watershed Models 



 Receiving Water 

Model 

 Load Duration 

Curves 













Can directly simulate regulated 

stormwater sources as distinct 

hydrologic units to facilitate better 

representation of source inputs. 

Enables source-level allocations 

Provides flexibility in expression of 

allocations, from gross to detailed, 

site-level (if detailed model is used). 

Provides time-variable simulation 

and results to better represent 

varying conditions for regulated 

stormwater source inputs and 

impacts (if dynamic model is used). 

Flexibility in how model is set up to 

represent land units allows for 

simulation of subareas or land uses 

within regulated stormwater source 

boundaries to define spatial inputs 

and impacts for targeting 

implementation. 

Allows users to simulate potential 

changes in water quality conditions 

that might result from 

implementation activities. 











Requires significant data and 

analysis if outlet-level allocation is 

necessary. 

Direct simulation of stormwater 

sources is dependent on accurate 

information on drainage areas and 

runoff. 

Moderate or general watershed 

models (e.g., those with monthly 

time-steps) have limited capabilities 

for temporal evaluation or highly 

variable stormwater sources. 

Model accuracy dependent on 

having sufficient water quality data 

for calibration. 

Requires trained or experienced 

staff to run the model and 

understand model assumptions and 

limitations. 

IC Method 



 Watershed 

Models 

 Load Duration 

Curves 





Because area of impervious 

surfaces is easily explained and 

tangible, can be more 

understandable to the public. 

Is translated more easily into 

management actions. 







Requires supplemental analysis and 

data to support linkage to a load . 

Requires sufficient data to support 

the link between IC and water 

quality standards. 

Is not appropriate for waterbodies 

affected by a mix of sources other 

than urban runoff. 
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68 DRAFT November 2008 

Approach 

Can be combined 

with... Advantages Disadvantages 

Export Coefficients 



 Percent 

Reduction 

 Load Duration 

Curves 

 Is simple to apply. 



Is usually based on regional or 

national literature values that might 

not be representative of local 

conditions. 

Does not provide a direct link to 

waterbody conditions and use 

support. 

Simple Method 



 Percent 

Reduction 

 Load Duration 

Curves 





Is useful in watersheds lacking flow 

data. 

Is specifically designed to evaluate 

stormwater pollutant loads from 

impervious areas. 







Assumes all loading originates on 

impervious surface during storm 

events, not accounting for runoff 

from impervious areas or subsurface 

inputs and baseflow loading. 

Because it uses a static runoff 

concentration, does not account for 

variability in loading or in-stream 

levels. 

Not appropriate for large watersheds 

(>1 mi2

) or non-urban areas. 

Waterbody-based Approaches 

Receiving Water  Watershed  Represents a high level of spatial  When applied independently, is 

Models 



Model 

 Mass Balance 



variability within a waterbody, 

allowing for detailed outfall-based 

allocations 

Provides time-variable simulation 

and results to better represent 

varying conditions for regulated 

stormwater source inputs and 

impacts (if dynamic model is used). 





limited to allocations set for specific 

input points; requires combination 

with a watershed model for landbased allocation analysis. 

Provides limited allocation 

opportunities when source is not 

discharging directly to the receiving 

water (e.g., for construction sites in 

upland locations). 

Provides limited allocation options 

when applied as a steady-state 

(non-dynamic) modeling application. 

Load Duration  Watershed 

Models 

 Simple Method 







Is based on observed monitoring 

data, providing a data-based 

representation of existing conditions. 

Identifies the allowable and existing 

loads for all flow conditions, 

providing insight into the critical 

conditions and accounting for the 

natural variations in loading and instream conditions. 

Because it is based on observed instream conditions, can capture the 

effects of unknown sources (e.g., 

failing septic systems, illicit 

connections). 









Provides limited information 

regarding the relative magnitude of 

source loads. 

Requires supplemental analysis to 

distribute loading capacity into 

source-based allocations. 

Requires robust and consistent 

records of flow and in-stream water 

quality data. 

Is applicable only to non-tidal 

streams or rivers. 

Percent Reduction  Simple Method  Is simply and quickly applied.  Assumes a 1:1 relationship between 

Method  Export 

Coefficients 

 Easy for the public to understand. 



reductions in pollutant loading and 

resulting reductions in concentration.

Does not calculate source-based 

loads, requiring supplementary 

analysis to identify stormwater 

WLAs. 
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November 2008 DRAFT 69 

Can be combined 

Approach with... Advantages Disadvantages 

Mass Balance or  Receiving Water  Relatively simple to apply.  Typically focuses on a single critical 

Steady-state Models  Is based on observed monitoring condition (e.g., critical flow) or longAnalysis data, providing a data-based term average conditions (e.g., 

representation of existing conditions. monthly loading and concentration), 

not allowing for evaluation of 

variability in pollutant loading or 

waterbody conditions. 

 Simple representative of pollutant 

fate and transport. 

4.3.1. Land-based Approaches 

This section discusses the modeling and non-modeling TMDL approaches that evaluate land-based 

loading or conditions and issues related to their use in stormwater TMDLs. 

4.3.1.1. Watershed Models 

Watershed models are commonly applied to provide a quantitative linkage between source contributions 

and waterbody response for TMDL development. This category of models loosely refers to numerical 

frameworks that address a combination of land-based rainfall/runoff and contaminant loading processes 

and conveyance of flow and contaminants within a stream, impoundment, or some other type of 

receiving waterbody. Commonly available data used to drive watershed models and accurately simulate 

conditions within receiving waters include land cover, soil type, meteorological characteristics, and 

stream/impoundment dimensions. The level of complexity associated with each of these models varies 

widely from annual flow and contaminant load delivered from a watershed to sub-hourly prediction of 

detailed hydrologic processes at the land use or site level. 

While structure and simulated processes vary from one model to the next, watershed models provide a 

useful basis for allocation to watershed sources contributing to waterbody impairment, including 

permitted stormwater sources. Most watershed models perform calculations on a land unit basis. That is, 

the modeler divides a drainage area into smaller land units to enhance representation of heterogeneities. 

These land units can represent variable soil characteristics, political boundaries, or more commonly, 

land use or cover. Many models take this land cover breakdown one step further and represent 

impervious and pervious land cover types using different mathematical formulations. Impervious land, 

for example, is not subject to infiltration and associated subsurface processes. Figure 16 illustrates how a 

typical watershed model categorizes each area of land by land use type (e.g., residential, forest, 

agriculture) and then routes flow and loads delivered from those within each watershed to the receiving 

waterbody. Watershed models typically have the capability to separate the watershed into a number of 

smaller subwatersheds. Some then have the added capability to route the flow and load contributed from 

each subwatershed to the corresponding stream reach to the downstream reach, creating a system of 

connected waterbody segments. Dividing a watershed into land cover-based units typically provides a 

logical basis for developing source-based allocations in a TMDL. For example, specific allocations (in 

terms of time-variable flow and contaminant loads) can be made to each land cover category represented 

in a watershed model. A watershed model might, for example, represent the following five land cover 

categories: forest, pasture, crop, residential–pervious, residential–impervious. In this situation each 
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70 DRAFT November 2008 

category might receive independent allocations, which collectively result in the receiving water for the 

drainage area to attain and maintain water quality criteria. 

Land Use Rivers/Stream

Units Segments

Land use 1

Land use 2

Land use 3

Land use 4

Subwatersheds

Subwatershed 1 Reach 1

Flow

Flow

Load

Load

Reach 2 Flow

Load

Land use 1

Land use 2

Land use 3

Land use 4

Subwatershed 1

Flow

Load

Figure 16. Typical watershed model elements for simulating runoff and pollutant loading from 

watershed land uses to receiving waterbodies. 

Of the approaches presented in this section, watershed models can have the greatest variation in the type 

and method of application of the approach. Watershed models can vary widely in their capabilities, 

influencing their applicability for a particular TMDL application. The applicability of a model for 

simulating stormwater sources in a TMDL analysis can be affected by the model’s capabilities, 

including the following: 

• Timestep (temporal resolution) 

• Ability to represent spatial variations (spatial resolution) 

• Processes that are simulated 

The temporal resolution can affect the model’s ability to capture the variations in stormwater loading 

and the waterbody response. Its spatial scale can determine how watersheds are divided and represented, 

including how stormwater sources are isolated. The processes simulated by a model will often be a 

primary determining factor in model selection for TMDL development. A model’s ability to simulate 

certain land-based processes (e.g., pollutant accumulation, runoff, erosion), waterbody processes (e.g., 

in-stream transport, nutrient dynamics, die-off), and management processes (e.g., stormwater detention) 

can determine whether it is appropriate for use in a certain TMDL 

project and also how the model will be applied. Table 9 identifies 

several model capabilities that can affect how it can be used to 

evaluate and estimate stormwater source loads. For each capability, 

the table rates a number of watershed models used for TMDL 

development, including: 

Resources: For more information 

on selecting watershed models 

based on needs for a given TMDL, 

refer to the Resources list at the end 

of this chapter in Section 4.5.2. 
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• Annualized Agricultural Non-Point Source (AnnAGNPS) 

• GWLF 

• HSPF 

• Loading Simulation Program in C++ (LSPC) 

• Program for Predicting Polluting Particle Passage through Pits, Puddles, and Ponds—Urban 

Catchment Model (P8-UCM) 

• Source Loading and Management Model (SLAMM) 

• Soil and Water Assessment Tool (SWAT) 

• Storm Water Management Model (SWMM) 

Table 9. Commonly used watershed models and select capabilities for evaluating stormwater 

sources in TMDLs 

Simulation capability 

AnnAGNPS GWLF HSPF LSPC P8-UCM SLAMM SWAT SWMM 

Simulates land-based pollutant accumulation and runoff z ~ z z ~ z z z

Simulates dynamic daily conditions (variable 

precipitation/flow) 

~ { z z ~ z ~ z

Simulates conveyance systems { { ~ ~ { ~ { z

Simulates sediment erosion and transport z ~ z z ~ z ~ ~

Simulates in-stream fate and transport { { z z { { ~ ~

Simulates BMPs z { ~ ~ z z z ~

Is appropriate for urban watersheds { { ~ ~ z z { z

Is appropriate for mixed land uses ~ ~ z z { ~ ~ ~

{ = Low 

~ = Medium 

z = High 

Other modeling references (USEPA 1997, 2005c) include summaries of model applicability for more 

general considerations, such as land uses simulated, pollutants simulated, and waterbody types simulated 

as well as for considerations related to their application. For example, TMDL Model Evaluation and 

Research Needs (USEPA 2005c) provides a series of tables rating the capabilities or applicability of 

more than 60 available watershed and receiving water models in the following categories: 

• TMDL Endpoints. Considers the model’s ability to simulate typical TMDL target pollutants (e.g., 

nutrients, toxics, bacteria) and expressions (e.g., load vs. concentration). Characterizes the models 

depending on the timestep of the simulation for the target (e.g., steady state, annual, daily, hourly). 

• General Land and Water Features. Rates models according to their ability to simulate general 

land uses (e.g., urban, agricultural) and waterbody types (e.g., river, lake, estuary). 

• Special Land Processes. Rates models on their ability to simulate more than 15 special land 

processes such as wetlands, hydrologic modification, urban BMPs (e.g., street sweeping, detention 

ponds), and rural BMPs (e.g., nutrient control practices, irrigation practices). 
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72 DRAFT November 2008 

• Special Water Processes. Rates models on their ability to simulate special processes occurring in 

receiving waterbodies such as air deposition, stream bank erosion, algae, and fish. 

• Application Considerations. Rates models on the following practical considerations affecting their 

application—experience required, time needed for application, data needs, support available, 

software tools, and cost. 

Regardless of what type of watershed model is used, TMDL writers can calculate WLAs for permitted 

stormwater sources (e.g., MS4s, industrial facilities, and construction sites) in a number of ways using 

watershed models. The primary issues with modeling approaches that include stormwater sources are 

similar to those for other approaches, i.e., how to most accurately represent the sources and how to 

isolate and calculate their pollutant load contributions. Because models typically provide continuous 

simulation of multiple land- and waterbody-based processes, they can provide more flexibility than nonmodeling approaches in representing stormwater sources. 

An essential part of defining the contributions from permitted stormwater sources depends on isolating 

the regulated boundary of the source, or the drainage area that delivers stormwater runoff to the source’s 

system. (See Chapter 3 about delineating the regulated area of a stormwater source.) Stormwater permits 

require all permittees to have documentation of the system configuration, its outfalls and boundaries. 

The level of detail of this information, however, will vary by permittee. Some might have detailed GISbased coverages of storm sewer outlets and the areas they drain. Others might have coarse, hand-drawn 

maps roughly identifying their outlet locations and property boundaries. Once the TMDL writer 

delineates source areas, the issue becomes how to account for the delineated source area in the modeling 

analysis. The selection of an approach to include permitted stormwater sources in a modeling analysis 

for load calculation and allocation primarily depends on the available data and the importance or 

magnitude of the types of permitted stormwater sources. The two primary approaches include the 

following: 

1. Including the permitted stormwater sources as discrete land units within the model—units that 

produce flows and loads separate from other watershed land areas and are hydrologically connected 

to the stream system 

2. Calculating the load from the regulated stormwater source in a supplementary analysis using the 

model results of loads generated by watershed land uses 

Which approach is used might depend on the significance of permitted stormwater sources to the 

impairment and the level of detail needed to characterize each source (e.g., evaluate as a whole vs. 

evaluate subareas within the regulated boundary). The first approach is most suitable for watersheds 

where stormwater sources are considered significant sources of impairment and sufficient data exist to 

define and isolate their regulated boundary, as well as discharge quality and quantity. The second 

approach can be used when regulated boundaries are not easily defined or because it is more important 

to evaluate the land use-specific effects on water quality rather than the effect of the cumulative 

contributions from a stormwater source (e.g., a regulated MS4) as a whole. For example, the regulated 

boundary of an MS4 might cover the majority of the watershed of the impaired waterbody. In such a 

case, it might be more efficient to set up the watershed model to evaluate the entire watershed area and 

the specific land uses or activities and then estimate and subtract the small portion that is delivered to the 

waterbody through nonpoint source runoff rather than the MS4. Both approaches are discussed in 

greater detail in the following sections. 
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4.3.1.1.1 Including Stormwater Sources as a Modeled Land Unit 

The most direct approach for calculating 

stormwater source allocations is to develop a 

watershed model including the regulated 

boundary. This means that the TMDL writer can 

isolate and represent the regulated area in the 

watershed model as a separate land unit—

representing the area that produces stormwater 

runoff that is delivered to the receiving waterbody

through the permittee’s stormwater conveyance 

system. Using this approach, the model would 

internally calculate flow and pollutant loading 

separately for each stormwater source’s area. For 

example, the TMDL writer could separate land 

covering each permitted industrial facility into 

individual land units to independently determine 

flow and contaminant contributions from each 

permitted industrial facility. The TMDL writer 

can use the same approach for construction sites 

(or areas expected to undergo construction) and 

areas covered by an MS4 permit. 

Figure 17 illustrates this process for calculating the WLA for an MS4. The example assumes that the 

overlay of the storm sewer system is available, along with the corresponding areas that drain to that 

system for delivery to watershed streams (i.e., the regulated boundary), as shown in the Subwatershed 

Characteristics panel of the figure. Therefore, the TMDL writer can isolate areas drained by the system 

and include these areas in the model as discrete units. As shown in the Model Setup panel, the model 

then simulates flow and loads contributed by the discrete MS4 units within each subwatershed to the 

respective stream segment. Because of this setup, the TMDL writer can directly calculate WLAs within 

the model, similar to the approach for other simulated land units (e.g., urban, agriculture), as shown in 

the Allocations based on Model Output panel. 

If the individual stormwater resources are represented in the model by their regulated boundaries, it will 

be necessary to characterize their land areas for simulation of runoff and loading. For example, when a 

watershed model is set up, the individual land use or other source areas are characterized by things such 

as soil type and characteristics (e.g., erodibility), perviousness, and measures of pollutant accumulation 

and generation. If available, monitoring data from the stormwater source should be used to characterize 

the source area and their associated runoff. If such data are not available, the areas included within the 

stormwater boundaries can be characterized in the same manner as other land uses. This is typically 

done by using GIS coverages such as land use, soil type, and topography to characterize the physical 

conditions of the land areas and site-specifc calculations or default values to represent pollutant 

accumulation for certain land use types. 

The extent to which TMDL writers can use this approach of including stormwater sources as modeled 

land units depends primarily on the level of detail available geographically for each stormwater source, 

the data available to characterize pollutant generation and washoff from the source area and, in some 

cases, the capabilities of the watershed model. 

Tip: Modeling surface runoff vs. baseflow loads 

in MS4s 

The use of watershed models allows a TMDL writer to 

estimate both surface runoff and baseflow loads from all 

sources, including MS4s. A strict reading of the MS4 

definition would suggest that only groundwater that enters 

the stream by reemerging or infiltrating into the 

conveyance system should be covered by the MS4 

permit, while groundwater that discharged directly to the 

stream would not be covered. However, in some 

situations groundwater discharge can be included within 

the WLAs because the entire watershed is within the MS4 

boundary and much of the groundwater load (e.g., for 

bacteria) was assumed to derive from leaky sewer 

systems which are also covered by NPDES permit. In the 

case of fecal coliform and a watershed entirely within 

MS4 boundaries it is probably safe (and easier) to include 

all the load within the WLAs. For other parameters, 

however, it might not be so clear. For example, for TDS 

the natural geologic background in groundwater would be 

inappropriate to lump into the MS4 WLA. 
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74 DRAFT November 2008 

Subwatershed Characteristics Model Setup 

Forest

Urban

Agriculture

Commercial

Residential

Regulated boundary

Storm sewer

Stream

Storm sewer discharge point

Municipality boundary

Subwatershed 1 Land Use Rivers/Stream

Units Segments

Permitted MS4

Stormwater

Forest

Urban

Agriculture

Commercial Subwatershed 1

Residential

Subwatersheds

Subwatershed 1 Reach 1 Reach 1

Reach 2 Reach 2

Permitted MS4

Stormwater

Forest

Urban

Agriculture

Commercial

Subwatershed 2

Residential

Subwatershed 2

etc. etc. etc. etc. etc.

Allocations based on Model Output 

29.57

Allocated 

Load (kg/yr)

3.87

15.37

15.43

3.6

29.16

1 MS4

1 Total 97

etc. etc. x

1 Residential

1 Commercial

1 Agriculture

1 Urban

Sub. Land Use

1 Forest

29.57

Allocated 

Load (kg/yr)

3.87

15.37

15.43

3.6

29.16

1 MS4

1 Total 97

etc. etc. x

1 Residential

1 Commercial

1 Agriculture

1 Urban

Sub. Land Use

1 Forest

Figure 17. Process for calculating MS4 allocation when the MS4 drainage area is included in the 

model as a discrete land unit. 

In Practice: Modeling Sediment Loads from Permitted Construction in the Potomac Direct 

Drains, West Virginia, TMDL (2008) 

West Virginia’s 2006 section 303(d) list includes 29 impaired streams in the 927-square-mile Potomac Direct 

Drains watershed in Berkeley and Jefferson counties. Many of the listed waters are biologically impaired on the 

basis of a narrative water quality criterion that prohibits the presence of wastes in state waters that cause or 

contribute to significant adverse effects on the chemical, physical, hydrologic, and biological components of 

aquatic ecosystems. SI analyses indicated sedimentation as a causative stressor in many of the streams, and 

sediment TMDLs were developed. 

In recent years, the eastern panhandle of West Virginia has undergone significant development, with 

agricultural land and open space being converted to roads and housing subdivisions. The increased 
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construction activity is a potential source of sediment to the impaired waterbodies. At the time the TMDLs were 

developed, 297 construction sites encompassing 8,470 acres were registered, or had registrations pending, 

under the general permit. 

The Mining Data Analysis System (MDAS) was used to represent the source-response linkage for sediment. 

Sediment TMDLs were developed using a reference watershed approach where MDAS was used to model 

sediment loading from a reference watershed. The reference loading rate was normalized for the Potomac 

Direct Drains watershed to establish the numeric TMDL target for sediment. 

Sites subject to the Construction Stormwater General Permit were represented in the model using precipitation 

and runoff from the registered disturbed area and an assumption that proper installation and maintenance of 

required management practices would achieve an approximate 60 percent reduction of the sediment loading 

contributed by barren land. All registered sites and sites with registrations pending as of October 2006 were 

incorporated. All active registered sites and pending site registrations as of October 2006 were provided 

individual WLAs in the TMDL. 

The Potomac Direct Drains TMDL reports are at www.wvdep.org/alt.cfm?asid=140. 

4.3.1.1.2 Calculating Stormwater Source Loads Outside of the Model 

More commonly, the TMDL writer develops the watershed model based on typical land cover 

categories, without specifically including the regulated boundaries of stormwater sources. This is 

typically done because the boundaries of the area draining to the source’s MS4 are not yet available or it 

is decided to focus on the land-use specific effects in more detail. In such situations, the TMDL writer 

will develop, calibrate, and apply the watershed model to TMDL development first, and then 

subsequently make allocations to permitted stormwater sources on the basis of their respective 

boundaries, whether represented by the regulated boundary or some alternative boundary (e.g, 

jurisdictional area) in the absence of the regulated boundary. The TMDL writer makes this allocation by 

overlaying the land cover represented in the model with applicable stormwater source area boundaries, 

such as the areas of an industrial facility exposed to stormwater runoff or the regulated MS4 boundary. 

The TMDL writer then determines the flow and pollutant load contribution associated with the various 

land cover categories that fall within the permitted stormwater source area boundary. This is often done 

by determining the unit area flow and pollutant loading for each land cover category on the basis of 

model results. The TMDL writer can then multiply the unit area rates by the corresponding area of the 

stormwater source that falls within that land cover category to calculate the portion of the land-use load 

that originates within the stormwater source boundary. To find the total load for the permitted 

stormwater source, the TMDL writer can then sum the 

land use-specific loads. 

Figure 18 illustrates this type of process for calculating 

the WLA for an MS4 when the regulated boundary for 

the stormwater source is not included as a discrete area 

in the model. Unlike the example in Figure 17, the 

exact coverage of the storm system and the regulated 

boundary draining to the system has not yet been 

delineated by the permittee. Therefore, the model is set 

up to include only watershed land uses (see Model 

Setup panel in the figure), regardless of the MS4’s 

boundary. The land-use loads are then used as the 

basis for calculating the load allocated to the MS4. 

This is done using an alternate boundary representing 

Tip: Regulated Boundary vs. Jurisdictional 

Boundary 

The most accurate way to represent a stormwater 

source’s runoff and pollutant load contributions in a 

watershed model is to use the regulated boundary—

the area actually generating and delivering runoff to 

the storm sewer system. However, many TMDLs 

represent the regulated stormwater source simply 

using the jurisdictional boundary (e.g., the municipal 

boundary for a small MS4) rather than the regulated 

boundary. As discussed in Chapter 3, the TMDL 

writer should work with the permitting authority to 

identify and obtain the regulated boundary to include 

in the modeling analysis or identify an appropriate 

alternative boundary if it does not exist. 
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76 DRAFT November 2008 

the MS4 (e.g., jurisdictional boundary) rather than the actual area draining to the system. As shown in 

the Allocations based on Model Output panel of the figure, the areas of each land use that fall within the 

regulated MS4 boundary is calculated and multiplied by the loading rate for that land use. These loads 

are then totaled to calculate the total load attributed to the MS4. 

Subwatershed Characteristics Model Setup 

Forest

Urban

Agriculture

Commercial

Residential

Stream

Municipality boundary

Subwatershed 1 Land Use Rivers/Stream

Units Segments

Forest

Urban

Agriculture

Commercial

Subwatershed 1

Residential

Subwatersheds

Subwatershed 1 Reach 1 Reach 1

Reach 2 Reach 2

Forest

Urban

Agriculture

Commercial

Subwatershed 2

Residential

Subwatershed 2

etc. etc. etc. etc. etc.

Allocations based on Model Output 

- 106 - - - 29.57

Modeled Area Unit Area MS4 Area MS4 Allocated 

Load (acre) Loading in Land Load Load 

(kg/yr) (kg/acre/yr Use (acres) (kg/yr) (kg/yr)*

5 110 0.045 25 1.13 3.87

25 65 0.385 25 9.63 15.37

20 35 0.571 8 4.57 15.43

12 40 0.300 28 8.40 3.6

35 120 0.292 20 5.84 29.16

1 MS4

1 Total 97 476 - 106 29.57 97

etc. etc. x x x x x x

1 Residential

1 Commercial

1 Agriculture

1 Urban

Sub. Land Use

1 Forest

- 106 - - - 29.57

Modeled Area Unit Area MS4 Area MS4 Allocated 

Load (acre) Loading in Land Load Load 

(kg/yr) (kg/acre/yr Use (acres) (kg/yr) (kg/yr)*

5 110 0.045 25 1.13 3.87

25 65 0.385 25 9.63 15.37

20 35 0.571 8 4.57 15.43

12 40 0.300 28 8.40 3.6

35 120 0.292 20 5.84 29.16

1 MS4

1 Total 97 476 - 106 29.57 97

etc. etc. x x x x x x

1 Residential

1 Commercial

1 Agriculture

1 Urban

Sub. Land Use

1 Forest

* Allocated load = modeled load – MS4 load

Figure 18. Process for calculating MS4 allocation when the specific stormwater drainage boundary 

is not included in the model and MS4 load is calculated based on percent of area within each 

modeled land use. 
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In Practice: Using watershed model-based approach for MS4 WLAs in the Christina River Basin 

The Christina River Basin High-Flow Nutrient TMDL (www.epa.gov/reg3wapd/tmdl/pa_tmdl/

ChristinaMeetingTMDL/index.htm), established by EPA in conjunction with Pennsylvania, Delaware, and 

Maryland, demonstrates the approach of calculating MS4 WLAs on the basis of watershed model output and 

jurisdictional boundary. TMDLs were determined for the Christina River and its tributaries using a watershed 

model (HSPF) combined with a receiving water model (Environmental Fluid Dynamics Code [EFDC]). The 

HSPF model represented contributions from 10 land cover categories: residential-septic, residential-sewer, 

agricultural-livestock, agricultural-rowcrop, agricultural-mushroom, open land, forested, wetlands/water, 

undesignated, and urban. 

After determining the allowable nutrient load contributions from each land cover category for each 

subwatershed, the allowable loading was summarized for each township/municipality covered under the MS4 

permit. To do this, the municipality boundary GIS layer was overlaid with the land use coverage. Nutrient 

loadings were then estimated for each land cover category within each municipality boundary within each of the 

modeled subwatersheds. 

Because data were not readily available to determine stormwater outfall locations or to distinguish between 

areas within the municipalities that collected/conveyed stormwater and those that are truly nonpoint sources, 

the WLAs were based on jurisdictional boundaries. It was assumed that as part of the Phase II process more 

detailed stormwater information would be collected. This would enable the LA portion (i.e., that representing the 

truly nonpoint sources not delivered to receiving waters through stormwater collection/conveyance) to be 

extracted from the initial MS4 WLA allocations. 

4.3.1.1.3 Modeling Specific Stormwater Conveyances and Discharges 

Detailed stormwater conveyance models might be available in urbanized areas to support identification 

of allocations to regulated stormwater sources, particularly MS4 infrastructure. Stormwater models, 

often developed using the SWMM modeling framework, are implemented to support engineering 

design. They are generally capable of representing the effects of different storm events on flow 

throughout surface and subsurface components of a stormwater network. Thus, they are able to predict 

flow at individual stormwater discharge points to a receiving water. 

It is possible to modify these models so that they also represent long-term conditions and contaminant 

loading. Employing this approach would provide the greatest level of detail for stormwater allocation in 

TMDLs (particularly for regulated MS4s and permitted industrial facilities and construction sites within 

regulated MS4 boundaries); however, it can be cost-prohibitive for TMDL development because of the 

time and effort needed to set up and calibrate a model at that level of detail. The number of outfalls to 

include could require the model be set up at a spatial level that is not supported by available data. For 

example, to include inputs from individual outfalls, it is necessary to have monitoring data to 

characterize the discharge from each outfall. 

4.3.1.2. Impervious Cover Method 

The IC method for stormwater TMDL development involves setting a target of percent IC for the 

watershed of the impaired waterbody to represent attainment of water quality standards. As discussed in 

Chapter 3, many areas affected by stormwater sources experience impairments due to the increase in the 

frequency and volume of surface runoff from impervious surfaces as well as the degraded quality of 

surface runoff. Therefore, the use of a target based on IC relates the amount of impervious surface to the 

resulting runoff volume and pollutant loading. This approach has been used in areas where waters are 

listed on the basis of biological impairment, and stormwater runoff has been determined to be the 
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78 DRAFT November 2008 

primary cause of impairment. The approach should focus on effective imperviousness (i.e., a calculation 

of imperviousness that reflects the extent to which water falling on impervious areas is infiltrated, 

evapotranspired or reused) rather than total imperviousness. The target can represent the cumulative 

effects of both flow quantity and flow quality or the runoff and loading of a particular pollutant (e.g., 

metals, sediment, nutrients). The target should be used with available data to establish a link between IC, 

associated pollutant loading, and resulting waterbody conditions to calculate the loading capacity for the 

TMDL. Because the approach relies on a linkage between IC and the resulting runoff and waterbody 

effects, the approach is not be appropriate in areas where multiple sources other than urban runoff (e.g., 

wastewater treatment plant discharging nutrients) are affecting biological impairment. 

In Practice: Using the Impervious Cover Method to Develop TMDLs in the State of Connecticut 

To support TMDL development for waters impaired by stormwater, the state of Connecticut evaluated the 

relationship among IC and stream biological health to establish a statewide target using the surrogate measure 

of IC to represent attainment of water quality standards and support for aquatic life uses. The evaluation of 

statewide GIS-derived estimates of IC and macroinvertebrate data to identify a relationship among the 

parameters showed that measures of biological community (e.g., taxa richness) decreased with increasing IC. 

Using this analysis Connecticut Department of Environmental Protection established a target of 12 percent IC 

as representing maintenance of the state’s aquatic life criteria. 

4.3.1.3. Export Coefficients 

Export coefficients are measures of typical loading rates from certain land uses or sources. In TMDLs 

they would typically be used to calculate existing loads on the basis of the land use distribution in a 

watershed and would often be combined with a supplementary approach that calculates an allowable 

load on the basis of in-stream targets (e.g., percent reduction). Export coefficients can be obtained from 

literature values from regional or national studies (e.g., EPA’s NURP study [USEPA 1983]) or based on 

site-specific sampling of stormwater from individual land uses. The TMDL writer should evaluate the 

applicability of the coefficients to the watershed of the impaired waterbody and decide whether they are 

representative and appropriate. In addition, it is important to use the export coefficients in conjunction 

with some type of analysis of in-stream conditions to support calculation of a loading capacity. 

4.3.1.4. Simple Method 

The Simple Method calculates pollutant loading on the basis of 

stormwater runoff and typically is used in combination with another 

waterbody-based approach to support TMDL calculations to meet a 

waterbody target. For example, the Simple Method can be used to 

calculate the existing loading and then the percent reduction method 

would be applied to calculate the corresponding loading capacity. In other instances, the Simple Method 

equation can be applied to represent conditions assumed to meet water quality standards, such as using 

the applicable criteria as the allowable runoff pollutant concentration or using impervious targets to 

calculate the runoff and resulting loads. Regardless of how it is applied, the Simple Method will likely 

result in a gross pollutant load for the drainage area of an impaired waterbody. It would be necessary to 

distribute that load among the sources to set WLAs and LAs using such characteristics as land use areas 

Resources: For more information 

on the Simple Method, refer to the 

Resources list at the end of this 

chapter in Section 4.5.3. 
CHAPTER FOUR: DEVELOPING TMDLS 

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November 2008 DRAFT 79 

or jurisdictional areas. Alternatively, the Simple Method can be applied for delineated source areas to 

calculate loads from the different sources in the watershed. 

In Practice: Using the Simple Method to Support Development of the Swamp Creek Fecal 

Coliform TMDL (2006) 

Washington’s Swamp Creek, north of Seattle, does not meet state water quality criteria for primary contact 

recreation due to high levels of fecal coliform bacteria. Data analysis using ambient monitoring and source 

inventories identified urban stormwater and nonpoint sources as the primary problem. The Washington State 

Department of Ecology developed a TMDL for Swamp Creek, including separate loading capacities for the wet 

season (October–April) and the dry season (May–September) to capture significant variations in in-stream 

concentrations and expected source loading. Daily dry-season and wet-season loading capacities were 

calculated at three points in the watershed by multiplying the average seasonal flows by the water quality 

criteria of 100 cfu/100 mL, representing the allowable 90th percentile concentration. To compare with the 

loading capacities and identify necessary load reductions, existing loads were estimated by multiplying the 

same season average flows by the observed 90th percentile bacteria concentration based on data collected at 

the respective station. The Simple Method was used to estimate the relative stormwater loading from each 

MS4 to assign WLAs to the multiple MS4s in the watershed. WLAs were then assigned on the basis of the total 

loading capacity and each MS4’s proportional loading contribution at that station. Washington’s Swamp Creek 

TMDL is at www.ecy.wa.gov/biblio/0610021.html. 

4.3.2. Waterbody-based Approaches 

Waterbody-based approaches rely on calculating an overall in-stream load, sometimes reflecting 

drainage areas with multiple sources. Therefore, it is necessary to use some supplementary analysis to 

distribute the calculated loading capacity among the identified sources, including permitted stormwater 

sources. To divide a cumulative load among watershed sources, it is necessary to gain a general 

understanding of the relative magnitude of the sources. The source characterization discussed in Chapter 

3 should provide a TMDL writer with enough information to identify the sources of concern and at least 

generally delineate their location. The TMDL writer should be able to list the sources requiring LAs and 

WLAs and should know what data or information is available to characterize that source. For example, 

are there outfall or stormwater monitoring data for any of the stormwater sources? If so, such data (e.g., 

flow and concentration) can be evaluated and used to calculate a representative load attributed to the 

source. Or can the approach (e.g., load duration) be applied at a location that represents a drainage area 

from only one source? If so, the calculations at this point can be allocated to that individual source. 

With any of the waterbody-based approaches, if stormwater source monitoring data are available, the 

TMDL writer should try to incorporate the data to most accurately represent the source inputs and 

calculate existing loads and subsequently distribute allowable loads. For example, storm event sampling 

can be used to estimate source loads, either through collecting data at major stormwater outfalls or 

through ambient monitoring at key locations representative of certain land use categories. However, 

monitoring data are not always available to characterize the discharge characteristics of individual 

sources. In such cases, common methods used to divide the total load into WLAs for stormwater include 

consideration of a stormwater source regulated area, jurisdictional area, land use, or IC: 

• Stormwater regulated boundary: loading capacity is allocated to permitted stormwater sources (and 

other land-based sources) based on the proportion of the total drainage area they represent. For 
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80 DRAFT November 2008 

example, if the loading capacity is 100 lbs/day and an MS4 conveyance system drains and transports 

runoff from 20 percent of the area draining to the assessment location, the MS4 WLA is specified as 

20 lbs/day. To use this approach, it is necessary to be able to delineate the area draining to the 

source’s stormwater conveyance system. Otherwise, an alternate estimate of the stormwater source’s 

drainage area can be used, such as jurisdictional area. (For information on delineating the area of a 

stormwater source, see Chapter 3.) 

• Jurisdictional area: loading capacity is allocated to permitted stormwater sources (and other landbased sources) on the basis of the portion of the drainage area included within their physical 

boundary. Without knowing the specific area draining to a stormwater conveyance system, the 

stormwater source area can be represented by the jurisdictional or operational area of the source 

(e.g., urbanized area for an MS4). For example, if the loading capacity is 100 lbs/day and the 

urbanized area of an MS4 represents 30 percent of the area draining to the assessment location, the 

MS4 WLA is specified as 30 lbs/day. 

• Land use: loading capacity is allocated to permitted stormwater 

sources on the basis of expected land use unit area loads derived 

from literature values. For example, if the loading capacity is 

100 lbs/day and an MS4 is estimated to contribute 25 percent of 

the load on the basis of the land uses within its boundary, the 

MS4 WLA is specified as 25 lbs/day. 

• IC: loading capacity is allocated to permitted stormwater sources on the basis of the proportion of 

the drainage area they represent modified to reflect the amount of IC. 

While the issue of how to distribute the cumulative loading capacity among sources is relevant to all the 

waterbody-based approaches, the following discussion identifies some issues and considerations specific 

to each approach. 

4.3.2.1. Receiving Water Models 

In some situations, the TMDL writer might use receiving water 

models alone to support TMDL development. Receiving water 

models differ from watershed models in that they only represent 

conditions within a receiving water, such as a stream or reservoir. 

Land-based contributions are typically addressed through 

designation of boundary conditions (which are often based on 

monitoring data) or through development of a separate watershed model. While these models provide 

many benefits for water column analysis, they pose limitations for stormwater allocation when applied 

independently from a land-based loading or watershed model. The inherent limitation is that they do not 

explicitly represent land-based sources (i.e., land units). This limitation is most pronounced when only 

limited geographical data are available for regulated MS4s, industrial facilities, and construction sites. 

In situations where detailed geographic data are available, they can potentially provide a level of 

stormwater allocation commensurate with that of a watershed model application. Receiving water 

models are robust in their ability to provide a high level of detail laterally, vertically, and longitudinally. 

They can represent a stream, lake, or estuary using numerous analytical elements—in some cases tens of 

thousands. Thus, model predictions can be very accurate at many locations along the length of a 

receiving water. With this capability, receiving water models can be used to most accurately determine 

Resources: For more information 

on obtaining land use and coverage 

information, refer to the Resources 

list at the end of Chapter 3 in 

Sections 3.4.6 and 3.4.7. 

Resources: For more information on 

selecting receiving water models 

based on needs for a given TMDL, 

refer to the Resources list at the end 

of this chapter in Section 4.5.4. 
CHAPTER FOUR: DEVELOPING TMDLS 

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the specific amount of a pollutant that can enter a receiving water at different locations while still 

attaining and maintaining water quality criteria throughout. Thus, allocations can potentially be made at 

the very detailed stormwater discharge level. This assumes that very detailed stormwater discharge 

location (i.e., outfall) and contribution (flow and contaminant levels) are available to support the 

receiving water analysis. Without this level of detail, a receiving water model is limited. 

In practice, TMDLs employing receiving water models often also employ a watershed model to support 

source-based allocations, including those to regulated stormwater sources. 

4.3.2.2. Load Duration Approach 

TMDLs developed using the load duration approach most often 

identify the portion of the loading capacity for the stormwater 

WLA(s) on the basis of jurisdictional area. However, because the 

duration curve framework establishes a series of individual flowvariable loading capacities, the portion of each loading capacity 

attributed to individual sources typically will also vary by flow. 

Figure 19 illustrates a TMDL that was developed using a duration curve framework. WLAs are specified 

for municipal treatment plants that reflect NPDES permit limits. In the case of Figure 19, these WLAs 

are based on technology-based effluent limits at facility design flows. The treatment plant WLAs are 

constant across all flow conditions and ensure that water quality standards will be attained. WLAs are 

also identified for MS4s, which reflect increased loads under higher flow conditions. In the Figure 19 

example, stormwater WLAs for MS4 communities are based on the percent jurisdictional area approach. 

In this case, 3 percent of the watershed falls within the jurisdiction of MS4 communities. Thus, the MS4 

WLA is 3 percent of the available allocation for each flow zone. The remaining 97 percent is designated 

for nonpoint sources and natural background as the LA for each zone. 

Because a load duration curve establishes a flow-variable loading capacity, the framework allows for 

source-specific allocations to be adjusted by flow zone. To target loading controls and put the load 

duration results in a more digestible format, the load duration curve is usually divided into different flow 

zones representing different conditions (e.g., low flow, high flow). Representative existing loads and 

allowable loads can then be identified for each of those intervals. Because some sources tend to produce 

pollutant loads and affect the stream under certain flow conditions, this can help to distribute the 

allowable load among expected sources specific to a flow zone. This can account for different source 

areas and delivery mechanisms that might dominate under different flow conditions. For example, some 

TMDLs developed using the load duration approach allocate WLAs to stormwater sources for only 

certain flow zones (e.g., a WLA of zero is specified for the low-flow zone under the assumption that no 

load is generated from this source during those periods; see Table 10 for example). During the 

characterization activity discussed in Chapter 3, the TMDL writer should have an understanding of the 

types of stormwater sources and under what conditions they are affecting the stream. Within the load 

duration framework, allocations within the TMDL can be set in a way that reflects dominant concerns 

associated with appropriate hydrologic conditions. 

Resources: For more information 

on the load duration approach, refer 

to the Resources list at the end of 

this chapter in Section 4.5.5. 
CHAPTER FOUR: DEVELOPING TMDLS 

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82 DRAFT November 2008 

Figure 19. Example TMDL using duration curve framework 

Table 10. Example summary of allocations calculated for a TMDL using the load duration framework 

Name TMDL 

Component 

Duration Curve Zone 

(Phosphorus expressed as pounds/day) 

Redrock River—Phosphorus High Moist Mid Dry Low 

 TMDL 19.93 10.84 8.53 6.53 3.44 

 MOS 5.63 1.47 0.72 0.57 0.43 

 LA 11.79 7.62 6.28 4.72 2.59 

Douglton (WWTP) WLA 0.38 0.38 0.38 0.38 0.38

Guilaine (Industrial) WLA 0.04 0.04 0.04 0.04 0.04

Douglton (MS4 / Phase 1) WLA 1.39 0.89 0.74 0.55 0.00

Westerbridge (MS4 / Phase 2) WLA 0.49 0.31 0.26 0.19 0.00

Thompath (MS4 / Phase 2) WLA 0.21 0.13 0.11 0.08 0.00

Stormwater Source Areas of Concern 

CSOs 

Riparian Areas 

Impervious Surfaces 

Illicit Discharges 

4.3.2.3. Percent Reduction Method 

The Percent Reduction method typically involves comparing ambient water quality data to applicable 

water quality criteria to identify a necessary percent reduction in observed concentrations to meet WQS. 

That percent reduction is then applied to an existing load to calculate the loading capacity. Depending on 

how the existing loads were calculated, that analysis can help to support distributing the loading capacity 

among the sources and identifying any stormwater WLAs. For example, if the existing load is based on 

export coefficients or literature values for watershed land uses or sources, the TMDL writer will already 
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Tip: Refining allocations using spatial 

and temporal considerations 

TMDL writers can consider further refining 

any of the four basic categories of 

stormwater source WLAs using spatial and 

temporal considerations. Spatial 

considerations can include jurisdictional 

boundary, subwatershed boundary, or land 

cover. Temporal considerations can include 

wet seasons versus dry condition (based on 

flow), seasonally, or monthly. 

have calculated the source-specific existing loads and can target percent reductions to individual sources 

to meet the overall loading capacity goal. If the existing load did not include specific calculation for 

stormwater sources, it will be necessary to use one of the approaches discussed previously to distribute 

the loading capacity (e.g., source drainage area, jurisdictional area) into LAs and WLAs. 

4.3.2.4. Mass Balance or Steady-State Analysis 

Like the Percent Reduction method, applying a mass balance or steady-state analysis to calculate a 

loading capacity relies on some calculation of the incoming existing pollutant load. This might be done 

by back-calculating an existing load on the basis of observed concentrations and stream flow (or volume 

for lakes and reservoirs) and accounting for any expected losses (e.g., die-off, settling). If this is the 

case, the existing load represents a cumulative load from all the sources contributing to the pollutant of 

concern. If there are no data available to directly calculate loads from individual sources, the most likely 

approach for distributing the loading capacity and identifying stormwater WLAs will be to use some 

measure of source area (e.g., jurisdiction, drainage area) as discussed previously. Another option for 

calculating the existing load for a mass-balance analysis is the use of export coefficients. Similar to the 

Percent Reduction method, this would produce source-specific existing loads that could be multiplied by 

an estimated load reduction to calculate LAs and WLAs that meet the overall loading capacity. 

4.4. Categorizing WLAs for Stormwater Sources 

This section provides a description of the various ways that TMDL writers can categorize and assign 

WLAs to permitted stormwater sources. The manner in which TMDL writers choose to calculate the 

WLAs for permitted stormwater sources can vary depending on data availability and quality, stormwater 

source characteristics, and permit implementation considerations. Four basic options for categorizing 

stormwater source WLAs include the following: 

1. Aggregated for all stormwater sources (i.e., one overall WLA that represents total allocation to all 

MS4s, construction activities, and industrial facilities) 

2. Aggregated by each type of stormwater source (i.e., one WLA for all permitted MS4s; one WLA for 

all permitted construction activities; one WLA for all permitted industrial facilities) 

3. Individual by each stormwater source 

4. Individual by each outfall 

It is important to note that the four categories listed above are 

just basic options for presenting allocations within the TMDL.

They do not represent all possibilities. TMDL writers can 

consider using one or more of these options in concert within 

a TMDL for various types of stormwater sources. In addition, 

TMDL writers can further refine these basic options using 

spatial and temporal considerations to make the allocations 

more meaningful to stormwater sources. For example, a 

TMDL writer could present individual or aggregated WLAs 

for sources by subwatershed or by land cover category. If the 

WLA has temporal variations, a TMDL writer could further 

refine the WLAs by flow conditions (i.e., wet versus dry), months, or seasons. 
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WLAs that reflect the way that stormwater sources implement their respective SWMPs and SWPPPs are 

likely to be more user-friendly than WLAs that do not closely align with how stormwater sources 

manage their programs and plans. The more detailed and refined the allocation, the easier it will be for 

permit writers to translate through the permit and for stormwater sources to implement through by 

complying with permit requirements. The goal is to ultimately develop WLAs that are accurate, 

equitable, and implementable. 

The details of each basic categorization option are discussed below. Table 11 summarizes advantages 

and disadvantages associated with the four basic options for categorizing and assigning WLAs to 

stormwater sources. Where available, examples illustrate how states and EPA have used each 

categorization approach. The Appendix of this Handbook contains additional examples of categorization 

approaches for WLAs. 

Table 11. Options for assigning WLAs to stormwater sources 

Option Advantages Disadvantages 

Single Aggregated WLA for All 

Permitted Stormwater Sources 

Example: Lake Champlain 

(Vermont) Phosphorus TMDL 

(2002) 









 

Requires fewer permitted stormwater 

source characterization data to 

calculate 

Requires fewer resources to calculate 

Allows use of less complex WLA 

development approaches 

Allows permitted stormwater sources to 

determine at the local level how to 

further subdivide the overall allocation 

without specific commitments that could 

translate into permit requirements 











More difficult to implement in permits 

Requires permit writers or sources to conduct 

further analyses to disaggregate the overall 

WLA to individual sources 

Potentially overlooks pollutant load contributions 

from all types of permitted stormwater sources 

throughout the watershed 

Does not promote individual permitted 

stormwater source accountability for pollutant 

load reductions to implement the WLA 

More potential to capture unpermitted 

stormwater sources (i.e., urban nonpoint source 

runoff) in the aggregated WLA 

Separate Aggregated WLA for 

Types of Permitted Stormwater 

Sources 

Examples: 

 Potomac Direct Drain (West 

Virginia) Sediment TMDL 

 Charles River 

(Massachusetts) Pathogen 

TMDL 

 Shingle Creek (Minnesota) 

Chloride TMDL 

 Columbia Slough (Oregon) 

TMDLs









 

Requires less permitted stormwater 

source characterization data to 

calculate but allows for specificity within 

each category of permitted stormwater 

sources 

Requires fewer resources to calculate 

Allows use of less complex WLA 

development approaches 

Allows permitted stormwater sources 

within a specific category to determine 

at the local level how to further 

subdivide the overall allocation without 

specific commitments that could 

translate into stormwater permit 

requirements 









Does not result in equitable and easy-tounderstand (i.e., implementable) WLAs for 

permit writers or stormwater sources 

Requires permit writers or stormwater sources 

to conduct further analyses to identify sources 

that will implement the WLA 

Does not promote individual permitted 

stormwater source accountability for pollutant 

load reductions to implement the overall WLA 

Potential for double-counting pollutant load 

reductions where categories of stormwater 

sources might overlap (e.g., MS4s with 

permitted construction activities that a TMDL 

writer could assign under the aggregated MS4 

WLA or under the aggregated construction WLA 
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Option Advantages Disadvantages 

WLAs to Each Individual 

Stormwater Source 

Examples: 

 Columbia Slough (Oregon) 

Lead TMDLs 

 Wissahickon Creek 

(Pennsylvania) Siltation 

TMDL 

 Swamp Creek (Washington) 

Pathogen TMDL 

 Potomac Direct Drain (West 

Virginia) Sediment TMDL









 Promotes transparency and 

accountability in TMDL implementation 

Provides permit writers enough 

information to include reasonable 

provisions in relevant permit(s) 

Allows permitted stormwater source to 

understand specific pollutant load 

reduction responsibility and take steps 

to analyze overall SWMP or SWPPP to 

achieve reduction 

Promotes following regulatory 

boundaries (e.g., regulated MS4 

boundary) that are familiar to permitted 

stormwater sources and used to 

develop and implement SWMPs and 

SWPPPs 





Has the potential to add time to the overall 

TMDL development process if each permitted 

stormwater source has concerns related to the 

separate WLA assigned to its respective 

regulated MS4, industrial facility, or construction 

site 

Requires data and information specific to each 

permitted stormwater source, depending on the 

WLA development approach selected by TMDL 

writers (e.g., specific regulatory boundaries for 

each permitted stormwater source) 

Individual WLAs on an Outfall 

Basis 

Example: Middle Rio Grande 

(New Mexico) Fecal Coliform 

TMDL (2002) 





Provides permit writers with detailed 

information to develop tailored permit 

provisions, particularly for individual 

permits 

Allows stormwater sources to target 

implementation efforts to address a 

specific area represented by a 

stormwater discharge from a specific 

outfall 









Requires data and information pertinent to each 

outfall addressed by the TMDL analysis 

Requires comprehensive spatial understanding 

of a permitted stormwater source area, including 

a detailed system map with location of all 

outfalls, and surrounding land uses 

Might not be feasible in watersheds with a large 

number of permitted stormwater sources, 

particularly permitted MS4s that might have a 

large number of outfalls draining the system 

Does not align with how many permitted 

stormwater sources approach SWMP and 

SWPPP implementation (i.e., focus on 

systemwide approach as opposed to an outfallby-outfall approach) 

4.4.1. Single Aggregated WLA for All Stormwater Sources 

When data and resources to develop a refined characterization of sources are extremely limited, the 

ability of TMDL writers to analyze and identify the pollutant load contributions from each type of 

source is also limited. As a result, the TMDL writer might opt to group all stormwater sources into one 

category and assign one all allocation to all stormwater sources in this generalized category. 

Although this approach alleviates some technical complexities for TMDL writers, the lack of specificity 

associated with this approach can result in a lack of transparency and accountability that can create 

implementation challenges for permit writers and sources. TMDL writers can mitigate some of the 

challenges by considering and addressing them at the outset of WLA development. To ensure that the 

single aggregated WLA promotes transparency and accountability, it is essential to generate a 

comprehensive inventory of all permitted stormwater sources represented under the single WLA as a 

means to help (1) stakeholders and EPA reviewers ensure that the WLA considers all relevant 

stormwater sources in the watershed boundary (2) permit writers to incorporate reasonable and clear 

provisons into the relevant permit(s), and (3) stormwater sources understand who has a responsibility to 

help conduct implementation activities to implement the WLA. As discussed in Chapter 3, 

Characterizing Impairments and Stormwater Sources, it is important that the TMDL writer has a 

comprehensive understanding of the type, number, and location of stormwater sources within the 

watershed boundary. The TMDL document should clearly identify all stormwater sources addressed by 
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86 DRAFT November 2008 

the single aggregated WLA. As with all WLAs, the 

TMDL writer should also document other 

assumptions used to generate the single aggregated 

WLA. Assumptions might include the following: 

• The analysis used municipal jurisdictional 

boundary versus the actual MS4 boundary that 

defines the permitted area 

• The analysis assumes that all construction sites 

within the boundary selected to represent the 

MS4 (i.e., jurisdictional boundary or actual 

MS4 boundary) are the responsibility of 

regulated MS4s and fall under the MS4 

allocation, not a separate construction 

allocation 

Under this approach, permit writers or stormwater 

sources captured in the single aggregated WLA 

likely will need to conduct further analyses to 

make sub-allocations to each source; therefore, it is 

important that the TMDL document clearly 

identifies which stormwater sources are included in the single aggregated WLA. While it is possible to 

use single aggregated WLAs for stormwater sources in TMDLs, this approach is not preferred because 

of the associated implementation challenges. To avoid using this approach, TMDL writers can work 

with permit writers and stormwater sources to collect the additional data necessary to disaggregate 

allocations and assign to specific types of stormwater sources or, ideally, individual sources. 

4.4.2. Separate Aggregated WLA for Each Type of Permitted Stormwater 

Source 

This approach is an option for the TMDL writer to consider when the data are available to identify and 

separate the general pollutant load contributions from each type of permitted stormwater source, but 

source-specific information is not available to support accurate individual source allocations. For 

example, the TMDL writer might have a complete inventory of industrial facilities within the watershed 

boundary compiled from data obtained from a Phase I MS4 required to maintain an inventory of 

regulated industrial facilities (for permitted industrial facilities within the regulated MS4 boundary) and 

from a state or EPA Regional Office database of industrial facilities covered by individual or general 

industrial stormwater permits (for permitted industrial facilities outside the regulated MS4 boundary but 

within the watershed boundary). Although the TMDL writer has compiled a comprehensive inventory of 

the location and number of permitted industrial facilities, more information would be necessary on 

facility-specific processes and practices to estimate the pollutant load contribution from each facility. In 

such a case, the TMDL writer can use a general set of assumptions about the inventory of industrial 

facilities, such as the aggregate land area occupied by all industrial facilities and the location of these 

facilities within the watershed boundary (to determine soil type, perviousness, proximity to receiving 

waterbody) to calculate an aggregated. 

Tips for Using a Single Aggregated WLA for All 

Permitted Stormwater Sources 

• Create a comprehensive inventory of all permitted 

stormwater sources within the watershed boundary 

and document this inventory in the WLA 

assumptions 

• Indicate which permitted stormwater sources are 

part of the single aggregated WLA and provide 

rationale 

• Consider providing guidance to permitted 

stormwater sources on ways to further sub-allocate 

the single aggregated WLA to promote equity and 

accountability in implementation strategies and 

approaches 

• Provide rationale for using this approach versus 

other approaches that assign WLAs to more 

narrowly defined categories (e.g., limited available 

data, limited TMDL development resources, desire 

to promote flexibility) 

• Highlight any plans to revisit and revise the single 

aggregated WLA based on future data availability 
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In Practice: Approaches and Rationale for Developing and Assigning Separate Aggregated 

WLAs for Specific Types of Stormwater Sources in West Virginia, Massachusetts, 

Minnesota, and Oregon 

TMDL writers might decide to develop and assign separate aggregated WLAs for specific categories of 

stormwater sources based on a variety of factors, such as data availability, links to other TMDLs, or 

implementation considerations. Three TMDLs that assign separate aggregated WLAs for categories of 

stormwater sources include the Potomac Direct Drain (West Virginia) Sediment TMDL, the Charles River 

(Massachusetts) Pathogen TMDL, and Shingle Creek (Minnesota) Chloride TMDL. A brief description of the 

approach and rationale for developing and assigning stormwater source WLAs under each TMDL is provided 

below. 

Potomac Direct Drain (West Virginia) Sediment TMDL (www.wvdep.org/alt.cfm?asid=140) 

This TMDL addresses construction activities in tributaries of the Potomac River in Berkeley and Jefferson 

counties, West Virginia. The TMDL provides allocations for 297 active and pending construction sites 

registered under the West Virginia CGP. The main TMDL report provides an aggregated WLA for construction 

activities by subwatershed. In addition, the TMDL’s appendices provide individual WLAs for each impending 

and active construction site. 

Charles River (Massachusetts) Pathogen TMDL (www.mass.gov/dep/water/resources/tmdls.htm#charles) 

According to the final TMDL, all 35 communities in the Charles River watershed have stormwater permit 

coverage under the Phase I and Phase II MS4 Stormwater program (34 are covered under Phase II, and one is 

covered under Phase I). The TMDL addresses stormwater runoff from Phase I and Phase II as a pathogen 

source and assigns two WLAs toall MS4s—one for each type of surface water classification—as daily 

concentration targets. In addition, the TMDL expresses WLAs for stormwater as a daily load (i.e., colonies/day) 

using flow data from the fraction of the watershed that has IC; areas with pervious cover are considered 

nonpoint source runoff and accounted for under the LA. WLAs expressed as daily loads for stormwater runoff 

from Phase I and Phase II also includes contributions from combined sewer overflows and are presented on a 

segment-by-segment basis. 

Shingle Creek (Minnesota) Chloride TMDL (www.pca.state.mn.us/water/tmdl/project-shinglecreekchloride.html) 

The Shingle Creek Chloride TMDL addresses chloride contributions from road deicing activities by nine 

municipalities, one county, and the Minnesota Department of Transportation that have road maintenance 

Tips for Using Separate Aggregated WLA for Each Type of Permitted Stormwater Source 

• Create a comprehensive inventory of all permitted stormwater sources within the watershed boundary under each 

category of stormwater sources and document this inventory by category in the WLA assumptions 

• Indicate which permitted stormwater sources are included under each permitted stormwater source category of the 

separate aggregated WLA for each category and provide decision criteria to ensure consistency and clarity (e.g., all 

separately permitted construction sites in and outside regulated MS4 boundaries should be represented in the 

aggregated construction WLA; all separately permitted industrial facilities in and outside of regulated MS4 boundaries 

should be represented in the aggregated industrial WLA; only stormwater discharges from within the regulated MS4 

boundary should be represented in the aggregated MS4 WLA) 

• Provide guidance to permitted stormwater sources on ways to further sub-allocate the separate aggregated WLA for 

each stormwater source category to promote equity and accountability in implementation strategies and approaches 

• Provide rationale for using this approach versus other approaches that assign WLAs to more narrowly defined 

categories (e.g., limited available data, limited TMDL development resources, desire to promote flexibility for 

purposes of implementation) 

• Highlight any plans to revisit and revise the separate aggregated WLAs for each stormwater source category and 

under what circumstances (e.g., availability of better data or additional resources) 
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responsibilities within the Shingle Creek watershed. All these entities are MS4s. Under this TMDL, the MS4s 

received one aggregated WLA in the form of a percent reduction. (It is important to note that the WLA also 

addressed road salt storage facilities, private application, and residential chloride sources.) These permitted 

stormwater sources worked together through the Shingle Creek Watershed Commission to develop an 

implementation plan to meet the overall percent reduction target. The approach of assigning a single 

aggregated WLA for permitted stormwater sources in the regulated MS4 category allowed for a more flexible 

approach to implementation. Per the Shingle Creek Chloride Implementation Plan, working collectively to 

achieve the overall percent chloride load reduction allowed the entities with permitted MS4s to allocate load 

reductions to meet the percent reduction target on the basis of unique factors such as financial constraints, 

feasibility limitations, and public concerns and perceptions. 

Columbia Slough (Oregon) TMDLs 

(www.deq.state.or.us/wq/TMDLs/docs/willamettebasin/columbiaslough/tmdl.pdf) 

The TMDLs developed for the Columbia Slough include WLAs for industrial facilities and MS4s to address 

numerous parameters including dissolved oxygen, phosphorus, bacteria, and lead. To address the pollutant 

load reductions needed from stormwater sources, the TMDLs for 5-day biochemical oxygen demand (BOD5) 

and lead group together the two MS4s in the Columbia Slough basin into an MS4 category and the industrial 

facilities with permitted stormwater discharges into an industrial category. 

4.4.3. Individual WLAs for Each Permitted Stormwater Source 

TMDL writers can develop and assign WLAs for each stormwater source within the impaired 

waterbody’s watershed boundary. Although this approach might require additional data, it can facilitiate 

the implementation of permits and is ideal for permitted stormwater sources that want to know their 

specific pollutant load reduction responsibility without the need for negotiating with other permitted 

stormwater sources to further allocate pollutant load reductions. In some instances, depending on data 

availability, TMDL writers might have the ability to assign individual WLAs to specific permitted 

stormwater sources under a category of permitted stormwater sources (e.g., each MS4, each industrial 

facility), but might find that data limitations require the use of aggregated WLAs for other permitted 

stormwater sources (e.g., all active construction sites). This approach has the potential to allow each 

permitted stormwater source, and other interested 

stakeholders, to more closely review and analyze the 

technical approach for WLA development, including 

the specific assumptions used to generate the 

individual WLA. It also provides more of a direct 

nexus between the WLA and stormwater permit 

requirements that must contain conditions consistent 

with the requirements and assumptions of the WLA, 

thus promoting more accountability and a more 

clearly defined expectation with regards to 

implementation. Individual WLAs also provide a 

foundation for transparent and accountable watershedbased trading because each permittee has its own 

WLA (which is enforceable through a permit), and the 

WLAs are available to all potential trading partners to 

review. 

Tips for Using WLAs Assigned to Each 

Permitted Stormwater Source 

• Provide clear description of assumptions and 

information used to calculate the WLA for each 

permitted stormwater source 

• Ensure that permitted stormwater sources listed 

in the TMDL source characterization links to the 

list of permitted stormwater sources assigned a 

WLA to ensure transparency and equity 

• Provide information on estimated baseline 

pollutant load contributions from each permitted 

stormwater source to help sources understand 

the required pollutant load reduction 

• Consider additional methods for presenting the 

WLA that will support implementation based on 

understanding of existing SWMP and SWPPP 

approaches (e.g., breaking down total WLA for 

each permitted stormwater source on a 

subwatershed basis) 
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November 2008 DRAFT 89 

In Practice: Approaches and Rationale for Developing and Assigning Separate WLAs for 

Individual Stormwater Sources in Oregon, Pennsylvania, Washington, and West 

Virginia 

Columbia Slough (Oregon) TMDLs 

(www.deq.state.or.us/wq/TMDLs/docs/willamettebasin/columbiaslough/tmdl.pdf) 

The lead TMDL developed for the Columbia Slough includes an aggregated WLA for the industrial permitted 

area and provides an approach for these industrial facilities to calculate their individual allocation. The 

approach involves calculating individual industrial facility allocations on a unit-area basis for each flow 

condition. While the TMDL does not contain specific individual WLAs for each industrial facility, the TMDL 

provides the equation necessary for each facility to calculate the load specific to its site. 

Wissahickon Creek (Pennsylvania) Siltation TMDL 

(www.epa.gov/reg3wapd/tmdl/pa_tmdl/wissahickon/index.htm) 

This TMDL calculated WLAs for 16 permitted MS4s (defined by municipal boundaries, as opposed to regulated 

MS4 boundaries, for purposes of this TMDL) using land-use-specific, unit-area loads determined in modeling 

analysis for specific regions of the Wissahickon Creek basin, as well as the streambank erosion within each 

municipality. TMDL writers divided the Wissahickon Creek watershed into five main subwatersheds to match 

the impaired watershed with the smaller reference watershed used in the analysis. Sediment loads were 

estimated for each of the five subwatersheds and then distributed among municipalities as MS4 stormwater 

WLAs for each individual 303(d)-listed watershed. The WLA assigned to each of the 16 permitted MS4s 

accounted for both overland flow and streambank erosion. The final TMDL report presented a summary table 

that presented the existing load, the WLA, and the associated percent reduction to implement the WLA for both 

overland flow and streambank erosion, as well as the total WLA, for each permitted MS4. Appendix G of the 

final TMDL report also presents this information for each permitted MS4 on a subwatershed basis, allowing 

permitted MS4s to see and understand WLA information in both formats. 

Swamp Creek (Washington) Pathogen TMDL (www.ecy.wa.gov/biblio/0610021.html) 

This TMDL estimated the relative bacteria loading from each MS4 permit holder (i.e., the permit holder’s 

jurisdictional boundary, not necessarily the loading from the actual MS4 boundary) using the Simple Method 

and assigned WLAs to MS4s at each water quality monitoring station on the basis of their proportional 

contribution at that station and the bacteria criterion needing the greatest reduction. Each MS4 is required to 

achieve a percent reduction of the loading capacity at each TMDL compliance point according to the estimated 

contribution from the MS4 permit holder. 

Potomac Direct Drain (West Virginia) Sediment TMDL (www.wvdep.org/alt.cfm?asid=140) 

Although the main body of the TMDL report provides an aggregated WLA for construction sites, the TMDL’s 

appendices provide individual WLAs for each impending and active construction site. The individual WLAs are 

provided on a subwatershed basis. This TMDL covers approximately 297 active and pending construction sites 

registered under the West Virginia CGP in tributaries of the Potomac River within Berkeley and Jefferson 

counties, West Virginia. 
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90 DRAFT November 2008 

Tips for Using Individual WLAs on an Outfall Basis 

• Use where information about the permitted stormwater 

source and watershed conditions indicate that an outfall 

approach is not only feasible, but would be supported 

through SWMP and SWPPP implementation (e.g., few 

known outfalls that allow permitted stormwater sources 

to isolate and track implementation activities associated 

with changes in discharge pollutant loads at each 

outfall) 

• Ensure that permitted stormwater source has up-to-date 

information about outfall locations 

• Ensure that an outfall-by-outfall approach is feasible for 

the TMDL development process 

• Ensure that effectiveness monitoring is required in 

relevant permits, as appropriate, on an outfall-by-outfall 

basis 

4.4.4. Individual WLAs on an Outfall Basis 

Stormwater permits require permitted 

stormwater sources to develop maps of 

regulated MS4s, industrial facilities, and 

construction sites that include locations of 

stormwater outfalls. Permitted stormwater 

sources are required to map the location of 

stormwater outfalls, and with increasing 

frequency, this information is available. It is 

still possible that some stormwater sources 

might not have the information necessary to 

develop and assign WLAs on an outfall-byoutfall basis. However, for those stormwater 

sources that do have adequate data, TMDL 

writers can consider the feasibility and benefits 

of assigning individual WLAs to specific 

outfalls. For example, if a TMDL addresses 

pathogens during dry-weather flow from a high-priority area within the boundary of a regulated MS4 

(i.e., the system boundary), an individual WLA for specific outfalls within the high-priority area could 

be more useful in terms of supporting implementation than a WLA assigned to the entire MS4. 

In Practice: Developing and Assigning WLAs to Individual Outfalls Under the Middle Rio 

Grande, New Mexico, Fecal Coliform TMDL (2002) 

The Middle Rio Grande fecal coliform TMDL established that stormwater conveyances are the primary sources 

of fecal coliform loading to the Middle Rio Grande. Specifically, the TMDL report identifies four discrete 

concrete transports of stormwater contributing to fecal coliform loads and assigns a WLA to these four 

conveyances. The city of Albuquerque is responsible for implementing the WLAs assigned to these four 

stormwater conveyances through its MS4 SWMP. The TMDL is at 

www.nmenv.state.nm.us/SWQB/Middle_Rio_Grande-Fecal_Coliform_TMDL-May2002.pdf

In Practice: Using Multiple Approaches to Categorize Stormwater Source WLAs in the Los 

Angeles River Metals TMDL (2007) 

The Los Angeles River Metals TMDL categorizes stormwater source WLAs using a variety of options to help 

permitted stormwater sources meet both dry-weather and wet-weather targets for cadmium, copper, lead, zinc, 

and selenium in impaired reaches of and tributaries to the Los Angeles River. 

For dry weather, the TMDL provides single aggregated WLAs for all permitted stormwater sources on a 

subwatershed basis for three pollutants—copper, lead, and zinc. The permitted stormwater sources included in 

the single aggregated dry-weather WLAs include Los Angeles County MS4, Long Beach MS4, Caltrans, 

industrial facilities subject to the general industrial stormwater permit, and construction activities subject to the 

CGP. However, per the TMDL, industrial facilities and construction activities received a WLA of zero for dry 

weather, so only those entities subject to MS4 permit requirements share the single aggregated dry-weather 

WLAs for copper, lead, and zinc in the six reaches of the Los Angeles River and the seven tributaries. 
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For wet weather, the TMDL presents single aggregated WLAs for cadmium, copper, lead, and zinc that apply to 

all reaches and tributaries. In addition to providing single aggregated WLAs for each of these pollutants, the 

TMDL breaks down the overall WLAs into different stormwater source categories “by their percent area of the 

portion of the watershed served by storm drains.” Under this approach, the TMDL assigns separate aggregated 

WLAs to MS4s (i.e., Los Angeles County and Long Beach MS4s), Caltrans, industrial facilities covered by the 

industrial general permit, and construction sites covered by the CGP. In addition to separate aggregated WLAs, 

the TMDL states that each permitted industrial facility and construction site will receive individual WLAs per 

acre on the basis of the total acres of their facility. 

The TMDL is available at www.swrcb.ca.gov/rwqcb4/water_issues/programs/tmdl/tmdl_list.shtml. 

4.4.5. Other Elements in a TMDL 

Regulation and guidance require that all TMDLs include minimum 

elements. In addition to the elements already discussed in previous 

chapters (water quality standards, loading capacity, WLAs, LAs), a 

TMDL must also include a MOS, seasonal variation, and daily 

loads, In addition EPA recommends that TMDLs include 

allocations for future growth and reasonable assurance. The process or decisions related to including 

these elements in a TMDL might not vary when stormwater sources are involved. However, TMDL 

writers should consider how these other minimum elements might change when addressing stormwater 

sources in the analysis. Summarized below is each additional minimum element and, where appropriate, 

the stormwater-specific considerations related to the minimum element. 

• MOS. MOS must be included in a TMDL to account for any lack of knowledge concerning the 

relationship between allocations and water quality. The MOS may be implicit, incorporated into the 

TMDL through conservative assumptions in the analysis, or explicit, expressed in the TMDL as 

loadings set aside for the MOS. 

• Seasonal Variation. TMDLs must be developed with a consideration of seasonal variation in the 

analysis. As discussed in Section 4.3, TMDLs addressing stormwater sources are likely to have 

seasonal variations related to wet and dry seasons included in the analysis. 

• Reasonable Assurance. For TMDLs developed for waters affected by a mix of point sources and 

nonpoint sources, the TMDL should include reasonable assurance that nonpoint source control 

measures can achieve expected load reductions. TMDLs addressing stormwater sources often use 

this element to describe the NPDES permit requirements that apply to MS4s, industrial facilities, and 

construction activities included in the analysis. 

• Future Growth. TMDLs can also include allocations for future nonpoint and point sources, acting 

as a reserve for future sources. This minimum element is particularly important to stormwater 

sources that are intermittent. For example, construction activities might obtain permit coverage and 

commence after developing a TMDL or an industrial facility might no longer certify to a condition 

of no exposure and require permit coverage. These types of stormwater sources would need to 

implement WLAs from the future growth allocation. 

• Daily Load. TMDLs should include allocations expressed in terms of daily time increments. In 

addition, TMDL submissions can include alternative, non-daily pollutant load expressions (e.g., 

monthly, annual) to facilitate implementation of the applicable water quality standards. 

Resources: For more information on 

the required elements of a TMDL, 

refer to the Resources list at the end 

of this chapter in Section 4.5.7.
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92 DRAFT November 2008 

TMDL writers should consider how to include these minimum elements early in the TMDL 

development process and evaluate them in context of the major sources, critical conditions, and the 

chosen TMDL development approach. 

4.5. Resources 

4.5.1. General TMDL Development 

1. USEPA (U.S. Environmental Protection Agency). 1991. Guidance for Water-Quality-based 

Decisions: The TMDL Process. EPA 440/4-91-001. U.S. Environmental Protection Agency, Office 

of Water, Washington, DC. www.epa.gov/OWOW/tmdl/decisions/ 

This guidance document explains the programmatic elements and requirements of the TMDL 

process as established by CWA section 303(d) and by EPA’s Water Quality Planning and 

Management Regulations (40 CFR Part 130). It discusses the process for developing a TMDL, 

roles of EPA and the states in the process, and supporting or related water programs. 

2. USEPA (U.S. Environmental Protection Agency). 1999. Protocol for Developing Sediment TMDLs. 

EPA 841-B-99-004. U.S. Environmental Protection Agency, Office of Water, Washington, DC. 

www.epa.gov/owow/tmdl/sediment/pdf/sediment.pdf

This technical guidance document provides information to support TMDL writers in developing 

TMDLs for sediment. The document includes information on how to complete each step of the 

TMDL process, including problem identification, source assessment, linkage of water quality 

targets and sources, allocation analysis, and monitoring. 

3. USEPA (U.S. Environmental Protection Agency). 2000. Protocol for Developing Pathogen TMDLs. 

EPA 841-R-00-002. U.S. Environmental Protection Agency, Office of Water, Washington, DC. 

www.epa.gov/owow/tmdl/pathogen_all.pdf

This technical guidance document provides information to support TMDL writers in developing 

TMDLs for pathogens. The document includes information on how to complete each step of the 

TMDL process, including problem identification, source assessment, linkage of water quality 

targets and sources, allocation analysis, and monitoring. 

4. USEPA (U.S. Environmental Protection Agency). 2000. Protocol for Developing Nutrient TMDLs. 

EPA 841-B-99-007. U.S. Environmental Protection Agency, Office of Water, Washington, DC. 

www.epa.gov/owow/tmdl/nutrient/pdf/nutrient.pdf

This technical guidance document provides information to support TMDL writers in developing 

TMDLs for nutrients. The document includes information on how to complete each step of the 

TMDL process, including problem identification, source assessment, linkage of water quality 

targets and sources, allocation analysis, and monitoring. 

4.5.2. Watershed Models 

1. USEPA. 2005. TMDL Model Evaluation and Research Needs. EPA/600/R-05/149. U.S. 

Environmental Protection Agency, Office of Research and Development, National Risk 
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Management Research Laboratory, Cincinnati, OH. 

www.epa.gov/nrmrl/pubs/600r05149/600r05149.htm

This report documents the review of more than 60 available watershed and receiving water 

models for their applicability to TMDL development and implementation. It discusses model 

selection on the basis of model capabilities and provides a series of tables rating the capabilities 

or applicability the models using the categories of TMDL endpoints, general land and water 

features, special land processes, special water processes, and application considerations. The 

document also provides individual fact sheets for each reviewed model. 

2. USEPA. 1997. Compendium of Tools for Watershed Assessment and TMDL Development. EPA 841-

B-97-006. U.S. Environmental Protection Agency, Office of Water, Office of Wetlands, Oceans, and 

Watersheds, Washington, DC. www.epa.gov/owow/tmdl/comptool.html

This document reviews more than 50 watershed, receiving water and ecological assessment 

models. The document provides factsheets for each model that describes model components, 

methods, applications, pollutants addressed, limitations, input data requirements, and type of 

output. The document also contains information on model selection for specific applications, 

model calibration, and model verification. 

4.5.3. Simple Method 

1. Center for Watershed Protection’s Web site on the Simple Method: www.stormwatercenter.net. 

Click “By Category.” Information on the Simple Method is included in the Impacts of Urbanization

category. 

4.5.4. Receiving Water Models 

1. USEPA. 2005. TMDL Model Evaluation and Research Needs. EPA/600/R-05/149. U.S. 

Environmental Protection Agency, Office of Research and Development, National Risk 

Management Research Laboratory, Cincinnati, OH. 

www.epa.gov/nrmrl/pubs/600r05149/600r05149.htm

This report documents the review of more than 60 available watershed and receiving water 

models for their applicability to TMDL development and implementation. It discusses model 

selection on the basis of model capabilities and provides a series of tables rating the capabilities 

or applicability the models using the categories of TMDL endpoints, general land and water 

features, special land processes, special water processes, and application considerations. The 

document also provides individual fact sheets for each reviewed model. 

2. USEPA. 1997. Compendium of Tools for Watershed Assessment and TMDL Development. EPA 841-

B-97-006. U.S. Environmental Protection Agency, Office of Water, Office of Wetlands, Oceans, and 

Watersheds, Washington, DC. www.epa.gov/owow/tmdl/comptool.html

This document reviews more than 50 watershed, receiving water and ecological assessment 

models. The document provides factsheets for each model that describes model components, 

methods, applications, pollutants addressed, limitations, input data requirements, and type of 
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94 DRAFT November 2008 

output. The document also contains information on model selection for specific applications, 

model calibration, and model verification. 

4.5.5. Load Duration Curves 

1. USEPA (U.S. Environmental Protection Agency). 2007. An Approach for Using Load Duration 

Curves in the Development of TMDLs. EPA 841-B-07-006. U.S. Environmental Protection Agency, 

Office of Water, Office of Wetlands, Oceans, and Watersheds, Washington, DC. 

www.epa.gov/OWOW/tmdl/duration_curve_guide_aug2007.pdf

This document provides an overview on the use of duration curves for TMDLs, describing the 

basic steps needed to develop duration curves and subsequently identify loading capacities, 

LAs, WLAs, MOS, and seasonal variations. The guide also discusses some considerations and 

limitations in using the approach and includes several case examples. 

4.5.6. WLA Expression Options 

1. Wayland, R.H., and J.A. Hanlon. 2002. Establishing Total Maximum Daily Load (TMDL) Wasteload 

Allocations (WLAs) for Storm Water Sources and NPDES Permit Requirements Based on Those 

WLAs. Memorandum from Robert H. Wayland, III, Director, Office of Wetlands, Oceans and 

Watersheds, and James A. Hanlon, Director, Office of Wastewater Management, U.S. 

Environmental Protection Agency, Washington, DC. www.epa.gov/npdes/pubs/final-wwtmdl.pdf

This memo clarifies existing EPA regulatory requirements for, and provides guidance on, 

establishing WLAs for stormwater discharges in TMDLs approved or established by EPA. 

4.5.7. Required Elements of a TMDL 

1. USEPA (U.S. Environmental Protection Agency). 1991. Guidance for Water-Quality-based 

Decisions: The TMDL Process. EPA 440/4-91-001. U.S. Environmental Protection Agency, Office 

of Water, Washington, DC. www.epa.gov/OWOW/tmdl/decisions/ 

This guidance document explains the programmatic elements and requirements of the TMDL 

process as established by CWA section 303(d) and by EPA’s Water Quality Planning and 

Management Regulations (40 CFR Part 130). 

2. Grumbles, B.H. 2006. Establishing TMDL “Daily” Loads in Light of the Decision by the U.S. Court 

of Appeals for the D.C. Circuit in Friends of the Earth, Inc. v. EPA, et al., No. 05-5015, (April 25, 

2006) and Implications for NPDES Permits. Memorandum from Benjamin H. Grumbles, Assistant 

Administrator. U.S. Environmental Protection Agency, Office of Water, Washington, D.C. 

www.epa.gov/owow/tmdl/dailyloadsguidance.html

This memo clarifies EPA’s expectations concerning the appropriate tirne increment used to 

express TMDLs in light of the recent decision by the U.S. Court of Appeals for the D.C. Circuit 

in Friends of the Earth, Inc. v. EPA, et al., No. 05-5015 (D.C. Cir. 2006), which held that two 

TMDLs for the Anacostia River did not comply with the CWA because they were not expressed 

as daily loads. 
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3. USEPA (U.S. Environmental Protection Agency). 2007. Options for Expressing Daily Loads in 

TMDLs (Draft). U.S. Environmental Protection Agency, Office of Wetlands, Oceans and 

Watersheds, Washington, D.C. www.epa.gov/owow/tmdl/draft_daily_loads_tech.pdf

This document provides technically sound options for developing daily load expressions for 

TMDLs calculated using allocation time frames greater than daily (e.g., annual, monthly, 

seasonal). 

4. Perciasepe, R. 1997. New Policies for Establishing and Implementing Total Maximum Daily Loads 

(TMDLs). Memorandum from Robert Perciasepe, Assistant Administrator, to Regional 

Administrators Regional Water Division Directors. U.S. Environmental Protection Agency, Office 

of Water, Washington, D.C. 

The memo supplements existing regulations and guidance by documenting two policies to 

establish a nationally consistent approach for establishing and implementing TMDLs. These 

policies, and will remain in effect unless they are specifically changed by the Office of Water. 
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5. PROMOTING EFFECTIVE STORMWATER MANAGEMENT

TMDLs identify and quantify the loading reductions needed to meet water quality standards and, to the 

extent possible, facilitate implementation of management measures to implement targeted loading 

reductions. For stormwater discharges, permittees will reduce loadings in most cases by implementing a 

suite of structural and nonstructural stormwater BMPs over a certain period of time. Permittees then 

need to evaluate the effectiveness of BMPs implemented to implement loading reductions, make 

adjustments where performance was less than expected, and incorporate lessons learned in future BMP 

implementation activities. In many cases TMDL writers can anticipate an iterative process for making 

progress toward implementing WLAs and reflect this type of process in the TMDL report. This type of 

iterative, information-based approach to continuous improvement is often referred to as adaptive 

management. Through TMDL recommendations and permit requirements, TMDL and permit writers 

can direct permittees to create SWMPs and SWPPPs that achieve progress toward implementing WLAs 

over time and demonstrating such progress. 

As shown in Figure 20, the step of TMDL 

implementation involves identifying management 

options to implement LAs and WLAs. This 

chapter discusses the activities related to TMDL 

implementation for stormwater sources. The goal 

of this chapter is to provide TMDL and permit 

writers with an understanding of (1) their 

potential role in developing implementation and 

adaptive management recommendations and 

requirements; (2) technical considerations for 

analyzing and selecting structural and 

nonstructural BMPs that are suitable for making 

progress toward implementing a WLA; and (3) 

technical considerations for monitoring and 

assessing implementation strategies to implement 

the WLA. 

5.1. Implementation Roles and Responsibilities 

Implementing TMDLs through stormwater permits will involve planning and coordination. 

Implementation planning activities might involve TMDL and permit writers or draw on the skills of 

other agency staff dedicated to implementation related activities. In some instances, implementation 

planning activities might only involve permittees. As a result, this chapter refers to those engaged in 

implementation planning activities as stormwater planners. 

The role of stormwater planners in implementation planning will vary. Potential roles and activities for 

stormwater planners can include the following: 

• Evaluating and interpreting the WLAs assigned to stormwater sources. As discussed in Chapter 

4, TMDL writers can use a variety of approaches for categorizing and calculating stormwater WLAs 

in TMDLs. Individual WLAs assigned to each stormwater source or a specific source’s stormwater 

outfall(s) are the most straightforward type of WLAs to interpret and implement. Permit writers can 

Figure 20. Illustration of the steps in the TMDL 

process, including activities related to TMDL 

implementation. 
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most directly use individual WLAs and translate them into permit requirements. Stormwater 

planners should have the ability to conduct implementation planning activities using these refined 

WLAs. Where TMDL writers use aggregated approaches to categorize WLAs, stormwater sources 

might need to implement disaggregated WLAs for the purpose of SWMP or SWPPP 

implementation. Who disaggregates an aggregated WLA might vary—in some instances, it could be 

the TMDL writer, the permit writer, or even the actual stormwater sources. Stormwater planners can 

play a role in conducting the activities necessary to refine aggregated WLAs. 

• Developing a recommended list of structural and nonstructural BMPs to include in TMDLs or 

permits. If the approach entails providing sources with a narrow suite of BMPs to promote 

implementation, TMDL and permit writers can play the role of stormwater planners and have the 

responsibility for analyzing and identifying a suite of BMPs that is most appropriate for addressing 

the impairment(s) for use by one or more types of sources. Under this option, TMDL and permit 

writers acting as stormwater planners can coordinate to determine where to list the suite of BMPs—

either in the TMDL as recommended BMPs or in the permit as part of the permit requirements. 

Chapter 6 of this Handbook provides a detailed discussion on options for language to connect BMP 

implementation recommendations and requirements through TMDLs and permits. 

• Identifying specific structural and nonstructural BMPs when no BMP recommendations or 

requirements are provided in the TMDL or permits. If the approach requires sources to analyze 

and select BMPs on their own, permittees can then act as the primary stormwater planners. Under 

this option, TMDL and permit writers might focus on providing permittees with technical assistance 

to assist in BMP selection. As mentioned throughout this Handbook, federal law does not require the 

development of TMDL implementation plans. Where state regulations require the development of 

TMDL implementation plans, or states voluntarily attempt to develop these plans as part of the 

TMDL development process, TMDL writers might consider providing technical assistance type 

information in the implementation plan. TMDL writers can also consider incorporating 

recommendations on BMPs to implement the load reductions as part of the TMDL report. Permit 

writers can consider providing TMDL implementation technical assistance to permittees through the 

permit fact sheet or compliance assistance documents. 

• Developing BMP performance standards to include in the TMDL or permit. In some instances, 

the TMDL or permit might focus on BMP performance standards, rather than actual BMP 

recommendations or requirements. TMDL and permit writers in the role of stormwater planners can 

develop BMP performance standards to implement WLAs. Performance standards provide sources 

with the flexibility to select structural and nonstructural BMPs that are locally suitable while 

achieving the required or recommended performance standard. For example, New Jersey’s 

Stormwater Management Rules (N.J.A.C. 7:8) requires major development projects that create at 

least 0.25 acres of new or additional impervious surface to include stormwater management 

measures that reduce the average annual TSS load in the development site’s post-construction runoff 

by 80 percent. This type of performance standard allows sources in the role of stormwater planners 

to identify, select, and implement the most effective structural and nonstructural BMPs for a site or 

facility. 

Regardless of who plays the role of stormwater planner and to what degree, it is important for TMDL 

and permit writers to understand the range of potential BMPs, technical information related to BMP 

performance, processes for BMP selection, and adaptive management techniques to ensure that 

implementation recommendations and requirements translate into effective, on-the-ground 

implementation actions by sources. It is also important that permit writers and other agency staff 
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involved in permitting activities (e.g., enforcement and compliance staff) have an understanding of BMP 

implementation and adaptive management for purposes of reviewing and approving (where applicable) 

permit information and documentation (e.g., SWMPs, annual reports, monitoring data). 

The focus of this chapter is on the technical considerations necessary for selecting appropriate BMPs, 

evaluating BMP effectiveness over time, and making modifications to BMP implementation to ensure 

continuous water quality improvements. The goal of this chapter is to present these technical 

considerations to help stormwater planners as they develop implementation recommendations and 

requirements for use in TMDLs and permits to form the basis for demonstrating that TMDLs are 

adequately addressed in permits and associated SWPPPs and SWMPs. Although the focus of this 

chapter is on TMDL and permit writers serving as stormwater planners, the information presented in this 

chapter might also benefit permittees playing the role of stormwater planners. 

5.2. Key Questions for Promoting Effective BMP Implementation 

Stormwater planners can ask a series of six key questions to promote effective BMP implementation 

through an adaptive management framework. The adaptive management framework consists of 

planning, implementing, evaluating and learning, and adjusting. The six key questions are as follows: 

1. What is the current pollutant loading from the stormwater source’s discharge to the impaired 

waterbody accounting for existing BMPs? 

2. What additional loading reduction is necessary to implement the WLA? 

3. What additional BMPs might provide the remaining pollutant load reductions necessary to 

implement the assigned WLA on the basis of the expected performance of these BMPs? 

4. How should permittees measure BMP performance as implementation proceeds? 

5. Are measured pollutant load reductions adequate to make progress toward the assigned WLA over 

time? 

6. What modifications to the overall implementation strategy are necessary to make further progress 

toward implementing the WLA? 

Stormwater planners can use these key questions to guide implementation and adaptive management 

activities to achieve progress toward implementing WLAs. Although the discussion of the key questions 

highlights potential roles for TMDL and permit writers, sources might find these key questions helpful 

in conducting their implementation planning activities to comply with permit requirements. 

5.2.1. Establishing the Baseline Load and Accounting for Existing Load 

Reductions (Key Questions 1 and 2) 

The planning phase of adaptive management focuses on selecting BMPs to achieve the WLA and 

associated performance indicators to aid in tracking progress. Because the goal is to make iterative 

progress toward implementing a WLA, it is first necessary to quantify the starting point from which a 

source should measure progress. This is often referred to as the baseline. The baseline for a source might 

vary, depending on the context of the TMDL analysis. Sometimes the baseline pollutant loads for a 
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source’s discharge might take into account existing BMPs, and sometimes it might not. Stormwater 

planners involved in implementation planning should address the issue of a source’s baseline by asking 

the first two key questions as follows: 

1. What is the current pollutant loading from the stormwater source’s discharge to the impaired 

waterbody accounting for existing BMPs? 

2. What additional load reduction is necessary to implement the WLA? 

5.2.1.1. Answering Key Question 1: Determining Current Pollutant Loading from 

Stormwater Source Discharge 

Stormwater planners can first review the TMDL to determine how the analysis defined a source’s 

pollutant baseline load. During the development of the TMDL, the TMDL writer characterizes the 

pollutant loads from existing stormwater discharges. It is important for stormwater planners to review 

the TMDL analysis to determine the approach used to characterize pollutant loads from stormwater 

discharges. For example, the TMDL report should indicate if the TMDL writer made this determination 

on the basis of modeling using literature values for land use types and other generalized assumptions 

related to the stormwater source or if the analysis included real-world data and information, such as 

monitoring data to calibrate the model to actual conditions. 

After identifying the stormwater discharge pollutant loading used in the TMDL analysis, stormwater 

planners can then determine if the analysis accounted for any existing BMPs and, if so, the assumed or 

measured pollutant load reductions from those BMPs. If the TMDL analysis does account for existing 

BMPs, stormwater planners can use the pollutant loading information contained in the TMDL analysis 

as the starting point for gauging progress toward the WLA. Stormwater planners should attempt to 

verify that the existing BMPs used in the TMDL analysis represent a comprehensive and accurate 

listing. 

It is important for TMDL writers to remember that the information provided in the TMDL, as well as the 

assumptions used in the analysis, can influence the BMP selection process. The information contained in 

the TMDL serves as the basis for identifying the pollutants of concern and their source, which broadly 

indicates the type(s) and locations of BMPs needed to implement the pollutant load reductions from the 

sources with assigned WLAs. Greater specificity in the characterization of existing stormwater loads can 

likely promote more effective implementation. 

If the TMDL analysis does not provide comprehensive information on existing BMPs, answering key 

question 1 requires conducting two activities: (a) accounting for existing BMPs and (b) quantifying the 

associated pollutant load reductions. The first activity involves developing an inventory of existing 

BMPs that would affect loads of the pollutant of concern. The second activity involves measuring or 

estimating the pollutant load reductions from the BMPs identified in the inventory. 

5.2.1.1.1 Key Question 1, Activity A: Inventorying Existing BMPs 

A BMP inventory is a comprehensive listing or database of existing structural and, if applicable, 

nonstructural BMPs that directly or indirectly address the impairment(s). For structural BMPs, the 

inventory should include information on the type of BMP, location, date of installation, area treated by 

the BMP, and design and maintenance issues. For nonstructural BMPs, the inventory should include 

information on type of activity, implementation schedule, area addressed, and performance related data. 
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Stormwater planners involved in implementation activities for sources covered by individual stormwater 

permits might feasibly engage in developing BMP inventories by working with sources. For sources 

covered by general permits, it is unlikely that stormwater planners will conduct this activity. Instead, 

permit writers might opt to include in general permits a requirement for developing a BMP inventory. 

Through this option, stormwater planners could provide criteria to assist sources in the development of a 

BMP inventory, such as defining the type or location of BMPs that are most appropriate to address the 

impairment(s). The TMDL writer could include these BMP inventory criteria in the TMDL or the permit 

writer could include the criteria in the permit with the BMP inventorying requirement. 

Sources can also develop an accurate and comprehensive BMP inventory without the aid of criteria in 

the TMDL or permit. Permit writers can include BMP inventory requirements in permits that instruct 

sources to review existing SWPPPs or SWMPs—depending on the type of permittee—and identify 

BMPs that are likely to address the impairment(s). Sources would develop BMP inventories in 

compliance with this requirement through inspections or desktop auditing, depending on the type and 

number of BMPs in place. 

Technical issues related to compiling a BMP inventory can vary 

depending on the type of stormwater source—industrial facility, 

construction site, or MS4. Therefore, stormwater planners—

particularly permit writers—should keep these technical issues in 

mind when developing permit requirements related to compiling 

BMP inventories. For industrial facilities and construction sites, compiling a BMP inventory might 

involve a review of SWPPPs or a thorough facility inspection or monitoring of influent and effluent of 

BMP structures. This approach would obviously be very complex within some MS4s, however, because 

of the sheer numbers of BMPs and the difficulty in locating and characterizing them. The level of detail, 

therefore, for the BMP audit at an MS4 might be less, and many assumptions might need to be made. 

For example, an MS4 responsible for conducting a BMP inventory as part of the implementation process 

to implement a TSS WLA might determine that it is necessary to include all erosion and sediment 

control BMPs on active construction projects. Because of the large number and dynamic nature of 

construction projects, however, the MS4 might determine that it is not feasible to do an actual count of 

the numbers of various types of BMPs. Therefore, the MS4 uses the assumption that all active 

construction sites implement certain standard BMPs (i.e., silt fence, construction entrances, sediment 

traps) due to local ordinance and state permit requirements. The MS4 could then assume a certain 

noncompliance factor after reviewing enforcement actions against existing sites to attempt to correct for 

improper implementation of BMPs in the field when modeling actual contribution. Using municipal 

planning tools, municipalities with permitted MS4s can also project the number and location of future 

construction sites on the basis of growth projections, comprehensive planning, and land use zoning. 

In Practice: Promoting the Development of an Existing BMP Inventory in Washington 

Resources: For more information 

on compiling a BMP inventory, refer 

to the Resources list at the end of 

this chapter in Section 5.3.1.

The draft Phase II MS4 General Permit for Western Washington (Appendix 2) contains the list of all TMDLs in 

western Washington that include specific implementation activities that go beyond the general permit 

requirements. The general permit requires permittees with WLAs under these TMDLs to implement the 

activities specified in Appendix 2. Some of these additional requirements include developing an inventory of 

existing BMPs. For example, the Snohomish River Tributaries fecal coliform TMDL (2001) in Washington 

requires (via the permit) permittees to implement baseline source control BMPs for (1) commercial animal 

handling areas and (2) commercial composting facilities. The TMDL requires (via the permit) that the 

permittees compile a list of the existing facilities and conduct inspections of them to ensure implementation of 
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source control BMPs. After meeting these baseline conditions, permittees must develop and implement a 

Bacterial Pollution Control Plan. 

In addition to the type of stormwater source, the effort of compiling a BMP inventory can have an 

additional set of technical issues if the inventory is to include nonstructural BMPs. Nonstructural BMPs 

are typically programmatic in nature, such as good housekeeping practices at a transportation facility. 

While it might prove easy to account for these activities, it is not always easy to determine the pollutant 

load reduction associated with these activities. In the case of programmatic BMPs, stormwater planners 

should consider the need to make assumptions about defining what it means to implement nonstructural 

BMPs according to plan and justifying associated assumptions. 

5.2.1.1.2 Key Question 1, Activity B: Quantifying Load Reductions from Existing BMPs 

After the BMP inventory is complete, stormwater planners can either undertake the second activity of 

key question 2 or develop recommendations or requirements for sources related to this activity. The 

second component of key question 2 focuses on measuring or estimating the pollutant load reductions 

associated with the existing BMPs. Quantifying existing pollutant load reductions can help to determine 

if sources have made any progress toward implementing the WLA if the TMDL had assumed no BMP 

implementation. 

It is important that planners consider how best to evaluate and quantify the effectiveness of BMPs and 

provide guidance to sources either through the TMDL or the permitting process. Evaluation techniques 

might vary by the type of BMP, pollutant type, and other factors. It is particularly important to provide 

this type of guidance in instances where multiple sources addressed under the same TMDL will 

participate in BMP evaluations to determine progress toward the same or related WLAs. Ensuring that 

sources are using similar evaluation techniques for similar BMPs can not only provide a more defensible 

pool of BMP performance data, but also provide a level of equity among sources as they determine 

progress toward implementing WLAs. 

Quantifying pollutant load reductions associated with existing BMPs can vary in complexity for sources, 

depending on the size of the source’s permitted boundary, the number and types of BMPs, and whether 

they are structural versus nonstructural. For example, it might be relatively easy to determine the 

effectiveness of BMPs at an industrial facility with a single defined outfall and four oil/water separators 

which treat the runoff from the processing facility and parking lots than it would be to quantify the 

effects of multiple, individual BMPs or an entire MS4 SWMP. 

For a source with many BMPs, such as a large MS4 or a large construction site, the most viable option 

might be a suite of evaluation techniques that include monitoring, researching applicable literature 

values, and modeling using justifiable assumptions. For example, a large construction site with several 

types of erosion and sediment control BMPs could choose to either monitor representative BMPs or use 

existing literature to determine optimum sediment removal efficiency. Using the values for 

representative pollutant removal, the source could then estimate erosion and sediment control BMPs 

throughout the entire construction site. 

The complexities only increase when trying to quantify the effects of more programmatic BMPs, 

including employee training on good housekeeping techniques, public outreach, and inspections of 

industrial/commercial facilities. Many of these activities focus on source reduction and pollution 

prevention or behavioral changes that are difficult to translate into pollutant load reductions. 
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BMPs are likely to vary among the types of sources; therefore, appropriate evaluation techniques and 

approaches are likely to also vary. Evaluation techniques might also vary depending on the specific 

pollutant of concern or impairment that a source is trying to address through BMP implementation. 

The evaluation of stormwater management BMPs and plans can be very complex because many of these 

BMPs and plans focus on nonstructural, source reduction activities that prevent pollutants from entering 

the storm sewer system to begin with. As a result, there might not be any easy means to quantify the 

associated pollutant load reductions. Stormwater planners attempting to quantify pollutant load 

reductions from BMPs can likely have to generate estimates using a wide range of information sources 

ranging from state and local studies, national BMP performance data, monitoring data, modeling, and 

assumptions based on best professional judgment. It is critical that TMDL and permit writers keep these 

challenges in mind when developing WLAs and recommendations or requirements for assessing 

progress toward WLAs. The TMDL itself needs to account for the complexities of quantifying the 

effects of BMPs on stormwater runoff and the permit writer needs to allow for flexibility in the 

development of assessment strategies. 

Regardless of the difficulty level, it is in the source’s best interest that stormwater planners conduct the 

inventory of existing BMPs thoroughly and justifiably. If the source is the primary stormwater planner 

and is conducting the BMP inventory, the permit writer might want to review the BMP inventory 

procedure before initiation to confirm that the pollutant load reductions attributed to existing BMPs are 

appropriate. If a review of the BMP inventory procedure is a permit requirement, the permit writer 

should consider explicitly stating what the source must submit for review and approval. Information in 

the final inventory can assist sources with future implementation efforts, including scheduling and 

tracking maintenance activities and compiling records and reports. 

In Practice: Conducting a BMP Effectiveness Evaluation in Portland, Oregon 

Portland conducted an evaluation to determine the effectiveness of existing stormwater BMPs. According to the 

summary document, Effectiveness Evaluation of Best Management Practices for Stormwater Management in 

Portland, Oregon (www.portlandonline.com/shared/cfm/image.cfm?id=133994), the purpose of the 

effectiveness evaluation was to “develop and document the effectiveness ranges and preferred values for all 

BMPs either currently in use or anticipated for use in the management of stormwater quality and quantity in the 

City of Portland.” The evaluation methodology focused on analyzing a list of BMPs that met specific criteria, 

including BMPs required to implement TMDLs. The methodology also focused on surrogate pollutants selected 

to represent whole classes of pollutants. Surrogates included TSS, dissolved zinc, E. coli, and total 

phosphorus. In addition to water quality parameters, the evaluation addressed flow rates and volume, 

temperature, and habitat issues. Through the evaluation, Portland derived values for BMP performance using a 

wide variety of information sources as well as best professional judgment. The evaluation provides a range of 

effectiveness values for each BMP to account for uncertainty and location-specific or application-specific 

conditions that result in various points in the range of values. The evaluation serves as the necessary 

documentation of methods and assumptions to facilitate future review of BMP effectiveness and to evaluate the 

applicability of a particular value to a specific BMP. 
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5.2.1.2. Answering Key Question 2: Determining Additional Load Reductions 

Necessary to Implement the WLA 

After quantifying the measured or estimated pollutant load reductions associated with existing BMPs, 

stormwater planners can calculate the remaining pollutant load reductions necessary to implement the 

WLA. This value can serve as the driver for addressing key question 3: selection of additional BMPs. 

By comparing the information generated to answer key question with the overall pollutant load 

reduction necessary to implement the WLA, stormwater planners can estimate the magnitude of the 

pollutant load not addressed by BMPs. The difference between the source’s actual contribution (defined 

as the baseline pollutant load minus the pollutant load reductions addressed through existing BMPs) and 

the assigned WLA is the additional amount of pollutant removal that the source can implement through 

additional BMPs. 

5.2.2. Selection of Additional BMPs to Implement WLAs (Key Question 3) 

The information generated under key question 2 can 

serve as a starting point for stormwater planners to 

answer key question 3: 

What additional BMPs might provide the remaining 

pollutant load reductions necessary to implement the 

assigned WLA on the basis of the expected performance 

of these BMPs? 

Answering key question 3 also entails two activities: (a) 

identifying the list of possible BMPs to address the 

pollutant of concern or impairment and (b) quantifing the 

expected performance of each BMP under consideration. 

The role of stormwater planners in the BMP identification and selection process can vary depending on 

the approaches discussed at the beginning of this chapter. In addition, factors such as the type of source 

and the type of permit under which the source has coverage can also affect the role of stormwater 

planners at this phase of implementation planning. For example, TMDL and permit writers playing the 

role of stormwater planners can work closely with sources covered under an individual permit (e.g., 

large and medium MS4s) to conduct an analysis of possible BMPs and select a final suite of prescribed 

BMPs to include in the permit. Where general permits are available to sources, TMDL and permit 

writers playing the role of stormwater planners might rely on the use of performance standards in 

TMDLs and permits, while providing broad technical assistance through guidance documents. For 

example, TMDL and permit writers could research a range of possible BMPs and provide some of the 

information (e.g., expected performance information, cost, maintenance requirements) that sources 

would need to evaluate and identify the most suitable additional BMPs to address the remaining 

pollutant load reductions necessary to implement the WLA. Stormwater planners should keep in mind 

the range of other factors that can ultimately influence a source’s decision to implement a BMP, such as 

location, cost, and maintenance. 

The topics of identifying possible management strategies and selecting final management strategies to 

implement pollutant load reduction targets are discussed extensively in Chapters 10 and 11 of EPA’s 

Handbook for Developing Watershed Plans to Restore and Protect Our Waters (USEPA 2008a). While 

Tip: Ensure proper O&M to maximize 

BMP performance and resource 

investment 

State planners can remind sources that O&M are 

extremely important factors in maximizing BMP 

effectiveness. Sources might maximize pollutant 

load reductions by improving the O&M of existing 

BMPs rather than investing in additional BMPs. 

TMDL and permit writers can develop 

recommendations and requirements that ensure 

sources focus on proper O&M of BMPs intended 

to achieve progress toward WLAs. 
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the information provided in this extensive EPA resource is geared toward sources with implementation 

responsibilities, the process for screening and selecting BMPs described in these chapters have 

applicability to planners involved in TMDL implementation planning activities considering developing a 

suite of recommended or required BMPs to include in the TMDL or the permit. This section provides an 

overview of BMP identification and selection information that would benefit stormwater planners 

playing a more active role in this step of the process. 

5.2.2.1. Key Question 3, Activity A: Identifying List of Possible BMPs 

Identifying and selecting BMPs to implement the remaining pollutant load reductions involves 

compiling a list of candidate BMPs appropriate for the pollutant of concern or the impairment and 

determining a BMP performance rating (i.e., expected pollutant load removal or flow rate and volume 

reduction). It is possible that much of this work might already be done by addressing key question 1 

focused on inventorying and estimating the pollutant load reductions from existing BMPs. However, the 

suite of BMPs available to implement a WLA might encompass BMPs beyond those already 

implemented by a source. Therefore, the list of candidate BMPs is likely to include both existing and 

new practices for consideration. 

Stormwater planners should consider providing a suite of 

BMPs that encompasses every possible category of controls: 

source controls, treatment controls, structural controls, and 

nonstructural controls. Source controls are BMPs that seek to 

reduce the presence, use, or exposure of pollutants to the weather; volume controls promote infiltration, 

evapotranspiration and reuse of water, detention, or retention of stormwater and its constituents. 

Treatment controls are BMPs that attempt to remove or inactivate pollutants through physical, chemical, 

or biological processes. Structural controls are built structures or facilities that capture runoff, treat it 

through chemical, physical, or biological means, and discharge the treated effluent to the soil, the 

stormwater conveyance system, or directly to surface waters. Nonstructural controls usually involve 

management practices that focus on changes in activities or behaviors, as well as focus on controlling 

pollutants at their source. Examples include developing and implementing erosion and sediment control 

plans, organizing public education campaigns, and practicing good housekeeping at municipal, 

commercial, and industrial facilities. Regulatory and 

policy tools, such as ordinances, operating procedures, 

and maintenance schedules, are also examples of 

nonstructural controls. 

It is important to note that for certain types of 

impairments in certain areas nonstructural controls and 

source reduction practices might be less expensive and 

more effective than structural controls. For example, 

implementing new policies for sweeping streets and 

addressing illicit discharges is often less costly and more 

protective of water quality than constructing ponds or 

other stormwater treatment facilities at down-gradient 

locations. For example, stormwater planners addressing 

areas with seasonal chloride impairments associated with 

deicing activities can consider nonstructural practices 

Resources: For more information on BMP 

selection, refer to the Resources list at the 

end of this chapter in Section 5.3.2. 

Tip: Addressing peak flows to reduce 

channel impacts 

To prevent downstream degradation and 

maintain stream channel stability, planners 

should promote maintenance of predevelopmen

runoff volumes and rates for new development 

and redevelopment. Recent research shows 

that simply requiring site designs not to exceed 

pre-development runoff rates for a specified 

storm size (e.g., a detention basin designed to 

limit release rates) will not adequately protect 

the hydrology and habitat of the receiving water.

Management practices that slow, detain or 

infiltrate the runoff and release it slowly via 

baseflow to the receiving stream systems can 

replicate predevelopment site conditions and 

ensure both adequate aquifer recharge and 

stream base flow. 

t 
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such as alternatives to salt or salt reduction management activities (e.g., calibrating salt trucks) rather 

than structural BMPs. 

Many BMPs do and should target volume reductions (i.e., flow) to reduce total pollutant load. In 

addition to pollutant load reductions, BMPs that provide volume reductions also reduce channel erosion, 

alleviating impacts related to increased runoff. Streambank erosion, channel deformation and downcutting, and loss of natural habitat are among the more common effects of increased runoff from 

impervious surfaces. Stormwater planners can promote BMPs that reduce volumes through infiltration 

and evapotranspiration to ensure that BMPs are most effective in protecting or restoring impaired 

waterbodies. Volume reduction can also play a significant role in overall pollutant load reduction, which 

might not be immediately apparent if a BMPs ability to reduce incoming effluent concentrations is the 

primary metric by which it is measured. When a BMP, such as a rain garden or bioretention practice, 

captures a portion of incoming runoff and infiltrates it into the soil, the pollutants in that portion are 

prevented from entering a nearby river, lake, wetland, or coastal water. Simply comparing the 

concentration in and the concentration out of a BMP does not account for this pollutant load reduction 

due to infiltration. 

Because flow is a critical factor to consider during BMP 

selection and implementation, stormwater planners can 

promote that sources first consider source controls and volume 

reduction controls before considering more traditional 

treatment controls. One option for doing so is to include only 

these types of BMPs on a list of prescribed BMPs (e.g., sources must implement all BMPs on the list) or 

on a required menu of options (e.g., sources must select one or more BMPs from the list). Another 

option could involve stormwater planners developing recommendations or requirements that indicate 

that sources must first examine source and volume reduction controls to implement remaining pollutant 

load reductions and provide associated assumptions that indicate treatment controls are more appropriate 

or effective at reducing remaining loads. 

In Practice: Promoting Maintenance of Predevelopment Runoff Volumes and Rate in New 

Jersey 

Resources: For more information on 

volume control, refer to the Resources list 

at the end of this chapter in Section 5.3.3. 

New Jersey’s Stormwater Management Rules (N.J.A.C. 7:8) recognize that predevelopment site hydrology is 

essential to maintain to protect downstream hydrology. The New Jersey Model Stormwater Control Ordinance 

for Municipalities, included in the New Jersey Stormwater Best Management Practices Manual

(www.state.nj.us/dep/stormwater/tier_A/bmp_manual.htm) as Appendix D, contains requirements pertaining to 

controlling stormwater runoff quantity impacts. The ordinance requires developers to select and comply with 

one of the requirements. Two requirements focus on conducting a hydrologic and hydraulic analysis that 

compares pre-construction runoff volume and rates with post-construction conditions. A third requirement 

focuses on performance standards for stormwater management measure design to control post-construction 

peak runoff rates. 
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In Practice: Promoting Consideration of Nonstructural Controls First in New Jersey 

The New Jersey Model Stormwater Control Ordinance for Municipalities (referenced above) encourages MS4s 

to require developers to first consider nonstructural controls into new development designs. The model 

ordinance provides a list of nonstructural controls that includes protecting natural vegetation and drainage; 

protecting areas susceptible to erosion; minimizing impervious surfaces and breaking up or disconnecting flow 

over impervious surfaces; providing low maintenance landscaping; and incorporating source controls to 

minimize use and exposure of pollutants into site designs. The ordinance language states that if the developer 

contends it is not feasible to include these nonstructural controls into a project, the developer must provide the 

engineering, environmental, or safety reasons that render use of the nonstructural controls unfeasible. 

In many cases, functions of the various BMPs can overlap and can 

reduce a pollutant in more than one way. For example, green 

infrastructure (GI) and low impact development (LID) practices 

control the volume of stormwater being delivered downstream, which 

can help reduce streambank erosion and sediment loading, while also 

retaining sediment. GI and LID are stormwater management approaches and practices intended to 

eliminate or reduce urban runoff and pollutant loadings by managing the runoff as close to its sources as 

possible. As a collection of small-scale practices, linked together on a site, GI/LID have the potential to 

reduce the effects of development and redevelopment activities on water resources by maintaining or 

replicating the predevelopment hydrology of the site. Through practices such as rain gardens, vegetated 

swales, pervious pavements, and green roofs, GI/LID promote on-site infiltration, evapotranspiration, or 

reuse of rainwater. 

The final mix of structural and nonstructural management practices selected can, most importantly, be 

determined by which pollutants each BMP can effectively address and, more importantly, what specific 

level of performance each can provide. This can depend upon several highly variable factors, including 

the concentration and total load of the pollutant in the runoff, the volume and various rates of the runoff, 

antecedent rainfall and runoff conditions, and even the 

season or time of year. The variability of both applicable 

pollutants and levels of treatment can be seen by 

reviewing the sampling results of actual structural 

facilities taken over a number of storm events. Depending 

on the pollutant, the reduction in pollutant load or mean 

concentration achieved by selected structural facilities can 

vary considerably from event to event, with even negative 

reductions achieved at times, particularly for nutrients. 

Such variability makes it extremely difficult to determine 

a structural facility’s exact pollutant removal rate and 

illustrates why pollutant removal criteria are typically 

based on average annual conditions. 

5.2.2.2. Key Question 3, Activity B: Determining Expected BMP Performance 

Evaluating the potential performance of a BMP is a potentially challenging activity, but the information 

generated through this activity is essential to selecting BMPs with the most potential for making 

progress toward WLAs and attainment of water quality standards. Although this activity is discussed as 

Tip: Calculating the savings associated 

with GI 

The Green Values® Stormwater Calculator 

(http://greenvalues.cnt.org/calculator), 

developed by the Center for Neighborhood 

Technology, helps users see how GI can help 

save money and increase hydrologic benefits. It 

can be used to estimate the benefits of LID and 

conservation practices and allows users to input 

site development characteristics and green 

practices and returns financial and hydrologic 

outcomes for different scenarios. 

Resources: For more information 

on GI and LID, refer to the 

Resources list at the end of this 

chapter in Section 5.3.4. 
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a separate activity after compiling the list of possible BMPs, planners can make assumptions and 

determinations about BMP performance while generating the list. 

There are three important factors to consider when evaluating the potential performance of BMPs: 

concentration, volume, and total load. Estimating the total pollutant load reduction associated with a 

BMP is probably the best way to evaluate overall BMP performance. To calculate the total load, 

multiply the volume of water discharged by the BMP over a given period by the mean or average 

concentration of the pollutant. EPA’s Urban BMP Performance Tool Web site, at 

http://cfpub.epa.gov/npdes/stormwater/urbanbmp/bmptopic.cfm, provides an in-depth discussion of 

these factors and other considerations for evaluating potential BMP performance. 

Stormwater planners can estimate load reductions from BMPs in a number of ways. The most desirable 

method would be to monitor a BMP for every rain event over the course of several years to determine 

the volume of water coming into and leaving the BMP and the associated changes in pollutant 

concentrations. Rarely, however, is this level of monitoring possible because of resource constraints or 

because the type of BMP does not lend itself to this kind of water in—water out type of monitoring. This 

is important for planners to keep in mind when considering options related to monitoring and assessing 

implementation progress as part of the overall adaptive management framework. 

Other options for evaluating potential BMP 

performance include using existing information 

from past BMP studies and evaluations or 

computer-based predictive tools. Stormwater 

planners can consider the strengths and challenges 

associated with each option for evaluating 

potential BMP performance to determine which 

approach to use for supporting internal 

implementation planning activities, or to promote 

through recommendations and requirements for 

sources. Planners should consider several factors 

when examining each potential approach, 

including water quality parameters, source types, 

local conditions, available data, and user 

experience. Approaches range from simple to 

complex. Simple approaches include the use of 

published literature values for typical BMP 

performance or simple, spreadsheet-based models 

that calculate loads delivered to and removed by management measures. Complexity increases with the 

use of watershed models, which require substantial data inputs on multiple management techniques and 

can evaluate the optimum placement of BMPs as well as their performance. Simpler models are often 

sufficient to meet the needs of an analysis and are advantageous when time and budget resources are 

limited. Considerations for the use of literature values versus computer-based predictive tools are 

provided below. 

5.2.2.2.1 BMP Literature Values 

One method used for predicting load reductions from BMPs is to 

use literature values of performance typically associated with type 

Tip: Understanding “Percent Removal” 

Percent removal is a common metric for gauging BMP 

effectiveness. The pollutant concentration of stormwater 

flowing into a BMP is compared to the pollutant 

concentration of stormwater discharged after treatment 

by the BMP. Stormwater planners should understand the 

advantages and challenges associated with using 

percent removal as a way to determine BMP 

performance. One advantage of using percent removal is 

that it is a relatively available estimation of BMP 

performance. The challenge, however, is that percent 

removal can be a misleading statistic. For example, 

percent removal depends primarily on the influent 

quality. A BMP treating very dirty runoff will have a 

higher percent removal than the same BMP treating 

cleaner runoff. Also does not take into account volume 

reductions achieved through the BMP. For more detail 

describing why percent removal is a poor measure of 

BMP performance, see the fact sheet developed for the 

international stormwater BMP database at 

www.bmpdatabase.org/Docs/FAQPercentRemoval.pdf. 

Resources: For more information on 

BMP performance literature values, 

refer to the Resources list at the end 

of this chapter in Section 5.3.5. 
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of management practice and pollutant (e.g., detention pond, sediment). The percent reductions 

commonly documented in the literature are normally estimated from one or more monitoring studies 

where performance of BMPs was measured using flow and chemical monitoring. For example, the 

effectiveness of management practice systems could be calculated using the relative effectiveness of 

individual practices. Stormwater planners using this approach should verify that the study carefully 

estimated inflow pollutant concentrations and should also take volume reductions into consideration. 

This approach can help with initial BMP scoping and screening. However, planners using this approach 

to estimate pollutant load removals should be aware that this approach might oversimplify and 

overestimate cumulative removal rates for BMP treatment trains. 

5.2.2.2.2 Modeling Tools 

Stormwater planners can select or refine a list of potential BMPs 

using computer-based predictive tools, or modeling. Modeling 

stormwater pollutant load reductions from BMPs commonly involves 

three primary measures—pollutant concentrations in the stormwater 

at some point in time, the total pollutant load conveyed over a time 

period, and the event mean concentration. Stormwater planners can refer to the TMDL analysis to 

determine what modeling approach the TMDL writer used and if it is possible to use the same model to 

narrow the range of BMP options and aid in siting and sizing them. Performance standards for 

management practices often include controls on the stormwater peak discharge rate, the total runoff 

volume, and the total pollution load, which often is the focus of the WLA. Load reductions for 

individual and multiple BMPs—functioning as groups of practices or consecutive facilities arranged in a 

treatment train—can be modeled in some cases by using BMP calculators built into watershed models. 

For example, planners can use SWMM to evaluate urban area management practices and has the 

capabilities to emulate the major management practice processes (i.e., storage, infiltration, first-order 

decay, and sediment settling). The recently added overland flow rerouting (land-to-land routing) options 

block can be used to mimic the parcel (individual lot) level sites. 

A variety of more BMP-specific modeling options exist to simulate specific BMPs or unique situations. 

In some cases, specialized watershed models and management practice models are used to perform 

small-scale analysis of BMPs. Table 12 provides a brief description of several specialized models that 

can aid planners in selecting and siting BMPs to implement progress toward WLAs. 

Those selecting BMPs can take into account the unique 

set of conditions associated with each stormwater 

source. In addition to performance, sources will 

undoubtedly need to consider other selection factors that 

have little to do with water quality improvement. If the 

planning is being conducted by someone other than the 

permittee, consultations with industrial, construction 

site, and MS4 permittees during this process can help to 

create a bridge for discussions regarding BMPs selected, 

sites, designs, size, and cost. 

 Tip: Determining if models used in TMDL 

development are appropriate for 

implementation planning. 

If the TMDL development process involved 

watershed modeling, planners might have an 

opportunity to use this type of model for selecting 

and siting BMPs. Planners can work with TMDL 

writers to determine if the watershed model 

contains the necessary data for BMP 

implementation analysis. TMDL writers can also 

keep in mind the potential a watershed model 

might have for implementation planning purposes 

early in the TMDL development process. 

Resources: For more information 

on model applicability, refer to the 

Resources list at the end of this 

chapter in Section 5.3.6. 
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Table 12. Description of modeling tools available for BMP selection, sizing, and siting decision 

making

Model/tool Description 

The Site Evaluation Tool 

(SET) 

The model was developed to assess the effects of development, including sediment and nutrient 

loading, on a site scale. The tool allows definition of pre- and post-treated land use, allowing for 

multiple drainage areas and various combinations of practices. Structural and nonstructural 

practices can be represented, giving the user a suite of options for evaluation. 

www.unrba.org/set/index.shtml

Prince George’s County 

BMP-Decision Support 

System (PG BMP-DSS) 

The PG BMP-DSS evaluates the effect of management practices or combinations of 

management practices on flow and pollutant loading. This module uses simplified process-based 

algorithms to simulate management practice control of modeled flow and water-quality time 

series generated from runoff models such as HSPF. These simple algorithms include weir and 

orifice control structures; storm swale characteristics; flow and pollutant transport; flow routing 

and networking; infiltration and saturation; and a general loss/decay representation for a 

pollutant. The module offers the user the flexibility to design retention-style or open-channel 

management practices; define flow routing through a management practice or management 

practice network; simulate Integrated Management Practices such as reduced or discontinued 

imperviousness through flow networking; and compare management practice controls against a 

defined benchmark such as a simulated predevelopment condition. 

Model for Urban 

Stormwater 

Improvement 

Conceptualization 

(MUSIC) 

MUSIC was developed to evaluate small and large-scale (100 square kilometer) urban 

stormwater systems using modeling time steps that range from 6 minutes to 24 hours. MUSIC 

provides an interface to help set up complex stormwater management scenarios. The stormwater 

control devices evaluated by MUSIC include ponds, bioretention, infiltration buffer strips, 

sedimentation basins, pollutant traps, wetlands, and swales. Major techniques used to evaluate 

management practices including settling in ponds and decay of pollutants. www.toolkit.net.au/cgibin/WebObjects/toolkit.woa/wa/productDetails?productID=1000000 

Integrated Design and 

Evaluation Assessment 

of Loadings (IDEAL) 

IDEAL provides a spreadsheet-based technique for assessing the benefits of urban management 

practices on flow, sediment, nutrients and bacteria. The model predicts watershed runoff, 

concentrations, and loads, using the user’s selection of vegetative filter strips, dry detention 

ponds, and wet detention ponds. Urban areas are defined as pervious, impervious connected, 

and impervious unconnected areas. Flow and loads can be directed to a pond that can be either 

dry (no permanent pool) or wet (permanent pool). The model then calculates the pollutant 

removal efficiencies of the practices using empirical equations. The model predicts single storm 

values and converts them to average annual storm values using a statistical process. The IDEAL 

model is designed to help managers estimate long-term management practice pollutant removal 

efficiencies and is not designed for looking at individual storms. 

www.greenvillecounty.org/land_development/Planning.asp 

Soil and Water SWAT is a river basin-scale model developed to quantify the effect of land management 

Assessment Tool practices in large, complex (primarily agricultural) watersheds. SWAT was developed to predict 

(SWAT) the effect of land management practices on water, sediment, and agricultural chemical yields in 

watersheds with varying soils, land use, and management conditions over long periods of time. 

www.brc.tamus.edu/swat/ 

Storm Water 

Management Model 

(SWMM) 

SWMM is a dynamic rainfall-runoff simulation model applied primarily to urban areas and for 

single-event or long-term (continuous) simulation using various time steps. It was developed for 

analyzing surface runoff and flow routing through complex urban sewer systems. In SWMM, flow 

routing is performed for surface and subsurface conveyance and groundwater systems, including 

the options of nonlinear reservoir channel routing and fully dynamic hydraulic flow routing. 

SWMM has a variety of options for quality simulation, including traditional buildup and washoff 

formulation as well as rating curves and regression techniques. SWMM can simulate storage, 

treatment, and other BMPs. www.epa.gov/ednnrmrl/models/swmm/index.htm 

Vegetative Filter Strip 

Model (VFSMOD) 

VFSMOD provides specialized modeling of field-scale processes associated with filter trips or 

buffers. This model provides routing of storms runoff from an adjacent field through a vegetative 

filter strip and calculates outflow, infiltration, and sediment trapping efficiency. The model is 

sensitive to characteristics of the filter including: vegetation roughness or density, slope, 

infiltration characteristics, and the incoming runoff volume and sediment particle sizes. 

http://carpena.ifas.ufl.edu/vfsmod/ 
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Model/tool Description 

Wetland water balance WETLAND is a dynamic compartmental model to simulate hydrologic, water quality and 

and nutrient dynamics biological processes and to assist the design and evaluation of wetlands. The model can 

model (WETLAND) simulate both free-water surface and subsurface flow wetlands. WETLAND is modular and 

includes hydrologic, nitrogen, carbon, dissolved oxygen, bacteria, sediment, vegetation, and 

phosphorous submodels. 

Virginia Field Scale 

Wetland Model 

(VAFSWM) 

VAFSWM is a field-scale model for quantifying the pollutant removal in a wetland system. It 

includes a hydrologic subroutine to route flow through the treatment system; precipitation, 

evapotranspiration, and exchange with subsurface groundwater. 

Delaware Urban Runoff 

Management Model 

(DURMM) 

The Delaware Department of Natural Resources (DNREC) created DURMM to provide a more 

rigorous hydrological design tool for Green Technology BMPs. Green Technology BMPs are 

designed to “intercept runoff from rooftops, parking lots and roads as close as possible to its 

source, and direct it into vegetative recharge/filtration facilities incorporated into the overall site 

design and runoff conveyance system.” They include conservation site design, impervious area 

disconnection, conveyance of runoff through swales and biofiltration swales, filtration through 

filter strips, terraces, bioretention facilities, and recharge through infiltration facilities. 

www.swc.dnrec.delaware.gov/Pages/SedimentStormwater.aspx

Basin Sizer The Basin Sizer program allows users to find information useful for sizing stormwater basins in 

California. It is built from the STORM model and performs continuous simulation. Elements sized 

include infiltration basins, detention basins, and flow-based BMPs. http://stormwater.waterprograms.com/BasinSizer/Basinsizer.htm

City of Emeryville 

Stormwater Sizing 

Worksheet 

This spreadsheet allows the user to size metered detention, bioretention, planter strip, flowthrough planter box, and biofiltration BMPs. www.ci.emeryville.ca.us/planning/stormwater.html 

In Practice: Predicting BMP performance using a BMP Decision Support System in Vermont 

The Vermont Department of Environmental Conservation has developed a BMP Decision Support System 

(BMP-DSS) to facilitate stormwater permit issuance and compliance under the state’s stormwater regulation. 

The BMP-DSS is an innovative decision-making tool for evaluating placement and selection of BMPs and LID 

techniques at strategic locations in urban watersheds. It uses GIS technology, integrates BMP process 

simulation models, and applies system optimization techniques for BMP placement and selection to address 

the cost/benefit issues associated with stormwater management. The model provides the continuous simulation 

of hydrographs and pollutant loads and concentrations so that the effectiveness of LID approaches can be 

simulated within large-scale watersheds models such as HSPF and SWMM. It offers the user the flexibility to 

design stormwater structural practices such as bioretention cells, rain barrels, roof gardens, vegetated swales, 

infiltration chambers, wetlands and off-line regional stormwater retention and detention ponds. It also includes 

the simulation of site design characteristics such as storm drains, building density, road and sidewalk 

dimensions, disconnection of impervious surfaces, and compares BMP controls against some defined 

benchmark such as a simulated predevelopment condition. 
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In Practice: Predicting BMP performance using a Site Evaluation Tool in the Upper Neuse River 

Basin, North Carolina 

The Upper Neuse River Basin Association (UNRBA), made up of 13 city and county governments in the 

Triangle region of North Carolina, works to protect water resources within the 770-square-mile watershed 

above Falls Lake dam. Critical issues in the watershed are risk of nutrient enrichment in water supply reservoirs 

and effects on stream aquatic life from sediment loading and stream channel erosion. The Upper Neuse Site 

Evaluation Tool (SET) was developed under the guidance of many of the participating governments that were 

interested in a tool that could be used for both UNRBA goals and for local stormwater programs. This version of 

the SET included the ability to predict storm event peak flows and hydrographs, with scoping-level assessment 

of BMP influence on the storm event hydrographs. Model output was also tailored to assess site performance 

against variable nutrient loading rate goals on the basis of the user’s selection of the whether the site was 

residential or nonresidential and whether it was in a predefined urban versus rural zone. 

5.2.2.2.3 Additional Screening Criteria 

After researching candidate BMPs with regard to the effect on the pollutant of concern, planners should 

have enough information to analyze each management opportunity using appropriate and locally 

applicable screening criteria (see example in Table 13). Screening criteria are typically based on 

pollutant type, source area(s), performance/effectiveness, capital and O&M costs, and so on. 

Table 13. Example of screening criteria for stormwater management practices 

Screening criteria Description 

Volume reduction/source 

controls 

Consider the role volume plays in contributing to elevated pollutant loads. Determine the 

extent to which rate and volume of flow can be retained or reduced on-site. 

Location of the management 

practice within the critical 

area/watershed landscape 

Check to see if the candidate management practice can help implement the load 

reductions that were identified in one of the critical areas of the watershed. 

Estimated load reductions Using the information you collected during desktop and field scoping, document whether 

the anticipated load reduction is low, medium, or high. 

Legal and regulatory 

requirements 

Identify legal or regulatory requirements for projects, and determine whether any pose 

significant constraints. 

Property ownership Determine the numbers of property owners that need to agree to installation or 

implementation of the management practice(s). It is often easier to obtain easements on 

lands in public ownership. 

Site access Consider whether you will be able to physically access the site and identify a contact to 

obtain permission if private property must be traversed to access the site. Consider 

whether maintenance equipment (e.g., front-end loaders, vacuum trucks) will be able to 

reach the site safely. Designs and cost estimates might require adjustment if a structural 

control requires hand-cleaning because of maintenance access constraints. 

Added benefits In addition to their intended design, management practices can also provide secondary 

benefits by controlling other pollutants, depending on how the pollutants are generated or 

transported. For example, practices that reduce erosion and sediment delivery often 

reduce phosphorus losses because phosphorus is strongly adsorbed to silt and clay 

particles. 

Unintended effects In some cases, management practices that are used to control one pollutant could 

inadvertently increase the generation, transport, or delivery of another pollutant. 
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Screening criteria Description 

Physical factors There are many physical factors that can determine whether you will be able to install 

management practices. Look for constraints such as steep slopes, wetlands, high water 

tables, and poorly drained areas. Also look for opportunities such as open space, existing 

management practices that can be upgraded, outfalls where management practices could 

be added, and well-drained areas. 

Infrastructure Look for sites that have few utilities, road crossings, buried cables, pipelines, parking 

areas, or other significant physical constraints that could preclude installation or cause 

safety hazards. 

Costs The appropriateness of a management practice for a site can be affected by economic 

feasibility considerations that encompass short- and long-term costs. Short-term costs 

include installation costs, while long-term costs include continued O&M. 

Social acceptance Consider how nearby landowners will perceive the management practice. Will it cause 

nuisances such as localized ponding of water or vector control problems? Can these 

issues be addressed in the siting and design of the practice? How can nearby residents be 

involved in selecting and designing the practice to address their concerns? The optimal 

method for evaluating site feasibility for both riparian and upland management practices is 

through a site visit. 

5.2.2.2.4 BMP Site Selection 

In most instances, it is likely that sources acting as stormwater planners, rather than TMDL and permit 

writers or other entities at the state or EPA Regional level, will focus on identifying the most appropriate 

geographic location for siting and installing structural BMPs or conducting nonstructural BMPs. Factors 

affecting BMP siting decisions might include local conditions such as slopes, soils, and critical areas; 

historical, current, and future land uses; property ownership; cost; site access; infrastructure 

considerations; and social acceptance. Therefore, state or EPA Regional stormwater planners should 

either work closely with sources if the strategy includes making siting recommendations or consider 

leaving siting decisions to sources altogether. 

Stormwater planners can use, recommend, or require desktop and field reconnaissance to scope the 

possible additional BMPs appropriate for addressing the pollutants of concern and the potential 

geographic locations for implementing these additional BMPs. This information can be gathered during 

the initial phase of determining the location of existing BMPs. The reconnaissance usually involves 

locating and mapping the likely source areas for the TMDL listed pollutants, identifying candidate 

BMPs that might be used to address them, and screening possible BMP installation sites—or, in the case 

of nonstructural BMPs (e.g., erosion control, pavement sweeping, IDDE, materials management)—

scoping potential policy practices to address the problems identified. 

After identifying pollutant sources cited in the TMDL, stormwater planners might consider 

recommending areas where appropriate management measures can likely implement the greatest 

pollutant load reductions, or requiring sources to identify these areas. These so-called critical areas are 

at or near pollutant source areas, and could include places with severe upland or channel erosion, sites 

generating oil and grease or other toxics, extensively paved subwatersheds or small catchments requiring 

runoff volume controls, areas with a high density of illicit connections, parks that generate significant 

bacteria loads from pets, industrial facilities generating high pollutant loads, and similar locations. 

Stormwater treatment via multiple, consecutive BMPs can significantly improve the quality of water 

discharged to urban rivers, lakes, streams, wetlands, and coastal waters. In general, stormwater treatment 
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 Tip: Consider best placement of treatment 

trains 

There is some uncertainty regarding whether 

treatment facilities should be placed upstream or 

downstream of detention facilities that are needed 

for flow control purposes. In general, all treatment 

facilities can be installed upstream of detention 

facilities, although presettling basins are needed for 

sand filters and infiltration basins. However, not all 

treatment facilities can function effectively if located 

downstream of detention facilities. Those facilities 

that treat unconcentrated sheet flows, such as filter 

strips and narrow biofilters, are usually not practical 

downstream of detention facilities because of a 

variety of factors, including the sheer volume that 

must be treated. 

trains should seek to address source controls and infiltration, evapotranspiration and reuse first, then 

large particles, and, finally, small particles. The specific pollutant removal role of the second or third 

facility in a treatment train often assumes that significant settling or removal of solids has already 

occurred. For example, phosphorus removal using a two-facility treatment sequence relies on the second 

facility (e.g., sand filter) to remove a finer fraction of solids than those removed by the first facility. Oil 

control facilities must be upstream of treatment facilities and as close to the source of oil-generating 

activities as possible. They should also be upstream of detention facilities, if possible. 

5.2.3. Monitoring and Assessing Implementation Progress (Key Questions 

4-5) 

After identifying the suite of BMPs appropriate for addressing the remaining pollutant load reduction to 

implement the WLA, stormwater planners will need to rely on the sources to undertake the necessary 

steps to implement the final set of BMPs. At this point, stormwater planners can provide 

recommendations and requirements that focus on assessing BMP performance to determine if the 

additional controls are producing the expected pollutant load reductions. To accomplish this, stormwater 

planners should consider what recommendations and requirements can promote iterative improvements 

using an adaptive management framework. The key questions that planners should help sources answer 

include the following: 

4. How should permittees measure BMP performance as implementation proceeds? 

5. Are measured pollutant load reductions adequate to make progress toward the assigned WLA? 

5.2.3.1. Answering Key Question 4: Monitoring and Assessing Implementation 

of Additional BMPs 

This set of questions summarizes the basic activities related to conducting an adaptive management 

approach. It is important to remember that the ultimate goal of stormwater management driven by 

implementing a WLA is to meet the water quality criteria associated with the designated use(s) of the 

relevant waterbody. The iterative approach of stormwater management implementation allows for 

sources to make progress toward that goal over time. Therefore, it is imperative that stormwater planners 

provide sources with recommendations and 

requirements related to assessing the performance of 

BMPs and overall SWMPs and SWPPPs and how to 

use assessment information to make meaningful 

changes to management strategies that can ensure 

further progress. 

Stormwater planners can generate monitoring and 

assessment recommendations and requirements that 

can encourage sources to periodically evaluate BMP 

implementation and review monitoring and 

assessment information to track progress toward 

implementing WLAs. To comprehensively evaluate 

implementation activities, stormwater planners need 

to consider a combination of both process and 
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summative analyses. A process analysis is one that tracks progress, assesses the quality of data relative 

to measurement quality objectives (i.e., whether the data are of sufficient quality to answer the 

monitoring question), and provides early feedback on trends, changes, and problems in the watershed. 

The summative analysis is more intensive and determines the status, changes, trends, or other issues that 

measure the environmental response to BMP implementation, as well as overall SWMP and SWPPP 

implementation. 

The California Stormwater Quality Association (CASQA) developed the Municipal Stormwater 

Program Effectiveness Assessment Guidance (CASQA 2007) to assist municipal stormwater program 

managers in designing and conducting program effectiveness assessments using a range of assessment 

methods. The document also describes how to use these methods on the basis of program-specific 

desired outcomes and goals. Figure 21 (labeled Figure 2 in the CASQA guidance document) shows the 

outcome levels described with associated effectiveness assessments. Levels 1 to 4 are evaluated through 

Implementation Assessments and Levels 5 to 6 are evaluated through Water Quality Assessments. An 

MS4 would typically use Levels 1-4 to determine if programmatic targets or goals are being met. 

Evaluations like the ones described in the CASQA document or 

those typically conducted by permitting authorities to determine an 

MS4’s compliance with permit requirements and SWMP 

obligations typically do not quantify the level of effectiveness. 

They typically assess whether the MS4 is meeting the goals of the 

permit to the MEP. (For a discussion of MEP, see Chapter 2) This 

quantification or characterization in a manner that can be compared to the goals in a TMDL is the 

greatest challenge when trying to determine if an MS4 program is making progress toward a WLA. The 

best, most appropriate method to accomplish this can vary greatly among MS4s according to the types 

of BMPs and pollutant being limited by the TMDL. 

Figure 21. Approaches to evaluating storm water program effectiveness. (Source: CASQA 2007) 

Resources: For more information 

on stormwater program evaluation, 

refer to the Resources list at the end 

of this chapter in Section 5.3.7. 
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In Practice: Developing a Monitoring Program to Assess BMP Effectiveness in Addressing 

Pathogens in the Middle Rio Grande, New Mexico (2002) 

Middle Rio Grande TMDL for fecal coliform recommends that MS4s, “[d]evelop and implement a monitoring 

program to assess BMP effectiveness and to compare loadings to the targets.” The Albuquerque MS4 permit, 

based on this recommendation in the TMDL, requires the city to develop a monitoring program to track trends 

in fecal coliform and BMP effectiveness to track compliance with the TMDL WLA; to use an adaptive 

management approach by implementing revisions to the required programs if deemed necessary on the basis 

of monitoring data; and to develop and submit BMP evaluations and assessments, as well as an Annual TMDL 

Progress Report that summarizes monitoring results and includes computations of annual percent reduction 

achieved from the baseline loads and comparisons with the target loads. 

In Practice: Working with Stakeholders on Adaptive Management for Swamp Creek, 

Washington 

The Swamp Creek TMDL (2006) includes language that requires the Washington Department of Ecology to 

annually meet with municipal stakeholders to, “share information on the state of water quality in the watershed 

and status of implementation activities. Water quality data, trends (where applicable), regulatory changes, new 

and innovative concepts, and funding sources will be discussed to evaluate the overall status of the TMDL. 

Ecology will solicit input from the workgroup at this time to help direct the adaptive management of this TMDL. 

Ecology will track implementation no less than annually using the tracking table in Appendix E and through 

municipal stormwater permit program audits.” 

In Practice: Permit Requirements for Establishing Pollutant Load Reduction Benchmarks for 

Columbia Slough, Oregon 

The Columbia Slough TMDL (1998) states that monitoring and implementation of BMPs will be done by MS4s 

to comply with the BOD5 WLA. The Phase I MS4 permit for the city of Gresham, city of Fairview, and 

Multnomah County specifies BMP requirements to implement the WLA to the MEP. The Oregon Department of 

Environmental Quality created an MS4 permit benchmarking approach that applies to all TMDL parameters for 

which stormwater WLAs were established. Benchmarks are estimates of future pollutant load reductions. The 

permit requires that the benchmarks and necessary BMPs be included in the MS4 SMWP. The permit defines a 

benchmark as follows: 

A benchmark is a total pollutant load reduction estimate for each parameter or surrogate, where applicable, 

for which a WLA is established at the time of permit issuance. A benchmark is used to measure the overall 

effectiveness of the stormwater management plan in making progress toward the WLA (this estimate will 

be related to the statistical variability of the underlying data and may be stated as a range), and is intended 

to be a tool for guiding adaptive management activities. A benchmark is not a numeric effluent limit; rather 

it is a goal that is subject to the maximum extent practicable standard. The co-permittee must provide the 

rationale for the proposed benchmark, which includes an explanation of the relationship between the 

benchmarks and the TMDL wasteload allocations. Any limiting factors related to the development of a 

benchmark, such as data availability and data quality, must also be included in this rationale. 

The Phase I permit requires that a monitoring plan be designed to track the long-term progress of the SWMP 

toward achieving improvements in receiving water quality, including progress toward implementing pollutant 

load reduction benchmarks associated with TMDL constituents. This requirement is addressed with the 

ambient and outfall monitoring that is conducted, and assessed as part of the data evaluation and reporting 

components of the program that occur during each permit renewal application. The permit also requires that 

results of the monitoring be used to support the adaptive management process and lead to refinements of the 

SWMP. 
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To ensure a successful adaptive management process, both water quality and BMP performance 

monitoring data should be assessed fairly frequently (e.g., as part of the SWPPP/SWMP evaluation 

process). Progress reports and regular team meetings are two effective ways to accomplish this. Even 

though frequent evaluations of BMP performance and water quality data might seem demanding, early 

indications of trends or problems can prevent major future problems. 

The best use of monitoring data can depend on a variety of factors, including the type of permittee, 

geographic size of the permittee and the impairment(s). However, there are three basic types of 

monitoring that could be used—BMP monitoring, outfall monitoring, and receiving water monitoring. 

• BMP Monitoring. There are several options for assessing BMP 

performance, including BMP monitoring. The University of 

Minnesota’s Assessment of Stormwater Best Management 

Practices provides a process for developing and implementing a 

BMP assessment program that includes four levels of 

assessment: (1) visual inspections, (2) capacity testing, (3) synthetic runoff testing, and (4) 

monitoring. Monitoring is considered the most accurate method for assessing BMP volume 

reduction, peak flow reduction, and pollutant removal efficiency. Procedures for each level of BMP 

assessment can vary by the type of BMP. As previously stated, certain types of structural BMPs lend 

themselves well to direct monitoring (e.g., discharge from a stormwater pond). Data from these types 

of BMPs should be collected to the extent possible and used to determine if the BMPs are 

performing as expected and if this is adequate. If there is a large number of BMPs, it will likely be 

more appropriate to sample a representative number and extrapolate the results to the rest. 

• Outfall Monitoring. Permittees of small geographic size (i.e., 

construction project or industrial facility) with few outfalls 

might be able to monitor the effluent and directly measure 

whether the facilities’ contribution has been reduced adequately 

to implement the applicable WLA. Monitoring outfalls of large 

permittees (i.e., MS4s) might not provide any useful data for adaptive management of specific BMPs 

but could help to determine geographic areas or isolated land use types that might need further 

attention if the monitoring reveals inadequate progress toward TMDL goals. Data from outfall 

monitoring can also serve as an indicator to determine if more structural or nonstructural BMPs are 

needed in the area contributing flow to the outfall (e.g., data from one outfall demonstrates inferior 

BMP performance versus other outfalls). 

• Receiving Water Monitoring. Because the ultimate purpose of 

the TMDL and subsequent implementation efforts is to improve 

receiving water quality, it might be in the best interests of 

permittees to conduct their own ambient monitoring. For 

example, larger permittees with adequate resources could 

regularly monitor receiving water to evaluate trends, confirm significant sources, and potentially 

update and improve existing models. As previously described, the CASQA Municipal Stormwater 

Program Effectiveness Assessment Guidance provides information on water quality assessments to 

help determine what effect BMPs are having on receiving water quality. A list of other monitoringrelated resources is also provided at the end of this section. 

Resources: For more information 

on BMP monitoring, refer to the 

Resources list at the end of this 

chapter in Section 5.3.8. 

Resources: For more information 

on receiving water monitoring, refer 

to the Resources list at the end of 

this chapter in Section 5.3.10. 

Resources: For more information 

on stormwater outfall monitoring, 

refer to the Resources list at the end 

of this chapter in Section 5.3.9. 
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In Practice: Using Receiving Water Monitoring to Demonstrate Progress in the Los Angeles 

River, California 

Receiving water monitoring is required to demonstrate a permittee is implementing the WLAs in the Los 

Angeles River and Tributaries Metals TMDL. This TMDL includes an implementation plan which states the 

following: 

Each municipality and permittee will be required to meet the storm water waste load allocations shared by 

the two MS4s and Caltrans permittees at the designated TMDL effectiveness monitoring points. [...] The 

MS4 and Caltrans stormwater NPDES permittees will be found to be effectively meeting dry-weather waste 

load allocations if the in-stream pollutant concentration or load at the first downstream monitoring location 

is equal to or less than the corresponding concentration- or load-based waste load allocation. Alternatively, 

effectiveness of the TMDL may be assessed at the storm drain outlet based on the waste load allocation 

for the receiving water. For storm drains that discharge to other storm drains, the waste load allocation will 

be based on the waste load allocation for the ultimate receiving water for that storm drain system. The MS4 

and Caltrans stormwater NPDES permittees will be found to be effectively meeting wet-weather waste load 

allocations if the loading at the downstream monitoring location is equal to or less than the wet-weather 

waste load allocation. 

5.2.3.2. Answering Key Question 5: Determining Whether Implementation 

Progress is Adequate 

After quantifying the pollutant load reduction gains made through implementing existing and additional 

BMPs, stormwater planners should assist sources in determining if the current SWMP and SWPPP are 

adequate to implement the WLA over time. To answer this question, however, both stormwater planners 

and sources need guidance or criteria for judging progress, information that ideally should be first 

presented in the TMDL and permit. Therefore, TMDL and permit writers should consider how to define 

progress, which is likely to vary by pollutant, permittee type, and other potential factors such as 

compliance determination methods by the permitting authority. 

Options for defining progress toward implementing the WLA are as follows: 

• Demonstrating pollutant load reductions by an agreed on date 

• Demonstrating pollutant concentration reductions by a particular date 

• Implementing a set of prescribed BMPs with specific performance standards by a particular date 

• Complying with the permit SWMP or SWPPP requirements during the permit term 

Stormwater planners should be aware that current TMDLs and permits use a variety of these approaches 

to define progress, while still other TMDLs and permits remain silent on the issue of progress. If the 

TMDL and the permit do not provide a definition of how to determine progress, stormwater planners 

can consider developing recommendations and requirements to help sources create and justify their own 

progress milestones or benchmarks. Stormwater planners can specify the nature of the benchmarks (e.g., 

quantifiable) and determine supporting information sources should submit to justify and verify the 

validity and accuracy of the selected benchmarks. Most importantly, TMDL and permit writers need to 

remember the iterative, dynamic nature of SWMP and SWPPP implementation when crafting TMDL 

and permit requirements. Where possible, defining a framework approach, including goals, for assessing 

progress and allowing the sources to determine the details (with adequate evaluation through the SWMP 

and SWPPP) is going to be more useful than trying to identify a detailed approach at the state level. 
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(Chapter 6 provides a brief discussion on compliance schedules—an important factor to consider when 

evaluating overall progress.) 

In Practice: Including Specific Criteria for Assessing Progress in the Los Angeles MS4 Permit 

The Santa Monica Bay Beaches dry weather bacteria TMDL is expressed in terms that are directly applicable 

to MS4s and the associated WLA is included in the Los Angeles MS4 permit. This provides the permittees a 

clear understanding of what must be accomplished to implement the WLA. The permit language (Provision 29 

of Order 01-182) is as follows: 

The Waste Load Allocations (WLAs) in the Dry Weather Bacteria TMDL are expressed as the number of 

allowable days that the Santa Monica Bay beaches may exceed the Basin Plan water quality objectives for 

protection of Water Contact Recreation (REC-1) in marine waters, specifically the water quality objectives 

for bacteria.... Tables 7-4.1, 7-4.2a, and 7-4.3 of the [Los Angeles River] Basin Plan set forth the pertinent 

provisions of the Dry Weather Bacteria TMDL. They require that during Summer Dry Weather there shall 

be no exceedances in the Wave Wash of the single sample or the geometric mean bacteria objectives set 

to protect the Water Contact Recreation (REC-1) beneficial use in marine waters. Accordingly, a prohibition 

is included in this Order barring discharges from a MS4 to Santa Monica Bay that result in exceedance of 

these objectives. Since the TMDL and the WLAs contained therein are expressed as receiving water 

conditions, Receiving Water Limitations have been included in this Order that are consistent with and 

implement the zero exceedance day WLAs. 

[Provision 32] These Receiving Water Limitations apply at the compliance monitoring sites identified in the 

Santa Monica Bay Beaches Bacterial TMDLs Coordinated Shoreline Monitoring Plan dated April 7, 2004.1 

Compliance with the Receiving Water Limitations shall be determined using shoreline monitoring data 

obtained in conformance with the Santa Monica Bay Beaches Bacterial TMDLs Coordinated Shoreline 

Monitoring Plan dated April 7, 2004. 

In Practice: Allowing Sources to Establish Benchmark Values for Pollutants in the City of 

Gresham, Oregon 

The MS4 permit for the City of Gresham requires that the permittees develop benchmark values for applicable 

TMDLs. The permit language is as follows: 

Schedule D.2.(d) 

i) Progress toward reducing TMDL pollutant loads must be evaluated by the co-permittee through the use 

of performance measures and pollutant load reduction benchmarks developed and listed in the SWMP. 

(1) ...

(2) A benchmark is a total pollutant load reduction estimate for each parameter or surrogate, where 

applicable, for which a [Waste Load Allocation] WLA is established at the time of permit issuance. A 

benchmark is used to measure the overall effectiveness of the storm water management plan in making 

progress toward the wasteload allocation (this estimate will be related to the statistical variability of the 

underlying data and may be stated as a range), and is intended to be a tool for guiding adaptive 

management activities. A benchmark is not a numeric effluent limit; rather it is a goal that is subject to the 

maximum extent practicable standard. The co-permittee must provide the rationale for the proposed 

benchmark, which includes an explanation of the relationship between the benchmarks and the TMDL 

wasteload allocations. Any limiting factors related to the development of a benchmark, such as data 

availability and data quality, must also be included in this rationale. The Interim Evaluation Report City of 

Gresham & Co-permittees (May 1, 2006, 4.0 Benchmark Evaluation for Gresham) summarizes the 

benchmarking process developed by the City of Gresham for TMDLs within the Columbia Slough. 
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In Practice: Specifying Pollutant Concentrations and BMPs Necessary to Make Adequate 

Progress in Oregon 

The state of Oregon includes two options in the general permit for construction activities 

(www.deq.state.or.us/wq/stormwater/constappl.htm) that must be implemented to meet the goals of any 

turbidity or sediment TMDL—either meeting a benchmark concentration in the runoff established by the 

permitting authority or implementing a set of BMPs specific to the pollutant of concern. The exact permit 

language follows: 

2. Water Quality Requirements for TMDL and 303(d) Listed Waterbodies 

In addition to other applicable requirements of this permit, if sediment or turbid water from a permit 

registrant’s construction project has the potential to discharge into waterbodies that are listed for turbidity 

or sedimentation on the most recently EPA-approved Oregon 303(d) list or that have an established Total 

Maximum Daily Load (TMDL) for sedimentation or turbidity, the permit registrant must implement one of 

the two following sets of actions, in accordance with Schedule C. 

a. Option #1: Collect and analyze samples for turbidity in stormwater runoff from the construction site as 

required by Condition B.2. (p. 12) and compare the results to the benchmark value of 160 Nephelometric 

Turbidity Units (NTUs). The benchmark is used to determine if best management practices are effective; it 

is not an effluent limit. If any stormwater sample exceeds the benchmark, then the permit registrant must 

evaluate the best management practices (BMPs) and the adequacy of the ESCP and take corrective 

actions. If after such actions have been implemented and sample results still exceed the 160 NTU 

benchmark, the requirements of Option #2 below must be followed, and the permit registrant must submit 

an Action Plan to the department identifying the selected BMP(s) that will be implemented and the rationale 

for choosing the selected BMP(s). 

b. Option #2: In addition to the applicable BMPs required by Condition A.7., implement one or more of the 

following BMPs to control and treat sediment and turbidity: 

i. Compost berms, compost blankets, or compost socks; 

ii. Erosion control mats (rolled or blown); 

iii. Tackifiers used in combination with perimeter sediment control BMPs; 

iv. Established vegetated buffers sized at 50 feet plus 25 feet per 5 degrees of slope; 

v. Water treatment by electro-coagulation, chemical flocculation, or filtration; or 

vi. Other substantially equivalent sediment or turbidity BMP approved by the department. 

The selected BMP(s) must be specifically identified in the ESCP [erosion and sediment control plan] as 

addressing this condition of the permit, and the rationale for choosing the selected BMP(s) must also be 

provided. 

In this example, the permittee has two choices for demonstrating that the construction project is implementing 

the goals of the TMDL. The permittee would either need to apply Option A and monitor the effluent per the 

requirements of the permit to show that the benchmark turbidity value is being met at the construction project. 

Or the permittee would need to demonstrate through inspection reports, that the BMPs required in a basic 

SWPPP as well as the additional BMPs defined by Option B are implemented and operating as designed on 

the site. 
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5.2.4. Adjusting Implementation for Continuous Improvements (Key 

Question 6) 

Adaptive management focuses on learning from the information gathered through monitoring and 

assessment and making changes to implementation strategies on the basis of the information collected. 

Sources can continue to implement BMPs, as well as overall SWMPs and SWPPPs, if the information 

gathered through the process and summative analyses show the necessary progress toward implementing 

the WLA. If progress is not adequate, planners should develop recommendations and requirements that 

will allow sources to answer key question 6: 

5.2.4.1. Answer Key Question 6: Identifying Modifications to Implementation 

Strategy 

Because the adaptive management approach is not linear but iterative, TMDL and permit writers can 

develop recommendations and requirements for sources that focus on taking the information from key 

questions 4 and 5 to adjust the current implementation strategy. This could entail conducting an 

efficiency audit of existing BMPs to determine if they are properly installed, operated, and maintained, 

or recommend that sources identify the most effective BMPs after a set number of years of operation (so 

that these most effective BMPs are used in the future). In addition, modifications could involve 

revisiting key question 3 to determine if additional BMPs are necessary to make continuous 

improvements. Adaptive management actions might include retrofitting or adjusting previously installed 

BMPs that are not performing as expected or compiling lessons learned from implementation activities 

to help in future implementation planning. 

It is important to note that permit writers include requirements that can facilitate adaptive management 

activities in existing individual and general stormwater permits, such as recording keeping and reporting 

requirements associated with inspections and, where applicable, monitoring activities. For example, 

construction stormwater permits might include requirements to conduct weekly stormwater monitoring 

and visual BMP inspections; where information from these activities show BMPs are not performing 

adequately, stormwater sources must modify the SWPPP accordingly. MS4 stormwater permits contain 

requirements for developing annual SWMP reports that assess the performance of BMPs and include 

detailed information on planned modifications. While these permit requirements might not have an 

explicit link to WLAs, sources should already have experience in collecting and analyzing performance 

information and applying that information to improve the effectiveness of SWPPPs and SWMPs. 

5.3. Resources 

5.3.1. BMP Inventory 

1. USEPA (U.S. Environmental Protection Agency). 2007. Municipal Separate Storm Sewer System 

(MS4) Program Evaluation Guidance. U.S. Environmental Protection Agency, Office of Wastewater 

Management. EPA-833-R-07-003 (field test version) 

www.epa.gov/npdes/pubs/ms4guide_withappendixa.pdf and 

www.epa.gov/npdes/pubs/ms4guide_appendicesb-d.pdf 
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This document is available to assist in assessing the compliance and effectiveness of Phase I and 

Phase II MS4 stormwater programs, including compiling a comprehensive BMP inventory. 

Sources can also use this document to conduct self-audits of SWMPs. 

5.3.2. BMP Selection 

1. ASCE (American Society of Civil Engineers). 2001. Guide for Best Management Practice Selection 

in Urban Developed Areas. American Society of Civil Engineers, Reston, VA. 

www.asce.org/bookstore/book.cfm?book=4058 

This guide examines permanent structural techniques which can be used for retrofitting the 

stormwater management systems in existing developed areas. 

2. Washington Department of Ecology. 2005. Stormwater Management Manual for Western 

Washington. Washington Department of Ecology, Olympia, WA. 

www.ecy.wa.gov/programs/wq/stormwater/manual.html

This manual focuses on the applicability, technical design, construction, and maintenance of a 

range of stormwater management practices for use in western Washington to achieve water 

quantity and water quality control. 

3. Maryland Department of the Environment. 2000. Maryland Stormwater Design Manual. Maryland 

Department of the Environment, Baltimore, MD. 

www.mde.state.md.us/Programs/WaterPrograms/SedimentandStormwater/stormwater_design/index.

asp

This manual focuses on the applicability, technical design, construction, and maintenance of a 

range of stormwater management practices for use in Maryland to achieve water quantity and 

water quality control. 

4. New Jersey Department of Environmental Protection. 2004. New Jersey Stormwater Best 

Management Practices Manual. New Jersey Department of Environmental Protection, Trenton, NJ. 

www.njstormwater.org/bmp_manual2.htm

This manual focuses on the applicability, technical design, construction, and maintenance of a 

range of stormwater management practices for use in New Jersey to achieve water quantity and 

water quality control. 

5. University of Wisconsin – Extension. 2000. Wisconsin Stormwater Manual: Technical Design 

Guidelines for Stormwater Management Practices. University of Wisconsin Extension, Madison, 

WI. http://learningstore.uwex.edu/Wisconsin-Storm-Water-Manual-P603C123.aspx. 

This manual focuses on the applicability, technical design, construction, and maintenance of a 

range of stormwater management practices for use in Wisconsin to achieve water quantity and 

water quality control. 

6. Center for Watershed Protection. 2003-2008. Urban Subwatershed Restoration Manual Series. 

Center for Watershed Protection, Ellicott City, MD. www.cwp.org/Store/usrm.htm. 
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This 11 manual series covers the seven major practices used to restore urban watersheds: 

stormwater retrofits, stream repair, riparian management, discharge prevention, pollution source 

controls, watershed forestry and municipal operations. In addition, the series outlines new 

methods for desktop and field assessment and stakeholder management to develop effective 

small watershed restoration plans, and presents an integrated framework for urban watershed 

restoration. 

5.3.3. Volume Control 

1. Zomodi, K. 2007. Effectiveness of Time of Concentration Elongation on Peak Flow Reduction. 2nd 

National Low Impact Development Conference, March 12–14, 2007. 

www.bae.ncsu.edu/topic/lidconference07/A6/A6.4.Effectiveness%20of%20Time%20of%20Concent

ration%20Elongationon%20on%20Peak%20Flow%20Reduction.pdf

This presentation discusses the findings of a study intended to theoretically evaluate the relative 

impact of time of concentration elongation on peak flow reduction under typical conditions of 

LID use. 

2. Stein, Eric D. 2005. Effect of Increases in Peak Flows and Imperviousness on Stream Morphology of 

Ephemeral Streams in Southern California. Technical Report 450. Southern California Coastal 

Water Research Project, Westminster, CA. www.environmentalexpert.com/files/19961/articles/4562/4562.pdf. 

This technical report investigates the effects of urbanization on ephemeral or intermittent streams 

in Southern California. The study seeks to (from the document) establish a stream channel 

classification system for Southern California streams, assess stream channel response to 

watershed change, develop deterministic/predictive relationship between changes in IC and 

stream channel enlargement and provide a conceptual model of stream channel behavior that 

may be used as the basis for a future numeric model. Eight watersheds and eleven sites were 

selected for study. 

5.3.4. Green Infrastructure and Low Impact Development 

1. Horner, R., C. May, E. Livingston, D. Blaha, M. Scoggins, and J. Tims. (n.d.). Structural and NonStructural BMPs for Protecting Streams. Watershed Management Institute, Crawfordsville, FL. 

www.chesterfield.gov/CommunityDevelopment/Engineering/LIDGrant/Studies/HornerMay2001Pap

er.pdf 

This paper presents findings from a study of the effects of non-structural BMPs, including 

riparian buffers and retention of natural wetlands, on three stream ecosystems to help guide 

application of these practices in low-impact urban design. 

2. Kloss, C, and C. Calarusse. 2006. Rooftops to Rivers: Green Strategies for Controlling Stormwater 

and Combined Sewer Overflows. Natural Resources Defense Council, New York City, NY. 

www.nrdc.org/water/pollution/rooftops/contents.asp
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This policy guide provides decisionmakers with ideas on how to incorporate green strategies into 

urban landscapes to address the effects of wet weather on water quality. Includes nine case 

studies. 

3. USEPA (U.S. Environmental Protection Agency). 2008. Incorporating Green Infrastructure 

Concepts into Total Maximum Daily Loads. U.S. Environmental Protection Agency, Washington, 

DC. www.epa.gov/owow/tmdl/stormwater/

This 11 page fact sheet provides recommendations for incorporating GI and LID concepts into 

various elements of a TMDL and provides two TMDL case studies. 

5.3.5. BMP Performance Literature Values 

1. American Society of Civil Engineers’ (ASCE) and EPA’s International Stormwater BMP Database: 

www.bmpdatabase.org/

The database contains more than 300 BMP studies, performance analysis results, tools for use in 

BMP performance studies, monitoring guidance and other study-related publications. The overall 

purpose of the project is to provide scientifically sound information to improve the design, 

selection, and performance of BMPs. Continued population of the database and assessment of its 

data will ultimately lead to a better understanding of factors influencing BMP performance and 

help to promote improvements in BMP design, selection and implementation. 

2. FHWA (Federal Highway Administration). 2002. Stormwater Best Management Practices in an 

Ultra-Urban Setting: Selection and Monitoring. Federal Highway Administration, Landover, MD. 

www.fhwa.dot.gov/environment/ultraurb/

The purpose of this document is to provide a planning-level review of the applicability and use 

of new and more traditional BMPs in ultra-urban areas. The document presents data, design 

criteria, and monitoring study results on BMPs implemented in ultra-urban areas 

3. Pitt, R.E., A. Maestre, and R. Morquecho. 2004. The National Stormwater Quality Database (NSQD, 

version 1.1). University of Alabama, Tuscaloosa, AL. 

http://unix.eng.ua.edu/~rpitt/Research/ms4/mainms4.shtml and 

http://unix.eng.ua.edu/~rpitt/Research/ms4/Paper/recentpaper.htm

These stormwater quality data and site descriptions are to describe the characteristics of national 

stormwater quality, provide guidance for future sampling needs, and enhance local stormwater 

management activities in areas having limited data. The monitoring data was collected over 

nearly a 10-year period from more than 200 municipalities throughout the country. This project 

is creating a national database of stormwater monitoring data collected as part of the existing 

stormwater permit program, providing a scientific analysis of the data, and providing 

recommendations for improving the quality and management value of future NPDES monitoring 

efforts. 

4. University of Massachusetts Amherst’s Massachusetts Stormwater Technology Evaluation Project 

(MASTEP), Stormwater Technologies Clearinghouse: www.mastep.net/ 

This searchable database contains validated performance data and technical information on 

innovative stormwater treatment technologies. The Technology Acceptance and Reciprocity 
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Partnership (TARP) protocol is the basis for evaluating treatment efficiencies for various 

pollutants and the Web site is designed to help stakeholders interpret information such as site and 

environmental considerations as well as whether performance studies meet the minimum TARP 

requirements. It also serves as a repository for test reports and performance data from a variety 

of sources. 

5. Fraley-McNeal, F., T. Schueler, and R. Winer. 2008. National Pollutant Removal Performance 

Database Technical Brief (Version 3.0). Center for Watershed Protection, Ellicott City, MD. 

www.cwp.org/Resource_Library/Center_Docs/SW/bmpwriteup_092007_v3.pdf 

The updated database was statistically analyzed to derive the median and quartile removal values 

for each major group of stormwater BMPs. The data are presented as box and whisker plots for 

the various pollutants found in stormwater runoff. 

6. EPA’s Urban BMP Performance Tool: 

http://cfpub.epa.gov/npdes/stormwater/urbanbmp/bmpeffectiveness.cfm

Based largely on data from the International Stormwater BMP Database, this tool allows users to 

search BMP study summaries based on the pollutants measured, BMPs examined, or total 

volume of stormwater runoff reduced. Search results are displayed in a tabular format sorted by 

effluent concentration, and additional details on each study can be accessed by clicking on the 

study title. The Urban BMP Performance Tool also includes background information about BMP 

performance, including basic information on understanding how BMP affect concentration, 

volume and total load. 

7. University of New Hampshire Stormwater Center, Nonpoint Education for Municipal Officials 

(NEMO), Innovative Stormwater Management Inventory Database: 

www.erg.unh.edu/stormwater/index.asp 

This searchable and amendable database highlights innovative stormwater management 

techniques such as low impact design, used in New England. The database contains information 

on bioretention areas, green roofs, rain gardens, detention pond retrofits, tree filters, and 

constructed wetlands. 

5.3.6. Model Applicability 

1. USEPA. 2005. TMDL Model Evaluation and Research Needs. EPA/600/R-05/149. U.S. 

Environmental Protection Agency, Office of Research and Development, National Risk 

Management Research Laboratory, Cincinnati, OH. 

www.epa.gov/nrmrl/pubs/600r05149/600r05149.htm

This report documents the review of more than 60 available watershed and receiving water 

models for their applicability to TMDL development and implementation. It discusses model 

selection on the basis of model capabilities and provides a series of tables rating the capabilities 

or applicability the models using the categories of TMDL endpoints, general land and water 

features, special land processes, special water processes, and application considerations. The 

document also provides individual fact sheets for each reviewed model. 

2. USEPA. 1997. Compendium of Tools for Watershed Assessment and TMDL Development. EPA 841-
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B-97-006. U.S. Environmental Protection Agency, Office of Water, Office of Wetlands, Oceans, and 

Watersheds, Washington, DC. www.epa.gov/owow/tmdl/comptool.html

This document reviews more than 50 watershed, receiving water and ecological assessment 

models. The document provides factsheets for each model that describes model components, 

methods, applications, pollutants addressed, limitations, input data requirements, and type of 

output. The document also contains information on model selection for specific applications, 

model calibration, and model verification. 

3. USEPA (U.S. Environmental Protection Agency).2008. Handbook for Developing Watershed Plans 

to Restore and Protect Our Waters. U.S. Environmental Protection Agency, Washington, DC. 

www.epa.gov/nps/watershed_handbook/

Chapter 8 of this document focuses on methods for estimating pollutant loads, including the use 

of watershed models. This chapter provides assistance in selecting and applying watershed 

models to estimate pollutant loads from existing conditions. 

5.3.7. Stormwater Program Evaluation 

1. CASQA (California Stormwater Quality Association). 2007. Municipal Stormwater Program 

Effectiveness Assessment Guidance. California Stormwater Quality Association, Menlo Park, CA. 

www.casqa.org

This document provides stormwater managers with a variety of stormwater program assessment 

strategies and methods appropriate for different stormwater program elements and outcome 

levels. 

2. USEPA (U.S. Environmental Protection Agency). 2007. Municipal Separate Storm Sewer System 

(MS4) Program Evaluation Guidance. U.S. Environmental Protection Agency, Office of Wastewater 

Management. EPA-833-R-07-003 (field test version) 

www.epa.gov/npdes/pubs/ms4guide_withappendixa.pdf and 

www.epa.gov/npdes/pubs/ms4guide_appendicesb-d.pdf 

This document is available to assist in assessing the compliance and effectiveness of Phase I and 

 Phase II MS4 stormwater programs, including compiling a comprehensive BMP inventory. 

 Sources can also use this document to conduct self-audits of SWMPs. 

5.3.8. BMP Monitoring 

1. Washington Department of Ecology. 2008. Guidance for Evaluating Emerging Stormwater 

Treatment Technologies, Technology Assessment Protocol (revised). Publication # 02-10-037. 

Washington Department of Ecology, Olympia, WA. www.ecy.wa.gov/biblio/0210037.html

This document’s primary purpose is to establish a testing protocol and process for evaluating and 

reporting on the performance and appropriate uses of emerging stormwater treatment 

technologies. This document is also intended for use in evaluating public domain practices 

possibly resulting in changes to the design standards for these practices in the state’s Stormwater 

Management Manual. 
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2. University of Minnesota. 2008. Assessment of Stormwater Best Management Practices. University 

of Minnesota, St. Paul, MN. 

http://wrc.umn.edu/outreach/stormwater/bmpassessment/assessmentmanual/ 

This manual provides information on four levels of a BMP assessment program and provides 

assessment procedures for source reduction BMPs, filtration practices, infiltration practices, 

sedimentation practices, and biologically enhanced practices. 

3. TARP (Technology Acceptance and Reciprocity Partnership). 2003. TARP Protocol for Stormwater 

Best Management Practice Demonstrations. Final August 2001. Updated July 2003. 

www.dep.state.pa.us/dep/deputate/pollprev/techservices/tarp/pdffiles/Tier2protocol.pdf 

This protocol describes a set of uniform criteria on evaluating stormwater BMP performance that 

is acceptable to the endorsing states (California, Massachusetts, Maryland, New Jersey, 

Pennsylvania, Virginia). The protocol primarily deals with the demonstration of BMPs that are 

designed for one or more of the following: (1) directing and distributing flows; (2) reducing 

erosive velocities; and (3) removing contaminants such as suspended or dissolved pollutants 

from collected stormwater through physical and chemical processes such as settling, mediafiltering, ion-exchange, carbon adsorption, and precipitation. 

4. Stricker, E., and M. Quigley (URS Greiner Woodward Clyde), ASCE (American Society of Civil 

Engineers), and USEPA (U.S. Environmental Protection Agency). 1999. Determining Urban 

Stormwater Best Management Practice (BMP) Removal Efficiencies. American Society of Civil 

Engineers, Urban Water Resources Research Council, City, ST, and U.S. Environmental Protection 

Agency, Office of Water, Washington, DC. www.bmpdatabase.org/docs/task3_1.pdf 

This technical memorandum provides an overview of methods for evaluating the efficiency, 

performance, and effectiveness of BMPs through analysis of water quality, flow, and 

precipitation data for monitored storm events as well as BMP design attributes collected and 

stored in the National Stormwater Best Management Practices Database. 

5.3.9. Outfall Monitoring 

1. USEPA (U.S. Environmental Protection Agency). 1992. NPDES Stormwater Sampling Guidance 

Document. EPA 833-B-92-001. U.S. Environmental Protection Agency, Washington, DC. 

www.epa.gov/npdes/pubs/owm0093.pdf 

5.3.10. Receiving Water Monitoring 

1. Stormwater Monitoring Coalition’s Model Monitoring Technical Committee. 2004. Model 

Monitoring Program for Municipal Separate Storm Sewer Systems in Southern California. August 

2004. Technical Report #419. ftp://ftp.sccwrp.org/pub/download/PDFs/419_smc_mm.pdf 

This report describes a model monitoring program for receiving waters affected by urban runoff 

in both wet and dry weather. 
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2. USGS (U.S. Geological Survey). Variously dated. National Field Manual for the Collection of 

Water-Quality Data. U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, 

chaps. A1-A9. U.S. Geological Survey, Reston, VA. http://pubs.water.usgs.gov/twri9A 

This manual includes information and detailed descriptions of water quality sampling methods. 
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6. COORDINATING TMDLS AND STORMWATER PERMITS 

Improving the communication between TMDLs and permit writers can result in more effective 

implementation of TMDLs into stormwater permits. Considerations for developing a connection 

between the TMDL and the stormwater permit can include the following: 

• What type of permit requirements (e.g., water quality controls and effluent limitations, monitoring, 

reporting) can permit writers consider to facilitate implementation of stormwater WLAs? 

• How can permit writers, with information provided by TMDL writers, describe these elements? 

• What can each programmatic document (e.g., TMDL reports or stormwater permits) say in relation 

to the other to facilitiate TMDL implementation? 

As discussed in Chapter 1 of this Handbook, TMDLs include technical analyses that are not selfimplementing. For point sources, permit limits that are consistent with the WLA are enforceable 

through NPDES permits. It is the permit writer’s 

responsibility to interpret and incorporate 

information from the TMDL into permits, 

thereby turning them into enforceable 

requirements. Stormwater permit writers 

therefore can incorporate data and information 

provided in the TMDL directly into the permit, 

or can include information indirectly (e.g., 

reference sections of the TMDL. As a result, 

TMDLs and permitscan work in tandem to 

identify effective implementation methods and 

make progress toward restoring impaired waters. 

As shown in Figure 22, the step of TMDL 

implementation involves identifying and 

implementing management options to implement 

the LAs and WLAs. This chapter discusses ways 

to coordinate development of permit language 

with the information and data provided in the 

TMDL report to promote effective 

implementation of TMDLs through permits. 

6.1. Options for Implementing TMDLs in Permits 

Stormwater permit writers often consider TMDL-related information as they develop permit 

requirements such as applicability, implementing water quality controls through SWMPs and SWPPPs, 

developing a monitoring plan, reporting, and assessing progress. Under each of these permit elements, 

permit writers can consider a variety of options for expressing requirements related to TMDL 

implementation. The two primary types of stormwater permits—individual and general permits—can 

affect options and approaches for coordinating TMDL language with the permit. 

Figure 22. Illustration of the steps in the TMDL 

process, including activities related to TMDL 

implementation. 
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Individual permits provide the permit writer with an opportunity to develop specific permit requirements 

because the permit focuses on one source, or a primary source and its co-permittees. As a result, permit 

writers can feasibly work with the TMDL writer in developing a permit that directly reflect elements of 

the TMDL analyses and, where available, implementation plans. This could include incorporating 

information provided in the TMDL directly into the permit. General permits apply to a broad category 

of sources in a geographic area. In most cases, general permits are statewide, but in some cases, permit 

writers can develop and issue general permits on a watershed-scale. While general permits provide 

permit writers with a degree of permitting efficiency, this type of permit may not provide the same 

opportunities for tailoring requirements and adding a level of specificity. To implement TMDLs through 

statewide general stormwater permits, permit writers can use broad requirements that direct sources to 

identify applicable TMDLs and modify their SWMPs or SWPPPs accordingly. Implementing TMDLs 

through watershed-scale general permits can provide permit writers with more flexibility to incorporate 

specific elements, such as BMP performance standards, that broadly apply to a group of sources. Permit 

writers could also add specificity into general permits to promote TMDL implementation, such as 

developing technical appendices that include TMDL-specific elements applicable to stormwater sources. 

Categories of permits elements discussed in this section include determining applicability, water quality 

controls, and monitoring. These categories apply to both individual and general permits, although the 

approaches used in each category will vary depending on the type of permit. Each category contains a 

description of options based on real-world examples, and are available in the Appendix. 

6.1.1. Requirements Related to Determining Applicability 

Stormwater permits often require permittees to first determine if an existing TMDL is applicable to its 

discharge. The key questions for making this determination are: 

Regulatory Requirements and EPA Guidance on Establishing Stormwater Permit Requirements to 

Implement WLAs 

In the 2002 memorandum, Establishing TMDL WLAs for Stormwater Sources and NPDES Permit Requirements Based 

on Those WLAs, EPA provides regulatory requirements and guidance on how stormwater permit requirements should link 

to stormwater source WLAs under approved TMDLs. With regard to stormwater permit requirements, such as effluent 

limitations and monitoring, the 2002 memorandum states the following: 

• NPDES permit conditions must be consistent with the assumptions and requirements of available WLAs. See 40 CFR 

122.44(d)(1)(vii)(B). 

• WQBELs for NPDES-regulated storm water discharges that implement WLAs in TMDLs may be expressed in the form 

of BMPs under specified circumstances. See 33 U.S.C. 1342(p)(3)(B)(iii); 40 CFR 122.44(k)(2)&(3). If BMPs alone 

adequately implement the WLAs, then additional controls are not necessary. 

• EPA expects that most WQBELs for NPDES-regulated municipal and small construction storm water discharges will 

be in the form of BMPs, and that numeric limits will be used only in rare instances. 

• When a non-numeric WQBEL is imposed, the permit’s administrative record, including the fact sheet when one is 

required, needs to support that the BMPs are expected to be sufficient to implement the WLA in the TMDL. See 40 

CFR 124.8, 124.9, and 124.18. 

• The NPDES permit must also specify the monitoring necessary to determine compliance with effluent limitations. See 

40 CFR 122.44(i). Where effluent limits are specified as BMPs, the permit should also specify the monitoring 

necessary to assess if the expected load reductions attributed to BMP implementation are achieved (e.g., BMP 

performance data). 

• The permit should also provide a mechanism to make adjustments to the required BMPs as necessary to ensure their 

adequate performance. 
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1. Is there a discharge to an impaired waterbody with an approved TMDL? 

2. Does the discharge include the pollutant of concern addressed in the approved TMDL? 

One option for determining applicability requirements could be for a state that the source should make a 

determination on the basis of the knowledge of their location and discharges, and provide infomration on 

how to determine if a discharge goes to an impaired waterbody, such as a link to a Web site that 

provides lists of impaired waterbodies. Another option could be to provide more detailed information to 

sources to help them determine if they contribute to one or more impairments, where to go for more 

information on a specific TMDL, or to even include the applicable TMDL data and information in an 

appendix to the permit. 

Applicability can go beyond whether the source is discharging to an impaired waterbody to determine if 

the discharge contributes to the impairment(s). A source might have a discharge to an impaired 

waterbody, but if the discharge does not contribute to the impairment(s) (e.g., does not contain the 

pollutant of concern) the additional controls related to the impaired waterbody might not apply. If permit 

writers want to use pollutant-specific considerations as a factor in determining applicability, there are 

various options to consider. One option could be for permit writers to limit permit coverage for sources 

that discharge to impaired waterbodies with impairments caused by pollutants commonly associated 

with that source. For example, permit writers could state that industrial facilities are not eligible for 

coverage under an industrial general permit if the facility discharges to an impaired waterbody with an 

approved TMDL for metals. Another option for permit writers could be to require sources to conduct 

discharge monitoring for a pollutant of concern or flow monitoring (or both) over a specified period of 

time to determine applicability. If the source can demonstrate that the discharge does not contribute to 

the impairment(s), the permit could then exempt the source from some or all additional TMDL 

implementation requirements. 

In Practice: Requiring Sources to Determine Applicability in Tennessee 

Tennessee’s Phase II MS4 General Permit simply states that a permitted stormwater source must determine if 

any discharges go to an impaired waterbody. The permit states the following: 

Determine whether stormwater discharge from any part of the MS4 significantly contributes directly or 

indirectly to a 303(d) listed (i.e., impaired) waterbody. Water quality impaired waters means any segment of 

surface waters that has been identified by the division as failing to support classified uses.... If you have 

303(d) discharges described above, you must also determine whether a Total Maximum Daily Load 

(TMDL) has been developed by the division and approved by EPA for the listed waterbody. 

In Practice: Providing Applicable TMDL Requirements as a Permit Appendix in Washington

The state of Washington’s final Phase II MS4 general permits for regulated small MS4s in eastern and western 

portions of the state compile the information permitted stormwater sources need to determine TMDL 

requirement applicability. Each permit contains a separate appendix that lists all the applicable TMDLs, the 

specific requirements, and a list of the potential permittees to which each of the TMDLs applies. 
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In Practice: Using Geographic Location to Determine Applicability for Georgia’s Industrial 

General Permit 

Georgia’s Industrial General Permit contains applicability requirements that are based on a permitted 

stormwater source’s geographic location, relative to an impaired stream segment. Permit language follows: 

Any operator who intends to obtain coverage under this permit for storm water discharges associated with 

industrial activity into an Impaired Stream Segment, or within one (1) linear mile upstream of and within the 

same watershed as, any portion of an Impaired Stream Segment, identified as partially supporting or not 

supporting designated uses on Georgia’s most current 303(d) list, must satisfy the requirements of Part III.C of 

this permit if the pollutant(s) of concern for which the Impaired Stream Segment has been listed may be 

exposed to storm water as a result of current or previous industrial activity at the facility. Those discharges that 

are within one (1) linear mile of an Impaired Stream Segment, but are not within the watershed of any portion of 

that stream segment are excluded from this requirement. 

Georgia’s 303(d) list is at www.gaepd.org. 

In Practice: Identifying Pollutants of Concern in Permit Eligibility Requirements 

Minnesota’s CGP lays out what are the pollutants of concern for construction sites that discharge to impaired 

waterbodies. Construction site operators that want coverage under Minnesota’s CGP must implement the 

applicable BMPs listed in the permit and any other specific implementation measures specified in the approved 

TMDL to which they are subject. The draft Minnesota CGP states the following: 

Discharges to waters identified as impaired pursuant to section 303 (d) of the federal Clean Water Act (33 

U.S.C. § 303(d)) where the identified pollutant(s) or stressor(s) are phosphorus (nutrient eutrophication 

biological indicators), turbidity, dissolved oxygen, or biotic impairment (fish bioassessment, aquatic plant 

bioassessment and aquatic macroinvertebrate bioassessment), and with or without a U.S. Environmental 

Protection Agency (USEPA) approved Total Maximum Daily Load (TMDL) for any of these identified 

pollutant(s) or stressor(s)" must meet the applicable requirements of Part III.A.9, which include specific 

BMPs to limit these stressors. 

6.1.2. Requirements Related to Identifying and Implementing Water Quality 

Controls 

After EPA has approved a TMDL, permit writers must develop effluent limitations and permit 

conditions that are consistent with the assumptions and requirements of the approved WLA. In the 

context of stormwater permits, effluent limitations are most often expressed as BMPs, but permit writers 

can also use numeric limits (e.g., loads, concentrations, or performance standards). Effective TMDL 

implementation often relies on selecting the right mix of control measures to achieve progress toward 

addressing the WLA. Chapter 5 of this Handbook focuses on the technical resources and approaches for 

evaluating and selecting BMPs to implement the WLA. 

Permit writers, in conjunction with information provided by TMDL writers, can consider a variety of 

approaches that involve either recommending BMPs or relying on sources to do the evaluation and 

selection on their own. Approaches for permit writers to consider are described below. 

• Requiring implementation of specific BMPs in the permit. Under this option, permit writers 

could develop a proposed list of BMPs that a source could implement to reduce pollutant loadings to 
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implement the WLA. Permit writers could consider this approach when stormwater planners have 

conducted an internal analysis of possible BMPs. Given the resource-intensive nature of this option, 

this approach might be suitable for geographic areas that need certain types of BMPs (e.g., 

developing areas versus built-out areas of an MS4), or discreet sources (e.g., the only MS4 or 

construction site discharging to the impaired waterbody). 

• Providing a recommended menu of potential BMPs in the TMDL, implementation plan, or the 

permit for sources to evaluate and select. This option is similar to the option described above in 

that TMDL and permit writers could develop a recommended list of BMPs. The difference between 

these options, however, is that this BMP list serves more as a menu of potential BMPs. Under this 

option, permit writers could provide some technical information related to each BMP to help sources 

evaluate and select appropriate BMPs. Sources would need to conduct a thorough analysis to select 

the appropriate suite of BMPs from the list to achieve progress toward implementing the WLA. 

• Referencing BMP performance standards in the TMDL, implementation plan, or the permit.

Under this option, TMDL and permit writers could recommend or reference performance standards 

for specific pollutants and allow sources to determine which BMPs will best meet the performance 

standard. One example might be a construction site must achieve a specific percent reduction in 

TSS, giving the source flexibility in the types of BMPs used to meet the standard. The TMDL and 

permit writer could provide recommendations on how sources could demonstrate that the selected 

BMPs can meet the required performance standards (e.g., using a combination of modeling and 

monitoring). 

• Recommending the selection of BMPs and developing benchmark values or performance 

measures. This option has similarities to the option described above in that it focuses on the use of 

performance standards. However, under this option, permit writers could develop permit 

requirements that give sources the responsibility for developing the performance standards, often 

referred to in this context as benchmark values or performance measures. Permit requirements can 

focus on selecting BMPs to achieve progress toward implementing the WLA and developing 

performance measures that indicate the expected level of BMP performance. Beyond BMP 

performance, requirements can focus on developing quantifiable benchmarks that track the overall 

success of SWMPs and SWPPPs in reducing pollutant loads. If permit writers choose this option, it 

is important to note that sources might have concerns about compliance implications associated with 

benchmark values and performance measures. As a result, permit writers might want to consider 

developing permit language that specifies the intended use of performance measures and benchmark 

values—not as numeric effluent limits but as guideline values to facilitate adaptive management. 

• Requiring the review of existing BMPs and selecting additional BMPs to achieve progress 

toward addressing the WLA. Under this option, permit writers could require sources to conduct an 

analysis of existing BMPs to determine the need for additional pollutant load reductions through 

improved BMP implementation or additional BMPs. Sources receive little technical guidance 

through the requirements, allowing them flexibility in conducting the analysis and justifying the 

selection of specific BMPs. Permit writers could consider including in the requirements a list of 

supporting documentation (e.g., calculations, assumptions, studies) that would provide the rationale 

for the proposed strategy to achieve progress toward addressing the WLA. This option is particularly 

effective when the the TMDL writer develops a WLA that permit writers can use as the basis for 

developing a performance standard. This approach provides permittees with flexibility in finding the 

optimal combination of existing and new BMPs to implement the WLA. 
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• Consider numeric effluent limitations. Permit writers might determine that BMPs are not an 

appropriate way to express effluent limitations and might choose to develop numeric effluent 

limitations as a feasible and appropriate way to incorporate the TMDL provisions into the permit. 

There are no guidelines for determining which approach is most appropriate to use. It is likely that a 

variety of factors, including type of source, type of permit, and availability of resources, will influence 

which approach makes the most sense. 

In Practice: Options for Water Quality Control Requirements 

Existing stormwater permits use different types of requirements to ensure SWMPs and SWPPPs integrate 

effective BMPs for addressing stormwater source WLAs under approved TMDLs and, in some cases, impaired 

waterbodies without approved TMDLs. Provided below are examples of differing SWMP and SWPPP 

requirements for developing and implementing BMPs to addressWLAs under various types of stormwater 

permits. 

Broad Requirements for Permitted Stormwater Sources to Develop SWMPs and SWPPPs to Address 

Impairments and Stormwater Source WLAs 

Tennessee Phase II MS4 General Permit

The general permit developed for use by regulated small MS4s in Tennessee contains a broad set of 

requirements for developing and implementing SWMPs. The requirements focus on evaluating whether the 

implementation of existing stormwater control measures is meeting the TMDL provisions, or if additional control 

measures are necessary. Permittees must document all control measures being implemented or planned to be 

implemented and provide a schedule of implementation for all planned controls. To demonstrate that control 

measures will meet TMDL requirements and assumptions, permittees must also provide associated 

calculations, assessments, reports, and other evidence that provided the rationale for selecting specific control 

measures. 

Wisconsin Phase II MS4 General Permit

Under Wisconsin’s Phase II MS4 general permit, permittees that discharge to an impaired waterbody must take 

steps to develop and implement an SWMP to reduce—and potentially eliminate—the pollutant of concern 

contributing to a waterbody impairment. Because the permit is a general permit and the requirements could 

apply to a wide range of pollutants, the permit uses broadly defined requirements to ensure permittees 

discharging to impaired waterbodies develop and implement SWMPs that will reduce the pollutant of concern. 

Specifically, the general permit language requires permittees to, “include a written section in its storm water 

management program that discusses the management practices and control measures it will implement as part 

of its program to reduce, with the goal of eliminating, the discharge of pollutant(s) of concern that contribute to 

the impairment of the waterbody. This section of the permittee’s program shall specifically identify control 

measures and practices that will collectively be used to try to eliminate the MS4’s discharge of pollutant(s) of 

concern that contribute to the impairment of the waterbody and explain why these control measures and 

practices were chosen as opposed to other alternatives.” 

Watershed-specific BMP and Performance Standard Requirements 

Big Darby Creek Watershed (Ohio) Construction General Permit

To implement the Big Darby Creek Watershed TMDL, Ohio EPA developed and issued the Big Darby Creek 

Watershed CGP. The overall permit is intended to implement the pollutant load reduction targets established 

under the TMDL; therefore, the permit states that the “general permit requires control measures/BMPs for 

construction sites that reflect recommendations set forth in the U.S. EPA approved Big Darby Creek TMDL.” 

The water quality control measures specified under the SWPPP requirements include a combination of 

management practices, effluent targets, and infiltration requirements necessary to support stream base flows 

and stream setbacks necessary to protect the stream channel. The permit also states that “the erosion, 

sediment, and stormwater management practices used to satisfy the conditions of this permit, should meet the 

standards and specifications in the most current edition of Ohio’s Rainwater and Land Development manual or 

other standards acceptable to Ohio EPA unless otherwise specified as a condition of this permit.” 
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Specific BMP Requirements for SWMPs and SWPPPs to Address Impaired Waterbodies and Implement 

Stormwater Source WLAs 

Oregon Construction General Permit (1200-C)

If a permitted construction site has the potential to discharge sediment or turbid water to a waterbody with an 

approved TMDL for sedimentation or turbidity, or is listed on the state’s 303(d) for impairment due to 

sedimentation or turbidity, Oregon’s CGP requires the permittee to implement one of two sets of options. Under 

Option 2, the CGP requires permittees to implement one or more of six BMPs identified to control and treat 

sedimentation and turbidity, in addition to implementing the standard set of BMPs required of all erosion and 

sediment control plans (ESCP). The BMPs identified in the permit are as follows: 

• Compost berms, compost blankets, or compost socks; 

• Erosion control mats (rolled or blown); 

• Tackifiers used in combination with perimeter sediment control BMPs; 

• Established vegetated buffers sized at 50 feet plus 25 feet per 5 degrees of slope; 

• Water treatment by electro-coagulation, chemical flocculation, or filtration; or 

• Other substantially equivalent sediment or turbidity BMP approved by the department. 

Permittees selecting Option 2 must indicate in the ESCP which of the six supplemental BMPs have been 

selected to address discharges to waterbodies impaired for sedimentation and turbidity. 

Georgia Industrial Stormwater Permit (GAR000000)

Georgia’s industrial stormwater permit contains requirements related to discharges into, or within 1 linear mile 

upstream of and within the same watershed as, any portion of an Impaired Stream Segment impaired for fecal 

coliform, as well as substances other than fecal coliform. Under the requirements for facilities discharging fecal 

coliform, the permit references an appendix that contains a list of BMPs specifically for animal processing 

plants that might be potential sources of fecal coliform. Permittees that do not meet the TSS benchmark value 

(used as a surrogate indicator for fecal coliform under this permit) in the first 12-month sampling period have 

one year to select, design, and implement supplemental BMPs from the list of BMPs provided in the appendix, 

or other appropriate BMPs. 

Quantifiable Performance Targets to Determine the Need for Additional BMPs 

City of Portland, Multnomah County, and Port of Portland (Oregon) Phase I MS4 Individual Permit No. 101314

Portland and its co-permittees are subject to an individual Phase I MS4 permit that contains a variety of 

requirements for developing and implementing an effective SWMP to address approved WLAs for the permitted 

MS4s. Permit conditions do not include recommended or required BMPs; instead, the permit requires copermittees to select BMPs to implement the approved WLAs and develop quantifiable performance measures 

for assessing BMP effectiveness. Performance measures are pollutant load reduction estimates to facilitate an 

adaptive management approach to SWMP implementation—the quantifiable performance measures for BMPs 

are not numeric effluent limitations. 

Washington Construction General Permit

The Washington Department of Ecology issued a CGP that contains requirements for three categories of 

permitted construction sites: (1) less than 1 acre; (2) between 1 and 5 acres; and (3) greater than 5 acres. 

Under this permit, permittees that discharge turbidity, fine sediment, high pH, or phosphorus to impaired 

waterbodies listed for one of these parameters is required to conduct regular sampling. If discharge sampling 

indicates that the permittee’s discharge exceeds the water quality standard for turbidity, all future discharges 

are subject to a numeric effluent limitation equal to the water quality standard for turbidity. Exceedances of the 

numeric effluent limitation then triggers requirements for evaluating and modifying the SWPPP to include BMPs 

that will allow the permittee to meet the numeric effluent limitation for turbidity. 

Numeric Effluent Limitations 

Draft Ventura County Phase I MS4 Individual Permit No. CAS004002 

The draft version (dated 08/28/07) of the Ventura County Phase I MS4 Individual permit addresses 

requirements under multiple TMDLs for multiple watersheds for a variety of pollutants of concern, including 

nitrogen compounds, bacteria, sediments, and toxicity. Rather than express effluent limitations in the form of 

BMPs, this version of the Ventura County Phase I MS4 individual permit addresses the TMDL WLAs as 

numeric WQBELs for dry weather and wet weather. 
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6.1.3. Monitoring Requirements 

As discussed in Chapter 5, monitoring and assessment are essential to help sources to make progress 

toward implementing WLAs. Information generated through monitoring and assessment activities are 

also key to promoting adaptive management and continuous improvement in implementation activities. 

In addition, monitoring and assessment activities are key to quantifying pollutant load reductions 

achieved through SWMP and SWPPP implementation—first to establish a baseline of pollutant load 

reductions expected or achieved through BMPs implemented under existing SWMPs and SWPPPs and 

then to determine pollutant load reductions from any additional BMPs deemed necessary to address 

WLAs. Requirements related to quantifying pollutant load reductions focus on developing and 

implementing monitoring plans. 

Many SWMP and SWPPP requirements related to meeting stormwater source WLAs include some type 

of monitoring requirements to gauge BMP effectiveness, overall SWMP and SWPPP effectiveness, and 

to facilitate adaptive management activities. Options for monitoring requirements include BMP 

performance monitoring, stormwater discharge outfall monitoring, and ambient in-stream water quality 

monitoring. Per EPA’s 2002 memorandum, where stormwater permits contain effluent limitations 

expressed as BMPs, these permits should specify the monitoring necessary to determine if BMPs are 

achieving the expected pollutant load reductions. Therefore, stormwater permits that implement TMDLs 

often contain monitoring requirements that focus on BMP performance, such as benchmark monitoring 

to determine progress toward a pollutant reduction goal, and support adaptive management activities 

related to modifying SWMPs and SWPPPs to improve overall effectiveness. Where stormwater permits 

do contain numeric effluent limitations, monitoring requirements are likely to focus on determining 

compliance with applicable numeric effluent limitations. 

It is important to note that many existing permits do not specifically require monitoring; however, 

monitoring might be warranted for these permits to assess TMDL implementation. Chapter 5 of this 

Handbook discusses monitoring and assessment issues and provides resources that can aid in the 

development of monitoring and assessment requirements. 

In Practice: Options for Monitoring Requirements 

General Monitoring Plan Development 

Tennessee Phase II MS4 General Permit

Under the general permit, regulated small MS4s must describe a method for evaluating whether stormwater 

controls are adequate for meeting the requirements of the TMDL. 

Performance and Benchmark Monitoring 

City of Portland, Multnomah County, and Port of Portland (Oregon) Phase I MS4 Individual Permit No. 101314

To gauge the effectiveness of the SWMP in reducing TMDL pollutant loads to the MEP, the Portland and its copermittees must include a specific strategy for implementing monitoring in the SWMP. Under this permit, copermittees measure effectiveness according to the quantifiable performance measures and benchmarks that 

the permit requires them to establish to support SWMP adaptive management. According to the permit, 

performance measures are estimates of the effectiveness of BMPs expressed as pollutant load reduction 

estimates and benchmarks are total pollutant load reduction estimates for each parameter or surrogate, where 

applicable, for which a WLA is established at the time of permit issuance. Neither performance measures nor 

benchmarks are numeric effluent limits, but rather tools for guiding adaptive management activities. 

Oregon Construction General Permit (1200-C) 
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If a permitted construction site has the potential to discharge sediment or turbid water to a waterbody with an 

approved TMDL for sedimentation or turbidity, or is listed on the state’s 303(d) for impairment due to 

sedimentation or turbidity, Oregon’s CGP requires the permittee to implement one of two sets of options. Under 

Option 1, the permittee must conduct stormwater monitoring to determine if discharges meet or exceed a 

benchmark value for turbidity (i.e., 160 NTUs) that indicates the effectiveness of BMPs. If any samples exceed 

the benchmark value, the permittee must evaluate existing BMPs and take corrective action. If stormwater 

discharge samples continue to exceed the benchmark value, the permittee must implement Option 2 (see 

description of Option 2 under the In Practice discussion related to water quality controls above) and submit an 

Action Plan that identifies additional BMPs to be implemented and the rationale for selecting the identified 

BMPs. 

Washington Construction General Permit 

Permittees covered under Washington’s CGP must conduct turbidity sampling to determine compliance with 

the water quality standard for turbidity. The permit requires permittees to measure background turbidity in the 

303(d)-listed receiving water immediately upstream (upgradient) or outside the area of influence of the 

discharge. For monitoring discharge turbidity, the permit requires permittees to measure at the point of 

discharge into the 303(d) listed receiving waterbody, inside the area of influence of the discharge or at the point 

where the discharge leaves the construction site, rather than in the receiving waterbody. Although the permit 

states that monitoring is used to determine compliance with the turbidity water quality standard, it appears that 

it functions more as a performance benchmark. If discharge turbidity exceeds the water quality standard for 

turbidity, all future discharges must comply with a numeric effluent limitation (equal to the water quality 

standard for turbidity). Future discharges exceeding the numeric effluent limitation triggers SWPPP modification 

requirements. 

Compliance Monitoring 

Draft Ventura County Phase I MS4 Individual Permit No. CAS004002 

Compliance with the dry-weather and wet-weather numeric WQBELs under the draft version of the Ventura 

County Phase I MS4 Individual permit is determined through end-of-pipe monitoring at major outfalls. The draft 

version of the permit requires co-permittees to develop a wet-weather and dry-weather monitoring workplan for 

review and approval by the Regional Water Board. 

6.1.4. Compliance Considerations 

[This section still under internal review – do not quote or cite] 

When developing permit requirements, permit writers will likely face issues and questions related to 

determining compliance. One key issue relates to establishing an appropriate compliance schedule for 

sources to implement a WLA (e.g., within a 5-year permit term or beyond). Permit writers can consider 

using interim limits or a phased implementation approach. Interim limits are a way for permit writers to 

schedule incremental progress toward implementing the WLA over time. One option is for the TMDL to 

reference, or for the permit to specify, the interim limits by 

providing a schedule with the required interim numeric 

targets in a specific timeframe. Another option to consider 

relates to the benchmarking approach (discussed earlier in 

this chapter) in which a source determines how much 

progress is feasible, over a specified time frame, to 

implement the WLA. The source could then submit the 

proposed benchmarks and associated schedule to the 

permit writer for review and approval. 

Resources: The Appendix of this Handbook 

contains the actual stormwater permit 

language referenced in this section, as well as 

relevant requirements from other stormwater 

permits, for TMDL and permit writers to review 

and consider when discussing and developing 

requirements to implement TMDLs. 
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In Practice: Incorporating Interim TMDL Numeric Targets into Permits in San Diego, California 

The San Diego Phase I MS4 permit (Order No. R9-2007-0001/NPDES No. CAS0108758) contains 

requirements to implement the Chollas Creek Diazinon TMDL. The permit includes a table that presents the 

WLA, interim TMDL numeric target, and percent reduction required over the permit duration. Permit language 

reads as follows: 

a. The Copermittees in the Chollas Creek watershed shall implement BMPs capable of achieving the 

interim and final diazinon Waste Load Allocation (WLA) concentration in the storm water discharge in 

Chollas Creek listed in Table 5. 

b. The Copermittees in the Chollas Creek watershed shall not cause or contribute to the violation of the 

Interim TMDL Numeric Targets in Chollas Creek as listed in Table 5. If the Interim TMDL Numeric Target is 

violated in Chollas Creek in more than one sample in any three consecutive years, the Copermittees shall 

submit a report that either 1) documents compliance with the WLA through additional sampling of the urban 

runoff discharge or 2) demonstrates, using modeling or other technical or scientific basis, the effectiveness 

of additional BMPs that will be implemented to achieve the WLA. The report may be incorporated into the 

Watershed Urban Runoff Management Program Annual Report unless the Regional Board directs an 

earlier submittal. The report shall include an implementation schedule. 

6.2. Options for Connecting Programmatic Documents 

TMDL and permit writers can not only work together to consider the appropriate types of conditions that 

could be developed to implement stormwater source TMDLs, but also can collaborate to decide how the 

associated programmatic documents, such as the TMDL report and the stormwater permit, can help 

articulate this information. The goal is to help ensure that no matter what document a permitted 

stormwater source refers to—the TMDL or the stormwater permit—each document has a clear and 

consistent connection to the information contained in the other. This section discusses options for 

TMDL and permit writers to consider when deciding how to develop a connection between the TMDL 

report and stormwater permits, as well as other related programmatic documents including 

implementation plans. 

Within the TMDL report and stormwater permit, TMDL and permit writers have several possible 

options for developing a connection between the information contained in the document. In the TMDL 

report, the TMDL writer might develop a connection to the permit through the WLA or the 

implementation plan (if included as part of the TMDL report) or other components that could be 

included in the TMDL report or implementation plan (e.g. optional monitoring section). In stormwater 

permits, the connection to TMDLs can also vary. Permit requirements might appear as a comprehensive 

set of detailed special conditions related to stormwater discharges to impaired waterbodies with 

approved TMDLs (or listed waterbodies without an approved TMDL), or the requirements ccould 

appear throughout the permit under existing permit conditions related to SWMP and SWPPPs, 

monitoring, reporting, and other categories of requirements. 

TMDL and permit writers can work together to develop the most appropriate section of each 

programmatic document to place stormwater-source-TMDL-related information. From there, TMDL 

and permit writers can decide the best approach for developing the connection, such as referencing 

elements contained in one programmatic document or directly including and referenceing them in each. 

A brief discussion these approaches related to each type of programmatic document and its associated 

components is provided below. 
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6.2.1. Using the WLA to Connect to Permits 

Per EPA’s 2002 memorandum, regulations state that when a TMDL has been approved, NPDES permits 

should contain effluent limits and conditions consistent with the requirements and the assumptions of the 

WLA. One option is to describe and reference all relevant permit requirements for implementing the 

WLA within the the TMDL report. Under this approach, stormwater permits could state that sources 

should comply with the numeric WLA and the associated elements included in the TMDL document. 

This approach basically incorporates the WLA and associated implementation information into the 

permit by reference. It is important to note, that although this type of approach could facilitate 

connections between WLAs and general permits, it may result in permits that are more challenging to 

tailor with specific permit requirements. 

Another option is to have stormwater permits directly incorporate the TMDL WLA information so that 

each programmatic document contains the same element using identical language, as opposed to just 

referencing the other document as is suggested above. While this more specific approach could be easier 

to apply when using individual or watershed-based stormwater permits, permit writers could also use 

this type of approach with general permits in conjunction with a regularly updated TMDL appendix that 

contains specific information for permittees. 

In Practice: Referencing and Integrating TMDL Provisions in Permits in Washington 

The Western Washington Phase II MS4 General Permit contains broad language to convey the requirements 

related to MS4s addressed by an approved TMDL. The body of the permit requires MS4s to comply with the 

specific actions described in applicable TMDLs. To aid permittees, the Washington Department of Ecology 

compiled all relevant, existing implementation actions from applicable TMDLs into a comprehensive appendix 

to the permit. By referencing the appendix in the permit, permit writers were able to ensure that the compilation 

of TMDL-specific activities would become enforceable permit requirements. The general permit language 

states the following: 

The following requirements apply if an applicable Total Maximum Daily Load (TMDL) is approved for 

stormwater discharges from MS4s owned or operated by the Permittee. Applicable TMDLs are TMDLs 

which have been approved by EPA on or before the date permit coverage is granted. All Permittees shall 

be in compliance with the requirements of applicable TMDLs. 

A. For applicable TMDLs listed in Appendix 2, affected permittees shall comply with the specific 

requirements identified in Appendix 2. Each Permittee shall keep records of all actions required by this 

Permit that are relevant to applicable TMDLs within their jurisdiction. The status of the TMDL 

implementation shall be included as part of the annual report submitted to Ecology. 

Where monitoring is required in Appendix 2, the Permittee shall conduct the monitoring according to a 

Quality Assurance Project Plan (QAPP) approved by Ecology. 

B. For applicable TMDLs not listed in Appendix 2, compliance with this Permit shall constitute compliance 

with those TMDLs. 

C. For TMDLs that are approved by EPA after this Permit is issued, Ecology may establish TMDL related 

permit requirements through future permit modification if Ecology determines implementation of actions, 

monitoring or reporting necessary to demonstrate reasonable further progress toward achieving TMDL 

waste load allocations, and other targets, are not occurring and shall be implemented during the term of 

this Permit or when this Permit is reissued. Permittees are encouraged to participate in development of 

TMDLs within their jurisdiction and to begin implementation. 
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6.2.2. Using Implementation Plans to Connect to WLAs and Permits 

While implementation plans are not a required component of a TMDL, many states do develop and 

include some form of an implementation plan in their TMDL document either to satisfy state regulatory 

requirements or to facilitiate TMDL implementation. If TMDL and permit writers intend to engage in 

implementation planning activities and develop an implementation plan for stormwater source TMDLs, 

there are a few ways to consider developing a stronger connection between WLAs and permits through 

the use of implementation plans. One option for using implementation plans to connect stormwater 

source WLAs with stormwater permits is to have the implementation plan serve as the primary vehicle 

for referencing and conveying information relating to implementation of the stormwater source WLAs 

(e.g., supplemental BMPs, SWMP and SWPPP assessment and modification, monitoring plan 

development and implementation, adaptive management measures). The TMDL could include these 

elements by reference and the associated stormwater permits could then incorporate the implementation 

plan information by reference. 

In Practice: Using Implementation Plans to Connect WLAs and Stormwater Permits 

Implementation plans with recommendations to guide stormwater permit requirements: 

Swamp Creek Fecal Coliform Bacteria TMDL (Washington) Water Quality Improvement Report and 

Implementation Plan 

The Water Quality Improvement Report and Implementation Plan includes recommendations for each MS4 as 

well as “anticipated actions” or requirements that the permitting authority intends to include in associated 

NPDES permits (Appendix D). The recommendations are those measures and practices that are intended to 

reduce bacterial pollution to Swamp Creek. 

Big Darby Creek (Ohio) Watershed TMDLs 

The implementation portion of the TMDL develops a tiered approach to monitoring progress and validating the 

TMDL: 

1. Confirmation of completion of implementation plan activities 

2. Evaluation of attainment of chemical water quality criteria 

3. Evaluation of biological attainment. 

A TMDL revision will be triggered if any one of the validation steps is not being completed or if the water quality 

standards are not being attained after an appropriate time interval. Once the majority of or the major 

implementation plan items have been carried out or the water quality monitoring shows consistent and stable 

improvements then a watershed assessment would be completed to evaluate attainment of the use 

designations. If water quality monitoring does not show improvement or waterbodies are still not attaining water 

quality standards after the implementation plan has been carried out, a TMDL revision would be initiated. 

Implementation plans with specific BMPs using adaptive management and phased implementation: 

Chloride TMDL Report: Shingle Creek, Minnesota 

The implementation portion of the TMDL report calls for the use of adaptive management principles. According 

to the report, adaptive management is appropriate because it is difficult to predict the chloride reduction that will 

occur from implementing strategies with limited amount of data available to predict expected reductions. The 

report requires continued monitoring and course corrections based on monitoring results. 

 
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Another possible option is for the stormwater permit, not the 

implementation plan, to include all additional requirements 

developed to implement the stormwater source WLAs. Under 

this approach, the implementation plan serves as a mechanism 

to link the TMDL and the stormwater permit without providing 

a significant amount of detail in the TMDL. This approach 

could be ideal for situations in which TMDL writers do not 

have the time or resources to engage in detailed implementation 

planning. 

6.3. Other Information to Consider 

Permit writers, watershed organizations, or municipalities could encounter situations where previously 

developed TMDLs and implementation plans are more difficult to implement in permits, including the 

following situations. 

6.3.1. No separate WLA for NDPES Stormwater source 

Permit writers might be working with older TMDLs that were approved prior to EPA’s guidance, 

Establishing Total Maximum Daily Load (TMDL) Wasteload Allocations (WLAs) for Storm Water 

Sources and NPDES Permit Requirements Based on Those WLAs (Wayland, R.H., and J.A. Hanlon, 

2002), or that include allocations for stormwater discharges that were considered to be nopoint sources 

at the time the TMDL was approved, but that currently are subject to NPDES permitting. For example, 

an older TMDL could group both urban nonpoint source and point source runoff into one overall 

category under an aggregated load allocation, or the older TMDL could include runoff from MS4’s 

under the load allocation that are now covered under the Phase II NDPES requirments. Because permit 

effluent limits have to meet water quality standards under 122.44(d)(1)(vii)(A), the permit writer will 

need to account for the current regulated stormwater discharges identified in the TMDL regarldless of 

how they were labeled in the older TMDL document, and should explain how they are being accounted 

for in the permit. The permit writer might be able to get additional information from TMDL writers to 

help implement these wasteload allocations into permits. 

6.3.2. Impaired Waterbody with No Approved TMDL 

Another scenario that TMDL and permit writers might encounter is an impaired waterbody that does not 

yet have an approved TMDL. Clearly, it is important to ensure that stormwater discharges do not further 

cause or contribute to exceedances of water quality standards. However, without a specific WLA, 

TMDL and permit writers might have questions about the appropriate implementation activities to 

recommend or, in the case of a permit, require until a TMDL is developed and approved. To provide 

some level of control on pollutants of concern associated with the impairment, TMDL and permit 

writers could work together to identify interim early action BMPs or performance standards that sources 

could implement until an approved TMDL becomes available. In such cases, it might be valuable to 

identify monitoring programs to evaluate contributions from stormwater sources to the impairment for 

use in future TMDL development. 

Resources: The Appendix of this 

Handbook contains excerpts of TMDLs, 

implementation plans, and stormwater 

permit requirements from the real-world 

examples provided in this section, as well 

as other TMDLs and stormwater permits 

that illustrate how states connect 

permitted stormwater source requirements 

among programmatic documents.
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In Practice: Addressing Impaired Waterbodies with No Approved TMDL in Permits in 

Wisconsin 

The Wisconsin Phase II MS4 General Permit contains requirements that address MS4 discharges to impaired 

waterbodies with no approved TMDL. The permit requires permittees to address the pollutant of concern 

contributing to the impairment in the SWMP and limits permittees’ ability to have a new or increased discharge 

of a pollutant of concern unless there is an approved TMDL. The general permit language states the following: 

1.5.2 If the permittee’s MS4 discharges to an impaired waterbody, the permittee shall include a written 

section in its storm water management program that discusses the management practices and control 

measures it will implement as part of its program to reduce, with the goal of eliminating, the discharge of 

pollutant(s) of concern that contribute to the impairment of the water body. This section of the permittee’s 

program shall specifically identify control measures and practices that will collectively be used to try to 

eliminate the MS4’s discharge of pollutant(s) of concern that contribute to the impairment of the waterbody 

and explain why these control measures and practices were chosen as opposed to other alternatives. 

Pollutant(s) of concern means a pollutant that is causing impairment of a waterbody. 

1.5.3 After the permittee’s start date of coverage under this permit, the permittee may not establish a new 

MS4 discharge of a pollutant of concern to an impaired waterbody or increase the discharge of a pollutant 

of concern to an impaired waterbody unless the new or increased discharge causes the receiving water to 

meet applicable water quality standards, or the Department has approved a total maximum daily load 

(TMDL) for the impaired waterbody. 
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APPENDIX: TMDL AND NPDES STORMWATER PERMIT LANGUAGE 

EXCERPTS

California 

Los Angeles County MS4 NPDES Permit 

10. On May 18, 2000, the USEPA established numeric criteria for priority toxic pollutants for 

the State of California (California Toxics Rule (CTR)) 65 Fed. Reg. 31682 (40 CFR 131.38), for 

the protection of human health and aquatic life. These apply as ambient water quality criteria for 

inland surface waters, enclosed bays, and estuaries. The State Board adopted the Policy for 

Implementation of Toxics Standards for Inland Surface Waters, Enclosed Bays, and Estuaries of 

California (SIP) – 2000, on March 2, 2000, for implementation of the CTR (State Board 

Resolution No. 2000-15 as amended by Board Resolution No. 2000-030). This policy requires 

that discharges comply with TMDL-derived load allocations as soon as possible but no later 

than 20 years from the effective date of the policy.

28. The Regional Board adopted the Santa Monica Bay Beaches Dry Weather TMDL for 

Bacteria (hereinafter “Dry Weather Bacteria TMDL”) on January 24, 2002. The TMDL was 

subsequently approved by the State Board, the Office of Administrative Law (OAL), and the 

USEPA and became effective on July 15, 2003. 

29. The Waste Load Allocations (WLAs) in the Dry Weather Bacteria TMDL are expressed as 

the number of allowable days that the Santa Monica Bay beaches may exceed the Basin Plan 

water quality objectives for protection of Water Contact Recreation (REC-1) in marine waters, 

specifically the water quality objectives for bacteria. Appropriate modifications to this order are 

therefore included in Parts 1 (Discharge Prohibitions) and 2 (Receiving Water Limitations), 

pursuant to 40 CFR 122.41(f) and 122.62, and Part 6.I.1 of this Order. Additionally, 40 CFR 

122.44(d)(1)(vii)(B) requires that NPDES permits be consistent with the assumptions and 

requirements of any available waste load allocation. Tables 7-4.1, 7-4.2a, and 7-4.3 of the Basin 

Plan set forth the pertinent provisions of the Dry Weather Bacteria TMDL. They require that 

during Summer Dry Weather there shall be no exceedances in the Wave Wash of the single 

sample or the geometric mean bacteria objectives set to protect the Water Contact Recreation 

(REC-1) beneficial use in marine waters. Accordingly, a prohibition is included in this Order 

barring discharges from a MS4 to Santa Monica Bay that result in exceedance of these 

objectives. Since the TMDL and the WLAs contained therein are expressed as receiving water 

conditions, Receiving Water Limitations have been included in this Order that are consistent 

with and implement the zero exceedance day WLAs. 

30. Pursuant to federal regulations at 40 CFR 124.8, and 125.56, a Fact Sheet was prepared to 

provide the basis for incorporating the Dry Weather Bacteria TMDL into this Order. The Fact 

Sheet is hereby incorporated by reference into these findings. 

31. The iterative approach to regulating municipal stormwater is not an appropriate means of 

implementing the Santa Monica Bay beaches Summer Dry Weather WLAs for any and all of the 

following reasons: (a) The WLAs do not regulate the discharge of stormwater; (b) The harm to 
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148 DRAFT November 2008 

the public from violating the WLAs is dramatic both in terms of health impacts to exposed 

beachgoers, and the economic cost to the region associated with related illnesses; (c) Despite 

the fact that more than a decade and a half has passed since MS4 permittees were required to 

eliminate illicit connections/discharges (IC/ID) into their MS4s, their programs have not 

eliminated standards violations at the beaches; and (d) Few permittees have ever documented 

revisions to their SQMP to address chronic exceedances of water quality standards. 

Georgia 

Phase II MS4 General Permit 

D. Stormwater Management Modifications 

1. The SWMP may be modified by the permittee at any time. Written notification of substantial 

SWMP modifications must be submitted 30 days prior to implementation of the SWMP modification. 

2. EPD may require the permittee to modify the SWMP as needed to: 

a. Include more stringent requirements as necessary to comply with new State or Federal statutory 

or regulatory requirements; 

b. Include other conditions deemed necessary by the Director to comply with the goals and 

requirements of the CWA and the State Act. The Director’s request for modifications shall be made 

in writing and set forth a time schedule for the permittee to develop the modification(s), and offer the 

permittee the opportunity to propose alternative SWMP modifications to meet the objective of the 

requested modification. 

General Permit for Stormwater Discharges Associated with Industrial Activity 

C. Discharges Into, Or Within One Mile Upstream Of And Within The Same Watershed As, Any 

Portion Of An Impaired Stream Segment. 

An operator is not eligible for coverage under this permit for discharges of stormwater 

associated with industrial activity to waters of the State for which a Total Maximum Daily Load 

(TMDL) is approved prior to or during the term of this permit, unless the facility develops, 

implements, and maintains a SWP3 that is consistent with the TMDL. The SWP3 must 

specifically address any conditions or requirements included in the TMDL that are applicable to 

the operator’s discharge within the timeframe specified in the TMDL. If the TMDL establishes a 

specific numeric wasteload allocation that applies to an operator’s discharge, or to stormwater 

discharges associated with industrial activity in general, then the operator must incorporate that 

allocation into the facility’s SWP3 and implement all necessary measures to meet that allocation. 

Any operator who intends to obtain coverage under this permit for stormwater discharges 

associated with industrial activity into an Impaired Stream Segment, or within one (1) linear 

mile upstream of and within the same watershed as, any portion of an Impaired Stream Segment, 

identified as “partially supporting” or “not supporting” designated uses on Georgia’s most 

current 303(d) list, must satisfy the requirements of Part III.C of this permit if the pollutant(s) of 

concern for which the Impaired Stream Segment has been listed may be exposed to stormwater 
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as a result of current or previous industrial activity at the facility. Those discharges that are 

within one (1) linear mile of an Impaired Stream Segment, but are not located within the 

watershed of any portion of that stream segment are excluded from this requirement. Georgia’s 

303(d) list can be viewed on EPD’s website at www.gaepd.org. 

1. Discharges into, or within one (1) linear mile upstream of and within the same watershed as, 

any portion of an Impaired Stream Segment Impaired by substances other than fecal coliform. 

a. Sampling schedule. 

Regulated industrial facilities that are subject to the requirements in Part III.C.1. of this permit 

must conduct stormwater discharge sampling for the pollutant(s) of concern two times per 

quarter for a period of twelve (12) months. The pollutant(s) of concern for each impaired stream 

segment are identified on Georgia’s 303(d) list. The sampling will only be required for those 

outfalls at the facility that have the potential to discharge the pollutant(s) of concern. The 

sampling must be conducted in accordance with Parts VI.A.3, 4, and 5 of this permit, except that 

composite samples may be collected in lieu of grab samples at the permittee’s discretion. The 

Director may require composite or grab sampling where deemed appropriate in order to ensure 

that representative samples are collected. 

Except as provided below, the sampling must begin no later than ninety (90) days after the later 

of the effective date of the permit or the date the facility becomes subject to the sampling 

requirements in Part III.C. However, if a facility with an existing stormwater discharge 

associated with industrial activity determines that additional time is needed to design and 

implement new or improved BMPs specifically for the pollutant(s) of concern, then that facility 

may delay commencement of the sampling program under this section of the permit for no more 

than twelve (12) months after the effective date of the permit in order to design and implement 

those BMPs. Facilities choosing this option must, no later than the date on which the Part III.C 

sampling would otherwise begin, provide a written notification, signed in accordance with Part 

VII.G of this permit, to EPD that they have elected to delay sampling and provide a schedule for 

BMP implementation. The Part III.C sampling program must begin immediately after the BMPs 

are required to have been implemented according to the schedule provided to EPD. 

A summary of the sampling results must be submitted to EPD’s Watershed Protection Branch 

with the Annual Report (see Appendix B of this permit). The report must also identify the 

applicable benchmark value(s) and state whether the facility has passed or failed the benchmark 

requirement for the twelve (12) month sampling period. 

If a facility is unable to conduct one or both of the Part III.C sampling event(s) during a certain 

quarter due to adverse climatic conditions (i.e. no qualifying rainfall event occurs), then the 

facility shall include a written explanation for the absence of the sampling event in the next 

Annual Report submitted to EPD. 

b. Applicable Benchmark Values. 

The applicable benchmark values for discharges into, or within one (1) linear mile upstream of 

and within the same watershed as, any portion of an Impaired Stream Segment shall be the same 

numeric value as the Instream Water Quality Criterion for the pollutant(s) of concern as 

specified in Georgia’s Rules and Regulations for Water Quality Control (Georgia Rule 39136. 
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03) unless otherwise established in Part III.C of this permit. The benchmark values are designed 

to assist permittees in determining if the BMPs established in a facility’s SWP3 are effective in 

minimizing the concentration of the pollutant(s) of concern in stormwater discharge(s) from their 

facility. These benchmark values are intended to be guideline concentrations rather than 

numeric effluent limitations or permit conditions. The exceedance of a benchmark value 

established in Part III.C of this permit is not a permit violation and does not of itself indicate a 

violation of instream water quality standards. However, an exceedance of a benchmark value 

may be used in conjunction with other information to demonstrate a violation of this permit or a 

violation of water quality standards. 

(1). Specific requirements for discharges into, or within one (1) linear mile upstream of and 

within the same watershed as, any portion of an Impaired Stream 

Segment impaired for DO (Dissolved Oxygen). 

Facilities discharging into, or within one (1) linear mile upstream of and within the same 

watershed as, any portion of an Impaired Stream Segment for which the listing criterion is 

identified as DO (Dissolved Oxygen) will only be required to conduct sampling under Part III.C 

if industrial materials that may contribute Five Day Carbonaceous Biochemical Oxygen 

Demand (CBOD5) or ammonia (NH3) may be exposed to stormwater as a result of current or 

previous industrial activity at the facility. These facilities must sample for Five Day 

Carbonaceous Biochemical Oxygen Demand (CBOD5) and NH3. The applicable benchmark 

value for these discharges shall be an Ultimate Oxygen Demand (UOD) of 125 mg/l. The UOD 

shall be calculated as [(CBOD5 x 1.5) + (NH3 x 4.57)]. 

(2). Specific requirements for discharges into, or within one (1) linear mile upstream of and 

within the same watershed as, any portion of an Impaired Stream 

Segment impaired by nonpollutant specific criteria. 

(i). Facilities discharging into, or within one (1) linear mile upstream of and within the same 

watershed as, any portion of an Impaired Stream Segment for which the listing criterion is 

identified as “Biota or Sediment” are required to conduct sampling for Total Suspended Solids 

(TSS) unless a TMDL has identified a different pollutant from nonpoint sources as causing the 

impairment, in which case sampling should be conducted for the pollutant(s) identified in the 

TMDL. The applicable TSS benchmark value for these discharges shall be 100 mg/l. 

(ii). Facilities discharging into, or within one (1) linear mile upstream of and within the same 

watershed as, any portion of an Impaired Stream Segment for which the listing criterion is 

toxicity, FCG (fish consumption guidelines), SB (shellfishing ban), CFB (commercial fishing 

ban) or TWR (trophic weighted residue value of mercury in fish tissue)” will only be required to 

conduct sampling under Part III.C if a TMDL identifying a specific water quality parameter has 

been approved for the stream segment. 

c. Evaluation of Part III.C sampling data 

The Part III.C stormwater discharge sampling is intended to measure the effectiveness of the 

Best Management Practices (BMPs) implemented at those facilities. If benchmark values are 

exceeded using the pass/fail determination provided below, then improved or additional BMPs 

are required at the facility. 
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November 2008 DRAFT 151 

The sampling data for the twelve (12) month period must be evaluated using one of the following 

criteria. This shall constitute the pass/fail determination for evaluating BMP effectiveness: 

(1). At least seventy-five (75) percent of the samples collected during the twelve (12) month 

period do not exceed the applicable benchmark value(s); or 

(2). The average of the samples collected during the twelve (12) month period does not 

exceed the applicable benchmark value(s). 

If a facility meets at least one of the above criteria then that facility has passed the benchmark 

requirement and may discontinue the Part III.C sampling but must thereafter properly maintain 

all of the BMPs that enabled the facility to meet the benchmark requirement. 

If a facility does not meet at least one of the above criteria, then that facility has failed the 

benchmark requirement. Those facilities that do not pass the benchmark requirement for the first 

twelve (12) month sampling period may take up to one year to budget, select, design and 

construct/implement additional supplemental BMPs at the facility. Once the supplemental BMPs 

have been implemented, an additional twelve (12) month (two samples per quarter) period of 

sampling must be conducted as described in Part III.C.1.a above. Those facilities that pass the 

benchmark requirement, using the above pass/fail determination, after implementing 

supplemental BMPs may discontinue the Part III.C sampling but must thereafter properly 

maintain all of the BMPs that enabled the facility to meet the benchmark requirement. 

Facilities that are not able to pass the benchmark requirement, using the above pass/fail 

determination, after implementing supplemental BMPs must continue the process of 

implementing additional supplemental BMPs at the facility and conducting a subsequent twelve 

month (two samples per quarter) period of sampling until the facility meets the benchmark 

requirement using the pass/fail determination provided above. If a facility is unable to pass the 

benchmark requirement after the twelve (12) month sampling period following a second round of 

implementing supplemental BMPs, then EPD will determine what further action is required, 

which may include, but is not limited to, applying for an individual NPDES permit. 

d. Written justification to cease Part III.C sampling. 

If a facility provides a written justification after the first twelve (12) month period of sampling 

(or after any subsequent twelve (12) month period of sampling) and EPD concurs that the 

facility’s stormwater discharges associated with industrial activity do not have a reasonable 

potential to cause or contribute to a violation of an instream water quality standard, then EPD 

may conclude that additional sampling under Part III.C is not required. Facilities that have 

passed the benchmark requirement are not required to submit a written justification in order to 

cease Part III.C sampling. 

2. Discharges into, or within one (1) linear mile upstream of and within the same watershed as, 

any portion of an Impaired Stream Segment impaired for fecal coliform. 

Facilities discharging into, or within one (1) linear mile upstream of and within the same 

watershed as, any portion of an Impaired Stream Segment for which the listing criterion is 

identified as fecal coliform must adhere to the following conditions if industrial materials or 
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152 DRAFT November 2008 

activities that are potential sources of fecal coliform (as defined in Part IV.D.9 of this permit) 

are, or may be, exposed to stormwater at the facility during the term of this permit. 

a. List of BMPs for animal processing plants that may be potential sources of fecal coliform. 

A list of BMPs designed to reduce fecal coliform levels in stormwater runoff has been developed 

for animal processing plants that may be potential sources of fecal coliform. Other facilities may 

find this list to be useful as well. The list is provided in Appendix C of this permit. 

b. Sampling schedule. 

Regulated industrial facilities that are subject to the requirements in Part III.C.2 of this permit 

must conduct stormwater discharge sampling for TSS two times per quarter for a period of 

twelve (12) months. Two of the sampling events must include simultaneous testing of TSS and 

fecal coliform. The sampling will only be required for those outfalls at the facility that have the 

potential to discharge stormwater associated with industrial activity where industrial materials 

or activities that are potential sources of fecal coliform (as defined in Part IV.D.9 of this permit) 

are, or may be, exposed to stormwater at the facility during the term of this permit. The sampling 

must be conducted in accordance with Parts VI.A.3, 4, and 5 of this permit. 

Except as provided below, the sampling must begin no later than ninety (90) days after the later 

of the effective date of the permit or the date the facility becomes subject to the sampling 

requirements in Part III.C. However, if a facility with an existing stormwater discharge 

associated with industrial activity determines that additional time is needed to design and 

implement new or improved BMPs specifically for the pollutant(s) of concern, then that facility 

may delay commencement of the sampling program under this section of the permit for no more 

than twelve (12) months after the effective date of the permit in order to design and implement 

those BMPs. Facilities choosing this option must, no later than the date on which the Part III.C 

sampling would otherwise begin, provide a written notification, signed in accordance with Part 

VII.G of this permit, to EPD that they have elected to delay sampling and provide a schedule for 

BMP implementation. The Part III.C sampling program must begin immediately after the BMPs 

are required to have been implemented according to the schedule provided to EPD. 

A summary of the sampling results for TSS and fecal coliform must be submitted to EPD’s 

Watershed Protection Branch with the Annual Report (see Appendix B of this permit). The report 

must also identify the applicable benchmark value(s) and state whether the facility has passed or 

failed the benchmark requirement for the twelve (12) month sampling period. 

If a facility is unable to conduct one or both of the Part III.C sampling event(s) during a certain 

quarter due to adverse climatic conditions (i.e. no qualifying rainfall event occurs), then the 

facility shall include a written explanation for the absence of the sampling event in the next 

Annual Report submitted to EPD. 

c. Applicable Benchmark Value 

A Total Suspended Solids (TSS) benchmark value of 100 mg/l will be used as a surrogate for 

evaluating fecal coliform levels in stormwater discharges associated with industrial activity. 

Fecal coliform sampling data collected simultaneously with TSS sampling data (as required in 
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November 2008 DRAFT 153 

Part III.C.2.b) is not subject to the pass/fail determination for benchmark sampling as 

established in Part III.C.2.d below. 

The TSS benchmark value is designed to assist permittees in determining if the implementation of 

the BMPs (as established in a facility’s SWP3) is minimizing the concentration of the pollutant(s) 

of concern in stormwater discharge(s) from their facility. These benchmark values are intended 

to be guideline concentrations rather than numeric effluent limitations or permit conditions. The 

exceedance of a benchmark value established in Part III.C of this permit is not a permit violation 

and does not of itself indicate a violation of instream water quality standards. However, an 

exceedance of a benchmark value may be used in conjunction with other information to 

demonstrate a violation of this permit or a violation of water quality standards. 

d. Evaluation of Part III.C sampling data. 

The Part III.C stormwater discharge sampling is intended to measure the effectiveness of the 

Best Management Practices (BMPs) implemented at those facilities. If benchmark values are 

exceeded using the pass/fail determination provided below, then improved or additional BMPs 

are required at the facility. 

The TSS sampling data for the twelve (12) month period must be evaluated using one of the 

following criteria. This shall constitute the pass/fail determination for evaluating BMP 

effectiveness. 

(1). At least seventy-five (75) percent of the samples collected during the twelve (12) month 

period do not exceed the TSS benchmark value; or 

(2). The average of the samples collected during the twelve (12) month period does not 

exceed the TSS benchmark value. 

If a facility meets at least one of the above criteria then that facility has passed the TSS 

benchmark requirement and may discontinue the Part III.C sampling but must thereafter 

properly maintain all of the BMPs that enabled the facility to pass the TSS benchmark 

requirement. 

If a facility does not meet at least one of the above criteria, then that facility has failed the TSS 

benchmark requirement. If a facility does not pass the TSS benchmark requirement for the first 

twelve (12) month sampling period then the facility may take up to one year to budget, select, 

design and construct/implement additional supplemental BMPs from the list provided in 

Appendix C, or other appropriate BMPs. Once the supplemental BMPs have been implemented 

at the facility, an additional twelve (12) month (two samples per quarter) period of sampling 

must be conducted as described in Part III.C.2.b above. Those facilities that pass the benchmark 

requirement, using the above pass/fail determination, after implementing supplemental BMPs 

may discontinue the Part III.C sampling but must thereafter properly maintain all of the BMPs 

that enabled the facility to pass the TSS benchmark requirement. 

Facilities that are not able to pass the TSS benchmark requirement after implementing 

supplemental BMPs must continue the process of implementing additional supplemental BMPs 

from the Appendix C list, or other appropriate BMPs, (within twelve (12) months after the end of 

the previous twelve (12) month sampling period) and conducting a subsequent twelve month (two 
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154 DRAFT November 2008 

samples per quarter) period of sampling until the facility passes the benchmark requirement 

using the pass/fail criteria provided above. 

e. Written justification to cease Part III.C monitoring. 

If a facility provides a written justification, after the first twelve (12) month period of sampling 

(or after any subsequent twelve (12) month period of sampling), and EPD concurs that the 

facility’s stormwater discharges associated with industrial activity do not have a reasonable 

potential to cause or contribute to a violation of an instream water quality standard, then EPD 

may conclude that additional sampling under Part III.C is not required. Facilities that have 

passed the benchmark requirement are not required to submit a written justification in order to 

cease Part III.C sampling. 

f. Demonstration of appropriate BMPs. 

If a facility with a stormwater discharge associated with industrial activity that may be a 

potential source of fecal coliform has implemented all technologically and economically feasible 

BMPs in the Appendix C list (for animal processing facilities), or other appropriate BMPs (for 

other facilities), and is still unable to pass the TSS benchmark requirement, the owner or 

operator of that facility may submit a demonstration to EPD that the facility has properly 

designed, installed and maintained all of the BMPs that are technologically and economically 

feasible for the facility and still cannot meet the benchmark. If, after reviewing the demonstration 

and conducting a site inspection, EPD concurs with the facility’s determination, then the facility 

will not be required to implement additional supplemental BMPs in order to comply with the 

permit. However, if new BMPs become technologically and economically feasible for the facility 

at a later date, then EPD may require the implementation of such BMPs at that time. EPD may 

also require an individual NPDES permit for a facility if that facility does not properly design, 

install and maintain technologically and economically feasible BMPs in a timely manner. 

New Mexico 

Phase I MS4 Permit for the City of Albuquerque 

B. Area-specific Stormwater Management Program Requirements. Permittees are required to develop 

and implement measures necessary to bring the discharge into compliance with the Middle Rio Grande 

Total Maximum Daily Load (TMDL) for Fecal Coliform. Specific permit requirements to implement the 

TMDL are included in Part III, Table III.B. [Note: Table III.B. includes the implementation activities 

required, the co-permittees responsible and the schedule of compliance. The implementation activities in 

the table have been incorporated as text as follows:]

1.0 Source Categories. Develop and submit a list of potential categories of fecal coliform sources by 

watershed and watershed density (undeveloped, low, moderate, high), covering the entire permit area. 

1.1 Legal Authority Evaluate adequacy of existing legal authority to implement the conditions 

included in this T able. Where existing ordinances are lacking, provide a schedule for obtaining 

the necessary legal authority. Ordinances shall be in place prior to the implementation of the 

programs. 
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2.0 Dry Weather Investigation. Develop and submit a dry weather field investigation program, by 

watershed, to identify and isolate fecal coliform sources that occur during dry weather so that they can 

be reduced/eliminated. The program shall address the sources identified in item 1.0 above. The program 

shall address the suitability of each of the following measures and shall include detailed description of 

activities and frequencies. 

2.1 Low Density Watersheds: 

2.1.1 Conduct dry weather channel survey 

2.1.2 Conduct survey of septic systems (e.g. aerial, ground, etc.) 

2.1.3 Conduct visual or tracer tests on suspected failing systems 

2.1.4 Investigate recreational and seasonal sewage dischargers 

2.1.5 Conduct ARA and study to determine whether fecal coliform s are of human or 

nonhuman origin 

2.1.6 Test ditch or channel sediments to see if they are a bacteria source or reservoir 

2.2 Moderate/High Density Watersheds: 

2.2.1 Conduct dry weather channel survey 

2.2.2 Test for Illicit connections 

2.2.3 Check integrity of major trunk lines for cracks and leaks 

2.2.4 Check for historic and unconnected septic systems 

2.2.5 Conduct ARA and study to determine whether fecal coliform s are of human or 

nonhuman origin 

2.2.6 Check ponds, lakes and impoundments for waterfowl concentrations 

3.0 Wet Weather Investigation Develop and submit a wet weather field investigation program, by 

watershed, to identify and isolate fecal coliform sources that occur during wet weather so that they can 

be reduced/eliminated. The program shall address the sources identified in item 1.0 above. The program 

shall address the suitability of each of the following measures and shall include detailed description of 

activities and frequencies. 

3.1 Low Density Watersheds 

3.1.1 Inspect septic systems for wet-weather failure 

3.1.2 Conduct comprehensive wet weather monitoring to isolate subwatershed hot spots 

3.1.3 Submit results of the Antibiotic Resistance Analysis and the study to determine 

whether fecal coliforms are of human or nonhuman origin 

3.1.4 Sample runoff from suspected source areas (e.g. hobby farms and livestock areas) 

3.1.5 Test storm drain or channel sediments to see if they are a bacteria sink or source 

3.2 Moderate/High Density Watersheds: 

3.2.1 Check for chronic sanitary sewer overflows at specific manholes and /or pumping 

stations 

3.2.2 Submit results of the Antibiotic Resistance Analysis and the study to determine 

whether fecal coliforms are of human or nonhuman origin 

3.2.3 Conduct comprehensive wet weather monitoring to identify key source areas or 

subwatersheds 

4.0 Submit certification of the full implementation of the dry and wet weather field investigation 

programs. 
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156 DRAFT November 2008 

5. 0 Fecal Coliform Reduction and Treatment Develop and submit a program for reducing or treating 

existing fecal coliform sources, by watershed and watershed density. The program shall address the 

sources identified in items 3.0 and 4.0 above. The program shall address the suitability of each of the 

following measures and shall include detailed description of activities and frequencies. Where activities 

are to be performed by entities other than the permittee, describe enforcement mechanism to be used to 

ensure compliance. 

5.1 Low Density Watersheds 

5.1.2 Rehabilitate failing septic systems 

5.1.3 Connect failing septic systems to sewer 

5.1.4 Increase sep tic system clean outs 

5.1.5 Retrofit stormwater ponds 

5.1.6 Retrofit ditches as dry swales 

5.1.7 Waterfowl management 

5.1.8 Install recreational vehicle sewage pumpouts 

5.1.9 Implement conservation plans at hobby farms 

5.2 Moderate/High Density Watersheds: 

5.2.2 Eliminate illicit connections to storm sewer 

5.2.2 Rehabilitate existing sewer system to eliminate sanitary sewer overflows 

5.2.3 Relocate storm outfalls 

5.2.4 Disinfect at the end of pipe 

5.2.5 Retrofit stormwater ponds 

5.2.6 Retrofit ditches as dry swales 

5.2.7 Waterfowl harassment 

5.2.8 Enforce pet waste disposal 

5.2.9 Implement conservation plans at hobby farms 

6.0 Submit certification of the full implementation of fecal coliform reduction and treatment program. 

7.0. Prevention of Future Fecal Discharges Develop and submit a program for preventing future fecal 

coliform discharges, by watershed. The program shall address at a minimum, the measures included 

below, with detailed description of activities and frequencies. Where activities are to be performed by 

entities other than the permittee, describe enforcement mechanism to be used to ensure compliance. 

7.1 Low Density Watersheds 

7.1.1 Land use management 

7.1.2 Stringent septic system requirements: 

7.1.2.1 Feasibility criteria 

7.1.2.2 Setbacks 

7.1.2.3 Reserve field requirements 

7.1.2.4 Minimum lot size 

7.1.2.5 Technology criteria 

7.1.2.6 Inspections 

7.1.2.7 Maintenance requirements 

7.1.3 Stream/ ditches buffers and access restrictions 

7.1.4 Livestock fencing 

7.1.5 Wildlife control 

7.1.6 Land application criteria for biosolids 
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7.1.7 Stormwater treatment for new development 

7.1.8 Public education 

7.1.9 Recreational vehicle and park sewage pump-out facilities 

7.2 Moderate/High Density Watersheds: 

7.2.1 New Sewer Testing 

7.2.2 Inspection of new sewer hookups 

7.2.3 SSO monitoring and prevention 

7.2.4 Stormwater treatment for new development 

7.2.5 Optimal stormwater outfall location 

7.2.6 Engineered stream buffers 

7.2.7 Pet Exclusion 

7.2.8 Waterfowl control /management 

7.2.9 Public education on pet waste 

7.2.10 Transient sewage disposal 

7.2.11 Septic system rehabilitation 

8.0 Submit certification of the implementation of the program to prevent future fecal coliform sources. 

9.0 Monitoring Program Develop a monitoring program, in consultation with the State of New Mexico, 

to assess BMP effectiveness and compliance with Fecal Coliform TMDL at North Diversion Floodway 

Channel, San Jose Drain, South Diversion Channel and Tijeras Arroyo. Target values and equation for 

comparison of loadings are included in Table III.B.2 below. While developing this monitoring program, 

the permittees should take into account the frequency of storm events, and the variation in Fecal 

Coliform levels, within individual storm event. Collection and analysis of samples shall be conducted in 

accordance with Part V requirements. Results shall be submitted in Discharge Monitoring Report 

(DMR) forms. 

10.0 Submit certification of the full implementation of the monitoring program to assess BMP 

effectiveness. 

11.0 BMP Assessment Submit BMP evaluations and assessment, and revisions to the programs above if 

deemed necessary, based on monitoring data obtained. 

12. 0 Annual TMDL Progress Reports The permittees shall submit annual reports describing progress 

on the activities required in Table III.B. to comply with the Fecal Coliform TMDL. The reports shall 

follow the requirements included in Part V.C, items 1, 4, 6 and 7, but shall be submitted separately from 

the Annual Report covering all other items of the permit. Results of the monitoring program shall be 

summarized in the Annual TMDL Progress Report and shall include graphic representation of fecal 

coliform trends. The Annual TMDL Progress Report shall also include computations of annual percent 

reduction achieved from the baseline loads and comparisons with the target loads.
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Ohio 

Big Darby Creek Watershed Construction General Permit 

F. Total Maximum Daily Load (TMDL) allocations 

This general permit requires control measures/BMPs for construction sites that reflect 

recommendations set forth in the U.S. EPA approved Big Darby Creek TMDL. 

G. SWP3 Requirements 

G2. Controls. 

2b. Riparian Setback Requirements. The SWP3 shall clearly delineate the boundary of required 

stream setback distances. No construction activity shall occur within the delineated setback 

boundary except activities associated with restoration or recovery of natural floodplain and 

channel form characteristics as described in Attachment B and storm water conveyances from 

permanent treatment practices. Such conveyances must be designed to minimize the width of 

disturbance. If intrusion within the delineated setback boundary is necessary to accomplish the 

purposes of a project then mitigation shall be required in accordance with Part III.G.2.c of this 

permit. Streams requiring protection under this section are defined as perennial, ephemeral or 

intermittent streams with a defined bed, bank or channel. National Resources Conservation 

Service (NRCS) soil survey maps should be used as one reference and the presence of a stream 

requiring protection should also be confirmed in the field. Any required setback distances shall 

be clearly displayed in the field prior to any construction related activity. 

Riparian setbacks shall be delineated based upon one of the following three methods: 

i. The setback distance from the centerline of the stream shall be sized as the greater of the 

following: 

1. The regulatory 100 year floodplain based on FEMA mapping; 

2. A minimum of 100 feet on each side; or 

3. Distance calculated using the following equation: 

W = 133DA0.43

where: 

DA = drainage area (mi2

) 

W = total width of riparian setback (ft) 

W should be divided by two (2) in order to calculate the setback for one side of the 

stream. If the DA remains relatively constant throughout the stretch of interest, then the 

DA of the downstream edge of the stretch should be used. Where there is a significant 

increase in the DA from the upstream edge to the downstream edge of the area of 

interest, the setback width shall increase accordingly. 

ii. Site Specific Riparian Setback Delineation. The total setback width shall be the streamway 

width centered over the meander pattern of the stream plus an additional 100 feet from the edge 

of the streamway per side. 
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November 2008 DRAFT 159 

The streamway width shall be calculated as described in Part III.G.2.b.i.3 or as ten times the 

bankfull width determined from sufficient site specific information adequate to characterize the 

channel through the site by a professional experienced in stream morphology. The average site 

specific bankfull width may be used if the bankfull width does not vary significantly throughout 

the reach of interest. Otherwise the streamway width should vary with bankfull width. Centering 

about the meander pattern can be thought of as determining where a line representing the 

streamway width would evenly intersect equal elevation lines on either side of the stream. 

iii. Stream Restoration with 100 feet (each side) Riparian Setback. Each stream segment within 

the proposed site boundaries can be assessed in accordance with Attachment B. In the event the 

stream segment is classified as a “Previously Modified Low Gradient Headwater Stream”, the 

permittee has the option to restore the stream segment in accordance with Attachment B and 

include a 100 feet water quality setback distance from the centerline of the stream on each side. 

In the event the stream segment exceeds the minimum criteria in Attachment B to be classified as 

a “Previously Modified Low Gradient Headwater Stream”, Part III.G.2.b.iii may be considered 

on a case-by-case basis. No structural sediment controls (e.g., the installation of silt fence or a 

sediment settling pond) or structural post-construction controls shall be used in a stream or the 

delineated setback. 

2i. Post-Construction Storm Water Management Requirements. So that receiving stream’s 

physical, chemical, and biological characteristics are protected and stream functions are 

maintained, post-construction storm water practices shall provide perpetual management of 

runoff quality and quantity. To meet the post-construction requirements of this permit, the SWP3 

must contain a description of the post-construction BMPs that will be installed during 

construction for the site and the rationale for their selection. The rationale must address the 

anticipated impacts on the channel and floodplain morphology, hydrology, and water quality. 

Detail drawings and maintenance plans shall be provided for all postconstruction BMPs. 

Maintenance plans shall be provided by the permittee to the post-construction operator of the 

site (including homeowner associations) upon completion of construction activities (prior to 

termination of permit coverage). A description of maintenance operations must be included in 

the maintenance agreement to ensure all Post Construction BMP’s will be maintained in 

perpetuity. For sites located within a community with a regulated municipal separate storm 

sewer system (MS4), the permittee, land owner, or other entity with legal control of the property 

may be required to develop and implement a maintenance plan to comply with the requirements 

of the MS4. Maintenance plans must ensure that pollutants collected within structural postconstruction practices, be disposed of in accordance with local, state, and federal regulations. 

Permittees, except for those regulated under the small MS4 program, are not responsible under 

this permit for operation and maintenance of post-construction practices once coverage under 

this permit is terminated. 

This permit does not preclude the use of innovation or experimental postconstruction storm 

water management technologies. However, the director may require discharges from such 

structures to be monitored to ensure compliance with Part III.G.2.i of this permit. The 

installation of structural controls in certain scenarios may also require a separate permit under 

section 404 of the CWA. Permittees are only responsible for the installation and maintenance of 

storm water management measures prior to final stabilization of the site and are not responsible 

for maintenance after storm water discharges associated with construction activity have been 
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160 DRAFT November 2008 

eliminated from the site. However, post-construction storm water BMPs that discharge 

pollutants from point sources once construction is completed, may in themselves, need 

authorization under a separate NPDES permit. 

Linear construction projects, (e.g., pipeline or utility line installation), which do not result in the 

installation of impervious surface, are not required to comply with the conditions of Part 

III.G.2.i of this permit. However, linear construction projects must be designed to minimize the 

number of stream crossings and the width of disturbance. 

Large Construction Activities. For all large construction activities (involving the disturbance of 

five or more acres of land or will disturb less than five acres, but is a part of a larger common 

plan of development or sale which will disturb five or more acres of land), the post construction 

BMP(s) chosen must be able to detain storm water runoff for protection of the stream channels, 

stream erosion control, and improved water quality. Structural (designed) post-construction 

storm water treatment practices shall be incorporated into the permanent drainage system for 

the site. The BMP(s) chosen must be sized to treat the water quality volume (WQv) and ensure 

compliance with Ohio’s Water Quality Standards in OAC Chapter 3745-1. The WQv shall be 

equivalent to the volume of runoff from a 0.75-inch rainfall and shall be determined according to 

one of the two following methods: 

i. Through a site hydrologic study approved by the local municipal permitting authority that uses 

continuous hydrologic simulation and local long-term hourly precipitation records or ii. Using 

the following equation: 

WQv = C * P * A / 12 

where: 

WQv = water quality volume in acre-feet 

C = runoff coefficient appropriate for storms less than 1 inch (see Table 5) 

P = 0.75 inch precipitation depth 

A = area draining into the BMP in acres 

An additional volume equal to 20 percent of the WQv shall be incorporated into the BMP for 

sediment storage and/or reduced infiltration capacity. Ohio EPA recommends that BMPs be 

designed according to the methodology included in the most current edition of the Rainwater and 

Land Development manual or in another design manual acceptable for use by Ohio EPA. 

BMPs shall be designed such that the drain time is long enough to provide treatment, but short 

enough to provide storage available for successive rainfall events as described in Table 6 below. 

The permittee may request approval from Ohio EPA to use alternative structural postconstruction BMPs if the permittee can demonstrate that the alternative BMPs are equivalent in 

effectiveness to those listed in Table 6 above. Construction activities shall be exempt from this 

condition if it can be demonstrated that the WQv is provided within an existing structural postconstruction BMP that is part of a larger common plan of development or if structural postconstruction BMPs are addressed in a regional or local storm water management plan. 

For redevelopment projects (i.e., developments on previously developed property), postconstruction practices shall either ensure a 20 percent net reduction of the site impervious area, 

provide for treatment of at least 20 percent of the WQv, or a combination of the two. 
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Small Construction Activities. For all small land disturbance activities (which disturb one or 

more, but less than five acres of land and is not a part of a larger common plan of development 

or sale which will disturb five or more acres of land), a description of measures that will be 

installed during the construction process to control pollutants in storm water discharges that 

will occur after construction operations have been completed must be included in the SWP3. 

Structural measures should be placed on upland soils to the degree attainable. 

i. Such practices may include, but are not limited to: storm water detention structures (including 

wet basins); storm water retention structures; flow attenuation by use of open vegetated swales 

and natural depressions; infiltration of runoff onsite; and sequential systems (which combine 

several practices). The SWP3 shall include an explanation of the technical basis used to select 

the practices to control pollution where flows exceed pre-development levels. 

ii. Velocity dissipation devices shall be placed at discharge locations and along the length of any 

outfall channel to provide non-erosive flow velocity from the structure to a water course so that 

the natural physical and biological characteristics and functions are maintained and protected 

(e.g., no significant changes in the hydrological regime of the receiving water). 

Big Darby Creek Watershed TMDL 

5.0 Implementation of the Big Darby Creek TMDL 

A key objective for preserving or restoring the high quality aquatic communities in the Big 

Darby Creek watershed is to determine ways for human activities to proceed without disrupting 

the existing natural system. Human intervention usually happens on a local scale. A small swale 

or ditch is often viewed locally as a conduit for exporting water so that the products of human 

pursuits can be maximized. But the system as a whole has a finite capacity. The cumulative 

impact of local interventions in the system has grown to the point that the system can no longer 

assimilate the changes, particularly in the upper Big Darby Creek watershed, Treacle Creek, 

Robinson Run and Hellbranch Run. These local interventions are happening from all aspects of 

our society, as such, solutions will need to come from all aspects of our society. This chapter of 

the TMDL report outlines the ways to implement the guidelines and loading reductions provided 

in Chapter 4. Achievement of these are necessary to maintain the Big Darby Creek watershed as 

a high quality aquatic system. 

5.1 Implementation Mechanisms 

Stream integrity concepts are discussed in Chapters 3 and 4, as well as the establishment of 

allowable loads for pollutants, and effluent limitations for point source dischargers. A variety of 

mechanisms will be evaluated and used to achieve these loading reductions. These mechanisms 

are discussed in more detail below. 

5.1.1 Storm Water Control 

Storm water control is largely achieved through the issuance of general permits under the 

NPDES program. These permits are issued for construction activities, and for industrial 

activities, and are issued to control storm water that is discharged from a discrete conveyance, 

such as pipes or confined conduits. NPDES individual and general permits are issued to 
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individuals, private entities, and local government entities. These permits function together to 

form a web of state and local authority under which storm water is controlled. 

General Permits For Construction Storm Water 

Ohio EPA has issued a draft general permit for runoff associated with construction activity that 

is specific to the Big Darby Creek watershed. Ohio EPA has used existing permit terms and 

conditions and has included new types of permit terms and conditions to ensure, to the extent 

authorized by law, that loading targets developed in Chapters 3 and 4 are achieved for storm 

water. These permit terms and conditions include management practices, effluent targets, 

infiltration requirements necessary to support stream base flows and stream setbacks necessary 

to protect the stream channel. The goal is to issue a permit that is protective of the aquatic life 

uses in the Big Darby Creek watershed. 

As is the case with the existing construction storm water general permit, construction companies 

will be expected to be co-permittees along with developers. This condition of the permit will be 

an area of emphasis by Ohio EPA in evaluating compliance with the general permit for storm 

water from construction activity. 

Phase I and Phase II MS4 Permits For Local Jurisdictions 

Federal storm water regulations call for the issuance of Phase I NPDES storm water permits to 

large municipalities, and the issuance of Phase II NPDES storm water permits to smaller 

municipalities. As with the general permits for construction storm water, Ohio EPA intends to 

revise the MS4 permits, to the extent authorized by law, so as to achieve the loading limitations 

established in Chapter 4 of this TMDL for storm water. Ohio EPA expects to exercise its 

authority to designate additional Phase II communities within the Big Darby Creek watershed 

and to ensure that the permits issued to those jurisdictions are protective of the aquatic life uses. 

Oregon 

Columbia Slough TMDL 

The DMAs will conduct monitoring of stormwater BOD5 loads and the instream response to 

those loads. Previous monitoring under the MS4 permits has measured BOD5 levels from urban 

runoff that do not correlate with the few instream BOD5 samples taken during storm events. The 

discrepancy between loads and instream concentration is likely due to processes such as 

deposition and decay during the transport to the receiving water. The monitoring data will be 

used to calibrate a dynamic water quality model to simulate the Slough’s response to stormwater 

and deicing fluid. The DMA WLA will not be included as an effluent limit. Achievement of the 

WLA will be through implementation of BMPs. Municipal discharges will be required to 

implement BMPs and demonstrate that the BMPs achieve the WLAs established. The DMAs will 

be required, through MOAs, to: 

1. Provide DEQ with a description of the program designed to reduce BOD5 loads to the Slough. 

2. Implement a program of BMPs that will reduce overall BOD5 load to achieve the DMA WLAs. 
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3. Implement coordinated monitoring to define stormwater loads to the Slough and the influence 

of stormwater BOD5 on receiving water quality. 

4. Implement monitoring to demonstrate compliance with BOD5 WLA targets. Instream 

monitoring will include grab samples of BOD5 and DO and continuous hydrolab monitoring. 

5. Implement water quality management plans as developed as part of the Lower Willamette 

Subbasin plan (projected completion spring 1999). 

Phase I MS4 Permit for City of Gresham, City of Fairview, and Multnomah County 

The requirements of this section [p. 17] apply to co-permittee’s MS4 discharges to receiving 

waters with established TMDLs and associated allocations as noted on page 1 of this permit. It 

is the intent of this section to ensure that pollutant discharges for those parameters listed in the 

TMDL are reduced to the maximum extent practicable. Adequate progress toward achieving 

assigned wasteload allocations (WLAs) will be demonstrated through the implementation of best 

management practices that are targeted at TMDL related pollutants. 

i) Progress towards reducing TMDL pollutant loads must be evaluated by the co-permittee 

through the use of performance measures and pollutant load reduction benchmarks developed 

and listed in the SWMP. 

(1) Performance measures are estimates of the effectiveness of various best management 

practices (BMPs) implemented by the co-permittees as per the SWMP; and they are not numeric 

effluent limits. Performance measures must, where appropriate, be pollutant reduction estimates. 

The performance measures for the BMPs addressing TMDL pollutants may be based on the same 

metrics developed in accordance with the program effectiveness monitoring requirements in 

Schedule B(1)(c)(i). 

(2) A benchmark is a total pollutant load reduction estimate for each parameter or surrogate, 

where applicable, for which a WLA is established at the time of permit issuance. A benchmark is 

used to measure the overall effectiveness of the stormwater management plan in making 

progress toward the wasteload allocation (this estimate will be related to the statistical 

variability of the underlying data and may be stated as a range), and is intended to be a tool for 

guiding adaptive management activities. A benchmark is not a numeric effluent limit; rather it is 

a goal that is subject to the maximum extent practicable standard. The co-permittee must provide 

the rationale for the proposed benchmark, which includes an explanation of the relationship 

between the benchmarks and the TMDL wasteload allocations. Any limiting factors related to the 

development of a benchmark, such as data availability and data quality, must also be included in 

this rationale. 

ii) The SWMP must describe a program that includes BMPs, monitoring triggers, narrative 

conditions, or other elements, designed to achieve reductions in the TMDL pollutants. The 

SWMP must include a specific strategy for implementing monitoring designed to enable the copermittee to gauge the effectiveness of the SWMP in reducing TMDL pollutant loads to the 

maximum extent practicable. 

iii) When the co-permittee applies for permit renewal, the co-permittee must include an 

evaluation of the effectiveness of the stormwater management plan with respect to all pollutant 
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parameters addressed in an applicable TMDL. This evaluation must assess progress towards 

meeting the pollutant load reductions (benchmarks) using the reporting and monitoring 

programs and other methods described in Schedules B(1), B(2) and D(2)(d)(v) of this permit. If 

the co-permittee has failed to meet the estimated pollutant load reductions during the permit 

term, they must use the adaptive management process described in Schedule D(2)(a)of this 

permit to reassess the SWMP and determine what additional or alternative BMPs are 

practicable. The co-permittee must update the SWMP to include these BMPs. The co-permittee 

must submit the evaluation and any SWMP revisions to the Department as specified in Schedule 

D(2)(d)(v). 

iv) If within three (3) years following permit issuance a TMDL is approved by the Environmental 

Protection Agency (EPA) and the TMDL has wasteload allocations assigned to stormwater 

within the geographic area covered by this permit, the co-permittee must, at the time of the next 

permit renewal application, complete a review and strategy development, and propose changes, 

if appropriate, to the SWMP to address the urban stormwater discharges. 

v) If, at the time of permit issuance, TMDL wasteload allocations have been established for 

pollutant parameters associated with the MS4’s discharges, each co-permittee must, as 

appropriate, review their SWMP to determine its adequacy in reducing TMDL pollutant 

discharges to the maximum extent practicable and develop pollutant load reduction 

benchmark(s) and performance measures in the SWMP as defined in Schedule D(2)(d)(i)(1) and 

(2). As part of the SWMP review and benchmark and performance measure development 

process, the co-permittee must document, and subsequently report in accordance with Schedule 

B(2)(b), the following information: 

(1) A description of the methodology and rationale used to develop and select pollutant 

reduction benchmarks and performance measures. The methodology must address current 

estimated discharge loadings and TMDL wasteload allocations. 

(2) Any proposed modifications to the SWMP resulting from the adaptive management process 

[Schedule D(2)(a)] necessary to give reasonable assurance that the SWMP is designed to reduce 

TMDL pollutants to the maximum extent practicable. This must include selection of BMPs and 

any assumptions related to the proposed BMPs. 

(3) Any proposed modifications to the monitoring component of the SWMP that are necessary to 

ensure adequate data and information are collected to assess SWMP implementation, BMP 

effectiveness, progress towards the pollutant load reduction (4) A description of the public 

participation process, including a summary of material public comments and the responses to 

those comments. 

The requirements of this section apply to receiving waters without established TMDL wasteload 

allocations. The co-permittee must qualitatively review the pollutants that are on the 2002 

303(d) list that are relevant to the co-permittee’s MS4 discharges. This review and 

corresponding summary of proposed actions must be incorporated into the interim evaluation 

report. The review and summary must accomplish the following: 

i) Determine whether there is a reasonable likelihood for stormwater from the MS4 to cause or 

contribute to water quality degradation of receiving waters through the discharge of pollutants 
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on the 2002 303(d) list. Provide the rationale for the conclusion, including the results of an 

evaluation. 

ii) If the discharges from the MS4 is a contributor to specific listed pollutants, determine and 

describe the relationship between the 303(d) listed pollutant and the MS4 discharges. 

iii) Determine whether the BMPs in the existing SWMP are effective to address the 303(d) 

pollutants. If not, describe how the plan could be adapted to more appropriately address these 

pollutants. A summary of the rationale for this determination must also be included in the report. 

If sufficient information is not available to make the determinations required above, the copermittee must compile pertinent information necessary to adequately complete these 

determinations. 

The Interim Evaluation Report is to include: i) An evaluation of, and proposed revisions to, the 

SWMP that addresses the requirements of Schedules D(2)(b) and B(1)(b), including the rationale 

supporting the proposed revisions. ii) A description of the current source identification 

components of the SWMP and the rationale regarding the adequacy of these components. iii) 

For each of the listed non-stormwater discharges [Schedule A(3)] expected to occur in a 

copermittee’s area, the co-permittee must identify the appropriate control measures and the 

rationale for the selection of these BMPs (or the rationale for why BMPs are deemed not 

necessary). iv) The required information regarding TMDL pollutants as described in Schedule 

D(2)(d)(v) and the corresponding proposed revisions to the SWMP, and/or the required 

information regarding 303(d) listed pollutants as described in Schedule D(2)(e) and the 

corresponding proposed revisions to the SWMP. v) An executive summary of the SWMP, no 

more than 15 pages in length, that describes the main elements of the SWMP. vi) Maps providing 

updated information as described in 40 CFR §122.26(d)(1)(iii)(B), where applicable. 

Draft Phase II MS4 Permit 

The requirements of this section apply to MS4 discharges to receiving waters with established 

TMDLs and associated wasteload allocations as noted on page 1 of this permit or if the 

permittee becomes subject to an approved TMDL, and following notice of such by the 

Department. If the permittee reduces applicable pollutant discharges for the parameters listed in 

the TMDL to the maximum extent practicable, this reduction is deemed to be adequate progress 

toward achieving assigned TMDL wasteload allocations (WLAs). 

 a) Progress towards reducing TMDL pollutant loads will be evaluated, in subsequent permit 

terms, by the permittee through the use of performance measures and pollutant load reduction 

benchmarks developed and listed in the SWMP. 

(1) Performance measures are estimates of the effectiveness of various best management 

practices (BMPs) implemented by the permittee as per the SWMP; and are not numeric effluent 

limits. Performance measures must, where appropriate, be pollutant reduction estimates. If 

appropriate, the performance measures for the BMPs addressing TMDL pollutants may be based 

on the same metrics developed to determine progress towards measurable goals, as described in 

the SWMP. 

(2) A pollutant load reduction benchmark is an estimate for each parameter or surrogate, where 

applicable, for which a WLA is established. A benchmark is used to measure the overall 
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effectiveness of the stormwater management program in making progress toward the WLA (this 

estimate will be related to the statistical variability of the underlying data and may be stated as a 

range), and is intended to be a tool for guiding adaptive management activities. A benchmark is 

not a numeric effluent limit; rather it is a goal. The permittee must provide the rationale for the 

proposed benchmark, which includes an explanation of the relationship between the benchmarks 

and the TMDL wasteload allocations. Any limiting factors related to the development of a 

benchmark, such as data availability and data quality, must also be included in this rationale. 

 b) The permittee must use adaptive management, as described in Schedule A(3), to focus and 

refine SWMP elements to address TMDL wasteload allocation(s) over the course of this permit 

cycle. 

 c) If, at the time of permit issuance or within three (3) years of permit issuance, a TMDL 

establishes municipal stormwater wasteload allocations for pollutant parameters associated with 

the MS4’s discharges, the permittee must develop and propose to the Department specific 

performance measures and pollutant load reduction benchmarks, as described in Schedule 

D(2)(a). Performance measures and pollutant load reduction benchmarks must be submitted to 

the Department as part of the permit renewal package described in Schedule B(3). 

Pennsylvania 

Wissahickon Creek TMDL 

The reference watershed approach is based on determining the current loading rates for the 

pollutants of interest from a selected unimpaired watershed that has similar physical 

characteristics (i.e., landuse, soils, size, geology) to those of the impaired watershed. The 

objective of this process is to reduce the loading rate of sediment (or other pollutant) in the 

impaired stream segment to a level equivalent to or slightly lower than the loading rate in the 

unimpaired reference stream segment. 

Tennessee 

Harpeth River E. coli TMDL 

SWMPs must include a section describing how discharges of pollutants of concern will be 

controlled to ensure that they do not cause or contribute to instream exceedances of water 

quality standards. Specific measures and BMPs to control pollutants of concern must also be 

identified. In addition, MS4s must implement the WLA provisions of an applicable TMDL and 

describe methods to evaluate whether stormwater controls are adequate to meet the WLA. In 

order to evaluate SWMP effectiveness and demonstrate compliance with specified WLAs, MS4s 

must develop and implement appropriate monitoring programs. Instream monitoring, at 

locations selected to best represent the effectiveness of BMPs, must include analytical 

monitoring of pollutants of concern. A detailed plan describing the monitoring program must be 

submitted to the Division of Water Pollution Control Nashville Field Office within 12 months of 

the approval date of this TMDL. Implementation of the monitoring program must commence 

within 6 months of plan approval by the Field Office. The monitoring program shall comply with 
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the monitoring, recordkeeping, and reporting requirements of NPDES General Permit for 

Discharges from Small Municipal Separate Storm Sewer Systems. 

Phase II MS4 General Permit 

1. Determine whether stormwater discharge from any part of the MS4 significantly contributes 

directly or indirectly to a 303(d) listed (i.e., impaired) waterbody. Water quality impaired 

waters means any segment of surface waters that has been identified by the division as 

failing to support classified uses. If you have discharges meeting these criteria, you must 

comply with Part 3.1.1.2 and 3.1.2; if you do not, the remainder of this Part 3.1 does not 

apply to you. 

2. If you have “303(d)” discharges described above, you must also determine whether a Total 

Maximum Daily Load (TMDL) has been developed by the division and approved by EPA for 

the listed waterbody. If there is a TMDL, you must comply with both Parts 3.1.2 and 3.1.3; if 

no TMDL has been approved, Part 3.1.3 does not apply until a TMDL has been approved. 

3. Water Quality Controls for Discharges to Impaired Waterbodies. The stormwater 

management program review submitted to the division must include a section describing how 

your program will control the discharge of the pollutants of concern. This section must 

identify the measures and BMPs that will collectively control the discharge of the pollutants 

of concern. The measures should be presented in order of priority with respect to controlling 

the pollutants of concern. 

4. Consistency with Total Maximum Daily Load (TMDL). If a TMDL has been approved for any 

waterbody into which you discharge, you must follow the procedure below and report on 

these activities in annual reports to the division: 

5. Determine whether the approved TMDL is for a pollutant likely to be found in stormwater 

discharges from your MS4. 

6. Determine whether the TMDL includes a pollutant wasteload allocation (WLA), 

implementation recommendations, or other performance requirements specifically for 

stormwater discharges from your MS4. 

7. Determine whether the TMDL addresses a flow regime likely to occur during periods of 

stormwater discharge. 

8. After the determinations above have been made and if it is found that your MS4 must 

implement specific provisions of the TMDL, evaluate whether the implementation of existing 

stormwater control measures is meeting the TMDL provisions, or if additional control 

measures are necessary. 

9. Document all control measures currently being implemented or planned to be implemented. 

Include a schedule of implementation for all planned controls. Provide your rationale (e.g., 

calculations, assessments, reports and/or other evidence) that shows that you will comply 

with the TMDL provisions. For control measures that are expected to be implemented and 

evaluated beyond the term of this permit, you should also include longer schedule of 

implementation as necessary to describe the control measure. 
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10. Describe a method to evaluate whether the stormwater controls are adequate to meet the 

requirements of the TMDL. 

11. If the evaluation shows that additional or modified controls are necessary, describe the type 

and schedule for the control additions/revisions. 

Vermont 

Phase II General MS4 Permit 

Your SWMP, including your operation and maintenance program for preventing or reducing 

pollutant runoff from municipal operations prepared pursuant to section 4.2.6, must include a 

section describing how your program will control to the maximum extent practicable the 

discharge of the pollutants of concern. This discussion must specifically identify measures and 

BMPs that will collectively control the discharge of the pollutants of concern. Pollutant(s) of 

concern refer to the pollutant identified as causing the impairment. 

As set forth in 1.3.7 in implementing the six minimum control measures set forth in 4.2 you must 

be consistent with recommendations applicable to your MS4 in the implementation section of the 

Lake Champlain TMDL and any future TMDLs for impaired waters affected by your MS4 

established or approved by EPA pursuant to section 303(d) of the federal Clean Water Act. The 

Lake Champlain Phosphorus TMDL recommendations for municipalities include: adoption of 

erosion controls (page 65), improved construction and maintenance practices for gravel 

backroads (page 69), promotion of riparian buffers and setbacks (page 76) and impervious 

surface minimization (page 76). 3.1.4. Determination of Consistency. The assessment of whether 

your Stormwater Management Program is consistent with TMDL recommendations will be 

based on your implementation and maintenance of best management practices not on estimates 

or measurements of pollutant loading does not authorize a direct discharge that is inconsistent 

with any EPA approved TMDL waste load allocation and any implementation plan for the 

waterbody to which the direct discharge drains. This general permit does not authorize a 

discharge to an impaired waterbody for which the Department has issued a watershed-specific 

general permit. 

Washington 

Draft Phase II MS4 General Permit (Western Washington)

An example of TMDL specific requirements is as follows: 

Name of TMDL: Snohomish River Tributaries 

Location of Original 303 (d) Listings – WA-07-1012, WA-07-015, WA-07-1052, WA-07-

1163WA-07-1163, WA-07-1030 and WA-07-040 

Area where TMDL Requirements Apply: 

For each waterbody listed, TMDL coverage includes areas draining to the WASWIS segment 

number, and all the upstream tributaries contributing to it: Allen Creek, YT94RF: Quilceda 

Creek, TH58TS: French Creek, XZ24XU: Woods Creek, FZ74HO: 
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Pilchuck River, NF79WA: Marshland Watershed, XW79FQ 

TMDL coverage includes the areas indicated in the Lower Snohomish River Tributaries 

Fecal Coliform Bacteria TMDL Detailed Implementation Plan dated June 2003, Figure 3, page 

7. This TMDL can be found at http://www.ecy.wa.gov/programs/wq/tmdl/watershed/tmdl_infonwro.html 

Parameter – Fecal Coliform 

Approval Date – 9 – Aug. 2001 

Potential MS4 Permittees – Phase I permit: Snohomish County 

Phase II permit: Granite Falls, Lake Stevens, Monroe, Snohomish, Marysville, Arlington, 

Everett 

WSDOT permit: WSDOT. 

Action Required – 

Baseline Requirements: Within 12 months after the effective date of this permit, all municipal 

stormwater permittees must adopt and enforce an ordinance or ordinances requiring the 

application of source control BMPs for the following existing land uses if they occur within their 

jurisdiction: 1) commercial animal handling areas, and 2) commercial composting facilities. 

Where these activities are not occurring, no action is required. BMPs shall be equivalent to 

those found in Volume IV of the 2001 Ecology Stormwater Management Manual for Western 

Washington. Ordinances shall also address illicit connections to storm drains. 

Where potential sources of bacterial pollution exist, operational source control BMPs shall be 

required for all pollutant generating sources. Only in those cases where a facility is 

demonstrated to be causing a violation of surface or ground water standards, or is discharging 

illegally, shall structural source control BMPs shall be required as related to this TMDL. The 

provision for structural source control BMPs is not intended to apply to individual municipal 

stormwater outfalls. 

No later than 12 months after the effective date of this permit, affected municipal permittees shall 

compile a list of the existing composting and animal waste handling facilities. This list shall be 

updated no later than 180 days prior to the expiration of the permit and submitted with the 

permit renewal application. Starting no later than 24 months after the effective date of this 

permit, conduct an inspection program for all the listed sites, with adequate enforcement 

capability to ensure implementation of source control BMPs. All facilities must be inspected with 

40 months of the effective date of this permit. 

Monitoring and Implementation Requirements: Permittees shall choose one or both of the 

following monitoring strategies. Strategy A is the default implementation strategy unless the 

permittee chooses to implement Strategy B in all or part of the area subject to the TMDL: 

Strategy A, Targeted Implementation Approach 

• Within 90 days of permit issuance, prepare and submit to Ecology for review, a Quality 

Assurance Project Plan (QAPP) for the sampling of streams and/or discharges from 

stormwater conveyances within the jurisdictions boundaries in order to determine areas with 

highest bacteria concentrations (high priority areas). Provisions for additional monitoring in 

high priority areas shall be included in order to locate pollution sources were they are not 

obvious. 
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• The QAPP shall be prepared following Ecology’s “Guidelines for Preparing Quality 

Assurance Project Plans for Environmental Studies, Feb. 2001, Ecology Publication No. 01-

03-003. Ecology will review and provide comments within 30 days the plan is received. The 

sampling plan shall include an adequate number of sampling points and adequate sampling 

frequency to reasonably characterize the receiving water or waste stream. Monitoring shall 

begin no later than 270 days after permit issuance. 

• No later than 365 days prior to permit renewal application, a Bacterial Pollution Control 

Plan shall be developed. The Bacterial Pollution Control Plan shall, at a minimum, consider 

the use of the following approaches: 

1) pet waste ordinance, 2) evaluation of water pollution control enforcement capabilities, 3) 

evaluation of CAO in relation to TMDL goals, 4) educational program directed at reducing 

bacterial pollution, 5) investigation and implementation of methods that prevent additional 

stormwater bacterial pollution through stormwater treatment, reducing stormwater volumes, 

and preventing additional sources of stormwater in association with new development, 6) 

implementation of activities in the Quilceda/Allen or French Creek Watershed Management 

Plans (as applicable), 7) ambient water quality and stormwater quality sampling to 

specifically identify bacterial pollution sources, and 8) livestock ordinance and compost 

ordinance (Phase I Permittees only.) 

• No later than 270 days prior to permit renewal application, conduct public review of the 

Bacterial Pollution Control Plan. 

• Submit the final Bacterial Pollution Control Plan to Ecology at the time of permit renewal 

application.

Strategy B: Early Action Approach. 

• Prepare Early Action BMP plan within 180 days of permit effective date. The Early Action 

Plan shall contain those BMPs that the permittee believes will be effective in reducing 

bacteria levels within the MS4 (or otherwise in local waters). The Early Action Plan must 

include implementation of the required baseline requirement for all municipal stormwater 

permittees including adoption and enforcement of ordinance(s) requiring the application of 

source control BMPs related to bacterial pollutants (equivalent to Volume IV of the 2001 

Ecology Stormwater Management Manual for Western Washington). 

• The Early Action BMP Plan shall, at a minimum, consider the use of the following 

approaches: 1) pet waste ordinance, 2) evaluation of water pollution control enforcement 

capabilities, 3) evaluation of CAO in relation to TMDL goals, 4) educational program 

directed at reducing bacterial pollution, 5) investigation and implementation of methods that 

prevent additional stormwater bacterial pollution through stormwater treatment, reducing 

stormwater volumes, and preventing additional sources of stormwater in association with new 

development, 6) implementation of activities in Quilceda/Allen or French Creek Watershed 

Management Plans (as applicable) Watershed Management Plan, 7) ambient water quality 

and stormwater quality sampling to specifically identify bacterial pollution sources, and 8) 

livestock and compost ordinances (Phase I permittees only) 

• Conduct and complete public review of the Early Action BMP plan within 270 days of permit 

effective date. 
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• Begin implementation of Early Action BMPs as specified in the plan within 360 days of permit 

issuance. BMPs shall be place within 36 months of permit issuance unless otherwise approved 

by Ecology. 

• Within 30 months of permit issuance, prepare and submit to Ecology for review, a Quality 

Assurance Project Plan (QAPP) for the sampling of streams and/or discharges from 

stormwater conveyances within the jurisdictions boundaries in order to assess whether or not 

affected water bodies and/or stormwater discharges, are meeting state water quality 

standards. 

• The QAPP shall be prepared following Ecology’s “Guidelines for Preparing Quality 

Assurance Project Plans for Environmental Studies, Feb. 2001, Ecology Publication No. 01-

03-003. Ecology will review and provide comments within 30 days the plan is received. The 

sampling plan shall include an adequate number of sampling points and adequate sampling 

frequency to reasonably characterize the receiving water or waste stream. Monitoring shall 

begin no later than 36 months after permit issuance. 

• No later than 270 days prior to permit renewal, a Bacterial Pollution Control Plan shall be 

developed. The Plan shall consider all available monitoring data and the approaches noted 

for the Early Action BMP Plan above. 

• No later than 270 days prior to permit renewal application, conduct public review of the 

Bacterial Pollution Control Plan. 

• Submit the Bacterial Pollution Control Plan to Ecology at the time of permit renewal 

application for review. 

Construction General Permit (state-wide) 

S8. DISCHARGES TO 303(D) OR TMDL WATERBODIES 

A. Sampling and Numeric Effluent Limitations For Discharges to 303(d)-listed Waterbodies 

1. Permittees that discharge to water bodies listed as impaired by the State of Washington under 

Section 303(d) of the Clean Water Act for turbidity, fine sediment, high pH, or phosphorus, shall 

conduct water quality sampling according to the requirements of this section. 

2. All references and requirements associated with Section 303(d) of the Clean Water Act mean 

the most current listing by Ecology of impaired waters that exists on November 16, 2005, or the 

date when the operator’s complete permit application is received by Ecology, whichever is later. 

B. Discharges to 303(d)-Listed Waterbodies (Turbidity, Fine Sediment, or Phosphorus) 

1. Permittees which discharge to waterbodies on the 303(d) list for turbidity, fine sediment, or 

phosphorus shall conduct turbidity sampling at the following locations to evaluate compliance 

with the water quality standard for turbidity: a. Background turbidity shall be measured in the 

303(d)-listed receiving water immediately upstream (upgradient) or outside the area of influence 

of the discharge; and b. Discharge turbidity shall be measured at the point of discharge into the 

303(d) listed receiving waterbody, inside the area of influence of the discharge; or Alternatively, 

discharge turbidity may be measured at the point where the discharge leaves the construction 

site, rather than in the receiving waterbody. 
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2. Based on sampling, if the discharge turbidity exceeds the water quality standard for turbidity 

(more than 5 NTU over background turbidity when the background turbidity is 50 NTU or less, 

or more than a 10% increase in turbidity when the background turbidity is more than 50 NTU), 

all future discharges shall comply with a numeric effluent limit which is equal to the water 

quality standard for turbidity. 

3. If a future discharge exceeds the water quality standard for turbidity, the Permittee shall: 

a. Review the SWPPP for compliance with Condition S9 and make appropriate revisions 

within 7 days of the discharge that exceeded the standard; 

b. Fully implement and maintain appropriate source control and/or treatment BMPs as 

soon as possible, but within 10 days of the discharge that exceeded the standard; 

c. Document BMP implementation and maintenance in the site log book; d. Notify the 

appropriate Ecology Regional Office by phone within 24 hours of analysis; 

e. Continue to sample daily until discharge turbidity meets the water quality standard for 

turbidity. 

C. Discharges to waterbodies on the 303(d) list for High pH 

1. Permittees which discharge to waterbodies on the 303(d) list for high pH shall conduct 

sampling at one of the following locations to evaluate compliance with the water quality 

standard for pH (in the range of 6.5 – 8.5): a. pH shall be measured at the point of discharge 

into the 303(d) listed waterbody, inside the area of influence of the discharge; or b. 

Alternatively, pH may be measured at the point where the discharge leaves the construction site, 

rather than in the receiving water. 

2. Based on the sampling set forth above, if the pH exceeds the water quality standard for pH (in 

the range of 6.5 – 8.5), all future discharges shall comply with a numeric effluent limit which is 

equal to the water quality standard for pH. 

3. If a future discharge exceeds the water quality standard for pH, the Permittee shall: 

a. Review the SWPPP for compliance with Condition S9 and make appropriate revisions 

within 7 days of the discharge that exceeded the water quality standard; 

b. Fully implement and maintain appropriate source control and/or treatment BMPs as 

soon as possible, but within 10 days of the discharge that exceeded the standards; 

c. Document BMP implementation and maintenance in the site log book; d. Notify the 

appropriate Ecology Regional Office by phone within 24 hours of analysis; and 

e. Continue to sample daily until discharge meets the water quality standard for pH (in 

the range of 6.5 – 8.5) or the discharge stops or is eliminated. 
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November 2008 DRAFT 173 

Parameter identified 

in 303(d) listing 

Parameter/Units Analytical Method Sampling Frequency 

Turbidity Phosphorus Turbidity/NTU SM2130 or Weekly, if discharging

Fine Sediment EPA180.1 If background is 50 

NTU or less: 5 NTU 

over background; or 

If background is more 

than 50 NTU: 10% 

over background 

High pH pH/Standard Units pH meter Weekly, if discharging 

D. Sampling and Limitations For Sites Discharging to Applicable TMDLs 

1. Discharges to a waterbodies subject to an applicable Total Maximum Daily Load (TMDL) for 

turbidity, fine sediment, high pH, or phosphorus, shall be consistent with the assumptions and 

requirements of the TMDL. 

a. Where an applicable TMDL sets specific waste load allocations or requirements for 

discharges covered by this permit, discharges shall be consistent with any specific waste 

load allocations or requirements established by the applicable TMDL. ii. The Permittee 

shall sample discharges weekly, or as otherwise specified by the TMDL, to evaluate 

compliance with the specific waste load allocations or requirements. iii. Analytical 

methods used to meet the monitoring requirements shall conform to the latest revision of 

the Guidelines Establishing Test Procedures for the Analysis of Pollutants contained in 

40 CFR Part 136. Turbidity and pH methods need not be accredited or registered unless 

conducted at a laboratory which must otherwise be accredited or registered. 

b. Where an applicable TMDL has established a general waste load allocation for 

construction stormwater discharges, but no specific requirements have been identified, 

compliance with Conditions S4 (Monitoring) and S9 (SWPPPs) will be assumed to be 

consistent with the approved TMDL. 

c. Where an applicable TMDL has not specified a waste load allocation for construction 

stormwater discharges, but has not excluded these discharges, compliance with 

Conditions S4 (Monitoring) and S9 (SWPPPs) will be assumed to be consistent with the 

approved TMDL. 

d. Where an applicable TMDL specifically precludes or prohibits discharges from 

construction activity, the operator is not eligible for coverage under this permit. 

2. Applicable TMDL means a TMDL for turbidity, fine sediment, high pH, or phosphorus, which 

has been completed and approved by EPA prior to November 16, 2005, or prior to the date the 

operator’s complete permit application is received by Ecology, whichever is later. TMDLs 

completed after the operator’s complete permit application is received by Ecology become 

applicable to the Permittee only if they are imposed through an administrative order by Ecology, 

or through a modification of permit coverage. 
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174 DRAFT November 2008 

Industrial General Permit (state-wide) 

Facilities that discharge to a waterbody with a control plan unless this general permit is 

adequate to provide the level of protection required by the control plan. Excluded facilities need 

to obtain coverage under another NPDES permit for stormwater discharges associated with 

industrial activity. Control plans may be total maximum daily load (TMDL) determinations, 

restrictions for the protection of endangered species, ground water management plans, or other 

limitations that regulate or set limits on discharges to a specific waterbody or groundwater 

recharge area. 

E. Stormwater Discharges to Impaired Waterbodies Except 303(d) Listings for Sediment and 

Tissue 

The Permittee’s discharge must not cause or contribute to an excursion of the State’s water 

quality standards, including the State’s narrative criteria for water quality. For 303(d) listings 

based on numeric water quality criteria, Permittees must comply with the State’s water quality 

standard for each pollutant named as a pollutant causing a violation of water quality standards 

at the location named on the State’s 303(d) list except for temperature which is not required and 

fecal coliform which is only required if there is a potential source from the industrial activity. 

Ecology will not require monitoring for fecal coliform if the Permittee can document that there is 

no potential source of fecal coliform from any of their industrial activities. A permittee’s 

requirements to comply with this condition will be listed on the cover sheet. Ecology will 

maintain an electronic list of permittees subject to this permit condition. This list, titled Appendix 

4, is available on Ecology’s web site. 

For waterbody segments listed as impaired by the State under Section 303(d) of the Clean Water 

Act, the applicable 303(d) list is the list which is in effect August 21, 2002, or the 303(d) list 

which is in effect at the date the first application for coverage is received by Ecology, whichever 

is later. 

Permittees must be in compliance with applicable Total Maximum Daily Load (TMDL) 

determinations. Applicable TMDLs or TMDL determinations are TMDLs which have been 

completed by the issuance date of this permit, or which have been completed prior to the date 

that the permittees application is received by Ecology, which ever is later. A permittee’s 

requirements to comply with this condition will be listed on their cover sheet. Ecology will 

maintain an electronic list of permittees subject to this permit condition. This list, titled Appendix 

5, is available on Ecology’s web site. Unless the first application for coverage is received after 

any updated 303(d) list is effective, changes associated with revised 303(d) lists completed after 

September 20, 2002 will only become effective if they are imposed through an administrative 

order issued by Ecology. 

Unless the first application for coverage is received after the TMDL is completed TMDL 

requirements associated with TMDLs completed after the issuance date of this permit will only 

become effective if they are imposed through an administrative order issued by Ecology. 

1. New Facilities and Significant Process Change New facilities that discharge either directly or 

indirectly via a stormwater conveyance system to waterbody segments listed as impaired by the 

State under Section 303(d) of the Clean Water Act must comply with the State’s water quality 

standards for the named pollutant(s) at the point of discharge. Facilities with coverage under 
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November 2008 DRAFT 175 

this permit, that implement a significant process change (see S1.D.1.) must either comply with 

the State’s water quality standards for the named pollutant(s) at the point of discharge or 

demonstrate no increase in loading from the entire facility as a result of the process change. All 

new discharges including new discharges associated with significant process changes must be in 

compliance with any applicable TMDL determination.

PARAMETER EFFLUENT LIMITATIONS: 

NEW FACILITIES TO 

IMPAIRED WATERS OR WATERS COVERED BY A 

TMDL 

Parameter(s) as identified for the 303(d) listed As listed on the coversheet, based on Chapter 173-201A 

segment or if applicable, TMDL determination or as identified in the TMDL or listing documentation 

2. Existing Facilities discharging to water bodies for which an applicable TMDL has been 

completed: 

PARAMETER EFFLUENT LIMITATIONS: 

EXISTING FACILITIES TO 

WATERS COVERED BY A TMDL 

Parameter(s) as identified in the applicable TMDL As listed on the cover sheet to comply with the applicable 

TMDL 

Note: A current listing of permittees subject to this permit condition and the specific effluent 

limitations and monitoring requirements, Appendix 5, is available on Ecology’s web site. 

3. Existing facilities which discharge either directly or indirectly via a stormwater conveyance 

system to waterbody segments listed as impaired by the State under Section 303(d) of the Clean 

Water Act are subject to the general compliance with standards provisions in S7. Additional 

monitoring and benchmarks apply as described in S.4.G Note: A current listing of permittees 

subject to this permit condition, and the associated benchmarks and monitoring requirements, 

Appendix 4, is available on Ecology’s web site. 

G. Monitoring Requirements for Facilities Discharging to 303(d) Listed Waters or Subject to 

TMDL Determination Except 303(d) Listings for Sediment and Tissue 

In addition to the requirements in S4.C. above, beginning January, 2005, all facilities that 

discharge to waterbody segments listed as impaired by the State under Section 303(d) of the 

Clean Water Act must conduct quarterly monitoring of authorized discharges of stormwater to 

surface water. Samples must be analyzed for the parameters named on the 303(d) as causing 

impairment of the listed waters except for temperature which is not required and fecal coliform 

which is only required if there is a potential source from the industrial activity. Note: A current 

Appendix 4 with a list of permittees subject to the monitoring requirements of this condition is 

available on Ecology’s web site. 

Discharges to a waterbody for which a TMDL has been completed must be consistent with the 

TMDL determination. Where the TMDL determination sets load allocations for new discharges 

or limits pollutant concentrations in the discharge, the Permittee must conduct quarterly 

monitoring for the named pollutant(s) and the monitoring must be consistent with TMDL 
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176 DRAFT November 2008 

requirements, if any. Reporting as required by this permit begins with the first quarter of the 

year 2005. Note: A current Appendix 5 with a list of permittees subject to the monitoring 

requirements of this condition is available on Ecology’s web site. 

1. Permittees may suspend monitoring for a listed parameter if: 

a. Eight consecutive samples fail to detect the presence of the listed parameter. Fail to 

detect does not apply to pH. For pH it is eight consecutive samples where the values are 

not outside of the water quality-based range of 6.5 to 8.5 (freshwater) or 7.0 to 8.5 

(marine). 

b. The Permittee can demonstrate to Ecology’s satisfaction after eight or more 

consecutive quarterly samples that there is no reasonable potential to violate water 

quality standards. For the purposes of suspending monitoring required under S4.G only, 

no reasonable potential to violate water quality is defined as a single sample exceeding 

eighty percent of the benchmark, and the average of the last eight consecutive quarterly 

samples is less than sixty percent of the benchmark. 

2. For existing permittees discharging to water bodies for which an applicable TMDL has been 

completed: 

Parameter Units Analytical Method Minimum Sampling 

Frequency 

Parameter(s) as identified in the As Applicable Appropriate EPA or Quarterly 

applicable TMDL. (See cover (see cover sheet) Equivalent Method (See cover sheet for 

sheet) specifics) 

Note: A current Appendix 5 with a list of permittees subject to the monitoring requirements of 

this condition is available on Ecology’s web site. 

3. Existing permittees discharging to water bodies that discharge to waterbody segments listed 

as impaired by the State under Section 303(d) of the Clean Water Act: 

Parameter Units Analytical Method 303(d) Benchmark Minimum 

Value Sampling 

Frequency 

Parameter(s) as identified As Applicable Appropriate EPA Based on Chapter Quarterly 

for the 303(d) listed (See cover sheet) or Equivalent 173-201A (See cover sheet for 

segment (See cover sheet) Method (See cover sheet) specifics) 

Note: A current Appendix 4 with a list of permittees subject to the monitoring requirements of 

this condition is available on Ecology’s web site. 

H. Monitoring Requirements for Facilities Discharging to 303(d) Waterbody segments listed for 

Sediment 

All facilities that discharge to waterbody segments listed for sediment must notify Ecology of any 

sediment data they may have collected. Upon request from Ecology they will submit the data. 
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November 2008 DRAFT 177 

In addition to the requirements in S4.A. above, beginning with the first quarter of the year 2005, 

all facilities that discharge to waterbody segments listed by the State for violations of sediment 

standards under Section 303(d) of the Clean Water Act must conduct quarterly monitoring of 

authorized discharges of stormwater to surface water for total suspended solids (TSS). 

Discharges that demonstrate TSS levels consistent with secondary treatment standards (30 mg/L 

monthly average not to exceed 45 mg/L) are considered unlikely to violate sediment quality 

standards. Permittees that can demonstrate consistent attainment TSS levels of secondary 

treatment standards may suspend monitoring for the duration of the permit term. Consistent 

attainment is defined as 8 consecutive quarterly samples (omitting any quarter where there is no 

discharge) with an average TSS of 30 mg/L and no sample exceeding 45 mg/L. 

Wisconsin 

Phase II MS4 general permit 

1.5 Impaired Water Bodies and Total Maximum Daily Load Requirements 

1.5.1 The permittee shall determine whether any part of its MS4 discharges to an impaired water 

body listed in accordance with section 303(d)(1) of the federal Clean Water Act, 33 USC 

§1313(d)(1)(C), and the implementing regulation of the US Environmental Protection Agency, 

40 CFR §130.7(c)(1). Impaired waters are those that are not meeting applicable water quality 

standards. A list of Wisconsin impaired water bodies may be found on the Department’s Internet 

site at: http://dnr.wi.gov/org/water/wm/wqs/303d/303d.html. 

1.5.2 If the permittee’s MS4 discharges to an impaired water body, the permittee shall include a 

written section in its storm water management program that discusses the management practices 

and control measures it will implement as part of its program to reduce, with the goal of 

eliminating, the discharge of pollutant(s) of concern that contribute to the impairment of the 

water body. This section of the permittee’s program shall specifically identify control measures 

and practices that will collectively be used to try to eliminate the MS4’s discharge of pollutant(s) 

of concern that contribute to the impairment of the water body and explain why these control 

measures and practices were chosen as opposed to other alternatives. Pollutant(s) of concern 

means a pollutant that is causing impairment of a water body. 

1.5.3 After the permittee’s start date of coverage under this permit, the permittee may not 

establish a new MS4 discharge of a pollutant of concern to an impaired water body or increase 

the discharge of a pollutant of concern to an impaired water body unless the new or increased 

discharge causes the receiving water to meet applicable water quality standards, or the 

Department has approved a total maximum daily load (TMDL) for the impaired water body. 

1.5.4 The permittee shall determine whether its MS4 discharges to an impaired water body for 

which the Department has approved a TMDL. If so, the permittee shall assess whether the 

TMDL wasteload allocation for the MS4 is being met through the existing storm water 

management controls or whether additional control measures are necessary. The permittee’s 

assessment of whether the TMDL wasteload allocation is being met shall focus on the adequacy 

of the permittee’s storm water controls (implementation and maintenance). Approved TMDLs 
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178 DRAFT November 2008 

are listed on the Department Internet site at: 

http://dnr.wi.gov/org/water/wm/wqs/303d/index.html. 

1.5.5 The storm water management program developed under section 2 of this permit shall be 

revised as necessary to achieve and maintain compliance with any Department approved-TMDL 

wasteload allocation for an impaired water to which the MS4 discharges. The redesigned storm 

water management programs shall be implemented as soon as possible. 

Federal Construction General Permit 

Part 1.3.C.: Eligibility, Limitations on Coverage 

5. Discharging into Receiving Waters With an Approved Total Maximum Daily Load Analysis 

a. You are not eligible for coverage under this permit for discharges of pollutants of concern to 

waters for which there is a total maximum daily load (TMDL) established or approved by EPA 

unless you incorporate into your SWPPP measures or controls that are consistent with the 

assumptions and requirements of such TMDL. To be eligible for coverage under this general 

permit, you must incorporate into your SWPPP any conditions applicable to your discharges 

necessary for consistency with the assumptions and requirements of such TMDL. If a specific 

wasteload allocation has been established that would apply to your discharge, you must 

incorporate that allocation into your SWPPP and implement necessary steps to meet that 

allocation. 

b. In a situation where an EPA-approved or established TMDL has specified a general 

wasteload allocation applicable to construction storm water discharges, but no specific 

requirements for construction sites have been identified in the TMDL, you should consult with 

the State or Federal TMDL authority to confirm that adherence to a SWPPP that meets the 

requirements of the CGP will be consistent with the approved TMDL. Where an EPA-approved 

or established TMDL has not specified a wasteload allocation applicable to construction storm 

water discharges, but has not specifically excluded these discharges, adherence to a SWPPP that 

meets the requirements of the CGP will generally be assumed to be consistent with the approved 

TMDL. If the EPA-approved or established TMDL specifically precludes such discharges, the 

operator is not eligible for coverage under the CGP. 

3.14 Documentation of Permit Eligibility Related to Total Maximum Daily Loads 

The SWPPP must include documentation supporting a determination of permit eligibility with 

regard to waters that have an EPA-established or approved TMDL, including: 

A. Identification of whether your discharge is identified, either specifically or generally, in an 

EPA-established or approved TMDL and any associated allocations, requirements, and 

assumptions identified for your discharge; 

B. Summaries of consultation with State or Federal TMDL authorities on consistency of SWPPP 

conditions with the approved TMDL, and 
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November 2008 DRAFT 179 

C. Measures taken by you to ensure that your discharge of pollutants from the site is consistent 

with the assumptions and requirements of the EPA-established or approved TMDL, including 

any specific wasteload allocation that has been established that would apply to your discharge. 

See section 1.3.C.5 for further information on determining permit eligibility related to TMDLs. 

Federal Multi-Sector General Permit for Stormwater Discharges Associated with Industrial 

Activity 

Impaired waters include both those with established TMDLs, and those for which TMDL 

development has been identified as necessary, but for which one has not yet been established. 

For a more detailed definition see Appendix A. 

1.4.4.1 Discharge to an Impaired Water with an Established TMDL. If a wasteload allocation 

(WLA) has been established that applies to your discharge, you must develop the SWPPP 

accordingly (Part 2.1.3.2), and implement all necessary controls to meet that allocation. You 

must verify that your discharge complies with the WLA through the appropriate discharge 

monitoring (Part 3.2.4.2). Failure to comply with a relevant WLA is a violation of this permit. 

If you have properly complied with the requirements of Part 2.1.3.2 and find that the applicable 

TMDL does not specify a wasteload allocation or other requirements either individually or 

categorically for your discharge (including disallowing such discharge), compliance with this 

permit will be deemed adequate to meet the requirements of the TMDL. 

1.4.4.2 Discharge to an Impaired Water without an Established TMDL. If a TMDL has not 

been established that applies to your discharge you must comply with the requirements of this 

permit and any additional conditions stipulated by the Secretary (Part 2.1.3.2). If you have 

properly complied with all such requirements then compliance with this permit will be deemed 

adequate to meet the requirements for discharging to an impaired water. You are also subject to 

the monitoring requirement of Part 3.2.4.1. Failure to comply with applicable conditions is a 

violation of this permit. 

3.2.4.1 Discharges to impaired waters with no applicable wasteload allocation. For discharges 

that are conveyed directly or indirectly to impaired waters, monitoring for the pollutant of 

concern must be conducted at a minimum of once each permit year throughout the term of the 

permit unless this permit already assigns your discharge an effluent limitation or a benchmark 

for the pollutant of concern. Your monitoring year begins on the day that your discharge is 

authorized. 

This monitoring requirement is waived after one year if the pollutant of concern is not detected 

in an amount expected to cause and contribute to a violation of Vermont Water Quality 

Standards in your stormwater discharge, and you document in your SWPPP that there is no 

exposure of the pollutant of concern to stormwater at your site. 

3.2.4.2 Discharges to impaired waters with an applicable wasteload allocation. For discharges 

that are conveyed directly or indirectly to waters for which a TMDL has been established with a 

wasteload allocation applicable to your discharge (either specifically or categorically), 

monitoring for the wasteload allocation pollutant of concern must be conducted, consistent with 
APPENDIX: TMDL AND NPDES

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180 DRAFT November 2008 

any instructions in TMDL documentation. If the TMDL documentation does not specify specific 

monitoring requirements, monitoring for the pollutant of concern must be conducted at a 

minimum of once each permit year throughout the term of the permit, unless this permit already 

assigns your discharge an effluent limitation or a benchmark for the pollutant of concern, in 

which case you must follow the effluent limitation or benchmark monitoring schedule. Your 

monitoring year begins on the day your discharge is authorized. This monitoring must be 

conducted in addition to all other monitoring requirements prescribed in this permit. Monitoring 

of a pollutant of concern for which your discharge has been assigned a wasteload allocation 

cannot be waived unless the WLA is specified only in terms of BMPs, in which case the 

monitoring requirement is waived after one year if the pollutant of concern is not detected in 

your stormwater discharge and you document in your SWPPP that you have adopted the 

required BMPs. 

If at any time your monitoring data exceed a relevant waste load allocation you are subject to 

the Corrective Action requirements of Part 3.3 and the Follow-up Monitoring and Reporting 

requirements of Part 3.4.
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November 2008 DRAFT 181 

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Academies. National Academies Press, Washington, D.C. 

Perciasepe, R. 1997. New Policies for Establishing and Implementing Total Maximum Daily Loads 

(TMDLs). Memorandum from Robert Perciasepe, Assistant Administrator, to Regional 

Administrators Regional Water Division Directors. U.S. Environmental Protection Agency, Office 

of Water, Washington, D.C. 

Pitt, R.E., A. Maestre, and R. Morquecho. 2004. The National Stormwater Quality Database (NSQD, 

version 1.1). University of Alabama, Tuscaloosa, AL. 

http://unix.eng.ua.edu/~rpitt/Research/ms4/Paper/recentpaper.htm 
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November 2008 DRAFT 183 

Stein, Eric D. 2005. Effect of Increases in Peak Flows and Imperviousness on Stream Morphology of 

Ephemeral Streams in Southern California. Technical Report 450. Southern California Coastal 

Water Research Project, Westminster, CA. www.environmentalexpert.com/files/19961/articles/4562/4562.pdf. 

Stormwater Monitoring Coalition’s Model Monitoring Technical Committee. 2004. Model Monitoring 

Program for Municipal Separate Storm Sewer Systems in Southern California. August 2004. 

Technical Report #419. ftp://ftp.sccwrp.org/pub/download/PDFs/419_smc_mm.pdf 

Stricker, E., and M. Quigley (URS Greiner Woodward Clyde), ASCE (American Society of Civil 

Engineers), and USEPA (U.S. Environmental Protection Agency). 1999. Determining Urban 

Stormwater Best Management Practice (BMP) Removal Efficiencies. American Society of Civil 

Engineers, Urban Water Resources Research Council, City, ST, and U.S. Environmental Protection 

Agency, Office of Water, Washington, DC. www.bmpdatabase.org/docs/task3_1.pdf 

TARP (Technology Acceptance and Reciprocity Partnership). 2003. TARP Protocol for Stormwater 

Best Management Practice Demonstrations. Final August 2001. Updated July 2003. 

www.dep.state.pa.us/dep/deputate/pollprev/techservices/tarp/pdffiles/Tier2protocol.pdf 

University of Minnesota. 2008. Assessment of Stormwater Best Management Practices. University of 

Minnesota, St. Paul, MN. 

http://wrc.umn.edu/outreach/stormwater/bmpassessment/assessmentmanual/ 

University of Wisconsin – Extension. 2000. Wisconsin Stormwater Manual: Technical Design 

Guidelines for Stormwater Management Practices. University of Wisconsin Extension, Madison, 

WI. http://learningstore.uwex.edu/Wisconsin-Storm-Water-Manual-P603C123.aspx. 

USEPA (U.S. Environmental Protection Agency). 1983. Results of the Nationwide Urban Runoff 

Program – Volume I Final Report. U.S. Environmental Protection Agency, Water Planning 

Division, Washington, DC. www.epa.gov/npdes/pubs/sw_nurp_vol_1_finalreport.pdf

USEPA (U.S. Environmental Protection Agency). 1991. Guidance for Water-Quality-based Decisions: 

The TMDL Process. EPA 440/4-91-001. U.S. Environmental Protection Agency, Office of Water, 

Washington, DC. www.epa.gov/OWOW/tmdl/decisions/ 

USEPA (U.S. Environmental Protection Agency). 1992a. NPDES Stormwater Sampling Guidance 

Document. EPA 833-B-92-001. U.S. Environmental Protection Agency, Washington, DC. 

www.epa.gov/npdes/pubs/owm0093.pdf 

USEPA (U.S. Environmental Protection Agency). 1992b. Stormwater Management for Industrial 

Activities: Summary Guidance on Developing Pollution Prevention Plans and Best Management 

Practices. EPA 833-R-92-002. U.S. Environmental Protection Agency, Office of Water, 

Washington, DC. www.epa.gov/npdes/pubs/owm0236a.pdf 

USEPA (U.S. Environmental Protection Agency). 1997. Compendium of Tools for Watershed 

Assessment and TMDL Development. EPA 841-B-97-006. U.S. Environmental Protection Agency, 

Office of Water, Office of Wetlands, Oceans, and Watersheds, Washington, DC. 
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USEPA (U.S. Environmental Protection Agency). 1999. Protocol for Developing Sediment TMDLs. 

EPA 841-B-99-004. U.S. Environmental Protection Agency, Office of Water, Washington, DC. 

www.epa.gov/owow/tmdl/sediment/pdf/sediment.pdf

USEPA (U.S. Environmental Protection Agency). 2000a. Protocol for Developing Nutrient TMDLs. 

EPA 841-B-99-007. U.S. Environmental Protection Agency, Office of Water, Washington, DC. 

www.epa.gov/owow/tmdl/nutrient/pdf/nutrient.pdf

USEPA (U.S. Environmental Protection Agency). 2000b. Protocol for Developing Pathogen TMDLs. 

EPA 841-R-00-002. U.S. Environmental Protection Agency, Office of Water, Washington, DC. 

www.epa.gov/owow/tmdl/pathogen_all.pdf

USEPA (U.S. Environmental Protection Agency). 2000c. Stressor Identification Guidance Document. 

EPA 822-B-00-025. U.S. Environmental Protection Agency, Office of Water and Office of Research 

and Development, Washington, DC. www.epa.gov/waterscience/biocriteria/stressors/ 

USEPA (U.S. Environmental Protection Agency). 2005a. Guidance for 2006 Assessment, Listing, and 

Reporting Requirements Pursuant to Sections 303(d), 305(b), and 314 of the Clean Water Act [2006 

Integrated Report Guidance (IRG)]. U.S. Environmental Protection Agency, Office of Water, Office 

of Wetland, Oceans and Watersheds, Washington, DC. 

www.epa.gov/owow/tmdl/2006IRG/#documents

USEPA (U.S. Environmental Protection Agency). 2005b. Stormwater Phase II Final Rule Fact Sheet: 

Small MS4 Stormwater Program Overview. EPA 833-F-00-002. U.S. Environmental Protection 

Agency, Office of Water. www.epa.gov/npdes/pubs/fact2-0.pdf

USEPA (U.S. Environmental Protection Agency). 2005c. TMDL Model Evaluation and Research Needs. 

EPA/600/R-05/149. U.S. Environmental Protection Agency, Office of Research and Development, 

National Risk Management Research Laboratory, Cincinnati, OH. 

USEPA (U.S. Environmental Protection Agency). 2006. Information Concerning 2008 Clean Water Act 

Sections 303(d), 305(b), and 314 Integrated Listing and Reporting Decisions. Memorandum from 

Diane Regas, Director, Office of Wetlands, Oceans and Watersheds to Regions 1-10 Water Division 

Directors. www.epa.gov/owow/tmdl/2008_ir_memorandum.html

USEPA (U.S. Environmental Protection Agency). 2007a. An Approach for Using Load Duration Curves 

in the Development of TMDLs. EPA 841-B-07-006. U.S. Environmental Protection Agency, Office 

of Water, Office of Wetlands, Oceans, and Watersheds, Washington, DC. 

www.epa.gov/OWOW/tmdl/duration_curve_guide_aug2007.pdf

USEPA (U.S. Environmental Protection Agency). 2007b. Developing Your Stormwater Pollution 

Prevention Plan: A Guide for Construction Sites. EPA 833-R-060-04. U.S. Environmental 

Protection Agency, Washington, DC. http://cfpub.epa.gov/npdes/stormwater/swppp.cfm

USEPA (U.S. Environmental Protection Agency). 2007c. Municipal Separate Storm Sewer System 

(MS4) Program Evaluation Guidance. EPA-833-R-07-003. U.S. Environmental Protection Ageny, 

Office of Wastewater Management, Water Permits Division, Washington, DC. 

www.epa.gov/npdes/pubs/ms4guide_withappendixa.pdf and 

www.epa.gov/npdes/pubs/ms4guide_appendicesb-d.pdf
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USEPA (U.S. Environmental Protection Agency). 2007d. Options for Expressing Daily Loads in 

TMDLs (Draft). U.S. Environmental Protection Agency, Office of Wetlands, Oceans and 

Watersheds, Washington, D.C. www.epa.gov/owow/tmdl/draft_daily_loads_tech.pdf

USEPA (U.S. Environmental Protection Agency). 2007e. Total Maximum Daily Loads and National 

Pollutant Discharge Elimination System Storm Water Permits for Impaired Water Bodies: A 

Summary of State Practices. U.S. Environmental Protection Agency, Region 5, Chicago, IL. 

www.epa.gov/region5/water/wshednps/pdf/state_practices_report_final_09_07.pdf 

USEPA (U.S. Environmental Protection Agency). 2007f. Total Maximum Daily Loads with Stormwater 

Sources: A Summary of 17 TMDLs. EPA 841-R-07-002. U.S. Environmental Protection Agency, 

Office of Wetlands, Oceans and Watersheds, Washington, DC. 

www.epa.gov/owow/tmdl/17_TMDLs_Stormwater_Sources.pdf 

USEPA (U.S. Environmental Protection Agency). 2008a. Handbook for Developing Watershed Plans to 

Restore and Protect Our Waters. EPA 841-B-08-002. U.S. Environmental Protection Agency, Office 

of Water, Nonpoint Source Control Branch, Washington, DC. 

www.epa.gov/nps/watershed_handbook/ 

USEPA (U.S. Environmental Protection Agency). 2008b. Incorporating Green Infrastructure Concepts 

into Total Maximum Daily Loads. U.S. Environmental Protection Agency, Washington, DC. 

www.epa.gov/owow/tmdl/stormwater/

USEPA. Undated. Urbanization and Streams: Studies of Hydrologic Impacts. U.S. Environmental 

Protection Agency, Office of Wetlands, Oceans and Watersheds, Washington, DC. 

http://epa.gov/owow/nps/urbanize/report.html. Accessed July 2008. 

USGS (U.S. Geological Survey). Variously dated. National Field Manual for the Collection of WaterQuality Data. U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, chaps. 

A1-A9. U.S. Geological Survey, Reston, VA. http://pubs.water.usgs.gov/twri9A

Wayland, R.H., and J.A. Hanlon. 2002. Establishing Total Maximum Daily Load (TMDL) Wasteload 

Allocations (WLAs) for Storm Water Sources and NPDES Permit Requirements Based on Those 

WLAs. Memorandum from Robert H. Wayland, III, Director, Office of Wetlands, Oceans and 

Watersheds, and James A. Hanlon, Director, Office of Wastewater Management, U.S. 

Environmental Protection Agency, Washington, DC. www.epa.gov/npdes/pubs/final-wwtmdl.pdf

Washington Department of Ecology. 2005. Stormwater Management Manual for Western Washington. 

Washington Department of Ecology, Olympia, WA. 

www.ecy.wa.gov/programs/wq/stormwater/manual.html

Washington Department of Ecology. 2008. Guidance for Evaluating Emerging Stormwater Treatment 

Technologies, Technology Assessment Protocol (revised). Publication # 02-10-037. Washington 

State Department of Ecology, Olympia, WA. www.ecy.wa.gov/biblio/0210037.html

Wong, T.H.F., H.P. Duncan, T.D. Fletcher, and G.A. Jenkins. 2001. A Unified Approach to Modeling 

Urban Stormwater Treatment. In Proceedings of the 2nd South Pacific Stormwater Conference. 

Auckland, New Zealand, June 27–29, 2001. pp.319–327. 
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Wright, T., C. Swann, K. Cappiella, and T. Schueler. 2005. Unified Subwatershed and Site 

Reconnaissance: A User’s Manual. Version 2.0. Ellicott City, MD. 

www.cwp.org/Store/usrm.htm#11 

Yu, S.L., G.M. Fitch, and T.A. Earles. 1998. Constructed Wetlands for Stormwater Management. VTRC 

98-R35RB.Virginia Department of Transportation Research Council, Charlottesville VA. 

Zomodi, K. 2007. Effectiveness of Time of Concentration Elongation on Peak Flow Reduction. 2nd 

National Low Impact Development Conference, March 12–14, 2007. 

www.bae.ncsu.edu/topic/lidconference07/A6/A6.4.Effectiveness%20of%20Time%20of%20Concent

ration%20Elongationon%20on%20Peak%20Flow%20Reduction.pdf

Web Sites and Databases 

American Society of Civil Engineers’ (ASCE) and EPA’s International Stormwater BMP Database: 

www.bmpdatabase.org/

Authorization Status for EPA’s Stormwater Construction and Industrial Programs by State: 

http://cfpub.epa.gov/npdes/stormwater/authorizationstatus.cfm 

Center for Watershed Protection’s Stormwater Manager’s Resource Center Web site: 

www.stormwatercenter.net/

Center for Watershed Protection’s Web site on the Simple Method: www.stormwatercenter.net. Click 

“By Category.” Information on the Simple Method is included in the Impacts of Urbanization

category. 

Conservation Technology and Information Center Web site, Know Your Watershed: Building Local 

Partnerships: www2.ctic.purdue.edu/KYW/Brochures/BuildingLocal.html

EPA’s Biocriteria Web site: www.epa.gov/waterscience/biocriteria/ 

EPA’s CADDIS Web site: http://cfpub.epa.gov/caddis/ 

EPA’s eNOI system: http://cfpub.epa.gov/npdes/stormwater/enoi.cfm

EPA’s Environmental Monitoring and Assessment Program (EMAP) database: 

www.epa.gov/emap/index.html 

EPA’s Guidelines for Reviewing TMDLs Under Existing Regulations Issued in 1992: 

www.epa.gov/owow/tmdl/guidance/final52002.html

EPA’s list of bioassessment publications from EPA and other federal agencies (e.g., U.S. Geological 

Survey [USGS], U.S. Department of Agriculture [USDA]): 

www.epa.gov/bioindicators/html/publications.html

EPA’s list of Regional stormwater contacts: 

http://cfpub.epa.gov/npdes/contacts.cfm?program_id=6&type=REGION 
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EPA’s list of sectors of industrial activity that require permit coverage: 

http://cfpub.epa.gov/npdes/stormwater/swcats.cfm

EPA’s list of state stormwater contacts: 

http://cfpub.epa.gov/npdes/contacts.cfm?program_id=6&type=STATE 

EPA’s NPDES Stormwater Discharges from Industrial Facilities Web site: 

http://cfpub.epa.gov/npdes/stormwater/indust.cfm

EPA’s NPDES Stormwater Discharges from Municipal Separate Storm Sewer Systems Web site: 

http://cfpub.epa.gov/npdes/stormwater/munic.cfm

EPA’s NPDES Stormwater Program Authorization Status: 

http://cfpub.epa.gov/npdes/stormwater/authorizationstatus.cfm

EPA’s NPDES Stormwater program Web site: http://cfpub.epa.gov/npdes/home.cfm?program_id=6

EPA’s NPDES Stormwater Discharges from Construction Activities Web site: 

http://cfpub.epa.gov/npdes/stormwater/const.cfm

EPA’s PCS: www.epa.gov/enviro/html/pcs/index.html 

EPA’s Storage and Retrieval Database (STORET): www.epa.gov/storet/

EPA’s TMDL and Stormwater Resources Web site: www.epa.gov/owow/tmdl/stormwater 

EPA’s TMDL Web site: www.epa.gov/owow/tmdl/ 

EPA’s Urban BMP Performance Tool: 

http://cfpub.epa.gov/npdes/stormwater/urbanbmp/bmpeffectiveness.cfm

Google Earth: http://earth.google.com/

Multi-Resolution Land Characteristics (MRLC) Consortium’s National Land Cover Database (NLCD): 

www.epa.gov/mrlc/

National Agriculture Imagery Program (NAIP) Aerial Photos: http://165.221.201.14/NAIP.html

University of Alabama’s (Pitt, R.E., A. Maestre, and R. Morquecho) The National Stormwater Quality 

Database (NSQD, version 1.1): http://unix.eng.ua.edu/~rpitt/Research/ms4/mainms4.shtml 

University of Massachusetts Amherst’s Massachusetts Stormwater Technology Evaluation Project 

(MASTEP), Stormwater Technologies Clearinghouse: www.mastep.net/ 

University of New Hampshire Stormwater Center, Nonpoint Education for Municipal Officials 

(NEMO), Innovative Stormwater Management Inventory Database: 

www.erg.unh.edu/stormwater/index.asp 

USDA’s PACFISH/INFISH Biological Opinion Program: http://fsgeodata.fs.fed.us/pibo/ 

USGS’s Earth Explorer: http://edcsns17.cr.usgs.gov/EarthExplorer/ 
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USGS’s National Water Information System Web site (NWISWeb): http://waterdata.usgs.gov/nwis 

USGS’s National Water-Quality Assessment (NAWQA) Program: http://water.usgs.gov/nawqa/ 

USGS’s Land Cover Institute: http://landcover.usgs.gov/ 
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November 2008 DRAFT 189 

GLOSSARY

Benchmark monitoring: The results of MSGP 2000 benchmark monitoring are primarily for the 

permittee’s use to determine the overall effectiveness of an SWPPP in controlling the discharge of 

pollutants to receiving waters. Benchmark values are not viewed as effluent limitations. An exceedance 

of a benchmark value does not, in and of itself, constitute a violation of the MSGP. According to EPA, 

while exceedance of a benchmark value does not automatically indicate that violation of a water quality 

standard has occurred, it does signal that modifications to the SWPPP may be necessary. In addition, 

permitting authorities may use the exceedance of benchmark values to identify facilities that would be 

more appropriately covered under an individual, or alternative general permit where more specific 

pollution prevention controls could be required. 

Best management practice (BMP): Policies or practices that prevent, reduce, or mitigate the effects of 

stormwater runoff. These methods can be structural (e.g., devices, ponds) or nonstructural (e.g., policies 

to reduce imperviousness). BMPs classified as nonstructural are those that rely predominantly on 

behavioral changes rather than construction to be effective. Structural BMPs are engineered or 

constructed to prevent or manage stormwater. BMPs are often further classified into (1) source-control 

BMPs to prevent pollution, (2) water quality BMPs to reduce or prevent pollutants in runoff, (3) flowcontrol BMPs to reduce the volume of stormwater and (4) infiltration BMPs to increase infiltration. 

Combined sewer system: Combined sewer systems are sewers that are designed to collect rainwater 

runoff, domestic sewage, and industrial wastewater in the same pipe. Most of the time, combined sewer 

systems transport all their wastewater to a sewage treatment plant, where it is treated and then 

discharged to a waterbody. During periods of heavy rainfall or snowmelt, however, the wastewater 

volume in a combined sewer system can exceed the capacity of the sewer system or treatment plant. For 

this reason, combined sewer systems are designed to overflow occasionally and discharge excess 

wastewater directly to nearby streams, rivers, or other waterbodies. 

Effluent limitation: Any restriction imposed on quantities, discharge rates, and concentrations of 

pollutants that are discharged from point sources into waters of the United States, the waters of the 

contiguous zone, or the ocean (40 CFR 122.2). 

Flow duration curves: Calculations of limits that analyze the cumulative frequency of historic flow 

data over a specified period. Flow duration curve development typically uses daily average discharge 

rates, which are sorted from the highest value to the lowest. Using this convention, flow duration 

intervals are expressed as a percentage, with zero corresponding to the highest stream discharge in the 

record (i.e., flood conditions) and 100 to the lowest (i.e., drought conditions). Duration curve analysis 

identifies intervals that can be used as a general indicator of hydrologic condition (i.e., wet versus dry 

and to what degree). Flow duration curve intervals can be grouped into several broad categories or 

zones. For example, many duration curves categorize the flow intervals into the following five zones: 

high flows, moist conditions, mid-range flows, dry conditions, and low flows. When water quality 

concentrations or loads are plotted on the basis of these flow zones, the resulting graphs can provide 

additional insight about conditions, patterns associated with the impairment, and potential sources 

contributing to the problem. Duration curves add value to the TMDL process by characterizing water 

quality concerns in terms of flow conditions, linking these concerns to key watershed processes, 

prioritizing source assessment efforts, and identifying potential solutions. 
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190 DRAFT November 2008 

General permit: An NPDES permit issued under 40 CFR 122.28 that authorizes a category of 

discharges under the CWA within a geographical area. A general permit is not specifically tailored for 

an individual discharger. 

Illicit discharge: Any discharge to a municipal separate storm sewer that is not composed entirely of 

stormwater, except discharges pursuant to an NPDES permit and discharges resulting from fire fighting 

activities. 

Individual permit: An NPDES permit specifically tailored for an individual discharger. 

Integrated Design and Evaluation Assessment of Loadings (IDEAL; Barfield 2002) provides a 

spreadsheet-based technique for assessing the benefits of urban management practices on flow, 

sediment, nutrients, and bacteria. The model predicts watershed runoff, concentrations, and loads on the 

basis of the user’s selection of vegetative filter strips, dry-detention ponds and wet-detention ponds. 

Urban areas are defined as pervious, impervious connected, and impervious unconnected areas. Flow 

and loads can be directed to a pond that can be either dry (no permanent pool) or wet (permanent pool). 

The model then calculates the pollutant removal efficiencies of the practices using empirical equations. 

The model predicts single storm values and converts them to average annual storm values using a 

statistical process. The IDEAL model is designed to help managers estimate long-term management 

practice pollutant removal efficiencies, and is not designed for looking at individual storms. 

Load allocation (LA): The portion of a receiving water’s loading capacity that is attributed either to one 

of its existing or future nonpoint sources of pollution or to natural background sources. LAs are best 

estimates of the loading, which can range from reasonably accurate estimates to gross allotments, 

depending on the availability of data and appropriate techniques for predicting the loading. Wherever 

possible, natural and nonpoint source loads should be distinguished [40 CFR 130.2(g)]. 

Loading capacity: The greatest amount of loading that a waterbody can receive without violating water 

quality standards. Loading capacity is equal to the TMDL. Loading capacities calculated using datadriven approaches are typically based on in-stream or delivered loads (i.e., in-stream flow multiplied by 

target and conversion factor at a location in the waterbody). Loading capacities developed using landbased modeling approaches can also be based on source loads (i.e., land-based loads before they are 

delivered to the stream). 

Margin of safety (MOS): The component of a TMDL that accounts for any lack of knowledge 

concerning the relationship between LAs, WLAs, and water quality [CWA section 303(d)(1)(C), 

40 CFR 130.7(c)(1)]. EPA’s Guidance for Water Quality-based Decisions: The TMDL Process (1991) 

explains that the MOS may be implicit (i.e., incorporated into the TMDL through conservative 

assumptions in the analysis) or explicit (i.e., expressed in the TMDL as loadings set aside for the MOS). 

If the MOS is implicit, the conservative assumptions in the analysis that account for the MOS must be 

described. If the MOS is explicit, the loading set aside for the MOS must be identified. 

Maximum extent practicable (MEP): The pollutant reduction standard applied to stormwater 

management programs developed to address stormwater discharges from regulated MS4s. 

Measurable goals: Quantifiable objectives for assessing program and best management practice 

effectiveness that regulated small municipal separate storm sewer systems must develop for the six 

minimum control measures to comply with Phase II MS4 permit requirements. 
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November 2008 DRAFT 191 

Model for Urban Stormwater Improvement Conceptualization (MUSIC; Wong et al. 2001): 

Software developed by the Cooperative Research Center (CRC) for Catchment Hydrology in Australia 

to evaluate small- and large-scale (100-square-mile) urban stormwater systems using modeling time 

steps that range from 6 minutes to 24 hours. MUSIC provides an interface to help set up complex 

stormwater management scenarios. It allows users to view results using a range of graphical and tabular 

formats. The stormwater control devices evaluated by MUSIC include ponds, bioretention, infiltration 

buffer strips, sedimentation basins, pollutant traps, wetlands, and swales. Major techniques used to 

evaluate management practices including settling in ponds and decay of pollutants (first order; see 

www.toolkit.net.au/music). 

Multi-Sector General Permit (MSGP): Authorizes the discharge of stormwater from industrial 

facilities, consistent with the terms of the permit, in areas of the United States where EPA manages the 

NPDES permit program. 

Municipal separate storm sewer system (MS4): A conveyance or system of conveyances (including 

roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, 

or storm drains): (i) Owned or operated by a state, city, town, borough, county, parish, district, 

association, or other public body (created by or pursuant to state law)...including special districts under 

State law such as a sewer district, flood control district or drainage district, or similar entity, or an Indian 

tribe or an authorized Indian tribal organization, or a designated and approved management agency 

under section 208 of the CWA that discharges into waters of the United States. (ii) Designed or used for 

collecting or conveying stormwater; (iii) Which is not a combined sewer; and (iv) Which is not part of a 

publicly owned treatment works (POTW) as defined at 40 CFR 122.2. 

Notice of Intent (NOI): Submitting a completed NOI constitutes notice that the entity intends to be 

authorized to discharge pollutants to waters of the United States from the facility or site identified in the 

form under a state or EPA general permit such as the Phase II MS4 General Permit, the Multi-Sector 

General Permit (MSGP) for industrial stormwater, or the Construction General Permit (CGP). 

Permitting authority: The U.S. Environmental Protection Agency (EPA), a Regional Administrator of 

EPA, or an authorized representative. 

PGBMP-DSS: Evaluates the effect of management practices or combinations of management practices 

on flow and pollutant loading. This module uses simplified, process-based algorithms to simulate 

management practice control of modeled flow and water-quality time series generated from runoff 

models such as HSPF. These simple algorithms include weir and orifice control structures; storm swale 

characteristics; flow and pollutant transport; flow routing and networking; infiltration and saturation; 

and a general loss/decay representation for a pollutant. Users have the flexibility to design retentionstyle or open-channel management practices; can define flow routing through a management practice or 

management practice network; can simulate Integrated Management Practices (IMPs) such as reduced or 

discontinued imperviousness through flow networking; and can compare management practice controls 

against a defined benchmark such as a simulated predevelopment condition. Because the underlying 

algorithms are based on physical processes, management practice effectiveness can be evaluated and 

estimated over a wide range of storm conditions, management practice designs, and flow routing 

configurations. 

Six minimum control measures: Categories of best management practices that regulated small MS4s 

must address under Phase II MS4 stormwater management programs. 
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192 DRAFT November 2008 

Storm sewershed: Land area in which all stormwater flows are conveyed to a single point, or outlet. 

Stormwater management program (SWMP): The program developed and implemented by EPA to 

minimize the discharge of pollutants from regulated MS4s to the maximum extent practicable using 

BMPs. 

Stormwater pollution prevention plan (SWPPP): A plan developed to minimize the discharge of 

pollutants from an industrial site (including construction activities) using BMPs. 

Subwatershed: Smaller division of a watershed, defined by the area draining to a tributary of the main 

waterbody. 

The Site Evaluation Tool (SET): Software developed to assess the effects of development, including 

sediment and nutrient loading, on a site scale. The SET provides a more robust environment for testing 

multiple management practices and site configurations than do simple export calculations. The tool 

allows definition of pre- and post-treated land use/land cover, allowing for multiple drainage areas and 

various combinations of practices. An important benefit of the SET is testing management practices in 

combination with each other, in the context of a site or small catchment. Structural and nonstructural 

practices can be represented, giving the user a suite of options for evaluation. 

Total Maximum Daily Load (TMDL): A calculation of the maximum amount of a pollutant that a 

waterbody can receive and still meet water quality standards. A TMDL is the sum of individual 

wasteload allocations for point sources (WLA), load allocations for nonpoint sources and natural 

background (LA), and must consider seasonal variation and include a margin of safety. The TMDL 

comes in the form of a technical document or plan. (40 CFR 130.2 and 130.7) 

Urbanized area (UA): A land area comprising one or more places—central place(s)—and the adjacent 

densely settled surrounding area—urban fringe—that together have a residential population of at least 

50,000 and an overall population density of at least 1,000 people per square mile used to identify 

regulated small MS4s under the Phase II MS4 Stormwater program. 

Vegetative Filter Strip Model (VFSMOD; Muñoz-Carpena et al. 2003): A model that provides 

specialized modeling of field-scale processes associated with filter trips or buffers. It provides routing of 

storm runoff from an adjacent field through a vegetative filter strip and calculates outflow, infiltration, 

and sediment trapping efficiency. It is sensitive to characteristics of the filter including vegetation 

roughness or density, slope, infiltration characteristics, and the incoming runoff volume and sediment 

particle sizes. VFSMOD includes a series of modules Green-Ampt infiltration, kinematic wave overland 

flow, and sediment filtration. The model can also be used to describe transport at the edge of the field 

when flow and transport are mainly in the form of sheet flow and the path represents average conditions 

across the vegetative filter strip. VFSMOD uses a variable time step that helps to more accurately solve 

the overland water flow equation. The model inputs are specified on a storm basis, and the model 

summarizes all the information after each event to generate storm outputs. 

Virginia Field Scale Wetland Model (VAFSWM; Yu, et al. 1998): A field-scale model for quantifying 

the pollutant removal in a wetland system. It includes a hydrologic subroutine to route flow through the 

treatment system; precipitation, evapotranspiration, and exchange with subsurface groundwater. The 

model adopted a Continuous Stirred Tank Reactor (CSTR) in series schema. VAFSWM models 

mechanisms of settling, diffusion, adsorption to plants and substrate, and vegetative uptake for a 

pollutant in dissolved and particulate forms in a two-segment (water column and substrate), two-state 
TMDLS TO STORMWATER PERMITS HANDBOOK GLOSSARY

November 2008 DRAFT 193 

(completely mixed and quiescent) reactor system by employing first-order kinetics. The governing 

equations for quiescent condition are identical to that of turbulent condition; however, far lower settling 

velocities are assumed to account for the greater percentage of finer particles during the quiescent state. 

VAFSWM is a relatively simple model that includes the most dominant processes in the wetland system. 

However, the users need to provide and calibrate the requisite kinetics parameters. 

Wasteload allocation (WLA): The portion of a receiving water’s loading capacity that is allocated to 

one of its existing or future point sources of pollution. WLAs constitute a type of water quality-based 

effluent limitation (40 CFR 130.2(h)). 

Water Quality Based-Effluent Limitation (WQBEL): An effluent limitation determined by selecting 

the most stringent of the effluent limits calculated using all applicable water quality criteria (e.g., aquatic 

life, human health, wildlife, translation of narrative criteria) for a specific point source to a specific 

receiving water for a given pollutant or on the basis of the facility’s WLA from a TMDL. 

Watershed: A land area that drains to a common waterway, such as a stream, lake, estuary, wetland or 

ultimately the ocean. 

WETLAND (Lee 1999; 2002): A dynamic, compartmental model to simulate hydrologic, water quality, 

and biological processes and help design and evaluate wetlands. WETLAND uses the continuously 

stirred tank reactor prototype, and it is assumed that all incoming nutrients are completely mixed 

throughout the entire volume. The model can simulate both free-water surface and subsurface flow 

wetlands. WETLAND is modular and includes hydrologic, nitrogen, carbon, dissolved oxygen, bacteria, 

sediment, vegetation, and phosphorous submodels. The strength of WETLAND lies on the linked 

kinetics for the water quality variables and considers seasonal variation (variable user-defined parameter 

by season/time period). The weaknesses of this model include the completely mixed assumption, which 

overlooks the effect of the system shape, and the needs for extensive kinetic parameters.