Consensus Study Report

Transportation Research Board Special Report 360

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COMMITTEE ON THE REVIEW OF APPROACHES FOR MANAGING POLLUTANT LOADS IN HIGHWAY STORMWATER RUNOFF

DIANE M. MCKNIGHT (NAE) (Chair), Distinguished Professor of Civil, Environmental and Architecture Engineering and Environmental Studies, University of Colorado

LAWRENCE BAND, Ernest H. Ern Professor, University of Virginia

JANE K. CLARY, Vice President, Wright Water Engineers, Inc.

JANA CRAWFORD, Stormwater Retrofits Project Manager, Jacobs Engineering Group

SARAH FAKHREDDINE, Assistant Professor, Carnegie Mellon University

ELIZABETH A. FASSMAN-BECK, Head of Engineering Department, Southern California Coastal Water Research Project

WILLIAM B. FLETCHER, II, Water Resources Specialist/Program Leader, Oregon Department of Transportation (retired)

STEPHANIE E. HURLEY, Professor of Ecological Landscape Design, University of Vermont

SUJAY KAUSHAL, Professor, University of Maryland

ANDREW H. MCDANIEL, Manager of Highway Stormwater Program, North Carolina Department of Transportation (retired)

ROBERT TRAVER, Professor and former Director of the Center for Resilient Water Systems, Villanova University

National Academies of Sciences, Engineering, and Medicine Staff

CHARLES R. BURGIS, Study Director, Earth Systems and Resources, Center for Health, People, and Places

DEBORAH GLICKSON, Senior Program Director, Earth Systems and Resources, Center for Health, People, and Places

TIMOTHY B. MARFLAK, Program Coordinator, Consensus and Advisory Studies, Transportation Research Board

THOMAS R. MENZIES, JR., Director, Consensus and Advisory Studies, Transportation Research Board

Reviewers

This Consensus Study Report was reviewed in draft form by individuals chosen for their diverse perspectives and technical expertise. The purpose of this independent review is to provide candid and critical comments that will assist the National Academies of Sciences, Engineering, and Medicine in making each published report as sound as possible and to ensure that it meets the institutional standards for quality, objectivity, evidence, and responsiveness to the study charge. The review comments and draft manuscript remain confidential to protect the integrity of the deliberative process.

We thank the following individuals for their review of this report:

Although the reviewers listed above provided many constructive comments and suggestions, they were not asked to endorse the conclusions or recommendations of this report nor did they see the final draft before its release. The review of this report was overseen by CHRIS T. HENDRICKSON (NAE), Carnegie Mellon University, and DANNY REIBLE (NAE), Texas Tech University. They were responsible for making certain that an independent examination of this report was carried out in accordance with the standards of the National Academies and that all review comments were carefully considered. Responsibility for the final content rests entirely with the authoring committee and the National Academies.

Acknowledgments

The committee wishes to thank Gregory Granato, who was the Federal Highway Administration’s liaison to the study. He oversaw the contract and provided helpful information to the committee.

The study committee met in person four times and virtually seven times over 20 months to gather information for the study. The committee members thank the following individuals for presentations during these meetings and for the submitted materials.

From state agencies: Charles Hebson and Cindy Dionne, Maine DOT; Nick Tiedeken, Minnesota DOT; Bhaskar Joshi, California DOT; Rhonda Thiele, Utah DOT; Benjamin Storey, Alaska DOT; Alex Forasté, Virginia DOT; Tom Martin, Montana DOT; Lance Grace, Florida DOT; Alisa Richardson, Rhode Island DOT; Madison Rose Bristol and Shelby Giltner, Washington State Department of Ecology; Jane Hann, Colorado DOT; Henry Barbaro, Massachusetts DOT (retired); and Cam McNutt, North Carolina Department of Environmental Quality.

From the U.S. Environmental Protection Agency: Heather Goss, Rosaura Conde, Amy King, and Justin Drew.

From other groups and organizations: Scott Taylor, Atkins Global; Rebecca Sutton and Diana Lin, San Francisco Estuary Institute; Charles Job, National Ground Water Association; Dan Yates, Ground Water Protection Council; Scott Andres, Delaware Geological Survey; Sheri Bozic, Jemez Pueblo; Andrew Earles, Wright Water Engineers; Heather Himmelberger, Southwest Environmental Finance Center; Hanna Miller, The Conservation Foundation; Greg Donofrio, University of Minnesota; Scott Berry, U.S. Water Alliance; Trevor Clements, Tetra Tech; Todd Redder,

Limno Tech; John Riverson, Paradigm Environmental; and Steve Carter, Paradigm Environmental.

PROJECT MANAGEMENT

Charles R. Burgis managed the study and assisted the committee in drafting the report under the guidance of Thomas R. Menzies, Jr., and Deborah Glickson. Timothy B. Marflak provided administrative and logistical support. Karen Febey, senior report review officer, managed the report review process.

Contents

BOXES, FIGURES, AND TABLES

PREFACE

ACRONYMS AND ABBREVIATIONS

EXECUTIVE SUMMARY

Study Charge

Conclusions and Recommendations

Concluding Comments

1 INTRODUCTION

Study Origins and Charge

Study Scope and Approach

Report Organization

References

2 FOUNDATIONAL BACKGROUND

Managing Water as an Interconnected Resource

Requirements of the Clean Water Act and their Implementation

Best Management Practices for Stormwater

Modeling

References

3 BACKGROUND ON HIGHWAY STORMWATER MANAGEMENT

Highway Stormwater Management Challenges

Sources and Types of Contaminants in Highway Runoff

State DOT Involvement in the TMDL Process

Best Management Practices for Highway Stormwater

Monitoring Highway Stormwater

Monitoring Data

Modeling Highway Stormwater

References

4 HIGHWAY STORMWATER MANAGEMENT CHALLENGES AND OPPORTUNITIES: BEFORE AND DURING TMDL DEVELOPMENT

Challenges Before TMDL Development

Challenges During TMDL Development

Opportunities for Meeting Challenges

Chapter Summary and Conclusions

References

5 HIGHWAY STORMWATER MANAGEMENT CHALLENGES AND OPPORTUNITIES: TMDL IMPLEMENTATION AND BEST MANAGEMENT PRACTICES

Challenges Ensuring Permit Compliance Capability

BMP Implementation Challenges

Opportunities for Implementation Improvements

Chapter Summary and Conclusions

References

6 CONCLUSIONS AND RECOMMENDATIONS

Before and During TMDL Development (Chapter 4)

TMDL Implementation and BMPs (Chapter 5)

Concluding Comments

APPENDIXES

A LEGISLATIVE REQUEST

Sec. 11520. Study on Stormwater Best Management Practices

B STUDY COMMITTEE BIOGRAPHICAL INFORMATION

Boxes, Figures, and Tables

BOXES

1-1 Legislative Text Calling for this Study

1-2 Statement of Task

2-1 Common TMDL Model Workflows

4-1 Contaminant Source Tracking Methods

4-2 Examples of USEPA Nonpoint Source Success Stories with Highway Involvement

5-1 BMP Performance for Solids, Bacteria, Nutrients, and Metals

5-2 Organizations with Testing Protocols for MTDs

FIGURES

1-1 Example watershed where shares of pollutant loads are ascribed to individual land uses by a TMDL

1-2 In addition to the pollutants that are generated on highways from vehicles, highways and their stormwater conveyance systems often serve as collectors for runoff and pollution that originates from beyond the highway environment

1-3 Climate regions of the contiguous United States

1-4 Annual average precipitation levels and temperatures in the contiguous United States, 1991–2020

2-1 The hydrologic cycle depicting interconnected forms of water and their interaction with human land uses

2-2 Relationship between water quality standards, water quality assessments, TMDLs, and permits

2-3 Existing pollutant loads from various land uses, the loading capacity determined by a TMDL, and load allocations to different land uses in a hypothetical watershed

2-4 A BMP collecting and treating stormwater after a rain event in a basin next to a highway in Northern Virginia

2-5 Unit treatment process effectiveness based on structural BMP type

2-6 Different spatial resolutions of stormwater models

2-7 Conceptual components of a stormwater model to generate pollutant loads and wasteload allocations

3-1 A highway traversing different land uses within a hypothetical watershed

3-2 TMDL review and potential implementation process

3-3 A bioswale BMP designed to manage stormwater from a section of Interstate 95 in Pennsylvania

3-4 Basic BMP effectiveness monitoring: Measure and compare inflow and outflow water quality

3-5 Example precipitation, stormwater flow rate, and total copper (Cu) concentration and loading rate data from a BMP in Southern California during a 1-inch storm on March 13, 2025

3-6 Example BMP influent (left) and effluent (right) monitoring stations along Lorton Road in Northern Virginia

4-1 A streamflow-nitrate (NO₃-N) concentration plot from a high-temporal-resolution sensor in a stream within a medium-density catchment dominated by local road runoff in Chapel Hill, North Carolina on January 23, 2018

4-2 Water protection opportunities in the Jordan Lake Watershed

5-1 An aerial view of a bioswale and a grass swale both draining into a central bioretention (center) along a highway in Northern Virginia

5-2 Boxplots of BMP influent and effluent event mean concentrations of total zinc and total suspended solids (TSS) from the International BMP database from BMPs receiving stormwater from highway land uses

5-3 Preliminary results from Spokane, Washington, showing boxplots of 6PPD-quinone concentrations (a tire wear–derived chemical) in influent and effluent water from two unvegetated bioretention BMPs

5-4 Percentage of the land area of the contiguous 48 states where a much greater than normal portion of total annual precipitation has come from extreme single-day precipitation events

5-5 Example of visual maintenance checklist for a bioretention basin as provided in a state DOT maintenance guide

5-6 Data from a real-time monitoring and control system in South Bend, Indiana

TABLES

1-1 Agencies and Organizations from 15 States Consulted

3-1 List of Highway Stormwater BMPs and Primary Functions

5-1 BMP Performance Statistics from Highway Drainage Areas

Preface

While not always evident to casual observers, stormwater infrastructure plays an important role in keeping roads safe and protecting water resources. Road surfaces are one of many land uses across a watershed that accumulate and mobilize pollutants through stormwater runoff. Increased understanding about the movement and effects of stormwater pollutants enables improved management strategies, which must evolve and adapt to external changes. Management approaches now include means to reduce pollutant sources, stormwater runoff, and mobilized pollutant loads. Nevertheless, some highway stormwater pollutants are not adequately removed by current management strategies, and there is a continuing need for improvements in the management of traditional contaminants (e.g., sediment, metals, nutrients), contaminants of emerging concern (e.g., tire wear particles and associated chemicals), and their varied sources.

Congress called for this study in Section 11520 of the Infrastructure Investment and Jobs Act of 2021. The legislative text seeks recommendations about methods that state transportation departments can use to evaluate and devise stormwater management compliance strategies and on ways that the U.S. Department of Transportation can assist states in carrying out and communicating these strategies. In accordance with this legislative direction, the Federal Highway Administration commissioned this study.

To conduct the study, the National Academies appointed an interdisciplinary committee of 11 members with expertise in stormwater management, civil and environmental engineering, green infrastructure, hydrology and hydraulics, water resources engineering and management, and environmental policy. Committee members’ biographical information is contained in Appendix B.

Acronyms and Abbreviations