Executive Summary

This report documents the results of a study, requested by Congress and commissioned by the Federal Highway Administration (FHWA), of how state departments of transportation (state DOTs) manage pollutant loads in highway stormwater runoff to comply with the federal Clean Water Act. The law’s regulations, which are administered by the U.S. Environmental Protection Agency (USEPA), require state environmental authorities to identify each water body that does not meet a water quality standard, determine the acceptable total maximum daily load (TMDL) for a pollutant to attain the standard, and allocate load reduction targets to specific discharge sources through the National Pollutant Discharge Elimination System (NPDES) permitting program. All state DOTs are subject to NPDES permitting because highways are both a source of and conveyance system for pollutants to water bodies through stormwater runoff and drainage outfalls. This report focuses on highways but acknowledges that the permitting and NPDES framework are designed for a broad array of parties within a watershed that discharge to waterbodies and are not customized to any one party.

The report—which was developed by a committee of experts in highway stormwater management, water resources engineering, hydrology, and other related fields—reviews the data sources and methods commonly used by USEPA and state environmental authorities to estimate and monitor pollutant loads, determine load sources, and allocate discharge allowances to these sources. For reasons that are explained, state DOTs face challenges in

complying with the NPDES permits that are established for their extensive highway systems that can cross numerous watersheds, topographies, land uses, and jurisdictions. The study committee’s consultations with highway stormwater managers from a regionally diverse set of states revealed the nature of these challenges, as well as many opportunities for state DOTs—especially with support from USEPA and FHWA—to address them through appropriately designed and wellimplemented stormwater best management practices (BMPs) and other means.

STUDY CHARGE

The first element of the study charge calls for a review of how maximum pollutant loads are calculated to inform the development of a TMDL applicable to a single pollutant and individual water body. The realism and accuracy of these calculations, as informed by stormwater runoff monitoring and modeling and by tracking contaminant sources, can have significant implications on the load limitations allocated to state DOTs and whether and how they can be met. An understanding of the highway environment by TMDL developers can be crucial to this accuracy and realism and to decisions about whether alternative means of water quality management are preferable to a TMDL, including watershedbased approaches to management.

The second element of the study charge calls for a review of how a TMDL is implemented, first by the creation of permits that authorize state DOTs to discharge pollutants into a water body and then by requiring state DOTs to adhere to these limits through the use of BMPs and other means of pollution control and prevention. Because TMDLs are the main approach for restoring water quality, the translation of TMDLs by USEPA and state authorities into permit conditions with which state DOTs can reasonably comply is a firstorder priority. BMPs are central to state DOT stormwater compliance strategies and bring their own complexities and challenges in their selection, design, siting, maintenance, and performance monitoring and assessment.

The third element of the charge asks that consideration be given to the means by which the federal government, including USEPA and FHWA, can assist in the communication and dissemination of best practices for highway stormwater management. Informed by the findings and conclusions reached from this threestage review, the committee is tasked with making recommendations regarding practices for devising, selecting, and carrying out stormwater management compliance strategies and on the federal government’s role in promoting best practices. The study’s key conclusions and recommendations for the fulfillment of this charge are summarized next.

CONCLUSIONS AND RECOMMENDATIONS

The study surfaced numerous findings about challenges and opportunities for effective highway stormwater management before and during the development of TMDLs and during TMDL implementation via the development of permit conditions and use of BMPs. These findings led the committee to reach the following conclusions, which are detailed in Chapters 4–6, about key needs and to propose means of addressing them through a series of recommended actions.

Before and During TMDL Development (Chapter 4)

Need for Models for TMDL Development that Represent the Highway Environment

Recommendation 6-1: To support the use of, and continued advances in, water quality modeling techniques for total maximum daily load (TMDL) development that more accurately represent the complexities of the highway environment, including adjacent land uses and external sources of pollution, the U.S. Environmental Protection Agency should review and update its guidance on available modeling options and input data collection methods. The guidance should contain criteria for selecting models that will account for different land uses and the uncertainties they can create for calculating TMDL and National Pollutant Discharge Elimination System permit loading capacity and pollutant load allocations. The guidance should also take into account the challenges that state agencies often face in acquiring the expertise and resources needed to validate models with representative empirical monitoring data.

Need to Elicit Community and Highway Agency Views during TMDL Development

Recommendation 6-2: The U.S. Environmental Protection Agency (USEPA) should encourage total maximum daily load (TMDL) developers to consult state departments of transportation, as well as other parties that have stormwater management responsibilities, early and regularly during TMDL development. Likewise, USEPA should encourage additional or expanded opportunities for public participation before the public comment period—for instance, by convening public meetings at the beginning of the TMDL development phase to gather input from community members and hear concerns specific to different parts of a watershed. While public involvement in developing TMDLs is

required by USEPA regulation, opportunities for public participation in the TMDL development process can be limited until the required public comment period after a TMDL has already been drafted.

Need to Incentivize More Comprehensive and Holistic Strategies for Water Quality Management

Recommendation 6-3: The U.S. Environmental Protection Agency (USEPA) should explore options for state departments of transportation (state DOTs) to prioritize the management of total maximum daily load (TMDL) pollutants that originate directly from the construction, operation, and maintenance of highways. For those TMDL pollutants that originate from highway operations through traffic activity (e.g., tire and brake wear contaminants such as 6PPD-quinone) and those TMDL pollutants that do not originate from highways and that state DOTs have limited control over (e.g., pollutants from adjacent land uses), USEPA should explore more effective source controls to accompany state DOT management. In addition, state DOTs should be encouraged to continually improve their monitoring of those pollutants that originate from or are mobilized by highway activity but that are not necessarily or consistently regulated (e.g., microplastics and deicing materials) to better inform management opportunities.

Recommendation 6-4: In collaboration with the Federal Highway Administration and watershed organizations that have expertise in holistic water resource management, the U.S. Environmental Protection Agency should develop guidance that includes specific considerations for state departments of transportation for participating in watershed-scale total maximum daily load (TMDL) alternative approaches that are available within existing regulatory frameworks for managing highway stormwater. Participation in these approaches should be viewed as a practical and potentially desirable alternative to the implementation of the traditional TMDL. The guidance should address

• The desirability of using alternative frameworks for prioritizing waters that are impaired primarily by stormwater-driven sources;
• Means of incentivizing alternative frameworks through model National Pollutant Discharge Elimination System permit language, such as for collaborative approaches that do not require load accounting methodologies; and
• Barriers to the preparation of alternative frameworks and advice on how to develop and implement collaborative partnerships among permittees to achieve water quality restoration goals.

TMDL Implementation and BMPs (Chapter 5)

Need to Facilitate Permit Compliance by State DOTs

Recommendation 6-5: To inform the development of total maximum daily loads (TMDLs) that are suited to the complexities and inherent uncertainties of the highway environment, the Federal Highway Administration and organizations that have expertise in stormwater management should collaborate with the U.S. Environmental Protection Agency on the creation of guidance for regulatory authorities on the development of TMDL implementation plans that are based on the principles of adaptive management, allowing for modifications based on collective experience and updated scientific consensus derived from changing conditions and information on pollutant sources, fate, and transport. The ensuing updated guidance could be aimed at a broader audience of permit holders in addition to state DOTs.

Need to Support BMP Performance Assessment

Recommendation 6-6: To improve the quality, coverage, availability, and stewardship of data on highway stormwater best management practice (BMP) performance and life-cycle cost, the Federal Highway Administration should partner with the U.S. Environmental Protection Agency and state departments of transportation through the American Association of State Highway and Transportation Officials to

• Develop guidance for highway agencies on monitoring and assessing BMP performance that includes standards for data gathering, sharing, reporting, and curating in accessible databases such as the International Stormwater BMP Database. This includes data collected for compliance with National Pollutant Discharge Elimination System permits.
• Make funding available for studies of more holistic BMP performance and the documentation of results in accessible databases, including the identification of data gaps and additional research needs, such as monitoring highway-specific emerging contaminants (e.g., 6PPD-quinone, microplastics, salt), BMP effects on surface water and groundwater, and BMP co-benefits (e.g., air quality or heat mitigation from green stormwater infrastructure).
• Investigate low-cost sensors that can be deployed by highway agencies for BMP performance and condition monitoring and assessment, including determining training requirements for agency staff to use them.

Need to Inform BMP Design, Selection, Siting, and Maintenance

Recommendation 6-7: To improve best management practice (BMP) design, construction, and maintenance procedures and guidance, the Federal Highway Administration should partner with the U.S. Environmental Protection Agency and state departments of transportation through the American Association of State Highway and Transportation Officials to

• Support the adoption of evidence-based and regionally relevant BMP design manuals and maintenance guidance using observations and data from BMP construction, performance, and maintenance;
• Develop regionally relevant workforce training guidance for BMP design, construction, inspection, and maintenance;
• Adopt more consistent stormwater infrastructure terminology; and
• Promote and provide funding as needed to support multidisciplinary peer exchanges, courses, and national practitioner forums for communicating and sharing information on BMPs, effective total maximum daily load development and implementation, permit negotiations, and integration with other environmental, landscape, and engineering elements.

CONCLUDING COMMENTS

The recommendations call for actions intended to help state DOTs better address challenges they face in meeting their stormwater management obligations while recognizing that other parties, including counties and municipalities, may face similar challenges. In this regard, the recommended actions may have broader applicability and benefits and are by no means meant to suggest that state DOTs, which were called out specifically for study, should receive special consideration.

The recommendations in this report are directed mainly toward FHWA, USEPA, and state DOTS and environmental authorities. The committee is fully aware, of course, that the request for this study originated in legislation. In many cases, the actions recommended will require a commitment of resources by the subject federal agencies and state agencies. In this regard, Congress may be called upon to help, in providing both the funding and direction for states to meet their highway stormwater management obligations under the Clean Water Act.