6

Conclusions and Recommendations

This report discusses how state departments of transportation (DOTs) strive to meet their responsibilities for managing the contaminants in stormwater discharged across their highway networks in compliance with the federal Clean Water Act. Given the wide array of topographies, watersheds, land uses, and jurisdictions that highway systems traverse, these public agencies face many challenges in fulfilling these responsibilities, as described in this report based on the study committee’s consultations with a varied array of highway agencies, other state and local authorities, academic experts, researchers, consultants, and advocacy groups.

These wide-ranging consultations informed the study committee’s conclusions (as documented in Chapter 4) about key challenges that state agencies face when developing total maximum daily load calculations that highway agencies can reasonably be expected to comply with once translated into National Pollution Discharge Elimination System permit conditions. The consultations also informed the committee’s conclusions (as documented in Chapter 5) about the challenges that state DOTs face in meeting these permit conditions by using best management practices, including issues related to their design, performance monitoring, construction, and maintenance.

In reviewing these challenges, and after identifying opportunities to address them, the committee reached a number of conclusions concerning the methods and practices used for devising, selecting, and carrying out highway stormwater management strategies. A recap of these conclusions is presented next followed by recommendations mainly to the Federal Highway Administration (FHWA), U.S. Environmental Protection Agency (USEPA), and state

agencies for steps that can be taken to improve information, policies, and practices to address key challenges that state DOTs face in managing highway stormwater and complying with Clean Water Act requirements.

BEFORE AND DURING TMDL DEVELOPMENT (CHAPTER 4)

Using Models for TMDL Calculations that Represent the Highway Environment

Conclusion 4-1: Stormwater discharges and associated loads from highway outfalls can be highly variable by location and over time. Furthermore, these discharges are often inconsistently monitored across a total maximum daily load watershed, creating uncertainty about a highway’s contribution of pollutants to an impaired water body. While advances in stormwater monitoring technologies and capabilities, including high-resolution sensors, source tracking technologies, and toxicity analysis, have the potential to expand and enrich the data available for understanding highway stormwater contaminant sources, loads, transport, and impacts, their cost and complexity can present practical challenges for application by state highway and environmental agencies.

Conclusion 4-2: While controlling pollutants at their source before they contaminate highway stormwater can be an effective and efficient means of preventing the pollution of water bodies, tracing pollutant sources can be resource intensive and involve the use of modeling techniques that have limited accuracy for the varied and distributed land use types that characterize the highway environment. Limitations in contaminant tracing and modeling accuracy can be especially problematic for state departments of transportation subject to total maximum daily loads covering pollutants that contribute to impairment but do not originate from highways or highway activities.

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 many 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.

Eliciting Community and Highway Agency Views During TMDL Development

Conclusion 4-3: The early and sustained engagement by environmental agencies of state departments of transportation and local communities during the development of total maximum daily loads (TMDLs) can improve the accuracy and reliability of the TMDL by incorporating highway agency expertise and citizenry knowledge of the transportation system and by increasing the potential for sustained public support for management practices and interventions that can take many years to restore impaired waters.

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.

Incentivizing More Comprehensive and Holistic Strategies for Water Quality Management

Conclusion 4-4: While total maximum daily loads (TMDLs) are usually developed for individual pollutant and water body combinations, the overall quality and ecological health of a watershed may be managed more effectively by using integrated, watershed-scale interventions, especially when impairments are driven primarily by discharges containing multiple (i.e., co-occurring) contaminants from multiple sources. To the extent allowed and facilitated by the U.S. Environmental Protection Agency, the use of watershed-scale alternatives to pollutant-specific TMDLs, such as Category 4b demonstrations and Category 5r designations (“Straight to Implementation”),

can have the advantage of enabling earlier actions by parties to control pollutants, including those that are highway-sourced but may not be subject to a TMDL or National Pollutant Discharge Elimination System permit (e.g., tire and brake wear particles and associated chemicals, highway deicing salts).

Recommendation 6-3: The U.S. Environmental Protection Agency (USEPA) should explore options for state departments of transportation (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 FHWA and watershed organizations that have expertise in holistic water resource management, the U.S. Environmental Protection Agency should develop guidance that is specific to state departments of transportation (DOTs) for participating in watershed-scale total maximum daily load (TMDL) alternative approaches that are available within existing regulatory frameworks for managing highway stormwater, such as Category 4b demonstrations and Category 5r designations. State DOT 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 Category 4b demonstrations for prioritizing waters that are impaired primarily by stormwater-driven sources;
• Means of incentivizing 4b demonstrations 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 4b demonstration plans and advice on how to develop and implement collaborative partnerships among permittees to achieve water quality restoration goals.

TMDL IMPLEMENTATION AND BMPS (CHAPTER 5)

Facilitating Permit Compliance Capability

Conclusion 5-1: Translating total maximum daily load (TMDL) guidance into clear and enforceable National Pollutant Discharge Elimination System permit conditions that highway agencies can reasonably be expected to comply with requires that TMDLs be written with compliance in mind. However, TMDL developers and permitting authorities may not routinely collaborate with highway agencies and other permittees when translating TMDLs into permit conditions and best management practice requirements; this can lead to interventions that are not informed by knowledge about highway contributions and that are rigid and difficult to revise when they do not work as intended. An evolution to more iterative and adaptive approaches can allow state departments of transportation to evaluate and adjust their stormwater management strategies to improve water quality outcomes in line with meeting their core safety and mobility missions while complying with other regulatory and policy interests. While reopening TMDLs to enable such adaptive management can be arduous, finding ways to develop TMDLs and permit conditions that adapt to the best available data and reflect the complexities of the highway environment can be essential to attaining water quality goals.

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 departments of transportation.

Supporting BMP Performance Assessment

Conclusion 5-2: Highway stormwater best management practice (BMP) performance and life-cycle cost data are needed for highway agencies to make evidence-based choices about the most appropriate BMP(s) to use

for specific contaminants and locations. These data will enable departments of transportation (DOTs) to make more reliable predictions for National Pollutant Discharge Elimination System permit compliance, design resilient BMPs, understand contaminant fate and transport in BMPs (including surface water and groundwater), and optimize BMP maintenance and asset management programs to maintain performance. Performance data are needed in accordance with the BMP’s intended functions and at different temporal and geographic scales (e.g., the levels of an individual outfall, receiving water, or watershed). However, limited data sharing and accessibility, lack of consistently applied performance metrics, limited funding for monitoring, and gaps in performance records for many commonly used BMPs are hindering such informed decision-making. Advances in sensors for performance monitoring have potential to help fill some information gaps. However, current sensor costs, maintenance requirements, and required technical capacity could slow deployment by state DOTs and they are not applicable to all highway-related pollutants.

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.

Informing BMP Design, Selection, Siting, and Maintenance

Conclusion 5-3: Highway stormwater managers face constraints when selecting the type of best management practice (BMP) to use and its location, requiring considerations about not only the targeted pollutants but also topography and soils, the characteristics of the landscape adjacent to the roadway, and the alignment and available space in the highway right-of-way. BMP selection and siting will also be affected by presence of protected resources, the need to ensure the safety of road users and maintenance workers, as well as community acceptance, especially in urban and suburban settings. Where space efficiency is needed or conditions are otherwise not suitable for conventional stormwater controls, compact and below-ground manufactured treatment devices may be options, including separation of water quantity and quality BMPs. Thinking more broadly and creatively, however, state departments of transportation may collaborate with other jurisdictions and authorities to take advantage of off-site treatment options (including mitigation banking), design BMPs to function as community assets, and make space for stormwater management and treatment when parks and other open spaces are created as part of major highway redesigns to provide community benefits.

Conclusion 5-4: Highway agencies need a workforce that is trained in a range of highway stormwater management topics and informed and guided by procedures and manuals that account for regional variations and factors such as rainfall and temperature patterns, resiliency needs, and National Pollutant Discharge Elimination System and other stormwater-related permit requirements. Regionally appropriate guidance and training must also be clearly communicated and based on the latest knowledge about best management practice (BMP) performance. However, because research results are continually being produced to inform BMP design, construction, and maintenance, updates in needed guidance, regulations, design manuals, and training curricula in many states and other jurisdictions can lag the state of knowledge.

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 in this report are directed mainly toward FHWA, USEPA, and state agencies. 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.