Approaches for Determining and Complying with TMDL Requirements Related to Roadway Stormwater Runoff (2019)

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1 Stormwater practitioners at state departments of transportation (DOTs) and other trans- portation agencies are faced with challenges of compliance with total maximum daily loads (TMDLs) due to the number of impaired water bodies and regulatory compliance deadlines. The state DOT highway system is unique in being a linear municipal separate storm sewer system (MS4) agency with a relatively small footprint scattered throughout the state and having generally limited impacts in a watershed. This NCHRP report provides guidance to state DOTs with the necessary scientific background and strategies to improve TMDL evaluations and assist in the decision-making process for the development of and compli- ance with TMDLs. This report equips the state DOT practitioner with practical information on how to determine and address TMDL requirements related to roadway stormwater runoff. An effective TMDL compliance strategy involves solutions within the state DOT right-of- way and watershed-based stormwater management efforts. This report lays out the foun- dation of various approaches for state DOT compliance with TMDLs by addressing the following objectives: • Analyze data, statistics, and information about stormwater runoff from roadways and other land uses or other pollutant sources. • Identify strategies for determining the significance and contribution of stormwater pollutants from roadways within a given watershed. • Identify practical feasibility of implementing traditional structural and nonstructural best management practices (BMPs). • Identify approaches for determining the relationship between performance and cost- effectiveness of BMPs. • Identify approaches for determining the efficiency and effectiveness of innovative water quality solutions—such as stormwater banking, pollutant trading, off-site mitigation, off-site source control, and other holistic compliance strategies—not commonly used by stormwater practitioners at state DOTs and other transportation agencies,. The groundwork for evaluating DOT compliance approaches first requires identifying pollutants that are being discharged from the highway environment into receiving water bodies. Most state DOT compliance analyses include determining the sources and loads of pollutants in highway runoff and then comparing the contribution of highway runoff loads to receiving waters relative to an applicable waste load allocation (WLA). If the state DOT’s contribution exceeds the applied WLA, the department must develop and imple- ment a strategy for reducing pollutant loads. Identifying loads enables the state DOT to S U M M A R Y Approaches for Determining and Complying with TMDL Requirements Related to Roadway Stormwater Runoff

2 Approaches for Determining and Complying with TMDL Requirements Related to Roadway Stormwater Runoff determine its contribution in comparison to other agencies and develop an effective strategy for achieving water quality compliance in the watershed. There are various methods to achieving TMDL compliance; however, the process of statewide pollutant categorization, BMP prioritization (structural or nonstructural), and watershed prioritization can provide the state DOT with enough quantification to imple- ment appropriate resources to a watershed with a greater benefit. In selecting BMPs for TMDL compliance, the state DOT may consider collaborative efforts and multibenefit proj- ects, evaluating the performance of selected BMPs, determining each BMP’s load reduction capability using applicable software tools and algorithms, comparing eligible BMPs and performance relationships, and assessing the costs and benefits of BMP implementation. Alternative compliance solutions include thinking beyond the right-of-way and exploring watershed-based approaches to determine the best and most effective options to mitigate the impact of stormwater stressors or pollutants. The state DOT practitioner should identify solutions with the greatest water quality and ecosystem benefits while considering the best stewardship of funds. Instead of constructing BMPs, the state DOT may consider partnering with other stakeholders through cooperative agreements, pursuing payment in lieu of con- struction BMPs, and sharing BMP construction, maintenance, and implementation costs. Developing a pollutant trading program, stormwater banking, restoration and preservation, and off-site compliance are also nontraditional alternatives to TMDL compliance.