Research is complete. The final report is available HERE
State departments of transportation (state DOTs) are increasingly subject to water quality improvement requirements derived from §303(d) of the federal Clean Water Act (CWA) related to Total Maximum Daily Loads (TMDL). A TMDL is a calculation of the maximum amount of a pollutant (the “load”) that a water body can receive while still meeting water quality standards; the load is allocated among the various sources of that pollutant in order to reduce the amount and nature of pollutants discharged to water quality-impaired receiving water bodies pursuant to §303(d) of the CWA. Although substantial effort has been put into characterizing highway runoff and identifying the sources of pollutants, additional effort is needed to support the development of appropriate TMDL requirements and implementation plans that accurately reflect a state DOT’s water quality impacts and its water quality mitigation capabilities. TMDL requirements are often applied broadly to a number of land uses. In the absence of detailed comparative studies, the impacts on water quality from roadway stormwater runoff has been approximately equated with water quality impacts from stormwater runoff from municipal, commercial, and industrial land uses. The portion of the total pollutant load attributable to the roadway is frequently calculated in a general manner—based on assumptions about relative contributions from different land uses—and it is often the case that no distinction is made between pollutants that originate from roadway stormwater runoff and sources outside the right-of-way. As a result, state DOTs may incur unnecessary compliance costs to construct, operate and maintain, inspect and validate stormwater treatment facilities, and implement other Best Management Practices (BMPs). This project developed guidance for state DOTs to evaluate pollutant loads from stormwater runoff from roadways and other land uses to inform development of appropriate TMDL requirements, and to provide guidance for selecting potential stormwater management and TMDL compliance strategies within a watershed.