State DOTs are increasingly subject to Total Maximum
Daily Load (TMDL) requirements for water quality improvement that are
implemented through National Pollutant Discharge Elimination System (NPDES)
permits.
As a result, state DOTs may incur significant costs to construct, operate,
maintain, and monitor performance of best management practices and other
stormwater treatment facilities that treat stormwater from sources outside the
right-of-way, as well as stormwater from roadway sources.
TRB’s National Cooperative Highway Research Program (NCHRP) Research Report
918: Approaches for Determining and Complying with TMDL Requirements Related to
Roadway Stormwater Runoff describes how to evaluate TMDLs and develop a plan to
comply with the requirements of a TMDL. The methods provide a robust approach to
determining the pollutants of concern and how to assess the contribution of the
roadway while understanding other important factors that affect overall
pollutant loads, including adjacent land uses and watershed conditions and
characteristics.
A set of presentation slides summarizing the project that developed the
report is available for download.