Impaired Waters and TMDLs

Impaired Waters and Stormwater

Throughout the United States there are thousands of waters listed for impairments from stormwater sources. The most common pollutants coming from stormwater sources include sediment, pathogens, nutrients and metals. These impaired waters need a Total Maximum Daily Load (TMDL), which identifies the total pollutant loading that a waterbody can receive and still meet water quality standards, and specifies a pollutant allocation to specific point and nonpoint sources.

The TMDL is implemented in part via the National Pollutant Discharge Elimination System (NPDES) stormwater permitting system. States and EPA Regions have used a variety of methods to develop stormwater-source TMDLs during the past decade. With the expansion of NDPES stormwater regulations to smaller municipalities and smaller construction activities, there has been increasing demand for more detailed quantification of stormwater allocations in TMDLs that are more useful for implementation in NPDES permits.

Resources for Developing Stormwater-Source TMDLs

Total Maximum Daily Loads with Stormwater Sources: A Summary of 17 TMDLs, July 2007, EPA 841-R-07-002

Total Maximum Daily Loads and National Pollutant Discharge Elimination System Storm Water Permits for Impaired Water Bodies: A Summary of State Practices, Sept. 15, 2007

TMDL-Stormwater Permit Case Studies:

The Minnehaha Creek Watershed District (Minnesota) Nine Lakes TMDL (2005)

Innovative TMDLs using Surrogates for Stormwater Pollutants

Three states in EPA Region 1 have developed TMDLs using impervious cover and flow as surrogates for stormwater-source pollutants in streams listed for biological impairment.