TMDLs in Chester County

"A TMDL is a calculation of the maximum amount of a pollutant that a waterbody can receive and still safely meet water quality standard"
-- U.S. Environmental Protection Agency

15 Total Maximum Daily Loads (TMDLs) are in effect or under development for the watersheds of Chester County. Those watersheds cover about 75% of the County's land area.

A TMDL is developed when a stream segment(s) or watershed has been assessed by PADEP and listed as impaired (not meeting water quality standards). This list (contained in "2010 Pennsylvania Integrated Water Quality Monitoring and Assessment Report") is updated approximately every two years and contains target dates for the development of a TMDL for impaired stream segment(s). Because the assessment process by PADEP is continuous, watersheds currently with very few or no impaired streams may become impaired in the future, most likely requiring that a TMDL be developed.
West Branch Brandywine Creek, West Brandywine Township Photo by Michael Lawrence
Working toward meeting water quality standards once a watershed or drainage area has been degraded is costly and regulations can be onerous. The best long-term strategy to meet water quality standards is to put initiatives in place to protect streams, before they become degraded.

The goal of a TMDL is to reduce the pollutant load from the contributing land area so that water quality standards can be met in the future. To improve the waters of Chester County, it is essential for communities to understand how they contribute to the impairments of streams.

Resources to Improve Implementation of TMDLs
Through the National Pollutant Discharge Elimination System (NPDES) permit program, municipalities in TMDL watersheds may be held responsible for meeting specific TMDL requirements. Municipalities, therefore, need to know which TMDLs are in effect and what their responsibilities may be toward meeting a pollutant load reduction. To assist with this, several resources are available here:

  • CCWRA Online Map Gallery (interactive online maps)
    There are online mapping tools which provide information on the TMDLS that are in effect for the different watersheds in Chester County. The "Watersheds with TMDLs" map and the "MS4 Pollution Reduction Tabbed Viewer" are the two tools that have information on TMDLs. The "Christina River Basin Subbasins with TMDL Load Reductions" identifies subbasins with nutrient or sediment load reductions.
  • Chesapeake Bay TMDL
    In Chester County, 6 watersheds, comprised of 142 square miles of land area, ultimately drain to the Chesapeake Bay Basin. These 6 watersheds include portions of 20 municipalities of Chester County.
  • Christina River Basin TMDL
    There are 3 TMDL Reports that have been developed for the Christina Basin in Pennsylvania and Delaware which set pollutant limits on nutrients and sediment. are provided.

Background Information



Additional Maps



GIS Coverages


  • GIS shapefiles are posted in the CCWRA GIS Files - Directory for Urbanized Areas, 2016 Integrated List (Impaired Streams), and Christina River Basin TMDL subbasins and stream impairments used to develop the TMDLs.

Other Resources


  • "Trout Creek TMDL Report" (not available on EPA/PADEP websites) Addresses impairment from one point source located in Montgomery County, PA.