TMDL: Total Maximum Daily Loads

To improve the quality of waters of Chester County, it is essential for municipalities to understand how their actions contribute to the impairment of streams. The Chester County Water Resources Authority (CCWRA) has compiled a list of TMDLs for waters in Chester County. Other information regarding TMDLs within the county, can be found on the CCWRA's page for TMDLs.

Impaired waters are polluted or are otherwise impaired to the point where they do not meet state water quality standards. One of the measurements used to determine the level of a waterway's level of impairment is its Total Maximum Daily Load, or "TMDL", which is the maximum amount of a pollutant that a water body can receive and still safely meet water quality standards. A TMDL establishes a water body's "loading capacity", so that the water body will meet and continue to meet water quality standards for that particular pollutant. The TMDL also allocates the pollutant load to point sources such as wastewater treatment plant discharges and to nonpoint sources such as agricultural runoff. The allocations reflect both societal and human induced impacts and natural sources of the pollutant.

TMDLs work differently in different landscapes. The type of TMDL designated for a stream segment will require different efforts to remove that pollutant, depending on the cause of the impairment. Fifteen TMDLs are in effect or under development for Chester County's watersheds, which will account for approximately 75 percent of the County's land area. The assessment process by PADEP is continuous, and therefore, watersheds currently with few or no impaired streams may become impaired in the future, and may require a TMDL to be developed. A map of Chester County's watersheds that have TMDLs is available online.


Addressing TMDLs offers the following benefits:


The following limitations may be associated with the implementation of TMDLs:

How to Use this Tool

tmdlThere are a variety of ways to implement TMDLs locally. Municipalities and residents can cooperate to protect waterways through regulatory controls such as stormwater management ordinances, riparian buffers, Best Management Practices, agricultural conservation plans, steep slope and woodland protection and erosion and sedimentation provisions. Comprehensive planning and regularly updated zoning and subdivision ordinances can also create the framework for municipalities to improve water quality while meeting regulatory requirements. The protection and improvement of water quality is a key component for natural resource preservation and the quality of life that residents of Chester County enjoy.


While most of the TMDLs in our area are newly established and are beginning to be implemented, they have proven to be effective in other parts of the country. The Carrier Creek in Michigan was severely impacted by increased impervious cover and, as a result, increased urban runoff was affecting the quality and quantity of stormwater being conveyed by the creek. The TMDLs for the Carrier Creek led to the installation of in-stream structures and the stabilization of stream banks and have resulted in the restoration of the stream, improved water quality, and decreased stream sedimentation.

In Chester County, the development of the Christina Basin TMDL has lead to a multi-municipal partnership, headed by the Brandywine Valley Association that includes 30 participating municipalities and various county agencies.

Related References

Related Tools