Jump to main content.

TMDL Primer

Chesapeake Bay TMDL Quick Finder

Chesapeake Bay TMDL Home Calendar of Events Contact Us Bay TMDL Fact Sheets & Key Documents Frequent Questions How Does it Work? Ensuring ResultsPartners' Links Press Room Subscribe to Recent Chesapeake Bay TMDL NewsSubscribe to Recent News
View State Impaired Waters ListsExit EPA Click for Disclaimer

The Clean Water Act sections 305(b) and 303(d) require that States assess their waters, determine whether waters meet water quality standards, and establish a schedule to restore those waters that are impaired.

What is a TMDL?

TMDL stands for Total Maximum Daily Load. The TMDL represents the maximum amount of a pollutant allowed to enter a waterbody by law so that the waterbody will meet and continue to meet the water quality standards for that particular pollutant. Pollutants are anything that prevents a waterbody from attaining the national goal of being "fishable and swimmable." Common pollutants include sediment, metals (often from mining activities), toxic chemicals, fecal coliform bacteria, pH, and excessive nutrients.

Section 303(d) of the Clean Water Act and regulations developed by EPA require states to identify all waters that do not meet water quality standards even after pollution controls required by law are in place. Waterbodies not meeting the appropriate water quality standards are considered to be impaired. The Impaired Segments identified by the states comprise each state's 303(d) list (or Section 303(d) list). The 303(d) list of impaired waters must be submitted to EPA for review and approval. TMDLs must be developed for all waterbodies on the approved 303(d) list.

In order to develop TMDLs for impaired waters, the state must calculate how much of the pollutant causing the impairment can enter a waterbody without exceeding the water quality standard for that particular pollutant. The calculated pollutant quantity is then distributed among all the pollutant sources. The pollutant quantity is known as the TMDL and is the sum of the individual wasteload allocations (WLAs) for point sources (e.g., sewage treatment plant and industrial discharges), load allocations (LAs) for nonpoint sources (e.g., pollutants carried by rainfall runoff from forests, agricultural lands, and abandoned mine lands) and natural background levels, and a safety factor to maintain the integrity of the water sources (margin of safety (MOS)). TMDLs can be developed to address individual pollutants or groups of pollutants and must clearly identify the links between the waterbody use impairment, the causes of the impairment, and the pollutant load reductions needed to meet the applicable water quality standards. TMDLs developed for impaired waters are to be submitted to EPA for review and approval. If EPA disapproves a state-developed TMDL, then the Clean Water Act states that the EPA must complete the TMDL.

TMDLs are used as planning tools by states to develop specific methods, or controls, used to meet water quality standards in the impaired waterbody.

Top of page

What is a Water Quality Standard? -

The Clean Water Act sets national minimum goals for all waters as "fishable and swimmable." To support this goal, states must adopt water quality standards. Water quality standards are state regulations. The main components of the water quality standards are as follows:

If a waterbody does not meet one or more of the components of a water quality standard, then the waterbody is considered to be exceeding that standard.

Top of page

How Are TMDLs Developed?

TMDLs are developed for the most sensitive environmental conditions (i.e., stream flow, temperature, weather conditions) using data from reliable sources compiled in a particular geographic area or watershed. Scientifically-accepted mathematical methods that represent what is happening in nature are used to develop the TMDL. Computer-based models, such as EPA's BASINS or EPA Region 3's MDAS, are used for the more complex situations to predict how certain pollutants behave in the waterbody. A GIS (geographic information system) format may also be used to provide the framework for the model's information, such as land use distributions and the locations of flow gages and water quality monitoring stations. Data and information may include the relative contribution of various point and nonpoint sources of pollution in the watershed. All of this information is then used to develop and propose a TMDL that is appropriate for the specific waterbody.

Top of page

What Does a TMDL Propose?

A draft TMDL will propose pollutant reductions from specific point (i.e., municipal wastewater treatment facilities and industrial waste treatment plants) and nonpoint sources (i.e., direct runoff from agricultural lands, urban areas, and forested lands) in the watershed. TMDLs will look at impacts on waterbodies from many kinds of contamination, while considering seasonal variations and background levels of pollutants (those levels naturally occurring in nature). TMDLs compare current pollutant loads and proposed pollutant loadings with expected levels of reduction to achieve the water quality standards. The reductions are proposed for those pollutant sources in which real changes can occur.

How Are TMDLs Implemented?

The TMDL program does not provide for any new implementation authorities. A state must implement the point source component of TMDLs through existing federal programs with enforcement capabilities (e.g., the National Pollutant Discharge Elimination System [NPDES]). Nonpoint source controls (e.g., Best Management Practices [BMPs]) required by a TMDL can be implemented through a voluntary approach or some state or local regulations or other authorities. A specific implementation plan is not required as part of an approved TMDL. The approved TMDL is a tool to be utilized in implementing pollutant controls.

Top of page

What About the Public?

The Clean Water Act provides for any individual or organization potentially impacted by the development and implementation of a TMDL to participate in the procedures. Either a state or EPA must provide a method by which local concerns are incorporated in the TMDL process. In EPA Region 3, the Water Protection Division holds public meetings for each TMDL project to explain the TMDL process and to get feedback from interested parties or stakeholders to ensure that the developed TMDLs accurately reflect all points of view. The public often contributes useful data and information about an impaired waterbody. The public can also often offer insights about their community that may ensure the success of one pollutant reduction strategy over another. EPA Region 3 posts all of its public meeting times and places in the most widely distributed local newspaper near the waterbodies undergoing TMDL development.

Top of page

EPA Water Home || Mid-Atlantic Water Home
Mid-Atlantic Water Topics A - Z

Local Navigation

water for kids

Jump to main content.