About TMDLs

What are TMDLs?

TMDLs are Total Maximum Daily Loads. A TMDL represents the maximum amount of a pollutant that can enter a water body so that the water body will meet and continue to meet state water quality standards. Section 303(d) of the Clean Water Act requires states to identify all waters that are not meeting their water quality standards. For these waters, the state must calculate how much pollution can be put in the water without violating the standard, and then distribute that quantity to all the sources. That quantity is the sum of the point sources plus the nonpoint sources plus a safety factor to maintain the integrity of the water sources. TMDLs serve as planning tools for developing specific controls needed to meet water quality standards.

How are TMDLs developed?

TMDLs are developed for the most sensitive environmental conditions, i.e., stream flow, temperature, weather conditions, etc., using data compiled in a particular geographic area or watershed from reliable sources. 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 system, are used for the more complex situations to predict how certain pollutants behave in the water body. A GIS (geographic information system) format may also be used to provide the framework for the model's information, particularly land use distributions. Data and information may include the relative contribution of different kinds of activities like point and nonpoint sources of pollutants. All of this information is then used to propose a TMDL that is appropriate for the specific water body.

What does a TMDL propose?

A TMDL will propose loads from specific point (such as municipal wastewater treatment facilities and industrial waste treatment plants) and nonpoint sources (such as direct runoff from agricultural lands, urban areas, forested lands, etc.) in the watershed. TMDLs will look at impacts on waterbodies from many kinds of contamination, will consider seasonal variations, and will account for background levels of pollutants (those levels occurring in nature). TMDLs compare current loads and proposed loadings with expected levels of reduction to achieve the water quality standards.

Why are we doing TMDLs?

The Sierra Club sued EPA Region 4 over TMDLs in Mississippi. A Notice of Intent was filed in June 1997. In December 1998, EPA Region 4 entered into a consent decree with the Sierra Club to have all TMDLs listed in the 1996 303(d) list completed within 10 years. As a part of the decree EPA Region 4 is required to complete any TMDLs that MDEQ does not complete. We have adopted the goal of doing all of the TMDLs for Mississippi.

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. The public may participate in the TMDL process through Mississippi’s Basin Management Approach. Also, anyone is welcome to comment on a TMDL during the public notice period for the TMDL. A 30-day public notice period will be held following the publication for each TMDL. Any comments received during the public notice period will become a part of the TMDL administrative record, and will be considered by MDEQ prior to finalizing the TMDL report.