Air & Waste Management Association (A&WMA)

What Every Environmental Professional Needs to Know About TMDLs


Much attention has been focused lately on the total maximum daily load (TMDL) program under Section 303(d) of the Clean Water Act (33 U.S.C.  § 1313(d)). This program has generated considerable controversy, largely because it is perceived that it will lead to control requirements for agricultural operations and other “nonpoint” sources that have not been regulated before under the act. (The Clean Water Act’s requirements generally apply to point sources, as defined in 33 U.S.C. § 1362(4).  See also 33 U.S.C. § 1311(e).) Yet, the most immediate and direct impact of the TMDL program will likely be felt by pointsource dischargers that operate under National Pollutant Discharge Elimination System (NPDES) permits. Over time, it is likely that many, if not most, of the effluent limits in NPDES permits will be based on water quality-based determinations made by states and the U.S. Environmental Protection Agency (EPA) in developing and implementing TMDLs.  These TMDL determinations will often be made years before the sources’ next permits are issued. This means that the dischargers will need to treat the TMDL development process as the initial stage of their permitting procedures, and should make plans to participate to the extent needed to fully protect their interests, just as they would if their permit limits were being issued.



The Clean Water Act contains two different control programs, one based on technology factors and another based on water quality concerns. Under the technology-based program, discharge limitations are set for various categories of industrial and municipal dischargers based entirely on the levels that can be achieved by those sources. Under the water quality programs, dischargers can be subject to control requirements beyond the technology-based limitations, if those additional control measures are needed to bring water quality to (or maintain it at) acceptable levels.  The TMDL program is an integral part of the water quality program. In this program, states first define the acceptable levels of water quality by adopting water quality standards, which consist of designated uses for the water body (e.g., swimming, boating, fishing) and criteria (e.g., numeric levels for particular pollutants) that are necessary to protect the designated uses. States then assess their waters, through sampling and review of other information, to determine whether those waters meet the applicable water quality standards. If a water body does not meet a standard, then it is placed on the state’s 303(d) list as “impaired,” and the state must develop a TMDL for that water body and pollutant. These steps are described below.

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