Jump to main content or area navigation.

Contact Us

Water: Total Maximum Daily Loads (303d)

What is a TMDL?

 Overview of Impaired Waters and Total Maximum Daily Loads Program


What is a TMDL?

TMDL is the acronym for Total Maximum Daily Load.

A TMDL is a calculation of the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards, and an allocation of that load among the various sources of that pollutant.  Pollutant sources are characterized as either point sources that receive a wasteload allocation (WLA), or nonpoint sources that receive a load allocation (LA).  Point sources include all sources subject to regulation under the National Pollutant Discharge Elimination System (NPDES) program, e.g. wastewater treatment facilities, some stormwater discharges and concentrated animal feeding operations (CAFOs).  Nonpoint sources include all remaining sources of the pollutant as well as anthropogenic and natural background sources.   TMDLs must also account for seasonal variations in water quality, and include a margin of safety (MOS) to account for uncertainty in predicting how well pollutant reductions will result in meeting water quality standards.

The TMDL calculates the maximum amount of a pollutant allowed to enter a waterbody so that the waterbody will meet and continue to meet water quality standards for that particular pollutant and allocates that load to point sources, (Wasteload Allocation or WLA), and nonpoint sources(Load Allocation or LA), which include both anthropogenic and natural background sources of the pollutant. TMDLs must also include a margin of safety (MOS) to account for the uncertainty in predicting how well pollutant reduction will result in meeting water quality standards, and account for seasonal variations.

The TMDL calculation is:


where WLA is the sum of wasteload allocations (point sources), LA is the sum of load allocations (nonpoint sources and background), and MOS is the margin of safety.

By regulation, each pollutant causing a waterbody to be impaired or threatened is referred to as a waterbody/pollutant combination, and a TMDL is developed for each waterbody/pollutant combination. For example, if one waterbody is impaired or threatened by three pollutants three TMDLs will be developed for the waterbody. However, in many cases, the word TMDL is used to describe a document that addresses several waterbody/pollutants combinations (i.e., several TMDLs exist in one TMDL document). More and more states are bundling TMDLs on a watershed scale.

Although regulations refer to a TMDL as strictly a calculation or formula used to address one pollutant in one particular part of a waterbody, as the 303(d) program has evolved, the concept of a TMDL has become more expansive both in scale and content for many states.

The regulations do not define the scale of TMDLs and states have been developing TMDLs on a waterbody/pollutant basis or have grouped several waterbody/pollutant combinations in larger watershed-scale analyses. Some states are also doing very large scale TMDLs that cover multiple watersheds.

Overall, the goal of developing a TMDL is to end up with an implementation plan or a watershed plan designed to meet water quality standards and restore impaired waterbodies. These plans can build on information in one or more TMDLs. Although not required by regulations, many states include in their TMDL analyses some or all of the elements of these implementation plans.

Top of page


When is a TMDL required?

According to the Clean Water Act, each state must develop TMDLs for all the waters on the 303(d) list. It is at the discretion of states to set priorities for developing TMDLs for waters on the 303(d) list. Timelines for the completion of TMDLs vary across the country. In approximately 20 states EPA is under court orders and consent decrees to ensure TMDL completion; required timelines range from five to fifteen years.

Top of page


Who is responsible for developing TMDLs?

Under the CWA, states are responsible for developing TMDLs and submitting them to EPA for approval and many states have taken the lead in developing all of their TMDLs. In some states, nonprofit statewide environmental groups or watershed organizations have taken on significant responsibility in the development of the TMDL document and supporting analysis, known as a third-party TMDL. These TMDLs must still be submitted to EPA by the states.

EPA reviews and issues approval or disapproval of all TMDLs. EPA has developed many TMDLs itself in response to court orders or upon requests from states.

Top of page


How are TMDLs developed?

The objective of a TMDL is to determine the loading capacity of the water body and to allocate that load among different pollutant sources so that the appropriate control actions can be taken and water quality standards achieved. The TMDL process is important for improving water quality because it links the development and implementation of control actions to the attainment of water quality standards.

TMDLs are developed using a range of techniques, from simple mass balance calculations to complex water quality modeling approaches. The degree of analysis varies based on a variety of factors including, the waterbody type, complexity of flow conditions, and pollutant causing the impairment.

All contributing sources of the pollutants (point and nonpoint sources) are identified, and they are allocated a portion of the allowable load that usually requires a reduction in their pollution discharge in order to help solve the problem. Natural background sources, seasonal variations and a margin of safety are all taken into account in the allocations.

The approach normally used to develop a TMDL for a particular waterbody or watershed consists of five activities:

Summary of TMDL Development Activities

  • Selection of the pollutant to consider
  • Estimation of the waterbody assimilative capacity (i.e., loading capacity)
  • Estimation of the pollutant loading from all sources to the waterbody
  • Analysis of current pollutant load and determination of needed reductions to meet assimilative capacity
  • Allocation (with a margin of safety) of the allowable pollutant load among the different pollutant sources in a manner that water quality standards are achieved

TMDLs must clearly identify the links between the waterbody use impairment, the causes of impairment, and the pollutant load reductions needed to meet the applicable water quality standards.

Top of page


Public participation in the TMDL development

The Clean Water Act requires public involvement in developing TMDLs, however, the level of citizen involvement in the TMDL process varies by state. Typically, the state will circulate a draft 303(d) threatened and impaired waters list and draft TMDLs and allow 30 to 60 days for public comment. In some cases, hearings will be held as well.

Local citizens sometimes know more about what is happening in their watersheds than state agencies, and this knowledge can be a valuable aspect of TMDL development. The public often contributes useful data and information about an impaired waterbody. The public can often offer insights about their community that may ensure the success of one pollutant reduction strategy over another. Citizen information and participation can improve the quality of TMDLs that are developed and can ultimately speed cleanup of impaired waters or secure protection of threatened waters.

Public/stakeholder role in TMDL Process:

  • Provide data and information to the states
  • Review and comment on impaired water list
  • Review and comment on draft TMDLs
  • Assist in the development of 3rd party TMDLs

Top of page


What are the components of a TMDL document?

EPA issued review guidelines for TMDL submissions in Guidelines for Reviewing TMDLs Under Existing Regulations Issued in 1992 | PDF (7 pp 89K, About PDF) document. Below is a TMDL Review Checklist with the minimum recommended elements that should be present in a TMDL document.

Elements of a Typical TMDL Document:

  • Identification of Waterbody, Pollutant of Concern, Pollutant Sources, and Priority Ranking
  • Applicable WQS & Numeric Water Quality Target*
  • Loading Capacity*
  • Load Allocations and Waste Load Allocations*
  • Margin of Safety*
  • Consideration of Seasonal Variation*
  • Reasonable Assurance for PS/NPS
  • Monitoring Plan to Track TMDL Effectiveness
  • Implementation Plan
  • Public Participation

*Required by 40 C.F.R. Part 130

Top of page


TMDL review and approval

Under the CWA, states are primarily responsible for developing TMDLs. EPA is required to review and approve or disapprove TMDLs developed and submitted by states within 30 days. If EPA disapproves a state TMDL, EPA must establish such TMDL within 30 days.

What happens after the TMDL is developed?

TMDLs not self implementing under 303(d):

  • Point Sources
    • Permit limits consistent with WLA are enforceable under CWA through National Pollutant Discharge Elimination System (NPDES)
    • Issued by EPA or states w/ delegated authority
  • Nonpoint Sources
    • No federal regulatory enforcement program
    • Primarily implemented through state/local NPS management programs (few w/ regulatory enforcement)

Section 303(d) of the CWA does not specifically require implementation plans for TMDLs; however, it requires that wasteload allocations be implemented through the National Pollutant Discharge Elimination System (NPDES) permit program. After a TMDL has been developed, water quality-based discharge limits in NPDES permits authorized under CWA section 402 must be consistent with the assumptions and requirements of the WLA. Additional information on NPDES permits is available on EPA's NPDES Web site.

Load allocations (LAs) are implemented by nonpoint sources through a wide variety of state, local, and federal programs (which may be regulatory, non-regulatory, or incentive-based, depending on the program), as well as voluntary action by citizens. For example, CWA section 319 establishes EPA's nonpoint source management program. As part of this program, states receive grant money, and often pass the funding along to counties and other local groups, to support a wide variety of activities for managing nonpoint sources. Additional information on nonpoint source and 319 funding is available on EPA's Nonpoint Source web site (www.epa.gov/owow/nps). Information on state-specific nonpoint source management activities may be obtained by contact the state's water quality management agency.

Although states are not required under section 303(d) to develop TMDL implementation plans, many states include implementation plans with the TMDL or develop them as a separate document. When developed, TMDL implementation plans may provide additional information on what point and nonpoint sources contribute to the impairment and how those sources are being controlled, or should be controlled in the future.

Top of page

Jump to main content.