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Indiana Department of Environmental Management


About TMDLs

TMDL Program Overview and Frequently Asked Questions

Who should read this?

This Indiana Total Maximum Daily Load (TMDL) Document is designed to assist anyone conducting watershed management projects, as well as those interested in learning about how state government works to improve and maintain the quality of Indiana’s lakes, rivers and streams. The intent of this information is to provide a clear, concise description of the TMDL development, review and approval processes.

What is a TMDL?

A TMDL is a calculation that determines the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards, as well as the allocation of that load among the various pollutant sources.

Pollutant sources are characterized as two types. Point sources are easily recognizable and that receive a wasteload allocation through regulation. Nonpoint sources include many different sources, usually difficult to track, and receive a load allocation. TMDLs must account for seasonal variations in water quality and include amargin of safety.

The TMDL calculation is:


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 that impairs or threatens a waterbody is called a waterbody/pollutant combination. 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 a calculation or formula used to address one pollutant in one particular part of a waterbody, the concept of a TMDL has become more expansive both in scale and content as the 303(d) program has evolved.

Current regulations do not define the scale of TMDLs, allowing states the flexibility to develop TMDLs on a single waterbody/pollutant or group together several waterbody/pollutant combinations. Some states are completing large scale TMDLs, covering multiple watersheds.

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

When is a TMDL required?

According to the Clean Water Act, each state must develop TMDLs for all the waterbodies on the 303(d) Impaired Waters List. Each state may prioritize waters on the 303(d) Impaired Waters List, in order to maximize resources.

Who is responsible for developing TMDLs?

Under the Clean Water Act, states are responsible for developing TMDLs and submitting them to U.S. EPA for approval. While many states develop 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 documents, known as a third-party TMDLs. These TMDLs must still be submitted to U.S. EPA by the states.

U.S. EPA reviews and issues approval or disapproval of all TMDLs. U.S. EPA has also developed many TMDLs in response to court orders or upon a state’s request.

How are TMDLs developed?

The objective of a TMDL is to determine the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards, as well as the allocation of that load among the various pollutant sources. The allocation is vital so that control actions can be identified to achieve water quality standards. The TMDL process is important for improving water quality because it links the development and implementation of control actions to meet water quality standards.

TMDLs are developed using a range of techniques from simple 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 the pollutant causing the impairment.

All contributing sources of the pollutants (point and nonpoint sources) are identified and are allocated a portion of the allowable load. The waterbody usually requires a reduction in 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; and,
  • Allocation (with a margin of safety) of the allowable pollutant load among the different pollutant sources in a manner that ensures 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.

Public participation in 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) Impaired Waters List and draft TMDLs, with a 30 to 60 day public comment period. In some cases, hearings will be held as well.

Local citizens sometimes have specific knowledge about their watersheds, which is a valuable aspect of TMDL development. The public often contributes useful data and information about an impaired waterbody, offering 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 developed TMDLs and can ultimately speed cleanup of impaired waters or secure protection of threatened waters.

Public/stakeholder role in the TMDL process:

  • Provide data and information to the states;
  • Review and comment on 303(d) Impaired Waters List;
  • Review and comment on draft TMDLs; and,
  • Assist in the development of third party TMDLs.

What are the components of a TMDL document?

U.S. EPA issued review guidelines for TMDL submissions in "Guidelines for Reviewing TMDLs Under Existing Regulations Issued in 1992" [HTML] [PDF]. 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
    • This section should provide a description of the TMDL study area, including the boundaries, as well any pollutants being addressed on each waterbody. The studied waterbodies, the applicable water quality standards for each waterbody, existing permit limits for point sources within the study area and other general descriptive or background information should also be included. The Indiana Waterbody Identification System should be used to identify specific waterbodies and watersheds, and GIS data should be used to provide a clear visual depiction of the study area. Please use Universal Transverse Mercator- North American Datum 1983 in your GIS applications.
  • Applicable water quality standards and numeric water quality target*
    • The water quality target is a description of the desired condition in the watershed or waterbody. Typically, targets are tied to specific water quality standards that provide measurable goals for the TMDL.
  • Loading capacity*
    • Once the maximum load capacity has been determined and the margin of safety has been identified, the next step is to distribute the remaining load to the various point and nonpoint sources in the system. Existing and future nonpoint sources are given load allocations. There are also allocations for background load, which is naturally occurring in the water, and margin of safety.
    • The manner of distributing allocations can sometimes be complex. Point sources are typically more easily regulated than nonpoint sources. As a result, the allocation process for a stream impacted only by point sources is much more straightforward than that of a stream with both point and nonpoint sources.
  • Load allocations and waste load allocations*
    • A stream or watershed with multiple stakeholders and pollution sources can be difficult to allocate. The methods for making these allocations are not regulated by the Clean Water Act or by federal regulations. In Indiana, implementation of point source allocations is done through permits issued under the National Pollution Discharge Elimination System (NPDES) Program. The state of Indiana was granted primacy from U.S. EPA to issue these permits on January 1, 1975 through implementation of a memorandum of agreement. From 1975 to 1986, the state managed the NPDES program through the Stream Pollution Control Board with staff provided by the Indiana State Board of Health (ISBH), Division of Water Pollution Control. On April 1, 1986, the Department of Environmental Management came into existence and responsibility of the NPDES program was transferred to the Office of Water Quality (OWQ).
  • Margin of safety*
    • Section 303(d) of the Clean Water Act and U.S. EPA regulations at 40 CFR 130.7 require that “TMDLs shall be established at levels necessary to attain and maintain the applicable narrative and numeric water quality standards with seasonal variations and a margin of safety which takes into account any lack of knowledge concerning the relationship between limitations and water quality.” U.S. EPA explains that the margin of safety may be considered in two ways. An implicit margin of safety is incorporated into the TMDL through conservative assumptions in the analysis, while an explicit margin of safety is expressed in the TMDL as loadings set aside for the margin of safety.
    • A moderate explicit margin of safety is applied as part of all Indiana watershed TMDLs by reserving ten percent of the allowable load.
  • Consideration of seasonal variation*
    • Water-quality data often shows variations depending on the season. These changes may be the result of a variety of conditions, including agricultural land use practices, biological activity or sediment sources. As an example, rain-based discharge may dominate during specific months of the year, whereas the natural flow of the stream may dominate at other times of the year. Another example is the increase in biological activity that occurs during summer months because of warmer temperatures as opposed to decreased activity during winter months, which might cause seasonal variation in nutrient concentrations.
  • Reasonable assurance for point source/nonpoint source pollution
    • Rural and urban run-off is reduced through the implementation of best management practices (BMPs). A BMP may involve building something, or changing landforms or equipment. BMPs may also be managerial, where there is a change in the specific way of using or handling infrastructure or resources. BMPs should be selected based on the goals of a watershed management plan, a TMDL implementation plan or an equivalent process. Livestock owners, farmers and urban planners can implement BMPs outside of a watershed management plan. However, the success of a BMP is typically enhanced if they are coordinated as part of a larger planning effort. The following is a partial list of BMPs that may be used to reduce pollutant loads:
      • Riparian area management: Creation and management of vegetation buffer zones including grasses, legumes or trees.
      • Manure collection and storage: Collecting, storing and handling manure in such a way that nutrients or bacteria do not run off into surface waters or leach down into ground water.
      • Conservation tillage: Use of tillage practices and residue management to control erosion and surface transport of pollutants from fields used for crop production.
      • Contour row crops: Farming with row patterns and field operations aligned with or nearly perpendicular to the slope of the land.
      • Manure nutrient testing: If manure application is desired, performing sampling and chemical analysis to determine nutrient content of the manure. This information is used to establish the proper application rates to avoid over-application and run-off.
      • Drift fences: These short fences or barriers can be installed to direct livestock movement. A drift fence parallel to a stream keeps animals out and prevents E. coli from entering directly into the stream.
      • Pet clean-up/education: Education programs for pet owners can improve water quality of run-off from urban areas.
      • Septic management/public education: Programs for management of septic systems can provide a systematic approach to reducing septic system pollution. Education on proper maintenance and illicit discharge removal can alleviate some sources of pollutants.
  • Monitoring plan to track TMDL effectiveness
    • Further monitoring takes place during IDEM’s five-year rotating basin schedule. In some cases, monitoring also occurs once TMDL implementation methods are in place. Monitoring is adjusted as needed to continue the identification and elimination of pollutant sources. When monitoring indicates that a waterbody is meeting water quality standards, the waterbody is removed from the 303(d) Impaired Waters List.
  • Implementation plan
    • Though not a required element of a TMDL report, implementation is the logical next step of a TMDL project. The results of a TMDL are recommendations on reductions in pollutant load and allocations of these loads among the different sources. In Indiana, the recommendations for point sources and nonpoint sources are applied in different ways.
    • Point sources are regulated through the state’s NPDES program. Permittees must have a wasteload allocation before a discharge permit is issued. This wasteload allocation is based on Indiana water quality standards.
    • Nonpoint source implementation is managed by locally-led watershed groups and often funded through IDEM’s Nonpoint Source/Section 319 program. Nonpoint source pollution controls are implemented on a voluntary basis. The primary mechanisms used by these groups are incentive programs for the installation of best management practices, as well as public education and outreach programs.
  • Public participation
    • Some public participation opportunities are required by federal regulations. A public notice of draft TMDL recommendations is required, as is a public meeting, and it is preferable to have the meeting located in the project watershed.
    • There is a tremendous resource of knowledge in local stakeholders that should be used throughout the TMDL process to ensure the success of a TMDL project. Stakeholders can provide resources such as water quality data, access to monitoring sites and information regarding local land management practices. These examples, and many other resources, dramatically improve the quality of TMDL recommendations.
    • Involving stakeholders in the planning stages of a TMDL project helps avoid mistakes later on. Involvement during the data collection and draft recommendation phases provides a sense of ownership, leading to better reception of final recommendations. Ultimately, having stakeholders involved in the process will result in much better success while implementing changes that improve water quality.
    • During the development process of the watershed management plan and implementation of nonpoint source load allocations, public participation can be vital. Stakeholder groups are typically involved in the beginning stages of plan development and public meetings are held following completion of the plan.

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

TMDL review and approval

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

What happens after the TMDL is developed?

  • Point Sources
    • Permit limits consistent with wasteload allocation are enforceable under the CWA through National Pollutant Discharge Elimination System (NPDES) permits.
    • Permits are issued by U.S. EPA or states with delegated authority.
  • Nonpoint Sources
    • No federal regulatory enforcement program.
    • Primarily implemented through state or local nonpoint source management programs. Few of these programs have regulatory enforcement.

Section 303(d) of the CWA does not specifically require implementation plans for TMDLs; however, it requires that point source wasteload allocations be implemented through the 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 wasteload allocation. Additional information on NPDES permits is available on the IDEM Water Quality website and the U.S. EPA NPDES website.

Load allocations are implemented by nonpoint sources through voluntary actions and a wide variety of state, local and federal programs which may be regulatory, non-regulatory or incentive-based depending on the program. For example, funding is available through the CWA’s Section 319 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 Section 319 funding is available on U.S. EPA's Nonpoint Source Web site. Information on state-specific nonpoint source management activities may be obtained by contacting the state's water quality management agency.

Although states are not required under Section 303(d) to develop TMDL implementation plans, many states develop them with the TMDL. These implementation plans often provide additional information on how point and nonpoint sources contribute to the impairment, as well as suggested control measures.

Water Quality