The Basics of Total Maximum Daily Loads (TMDLs)

This guide explains TMDLs, why they are important, how MassDEP develops them, and how the public can help.

Table of Contents

Overview: What is a TMDL?

Many waterbodies have good water quality, but approximately 40 percent of the lakes, ponds, rivers, wetlands, and coastal waters in the nation are listed as impaired due to pollution. For nearly 30 years, states focused on industrial and municipal discharges of pollution from point sources such as discharge pipes to address these impairments. States across the nation, including Massachusetts, now include nonpoint sources - such as stormwater runoff, septic systems, and erosion - in a more comprehensive framework to address pollution, using both the watershed approach and the total maximum daily load (TMDL) provision in the federal Clean Water Act.

What is a TMDL?

Water resources such as rivers, lakes, and estuaries are vital to the environment, public health, and economy of the Commonwealth. If healthy, these systems provide a variety of beneficial uses and services that support aquatic life, sustain natural habitat, promote public health, and facilitate recreational and commercial opportunities. A waterbody is considered impaired (i.e., not healthy) if pollutants limit or prevent beneficial uses and services. The Massachusetts Surface Water Quality Standards (314 CMR 4.00) regulation establishes both designated uses for surface waters and water quality criteria to protect and sustain waterbodies that may otherwise be impaired by pollutants. 

If a waterbody is assessed as impaired, the federal Clean Water Act requires states to develop a TMDL: a calculation of the maximum pollutant level (i.e., load) that a waterbody can receive while continuing to meet water quality standards. The Massachusetts Department of Environmental Protection (MassDEP), through the TMDL Section in the Watershed Planning Program, develops TMDLs and other watershed plans to restore impaired surface waters and to protect high quality waters throughout the Commonwealth. TMDLs are therefore critical to ensure impaired waters are restored and healthy waters are protected.

A TMDL is the sum of allowable loads from all contributing point and nonpoint sources of pollution. Point sources are primarily wastewater treatment plants that discharge to surface waters or groundwater. Nonpoint sources include septic systems, stormwater discharges via runoff over the land surface, and fertilizer runoff from lawns and golf courses.

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MassDEP's TMDL Strategy

MassDEP has developed a TMDL Strategy that prioritizes impaired waterbodies. The Massachusetts Vision 2.0: Clean Water Act Section 303(d) and Total Maximum Daily Load (TMDL) Development outlines MassDEP’s approach to TMDL prioritization. The approach outlines priority concerns based on impairments listed for the Final Massachusetts Integrated List of Waters for the Clean Water Act 2018/2020 Reporting Cycle. Given the priority concerns, waterbodies were sorted by waterbody type to identify metrics best suited to create a prioritization framework. In addition to the core document describing the overall approach, there are three appendices that describe the prioritization approach for estuaries, lakes, and rivers. Like previous prioritization efforts, the focus largely remains on nutrient and pathogen impairments. Elements of this strategy will evolve over time as priorities change, technology improves, and information is updated. 

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How can you get involved?

Stakeholders can help prioritize listed waters for TMDL development. Suggestions for TMDL development can be made via email to the MassDEP Watershed Planning Program. Consideration will be given to the severity of pollution, the beneficial uses of waterbodies, and the availability of acceptable methods to correct pollution problems.

MassDEP encourages public input and volunteer participation throughout the TMDL process. Local support is needed to develop recommendations and obtain funding. As each TMDL is developed, there is a comment period on the draft TMDL Report in order to involve the public in implementation planning. In addition, a meeting to receive public comments may be scheduled.

What is the TMDL Process?

Developing a TMDL

Developing a TMDL requires delineating watersheds, analyzing pollutant sources, and collecting comprehensive water quality and land use data. This information comes from communities, regional planning groups, and state and local monitoring programs. Following the TMDL requirements, MassDEP will allocate pollutant loads equitably to contributing point and nonpoint sources. Computer modeling may be used to consider pollution allocation alternatives that are feasible and cost-effective. In addition, naturally occurring pollution sources and seasonal variations will be taken into consideration, and a margin of safety will be included to ensure that water quality standards are met. The U.S. Environmental Protection Agency (EPA) also requires an implementation plan with control measures for each TMDL.

Review and Approval Process

MassDEP publishes each draft TMDL Report to facilitate public review in advance of a public meeting. At the meeting, staff explain the TBDL Report, development process, and recommendations, in addition to receiving comments from the public. MassDEP generally accepts written comments on each TMDL for four weeks after the meeting.

MassDEP then considers all comments and responds in writing, finalizes the TMDL Report, and submits it to EPA for formal approval. EPA reviews the TMDL Report, including public comments, and approves or denies it, typically within 30-60 days.

Community Input

Communities provide essential input on land use, water quality, and other important local factors. The target thresholds and TMDL calculations are science-based and driven by the Massachusetts Surface Water Quality Standards (314 CMR 4.00), including the classification and designated uses for each water body.


MassDEP has engaged in policy efforts to reduce nitrogen pollution and implement EPA-approved total nitrogen TMDLs on Cape Cod. In July 2023, MassDEP promulgated revisions to its septic system regulations (Title 5), 310 CMR 15.000, and new Watershed Permit regulations, 314 CMR 21.00, to compel nitrogen controls in 31 watersheds with EPA-approved TMDLs on Cape Cod. MassDEP is working with municipalities to address nitrogen pollution in these watersheds through a variety of nutrient reduction strategies, including sewering, upgrades to septic systems to incorporate Best Available Nitrogen Reducing Technology (BANRT), permeable reactive barriers, restoration of wetlands to improve nitrogen attenuation, and other measures.

Once the TMDL is complete, communities decide through Comprehensive Wastewater Management Planning (CWMP) how best to implement the TMDL to achieve the desired water quality goals. MassDEP reviews and approves a community's CWMP, and makes subsequent permitting decisions based on its approved Plan. Other tools to implement the TMDL include using watershed permits, watershed management plans, voluntary actions, and commitments for best management practices to restore water quality. 

One type of watershed management tool is a Nine Element Watershed-Based Plan (WBP). Grant funding is currently available to develop these plans through the Nonpoint Source Management Section in MassDEP’s Watershed Planning Program. The Nonpoint Source Management Section administers both the federal Clean Water Act Section 604(b) and 319 grant programs. The 604(b) grant program supports development of a WBP for local watershed planning, which can support future 319 grant implementation projects. Currently 319 grant program funds can also help develop WBPs for successful grant applicants. Development of many WBPs could be also funded locally through municipal and/or watershed association budgets. 

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Appeals and Enforcement

Can a TMDL be appealed?

Whenever a TMDL-derived pollutant reduction is incorporated into a federal or state discharge permit, the permit limit may be appealed following the federal or state administrative appeal process. After a final adjudication of the administrative agency appeal, eligible parties may appeal the agency decision to court. TMDLs themselves are not subject to appeal to MassDEP. Given that TMDLs are a relatively new approach to regulating pollutant loadings to Massachusetts waters, certain legal implications of appealing the TMDL directly to EPA have not been developed enough to provide definitive guidance.

How will MassDEP enforce a TMDL?

MassDEP prefers to work cooperatively with communities to protect and restore impaired waters. This is especially true when pollution comes from nonpoint sources such as stormwater runoff and on-site wastewater disposal, and where solutions are less straightforward than additional treatment of a point source discharge.

As long as a plan is developed and actions are being taken at a reasonable pace to achieve the goals of the TMDL, MassDEP will use discretion in taking enforcement steps. However, in the event that reasonable progress is not being made, MassDEP can take enforcement action through the broad authority granted by the Massachusetts Clean Waters Act, the Massachusetts Water Quality Standards, and through point source discharge permits.

TMDLs and Estuaries

Why do we need TMDLs for estuaries?

Most southeastern Massachusetts estuaries are impacted by excessive loads of nitrogen. Water quality is getting worse, and many estuaries do not meet the Massachusetts Water Quality Standards. Rapid population growth in the area has resulted in more nitrogen than estuaries can accept, causing poor water quality. Resulting problems include

  • Loss of eelgrass beds
  • Algae blooms, unpleasant odors, and scum
  • Fish kills
  • Reductions in important animal life on the ocean bottom, such as lobster, shrimp, scallops and mussels.

The Massachusetts Estuaries Project (MEP) determines which estuaries are being impacted by excessive nitrogen and identifies the sources of nitrogen pollution, so that communities have the scientific basis for common sense, cost-effective decisions on how to protect and restore their estuaries. TMDLs will guide the changes needed to restore water quality in these waterbodies.

How are TMDLs calculated for MEP Estuaries?

As with other TMDLs, developing a nitrogen TMDL for an MEP estuary requires delineating watersheds, analyzing nitrogen sources, and collecting comprehensive water quality and land use data. This information comes from communities, regional planning groups, and state and local monitoring programs.

The MEP integrates the data on water quality, nutrient loading, and hydrodynamics to model the impact of nitrogen loads on estuaries. The MEP models can be used to illustrate how changes in land use will affect the nutrient load and water quality in estuaries. For example, the model can illustrate how much the nitrogen load will drop if flows from on-site wastewater systems decrease or if fertilizer use is reduced.

SMAST, the School for Marine Science and Technology at UMass Dartmouth, prepares a Technical Report for each estuary. The Technical Report forms the basis of the TMDL by documenting the MEP model results and identifying potential nitrogen reduction approaches. To determine the most cost-effective solutions, communities may request modeling of additional nitrogen reduction approaches for a small additional cost.

TMDLs and Permitting

What if a TMDL is not achievable?

MassDEP and EPA recognize that restoring polluted waters, particularly where nonpoint sources are the primary contributor, is a long-term process. Rather than requiring that Water Quality Standards be met in a fixed time frame, the TMDL process requires communities to develop a plan to restore waterbodies and make progress toward implementing the plan. This "adaptive management" approach is based on taking action, measuring its impact, and adjusting future steps as necessary. MassDEP will work with communities on the implementation process to determine what is realistic and achievable.

If a TMDL is not achievable, the federal Clean Water Act has a process for downgrading the classification of a waterbody. EPA's "Use Attainability Analysis" must be used to scientifically demonstrate that a change in the designated use of the waterbody is appropriate. However, this is an extreme measure that is not undertaken until all feasible measures to meet the Water Quality Standards have been exhausted.

Will a TMDL trigger the need for a state permit?

This depends on the local situation and the implementation strategy chosen by the community. For example, actions such as building or expanding wastewater treatment facilities will require a permit.

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