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 about 40 percent of the lakes, ponds, rivers, wetlands, and coastal waters in the nation are listed as impaired waters due to pollution. For nearly 30 years, states have concentrated on industrial and municipal discharges of pollution from point sources such as discharge pipes. Now Massachusetts and other states are expanding efforts and including nonpoint sources - such as stormwater runoff, septic systems, and erosion - to clean up our water, using the watershed approach and the total maximum daily load (TMDL) provision in the federal Clean Water Act.

What is a TMDL?

TMDL stands for "Total Maximum Daily Load". A TMDL is a calculation of the maximum amount of a pollutant that a waterbody can accept and still meet the state's Water Quality Standards for public health and healthy ecosystems. The federal Clean Water Act requires all states to identify waterbodies that do not meet state standards and develop TMDLs for them.

Massachusetts sets statewide Water Quality Standards for a variety of uses, such as drinking water supplies, fishing, recreational swimming and boating, and healthy ecosystems for plants and animals. Clean water is vital to human health and recreation, and TMDLs are a critical step in ensuring that degraded waters are restored and protected so that all citizens enjoy a clean and healthy environment.

A TMDL is the sum of loads that are allowable 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.

MassDEP's TMDL Strategy

The Massachusetts Department of Environmental Protection (MassDEP) has developed a TMDL Strategy that prioritizes all listed waterbodies, establishes TMDLs for degraded waters, and plans for implementation of best management practices (BMPs) to clean up polluted waterbodies. The TMDL requires coordination with existing and proposed regulations and programs, such as NPDES permits for point source control and the stormwater management performance standards administered by conservation commissions under the Wetlands Protection Act. In addition, funding priority for Section 319 grants and SRF loans will be given to watershed cleanup projects that advance TMDL program requirements.

map of nutrient management projects for 2019
Nutrient management in Massachusetts as of 2019. See "Additional Resources" below for a PDF of this map.

MassDEP is integrating the Massachusetts TMDL Strategy into the state's five-year planning and permitting cycle. Surveys of water quality and aquatic life, conducted on a rotating basis within Massachusetts river basins, support MassDEP in its development of the 303(d) list of impaired waterbodies. Watershed teams, volunteers, and the public will help prioritize listed waters for TMDL development. Consideration will be given to the severity of pollution, the beneficial uses of waterbodies, and the availability of acceptable methods to correct pollution problems.

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 within reach. The U.S. Environmental Protection Agency (EPA) also requires an implementation plan with control measures for each TMDL. Using watershed management plans, voluntary actions, and commitments for best management practices to restore water quality, MassDEP will fulfill these EPA requirements. Assessment and monitoring of the control measures' effectiveness will complete the TMDL Strategy.

Additional Resources for MassDEP's TMDL Strategy

How can you get involved?

MassDEP encourages public input and volunteer participation throughout the TMDL process. Local support is needed to develop recommendations and obtain funding. Public involvement is sought during waterbody assessments and as the 303(d) list is revised. Beginning in 2000, the 303(d) process has occurred every two years. 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.

Additional Resources for How can you get involved?

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. MassDEP establishes the TMDL number and writes the TMDL Report, which documents the basis for the TMDL number and allocates the allowable loadings to point and nonpoint sources.

Review and Approval Process

MassDEP makes the draft TMDL Report available for public review in advance of a local public meeting. At the meeting, staff explain the Report, the TMDL process, and the TMDL recommendations, and receive comments from local officials and citizens. MassDEP generally accepts written comments on the TMDL for 2-4 weeks after the meeting.

MassDEP then considers all comments and responds to them 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 to the Report on land use, water quality, and other important local factors. The nitrogen thresholds and TMDL figures are science-based calculations driven by statewide Water Quality Standards and the classification of a waterbody for a particular use, and are established by MassDEP. Once the TMDL is complete, communities decide through Comprehensive Wastewater Management Planning (CWMP) how best to implement the TMDL in order 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.

Additional Resources for What is the TMDL Process?

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.

Additional Resources for TMDLs and Permitting

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