TMDL Background Information

What is a TMDL?

A TMDL or Total Maximum Daily Load 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 amount to the pollutant's sources. TMDLs are based on the relationship between pollution sources in the watershed and instream water quality conditions. A TMDL addresses a single pollutant or stressor for each waterbody or waterbody segment.

How does the Clean Water Act set up the framework for TMDL Studies?

Total Maximum Daily Loads (TMDLs) are a requirement found in §303(d) of the federal Clean Water Act.

The Clean Water Act requires that Rhode Island:

  • Establish Water Quality Regulations (including Water Use Classification and class-specific water quality criteria) for the state's waters.
  • Monitor and assess water quality conditions in the state's waters (i.e. lakes, ponds, rivers, streams, estuaries, and other marine waters) to determine whether these waters support designated uses. Findings are reported biennially in the state's Integrated Report.
  • Identify and list impaired waters (that is those waters that do not meet water quality standards with existing required technology-based controls alone) in the state's 303(d) list.
  • Set priority rankings (a schedule for development of TMDLs) for all impaired waters included on the 303(d) List.
  • Determine TMDLs that establish acceptable pollutant loads from both point and non point sources of pollution which allow the impaired waterbody to meet water quality standards for each listed waterbody and each cause of impairment.
  • Submit the 303(d) List and all TMDLs to U.S. Environmental Protection Agency for approval.
  • Incorporate TMDLs into the State's Continuing Planning Process.

Which waterbodies require TMDLs?

Waterbodies require TMDLs when waters do not meet water quality standards with existing required technology-based pollution controls alone.

Waterbodies that do not meet water quality standards are placed on Rhode Island's list of impaired waterbodies. This list is called the 303(d) list after the section in the Clean Water Act that requires it. The 2014 list identifies 96 named waterbodies having at least one impairment in need of a TMDL.

TMDL Study Development Process

Step 1 - Characterization of Water Quality Impaired Waters

  • Identify the pollutant of concern.
  • Compile and assess readily available data and information.
  • Choose appropriate model or other allocation approach.
  • If necessary, conduct field surveys to further characterize impairment, identify pollution sources and pollution source strength.
  • Document water quality impairments and pollutant sources.
  • Seek key stakeholders’ input and where appropriate, assistance throughout TMDL development process.

Step 2 - TMDL Computation and Load Allocation

  • Assemble the required input data for chosen model or allocation approach.
  • Determine waterbody's assimilative capacity (i.e. amount of pollutants waterbody can receive and still meet water quality standards).
  • Estimate current delivered loads and/or concentrations from point and nonpoint sources.
  • Determine necessary load and/or concentration reductions.
  • Allocate allowable loads between point and nonpoint sources, while accounting for margin of safety.
  • Identify pollution control measures necessary to achieve required load reductions.

Step 3 - TMDL Documentation and Review

  • Develop draft TMDL report, including a plan for implementation.
  • Conduct internal State agency review and preliminary review by US EPA.
  • Hold public workshop on draft TMDL.
  • Solicit stakeholder comments on draft TMDL report.
  • Prepare formal response to comments.

Step 4 - Submission and Approval of TMDL

  • Submit final TMDL to EPA Region I for approval.

How are TMDLs calculated?

A TMDL or Total Maximum Daily Load is the sum of the allowed pollutant loads for point sources, nonpoint sources, and a margin of safety

TMDL = Wasteload Allocation (Point Sources) + Load Allocation (Nonpoint Sources) + Margin of Safety

Point sources include industries and municipal wastewater treatment plants and regulated storm water. Nonpoint sources include septic systems and unchannelized urban or agricultural runoff. The Margin of Safety is used to help account for the uncertainties in monitoring and modeling a complex natural system.

Load and/or concentration allocations are determined through the review of monitoring data and watershed modeling. The tools used depends upon the complexity of the problem and could range from an expensive, complicated mathematical water quality model to a simple empirical equation based on available data.

Implementing TMDLs

Water quality-based limits are issued for point source dischargers including industrial and municipal wastewater treatment facilities and regulated storm water dischargers. In Rhode Island, point sources are regulated by the DEM RIPDES Program.

The RIPDES Storm Water Program requires regulated communities and industrial facilities to develop and implement Storm Water Management Program Plans (SWMPP). Affected communities have 180 days after being notified that a TMDL is final to incorporate relevant TMDL recommendations in their SWMPP.

Nonpoint source controls are implemented through Best Management Practices (BMPs). DEM utilizes various mechanisms to foster control of nonpoint sources of pollution including:

  • Technical assistance.
  • Financial assistance.
  • Education.
  • Training.
  • Technology transfer.
  • Demonstration projects.
  • Enforcement.

Typically DEM works with the responsible party to achieve voluntary control of nonpoint source problems. However, depending on the situation mandatory compliance via an enforcement action may be warranted. When allowing for nonpoint source controls, implementation plans should provide a reasonable assurance that the controls will be implemented and maintained or an effective monitoring program to demonstrate that nonpoint source reductions are taking place.

The effectiveness of a TMDL's pollution control strategy is evaluated using a variety of strategies including:

  • Monitor pollutant loadings.
  • Track implementation and effectiveness of controls.
  • Assess water quality trends and compliance with water quality standards.

Public Education and Participation in the TMDL Process

There are several steps within the TMDL process where the public may participate and comment on the DEM's work with impaired waterbodies.

Listing Process

Rhode Island is required by the federal Clean Water Act (CWA) to a develop prioritized list of impaired waterbodies (waters not meeting state water quality standards). This list is revised by DEM every two years and is known as the 303(d) list after the section of the CWA that requires it. The 303(d) list serves as the basis for TMDL program, since it sets the priorities for the TMDL program and identifies the waterbodies for which TMDLs will be developed over the course of the next several years.

There is usually one or more public meetings held to present and discuss the draft 303(d) list prepared by DEM. In addition, there is a formal comment period that allows the public to make written recommendations for changes. DEM responds to comments received and makes the appropriate changes to the list. The revised list is then submitted to EPA for approval.

TMDL Development Process

Participation is vital to making the TMDL process a success. In the initial stages of developing the TMDL, stakeholders can play an important role by contributing both water quality data and their in-depth local knowledge of the watershed. This information will help DEM to better characterize conditions in the waterbody and more easily identify pollution sources in the watershed.

DEM will seek input from the public at key points in the TMDL development process. A public meeting will be held at the beginning of the project to inform stakeholders that a TMDL is underway and to solicit input. At the midpoint of the process, typically after supplemental monitoring has been conducted, another meeting may be held to discuss the water quality data collected and to identify potential pollution sources in the watershed. If an extensive water model is to be utilized, another meeting, or series of meetings, may be held to discuss the development of the model. Finally, a public meeting will be held in conjunction with the formal 30-day public comment period required before the TMDL can be submitted to EPA for approval.

TMDL Implementation

Public participation is also crucial when a TMDL is implemented. Without public support, local government leaders may not be willing to commit the public resources or establish the added local requirements to improve water quality. Therefore, an engaged citizenry is a critical component in these restoration projects. If a citizen group has already been formed in the watershed, or if an ad-hoc group emerges from the TMDL process, DEM will continue to work with local stakeholders to support the implementation of the TMDL recommendations.