Overview of Total Maximum Daily Loads (TMDLs)
- What is a TMDL?
- What triggers the need for a TMDL?
- Who is responsible for developing a TMDL?
- How are TMDLs developed?
- Public participation in TMDL development
- What are the components of a TMDL document?
- What happens after the TMDL is approved by EPA?
A TMDL is the calculation of 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. A TMDL determines a pollutant reduction target and allocates load reductions necessary to the source(s) of the pollutant.
Pollutant sources are characterized as either point sources that receive a wasteload allocation (WLA), or nonpoint sources that receive a load allocation (LA). For purposes of assigning WLAs, 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). For purposes of assigning LAs, nonpoint sources include all remaining sources of the pollutant as well as 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.
Expressed mathematically, the TMDL equation is:
TMDL = ΣWLA + ΣLA + MOS
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.
Each pollutant causing a waterbody to be impaired or threatened is referred to as a waterbody/pollutant combination, and typically a TMDL is developed for each waterbody/pollutant combination. For example, if one waterbody is impaired or threatened by three pollutants, three TMDLs might be developed for the waterbody. However, in other cases, a single TMDL document may be developed to address several waterbody/pollutants combinations. Neither the CWA nor EPA’s regulations define or limit the scale of TMDLs. Some states have been developing TMDLs on a watershed-scale basis. Such state TMDLs may also cover multiple watersheds.
According to the Clean Water Act, each state must develop TMDLs for all the waters identified on their Section 303(d) list of impaired waters, according to their priority ranking on that list.
As a general matter, states are responsible for developing TMDLs and submitting them to EPA for approval. Even if third parties assist in the development of the TMDL or its supporting analysis, such TMDLs must still be submitted to EPA by the states.
Under the CWA, the EPA reviews and either approves or disapproves the TMDL. If EPA disapproves a state TMDL, EPA must develop a replacement TMDL.
The objective of a TMDL is to determine the loading capacity of the waterbody 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 serves as a link in the chain between water quality standards and implementation of control actions designed to attain those 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 contemplates 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:
- Selection of the pollutant(s) to consider.
- Estimation of the waterbody's 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 such that water quality standards are achieved.
TMDLs should 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.
EPA’s regulations require public involvement in developing TMDLs, however, the level of citizen involvement in the TMDL process varies by state.
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 roles in the TMDL process can include:
- Providing data and information to the states.
- Reviewing and commenting on impaired water list.
- Reviewing and commenting on draft TMDLs.
- Assisting in the development of TMDLs.
EPA issued review guidelines for TMDL submissions in Guidelines for Reviewing TMDLs under Existing Regulations Issued in 1992. Below is a TMDL Review Checklist with the minimum recommended elements that should be present in a TMDL document.
- Identification of Waterbody, Pollutant of Concern, Pollutant Sources and Priority Ranking.
- Applicable WQS and 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.
TMDL wasteload allocations (those pollutant allocations assigned to point sources) are generally implemented through EPA’s National Pollutant Discharge Elimination System (NPDES) permits under CWA section 402. This section of the Act requires that point source discharges be controlled by including water quality-based effluent limits in permits issued to point source entities. Under EPA’s permitting regulations, water quality-based discharge limits in NPDES permits must be “consistent with the assumptions and requirements” of wasteload allocations in EPA-approved TMDLs.
Non-point source load reduction actions are implemented through a wide variety of programs at the state, local and federal level. These programs may be regulatory, non-regulatory or incentive-based e.g., a cost-share program. In addition, waterbody restoration can be assisted by voluntary actions on the part of citizen and/or environmental groups. The EPA section 319 program provides grant money to the states to fund specific projects aimed at reducing the nonpoint source pollution.
Although states are not explicitly required under section 303(d) to develop TMDL implementation plans, many states include some type of implementation plan with the TMDL. 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.