Total Maximum Daily Load (TMDL)

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Total maximum daily loads (TMDLs) are part of the Clean Water Act and refers both to the legislation and regulatory documents which require assessment and listing of "impaired" waterbodies (section 303 d). TMDLs are also the calculations and numeric standards to meet water quality standards mandated by state, federal, or authorized tribal regulatory bodies.

Total mean daily loads (TMDLs) are a regulatory tool created by the US EPA to meet water quality goals for impaired water bodies under Section 303(d)(1)(A) of the Clean Water Act (CWA). Water quality goals are created with the goal of supporting the "beneficial uses" (BUs) of a waterbody, including water contact recreation, wildlife habitat, and others.

Total mean daily loads also refer to the specific Load and Waste Load Allocations (LAs and WLAs respectively) for each pollutant load, plus a margin of safety. These load allocations are the calculated amount of a given pollutant that a given waterbody can receive while still preserving and improving water quality [1].

Total maximum daily loads set upper limits on how much of a pollutant can enter a waterbody within a day from point sources (Waste Load Allocation--WLA) and nonpoint sources (Load Allocations--LAs) of pollution, plus a margin of safety (MOS).

Regulatory background

Under section 303(d) of the 1972 Federal Clean Water Act, all waterbodies within the US must be evaluated for water quality by States, territories and authorized tribes.

Those waterbodies not meeting water quality standards must be classified as "impaired". This impairment list, also known as the 303d list, requires that total maximum daily loads (TMDLs) be established for all pollutants identified as causing impairment. These TMDLs must be created with the goal of defining and achieving a total maximum daily load for each pollutant which will help achieve water quality goals within each 303d-listed waterbody. Responsible parties must then develop and implement a remediation plan for each 303d-listed water body.

An updated list of impaired waterbodies (303d list)must be submitted every two years. The list also identifies where TMDLs have been approved by the Environmental Protection Agency (EPA) and implemented but where water quality standards have not yet been met, and further action beyond TMDLs are needed address impairment. Water quality standards includes "beneficial uses" (BUs) which include water contact recreation, wildlife habitat, among others.

Total maximum daily loads also refer to the specific Load allocations and Waste Load Allocations (LAs and WLAs respectively) for each pollutant load, with an added margin of safety. These load allocations are the calculated amount of a given pollutant that a waterbody can receive while still preserving and improving water quality [1].

Once a TMDL is developed and approved for a water body, it can be removed from the 303d list[2]

General overview of TMDL development process

Overview of TMDL development process from US EPA[3].

TMDL development involves many stages from problem identification and problem statement development, through development of numeric standards to restore water quality goals, source assessments and linkage analysis, to load allocations, monitoring/evaluation, and finally implementation.

The US EPA provides protocols for developing TMDLs for nutrients[4], pathogens [5], and sediments [6]. These protocols provide guidance to staff, stakeholders, and consultants as to the process of TMDL development.

Problem identification

Problem identification starts the process of the TMDL by summarizing existing impairments, where they are located, and known probable sources. Compilation of existing data, identification of designated uses, and evaluating seasonal factors are parts of this early summary.

Beneficial uses

Beneficial uses (BUs) are those uses which a waterbody is expected to provide for human enjoyment and/or habitat value. See the main article for more information on BUs.

TMDLs establish the concentration or daily load of nutrients a body of water can contain while continuing to support its defined beneficial uses. The responsible agency defines TMDLs as either a mass load or a set unit of concentration. During TMDL development, it is necessary to consider ‘’secondary indicators’’ of water quality, such as dissolved oxygen levels, transparency and algal biomass, that are linked to the beneficial uses for the water body. The regulatory body may use a modeling tool to link secondary indicators to concentrations in the water column and predict levels of nutrients that would impair specific beneficial uses.

Numeric targets

Numeric targets are concentrations of specified nutrients that would not impair designated beneficial uses of a given water body. Water quality objectives given in the Basin Plan [7]. Water quality objectives for the Basin Plan are attempts to quantify the allowable nutrient concentrations, objectives listed in the Plan are used as a starting point during TMDL development and adoption.

Numeric targets are concentrations of specified nutrients that would not impair designated beneficial uses of a given water body. Allowable nutrient concentrations dictate numeric targets. For example, the numeric target often given for nitrate in TMDLs is 45 mg/L as the California drinking water standard[1].

Steps taken in the process to developing nutrient standards include:

  • Researching applicable water quality standards for the waterbody depending on jurisdictions and uses.
  • Selecting indicators
  • Identifying technical and general scientific validity
  • Considering time and cost considerations in selecting indicators.
  • Identifying target values
  • Comparing existing and target conditions

Because choosing target values or conditions is complex, local examples and experience are useful in setting target values (CITE).

Historical pollutant levels (especially reference reach conditions) can help start the process. Other factors to consider when setting values are seasonal fluxes and spatial variability in the indicator. Values can be graduated by categorical type (e.g. tributary vs. main stem), with a margin of safety. The margin of safety is based on reliability and accuracy of indicator measurements, degree of impairment, direct impacts, and best professional judgment based on experience and literature.

Source assessment

Source assessment diagram from EPA TMDL Protocol Document for Nutrients (2000)

Source assessment (also called source analysis) is the process whereby responsible regulatory agencies characterize the type and magnitude of loading sources in a watershed with the goal of creating inventories used to determine workable strategies to creating load allocations for dischargers into a given waterbody.

Examples of key questions:

  • Are selected indicators capable of detecting impacts to designated use and responding in a measurable way to control actions?
  • Have baseline or background conditions been adequately characterized?
  • Are numeric targets set at levels that reasonably promote desired water quality for designated uses of concern?
  • Have important pollutant sources been identified?
  • Have important pollutant sources been accurately estimated?

Source analysis steps:

  • Identify sources
    • Characterize source type and possible pathways
  • Estimate source loadings
    • Monitoring data, statistical relationships, and models are used to estimate loads.

Linkage Analysis

In a TMDL document, the aim of the linkage analysis is to link pollutant loads to water quality. This link supports TMDLs (loading capacity per day) specified to meet the numeric target (overall load concentration).

No explicit linkage analysis was given in the 2012 draft report because the numeric target concentration is not finalized. Once a target has been established, the linkage analysis will be used to convert that target concentration to a recalculated daily load.

  • Identify pathways of cause and effect between pollutant source and water quality target. Define needed level of analysis and then use monitoring data, variability, hydraulic characteristics, and type of indicator to create links. Simulation models can be utilized when other means are not sufficient to link indicators and sources.
  • Assess linkages

Margin of Safety

A MOS is used to account for the uncertainty in the linkage between nutrient loads and nutrient pollutant concentrations in the receiving water body.

There are two methods to approach the margin of safety in a TMDL report. One incorporates uncertainties implicitly by using conservative estimates in the model. The other option for incorporating uncertainty (MOS) is to quantify a portion of the total TMDL, leaving the remainder as sources (allocations are important here). It is important to not only make conservative assumptions about the parameters, but the thresholds as well. Uncertainties useful to take into consideration in regards MOS development are that:

  • there is finite data availability
  • concentrations are discrete values, estimated based upon limited parameters not necessarily accounting for interactions between parameters
  • the model may not be representative of the actual environmental conditions
  • rain patterns vary from year to year, therefore dilution varies from year to year also
  • nutrient bioavailability is more important than simple measured nutrient concentrations with regards to macrophytes in biostimulatory-impaired waterbodies

Some reports employ conservative estimates within their water quality objectives, thus implicitly accounting for the margin of safety. An important factor to consider when using conservative estimates in lieu of quantitative margins is to explicitly address the individual assumptions that are being accounted for. An efficient way to make conservative assumptions on a in an efficient way is to assume low-flow conditions.

If all sources of variability have been accounted for in the model by incorporating conservative assumptions, then that may be stated as the margin of safety. However, if any significant uncertainties have not been incorporated in the model, an explicit margin of safety should be identified. The typical margin of safety that has been accepted in California is about 20% (CITE), but this can vary depending on land use and other variables affecting nutrient loading into water bodies.

Load and Waste Load Allocations (LAs and WLAs)

Total Maximum Daily Loads (TMDLs) allocate the total concentration of nutrients and other pollutants (i.e. loading capacity) allowed to be discharged into a water body divided among all possible sources[2] without reducing water quality standards.

These allocations are typically comprised of Load Allocations (LAs) for "nonpoint" sources and Waste Load Allocations (WLAs) for "point sources". These allocations include a margin of safety (MOS) and "future growth" allowances set aside from the total pool of allocations to account for unanticipated sources of a particular contaminant.

Load allocations are divided among parties discharging pollutants into a given water body based on a process described here [3].

as a formula:

TMDL = LC* = LAs + WLAs + MOS + FG 
  • Loading Capacity (LC) is the total amount of a pollutant load a waterbody can receive without exceeding water quality standards.
  • Future Growth (FG) is a calculation of anticipated future developments (those or other changes to the watershed impacting the waterbody being considered.


On August 8, 1997, the Environmental Protection Agency (EPA) issued a memorandum [8] , “New Policies for Establishing and Implementing Total Maximum Daily Loads (TMDLs),” which directs EPA regions to work in partnership with states to achieve nonpoint source load allocations established for 303(d)-listed waters impaired solely or primarily by nonpoint sources. To this end, the memorandum asks that regions assist states in developing implementation plans that include reasonable assurances that the nonpoint source load allocations established in TMDLs for waters impaired solely or primarily by nonpoint sources will in fact be achieved.

This includes a public participation process and recognition of other relevant watershed management processes. Although implementation plans are not approved by EPA, they help establish the basis for EPA’s approval of TMDLs.

The purpose of an Implementation Plan is to describe the steps necessary to reduce pollutant loads to achieve TMDLs. Implementation Plans identify the following:

  • 1) actions expected to reduce pollutant loading
  • 2) parties responsible for taking these actions
  • 3) regulatory mechanisms by which the Central Coast Water Board will assure these actions are taken
  • 4) reporting and evaluation requirements that will indicate progress toward completing the actions
  • 5) a timeline for completion of implementation actions.

Implementation Plans also address economic considerations to achieve compliance.

The 2012 draft TMDL report suggests:

"SWRCB, California Coastal Commission and other State agencies have identified management measures (MMs) to address agricultural sources of nutrient pollution that affect State waters."

To this end, they recommend nutrient management as outlined in the California Nonpoint Source Pollution Control Program [4].

!This can involve lowering permitted discharges if present, and using as many other strategies as possible. Other strategies could be regulations, educational programs, best management practices, cost-sharing or other assistance, and use of volunteer groups[2].

Monitoring/Evaluation Plan

The protocol document[5] suggests that monitoring plans describe the timing, location, responsible parties, and quality assurance and control procedures.

The level of rigor required for a monitoring plan is dependent on confidence in the TMDL analysis. A greater level of uncertainty requires higher rigor in monitoring, while allowing space for future revision. Since watershed process drivers are not identical before and after implementation, models are useful for evaluating results of monitoring.

Models can be calibrated to pre or post-implementation to better compare results of monitoring actions. Coordination with other existing or planned monitoring activities can be particularly helpful for long-term monitoring programs, large study areas, or if the water quality agency has limited resources for monitoring programs. The type of monitoring is chosen by fit to desired goals, followed by development of a quality assurance plan to ensure data can support future analysis.

Overall a monitoring plan is created to evaluate the effectiveness of the implementation strategies and TMDL elements such as numeric targets and pollutant estimates.

  • Develop monitoring plan
  • Develop review plan
  • Develop schedule
  • Implementation Plan
  • Develop plan

Public Participation

Image taken from California Regional Water Quality Control Board Development of total maximum daily loads for nutrients and nutrient-related impairments: Lower Salinas River Watershed Factsheet

Public participation is a requirement [8] of the TMDL process and is vital to a TMDL’s success. The August 23, 1999, proposed regulation states that the public must be allowed at least 30 days to review and comment on a TMDL prior to its submission to the EPA for review and approval. In addition, with a TMDL submittal, the EPA must be provided with a summary of all public comments received regarding the TMDL and staff response to those comments, indicating how comments were considered in the final decision.

Typically a TMDL report may go through one or more cycles where staff receive public comments and revise the report to reflect those comments appropriately. The figure (right) is an example of the iterative development process (Factsheet).

Example TMDL

Previous TMDLs in the Monterey Bay Region of California have not included critical conditions key to development of water quality problems impacting beneficial use . The 2003 Santa Clara River Nitrogen TMDL included critical conditions. Although the climate around the Santa Clara River is drier, that water body is similar to the Lower Salinas River in seasonal flow and the effects of the first large storm ("first-flush").

Although not an ideal approach, the Santa Clara River TMDL makes an attempt to incorporate the increased impairment hazard presented by seasonal variation [9]. Specifically, critical conditions were evaluated on the basis of those conditions that would cause an increase in inorganic nitrogen species due to either (1) low flow conditions, (2) the effects of the first big storm of the season (first flush), or, (3) rising groundwater effects.

See also


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