Total Maximum Daily Load (TMDL)

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Total maximum 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 include "beneficial uses" (BUs) which include water contact recreation (REC-1), wildlife habitat ('WILD'), and others.

Total maximum 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 (MOS). 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].

General overview of TMDL development process

Overview of TMDL development process from US EPA[2].

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.

Beneficial uses

Beneficial uses (BUs) are those uses assigned to water bodies which have corresponding water quality goals which must be met to allow for those beneficial uses to be enjoyed. A common example of beneficial uses is municipal water (termed MUN), which has stringent requirements for low or non-existent levels of pollutants/contaminants (e.g. nitrate must be lower than 10mg/L for drinking water standards).

TMDLs establish the concentration or daily load of nutrients a body of water can contain while continuing to support its defined beneficial uses. TMDLs can be defined in terms of a mass load or can be set as a 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. A modeling tool can be used 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 [3]. 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.

Source analysis

The protocol document 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 more rigorous monitoring actions and must allow room 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’s monitoring resources are limited. It is also important to choose the type of monitoring that will be most appropriate to yield desired goals and then develop a quality assurance plan to ensure the 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.

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?

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.

Margin of Safety

The margin of safety (MOS) for the Lower Salinas River Watershed is still being developed. 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 that would be useful to take into consideration in regards to the LSRW include:

  • finite data availability
  • concentrations are discrete values that have been estimated based on a specific amount of parameters and do not necessarily account for the interactions of parameters
  • the model may not be representative of the actual environmental conditions
  • rain patterns vary from one year to the next so dilutions vary
  • nutrients bioavailability to macrophytes

In order to develop a proper margin of safety it is useful to reference other TMDL reports(ELABORATE..).

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.

The MOS for the XXXXX is dependent on the assumptions put into the INN benthic biomass modeler.

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%, but this can vary depending on land uses and other variables affecting nutrient loading into water bodies.

Load allocations (LAs)

TMDLs include the total concentration of nutrients allowed to be discharged into a water body by all possible sources. The allocations are typically comprised of Load Allocations (LAs) from nonpoint sources and Waste Load Allocations (WLAs) from point sources and also includes a Margin of Safety (MOS) to account for unexpected sources of a particular contaminant and Future Growth (FG), or as a formula:

TMDL = LAs + WLAs + MOS + FG


The 2012 draft TMDL report suggests that, "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 [1].

On August 8, 1997, the Environmental Protection Agency (EPA) issued a memorandum [4] , “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; 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 these 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.

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 [4] 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. 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 [5]. 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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