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Water: Total Maximum Daily Loads (303d)

Chapter 3 - Development And Implementation Of The TMDL

Guidance for Water Quality-Based Decisions:
The TMDL Process

Development of the TMDL

The TMDL process is an important element of the water quality-based approach. It links the development and implementation of control actions to the attainment of water quality standards. This chapter expands the discussion introduced in Chapter 2 on how to develop TMDLs and implement controls for water quality-limited waters. Appendix D and E provide supporting information on some important technical considerations and EPA supported models for TMDL development.

The TMDL Objective

As stated in 40 CFR 131.2, "[water quality] standards serve the dual purposes of establishing the water quality goals for a specific waterbody and serve as the regulatory basis for the establishment of water-quality-based treatment controls and strategies beyond the technology-based levels of treatment required by section 301(b) and 306 of the Act." Standards also contain antidegradation provisions to prevent the degradation of existing water quality.

The objective of a TMDL is to allocate allowable loads among different pollutant sources so that the appropriate control actions can be taken and water quality standards achieved. The TMDL provides an estimate of pollutant loadings from all sources and predicts the resulting pollutant concentrations. The TMDL determines the allowable loads and provides the basis for establishing or modifying controls on pollutant sources.

The TMDL Process

The total pollutant load to a waterbody is derived from point, nonpoint, and background sources. Pollutant loads may be transported into waterbodies by direct discharge, overland flow, ground water, or atmospheric deposition. The TMDL concept has successfully been applied to develop wasteload allocations for point source discharges in low flow situations where nonpoint sources are not a concern. TMDLs can and should be used, however, to consider the effect of all activities or processes that cause or contribute to the water quality-limited conditions of a waterbody. Activities may relate to thermal changes, flow changes, sedimentation, and other impacts on the aquatic environment. Control measures to implement TMDLs, therefore, are not limited to NPDES authorities but should also be based on State and local authorities and actions to reduce nonpoint source pollution.

An example of how to apply such a TMDL might be in the control of excess sediment which causes loss of a beneficial use of a waterbody. If standards, established to protect against the loss of a beneficial use (e.g., fish spawning), are not met and, if the process causing the problem (i.e., excess sedimentation) can be quantified, then it may be appropriate to use the TMDL process to assess the adverse impacts and potentially set controls on the problem activity. In this example, the activity might be urban development for which effective controls can be implemented to reduce sediment loading to the impacted waterbody.

The TMDL process distributes portions of the waterbody's assimilative capacity to various pollution sources -- including natural background sources and a margin of safety -- so that the waterbody achieves its water quality standards. The analyst may use predictive modeling procedures to evaluate alternative pollution allocation schemes in the same waterbody. By optimizing alternative point and nonpoint source control strategies, the cost effectiveness and pollution reduction benefits of allocation tradeoffs may be evaluated (see Appendix D). The approach normally used to develop a TMDL for a particular waterbody or watershed consists of five activities (see box).

TMDL Development Activities
  • Selection of the pollutant to consider.
  • Estimation of the waterbody assimilative capacity.
  • Estimation of the pollution from all sources to the waterbody.
  • Predictive analysis of pollution in the waterbody and determination of total allowable pollution load.
  • Allocation(with a margin of safety) of the allowable pollution among the different pollution sources in a manner that water quality standards are achieved.

In developing a TMDL it is important to keep in mind certain constraints on the WLA portion that are imposed by antibacksliding regulatory provisions. The WLA will normally result in new or more stringent water quality-based limits than those contained in a previously issued permit. In a limited number of cases, however, it is conceivable that less stringent water quality-based limits could result. In these cases, permit limits must conform to the antibacksliding provisions contained in section 402(o) of the CWA.

Selection of Approach

Figure 2 illustrates the critical decisions and the appropriate steps in the TMDL process for developing load allocations and implementing and evaluating control actions. In some cases, as illustrated by the left side of the diagram, TMDL development can be straight-forward and relatively simple. In other cases, as depicted by the right side of the diagram, a phased approach may be more appropriate. Regardless of which path is followed, the allocation of loads and establishment of control actions should ensure that all water quality-limited waters will meet their standards.


Once a waterbody is selected for action, an analyst must decide if the available data and information about the sources, fate, and transport of the pollutant to be controlled is adequate. The level of effort and scientific knowledge needed to acquire adequate data and perform meaningful predictive analyses is often a function of the pollutant source, pollutant characteristics, and the geographical scale of the pollution problem. As described in Chapter 2, modeling the fate and transport of conventional pollutants (e.g. biochemical oxygen demand) and point source contributions is better developed than modeling for non-traditional pollution problems. For certain non-traditional problems, if there are not adequate data and predictive tools to characterize and analyze the pollution problem with a known level of uncertainty, a phased approach may be necessary.

The phased approach is required when the TMDL involves both point and nonpoint sources and the point source WLA is based on a LA for which nonpoint source controls need to be implemented. There must be assurances that nonpoint source control measures will achieve expected load reductions in order to allocate a wasteload to a point source with a TMDL that also allocates expected nonpoint source load reductions. In this case, a phased approach is required because the TMDL that is developed has additional requirements that provide these assurances.

Despite the additional requirements of the phased approach, States may actually prefer it because the additional data collected can be used to verify expected load reductions, evaluate effectiveness of control measures, and ultimately determine whether a TMDL needs to be revised.

The Phased Approach

Under the phased approach, the TMDL has LAs and WLAs calculated with margins of safety to meet water quality standards. The allocations are based on estimates which use available data and information, but monitoring for collection of new data is required. The phased approach provides for further pollution reduction without waiting for new data collection and analysis. The margin of safety developed for the TMDL under the phased approach should reflect the adequacy of data and the degree of uncertainty about the relationship between load allocations and receiving water quality.

The TMDL, under the phased approach, includes (1) WLAs that confirm existing limits or would lead to new limits for point sources and (2) LAs that confirm existing controls or include implementing new controls for nonpoint sources. This TMDL requires additional data to be collected to determine if the load reductions required by the TMDL lead to attainment of water quality standards. Data collection may also be required to more accurately determine assimilative capacities and pollution allocations.

In addition to the allocations for point and nonpoint sources, a TMDL under the phased approach will establish the schedule or timetable for the installation and evaluation of point and nonpoint source control measures, data collection, the assessment for water quality standards attainment, and, if needed, additional predictive modeling. The scheduling with this approach should be developed to coordinate all the various activities (permitting, monitoring, modeling, etc.) and involve all appropriate local authorities and State and Federal agencies. The schedule for the installation and implementation of control measures and their subsequent evaluations will include descriptions of the types of controls, the expected pollutant reductions, and the time frame within which water quality standards will be met and controls re-evaluated.

Where no monitoring program exists, or where additional assessments are needed, it is necessary for States to design and implement a monitoring plan. The objectives of the monitoring program should include assessment of water quality standards attainment, verification of pollution source allocations, calibration or modification of selected models, calculation of dilutions and pollutant mass balances, and evaluation of point and nonpoint source control effectiveness. In their monitoring programs, States should include a description of data collection methodologies and quality assurance/quality control procedures, a review of current discharger monitoring reports, and be integrated with volunteer and cooperative monitoring programs where possible. If properly designed and implemented, the monitoring program will result in a sufficient data base for assessment of water quality standard attainment and additional predictive modeling if necessary.

Approval of TMDLs by EPA

TMDLs developed for all water quality-limited waters are submitted to EPA for review and approval. States are encouraged to coordinate with EPA prior to formal submission of their TMDLs. Chapter 4 explains EPA and State responsibilities for the review and approval process.

Implementation of the TMDL

After identifying the necessary pollutant load reductions through the development of TMDLs and after approval by EPA, State water quality management plans should be updated and control measures implemented. This section provides a brief review of point and nonpoint source control implementation. Additional guidance is available and is referenced throughout the remainder of this chapter.

NPDES Process for Point Sources

Both technology-based and water quality-based controls are implemented through the National Pollutant Discharge Elimination System (NPDES) permitting process. Permit limits based on TMDLs are called water quality-based limits.

Wasteload allocations establish the level of effluent quality necessary to protect water quality in the receiving water and ensure attainment of water quality standards. Once allowable loadings have been developed through WLAs for specific pollution sources, limits are incorporated into NPDES permits. It is important to consider how the WLA addresses variability in effluent quality. On the one hand, allocations for nutrients or bioaccumulative pollutants could be expressed as the required average effluent quality because the total loading of these pollutants is of concern. On the other hand, an allocation for toxic pollutants should be expressed as a shorter-term requirement because the concentration of these pollutants is typically of more concern than the total loading.9

As a result of the 1987 Amendments to the Act, Individual Control Strategies (ICSs) were established under section 304(l)(1) for certain point source discharges of priority toxic pollutants. ICSs consist of NPDES permit limits and schedules for achieving such limits, along with documentation showing that the control measures selected are appropriate and adequate (i.e., fact sheets including information on how water quality-based limits were developed, such as total maximum daily loads and wasteload allocations). Point sources with approved ICSs are to be in compliance with those ICSs as soon as possible or in no case later than three years from the establishment of the ICS (typically by 1992 or 1993).

The Clean Water Act (and corresponding State statutes) authorizes imposition of monitoring and data collection requirements on the owner or operator of a point source discharge. Requirements may include ambient and biological assessments, toxicity reduction evaluations, in-plant monitoring, etc. Needed data collection may be initiated through a direct request under Section 308 if there is a reasonable need for the information for EPA to carry out the objectives of the Clean Water Act. The request must also meet the Paperwork Reduction Act requirements. Information may also be collected through permit reporting requirements, or an administrative order. These authorities can be used to collect data from point sources when developing or assessing the effectiveness of a TMDL.

Examples of Best Management Practices
Animal waste management
Conservation tillage
Contour farming
Contour strip cropping
Cover crops
Crop rotation
Fertilizer management
Integrated pest management
Livestock exclusion
Range and pasture management
Sod-based rotations

Disturbed area limits
Nonvegetative soil stabilization
Runoff detention/retention
Surface roughening

Flood storage
Porous pavements
Runoff detention/retention
Street cleaning

Ground cover maintenance
Limiting disturbed areas
Log removal techniques
Pesticide/herbicide management
Proper handling of haul roads
Removal of debris
Riparian zone management
Road and skid trial management

Block-cut or haul-back
Water diversion

Buffer Strips
Detention/sedimentation basins
Devices to encourage infiltration
Grassed waterway
Material ground cover
Sediment traps
Streamside management zones
Vegetative stabilization/mulching

Permit requirements for data collection should be established when longer term data (e.g., for several seasons) are needed. The permit should include a statement that the permit can be modified or revoked and reissued if the data indicate an exceedance of State water quality standards.

State or Local Process for Nonpoint Sources

In addition to permits for point sources, nonpoint source controls may be established by implementing Best Management Practices (BMPs) so that surface water quality objectives are met. These controls should be based on LAs developed using the TMDL process. When establishing permits for point sources in the watershed, the record should show that in the case of any credit for future nonpoint source reductions, (1) there is reasonable assurance that nonpoint source controls will be implemented and maintained or (2) that nonpoint source reductions are demonstrated through an effective monitoring program. Assurances may include the application or utilization of local ordinances, grant conditions, or other enforcement authorities. For example, it may be appropriate to provide that a permit may be reopened for a WLA which requires more stringent limits because attainment of nonpoint source load allocation was not demonstrated.

In order to fully address waterbodies that are impaired or threatened by nonpoint source pollution, States should implement their nonpoint source management programs and ensure adoption of control measures (best management practices) by all contributors of nonpoint source pollution in those watersheds. Example BMPs are listed on the following page. State nonpoint source management programs may include, as appropriate, nonregulatory or regulatory programs for enforcement, technical assistance, financial assistance, education, training, technology transfer, and demonstration projects.

It is difficult to ensure, a priori, that implementing nonpoint source controls will achieve expected load reductions. Nonpoint source control measures may fail to achieve projected pollution or chemical load reductions due to inadequate selection of BMPs, inadequate design or implementation, or lack of full participation by all contributing sources of nonpoint pollution. 10States should describe nonpoint source load reductions and establish a procedure for reviewing and revising BMPs in TMDL documentation. The key objective for documenting load reduction goals and review procedures is to establish a rational procedure for site-specific evaluation of waterbodies with significant nonpoint source pollution loads. States should consult additional nonpoint source guidance for assistance in developing appropriate monitoring and evaluation approaches.11 12

Assessment of the TMDL

Once control measures have been implemented, the impaired waters should be assessed to determine if water quality standards have been attained or are no longer threatened. The monitoring program used to gather the data for this assessment should be designed based on the specific pollution problems or sources. For example, past experience has shown that several years of data are necessary from agricultural nonpoint source watershed projects to detect trends (i.e., improvements) in water quality. As a result, long term monitoring efforts must be consistent over time in order to develop a data base adequate for analysis of control actions.

As shown in Figure 2, a TMDL that allocates loads and wasteloads to meet water quality standards must be established. If the waterbody does achieve the applicable State water quality standards, the waterbody may be removed from the 303(d) list of waters still needing TMDLs. If the water quality standards are not met, the TMDL and allocations of load and wasteloads must be modified. This modification should be based on the additional data and information gathered as required by the phased approach for developing a TMDL, where appropriate, as part of routine monitoring activities, and when assessing the waterbody for water quality standards attainment.


-- the reader is referred to the Permit Writer's Guide to Water Quality-based Permitting for Toxic Pollutants (July, 1987) and the Technical Support Document for Water Quality-based Toxics Control (1985) for additional information on deriving actual permit limits. Back

10 -- USEPA. July, 1987. Setting Priorities: The Key to Nonpoint Source Control. OW/OWRS, EPA. Washington D.C. Back

11 -- USEPA. February, 1988. Draft Nonpoint Source Monitoring and Evaluation Guide. OW/NPS Branch, Washington D.C. Back

12 -- USEPA. September 19, 1989. Nonpoint Source Monitoring and Reporting Requirements for Watershed Implementation Grants. OW/NPS Branch, Washington D.C. Back

Table of Contents Chapter 1 Chapter 2 Chapter 3
Chapter 4 Appendix A Appendix B Appendix C
apd ape apf apg
acro sections

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