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Water: Total Maximum Daily Loads (303d)

Chapter 2 - The Water Quality-Based Approach To Pollution Control


Guidance for Water Quality-Based Decisions:
The TMDL Process

The Water Quality Planning and Management Regulation (40 CFR 130) links a number of Clean Water Act sections, including section 303(d), to form the water quality-based approach to protecting and cleaning up the nation's waters (diagrammed in Figure 1). This chapter describes the overall approach for the development of TMDLs and subsequent implementation of water quality-based point and nonpoint source pollution control measures based on water quality standards. Other related guidance on various aspects of the water quality-based approach are described in Appendix A.

The water quality-based approach emphasizes the overall quality of water within a waterbody and provides a mechanism through which the amount of pollution entering a waterbody is controlled based on the intrinsic conditions of that body of water and the standards set to protect it. This approach begins with the determination of waters not meeting (or not expected to meet) water quality standards after the implementation of technology-based controls (such as BPT and secondary treatment). Waters identified through this process are considered water quality-limited and must be prioritized. An overall plan to manage the excess pollutants in each waterbody can then be developed. The necessary limitations on the introduction of pollutants to the waterbody are identified through the development of a TMDL under section 303(d).

Previous practices for implementing 303(d) have focused primarily on point sources and wasteload allocations (WLA). All water quality-based permit limits are based on a WLA. The WLA is either reviewed individually by EPA or where there exists a State/EPA technical agreement, is developed consistent with that agreement.1 In recent years nonpoint source contributions to water quality problems have become better understood and it is now clear that EPA and State implementation of 303(d) must encompass nonpoint source pollution problems and seek to address problems occurring over large geographic areas. As a consequence, this document describes a more rigorous process for implementing 303(d) and reinforces the need to develop TMDLs that include load allocations (LA) as well as wasteload allocations.

As shown in Figure 1, the water quality-based approach contains the following steps:

  1. Identification of water quality-limited waters still requiring TMDLs.
  2. Priority ranking and targeting.
  3. TMDL development.
  4. Implementation of control actions.
  5. Assessment of water quality-based control actions

Steps 1, 2, and 3 are addressed by the CWA in section 303(d). Steps 4 and 5 are integral parts of the process and are briefly described in this document.

States are to review and revise water quality standards, as necessary, every three years and NPDES permits are to be re-evaluated and issued every five years. The water quality-based approach links these two processes and is, therefore, an ongoing process of evaluation and modification. In addition to standards and permits revisions, section 319(b) nonpoint source (NPS) management plans can and should be continually updated as well.

figure1

Step One: Identification of Water Quality-Limited Waters

The water quality-based approach to pollution control begins with the identification of problem waterbodies. State water quality standards form the basis and "yardstick" by which States can assess the waterbody status and implement needed pollution controls. State water quality standards include three elements: designated uses for the waterbody, criteria (physical, chemical, and biological) to protect the designated uses, and an antidegradation statement. States need to identify those waters not meeting any one of these components of water quality standards.

EPA's Water Quality Planning and Management Regulation establishes the process for identifying water quality-limited segments still requiring TMDLs. Waters require TMDLs when certain pollution control requirements (see box) are not stringent enough to implement water quality standards for such waters.

Identifying Waters Still Requiring
TMDLs: 40 CFR 130.7(b)
(b)(1) Each State shall identify those water quality segments still requiring WLAs/LAs and TMDLs within its boundaries for which:
(i) Technology-based effluent limitations required by sections 301(b), 306, 307, or other sections of the act;

(ii) More stringent effluent limitations (including prohibitions) required by either State or local authority preserved by section 510 of the Act, or Federal authority (e.g., law, regulation, or treaty); and

(iii) Other pollution control requirements (e.g., best management practices) required by local, State, or Federal authority

are not stringent enough to implement any water quality standard applicable to such waters.


The most widely applied water pollution controls are the technology-based effluent limitations required by section 301(b) and 306 of the Clean Water Act. In some cases, a State or local authority may establish enforceable requirements beyond technology-based controls. Examples of such requirements may be those that (1) provide more stringent NPDES permit limitations to protect a valuable water resource or (2) provide for the management of certain types of nonpoint source pollution.

To exempt a water quality-limited water from the TMDL process, the pollution control requirements cited in the regulation under 130.7(b)(i),(ii), and (iii) (see box) must be established and enforced by Federal, State, or local laws or regulations and be stringent enough that, when applied, the receiving waterbody will meet water quality standards. These requirements must also be specifically applicable to the particular water quality problem and, if not yet implemented, a schedule for the timely implementation of such requirements must be established. Chapter 4 contains more specific requirements pertaining to identification of water quality-limited waters still requiring TMDLs (see Chapter 4).

Identification of threatened good quality waters is an important part of this approach. Adequate control of new discharges from either point or nonpoint sources should be a high priority for States to maintain the existing use or uses of these waterbodies. In the identification of threatened waters it is important that the 303(d) process consider the water quality standards program to ensure that a State's antidegradation policies as established in State law are followed.

By identifying threatened good quality waters, States take a more proactive, "pollution prevention" approach to water quality management (see box).

Pollution Prevention Advantages
Consistent with 40 CFR 130.7 (c)(1)(ii) which requires that TMDLs be established for all pollutants that prevent or are expected to prevent water quality standards from being achieved.

Encourages States to maintain and protect existing water quality.

Easier and less costly in the long term to prevent impairments rather than retrofit controls to clean up pollution problems.

Meets EPA objectives to support the State's collection of data on impacted or threatened waters.



Each State may have different methods for identifying and compiling information on the status of its waterbodies depending on its specific programmatic or cross-programmatic needs and organizational arrangements. Typically, States utilize both existing information and new data collected from ongoing monitoring programs to assess whether water quality standards are being met, and to detect trends.

States assess their waters for a variety of purposes, including the targeting of cleanup activities, assessing the extent of contamination at potential Superfund sites, and for meeting federally mandated reporting requirements. While the identification of water quality-limited waters may appear to be a major task for the States, a significant amount of this work has already begun or has been completed under sections 305(b), 304(l), 314(a), and 319(a) of the Clean Water Act as amended in 1987. (Appendix B provides a summary of these supporting CWA programs.)

Section 305(b) requires States to prepare a water quality inventory every two years to document the status of waterbodies that have been assessed. Under section 304(l), States identified all surface waters adversely affected by toxic (65 classes of compounds), conventional (such as BOD, total suspended solids, fecal coliform, and oil and grease), and nonconventional (such as ammonia, chlorine, and iron) pollutants from both point and nonpoint sources. Under section 314(a), States identified a list of publicly owned lakes for which uses are known to be impaired by point and nonpoint sources. Section 319 State Assessment Reports identified waters adversely affected by nonpoint sources of pollution. Lists prepared to satisfy requirements under section 305(b), 304(l), 314(a) and 319 should be very useful in preparing 303(d) lists.

Other existing and readily available data and information sources should be utilized in preparing section 303(d) lists. See, for example, Appendix C, which presents screening categories similar to those found in current regulations promulgating the 304(l) requirements.2 Figure C-1 in the Appendix depicts a sample process for identifying 303(d) waters. Other data sources are listed as an appendix of the Final Guidance for Implementation of Requirements Under Section 304(l) of the Clean Water Act as Amended, March 1988. The Toxic Chemical Release Inventory (TRI) developed under Title III, Superfund and Reauthorization Act (SARA) is an important information source as well as any relevant State-run database.

Section 303(d) requires States to identify those water quality-limited waters needing TMDLs. States should regularly update their lists of waters (or the databases which store the information to produce the lists) as assessments are made and report these lists to EPA once every two years. States should include, in their biennial 303(d) lists, information on which waterbodies have been added or deleted from the list and which waterbodies were assessed since the last reporting period. (See Chapter 4 for further details on submission of lists to EPA.)

Step Two:Priority Ranking and Targeting

Once waters needing additional controls have been identified, a State prioritizes its list of waters using established ranking processes that should consider all water pollution control activities within the State. Priority ranking has traditionally been a process defined by the State and may vary in complexity and design. A priority ranking should enable the State to make efficient use of its available resources and meet the objectives of the Clean Water Act.

The Clean Water Act states that the priority ranking for such waters must take into account the severity of the pollution and the uses to be made of such waters. Several documents (see box) are available from EPA to assist States in priority setting.

Priority Setting Documents
Setting Priorities: The Key to Nonpoint Source Control (OWRS, July 1987).

Selecting Priority Nonpoint Source Projects: You Better Shop Around (OW and OPPE, August 1989, EPA 506/2-89/003).

The Lake and Resevoir Restoration and Guidance Manual, First Edition (OWRS, EPA 440/5-88-002).

The Lake and Reservoir Restoration and Guidance Manual, Second Edition (OWRS, EPA 440/4-90-006).

State clean Water Strategies: Meeting the Challenges for the Future (OW, December 1988).



According to EPA's State Clean Water Strategy document: "Where all water quality problems cannot be addressed immediately, EPA and the States will, using multi-year approaches, set priorities and direct efforts and resources to maximize environmental benefits by dealing with the most serious water quality problems and the most valuable and threatened resources first."

Targeting high priority waters for TMDL development should reflect an evaluation of the relative value and benefit of waterbodies within the State and take into consideration the following:

  • Risk to human health and aquatic life.
  • Degree of public interest and support.
  • Recreational, economic, and aesthetic importance of a particular waterbody.
  • Vulnerability or fragility of a particular waterbody as an aquatic habitat.
  • Immediate programmatic needs such as wasteload allocations needed for permits that are coming up for revisions or for new or expanding discharges, or load allocations for needed BMPs.
  • Waters and pollution problems identified during the development of the section 304(l) "long list."
  • Court orders and decisions relating to water quality.
  • National policies and priorities such as those identified in EPA's Annual Operating Guidance.

States are required to submit their priority rankings to EPA for review. EPA expects all waters needing TMDLs to be ranked, with "high" priority waters -- targeted for TMDL development within two years following the listing process -- identified. (See Chapter 4 for further details on submission of priorities to EPA.)

In order to effectively develop and implement TMDLs for all waters identified, States should establish multi-year schedules that take into consideration the immediate TMDL development for targeted waterbodies and the long-range planning for addressing all water quality-limited waters still requiring TMDLs. While it would be expected that these schedules would change when a State's priorities change in response to "hot spots" or critical situations at any given time, a long-range schedule provides several advantages to a State (see box).


Advantages to Long-range Schedules
  • Encourages integration with the permitting cycle, the water quality standards revisions, and other required water quality management activities.
  • Allows for long-term monitoring which may be needed to assess control action.
  • Sets consistency in developing TMDLs.
  • Establishes a basis for setting overall water quality management priorities.
  • Supports a geopgraphic approach for TMDL development for targeted waterbodies.

Step Three: TMDL Development

For a water quality-limited water that still requires a TMDL, a State must establish a TMDL that quantifies pollutant sources and allocates allowable loads to the contributing point and nonpoint sources so that the water quality standards are attained for that waterbody. The development of TMDLs should be accomplished by setting priorities, considering the geographic area impacted by the pollution problem, and, in some cases, using a phased approach to establishing control measures based on the TMDL.

The TMDL is developed using one or a combination of three technical approaches to protect receiving water quality: the chemical specific approach, the whole effluent toxicity approach, and the biocriteria/bioassessment approach. The chemical specific approach is one where loadings are evaluated in terms of the impact on physical-chemical water quality conditions (e.g., dissolved oxygen or toxicant concentrations). While an integrated approach that considers all three techniques is preferred for the protection of aquatic life, the chemical specific approach is usually the one used to address loads that affect those water quality standards which protect human health.

Many water pollution concerns are area-wide phenomena that are caused by multiple dischargers, multiple pollutants (with potential synergistic and additive effects), or nonpoint sources. Atmospheric deposition and ground water discharge may also result in significant pollutant loadings to surface waters. As a result, EPA recommends that States develop TMDLs on a geographical basis (e.g., by watershed) in order to efficiently and effectively manage the quality of surface waters.

The TMDL process is a rational method for weighing the competing pollution concerns and developing an integrated pollution reduction strategy for point and nonpoint sources. The TMDL process allows States to take a holistic view of their water quality problems from the perspective of instream conditions. Although States may define a waterbody to correspond with their current programs, it is expected that States will consider the extent of pollution problems and sources when defining the geographic area for developing TMDLs. In general, the geographical approach for TMDL development supports sound environmental management and efficient use of limited water quality program resources. In cases where TMDLs are developed on watershed levels, States should consider modifying permitting cycles so that all permits in a given watershed expire at the same time.

For traditional water pollution problems, such as dissolved oxygen depletion and nutrient enrichment, there are well validated models that can predict effects with known levels of uncertainty. This is not true for such non-traditional pollution problems as urban stormwater runoff and pollutants that involve sediment and bioaccumulative pathways. Predictive modeling for these problems therefore uses conservative assumptions, but in many cases the degree of certainty cannot be well quantified until more data becomes available to develop sensitivity analyses and model comparisons. For TMDLs involving these non-traditional problems, the margins of safety should be increased and additional monitoring required to verify attainment of water quality standards and provide data needed to recalculate the TMDL, if necessary.

EPA regulations provide that load allocations for nonpoint sources and/or natural background "are best estimates of the loading which may range from reasonably accurate estimates to gross allotments...".3 A phased approach to developing TMDLs may be appropriate where estimates are based on limited information. The phased approach is a TMDL that includes monitoring requirements and a schedule for re-assessing TMDL allocations to ensure attainment of water quality standards. Uncertainties that cannot be quantified may also exist for certain pollutants discharged primarily by point sources. In such situations a large margin of safety and follow-up monitoring is appropriate.

Where nonpoint source controls are involved, the phased approach is also necessary. Under the CWA, the only federally enforceable controls are those for point sources through the NPDES permitting process. In order to allocate loads among both nonpoint and point sources, there must be reasonable assurances that nonpoint source reduction will in fact be achieved. Where there are not reasonable assurances, under the CWA, the entire load reduction must be assigned to point sources. With the phased approach, the TMDL includes a description of the implementation mechanisms and the schedule for the implementation of nonpoint source control measures.

By pursuing the phased approach where applicable, a State can move forward to implement water quality-based control measures and adopt an explicit schedule for implementation and assessment. States can also use the phased approach to address a greater number of waterbodies including threatened waters or watersheds which would otherwise not be managed. Specific requirements relating to the phased approach are discussed in Chapter 3.

Step Four:Implementation of Control Actions

Once a TMDL or a phased TMDL has been established for a waterbody (or watershed) and the appropriate source loads developed, implementation of control actions should proceed. The State or EPA is responsible for implementation, the first step being to update the water quality management plan. Next, point and nonpoint source controls should be implemented to meet wasteload allocations and load allocations, respectively. Various pollution allocation schemes (i.e., determination of allowable pollution among different pollution sources in the same waterbody) can be employed by States to optimize alternative point and nonpoint source management strategies.

The NPDES permitting process is used to limit effluent from point sources. Chapter 3 provides a more complete description of the NPDES process and how it fits into the water quality-based approach to permitting. Construction decisions regarding publicly owned treatment works (POTWs) and advanced treatment facilities must also be based on the most stringent of technology-based or water quality-based limitations. These decisions should be coordinated so that the facility plan for the discharge is consistent with the limitations in the permit.

In the case of nonpoint sources, both State and local laws may authorize the implementation of nonpoint source controls such as the installation of Best Management Practices (BMPs). Section 319 State management programs can be a useful tool to implement nonpoint source control measures and ensure improved water quality. Many BMPs, however, may be implemented even where regulatory programs do not exist. In such cases, a State needs to document the coordination which may be necessary among State and local agencies, landowners, operators, and managers and then evaluate BMP implementation, maintenance, and overall effectiveness to ensure that load allocations are achieved. Chapter 3 discusses some of the technical issues associated with implementation of nonpoint source control measures.

Step Five:Assessment of Water Quality-Based Control Actions

Throughout the previous four steps, monitoring is a crucial element of water quality-based decision making. In this step, monitoring provides data for an independent evaluation of whether the TMDL and control actions that are based on the TMDL protect or improve the environment and are sufficient to meet changing waterbody protection requirements such as revised water quality standards or changing pollution sources (e.g., urbanization).

Monitoring programs often begin with baseline monitoring. Such monitoring should not be regarded as a prerequisite to implementing control measures for a waterbody. If monitoring has not yet begun, control measures and monitoring should be implemented simultaneously to assure that pollution abatement activities are not delayed.

In the case of point sources, assessments are facilitated in that dischargers are required to provide reports on compliance with NPDES permit limits. In some instances, dischargers may also be required in the permit to assess impact of their discharge on the receiving water. A monitoring requirement can be put into the permit as a special condition as long as the information is collected for purposes of writing a permit limit. States are also encouraged to use innovative monitoring programs (e.g., cooperative monitoring4 and volunteer monitoring5) to provide for adequate point and nonpoint source monitoring coverage.

States should also ensure that effective monitoring programs are in place for evaluating nonpoint source control measures. EPA recognizes monitoring as a high priority activity in a State's nonpoint source management program. 6 To facilitate the implementation and evaluation of NPS controls States should consult current guidance. 7 8



1 -- USEPA. 1985. Guidance for State Water Monitoring and Wasteload Allocation Program. OW/OWRS, EPA 440/4-85-031. Washington D.C. Back

2 -- 40 CFR 130.10 (d)(6) Back.

3 -- 40 CFR 130.2(g) Back

4 -- USEPA. 1984. Planning and Managing Cooperative Monitoring Projects. OW/OWRS. EPA 440/4-84-018. Washington D.C. Back

5 -- USEPA. 1990. Volunteer Water Monitoring: A Guide for State Managers. OW, EPA 440/4-90-010. Washington D.C. Back

6 -- 55 FR 3563, August 28, 1990 Back

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

8 -- 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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