Understanding TMDLs: A primer for permit writers

Understanding TMDLs: A primer for permit writers
Welcome to this presentation on understanding the process for developing Total Maximum Daily Loads, or TMDLs. This presentation is the first of three Web-based training modules sponsored by the U.S. Environmental Protection Agency on the topic of TMDLs and National Pollutant Discharge Elimination System, or NPDES, permitting. This module focuses on providing permit writers a basic understanding of the TMDL program and how TMDLs are developed, specifically what goes into establishing wasteload allocations for permitted point sources. While it isn’t necessary to complete the other modules in this Web-based training series, the information in the other modules do cover fundamental TMDL and NPDES permitting issues that are related to the topics addressed in this presentation. The second module covers implementation of wasteload allocations, referred to as WLAs, through the NPDES permitting program, while the third module focuses on TMDL and NPDES stormwater permit integration. Before we get started, I’d like to cover one important housekeeping item. All the materials used in this presentation have been reviewed by U.S.EPA staff for technical accuracy. Development and implementation of WLAs are governed by the existing requirements of the Federal Water Pollution Prevention and Control Act, known as the Clean Water Act, and U.S. EPA’s NPDES implementing regulations. These statutory and regulatory provisions contain legally binding requirements. The information in this presentation is not binding. Furthermore, it supplements, and does not modify, existing U.S. EPA policy, guidance, and training on TMDLs and NPDES permitting. U.S. EPA may change the contents of this presentation in the future. Now let’s take a look at the objectives of this module. U.S. Environmental Protection Agency

Module Objectives Provide a basic understanding of the TMDL program
Illustrate the technical steps in developing TMDLs Discuss typical approaches for developing TMDLs Highlight connection between TMDLs and NPDES permits This presentation has four primary objectives. First, to provide you with an overview of the TMDL program, including its regulatory foundation and requirements and how it fits in the broader context of the Clean Water Act. Second, to illustrate the steps in developing a TMDL. Third, to discuss some of the commonly used approaches for completing those steps and developing a TMDL, specifically the incorporation of point sources and development of associated WLAs. Finally, this module highlights the connection between TMDLs and NPDES permits. To help achieve these objectives, this module also presents a few example TMDLs, illustrating the development process and final elements of a typical TMDL.

Module Roadmap Definition of a TMDL Clean Water Act context
TMDL Elements TMDL Development Process Emerging or Evolving Issues Tips for Engaging in the TMDL Process Resources To accomplish these objectives we are first going to define a TMDL. We'll then discuss how the TMDL program fits into the Clean Water Act context and what the Act and its implementing regulations say about TMDLs within the broader water quality management framework. After understanding this bigger picture, we'll talk about the specific elements of a TMDL and how those elements are established or defined through the TMDL development process. We'll also talk about a few emerging or evolving issues currently of interest in the TMDL community, including issues that also pertain to the NPDES community. Finally we'll go over some ways that permit writers can become more involved in the TMDL process to ensure more appropriate WLAs. For additional technical details on TMDLs and related topics and approaches, EPA has issued a number of guidance documents and technical support documents. A list of helpful resources is provided at the end of this module.

What is a TMDL? Greatest amount of loading that a water can receive without violating water quality standards (i.e., loading capacity). [40 CFR 130.2(f)] TMDL = SWLAi + SLAi + MOS SWLAi: Sum of waste load allocations for existing and future point sources SLAi: Sum of load allocations for existing and future nonpoint and background sources MOS: Margin of safety To understand the TMDL program, let’s first start with the definition of a TMDL. TMDLs are developed for impaired waterbodies, which are waterbodies that do not meet their applicable water quality standards. TMDLs identify the greatest amount of loading that a waterbody can receive without violating water quality standards. This allowable load is the TMDL and is also known as the loading capacity. TMDLs are also required to include a safety factor as an extra measure of environmental protection, taking into account uncertainties associated with estimating the pollutant sources and their effect on the waterbody. This is referred to as the margin of safety. Once the loading capacity has been identified (with the margin of safety factored in), the TMDL is divided among the pollutant sources. Loads assigned to existing and future point sources are called wasteload allocations, or WLAs. Those assigned to existing and future nonpoint sources and natural background are called load allocations, or LAs. This concept is often illustrated with this equation showing that the TMDL is the sum of the individual WLAs and LAs as well as the incorporation of the margin of safety.

What is a TMDL? WLAs set pollutant loading cap for point sources
LAs set pollutant loading cap for nonpoint sources Reserve capacity sets aside allocation for future development Margin of Safety (MOS) allocation accounts for uncertainty Reserve Capacity Savings Account Point Source #1 Mortgage Contingencies MOS Point Source #2 Water & Electricity Car Payments Nonpoint Source #2 Another way to think of this concept is seeing the TMDL as a “pie” where the sum of all of the slices equals the total load that can be discharged to the waterbody. The portions of the TMDL pie assigned to point sources are the WLAs. Every point source that discharges to a water for which a TMDL has been developed receives a WLA. A point source’s WLA places a cap on the amount of pollutant that a particular point source can discharge to the waterbody and are implemented through NPDES permits. The portions of the TMDL pie assigned to nonpoint sources or background loads are the LAs. Other portions of the TMDL pie can include an allocation set aside to allow for future development, typically referred to as the reserve capacity. This is not required, but sometimes is included so that the TMDL doesn’t have to be reopened to accommodate new sources. Another portion of the TMDL pie might include an allocation for the Margin of Safety. The Margin of Safety can also be addressed through conservative assumptions in the TMDL calculations rather than as an explicit portion of the loading capacity. In essence a TMDL is a pollutant budget for a specific watershed that when met will result in water quality standards. You can think of this similar to a household financial budget that is set to allow for certain expenses while meeting an overall limit based on what you can afford. To meet your financial goals, you have an allowable amount of money to spend, similar to the loading capacity. This total budget is divided among certain expense categories such as your mortgage, car payments, electricity and water bills and budgets for groceries. These budget allocations would be similar to the wasteload and load allocations set for known point and nonpoint sources. In a TMDL, the margin of safety can represent an explicit portion of the load for uncertainty or an implicit inclusion through conservative assumptions. In the household budget, you might set aside money to account for unexpected expenses or make conservative assumptions about the cost of expenditures you are expecting to ensure that you have enough money remaining to cover all of your expenses within your allotted budget. In addition, you might have a savings account set aside for known or unknown future expenditures such as needed home repairs or a new car. This account would be similar to the reserve allocation in the TMDL for future sources. Before we proceed, I’d like to highlight some terminology from the TMDL world. The term “TMDL” refers to the loading capacity and associated allocations for each individual pollutant-waterbody combination. However, many people also use the term TMDL to refer to the TMDL report with all of the required elements and associated analysis that might encompass multiple impaired segments and impairments. Throughout this module we have tried to consistently use TMDL to mean the TMDL itself and use separately the phrases “TMDL report” and “TMDL analyses” where appropriate. Now that we’ve defined what TMDLs are, let’s briefly look at how they fit into the bigger picture of the Clean Water Act and related programs. Groceries & Other Essentials Nonpoint Source #1

Steps in the Water Quality-based Approach of the Clean Water Act
Relevant Statutes and Regulations Adopt Water Quality Standards CWA 303(a)–(c) 40 CFR 103.3 Monitor and Assess Waters 40 CFR 103.4 List Impaired & Threatened Waters CWA 303(d) 40 CFR 103.7 Develop TMDLs (TMDL = WLA + LA + MOS) CWA 303(d) 40 CFR 103.7 The TMDL program is established in Section 303(d) of the Clean Water Act. The overall goal of the Clean Water Act is “to restore and maintain the chemical, physical, and biological integrity of the Nation’s waters.” As shown in this figure, the water quality-based planning and management process relies on first adopting water quality standards, then monitoring waterbodies for compliance with standards, identifying those that do not meet standards and are therefore impaired and then developing TMDLs for restoring the waterbody to meet standards. The wasteload allocations established in the TMDLs are then implemented through federal and state permitting programs while load allocations are typically implemented through nonpoint source management programs that might include grants or voluntary programs for implementation of nonpoint source controls. Alongside this figure we’ve noted the relevant sections of the Clean Water Act and the associated regulations in case you’d like to read the regulatory language related to the TMDL program. We’ll now quickly walk through each of the steps illustrated in this figure, leading to the development of a TMDL. We’ll start with a brief overview of water quality standards. Control Point Sources via NPDES Permits Manage Nonpoint Sources through Grants, Partnerships and Voluntary Programs CWA 303(e) 40 CFR 103.5 40 CFR 103.6 Trading

Water Quality Standards
Designated use (e.g., aquatic life, recreation, drinking water) Water quality criteria to protect uses Narrative or numeric Magnitude, duration, frequency (e.g., DO: 5 mg/L daily minimum; fecal coliform: 200 counts/100 mL 30-day geometric mean) Antidegradation provisions General implementation policies Water quality standards are the foundation of the water-quality based control program mandated by the Clean Water Act. State-adopted water quality standards define the goals for a waterbody, thereby acting as the driver for all protection and restoration efforts through the CWA. Water quality standards consist of four basic elements. The first element is designated uses of the waterbody. These typically include uses for water supply, aquatic life, agriculture and recreation such as swimming and boating. The second element is water quality criteria to protect designated uses. Criteria can be narrative or numeric in nature. Numeric criteria are pollutant concentrations representing levels protective of human health, aquatic life or wildlife. Numeric criteria have an associated magnitude, duration and frequency. Examples include a criterion for dissolved oxygen set at a daily minimum of 5 mg/L or for fecal coliform as a maximum geometric mean of 200 counts/100 mL over a 30-day period. Narrative criteria typically are qualitative descriptions of conditions or prohibitions necessary to protect uses. Examples include statements like “surface waters shall be free from nutrients in concentrations that would cause or contribute to impairment of existing or designated uses” or “Surface waters must be virtually free from floating oils or sheens”. The third element of water quality standards is an antidegradation policy to maintain and protect existing uses and high quality waters. And finally, the fourth element is general policies addressing issues related to the implementation of the standards. These could include statements on design low flows, variances, or mixing zones.

Monitoring and Assessment
Establish monitoring programs to assess the quality of waters Include physical, chemical and biological data Include appropriate quality assurance and control Support a variety of CWA programs: Abatement and control Water quality standards TMDLs NPDES Waterbody health The next step is monitoring and assessment. The Clean Water Act requires states to establish appropriate monitoring methods and procedures to compile and analyze data on the quality of waters of the United States. The state’s water monitoring programs should be designed to collect and analyze physical, chemical and biological data and also include quality assurance and control programs to assure collection of scientifically valid data. The uses of these data support a variety of Clean Water Act programs and activities. These include identifying impaired or polluted waters; determining abatement and control priorities; developing and reviewing water quality standards; developing and reviewing TMDLs; assessing compliance with NPDES permits; and determining the health of the state’s waters.

303(d) List of Impaired Waters
Identify waters that do not meet WQS after: Technology-based effluent limitations More stringent effluent limitations Other pollution control requirements Include a priority ranking for all listed segments Identify TMDLs scheduled for next 2 years Identify the pollutants causing or expected to cause violations of the applicable WQS Provide documentation to support determination to list or not to list its waters States use data collected through their monitoring programs to determine the health of their waters by analyzing the levels of different pollutants such as metals and bacteria. Physical parameters such as pH and dissolved oxygen, measures of habitat quality, and the composition of biological assemblages such as fish, macroinvertebrates, and/or algae are also evaluated. States analyze the monitoring data to identify violations of water quality criteria, including nonsupport of designated uses, to determine whether waters are impaired. Impaired waters are those waters that do not meet water quality standards even after the implementation of technology-based limits such as required treatment limits at wastewater treatment facilities and after implementation of other effluent limitations or pollution control requirements required by federal, state or local agencies. States are required to develop a list of those impaired waters every two years. This list is referred to as the 303d list and represents those waters for which TMDLs must be developed to restore the waterbody to water quality standards. The list must include a priority ranking for each listed segment, including identifying those segments for which TMDLs are scheduled within the next two years. The list also must identify the pollutant causing or expected to cause the observed impairment in each listed segment. The 303d list submittal must also include documentation to support the state’s decisions on which waterbodies to include on the list. This includes a description of the methodology used to develop the list, a description of the data and information used to identify impaired waters, and, if a state decides not to use certain data in the listing process, a rationale for why the data were not used.

303(d) List of Impaired Waters (cont.)
Developed every 2 years Submitted to EPA for review/approval Available on state websites and summarized on EPA’s TMDL website: The first 303(d) lists were developed in 1992 and since then states develop and submit their lists every two years. States submit their lists to EPA who must approve or disapprove the list. If EPA disapproves the list, they will then develop the list. States are required to provide a public notice of the list development process for public review. In doing this they typically post their draft and final 303(d) lists on their websites. In addition, EPA’s TMDL program website includes nationwide and state summaries of 303(d) listed waters and their causes of impairment.

Integrated Report Integrated Report
303(d) list (impaired/threatened waters) 305(b) report (overall health of waterbodies) 314 report (health of lakes/reservoirs) Integrated Report Since 2002, due April 1, every even-numbered year Reporting guidance issued for each listing cycle (2002, 2004, 2006 and 2008): Beginning with the 2002 listing cycle, EPA issued guidance encouraging states to prepare a report that integrates the information reported under Sections 303(d), 305(b) and 314. As we’ve discussed, Section 303(d) requires states to submit their lists of impaired and threatened waters by April 1 of all even numbered years. Section 305(b) requires states to submit a description of the water quality of all waters of the state including rivers/stream, lakes, estuaries/oceans and wetlands. States may also include in their section 305(b) submittal a description of the nature and extent of ground water pollution and recommendations of state plans or programs needed to maintain or improve ground water quality. These reports are also required to be submitted by April 1 of all even numbered years. In addition, under Section 314, states are required to include in each 305(b) submittal an assessment of the status and trends of the health of significant publicly owned lakes, including the extent of point source and nonpoint source impacts due to toxics, conventional pollutants, and acidification. Since the 2002 listing cycle, states have been preparing a single report to satisfy reporting requirements for all three sections. This report is referred to simply as the Integrated Report.

Integrated Report (cont.)
Category Description 1 All designated uses met 2 Some, but not all, uses met 3 Cannot determine if any uses met (insufficient data) 4 Impaired/threatened – TMDL not needed 4a TMDL completed 4b TMDL alternative 4c Non-pollutant causes 5 Impaired/threatened by pollutant –TMDL needed The Integrated Report documents the water quality status of all assessed waters, as well as the availability of data and information for each waterbody, identifies certain trends in water quality conditions, and provides information to managers in setting priorities for future actions. Those waters that have been assessed by the state are included in 1 of five categories of the Integrated Report depending on their water quality status. States place waters in Category 1 when they are attaining all designated uses and no use is threatened. Waters are placed in Category 2 when the state determines that available data and information indicate that some, but not all, of the designated uses are supported. This category typically represents those waters where sufficient data are not available to assess all uses, but those uses that can be assessed are being supported. Waters are placed in Category 3 when there is insufficient data or information available to determine whether uses are supported. For waters in this category, EPA recommends that the state identify those segments that are higher and lower priority for follow-up monitoring or modeling to further evaluate use support and potential impairment. Category 4 represents those waters that are impaired or threatened but do not require the development of a TMDL. Because there are a variety of reasons recognized by EPA why a waterbody could be impaired but not need a TMDL, this category is divided into three subcategories. First, Category 4a are those waters that are impaired but have already had a TMDL developed. Next, Category 4b is for those waters where other pollution control requirements are being implemented and are expected to result in the attainment of water quality standards. Examples of other pollution control requirements have included Superfund clean-up activities that have addressed contaminated sites that were causing water quality impairments, reissuance of more stringent point source permits where single facilities were the cause of the observed impairment, and development and implementation of local ordinances that address site-specific sources of pollutants. The final subcategory of Category 4 is 4c, which is reserved for those waters where the failure to meet water quality standards is not caused by a pollutant, but instead by other types of pollution. Examples of circumstances where an impaired segment may be placed in Category 4c include waterbody segments impaired solely due to stream channelization or lack of adequate flow. The final category of the Integrated Report is Category 5. This category includes waters where data indicate that water quality standards are not supported, even after implementation of required technology-based effluent limitations, more stringent effluent limitations, and other pollution control requirements. Therefore, Category 5 represents the traditional Section 303(d) list of impaired waters that require TMDLs. A state’s 303(d) list typically identifies the basic information regarding the impaired waterbody and the observed impairment, usually including identifying information such as waterbody name, location, and size, as well as the water quality standard that was violated, the pollutant of concern, and the suspected causes and sources of impairment. Section 303(d) List

TMDL Roles States develop TMDLs for each “pollutant/waterbody combination” Public review and comment EPA reviews/approves If disapprove, EPA develops TMDL EPA has developed TMDLs in response to court orders or at request of states States develop TMDLs based on their 303d list and the associated priority rankings and TMDL schedules. Once a state develops a TMDL and associated report, the state must release it for public review and comment, often holding public meetings on draft TMDLs. The state then develops the final TMDL report, including a response to comments received during the public participation process, and submits the TMDL report to EPA for review. EPA is required to approve or disapprove a TMDL. If EPA disapproves a state TMDL, EPA must then establish the TMDL. In some cases EPA has established TMDLs instead of the state. For example, EPA has established a number of TMDLs in response to court orders or at the request of a state in situations such as large multi-jurisdictional waterbodies.

The TMDL Program Today 71,000+ impairments (waterbody-pollutant combinations) Top 10 Listed Impairments Pathogens Metals Nutrients Organic enrichment/ low DO Sediment PCBs Mercury pH Impaired biota Turbidity As of January 2012 While the CWA has required TMDLs since 1972, the early focus was on developing WLAs for individual point sources. In the late 1990s, environmental groups began bringing legal action against EPA seeking more accurate listing of waters and development of TMDLs. The litigation resulted in court-ordered schedules or consent decrees outlining TMDL development schedules in a number of states and requiring EPA to develop the TMDLs if the states did not. A significant effect of the litigation is that EPA and states began focusing TMDL efforts on waters impaired by not only point sources, but also by nonpoint sources or a combination of point and nonpoint sources. Since the increased activity in the TMDL program in the 1990s, the number of impaired waters has increased as has the number of TMDLs developed annually. As of January 2012, there are more than 71,000 impairment listings, representing more than 41,000 waterbody segments listed for one or more pollutants. The top 10 listed impairments across the nation include pathogens, metals, nutrients, organic enrichment and low dissolved oxygen, sediment, PCBs, mercury, pH, impaired biota and turbidity.

TMDL Elements Water Quality Standards Loading capacity
Mass per time, toxicity or other appropriate measure (40 CFR 130.2(f) and 130.2(i)) WLAs (40 C.F.R. §130.2(h)) LAs (40 CFR 130.2(g)) Range from reasonably accurate estimates to gross allotments MOS (CWA 303(d)(1)(C), 40 CFR 130.7(c)(1)) Implicit or explicit (USEPA 1991) Critical conditions (40 CFR 130.7(c)(1)) Seasonal variation (CWA 303(d)(1)(C), 40 CFR 130.7(c)(1)) Each TMDL and associated report must meet a minimum set of requirements as established in the Clean Water Act and its implementing regulations. We’ll now discuss each of these required elements and then discuss the process used to develop a TMDL, including each of these elements. The first required element is the applicable water quality standards that the TMDL is calculated to restore. The next required element is the TMDL itself... that is, the loading capacity. As we’ve already discussed, the loading capacity is that amount of a pollutant that can be assimilated by the waterbody and still result in attainment of water quality standards. The loading capacity is typically calculated as a mass per time, such as pounds per day, but according to regulation, it can also be expressed as a toxicity or other appropriate measure depending on the impairment. That loading capacity is divided among allocations for point sources and nonpoint and background sources, leading to our next required elements of wasteload allocations and load allocations. WLAs are assigned to individual existing or future point sources. Unpermitted nonpoint sources receive load allocations, which EPA recognizes can range from specific allocations for individual sources to gross allotments across a watershed. The required margin of safety must be incorporated in the TMDL analysis to account for any uncertainty about the sources and their impact on the impaired waterbody. This margin of safety can be included explicitly, with a portion of the loading capacity allocated to the MOS, or it can be implicit, incorporated through conservative assumptions in the TMDL analysis. Regulations also require the TMDLs be developed for critical conditions, including taking into account flow characteristics and pollutant loading. Critical conditions can be thought of as the worst case scenario for impairment in that particular waterbody. When the TMDL is developed to meet water quality standards under those conditions, it ensures that standards will be met at all times. Critical conditions are usually characterized by some combination of environmental factors such as flow, temperature, pollutant loading, or weather conditions that results in the worst times of impairment. Often evaluated in concert with the critical conditions, the consideration of seasonal variation accounts for patterns or variations in source pollutant loading or resulting impairment throughout the year.

TMDL Elements (cont.) Reasonable assurance Future growth or sources
When impaired by a blend of point and nonpoint sources, provide "reasonable assurances" that LAs will be achieved (USEPA 1991) Future growth or sources Daily load (Grumbles 2006) TMDLs should be expressed in terms of daily time increments TMDLs may include alternative, non-daily load expressions to facilitate implementation of WQS In addition to those TMDL requirements established in regulation, there are also other elements required or recommended by TMDL program policy, including reasonable assurance, future growth and identification of daily loads. For TMDLs that include both point and nonpoint sources, reasonable assurance must be provided that load allocations are expected to occur. This is necessary to support wasteload allocations that are based on an assumption that nonpoint source load reductions will occur. Reasonable assurance can include how and when actions will be implemented to achieve nonpoint source reductions. Allocations for future sources or future growth are included in some but not all TMDLs. Regulatory definitions for load allocation and wasteload allocations mention both existing and future sources. And it is typically determined on a TMDL-by-TMDL basis whether it’s necessary or appropriate to allocate a portion of the loading capacity as a reserve for future growth or even as an allocation to a specific source expected to discharge in the future. Although TMDL stands for “total maximum daily load”, TMDLs sometimes are presented as monthly, seasonal or even annual loads depending on the given impairment. However, in November 2006 EPA issued a memo recommending that all TMDLs and associated load allocations and wasteload allocations include a daily time increment in conjunction with other non-daily expressions. EPA subsequently issued a companion technical document entitled Options for Expressing Daily Loads in TMDLs.

TMDL Process Elements in a TMDL Submittal
1. Description of waterbody, pollutant of concern, pollutant sources, and priority ranking Problem Understanding TMDL Target Identification Source Assessment 2. WQS and numeric WQ target* Loading Capacity* (including critical conditions*) Linkage between Loading and Waterbody Response 4. LAs* 5. WLAs* 6. MOS* Seasonal Variation* Reasonable Assurance+ Stakeholder Involvement & Public Participation Allocation Analysis Now that we’ve identified what a TMDL is and also discussed all the required elements of a TMDL, we’ll walk through the steps that are typically followed to identify the TMDL and associated required elements. First we’ll provide an overview of the entire process and then provide more detail on each of the steps. The first step in developing a TMDL typically involves identifying the problem the TMDL will address. This involves compiling and analyzing available data, including GIS, monitoring, and weather data, to gain a basic understanding of the listed impairment as well as the impaired waterbody and its watershed. In understanding the problem, it’s also important to identify the goals of the TMDL, that is, identify the applicable water quality standards the TMDL is designed to meet. Based on those standards, the TMDL developer will identify a numeric TMDL target that will be used to calculate the loading capacity. Another initial step in the TMDL process is identifying and characterizing the potential sources contributing to the impairment. These three steps together are sometimes referred to as the Watershed Characterization. They provide the foundation for the entire TMDL process, providing the TMDL developer the necessary background on the nature of the impairment, water quality goals and sources to be able to make decisions throughout the process on what approaches to use, how to determine the loading capacity and how to distribute allocations. With a basic understanding of the impairment and all the sources and conditions contributing to that impairment, the TMDL developer can move on to calculating the TMDL. To do this, it’s necessary to establish a relationship or link between the watershed pollutant loading and the resulting waterbody impairment. Identifying this cause-and-effect relationship is referred to as the linkage analysis and it is necessary for identifying the loading capacity that will result in attainment of water quality standards. A number of approaches can be used to conduct the linkage analysis, as we’ll discuss later in our step-by-step discussion. Once the TMDL developer calculated the loading capacity, it’s divided among the point and nonpoint sources. The identification of allocations also involves the consideration of the margin of safety and seasonal variation. At this point in the TMDL process, the TMDL developer will also determine how to incorporate reasonable assurance depending on the sources and chosen allocations. Although not required by regulation, TMDLs typically include some information on how to implement the TMDL and what follow-up monitoring should be conducted to track water quality improvements and progress toward attainment of water quality standards. The final step in developing a TMDL is preparing the TMDL report and submitting it to EPA for review and approval. The TMDL report will document the entire TMDL development analysis and resulting allocations, including all of the required TMDL elements listed here. In addition, public participation is a required part of the TMDL development process. The typical activities for public participation include releasing the TMDL report for public review and comment as well as presenting the TMDL at a public meeting in the watershed. While those are the minimum requirements for TMDL public participation, many TMDL developers try to involve stakeholders and the public throughout the process to obtain data and input as well as facilitate stakeholder buy in and implementation. As a reminder, this slide notes those TMDL elements that are required by regulation and those recommended through EPA guidance. For further information on the required elements of a TMDL and what is expected in the documentation of those elements, you can refer to EPA’s guidelines on reviewing TMDLs. These guidelines are available on EPA’s TMDL program website. This document and the website are also included in a list of resources at the end of this module. Now let’s move on to the detailed discussions of each step of the TMDL development process. Implementation and Monitoring Plan 9. Monitoring Plan+ 10. Implementation Plan+ 11. Public Participation* TMDL Report and Submittal * Required by regulation (40 CFR 130.7) + Recommended through guidance From Guidelines for Reviewing TMDLs under Existing Regulations issued in 1992 (May 20, 2002):

Problem Understanding – Questions to Answer
What was the basis for listing (e.g., supporting data)? What pollutant is causing the problem? Under what conditions does the problem occur (i.e., critical conditions)? What characteristics of the waterbody and/or its watershed might be exacerbating or mitigating the problem? What are the known or potential sources? What efforts to protect the watershed are already underway? The goal of the problem understanding is to gain a basic understanding of the impairment being addressed by the TMDL so that the TMDL developer can proceed with the TMDL analysis and calculation in a way that is appropriate and accurate given the particular impairment, relevant water quality standards, waterbody, sources of pollutants and watershed characteristics. A state’s 303(d) list typically identifies the basic information regarding the impaired waterbody and the observed impairment. However, in most cases, the listing information needs to be supplemented with review of additional data and information to fully understand the problem. To do this, there are some primary questions that should be answered during the Problem Understanding step. First being, what is the basis for the original 303d listing? To develop the TMDL it’s also necessary to identify what pollutant is causing the problem. Typically the information included for the 303d listing will identify the pollutant causing the impairment. However, some listings don’t identify a specific pollutant, such as listings for biological impairments, listings for broader pollutant categories such as nutrients or metals, or listings that reflect a response to pollutant loadings and other conditions, such as low dissolved oxygen or sediment toxicity. In such cases, it is necessary to use additional data analyses to identify the specific pollutant or pollutants causing the observed impairment. Further defining the impairment involves identifying those conditions under which the impairment occurs. This helps to identify the critical conditions and define when the impairment is worst. Similar to this is the next question of what characteristics of the waterbody or its watershed might be affecting the impairment. Other questions that will help define the problem and support subsequent analyses in the TMDL development process are what are the known or potential sources and what efforts are already underway to protect the watershed or restore water quality standards? Identifying the sources and existing restoration or planning efforts can also help to identify stakeholders and support later implementation planning. Answering these questions often relies on compilation and analysis of a variety of waterbody and watershed data.

Problem Understanding – Data Compilation & Analysis
Waterbody monitoring data Point source monitoring data Watershed data and GIS coverages Previous studies Data Analysis What is the problem? Where does it occur? When does it occur? How does it occur? The types of data typically compiled and reviewed to characterize the impairment and better define the problem include a variety of monitoring data as well as GIS information and existing watershed reports. Monitoring data can include a variety of physical, chemical and biological monitoring data as well as point source effluent monitoring data from discharge monitoring reports, or DMRs. Typical GIS data that are compiled and evaluated include watershed land use or land cover, stream networks, soils, location of permitted point sources and topography and elevation. In addition, weather data collected within the watershed or nearby are compiled to support water quality data analyses. Many of the documents included in the resources list at the end of this module provide details on the types of data used and from where they are typically obtained. It should be noted that, TMDLs are developed based on the best available data. There are no legal requirements that dictate how much data are needed for TMDL development. Therefore, TMDLs are sometimes developed with limited data and if necessary can be revised when more data become available. Analysis of data to better define the impairment and water quality and watershed conditions is the primary activity of the problem understanding phase. Identifying the critical issues for water quality problems and dynamics can be accomplished by answering the questions of what, when, where, and how. That is.... What is the problem? Where does it occur? When does it occur? And how does it occur? The answers to these questions help to define many of the technical aspects of the TMDL, including what sources are included, what approaches can be used, how allocations are determined, and on what time and spatial scale the analysis is conducted. Now let’s go over each of these analyses in a little more detail. It’s important to also note that, while we are discussing these analyses separately, often times they are all conducted simultaneously so that the TMDL developer can look at the different results together to inform a larger perspective and understanding of the impairment and sources.

Data Analysis – Impairment Analysis
Confirm impairment Understand magnitude/frequency of exceedances The first focus of the data analysis phase is to answer the question of what is the problem? To do this, water quality monitoring data are analyzed to confirm the listed impairment by evaluating the presence of the pollutant of concern and how concentrations compare to applicable water quality criteria. For example, this figure compares observed monitoring data for fecal coliform to the applicable water quality criteria, including an instantaneous maximum concentration as well as a maximum 30-day geometric mean. The graph confirms that exceedances of the criteria have occurred and an impairment does exist. Once the impairment is confirmed, the TMDL developer can move on to evaluating where it occurs.

Data Analysis – Spatial Analysis Examples
Understanding spatial variations in waterbody and watershed conditions can help to identify pollutant sources and waterbody or environmental conditions that are contributing to the impairment. For instance, areas of relatively higher pollutant concentrations can indicate areas of source activity or perhaps areas where waterbody conditions are resulting in increased degradation. This map shows an example of a spatial analysis of pollutant concentrations throughout a watershed, noting the relative magnitude of concentrations across all sample sites. In a similar analysis, this graph shows statistics for data collected at a number of stations located along the length of an impaired river. These types of analysis of general trends throughout the impaired waterbody and its watershed paint a picture of what areas are experiencing the worst water quality and provide clues to the location and type of sources affecting impairment. These data can be evaluated along with information on the location of point and nonpoint sources and watershed land uses as well as other characteristics to identify sources and conditions potentially contributing to impairment. Understanding the areas that are exhibiting the worst impairments not only helps to identify the potential sources in the watershed but also to prioritize areas for future implementation, monitoring, enforcement & compliance, and restoration.

Data Analysis – Spatial Analysis Examples (cont.)
In addition to general trends across a watershed, spatial analyses can be used to take a closer look at the impact of individual sources, provided sufficient data are available. For example, this graph shows matching data collected upstream and downstream of an expected source to evaluate the potential impact of the source on downstream water quality in an impaired segment. Plotting the upstream data versus the downstream data shows that TSS values at the downstream station are consistently higher than those measured at the same time upstream, indicating that sediment inputs between the stations are causing an increase in instream concentrations. This next graph is an alternate representation of the same data from the first figure, simply plotting the data chronologically at the two stations to visually determine whether upstream and downstream stations are comparable and follow similar patterns. Again, the data show that downstream concentrations are consistently higher than those measured upstream, indicating the source discharging between the stations is having a considerable impact on instream concentrations. This type of upstream-downstream comparison can be helpful in evaluating the influence of tributary inputs as well individual sources, which can be especially useful when developing watershed TMDLs that evaluate multiple impaired segments.

Data Analysis – Temporal Analysis Examples
In addition to evaluating the spatial trends, another question to tackle is when the impairment is occurring. Temporal trends and patterns in water quality and watershed conditions can occur over a variety of timeframes, including daily, month to month or even year to year. The variations can be the result of patterns in environmental conditions, such as weather and resulting runoff and flows, or from changes in loading because of variations in source activities. Examples of temporal changes in source activities are varying schedules and locations for grazing livestock, road sanding during winter months, seasonal recreational activities such as boat use, and schedules for crop fertilization. Because of the variable influences, temporal analysis for a TMDL typically evaluates not only the patterns in the water quality conditions and the impairment but also weather patterns and variations in source activity and other watershed conditions. This first example of a temporal analysis shows monthly statistics for a long-term record of fecal coliform measurements, illustrating the variation in the typical range of bacteria concentrations from month to month. In evaluating the trends along with weather data and watershed information provided likely explanations for the variations. For example, bacteria levels are lowest during winter because the stream is typically frozen during January and February and begin to increase during the spring melt in March and April. The concentrations then dramatically increase during summer months when there is a large population of grazing livestock in the area. As illustrated by this next graph, analysis of longer-term, temporal variations such as trends over a decade, rather than across months or seasons, can also provide clues about watershed sources. This analysis also evaluates both spatial and temporal variations by using data from two stations on the same stream and collected over a 4-year period. The data show a significant increase in turbidity [3] downstream between the spring and fall of Because this suggested the introduction of a new source discharging between the two stations, TMDL developers investigated the issue further and determined that a private landowner was clear cutting forested areas and doing construction on his property, causing significant erosion and sediment delivery to the stream.

Data Analysis – Relationship among Parameters
Evaluate relationship among pollutants contributing to impairment(s) (e.g., sediment and phosphorus) Evaluate relationship among pollutants and other water quality response measures (e.g., nutrients and DO) Evaluate water quality and other waterbody conditions (e.g., flow, temperature, depth) The final piece of data analysis we’ll discuss is the analysis of relationships among multiple pollutant parameters or other waterbody measures. This can include evaluating the occurrence and magnitude of multiple pollutants of concern. Pollutants that follow similar trends can indicate a common source or perhaps that they are related, such as phosphorus delivered to a waterbody attached to sediment. Pollutant data can also be evaluated together with parameters that might exacerbate the impairment. For example, when pollutants are input to a waterbody, a number of physical, chemical and biological processes can occur in response to or along with those pollutants to result in the observed impairment. Examples of impairments that are dependent on pollutant loading as well as a number of complex and varied processes are nutrient-related impairments such as excessive plant growth, low dissolved oxygen, and toxicity from plant decay and decomposition. In addition, how nutrients are used within a waterbody and the impact they have on plant growth and subsequent waterbody responses depends on things like waterbody flow, depth, temperature, and clarity. Conducting these types of analysis to identify how pollutants and waterbody conditions interact and relate to waterbody response can help to understand how and why the impairment is happening, as well as identify potential sources and the critical conditions for impairment. This will all support selection of an approach for the TMDL to most appropriately identify the necessary reductions and controls to meet water quality standards. One of the primary analyses that is conducted for most types of impairments is an evaluation of water quality versus flow. We’ll now show a few examples of this important analysis.

Data Analysis – Water Quality and Flow Example
Types of sources are typically divided among those that are precipitation-driven and those that are not. And because flow is a direct result of the amount of precipitation, flow patterns in flowing waters like streams and rivers can be correlated to times of pollutant loading depending on the types of sources in a watershed. The pollutants delivered through surface runoff come from a variety of sources, some nonpoint sources such as cropland and other agricultural activities and some point sources such as regulated stormwater discharges from municipal separate storm sewer systems, referred to as MS4s, construction areas or industrial sites. Other sources are not dependent on precipitation and resulting runoff for delivery and input to a waterbody. For example, traditional point sources discharge their effluent based on the facility operation and characteristics. An example of a nonpoint source that directly discharges to a stream includes wildlife or livestock with access to waterways as well as failing septic systems. Because of these distinctions among source delivery mechanisms, evaluating flow and water quality can provide insight into the types of sources affecting impairment. Evaluating the relationship between water quality and flow can be done using a variety of techniques, including simple visual comparison using graphed time-series data, regression analyses, or the use of flow duration curves. These three examples show different ways of presenting and reviewing the same dataset, all showing that elevated bacteria concentrations occur during times of higher flows. Some or all of the types of data analyses we’ve discussed are conducted as part of the problem understanding phase of the TMDL development process. As we’ve discussed, the results of all of these analyses are evaluated within the context of broader waterbody and watershed conditions to understand the problem being addressed by the TMDL, providing a foundation for more informed decisions later in the TMDL process for setting targets, characterizing sources, selecting TMDL calculation approaches and defining allocations. Now that we’ve covered the Problem Identification, let’s move on to identifying a water quality target for developing the TMDL.

Water Quality Target Identification – Goals
Understand applicable water quality standards Identify numeric target for calculation of loading capacity Based on numeric water quality criterion (e.g., 750 ug/l aluminum) Based on interpretation of narrative criterion The goal of any TMDL is to restore an impaired waterbody to conditions that meet water quality standards and support designated uses. Therefore, it’s critical to review and fully understand the applicable water quality standards before proceeding with calculation of the loading capacity. Furthermore, development of a TMDL for a given waterbody requires some defined numeric target for which the loading capacity is calculated. And because the TMDL goal is to meet water quality standards, that numeric target must represent attainment of water quality standards. Often, the numeric target for the TMDL will be the numeric water quality criterion for the pollutant of concern. In some cases, however, TMDLs are developed for pollutants that do not have established numeric water quality criteria. In these cases, impairment is determined on the basis of narrative water quality criteria or identifiable degradation of designated uses such as an impaired fishery. It is then necessary to develop a numeric TMDL water quality target that represents attainment of the water quality standards. We will walk through this process on the next slide.

Water Quality Target Identification – Process
Select an indicator Consistent with WQS and impairment Quantifiable Compatible with local conditions, critical conditions, sources Select a target value Reference sites Historical or background conditions Literature/guidance values Functional relationships (e.g., algal mass & total phosphorus, TSS & measures of fishery health) A measurable parameter for which a target value can be set to represent attainment of WQS (e.g., nitrogen, TSS) Value or magnitude (e.g., concentration) established for indicator; level at which WQS will be supported When no numeric criteria exist, the first step of identifying the water quality target is to identify the water quality indicator. The indicator is some water quality parameter that can be measured in the waterbody and can be used to represent attainment of water quality standards. For example, for impairments by excess plant growth, the indicator could be nitrogen or phosphorus. Similarly, for impairments from general degradation of habitat quality, the cause might be excessive sedimentation and the indicator chosen could be total suspended solids, or TSS. A number of factors can be considered when selecting the indicator for a given TMDL and associated impairment. Most importantly the indicator should be able to represent water quality standards and the conditions that lead to the impairment. The indicator should also be quantifiable and measureable. Other considerations that can affect how well the indicator will represent the given impairment include whether the indicator is sensitive to local environmental conditions and the given critical conditions and also whether it can be related or linked to the key sources. Once an indicator is identified for developing the water quality target, it’s necessary to select the target value. The target value is the actual number associated with the indicator that will represent attainment of water quality standards. The target is often represented as a concentration for a particular pollutant. Deciding what number is appropriate can rely on a variety of information or analyses. Monitoring data are often analyzed to identify an appropriate target value, whether based on measured values at similar waterbody sites that are not impaired, called reference sites, or based on historical measurements taken prior to impairment or based on data that represent natural background conditions in the impaired waterbody. Target values might also be based on recommended values from scientific literature or available federal, state or local guidance documents. Occasionally, targets are based on relationships that relate the chosen indicator to other waterbody measures. For example, empirical equations exist that relate the level of phosphorus in a lake to the trophic state. Therefore, a target value for phosphorus can be selected based on the desired conditions for that lake. Similarly, relationships have been published on the level of TSS in a stream and the likely effect on fisheries.

Water Quality Target Identification – Examples
Sediment – “no adverse impacts to aquatic life” TSS concentration (literature values, reference conditions, background/historical data) Loading target based on reference conditions Biology – “maintain healthy benthic communities” Stressor Identification to identify pollutant Identify target based on statistical analysis, literature values, reference conditions, etc. Nutrients – “not in concentrations that cause objectionable conditions…” Concentration or loading target for limiting nutrient (modeling, statistical analysis of stressor-response, literature values, reference conditions, background/historical data) Let’s go through some examples of identifying a numeric water quality target to represent narrative water quality criteria. One example is for a sediment impairment. This could be based on observations of excessive deposition of fine sediment that has degraded the aquatic habitat, thereby violating the general narrative criteria prohibiting adverse impacts to aquatic life. A numeric target can be an instream concentration for TSS to represent levels of sediment loading and therefore deposition. Appropriate concentrations could be based on literature values, reference conditions or historical data. An alternative approach for a numeric target could be defining acceptable levels of sediment loading based on modeling a similar, but unimpaired, watershed. Another common example of impairments of narrative criteria include biological impairments, often represented by decreased measures of biological diversity and benthic communities. In these cases, a stressor identification analysis can be used to determine what pollutant or pollutants are causing the impacts to the biological community and then a target can be identified for the pollutant of concern based on statistical analysis, literature values or reference conditions. The last example we’ll discuss are impairments from nutrients. While there are a number of efforts underway throughout the country to establish numeric nutrient criteria, many states currently still rely on narrative criteria. The narrative criteria typically prohibit nutrient concentrations at levels that cause nuisance plant growth or adverse impacts to designated uses or more generally prohibit materials or substances that cause objectionable conditions or similar language. In these cases, concentration targets and sometimes loading targets for the limiting nutrient are identified based on modeling or statistical analyses of stressor-response relationships and also using the methods already mentioned for literature values and data analysis of reference, historical or background conditions.

Source Assessment – Goals
Identify potential point and nonpoint sources (type, location, pollutants, etc.) Understand how sources affect waterbody condition (e.g., delivery mechanisms) Understand relative magnitude or influence of major sources The next step in the TMDL process we’ll talk about is the source assessment. The source assessment should be an extension of the analyses conducted during the problem understanding step to further characterize the nonpoint and point sources. This step will provide a basic characterization of these sources, defining what they are, where they are, what pollutants they are discharging and when they are discharging. Further characterization of the sources will identify how those sources are affecting water quality and contributing to the observed impairment. This can include identifying how the sources generate the pollutant of concern and how the pollutants are in turn delivered to the waterbody. In evaluating all of the potential sources in the watershed, it can be important to understand the relative magnitude of the sources’ loadings and their influence on water quality. This can support identification of appropriate TMDL calculation approaches and guide the selection of allocations.

As illustrated by this picture, there can be a variety of sources that contribute the pollutants of concern to an impaired waterbody. These could include erosion and runoff from agricultural activities, wastewater treatment plant discharges, permitted industrial discharges, erosion and runoff from construction activities, erosion from forestry activities, runoff from residential areas and even inputs from recreational activities. With all the potential sources in a watershed, a TMDL developer will rely on the data analysis conducted during the problem understanding step, as well as available information from national and state databases, local reports, and stakeholders to identify and characterize the sources contributing to an observed impairment.

Source Assessment – Types of Sources
Point sources: Discharge effluent through discrete conveyance such as pipes or man-made ditches Permitted through NPDES To identify the sources and later decide how to represent them in the TMDL analysis and resulting allocations, it’s necessary to understand the different types of sources. Sources fall into one of two categories, either point sources or nonpoint sources. Because this audience is primarily permit writers, you likely already know what point and nonpoint sources are. But we’ll quickly go over them for those that are new to the program. As defined in the Clean Water Act, point sources are those sources that discharge pollutants into waters of the United States through discrete conveyances such as pipes or man-made ditches. Point sources are regulated through discharge permits issued by EPA or an authorized state through the NPDES program. Historically, point sources were typically industrial, municipal or other facilities that produced some effluent as a by-product of their operations and discharged that effluent to a waterbody. Examples of such facilities include wastewater treatment plants and a variety of industrial facilities like paper mills and manufacturing plants. The scope of the point source regulations have broadened over the years so that today, point sources also include vessel discharges, discharges from certain types of animal feeding operations, and certain types of stormwater discharges. Regulated stormwater sources include municipal separate storm sewer systems, referred to as MS4s, construction activities, and industrial activities. In a TMDL, point sources receive wasteload allocations. More information on the types of point sources that are regulated through the NPDES program and the specific regulations and requirements are included on EPA’s Office of Wastewater Management web site at

Source Assessment – Types of Sources (cont.)
Nonpoint sources: Diffuse pollution sources (i.e., without a single point of origin or not introduced into a receiving stream from a specific outlet) Generally carried from the land surface to waterbodies through stormwater runoff Not permitted through NPDES As opposed to point sources, nonpoint sources typically represent sources that are diffuse in nature, without a single point of origin or discharge. The Clean Water Act defines the term “nonpoint source” as any source of water pollution that does not meet the legal definition of “point source”. Therefore, essentially any source that is not regulated under the NPDES program is a nonpoint source. Nonpoint source pollution generally is the result of rainfall or snowmelt moving over the surface of a watershed, picking up natural and anthropogenic pollutants and eventually delivering the pollutants to a receiving waterbody. Examples of these types of precipitation-driven nonpoint sources include runoff from cropland and pastures, erosion and runoff from forestry activities, runoff from abandoned mine lands, and runoff from developed areas such as residential and commercial areas. However, some nonpoint sources are not precipitation-driven and discharge directly into a waterbody, such as watering livestock, leaking septic systems, wildlife, illegal dumping, atmospheric deposition, and recreational boat discharges. In a TMDL, nonpoint sources receive load allocations. For more information on nonpoint sources, go to EPA’s nonpoint source program web page at In addition, you can visit your state’s nonpoint source management program web site.

Source Assessment – Process
Identify potential nonpoint sources Field surveys, land use coverages, aerial photos, previous studies, local knowledge Location, pollutants, activity types/timing Identify potential point sources NPDES PCS/ICIS, state permitting staff Facility and permit information (e.g., type, design flow, pollutants discharged, permit limits, etc.) Use results of data analysis to identify primary sources expected to be contributing to impairment Identify source dynamics that need to be represented in linkage analysis On-the-ground information from field surveys and previous studies and reports for the area can be important for identifying nonpoint sources. However, this type of information is not always available. In these cases, identification of nonpoint sources is usually based on review of aerial photos, satellite imagery, and GIS coverages of land use, land cover and soils. In addition, local knowledge and input from stakeholders can be invaluable when identifying nonpoint sources. For example, staff at conservation districts, county planning or environmental agencies, and private citizens living in the watershed can have supplemental or more up-to-date information on land uses and sources that are not noted or described in readily available data and reports. When characterizing nonpoint sources, it’s useful to identify their location within the watershed and in relation to the impaired segment, to determine what pollutants they are discharging and to understand the types of activities that are occurring to generate and deliver the pollutants of concern and the timing of those activities. Point sources are generally easier than nonpoint sources to identify and quantify since they are permitted and tracked under the NPDES program. Identifying the number, type and location of NPDES permitted point sources in the watershed of the impaired waterbody typically starts with searching available databases like EPA’s Permit Compliance System, called PCS, or Integrated Compliance Information System, called ICIS. It’s also good practice for the TMDL developer to coordinate with relevant state or tribal permitting staff to obtain further information about the point sources and their discharges to make sure that all sources are captured in the analysis and that they are represented accurately. Typical information used to characterize a point source includes facility type, design flow, permit limits, number and location of permitted outfalls, and available discharge monitoring data. Once the potential nonpoint and point sources have been identified, they are typically evaluated within the context of the results of the data analyses to determine which sources are the primary causes of the impairment. Evaluating their location, behavior and timing along with the water quality conditions can provide insight on the relative influence of the sources. In addition, identifying the sources and when, where and how they generate and deliver the pollutant of concern provides information important for deciding what approach to use for the linkage analysis and also how to accurately represent the sources within that approach.

Atmospheric Deposition
Cropland Pastures Watering Livestock Atmospheric Deposition Expected Sources MS4 Stormwater Permitted Industrial Facility WWTP = NPS = PS Precipitation-driven Inputs Build-up and wash-off of pollutants Surface water runoff Direct Inputs Arbitrary, sometimes constant, discharge Discrete, direct discharge to waterbodies Delivery Mechanism For example, assume a watershed with a variety of point and nonpoint sources that have all been identified as potentially contributing the pollutant of concern to the impaired waterbody. Nonpoint sources include runoff from cropland and pastures, livestock that enter the stream for drinking and cooling, and atmospheric deposition. For point sources in the watershed, there is an MS4 that discharges regulated stormwater to the waterbody as well as an industrial facility and a wastewater treatment plant discharging to the impaired waterbody. Among the nonpoint and point sources, some represent activities that deposit pollutants on the watershed surface where they accumulate and are then delivered to the waterbody through surface runoff. These precipitation-driven sources include nonpoint source runoff from cropland and pastures as well as regulated stormwater from the MS4. Other types of sources discharge depending on source activity or operation rather than on precipitation and runoff. These sources include nonpoint source inputs from watering livestock and atmospheric deposition and point source inputs from the industrial facility and wastewater treatment plant. By understanding the source behavior and associated delivery mechanism for the pollutants of concern, it’s possible to identify the critical times for pollutant loading, those times when pollutant loading from the particular source will have the biggest impact in the receiving waterbody. For the precipitation-driven sources, that time is following storm events, during high flow conditions, when all the accumulated pollutants on the land surface have been washed off into the waterbody. For direct input sources, the critical time for loading is typically low flow or base flow conditions, when there is less volume of water to dilute the incoming load. Understanding these source behaviors and dynamics is critical for moving on to the linkage analysis and selecting and accurately applying an approach for establishing that cause-and-effect link between sources and resulting water quality and impairment. Critical Loading Conditions High flows, storm events Base flows

Linkage Analysis – Goals
Evaluate receiving water response to pollutant loadings Establish “link” between sources and water quality standards Identify loading capacity The linkage analysis step is where the TMDL, the actual number, is identified by evaluating the waterbody’s response to pollutant loadings. To do this, it’s necessary to establish a quantitative link or relationship between pollutant sources and receiving water response. This link allows for the prediction of water quality conditions based on certain levels of pollutant loading and, depending on the approach selected, also considering various conditions or processes in the waterbody. This linkage analysis is used to identify the loading capacity that will result in meeting water quality standards in the impaired waterbody.

Linkage Analysis – Process
Select approach Apply approach Establish “existing” loading and conditions Calculate loading capacity To complete the linkage analysis, first an approach is selected, and then applied. When applying the approach, typically a TMDL developer will first evaluate the existing loadings to the impaired waterbody, basically recreating the conditions of impairment to determine the starting point for restoration. Then the approach is applied to calculate the loading capacity for the impaired water. This allowable loading can be compared to the existing loadings to identify the reductions needed to restore the waterbody to water quality standards.

Linkage Analysis – Approach Selection
User and Application Considerations What experience or training is required to apply the approach? What level of effort is needed for application? What are the data needs? What is the expected cost (of necessary software and of time and labor for application)? Programmatic Considerations What is the schedule? Are there existing tools available for the waterbody/watershed? Are there any planned future uses for the approach (e.g., linkage to other analyses, implementation planning)? Are there proven and accepted methods applied for similar projects? Technical Considerations What are the applicable water quality criteria? What are the impairments and critical conditions? What are the sources and their behavior and characteristics? How are the multiple sources, impaired waterbodies or impairments related? There are a number of approaches that can be used to support the linkage analysis. Selecting which approach to use for any given TMDL is often guided by a number of technical and practical factors. These can include user needs or requirements, programmatic considerations, and technical needs. User and application considerations address those practical considerations for what the project staff needs to apply the approach. Applicable questions include: What experience or training is required to apply the approach? What level of effort is needed for application? What are the data needs? And finally, what is the expected cost, both in terms of the necessary computer software and of the time and labor for application? Next are programmatic considerations that evaluate how an approach fits within larger program goals. Questions include: What is the project schedule? Are there existing tools or analyses available for the waterbody and/or watershed that could be used for the TMDL? Are there any planned future uses for the approach such as linkage to other analyses or use for implementation planning subsequent to the TMDL? And also ... Are there proven and accepted methods that have been applied for similar projects? The last, and often most important, questions relate to the technical considerations for selecting an approach. These represent the technical elements or processes that should be captured in the approach. Primary questions include: What are the applicable water quality criteria? What are the impairments and critical conditions? What are the sources and their behavior and characteristics? And finally, how are the multiple sources, impaired waterbodies or impairments related? While user needs and programmatic considerations will often guide the general type of approach such as whether or not to use a model, the technical considerations will weigh heavily in the selection of a specific approach, such as which particular model to use.

Approach Selection Technical needs of approach Technical considerations for approach selection Water quality criteria and TMDL targets Impairments and critical conditions Sources Are different criteria or TMDL targets applicable in different locations within the watershed? How many impaired segments are being addressed? What are the location and distribution of impaired segments? What type of sources/land uses exist in the watershed? What are the location and distribution of sources? At what level do the sources need to be isolated (e.g., gross loading vs. land use specific loading)? Spatial Needs What are the duration and frequency of applicable criteria or targets? What is the timing associated with impairment (e.g., instantaneous vs. chronic or cumulative effects)? Are there any temporal trends to capture (e.g., seasonality in waterbody conditions)? Are the effects due to cumulative or acute loading conditions? Are there temporal variations in source loading (e.g., due to weather patterns, seasonal activities)? At what temporal scale do the sources need to be estimated? Time-scale Needs The technical considerations define the technical needs of the approach, namely the spatial scale, the temporal resolution, and also the processes or features that need to be included or represented. To help define these needs there are a number of questions that can be asked related to the water quality target used in the TMDL, the impairments and associated critical conditions and also the expected or known sources. We won’t go through each of these questions in detail during this training. When you have time, you can read through them on this slide or in EPA’s Handbook for Developing Watershed TMDLs or EPA’s draft TMDLs to Stormwater Permits Handbook. These resources will give you some additional insight into the issues that TMDL developers consider when selecting an approach for the linkage analysis. Examples of questions related to the spatial scale of the analysis include identifying the locations of the impaired segments in relation to the location of known or expected sources in the watershed. This can be especially important when evaluating multiple impaired segments within a single watershed TMDL analysis. For understanding the needs for temporal resolution or analysis time scale, questions include identifying the time scale or duration and frequency of the water quality target and the timing of impairment and source loading. When defining the environmental processes to include in the approach, questions include what pollutants are considered; what physical, chemical or biological waterbody measures or processes are needed to represent the impairment; and what are the delivery mechanisms for the sources of concern such as direct inputs versus precipitation-driven loading. The answers to the questions will help support such decisions as whether to evaluate the watershed as a whole or as multiple subwatersheds, or how to group or divide the multiple sources for representation in the analysis and also for allocation, what time increment should be used such as hourly, daily or even monthly or annual, and which types of physical, chemical and biological processes must be represented to accurately assess the impairment and calculate allocations. The watershed characterization step of the TMDL development process should generate the necessary information to answer these questions and define the analysis needs to support selection of an approach for the linkage analysis. It’s important to note that not all of these questions are always relevant to a particular TMDL. And some questions might weigh more heavily in the approach selection than others. It’s also important to always evaluate these technical needs within the context of the user and programmatic needs. For example, say there is a situation where the impairment is well-defined, sources are understood and controls are already underway or planned to address the problems. This situation likely won’t merit a detailed, time-intensive and expensive modeling analysis. Rather it could be appropriate to use a simpler approach that addresses or captures the major issues. Is criterion based on pollutant level (e.g., concentration) or a measure of response or condition (e.g., flow, habitat quality, eutrophication)? What are the pollutants? Is meeting the target dependent on or affected by multiple waterbody measures (e.g., nutrient levels, temperature, pH)? What are the waterbody critical conditions for loading response (e.g., dynamic, flow variable vs. steady-state)? If dealing with multiple pollutants, how are they related? What is the source loading behavior (e.g., precipitation-driven, direct discharge)? Do sources impact multiple impaired segments (i.e., need for in-stream routing and transport)? Does the analysis need to evaluate individual and/or cumulative impact of sources? Processes to Include

Types of Approaches – Examples
Mass balance Load duration Modeling Watershed Receiving water Now that we’ve talked about how a TMDL developer selects an approach for their specific analysis, let’s talk about a few of the commonly used approaches for developing TMDLs. We’ll talk about using mass balance approaches, using load duration curves, and also using watershed and/or receiving water models. It’s also important to note, because this module is designed to educate permit writers on how TMDLs are developed, the discussion of TMDL approaches does not provide much detail on the applicability or advantages or disadvantages of using a particular approach. That information can be critical when selecting an approach for developing a TMDL. However because you as permit writers will not be selecting a TMDL approach, the information included here focuses instead on giving you a basic understanding of what that particular approach is and how sources are typically represented within that approach. More detail, including pros and cons of each of these and other typical TMDL approaches, is provided in some of the technical support documents we’ve included in the resources list at the end of this module, including the Handbook on Developing Watershed TMDLs and EPA’s draftTMDLs to Stormwater Permits Handbook.

Types of Approaches – Mass Balance
Inputs – Losses = Outputs Relies on the assumption of conservation of mass into a waterbody Identifies the allowable load input to meet water quality target after the consideration of losses Typically “back-calculates” allowable loads based on target concentrations and waterbody volume First let’s talk about mass balance approaches. These types of approaches rely on the assumption of conservation of mass in a waterbody, basically that any load inputs must eventually become outputs with some loss to waterbody processes such as settling, decay or uptake. For a TMDL, the approach relies on identifying the load entering a waterbody that will meet the desired waterbody target after the consideration of all other inputs and losses. This is commonly done by “back-calculating” an allowable load. That is, using the desired endpoint of the water quality target to then calculate a corresponding pollutant load based on the volume of water and adjusting the load given any losses or other inputs. When using a single condition for the waterbody volume, this would represent a steady-state calculation and would require a representative measure of flow or volume appropriate for the given project, typically a critical low flow or volume or an average flow or volume. In its most basic form, the mass balance approach is simply a calculation of loading capacity based on multiplying the target concentration by a design flow or volume, without any adjustments for other inputs or losses. This is used in many TMDLs, especially ones where low flow is the critical period and runoff related nonpoint source inputs are minimal.

Types of Approaches – Mass Balance Example
Large river impaired by PCBs Monitoring confirmed two major sources Permitted industrial facility Permitted runoff from landfill Developed a spreadsheet model to represent a simplified mass balance for the system Now we’ll provide an example mass balance analysis for TMDL development. This particular TMDL was for a large river with historical and current impairments from PCBs. After initial identification of potential sources, monitoring data and analysis indicated two primary sources of PCBs. One is a permitted industrial facility that produces fiber textiles and the other is a landfill. To represent the linkage between source contributions and in-stream response, a simplified mass balance analysis was used to simulate input and transfer of PCBs through the river. This approach was chosen for a number of project-specific considerations. One being that the critical condition associated with this impairment is low flow. Because the mass balance approach represents a steady-state condition, it is appropriate for representing and analyzing source inputs and water quality impacts during the critical low flow condition. In addition, PCBs are a conservative pollutant with minimal decay. Therefore, instream processes and dynamics can be represented through a simplified mass balance and do not merit the use of a complex model. This approach also allows for easy incorporation of source inputs because the sources are well-defined, localized, and have recent sampling data available to represent their inputs.

Types of Approaches – Mass Balance Example (cont.)
Upstream Boundary Landfill runoff (flow, concentration) Industrial facility outfall (flow, concentration) Tributary #1 (flow, concentration) Tributary #2 As illustrated by this figure, the predictive mass balance analysis was constructed as a series of cells representing segments of the river. The river was segmented to simulate the distribution of PCBs, accurately account for the water balance throughout the river, and to capture the localized impact of point sources and tributaries. The model represents the segmented systems in one dimension, representing longitudinal flow under a steady-state condition. For TMDL development purposes, the steady-state condition represented the “critical condition” which was defined as the 7Q10 low flow. The starting point for the analysis is the upstream boundary condition, which was based on measured river flows and background PCB levels measured at an upstream monitoring station. Outflow concentrations are then calculated for each cell based on all inputs and losses. These outflow concentrations act as the inflow concentration for the next downstream cell. Inputs from tributaries and sources were added to the respective river segment based on user-defined flow and concentration. Tributary inputs were defined based on sampling data for flow and PCB concentrations. Inputs from the two critical sources confirmed during the pre-TMDL monitoring were also user-defined inputs. Input from the industrial facility was based on the average daily flow reported for the year of the sampling and PCB concentrations reported in DMR data and measured during pre-TMDL sampling. Flow and PCB concentration for inputs from the landfill were also based on measurements during the pre-TMDL sampling events. (flow, concentration) Downstream Boundary

Water INFLOW Dissolved Particulate OUTFLOW (upstream, tributaries, direct sources) Settling Burial Diffusion Resuspension Sediment/Water Interface This figure illustrates the processes and interactions that are represented in each of the river segments. Each segment defines a mass balance for PCBs distributed between the sediment of the riverbed and the overlying water column. PCBs are also partitioned into dissolved and particulate fractions in both the water and sediment layers. The outflow concentrations are computed based on the incoming PCBs, inputs from tributaries or sources, inputs from resuspension and losses to burial and diffusion. Calculations of resulting concentrations can depend on a number of physical and chemical factors. Equations for representing these dynamics for supporting a mass balance can vary in complexity, depending on what processes are taken into account. Equations are typically obtained from scientific literature and textbooks. Sediment

Reduce source contributions until achieve criteria Allocate to primary sources and background based on successful scenario WLA as concentration and annual load The last couple of slides presented how the mass balance analysis was set up to represent the existing conditions reflective of the impairment and accounting for source inputs. Once that was done, the predictive mass balance calculations could be used to determine the TMDL and associated allocations. To do this, source inputs were adjusted until resulting PCB concentrations in the river met the applicable water quality criteria. TMDL source allocations were then presented as both discharge concentrations in micrograms per liter and annual loads.

Types of Approaches – Load Duration
Uses observed flows and water quality target to establish a flow-variable curve of loading capacities Builds on using flow duration curves, which evaluate cumulative frequency of historic flow data Multiply target concentration by observed flows to create a “loading capacity curve” The next type of TMDL approach we’ll discuss is the load duration curve. The load duration curve approach for TMDL development is commonly used for impaired streams and calculates a series of allowable loads corresponding to the range of flow conditions expected to occur in the stream. The series of allowable loads are graphically presented as a curve of individual loading capacities as a function of flow. Developing load duration curves builds on the use of flow duration curves, which is a curve of measured flows arranged according to their frequency of occurrence within the given dataset. The flow duration curve can then be translated into a load duration curve by multiplying each of the individual flows by the water quality criterion or target concentration. For example, on this load duration curve for fecal coliform, the loading capacity calculated at the 5% flow is equal to 90,551 organisms per day. This is calculated based on a flow of 18,500 ft3/s and the criterion of 200 organisms/100 mL. The entire curve can be used to represent a series of flow-variable loading capacities, or individual loading capacities can be identified for specified flow intervals, used as a general indicator of hydrologic condition. For example, this graph shows typical flow zones used in the load duration curve approach. These are high flows, moist conditions, mid-range flows, dry conditions, and low flows. Each of these zones has the median loading capacity for the zone highlighted on the curve.

Types of Approaches – Load Duration (cont.)
Plotted with existing loads to identify needed reductions Existing load: 290,022 G-org/day Loading capacity: 90,511 G-org/day Load reduction: 69% The load duration curve can also be used to compare existing loads to the loading capacity and identify necessary load reductions. To do that, water quality data are converted to loads by multiplying observed pollutant concentrations by the corresponding flow at the time of sample collection. The existing loads are then plotted on the same graph as the load duration curve. Existing loads that plot above the load duration curve represent those exceeding the allowable water quality target and corresponding daily allowable load. Necessary reductions can then be identified for the different flow zones to target source controls. For example, the median existing load for the high flow zone in the load duration example for fecal coliform is 290,022 while the corresponding loading capacity is 90,511, representing a necessary load reduction of 69 percent.

Types of Approaches – Load Duration (cont.)
Loading Capacity LA Loading Capacity minus MOS MS4 WLA An underlying premise of the load duration curve approach is that water quality conditions, and therefore impairment, correlate to flow. Understanding this relationship can be helpful in identifying the types of sources contributing to impairment. However, because the curve is based on observed instream water quality and flow measurements, the approach on its own does not explicitly calculate or consider loads from individual point or nonpoint sources. Therefore, loads from and allocations to sources are typically calculated in a supplemental analysis. For example, this graph shows the load duration curve for loading capacities calculated for fecal coliform shown on previous slides. The curve can then be broken down into the corresponding parts for allocations to point sources, nonpoint sources and margin of safety. The starting point in our example is the total loading capacity, represented by the red line. The margin of safety was included as 10 percent of the loading capacity. The blue area on this graph now shows the adjusted loading capacity after removing the margin of safety. Next, the WLA for a wastewater treatment plant was calculated reflecting the permit design flow and effluent limit, which is equal to the water quality criteria for fecal coliform. The plant’s input is static across all flow conditions. This WLA is subtracted from the available loading capacity and the remaining load is available for the precipitation-based sources in the watershed. One of those sources is an MS4. Rather than setting a particular concentration associated with the MS4 to calculate a corresponding WLA, the WLA was estimated based on the relative area of the MS4 versus the watershed area. Fifteen percent of the watershed falls within the regulated boundary of the MS4 and therefore an assumption was made to allocate a corresponding 15 percent of the runoff-related load to the MS4. The remaining portion of the loading capacity is then designated as the load allocation for nonpoint sources and natural background. The load duration curve methodology is discussed in detail in EPA’s 2007 document An Approach for Using Load Duration Curves in the Development of TMDLs. WWTP WLA

Types of Approaches – Watershed Models
Watershed hydrologic and water quality processes Some simulate only the land-based processes Some include linked river segments and simulate in-stream transport and water quality processes Include direct inputs (e.g., septics, point sources) Vary in the level of detail (e.g., processes, simulation timestep) Range in complexity from the use of empirically based loading functions (e.g., GWLF) to physically based simulations (e.g., HSPF) The final TMDL approach we’ll talk about is modeling. This can encompass numerous individual models that can be used for a variety of applications. Models used in developing TMDLs generally fall into two categories, either watershed models or receiving water models. Watershed models simulate watershed hydrologic and water quality processes, including surface runoff, erosion, and buildup and washoff of sediment and pollutants. Some models simulate only these land-based processes while some can also include linked river segments and simulate in-stream transport and water quality processes. While surface runoff and pollutant delivery are primary model outputs for watershed models, some also provide the capability to simulate sources that discharge directly to the stream and are not dependent on surface runoff for delivery, such as septic systems and traditional point sources. Watershed models vary in the level of detail, including what processes they simulate and the simulation timestep, from sub-daily to daily to monthly. The complexity of watershed models can also range from the use of loading functions to physically based simulations. Loading functions represent empirically based estimates of loading based on meteorologic factors such as precipitation and temperature, while physically based simulations use scientific equations representing the relevant physical, chemical, and biological processes to predict runoff, pollutant accumulation and washoff, and sediment detachment and transport.

Types of Approaches – Receiving Water Models
Simulate conditions within a receiving waterbody (e.g., lake, stream, estuary) Based on representation of physical, chemical and biological processes Typically include inputs as user-defined boundary conditions (monitoring data, watershed model output) Steady-state or dynamic models Steady-state: operate under a single, nonvariable flow condition with constant inputs (e.g., design or critical flow) Dynamic models: time-variable simulation Varies in level of complexity in spatial detail (1-, 2-, or 3-D) In some cases, receiving water models are used to support TMDL development, either alone or in combination with a watershed model. As the term suggests, receiving water models only represent conditions within a receiving water, including streams, lakes, reservoirs and estuaries, and can simulate a variety of physical, chemical and biological processes. Because receiving water models do not explicitly represent land-based processes, pollutant and flow inputs to the waterbody are often defined in the model as boundary conditions, typically based on monitoring data or using linked dynamic output from a watershed model. Most receiving water models are either steady-state or dynamic in nature. Steady-state models, such as QUAL-2K and BATHTUB, operate under a single nonvariable flow condition with constant inputs, typically used to evaluate conditions for single or multiple design or critical flows. Dynamic models, such as EFDC, WASP and CE-QUAL-W2, allow for time-varying conditions on a small time step, typically shorter than a day. In a dynamic model, any process that is considered can vary across the time step. Therefore, the model can accept time-variable inputs and also calculate the resulting hydrology and water quality conditions dynamically. The level of complexity in receiving water models is also determined by spatial detail, depending on the capability to simulate a waterbody in one, two or three dimensions. As with watershed models, there is a wide variety of available receiving water models with different capabilities and levels of complexity or resolution. In addition, individual models can be applied with varying levels of complexity depending on how they are set up and applied for a given TMDL. For more information on watershed and receiving water models used in TMDL development and their applicability to different waterbody types, pollutants and sources, refer to EPA’s Compendium of Tools for Watershed Assessment and TMDL Development and TMDL Model Evaluation and Research Needs. Both of these documents are listed in the Resources at the end of this module.

Types of Approaches – Modeling Process
Loading Capacity Total Load Load Reduction Permitted Conditions (2) TMDL Scenario C TMDL Scenario B TMDL Scenario A Existing Conditions (1) Future Growth (3) Permitted Conditions (2) Existing Conditions (1) TMDL Scenario C Loading Capacity TMDL Scenario B TMDL Scenario A Future Growth (3) Depending on what model or combination of models is selected to support TMDL development, the specifics of the application and how it is used to represent source inputs and simulate resulting water quality conditions can vary widely. However, there is a general process for how the model is used within the TMDL framework to identify the loading capacity and identify associated allocations, as shown in this figure. The first step in the modeling process is typically to simulate existing conditions. This essentially recreates the impaired conditions, including current loads from point and nonpoint sources and calibrated to physical, chemical and/or biological monitoring data. In the existing conditions model scenario, point sources are typically represented as their actual discharge based on flow and concentration data in discharge monitoring reports, or DMRs. The next step in the process, denoted as #2 in the figure, is application of the model to establish the baseline conditions, which will serve as the starting point for reducing loads to meet the TMDL and water quality standards. In this scenario, point sources are set at permit conditions using the permitted flow and applicable effluent limit. If future growth is considered in a TMDL, it can be added to the nonpoint and/or the point source loading contributions to simulate what the total loading would be under future scenarios, without any load reductions or source controls yet in place. This is shown as #3 on the figure, denoting an increased baseline load to include the estimated loadings from future sources. Working from the baseline condition, either #2 or #3 depending on whether future sources are included, various scenarios can then be simulated to represent different combinations of load reduction from the various sources. The scenarios, shown here as A, B, and C, would therefore result in different total loads and water quality responses. The results of each scenario are compared with the TMDL water quality target and iteratively adjusted until the target is met, representing the loading capacity. The necessary load reduction can also be calculated as the difference between loads under permitted conditions and the loading capacity. Total Load Load Reduction

Allocation Analysis – Goals
Allocate loading capacity among WLAs, LAs and MOS Meet legal requirements Facilitate implementation Identify necessary source load reductions Regardless of which approach is used, after it’s applied to identify the loading capacity of the impaired waterbody, the TMDL developer will divide the loading capacity among source allocations. The allocation analysis will identify WLAs for point sources, LAs for nonpoint and background sources, the margin of safety and optional components such as a reserve capacity. If existing source loads were identified during the source assessment and linkage analysis, the allocation analysis will also compare those existing loads to allocations to identify needed load reductions from each source.

Allocation Analysis – Decisions
Identify appropriate expression of allocations Mass per time, toxicity, “other appropriate measure” Determine how to include MOS Implicit as conservative assumptions Explicit portion of load Determine whether future growth requires allocation “Reserved” portion of load for future NPS or point sources Based on amount of certain land areas (e.g., future conversion of ag to urban) There are several considerations when determining allocations in a TMDL. The first is how the allocations should be expressed. As the name “total maximum daily load” implies, most TMDLs are expressed as a load... that is, some mass of a pollutant for a given time period, such as pounds per day or kilograms per month. However, TMDL regulations also allow for TMDLs to be expressed in terms of toxicity or other appropriate measures. Other appropriate measures have included expressing allocations as a concentration and more recently as a surrogate measure such as flow or percent of imperviousness in the watershed. The use of surrogates is discussed more in module 3 on integrating TMDLs and stormwater permits. Before distributing the loading capacity among any of the sources, it’s necessary to determine how the margin of safety is included. This is typically decided earlier in the TMDL process because it can affect how the approach is applied. If it is included implicitly, the TMDL developer documents the conservative assumptions in the TMDL report and moves on to setting the source allocations. But if the MOS is explicit, now is the time to identify the quantified load assigned to the MOS, which is typically set at some percent of the loading capacity. The remaining portion is then available for distribution to WLAs and LAs. A basic first step in setting source allocations is confirming what sources will receive allocations as identified during the source assessment. One of the considerations for this is whether there are future sources in the watershed that are significant enough to receive allocations or whether it’s appropriate to set aside a portion of the loading capacity more generally for future growth. Examples where this might be relevant are watersheds where it’s known that a future facility will discharge to the waterbody or where rapid urbanization or other land use changes are expected to increase loads of the pollutant of concern.

Allocation Analysis – Decisions (cont.)
Identify appropriate spatial or source scale of allocations Location of impaired segment(s) Location/distribution of sources Location of monitoring stations Areas of targeted implementation Identify appropriate time scale for allocations Expression of water quality standards Nature of impairment/impact (e.g., acute vs. chronic) Temporal variations in loading Implications for monitoring or implementation If non-daily, also include daily In determining allocations, it’s also important to define the spatial or source scale of the allocations. For traditional point sources, this is not typically an issue since each facility receives a WLA. However, when dealing with nonpoint sources, it is sometimes a critical decision to decide whether and how to subdivide a watershed into smaller allocation areas like subwatersheds and also how to group or categorize individual sources for allocations. While load allocations can be expressed as a gross allocation for all nonpoint sources in a watershed, a common approach is to assign an allocation for each land use type, such as agriculture or residential. Spatial considerations might also affect how allocations are set for an MS4 area. This topic is also discussed in more detail in module 3 on integrating TMDLs and NPDES stormwater permits. In addition to the spatial scale of the allocations, it’s necessary to identify the time scale. The decision of the allocation time scale, whether daily, monthly, seasonal, or annual, will depend on consideration of a number of factors including the pollutant, the applicable water quality standards, the nature of the impairment, and the types and behavior of the sources. For example, pollutants such as bacteria tend to have acute rather than chronic or cumulative impacts. If bacteria are elevated in a stream it is an immediate threat to human health. In this case, using a daily load is relevant and meaningful given the nature of the impairment and pollutant. However, in other situations it might be more appropriate to use a non-daily allocation. For example, for impairment to narrative water quality criteria due to excessive sedimentation, assessment of longer-term cumulative loading impacts is necessary to understand how to achieve water quality standards and to evaluate the loading capacity. In this case, it might be appropriate to use monthly or even annual allocations. As a reminder about the elements of a TMDL, for TMDLs that include non-daily allocations, EPA recommends that it also include a daily load expression with the final TMDL submission. Resources at the end of this module can be reviewed for more information on daily loads.

Allocation Analysis – Process
Identify potential allocation scenarios Equal concentrations Equal percent removal Target sources Relative impact of individual sources (e.g., sensitivity) Potential for multiple benefits (e.g., control multiple pollutants from one source) “Controllability” of sources Stakeholder priorities Once the decisions are made regarding the expression and nature of the allocations, a TMDL developer can move on to identifying the specific allocations. How this is done depends on the approach used. This was briefly discussed during the slides on commonly used TMDL approaches. For example, when using a watershed and receiving water model, source loads can be reduced iteratively until the resulting water quality conditions meet the water quality target. Additionally, certain states or TMDL developers might have preferred allocation methods for how to identify allocations among sources. Many of these approaches have been historically established in EPA Technical Support Documents and applied when evaluating the impacts and needed controls for multiple point source effluent discharges. But some methods are used in the context of mixed source TMDLs as well. Examples of those methods include setting allocations so that all sources discharge at the same pollutant concentration. Another example is applying equal percent reductions across all sources. For example, if the TMDL analysis identifies that the total existing load needs to be reduced by 40 percent to meet the loading capacity, the source allocations would be calculated by applying a 40 percent reduction to each of their respective existing loads. Another approach that is sometimes used in mixed source TMDLs is to target source allocations based on a more subjective evaluation of the sources and their characteristics. This is primarily used for establishing load allocations among the various nonpoint sources. Factors considered in how to target the allocations include the relative impact of each source on the impairment and the potential for multiple or added benefits from controlling a source. An example of added benefit is the reduction in multiple pollutants that occur from implementing controls on a single source, such as reducing both nutrients and bacteria when controlling failing septic systems. Another consideration is the controllability of a source. This represents how easy, whether technically or economically, it is to achieve reductions from the source. For example, some sources that already contribute a small portion of the overall load might not be able to reduce the load any further without making financially impractical investments. While other sources that represent a larger percentage of the total load and have a greater opportunity for reductions, such as more land area for BMP opportunities, can be targeted with larger reductions. Stakeholder concerns and priorities can also be a factor when establishing targeted allocations. Stakeholders can provide information on planned or expected implementation activities and the likelihood of meeting necessary reductions that could affect how allocations are distributed.

Allocation Analysis – Process (cont.)
Represent different combinations of source reductions that all meet WQS Allocation approaches that don’t focus on an equal distribution of the loading capacity or reductions likely produce a number of scenarios that could result in attainment of water quality standards. This concept is illustrated by this graphic showing model output for three different allocations scenarios. The scenarios represent different levels of load reduction across the various watershed sources and all result in concentrations below the water quality target. Selection of the final allocation scenario would be based on project goals or characteristics for targeting allocations, identified on the previous slide.

Reasonable Assurance Required in mixed-source TMDLs — with nonpoint and point sources Provides assurance that NPS reductions will occur Should be case-specific Can utilize an adaptive management approach toward meeting WQS Another issue considered in setting allocations is reasonable assurance. EPA guidance requires that reasonable assurance be included in TMDLs that include both point and nonpoint sources, referred to as mixed source TMDLs. In mixed sources TMDLs, where WLAs are based on an assumption that nonpoint source load reductions will occur, the TMDL should provide reasonable assurances that nonpoint source reductions are expected to occur. The reasonable assurance provides the roadmap for nonpoint source reductions. This can include what control measures will be implemented and when. Demonstrating reasonable assurance is not one-size-fits-all and should be specific to an individual TMDL. Factors to be considered in providing reasonable assurance include the nature of the receiving waterbody, the types of pollutants causing the impairment, the relative mix of nonpoint and point source loadings, and the nature of the sources of those loadings. Reasonable assurance can utilize an adaptive management approach for implementing allocations and meeting water quality standards. Adaptive management relies on proceeding with initial TMDL implementation and using subsequent monitoring data to periodically evaluate implementation progress and potentially adapt the plan for implementation. Effective adaptive implementation relies on establishing a schedule and milestones for implementing actions for achieving the allocations and designing a monitoring program that will track and evaluate progress over time.

TMDL Implementation Nonpoint Sources: Point Sources:
No federal regulatory enforcement program State/local NPS management programs (few w/ regulatory enforcement) Point Sources: Permits enforceable under CWA through NPDES Issued by EPA or states w/ delegated authority The implementation of nonpoint source LAs is not required by regulation. The primary mechanisms for implementing nonpoint source LAs are through state and local nonpoint source control programs under Clean Water Act Section 319. Most nonpoint source control occurs through best management practices, referred to as BMPs, including structural BMPs such as detention basins and vegetated buffers and non-structural BMPs such as education, training, and good housekeeping to reduce polluted runoff from activities or sites that drain to waterways. WLAs for point sources are implemented through discharge permits issued under the NPDES program. Permits are developed and issued either by EPA or states with delegated authority for their permitting program. Developing permit limits consistent with WLAs are discussed in detail in Module 2, Understanding WLA Implementation Through NPDES Permits: A Primer for TMDL Developers and special considerations for implementing WLAs for regulated stormwater are discussed in Module 3, TMDLs with Stormwater Sources and the NPDES Stormwater Permitting Process. As a preview of those modules, the following slide provides a quick overview of how WLAs are translated into permits.

Translating WLAs into Permits
Permit writers should, as appropriate, translate WLAs to water quality-based effluent limitations (WQBELs) WLAs Derived directly from water quality criteria through TMDLs, watershed analyses, or facility-specific analyses Expression can vary (concentration vs. load; daily, monthly, annual) WQBELs Derived from WLAs using EPA or state-specific limit development procedures Must be consistent with assumptions used to derive applicable WLAs [40 CFR (d)(1)(vii)(B)] Typically have different duration/averaging period than WLAs Permits include water quality-based effluent limits for the protection of water quality standards. Water quality based effluent limits are often referred to as WQBELs. While WQBELs must be consistent with the assumptions used to derive applicable WLAs, they typically are expressed in different terms than WLAs. As we’ve discussed, how a WLA is expressed is dependent on a number of TMDL-specific factors, such as the nature of the impairment, applicable water quality criteria, and the type and behavior of major sources. WLAs can be expressed as a concentration or load, and their duration, whether daily, monthly or even annual, can vary by TMDL. However, based on guidance from EPA in recent years, it is also recommended that TMDLs include a daily expression of any non-daily allocations. WQBELs for continuous discharges on the other hand are typically expressed as average monthly limits and maximum daily limits, consistent with NPDES regulations. While WQBELs must be consistent with the assumptions used to derive WLAs, they typically have different durations and averaging periods than WLAs. Consequently, the WLAs must be translated into WQBELs and this is done using established statistical procedures.

Translating WLAs into Permits
Calculating WQBELs Translating WLAs into Permits Steps in Developing Chemical-Specific WQBELs from Aquatic Life Criteria Step 1: Determine Wasteload Allocation (WLA) from Aquatic Life Water Quality Criterion Step 2: Calculate Long-Term Averages (LTAs) and Select Lowest Most states that administer the NPDES permits program will have some detailed procedures for calculating water quality-based effluent limitations from wasteload allocations. However, EPA has established its recommendations in the Technical Support Document for Water Quality-based Toxics Control, referred to as the TSD. This flow chart summarizes the process for developing water quality-based effluent limitations for aquatic life criteria based on the procedures in the TSD. The first step is to identify the WLA. In the context of this module, the WLA would be available in an approved TMDL. The next step after reviewing the WLA from an approved TMDL is to calculate the long-term average effluent concentration that will ensure that the wasteload allocation is met. After a permit writer has defined the desired frequency distribution for the effluent in terms of the long-term average and coefficient of variation of pollutant concentrations, he or she can use that information to calculate effluent limitations expressed as a maximum daily limit and an average limit, generally an average monthly limit. Step 3: Calculate Maximum Daily Limit (MDL) and Average Monthly Limit (AML)* *Other averaging periods used where appropriate (e.g., instantaneous maximum and instantaneous minimum for pH) NPDES Permit Writers' Workshop: Module 6

TMDL Monitoring Plans Recommended through EPA guidance
Provides data to demonstrate water quality improvements associated with TMDL implementation Supports adaptive management Choose approach appropriate to TMDL goals and targets Before/after control study Paired watershed study Upstream/downstream study Trend monitoring Although not a required TMDL element, TMDL monitoring plans are recommended by EPA through guidance. The primary goal of TMDL monitoring is to identify water quality improvements (or lack thereof) that result from TMDL implementation. This information serves as an important source of feedback for refining and optimizing management approaches, often referred to as adaptive management. It is important to craft a TMDL monitoring plan based on the TMDL goals and targets. TMDL monitoring can be done in a variety of ways, including a before/after study, paired watershed study, an upstream/downstream study, or trend monitoring. For more information, refer to U.S. EPA Region 10’s Technical Guidance for Designing a TMDL Effectiveness Monitoring Plan available on U.S. EPA Region 10’s TMDL Program website.

Emerging or Evolving Issues
Regulated stormwater (subject of Module 3) Nutrient criteria Climate change PCBs Invasive species Ocean acidification Water quality trading TMDL revisions/reopening The TMDL program has evolved over the years, often in response to certain types of impairments or sources. Today there are a number of topics that are of interest throughout the TMDL community and could have subsequent impacts on the permitting program. Many are the focus of existing efforts within EPA and states to develop approaches to address these sometimes challenging issues. For example, regulated stormwater has presented challenges to TMDL developers because they are permitted point sources that have the variable, precipitation-driven nature of nonpoint sources. This issue is the subject of Module 3 which discusses many of the unique considerations for developing and implementing TMDLs with regulated stormwater sources. As mentioned earlier in the presentation, the lack of nutrient criteria is a major issue in the TMDL and permitting communities. As you saw at the beginning of this module, nutrients are the #3 listed impairment in the country, with nearly 7,000 waterbodies listed. Organic enrichment, which is often linked to nutrients, is the #4 listed impairment, with an additional 6,300 waterbodies. There are a number of efforts, both at EPA and state levels, currently underway to address this issue. Another current topic of interest is climate change. The general concerns surrounding climate change impacts to hydrologic patterns, flows and water quality conditions extend to future accuracy of TMDLs. Specifically whether TMDL allocations developed under current conditions will be sufficient or even relevant in the future as climate change potentially impacts the behavior and response of the waterbody of concern. PCBs have also been the subject of recent EPA efforts. PCBs represent one of the top 10 impairments, with more than 5,000 listed waters. However, relatively few TMDLs have been developed for PCBs….currently less than 400. Recently EPA developed a handbook to help TMDL developers in developing PCB TMDLs. The handbook is available on the TMDL program website and is included in the Resources at the end of the module. Ocean acidification and invasive species are also issues currently of interest, both for listing impaired waters as well as developing the subsequent TMDLs. EPA released a draft memo in November 2010 on listing waters for ocean acidification and have several supporting documents available on their website. EPA has developed a number of resources to support the use of water quality trading in meeting water quality goals, including implementing TMDLs. Trading is based on establishing a program where facilities facing higher pollution control costs to meet their regulatory obligations can purchase environmentally equivalent (or superior) pollution reductions from another source at a lower cost, thus achieving the same water quality improvement at lower overall costs. EPA’s Water Quality Trading website includes a trading toolkit and an online training module to support development of effective trading programs. Another topic raising questions throughout the TMDL community is the need for revising or reopening an approved TMDL. EPA is currently developing a memo on revising and withdrawing TMDLs. The memo outlines the types of changes that would (or would not) be considered a TMDL revision subject to EPA review and approval. The memo also lists the situations when a TMDL may be withdrawn.

Tips for Engaging in the TMDL Process
Understand format/content of your state’s 303(d) list Review TMDL development schedule Provide data and information on point sources Provide input on selection of linkage analysis approach and how point sources are represented Participate in allocation analysis Understand that the WLA in a TMDL may not provide the most stringent effluent limit Now that we’ve gone through the TMDL development process, let’s review things that permit writers can do to better integrate TMDLs and permits. Some include ways of becoming more involved in the TMDL process to ensure that point sources are accurately represented and WLAs are developed appropriately. Other activities are to help permit writers better understand the TMDL and associated WLAs to more effectively translate those WLAs into permits. First it can be helpful to become familiar with your state’s Integrated Report, particularly the 303(d) list. In addition, reviewing the state’s schedule for developing TMDLs will let permit writers know when TMDLs are being developed that will include permitted sources. The next few tips focus on how permit writers can contribute to the TMDL development process. Permit writers can provide valuable data and information on point sources that a TMDL developer might not know exists or have access to. This can include effluent monitoring data that are not available in national or state databases, regulated boundaries for stormwater sources, and special studies that dischargers conducted such as water effect ratios or translator studies. This type of information is important for accurately representing and characterizing the point sources within the TMDL analysis. Another step permit writers should be involved in is the selection and application of the approach for the linkage analysis. Permit writers can provide input on which approach should be used and also how point sources should be included in the approach. It will also be important for you to participate in the allocation analysis step of the TMDL process. Permit writers can provide critical input to ensure WLAs are realistic and feasible, that assumptions are appropriate, and that WLAs are expressed in a way that makes them easily understandable by permit writers for translation in WQBELs. Finally, it’s important to understand that the WLA presented in a TMDL might not provide the most stringent effluent limit. If a pollutant has an applicable numeric water quality criterion, the permit writer should determine reasonable potential and calculate a separate WLA to compare against TMDL. In addition, the permit writer should confirm that technology-based effluent limits for the pollutant of concern are not more stringent than the TMDL’s WLA.

Resources Technical Resources
TMDL Process and Background Guidance for Water Quality-based Decisions: The TMDL Process. EPA 440/ Protocol for Developing Sediment TMDLs. EPA 841-B Protocol for Developing Pathogen TMDLs. EPA 841-R Protocol for Developing Nutrient TMDLs. EPA 841-B Modeling for TMDLs Compendium of Tools for Watershed Assessment and TMDL Development. EPA841-B TMDL Model Evaluation and Research Needs. EPA/600/R-05/149. Special Topics Options for Expressing Daily Loads in TMDLs. Draft: June 22, 2007. Handbook for Developing Watershed TMDLs. Draft: December 15, 2008. TMDLs to Stormwater Permits Handbook. Draft: November 2008. An Approach for Using Load Duration Curves in the Development of TMDLs. EPA 841-B PCB TMDL Handbook. EPA 841-R (under TMDL Technical Resources) Policy and Guidance Resources (listing, special issues) (under TMDL Guidance) This slide presents a list of resources related to the information presented in this module. Most of the resources are available at EPA’s TMDL web page at Now it is time to take a brief quiz based on what we’ve covered in this module.

TMDL module

Understanding TMDLs: A primer for permit writers

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