Ohio State University Extension Fact Sheet

Ohio State University Fact Sheet

Agricultural Environmental and Development Economics

2120 Fyffe Road, Columbus, OH 43210-1061

Total Maximum Daily Loads (TMDLs)


Brent Sohngen
Chia-Yu Yeh

Agricultural Environmental and Development Economics
The Ohio State University

What Is a TMDL?

In 1996, the United States Environmental Protection Agency (U.S. EPA) issued its Healthy Watershed Strategy to increase the development and implementation of Total Maximum Daily Load (TMDL) calculations for watersheds. Briefly, TMDLs involve collecting data in waterbodies (a stream, stream segment, or lake) to determine aggregate point and nonpoint source pollution loads and using the data to set maximum allowable loads from each of these sources. Loads are the quantity of pollution contributed to a stream by a single source (i.e., a waste-water treatment plant) or by a group of sources (i.e., all developments or agricultural fields along a stream). Point sources of pollution typically involve pipes that convey discharges directly into streams, such as waste-water treatment plants. Nonpoint sources, such as farms, lawns, or construction sites contribute pollution diffusely through run-off.

TMDLs generally will be developed one pollutant at a time for particular waterbodies. For example, a TMDL may be developed for nitrogen in a particular stream segment. In some cases, multiple TMDLs for different pollutants will be developed for particular waterbodies. Pollution load data already exists for most point sources of pollution under the National Pollutant Discharge Elimination System (NPDES) permitting process. Thus, the major steps in this new initiative are (1) to develop similar load calculations for nonpoint sources of pollution; (2) to calculate natural loads; (3) to calculate maximum total loads from all sources; and (4) to determine how to reduce the maximum load in order to meet water-quality standards.

Because it is difficult to identify specific nonpoint sources of pollution, it is unlikely that data will be collected on individual nonpoint sources (or landowners) along a waterbody. Instead, TMDLs focus on estimating the aggregate contribution of all nonpoint sources along a waterbody. For example, if the total load, the point source load, and the natural load of nitrogen for a stream are known, aggregate nonpoint source loads can be determined as:

Nonpoint N = Total N - Point Source N - Natural N

In many circumstances, nonpoint source contributions will be broken down into additional categories, such as agriculture, development, forestry, or mining.

TMDLs are focused primarily on developing better estimates of the contribution of nonpoint sources to total pollution loads in streams. In Ohio, and indeed in many states, the process of identifying streams for TMDL development, developing the proper methods to calculate loads from all pollution sources, and implementing programs to reduce loads in order to meet water- quality goals is just beginning. Although it is expected that this entire process could take 10 to 15 years to complete for all waterbodies requiring a TMDL, some will be completed much more quickly, depending on the cause of impairment and whether or not the waterbody attains water-quality standards. Indeed, TMDL development is already underway in several streams in Ohio.

Why Develop TMDLs?

Over the past 25 years, pollution control under the Clean Water Act (CWA) has focused on point sources of pollution through the NPDES permitting process. While water quality has improved, the goals of the CWA have not been met in a number of streams. Data from the U.S. EPA suggest that nonpoint sources are now the largest source of pollution in streams and lakes (U.S. EPA, 1998), and data from Ohio supports this conclusion (Ohio Environmental Protection Agency [Ohio EPA], 1998). Specifically, the Ohio EPA ranks the leading sources of impairment in the following order: hydromodification, point sources, agriculture, mining, other/unknown, and urban runoff (Ohio EPA, 1998).

Although the Ohio EPA's data rank the sources of impairment on rivers, these estimates are not based on specific links between sources and aggregate pollution loads in streams. These links are difficult to establish with current data sources. TMDLs, however, are expected to provide the data to fill this gap in knowledge. The U.S. EPA expects that the data collected will help target local, state, and federal efforts on improving water quality enough to meet regulatory standards.

How Are TMDLs Being Developed in Ohio Today?

The TMDL process is dynamic, and it is under development in Ohio. TMDLs are required by federal law, and the U.S. EPA has just published preliminary guidance for states to use in developing their TMDL programs. Under federal regulations, each state is required to develop a plan for instituting TMDLs within its boundaries. As TMDLs are developed in Ohio, they must be submitted to U.S. EPA for approval. A draft of the U.S. EPA's suggested methods for TMDL development is available from the U.S. EPA websites, listed later in this publication, for interested individuals to review.

Currently, the Ohio EPA is developing methods for TMDL development in Ohio, with the help of an External Advisory Group (EAG). This advisory group will present recommendations to the Ohio EPA director by the end of 1999 for Ohio's TMDL process. Although the process for developing and implementing TMDLs may continue to change, certain general steps are likely to be part of any final process.

Steps for TMDL Development in Ohio

  1. Use existing data to identify waterbodies with water quality problems.

    The Ohio EPA has divided the water of the state into approximately 5,000 waterbody segments. Data from the 1998 Ohio Water Resource Inventory (Ohio EPA, 1998) and its accompanying analyses are the primary source of information on water quality. This assessment is intended to reflect the biological, habitat, sediment, and chemical quality of Ohio's surface waters. In addition to this, data and information from Section 319 project areas, Remedial Action Plan (RAP) areas, and Fish Consumption and Human Contact Advisory Areas will be used.

  2. Prioritize watersheds for TMDL development.

    TMDL development will be no small task, and it will not occur at once. Thus, the Ohio EPA has begun a prioritization process. This process may change in the future, depending on suggestions from the external advisory committee, the final regulatory guidance from the U.S. EPA, or as new data becomes available. For example, benefit cost analysis could be used to help set priorities, rather than the use of impairment rankings alone. Understanding the current process, however, can provide important background for interested individuals, and it is therefore described here briefly.

    The current process begins with the data on waterbody segments. Priority waterbody segments are those where the designated use is impaired or threatened, or the segment is within a fish consumption or human contact advisory area designated by the Ohio Department of Health. Designated uses for waterbody segments are set in accordance with the Clean Water Act. Based on these criteria, 881 out of approximately 5,000 waterbody segments have been selected as requiring TMDL development because waters in them are not meeting existing water-quality standards.

    To set priorities consistently with existing water-quality efforts in the state, the Ohio EPA mapped the segments to determine how many of the 326 watersheds in Ohio contained impaired waterbodies. Of these watersheds, 276 contain one or more of the impaired waterbody segments. These 276 watersheds were then ranked according to their impairment level to develop a prioritized list of watersheds targeted for TMDL development in Ohio. This list can be found on the Ohio EPA website listed later in this fact sheet.

    This process has already led to TMDL development in Ohio (and in other states). A TMDL for the middle Cuyahoga River will soon be submitted to the U.S. EPA for approval. Four additional TMDLs are to be completed in the year 2000: Mill Creek in Cincinnati, Sugar Creek, upper Little Miami River, and Rocky River. The upper Stillwater River, Bokes Creek, Mill Creek in Marysville, and Raccoon Creek are also in process and to be completed in 2001.

  3. Determine existing pollution loading from point and nonpoint sources.

    While pollution loads from point sources are generally known from NPDES permits, pollution loads from nonpoint sources and background sources must be estimated. Because these sources are difficult to identify precisely, TMDLs will focus on estimating total, or aggregate, loads from nonpoint and background sources for the waterbody under consideration. If possible, TMDLs will further break nonpoint sources into specific categories, such as agriculture, development, forestry, or mining. Clearly, there may be instances where it is difficult to determine loading from all sources with existing data. In these cases, the first efforts will focus on developing the proper data for TMDLs before loads are fully assigned.

  4. Allocate pollutant loading among point and nonpoint sources.

    To allocate loads, TMDLs will also estimate the assimilative capacity of the waterbody. Assimilative capacity is the quantity of pollution a stream segment can accept without causing water quality to exceed standards imposed by the Clean Water Act. To determine appropriate loading, actual data or models may be employed to calculate load reductions necessary to achieve water-quality standards. In addition, agencies must establish margins of safety around the maximum loads to account for uncertainty in load and assimilative capacity estimates.

    While determining the maximum total pollution load in a watershed may be relatively easy, the more difficult step will be to allocate the loads to different sources, i.e., NPDES permit holders, agricultural producers, or development. Where waterbodies have predominately either point or nonpoint sources in them, allocation may also be easy. If waterbodies have a mixture of sources, however, resource managers must approach the difficult task of deciding whose load must be reduced. Existing regulations do not provide guidance on determining how best to allocate pollution loads. For example, should the largest total source be targeted for reduction, should the cheapest source be targeted, or should all sources be reduced equally?

  5. Use existing federal, state, or local laws or programs to reduce pollution loads where necessary.

    Importantly, TMDLs do not establish new regulations beyond those that currently exist. Nevertheless, TMDL development is occurring as the U.S. EPA also pursues national strategies for particular sources, such as the recently announced "Joint Strategy for Animal Feeding Operations."

    Under TMDLs, point sources will be expected to implement their loading allocation through enforceable water-quality-based discharge limits in NPDES permits. Nonpoint sources will be expected to implement their loading allocation through voluntary incentive programs for best management practices or other existing statutes or programs like the animal feeding operation strategy discussed earlier. Examples of voluntary incentive programs include the U.S. EPA Section 319 program or the USDA Environmental Quality Incentive Program. TMDLs will also generate data and information that can be used to assist locally led watershed-protection efforts. Obviously, additional regulatory or nonregulatory programs for point or nonpoint source pollution may be developed in the future.

How Will TMDLs Affect Farmers?

The ultimate effect of TMDLs on farmers is unclear at this point. What is clear is that TMDLs will provide more data and information than currently exists on the contribution of nonpoint sources to total pollution loadings in Ohio waterbodies. This may show that agriculture contributes a large or small share of the pollution to waterbodies, depending on the particular place.

Currently, there are no new nonpoint source pollution regulations associated with TMDL development that would affect farmers today. The data collected may be used to help environmental and regulatory agencies better target funds provided by voluntary incentive payment programs. Further, with continued monitoring of TMDLs, agencies will be better equipped to monitor the effectiveness of these voluntary programs. The data may also be used to consider more stringent measures in the future to regulate nonpoint sources of pollution.

Related OSU Extension Bulletins and Fact Sheets

For a complete listing of OSU Extension Fact Sheets and Bulletins, please visit Ohioline at: http://ohioline.ag.ohio-state.edu

Nonpoint Source Pollution: Water Primer. OSU Extension Fact Sheet AEX-465-93

Nitrogen and the Hydrologic Cycle. OSU Extension Fact Sheet AEX-463-91

Surface and Ground Water Terminology. OSU Extension Fact Sheet AEX-460-94

Water Quality Project Evaluation: A Handbook for Objectives-Based Evaluation of Water Quality Projects. OSU Extension Bulletin 868-98


World Wide Web Resources

U.S. EPA: Total Maximum Daily Load Program Description

U.S. EPA: Case Studies on 13 TMDLs

U.S. EPA: Nonpoint Source Pollution Control Program Description

Ohio EPA, Section 303(d) TMDL Priority List for 1999-2000

Ohio EPA, Explore water quality in your watershed

Conservation Technology Information Center, Purdue University


Ohio Environmental Protection Agency.1998. 1998 Ohio Water Resources Inventory: Volume I. Addendum: Updated Aquatic Life Use Statistics. Ohio EPA Technical Bulletin MAS/1998-6-2.

United States Environmental Protection Agency. 1998. National Water Quality Inventory: Report to Congress. EPA-841-F-97-003.


This Fact Sheet was supported in part by the USDA Natural Resource Conservation Service EQIP education funds.The authors would like to thank Larry Antosch (Ohio Farm Bureau), Anne Baird (OSU Extension), Vicki Deisner (Ohio Environmental Council), Laurel Hodory (Ohio EPA), Constance Jackson (Ohio Farm Bureau), and Trinka Mount (Ohio EPA) for providing useful comments on this document. Brent Sohngen is an assistant professor and Chia-Yu Yeh is a graduate student in the Department of Agricultural, Environmental, and Development Economics at The Ohio State University. Please direct all correspondence to Brent Sohngen at 322 Agricultural Administration Building, 2120 Fyffe Road, Columbus, OH 43210.

All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.

Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.

TDD No. 800-589-8292 (Ohio only) or 614-292-6181

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