Water Quality and Total Maximum Daily Loads (TMDLs)

By Dr. Paul Risser
University of Oklahoma
and Dr. Robert Puls
Oklahoma Water Survey

The Total Maximum Daily Load (TMDL) is a calculation of the maximum amount of a pollutant a water body (for example, Oklahoma’s lakes, rivers or streams) can receive and still meet the applicable water quality standard.  The TMDL calculation, which is a detailed process requiring data and mathematical models, is based on both the properties of the water body itself and the pollutant sources leading to a water body’s failure to meet applicable water standards. The calculated TMDL is then used to describe the necessary reduction in pollutants or changes in land management to reduce the pollutant to acceptable levels.  This information is used as the basis for a plan to ensure that the water quality is restored.  Through the work of the Oklahoma Department of Environmental Quality, Oklahoma has conducted TMDL studies of several basins and has plans to complete many more.


For many years the U.S. Environmental Protection Agency (USEPA) and state water quality agencies across the country have used TMDLs in implementing the Clean Water Act by establishing maximum pollution limits for industrial wastewater discharges (point sources).  In the last two decades TMDLs have been widely applied to both point sources and non-point sources within watersheds.

The Oklahoma Department of Environmental Quality, Water Quality Division, Watershed Planning Division, has produced the detailed Oklahoma Total Maximum Daily Load Practitioners Guide ( along with a more general explanatory pamphlet (


The Total Maximum Daily Load (TMDL) is a regulatory term from the U.S. Clean Water Act (CWA) of 1972.  Water bodies that do not meet applicable water standards with best practice technology-based controls are placed on the section 303d list (based on section 303d of the Clean Water Act) and as a result of that designation, require development of a Total Maximum Daily Load (TMDL) calculation.

TMDLs are developed in the context of a watershed, including the quality of the water in the water body or bodies, the water that flows into and through the watershed and the point and non-point sources of pollution. The general steps are listed below.

  1. Define Watershed Goals – Describe the study area and spatial boundaries; define water quality standards; address key issues such as characterization of the impairment with respect to water quality standards; seasonal variability of concentration of water quality constituents; relationship between observed concentration of water quality constituents and flow; definition of long-term trends of constituents; existing permit limits for point sources within the study area; other general descriptive or background information and identification of the water quality monitoring stations.
  2. Complete Watershed Assessment to define water quality targets (or TMDL endpoints) – Develop the field sampling plan (including quality assurance and quality control protocols) for describing the type, magnitude and location of sources of pollutant loading and loading conditions for point sources, nonpoint sources, background contributions, tributaries, and any upstream flows; selecting appropriate data and mathematical models for estimating water flows, sediment dynamics and pollutants from point and non-point sources.
  3. Quantify loading and allocations – Use data to decide the data sources and time steps for analyses; calibrating and validating hydrologic, sediment and water quality models; estimating future growth in pollutant sources, where applicable; and deciding upon the numerical margin of safety to be used in the analyses.
  4. Develop TMDL – Conduct the calculation for pollutant loadings as described in the next section.
  5. Plan monitoring – Develop a plan for monitoring the water body in the context of the watershed as defined by the point and non-point pollutant loadings.
  6. Plan implementation – Implement best practice managements and describe the watershed plan so the water body will meet the applicable water quality standards.

The TMDL for a water body, the Maximum Load Capacity, includes four components:

  • Background Allocation (BA) – The amount of pollutant that occurs naturally in the watershed
  • Waste Load Allocation (WLA) – The fraction of the total pollutant load apportioned to point sources, including storm water discharges regulated under the National Pollutant Discharge Elimination System (NPDES)
  • Load Allocation (LA) – The fraction of the total pollutant load apportioned to non-point sources
  • Margin of Safety (MOS) – A percentage of the TMDL set aside to account for the uncertainty associated with natural process in aquatic systems, model assumptions, and data limitations.

Thus, the Total Maximum Daily Load can be described as follows:


Point sources of bacteria, for example, include municipal wastewater treatment plants (WWTPs), municipal no-discharge wastewater treatment plants, municipal separate storm sewer discharge, and concentrated animal feeding operations (CAFOs).  Non-point sources include pollutants that may arise from many sources, e.g., septic systems or nutrients or manure from fields, and are typically separated into urban and rural categories.

TMDLs can be established for large basins or for smaller watersheds.  And, they can be set for one or more conditions, including bacterial pathogens, temperature, dissolved oxygen, chlorophyll a, turbidity, pesticides or nutrients such as nitrogen or phosphorous.

The Illinois River Watershed in Arkansas and Oklahoma is an example that focuses on nutrients  (  Scientists in Region 6 of the U.S. EPA are developing a watershed model for the Illinois River watershed in Oklahoma and Arkansas to address nutrient water quality impairments. The purpose of this project is to develop a scientifically robust watershed model to determine the reductions in phosphorus loads that are needed to meet water quality standards in both States. This watershed model would identify nutrient reductions needed to ensure that water quality standards for phosphorus are protected in both States, and as such could serve as the basis for one or more TMDLs for the Illinois River Watershed.

Oklahoma has completed TMDLs on a number of basins or watersheds ( For the last three years, Oklahoma has been in the top 10 of states in reducing pollution of its streams and rivers and in 2011 was ranked number two in that category by the USEPA.

Figure 1. Number of TMDLs completed per fiscal year (EPA fiscal year starts Oct. 1 ends Sep. 30)

The Canadian River Basin is another example. It exceeded the limits for primary or secondary body contact for recreation water quality standards because three pollutants (fecal coliform, E. coli, Enterococcus spp.) exceeded Oklahoma’s current pathogen criteria based on USEPA guidelines (See Implementation Guidance for Ambient Water Quality Criteria for Bacteria, May2002 Draft; and Ambient Water Quality Criteria for Bacteria-1986, January 1986). The details of the calculation of TMDL for the Canadian River Basin can be found in the final report. (

Using data from 2008, which was a representative reporting year, the major causes of impairment for the state as a whole were pathogens, total dissolved solids, turbidity and dissolved oxygen. (


Oklahoma has a well-established TMDL process, the process has been used in several river basins and watersheds, and the state has a chronological plan to continue to address the 303d-listed water bodies. Implementing the TMDL procedures has identified a number of opportunities for future consideration with respect to future analyses of relevant data and information.

Several agencies are involved with collecting water quality data and the associated policies. The Oklahoma Department of Environmental Quality is responsible for the TMDL process and the Oklahoma Water Resources Board is responsible for developing the State’s water quality standards and also for the large-scale Beneficial Use Monitoring Program (BUMP).  The Oklahoma Conservation Commission conducts an extensive water quality-monitoring program, primarily in support of the non-point source program. The Oklahoma Corporation Commission is also involved in monitoring water quality, particularly near oil and gas fields.  In completing the Fort Cobb watershed TMDL, for example, data on water quality were used from sampling done by the U.S. Geological Survey, U.S. Bureau of Reclamation, U.S. Fish and Wildlife Service, Oklahoma Water Resources Board and Oklahoma Conservation Commission.  Although each of these monitoring programs fits the needs of the specific agency, there may be opportunities to increase coordination among the data collection processes.

Models and Tools

Various mathematical models are calibrated for each site (watershed) and used to predict the components of the TMDL. For example, the SWAT (Soil and Water Assessment Tool) model is used to simulate nutrient loads into a water body.  The EFDC (Environmental Fluid Dynamic Code) or the Water Quality Analysis Simulation Program (WASP) models are used to predict water quality in the water body.  As more data are collected and models are tested, current models may be improved and new models will be developed and available as potential tools to use in Oklahoma.

Many models used in the TMDL process are focused on prescribed sets of processes, such as loadings from non-point sources.  However, watersheds have many components including many land-uses and changing weather/climate.  Future models will integrate these components and use a large array of input variables to continue to improve the accuracy and precision of the model results both in the water body and the watershed.

Data reconciliation

Long-term trends in monitoring of water quality are very important in describing the temporal dynamics of the conditions of water bodies.  Among the key issues is how to address model validation when hourly or daily flow data are not available. Similarly, measuring conditions in the watershed is important, for example in assessing the impacts of the implementation of best management practices.  In many cases, the density and frequency of data collections are not sufficient to support confident conclusions from the analyses and model predictions. In addition, because data are collected at various times before the data are used in models, it is important to be sure that the loadings are properly aligned with the model results.

The results of the TMDL process are recommendations on the necessary reductions in pollutant loads and the allocation of these loads among different sources.  And yet, the important step in many ways is the plan for actions to reduce the offending pollutants.  Future TMDL processes in Oklahoma might incorporate both economic models to assess the impacts of various responses to the TMDLs along with modeling various restoration scenarios.


Oklahoma’s Water Quality Standards (OWQS) are rules (Oklahoma Administrative Code, Title 785, Chapter 45) that provide the baseline against which the quality of waters of the state are measured. These standards, which are the statutory responsibility of the Oklahoma Water Resources Board (, include two primary components:

  1. The beneficial uses ascribed to a water body, such as agriculture, hydroelectric power, fish and wildlife propagation, drinking water, or recreation.
  2. The numerical or narrative criteria that are assigned to each beneficial use.

Narrative criteria are descriptions of desired conditions.  Numerical criteria are usually maximum or minimum concentrations of factors such as nutrients, suspended solids, siltation, Trophic State Index (chlorophyll a), dissolved oxygen, pesticides or conditions of the water column.  The criteria of one factor may be contingent on other characteristics of the water body, such as pH, temperature or season. And, the same criteria in a water body may be different for different beneficial uses.  In establishing TMDLs, the most stringent criterion is usually applicable.

Decisions about whether a water body is impaired are based on a set of rules, described by the Use Support Assessment Protocols (USAP) ( that define how beneficial uses are assessed and whether the beneficial use for a particular water body is fully supported, partially supported or not supported.  Whether a water body is impaired is based on the Integrated Water Quality Assessment Report (Integrated Report) that includes the Assessment Methodology describing the data and the decision tree to be used in deciding the conditions under which a water body may be considered impaired. There are five possible attainment conditions:

Category 1 – all beneficial uses assessed and attained

Category 2 – some beneficial uses assessed, no impaired uses

Category 3 – not enough information to assess beneficial uses

Category 4 – one or more uses impaired, but no TMDL required

Category 5 – one or more uses impaired, TMDL required

Category 5 is the State’s 303(d) List of Impaired Waters.

Beneficial use designations include the following:

  • Drinking water
    – Treated/untreated
  • Water-based recreation
    – Noncontact/short-term/long-term
  • Fishing/eating
  • Aquatic life
    – Warm water species/habitat
    – Cold water species/habitat
  • Agriculture water supply
  • Industrial water supply

Oklahoma’s 2008 “303d” listed water bodies, along with the dates for TMDL analyses and the priority scale for future TMDL analyses can be found at In 2010, four streams were removed from this list (

In Oklahoma, the Oklahoma Department of Environmental Quality (ODEQ) sets the TMDLs (  ODEQ is required to submit all TMDLs to U.S. Environmental Protection Agency for review and approval.  Once the USEPA approves a TMDL, the water body may be moved to Category 4a of Oklahoma’s Integrated Water Quality Monitoring and Assessment Report (, where it remains until compliance with water quality standards is achieved.

Several state agencies have some responsibility to address point and non-point pollutant source reduction goals established by TMDLs. Nonpoint source pollution is managed by the Oklahoma Conservation Commission (, primarily though incentive-based programs that support the installation of best management practices. The Oklahoma Water Resources Board ( provides support, public education and outreach.  The Agricultural Environmental Management Services (AEMS) of the Oklahoma Department of Agriculture, Food and Forestry (ODAFF) ( oversees environmental policies and programs aimed at pollutants associated with agricultural animals and their waste through regulations established by the Oklahoma Concentrated Animal Feeding Operation Act.

As authorized by Section 402 of the CWA, the Oklahoma Department of Environmental Quality manages the NPDES Program in Oklahoma, except for certain jurisdictional areas related to agriculture and the oil and gas industry retained by State Department of Agriculture and Oklahoma Corporation Commission, for which the USEPA has retained permitting authority. The NPDES Program in Oklahoma is implemented via agreement between ODEQ and U.S. Environmental Protection Agency ( Implementation of point source Waste Load Allocation (WLA) for the TMDLs is done through permits issued under the OPDES program (

The Oklahoma Department of Environmental Quality (ODEQ) targets available funding and technical assistance to support pollution controls and management measures designed to achieve the reductions required by TMDLs. ODEQ’s Continuing Planning Process (CPP) ( 2002_cpp_final.pdf), required by the CWA §303(e)(3) and 40 CFR 130.5, summarizes Oklahoma’s commitments and programs aimed at restoring and protecting water quality throughout the State.


The true value and benefit of the TMDL process lies in the effectiveness of the implementation plans to achieve attainment goals for water quality.  These plans are generated by the states to secure federal funding under the Clean Water Act and TMDL program.  A major drawback is the lack of sufficient guidance to prepare comprehensive watershed implementation plans.  As a result of this lack of guidance the states are given flexibility in the development of these plans.  This results in significant variability in the plans among the different states.  The USEPA continues to update its guidance and the recent development of “Watershed Central” and other efforts by EPA should improve the effectiveness of the TMDL process (



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