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Mission and Aransas Rivers

This community project keeps recreation safe in segments of the Mission and Aransas Rivers by reducing bacteria levels. Together, the adopted total maximum daily loads (TMDLs) and approved implementation plan (I-Plan) provide the road map for improving water quality.
photo of copano bay
Aransas River Tidal
Photo courtesy of Texas AgriLife

Watershed Counties: Aransas, Bee, Goliad, Karnes, Refugio, and San Patricio
Parameter: Bacteria
Basin: San Antonio-Nueces Coastal
Segments: 2001, 2003, 2004, 2004B

On this page:

Background and Goals

High concentrations of bacteria are often found in the tidal portions of the Mission and Aransas Rivers. High concentrations of bacteria may indicate a health risk to people who swim or wade in the rivers—activities called “contact recreation” in the state’s standards for water quality.

Bacteria are commonly found in the intestines of warm-blooded organisms such as humans, livestock, cats, and dogs. These bacteria in water may indicate the presence of disease-causing microorganisms.

The goal of the project is to reduce bacteria levels to protect people who swim or wade in the rivers. The Texas Water Resources Institute (TWRI) coordinated stakeholder participation in development of the TMDLs and I-Plan. Stakeholders are also developing a watershed protection plan (WPP).

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Get Involved

Project staff informs the public about this project through a series of public meetings. The meetings are open to everyone.

In all its projects, the TCEQ seeks to gather opinion and information from people who represent government, permitted facilities, agriculture, business, environmental, and community and private interests in the watershed. The TWRI coordinates public participation in monitoring progress of activities to improve water quality in the watershed.

Educational Resources

See the Texas Water Resources Institute website for the Copano Bay Water Quality Education Project , which provided information about educational programs and demonstrations for land and livestock owners. These programs and demonstrations were designed to increase awareness of water quality issues and best management practices for reducing bacteria in stormwater runoff.

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Approved I-Plan

The commission approved the stakeholders' I-Plan on May 25, 2016.

Status of Activities

The TWRI watershed coordinator and stakeholders meet annually to evaluate the progress of their I-Plan and WPP. The I-Plan sets forth nine voluntary management measures and two regulatory control actions. Tracking of implementation progress allows stakeholders to evaluate actions taken, identify actions that may not be working, and make any changes as necessary.

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Adopted TMDLs

The commission adopted these TMDLs on May 25, 2016.

On August 9, 2016, the EPA approved the TMDLs, at which time they became part of the state’s Water Quality Management Plan. Learn more about the Water Quality Management Plan.

Revisions to the TMDLs

From time to time, it is necessary to revise TMDLs to account for changing conditions in the watershed. Revisions to the load allocations in TMDLs are made via the state’s Water Quality Management Plan (WQMP), which is updated quarterly. Revisions are made to TMDLs in one of two ways:

  • Updates are made to account for changing conditions in the watershed, such as new or revised wasteload allocations, permits that have been canceled or have expired, or changed facility names.
  • Addenda are created to add load allocations for assessment units that are impaired by the same pollutant or condition, within the same watershed as in the original TMDL report.


Additional TMDLs have been added for the watershed by addendum:

  •  Addendum One: Two TMDLs for Bacteria in the Aransas River Above Tidal and Poesta Creek
    Segments: 2004 and 2004B; Assessment Units: 2004_02 and 2004B_02
    October 2017 WQMP Update. Approved by EPA February 8, 2018.


  • April 2022, Appendix V, pages 30-31
  • April 2021, Appendix V, pages 21-22
  • July 2020, Appendix I, page 12

WQMP updates may be viewed in person at the TCEQ Library, Building A, 12100 Park 35 Circle, Austin, Texas. Electronic versions of updates published from 2014 through 2020 are available on the Texas State Library and Archives Commission website .

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Project History

The state and its partners collected and analyzed extensive amounts of data about the project watersheds from 2004 through 2010. While the TCEQ, the Texas State Soil and Water Conservation Board (TSSWCB), and stakeholders in the watershed evaluated the most appropriate strategy for reducing bacteria concentrations, agricultural producers voluntarily participated in educational activities aimed at better understanding the water quality issues and various methods for reducing the amount of bacteria that can wash off agricultural property and into waterways.

In 2011, stakeholders and the state agencies decided to develop TMDLs and an implementation plan for the tidal portions of the Mission and Aransas Rivers in the Copano Bay watershed. The goal of a TMDL is to determine the amount (or load) of a pollutant that a body of water can receive and still support its designated uses. The allowable load is then allocated among the categories of sources within the watershed. Stakeholders work with the state to develop an implementation plan (I-Plan) with measures that reduce pollutant loads.

Copano Bay, which is identified as having an impaired oyster waters use, was considered in the assessment stage of this project. Designation of the oyster waters impairment is based on a risk assessment by the Department of State Health Services (DSHS). These risk assessments are not based on concentrations of bacteria in the water; rather, they are based on certain risk factors such as the likelihood of high rainfall and runoff or the influx of sewage from failing or inefficient domestic treatment systems.

Actual concentrations of bacteria in the waters of the bay are within acceptable levels. Consequently, development of a bacteria TMDL for Copano Bay is not necessary, and would not be effective in reducing the risk assessed by the DSHS.

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Contact the TMDL Program

Please email, and mention the Mission and Aransas Rivers project in the subject. Or call us at 512-239-6682.

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Communities Working Together