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Arroyo Colorado: Legacy Pollutants and Organics

This is a completed project to reduce legacy pollution from chemicals and make it safe to eat fish from the waterways. The total maximum daily loads (TMDLs) were adopted, and the implementation plan (I-Plan) has approved. Together, the TMDLs and I-Plan are the the road map for improving water quality.

photo of the Arroyo Colorado
Arroyo Colorado
map of the Arroyo Colorado watershed

Counties: Cameron and Hidalgo
Parameters: Legacy pollutants and organics in fish tissue
River Basin: Nueces-Rio Grande Coastal
Segments: 2202, 2202A

On this page:

Background and Goal

The Arroyo Colorado, an ancient distributary channel of the Rio Grande, extends 90 miles from Mission, Texas to the Laguna Madre in the Rio Grande Valley. Water quality and fish tissue analyses in 1998 showed that upstream of the Port of Harlingen, legacy pollutants in fish tissues were at levels that posed an unacceptable risk to consumers. The Texas Department of State Health Services (DSHS) evaluates the health risk to consumers from toxic substances that have accumulated in fish tissue and issues consumption advisories or bans when needed.

Legacy pollutants are chemicals whose use has been banned or severely restricted, but which still remain in the environment. The pollutants found in fish tissue included chlordane, DDE, DDT, dieldrin, endrin, hexachlorobenzene, heptachlor, heptachlor epoxide, lindane, PCBs, and toxaphene.

The goal of this project was to establish acceptable levels of legacy pollutants in the Arroyo and to monitor conditions over time to ensure that toxic substances in fish tissue were reduced to acceptable risk levels. 

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Watershed Description

The Arroyo Colorado is the major source of fresh water flowing into the lower Laguna Madre, which is an economically and ecologically important resource in the region. The fertile farmland, long growing season, and access to water for irrigation from the Rio Grande make this region one of the most productive agricultural areas in the United States.

One-third of the stream is used for shipping from the Gulf Intracoastal Waterway to the Port of Harlingen. The Arroyo is also an integral part of a major floodwater system; water is diverted from the Rio Grande into the Arroyo Colorado during major flood events. 

The watershed is 1,828 square kilometers of flat coastal plain that slope gently toward the Gulf of Mexico. The Laguna Atascosa National Wildlife Refuge and several county and city parks are within the Arroyo watershed. The mild climate, semi-tropical plants and animals, and many recreational opportunities draw large numbers of people to the area. 

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DSHS first published an advisory in 1993, recommending that consumers limit the amount of fish they ate from the Arroyo Colorado because of legacy pollutants and PCBs in fish tissues. In December 2005 and January 2006, DSHS extended the area of risk to include the Donna Canal and Reservoir system.

In January 2008, DSHS modified the Arroyo Colorado advisory, suggesting that consumers further limit the amount of longnose gar (due to DDE, mercury and PCBs) and smallmouth buffalo (due to DDE and PCBs) they ate from the stream. 

As of December 2015, in its most recent advisory, DSHS found that levels of PCBs and mercury in some fish species still posed an unacceptable risk to consumers. DSHS also noted in that advisory that pesticide concentrations had decreased to acceptable levels.

In March 2008, at recommendation of TCEQ, EPA added the Donna Canal and Reservoir system to its Superfund Program National Priorities List. In May 2008, the EPA began investigating the source of PCBs in the canal and reservoir system, during which they captured and removed fish in August 2008, February 2009, August 2009, October 2012, and June 2017.

In October 2018, the EPA issued its Record of Decision, which specified remedies that included removing contaminated sediment and fish and building new infrastructure at the source of the contamination, among other measures. These actions should address risks to people’s health associated with eating fish from the reservoir and canal system.

Check EPA’s progress on remediating pollution in the watershed on their webpage Superfund Site: Donna Reservoir and Canal System .

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Public Participation

In all its projects, TCEQ gathers opinions and information from people who represent government, permitted facilities, agriculture, business, environmental groups, and community and private interests in the watershed. Two committees advised TCEQ about the TMDL project. The Watershed Steering Committee ensured local involvement. The Science and Technical Advisory Committee provided technical guidance.

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TMDLs and I-Plan

The Commission adopted these TMDLs on January 17, 2001. EPA approved them on June 14, 2001 , at which time they became part of the state’s Water Quality Management Plan.

The Commission adopted these additional TMDLs on July 25, 2003 . EPA approved them on May 13, 2004, at which time they became part of the state’s Water Quality Management Plan.

The Commission approved the I-Plan on September 14, 2001.

For More Information

Please email and mention the Arroyo Colorado in the subject line, or call us at 512-239-6278.

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