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Llano Grande Lake bottom sediments; a chronicle of water-quality changes in the Arroyo Colorado, South Texas, 1989-2001

October 1, 2002

The Arroyo Colorado, an ancient channel of the Rio Grande, extends 90 miles from Mission, Tex., to the Laguna Madre. The Arroyo Colorado flows through areas of intense agricultural cultivation and through important habitat for migrating birds and other wildlife, including several wildlife sanctuaries and refuges. The above-tidal segment of the Arroyo Colorado is included in the State of Texas 2000 Clean Water Act 303(d)1 list in part because of elevated concentrations of the hydrophobic legacy pollutants DDE (a DDT breakdown product), chlordane, and toxaphene in fish tissue. This report addresses three questions:

  • Do legacy pollutants (organochlorine compounds, major and trace elements) occur in the Arroyo Colorado at present and at what concentrations?
  • How has the occurrence of selected legacy pollutants in the Arroyo Colorado changed over time?
  • Are current concentrations of legacy pollutants in bottom sediments at levels of concern for the health of aquatic biota?

To answer these questions, the U.S. Geological Survey (USGS), in cooperation with the U.S. Environmental Protection Agency (USEPA), collected and analyzed a sediment core from Llano Grande Lake on the Arroyo Colorado (fig. 1). Sediment cores can be used to reconstruct historical trends in concentrations of hydrophobic contaminants (Eisenreich and others, 1989; Van Metre and others, 1997, 2000). The lake is part of the Rio Grande delta drainage system (fig. 1). The lake is 6 miles long and has a maximum width of 600 feet.