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Barataria-Terrebonne Basin Herbicide and Nutrient Study - Acadian-Pontchartrain NAWQA
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Barataria-Terrebonne Basin Herbicide and Nutrient Study

A study of nutrient and atrazine flows and levels in a drinking-water source and coastal freshwater marshes

Study Description

The Barataria and Terrebonne basins are experiencing severe land losses due to the degradation or death of freshwater wetlands and their subsequent conversion to open water. Theories of wetland loss in the area include subsidence due to faulting or compaction, erosion, drought, saltwater intrusion, nutrient imbalances, and damage to plants from herbicide runoff from upstream agricultural areas. Routing of water through this hydrologically complex area is not well understood, contributing to the lack of a clear cause of wetlands loss. Diversion of Mississippi River water as a method of slowing and possibly reversing wetlands loss is used with success in some areas of coastal Louisiana, notably the Violet Canal project which replenishes sediment in marshes of St. Bernard Parish. This appears to not be the case in the Penchant basin (part of the Terrebonne Basin), where floating marsh (also known as flotant) is disappearing in spite of water and sediment inputs from the Mississippi via the Atchafalaya.

Additionally, the upper part of the Terrebonne basin, known as the Verret Basin, includes bayous that are the sole drinking water supply for over 250,000 people. The Louisiana Department of Environmental Quality (LDEQ) has identified the Grosse Tete/Verret basin as an area of concern due to elevated atrazine levels. Sugarcane and corn are the major crops grown throughout this basin, and these two monocotyledon crops are extensively treated with atrazine and related herbicides (triazines). Atrazine is a broad-spectrum triazine herbicide with both pre- and post-emergence activity.

The objectives of our March-August 1999 herbicide surveys of the Barataria-Terrebonne system were to determine the occurrence, distribution, and transport of triazines during different seasons. Triazine herbicide analyses included both gas chromatograph and enzyme-linked immunoassay techniques. The surveys have been conducted in conjunction and consultation with the Louisiana Department of Agriculture and Forestry's Pesticide and Environmental Program and the Louisiana Department of Environmental Quality's Atrazine Sampling Program in the Bayou Grosse Tete area. Preliminary results show the presence of interbasin flow from the Mississippi and Atchafalaya Rivers into the Gulf Intracoastal Waterway (GIWW). The GIWW then functions as an east-west conveyance channel. Additionally, local inputs are greater than previously documented, particularly earlier than the Mississippi River atrazine peak.


Sites were selected to capture the complex routing of water through the Verret Basin, and on out to the Penchant area. Bayou Boeuf at Amelia is a monthly-sampled fixed site.

Small graphic of Verret Basin nutrient and pesticide site locations.
Click on the image to view a larger version in GIF format.
You may also view a large PNG version of the above graphic.

The following sites have been sampled in March, May, June, and August of 1999:
Site NameStation ID
Bayou Chevreuil near Chackbay at LA-Hwy 2007380270
Bayou Lafourche below weir at Thibodaux07381002
Bayou Grosse Tete at Rosedale at LA-Hwy 7607381440
Bayou Boeuf at Amelia at US-90073814675
Bluebird Canal near Bayou Copesaw293305090591200
Turtle Bayou near Bayou Penchant093524091041300
Salvador Wildlife Management Area interior canal294821090185200
Keyhole 6 Canal south of Westwego295031090082900
Grassy Lake at Simon Pass294605091065000
Cancienne Canal at LA-Hwy 400295215091045000
Belle River at LA-Hwy 70295435091123000
Bayou Pierre Part at LA-Hwy 70295800091125000
Gulf Intracoastal Waterway at Jack Miller Landing301315091190000
Choctaw Bayou near Brusly302510091203200
Bayou Grosse Tete at Frisco at LA-Hwy 411303520091314500

Methods and Analysis

Surface water was collected every four to six weeks. Constituents assayed at these sites include major ions, nutrients, dissolved and organic carbon, chlorophyll, and a variety of soluble and less-soluble organic pesticides and compounds. Methods for collecting and processing surface water are described in the "Field Guide for Collecting and Processing Stream-Water Samples for the National Water-Quality Assessment Program, Open-File Report 94-455".

Constituents analyzed:

  • Dissolved Major Ions
  • Dissolved Nutrients
  • Phytoplankton, Chlorophyll a and b
  • Dissolved and Suspended Organic Carbon
  • Pesticides

For questions about our Barataria-Terrebonne Herbicide and Nutrient Study, please contact Dennis Demcheck [ddemchec@usgs.gov], or Chris Swarzenski [cswarzen@usgs.gov], 225-298-5481. Floating marsh sites are located in various parts of the Jean Lafitte National Historical Park and Salvador Wildlife Management Area, and parts of the Penchant Basin. Contact Chris Swarzenski for information on these sites.

U.S. Department of the Interior, U.S. Geological Survey, Baton Rouge, Louisiana, USA
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Page created and maintained by Patricia D'Arconte.
URL http://la.water.usgs.gov/nawqa/atrazine.htm
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