Movement of Agricultural Chemicals and Sediment Through the Lower Mississippi River Basin During the 2011 Flood, April through July

Author(s): Welch, H.; Aulenbach, B.; Coupe, R.

Extreme hydrologic events, such as floods, can overwhelm a surface water system's ability to process agricultural chemicals (nutrients and pesticides) and can move large amounts of sediment downstream to larger surface water bodies. The Mississippi-Atchafalaya River basin drains approximately 41% of the conterminous United States and is the largest contributor of nutrients to the hypoxic zone that develops along the inner continental shelf of the Gulf of Mexico each spring. From March through April 2011, the lower Mississippi River basin received more than five times more precipitation than normal, which combined with snow melt from the upper Mississippi River basin, created a historic flood event that lasted from April through July. The U.S. Geological Survey, as part of the National Stream Quality Accounting Network (NASQAN), collected samples from six sites located in the lower Mississippi-Atchafalaya River basin, as well as, samples from the three flow-diversion structures: the Birds Point-New Madrid in Missouri and the Morganza and Bonnet Carré in Louisiana, from April through July. Samples were analyzed for nutrients, pesticides, suspended sediments, and particle size; results were used to determine the water quality of the river during the 2011 flood. Monthly loads for nitrate, phosphorus, pesticides (atrazine, glyphosate, fluometuron, and metolachlor), and sediment were calculated to quantify the movement of agricultural chemicals and sediment into the Gulf of Mexico. Nutrient loads were compared to historic loads to assess the effect of the flood on the zone of hypoxia that formed in the Gulf of Mexico during the spring of 2011.

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Technical Sessions

1 Best Management Practices #1
2 Delta Water Assessment
3 Flood Assessment & Mgmt.
4 Wetlands
5 Watershed Mgmt. #1
6 Non-Point Source Assessment
7 Modeling
8 Water Quality
9 Best Mgt. Practices #2
10 Delta Water Conservation
11 Sedimentation
12 Storm Water
13 Watershed Mgt. #2
14 Public Water Systems
15 Surface Water Assessment & Evaluation

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