Nitrogen dynamics within the Big Sunflower River Basin in northwestern Mississippi

Author(s): Barlow, J.; Coupe, R.; Kröger, R.

Two important water issues in northwestern Mississippi are: (1) the export of nutrients to the Mississippi River and eventually to the Gulf of Mexico, and (2) the availability of water for irrigation and to sustain baseflow in streams. Recently, the Yazoo River Basin in Mississippi was identified as a significant contributor of total nitrogen and phosphorus to the Gulf of Mexico. The Big Sunflower River Basin, located within the Yazoo River Basin, receives large annual inputs of nitrogen from agriculture, atmospheric deposition, and point sources. Recent publications indicate that nitrate, once it enters the surface waters of the Big Sunflower River Basin, acts conservatively and does not undergo significant losses. Stream flow in the Big Sunflower River has been substantially altered by loss of base flow due to declining water levels in the underlying alluvial aquifer. Therefore, instead of being a predominantly gaining stream, the Big Sunflower River is now a predominantly losing stream allowing for surface-water to move through the streambed. Nitrate transported with surface water through the streambed, with its generally low oxygen environment, is susceptible to denitrification, thereby removing nitrate from the system. Over the past 2 years, the USGS, in cooperation with the USACE, has conducted a study to determine the relative roles of in-stream processing and groundwater/surface-water exchange on the transport and fate of nitrate in the Big Sunflower River. Preliminary results from this study indicate that the transport of nitrate in surface water moving through the streambed is removed by denitrification, whereas the in-stream transport of nitrate is relatively conservative.

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