| Summary: | <p>Watersheds within northwestern Mississippi, a productive agricultural region referred to as the Delta, were recently identified as contributors of total nitrogen and phosphorus fluxes to the Gulf of Mexico. Water withdrawals for irrigation in the Delta have altered flow paths between surface-water and groundwater systems, allowing for more surface-water losses to the underlying alluvial aquifer. In order to understand how to manage nitrogen in a watershed, it is necessary to identify and quantify hydrologic flow paths and biogeochemical conditions along these flow paths, which ultimately combine to determine transport and fate.</p>
<p>In order to evaluate the extent and role of surface-water losses to the alluvial aquifer on the transport of nitrate, a two-dimensional groundwater/surface-water exchange model was developed for a site within the Delta. Results from this model determined that groundwater/surface-water exchange at the site occurred regularly and recharge was laterally extensive into the alluvial aquifer. Nitrate was consistently reported in surface-water samples (n= 52, median concentration = 39.8 micromol/L),
although never detected in samples collected from instream or near stream piezometers (n=46). Coupled model and water-quality results support the case for denitrification/ nitrate loss from surface water moving through an anoxic streambed.</p>
<p>At larger scale, recent results from two Spatially Referenced Regressions on Watershed attributes (SPARROW) models imply that nitrogen is transported relatively conservatively once it enters the main channel of the Big Sunflower River Basin, which contributes much of the water discharging from the Yazoo River Basin to the Mississippi River. Net loss of nitrogen was assessed by comparing total nitrogen data from Lagrangian sampling events to chloride, drainage area, and predicted total nitrogen flux results from the SPARROW models. Results indicated relatively conservative instream transport of nitrogen at the scale of the Big Sunflower River Basin; however, two potential nitrogen loss mechanisms were identified: (1) transport and transformation of nitrogen through the streambed, and (2) sequestration and transformation of nitrogen above the drainage control structures downstream of Anguilla.</p>
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