Denitrification in the Shallow Groundwater of a Tile-Drained Agricultural Watershed

Non-point source pollution of surface water by nitrogen is considered a major cause of hypoxia in coastal waters. Because Corn Belt watersheds have been identified as major sources of nitrogen in the Mississippi River basin, the fate and transport of nitrogen from Midwestern agricultural watersheds need additional research. Over a 3-year period, nitrogen was monitored in the surface water and groundwater of a tile-drained Corn Belt watershed in central Illinois. Using geochemistry, isotope geochemistry, microbiology, and groundwater modeling techniques, researchers found that nitrogen was transported past the tile drains and into shallow groundwater. Denitrification in shallow groundwater was estimated to be equivalent to 0.3 to 6.4% of the applied nitrogen or 9 to 27% of nitrogen exported via surface water. These estimates varied by water year and peaked during a year of normal precipitation after 2 years of below average precipitation. Analysis of data collected over 3 years of monitoring indicated that shallow groundwater in watersheds with fine-grained soils may be a significant nitrogen sink compared with the fate of nitrogen exported via surface water. These results were recently published in the Journal of Environmental Quality.