Nitrate-nitrogen patterns in engineered catchments in the upper Mississippi River basin
2012
Schilling, Keith E. | Jones, Christopher S. | Seeman, Anthony | Bader, Eileen | Filipiak, Jennifer
Many extensive subsurface tile drainage networks in the Corn Belt region of the United States are organized into quasi-governmental drainage districts. Approximately 3000 of these engineered watersheds exist in the state of Iowa. Tile discharge is the source of many headwater streams and contributes to loss of nitrate-nitrogen (NO₃-N) from cultivated fields. Downstream water use by municipal suppliers is impaired by stream concentrations>10mg/l, and load reductions>30% may be necessary to mitigate Gulf of Mexico hypoxia. The objectives of this study were to evaluate NO₃-N concentrations and loads discharged from three typical drainage districts in north-central Iowa and explore the relation of drainage district NO₃-N concentrations to the downstream drainage network. NO₃-N concentrations averaged approximately 13mg/l over a two-year period and exceeded 10mg/l (the standard for safe drinking water in the U.S.) nearly 90% of the time. NO₃-N yields from the studied drainage districts ranged from 33 to 77kg/ha per year. NO₃-N concentrations and episodes>10mg/l were observed to decrease downstream in a linear manner with log drainage area. A load reduction of 55% would be needed at the tile discharge to meet downstream water quality objectives. In-stream NO₃-N processing was observed immediately downstream of the tile outlet, but would appear to offer little potential for meaningful downstream reductions because the time period for NO₃-N processing was poorly timed with seasonal loading patterns. Study results suggest that focusing on NO₃-N reductions at the drainage district scale using best management practices, such as in-field nitrogen management or edge of field treatment, with constructed wetlands, would achieve significant downstream reductions.
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