Nitrate-nitrogen patterns in engineered catchments in the upper Mississippi River basin

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 > 10 mg/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 13 mg/l over a two-year period and exceeded 10 mg/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 77 kg/ha per year. NO3-N concentrations and episodes > 10 mg/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.