title: Illinois River Nitrate-Nitrogen Concentrations and Loads: Long-term Variation and Association with Watershed Nitrogen Inputs.
abstract: The Illinois River is a major contributor of nitrate-N to the Mississippi River and the Gulf of Mexico, where nitrate is a leading cause of summertime benthic hypoxia. Corn-soybean production on tile-drained land is a leading source of nitrate-N in this river system, in addition to municipal wastewater discharge. We calculated annual nitrate-N loads in the Illinois River at Valley City from 1976 to 2014 by linear interpolation. Although there was not a significant trend in annual loads during the entire study period, there was a significant downward trend in flow-weighted nitrate-N concentration after 1990 despite high concentrations in 2013 after the 2012 drought. Multivariate regression analysis revealed a statistically significant association between annual flow-weighted nitrate-N concentration and cumulative residual agricultural N inputs to the watershed during a 6-yr window. This suggests that declines in flow-weighted nitrate-N concentration may reflect increasing N use efficiency in agriculture and a depletion of legacy N stored in the watershed. The watershed appears to have transitioned from a state of stationarity in nitrate concentration to nonstationarity. The average annual nitrate-N load at Valley City from 2010 to 2014 was 10% less than the 1980-1996 average load, indicating recent progress toward Illinois' nutrient loss reduction milestone of 15% reduction by 2025 and ultimate target of 45% reduction.
authors.forename: Gregory F
authors.initials: GF
authors.lastname: McIsaac
authors.forename: Mark B
authors.initials: MB
authors.lastname: David
authors.forename: George Z
authors.initials: GZ
authors.lastname: Gertner
mesh_terms: D000383:Agriculture; D004784:Environmental Monitoring; D007087:Illinois; D008922:Mississippi; D009566:Nitrates / Q000032:analysis*; D009584:Nitrogen / Q000032:analysis*; D045483:Rivers; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D009566:Nitrates; D014874:Water Pollutants, Chemical; D009584:Nitrogen