Illinois River Nitrate‐Nitrogen Concentrations and Loads: Long‐term Variation and Association with Watershed Nitrogen Inputs

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. Core Ideas Downward trend in annual flow‐weighted nitrate‐N concentrations since 1990. Nitrate‐N concentrations correlated with residual agricultural N during the previous 6 yr. Nitrate‐N concentrations correlated with current year Chicago wastewater discharge. Nitrate‐N load highly variable due to variation in precipitation and water discharge. Average nitrate‐N load during 2010–2014 was 10% less than the 1980–1996 baseline period.