Sources and fate of nitrate in the Illinois River Basin, Illinois

We conducted a two-year investigation into the sources and fate of nitrate (image) in the Illinois River from the Chicago area to the river’s confluence with the Mississippi River. Samples from waterways in the Chicago area (Des Plaines River and the Sanitary and Ship Canal) had relatively high concentrations of nitrogen (N) species and image isotopic compositions indicative of treated wastewater (TWW). Downstream of the Brandon Road Lock and Dam, image in tributaries discharging to the Illinois River primarily comes from tiles draining row crops. Nitrate isotopic signatures from these tributaries as well as drain tiles were indicative of synthetic fertilizer and/or soil organic matter (SOM) at various stages of denitrification. Nitrate–N concentrations generally decreased in the Illinois River with distance from the Chicago area primarily due to dilution. The decrease in NO3–N concentrations was especially conspicuous during the summer, when there is minimal discharge from drain tiles and NO3–N concentrations in the tributaries were low. In August 2005, when conditions were very dry, NO3–N concentrations decreased from 7.4 mg/L in the Chicago area to less than 1 mg/L near where the Illinois River discharges to the Mississippi River.