SOURCES AND FATE OF NITRATE IN THE ILLINOIS RIVER

The Illinois River is a main tributary of the upper Mississippi River (MR) and has one of the highest fluxes of nitrogen in the MR Basin. The Illinois River watershed drains about 44% of Illinois which is dominated by row-crop agriculture. The sources and fate of nitrate (NO₃^-) in the Illinois River from Chicago to the MR were investigated using chemical and isotopic analyses. The main sources of NO₃^- were assumed to be from treated wastewater (TWW) primarily from the Chicago area and synthetic fertilizer from the agriculturally dominated land between Chicago and the MR. Water was collected from 14 sites along the Illinois River and selected tributaries on a seasonal basis. Water was also collected from wastewater treatment plants, an agricultural tile drain, and precipitation. Total nitrogen concentrations were as high as 11.7 mg-N/L near the Chicago area and had isotopic compositions indicative of TWW. The concentration of NO₃-N generally decreased in the river with distance from Chicago as discharge from groundwater, tributaries, and tile drains added to the river. The decrease in NO₃-N concentrations was greatest during the summer, when flow rates were low, field tiles were usually dry, and the tributaries had very low NO₃-N concentrations. The isotopic composition of the NO₃^- in the Illinois River showed variations seasonally and with the overall flow rate of the river, reflecting different dominant source inputs and denitrification trends. During the winter and early spring when flow rates were high, the isotopic composition indicated the NO₃^- was primarily from fertilizer-N/soil-N source inputs. During the late summer/early fall when flow rates were low, the isotopic composition of NO₃^- reflected significant proportions of TWW. Denitrification appears to predominantly occur in the groundwater prior to discharging into the Illinois River. However, during low flow conditions a positive shift in the isotopic composition between the upper and lower portions of the river was observed, suggesting possible in-stream denitrification or perhaps fractionation due to significant biota uptake in the broad shallow reaches of Peoria Lake, a 50 km long man-made lake created by a dam across the river south of Peoria.