Sources and Fate of Nitrate in the Illinois River Determined
The Illinois River is a main tributary of the upper Mississippi River and has one of the largest fluxes of nitrogen in the Mississippi River 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 Mississippi River were investigated using chemical and isotopic analyses. The major nitrate sources were assumed to be treated wastewater from the Chicago area primarily and synthetic fertilizer from the agriculturally dominated land between Chicago and the Mississippi. Samples were collected on a seasonal basis from 14 sites along the Illinois River and selected tributaries. Samples were also collected from treated wastewater plants, tile drains, and precipitation.
Total nitrogen concentrations in the river were as high as 11.7 mg of N/L near the Chicago area and generally decreased with distance from Chicago, as discharge from groundwater, tributaries, and tile drains added to the river. Nitrate concentrations decreased most during the summer, when flow rates were low, field tiles were usually dry, and tributaries showed little to no nitrate concentration.
The isotopic composition of the nitrate 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 was dominated by nitrate from fertilizer nitrogen and soil nitrogen sources. During late summer early fall, when flow rates were low, the isotopic composition indicated that the dominant nitrate input was from treated wastewater. Most of the data suggested that denitrification predominantly occurred in the groundwater prior to discharge 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, reflecting denitrification or fractionation due to significant biota uptake in the broad shallow reaches of Peoria Lake, a manmade lake in the central part of river system. This finding is significant because in stream denitrication has typically not been observed in larger river systems.