SOURCES OF NITRATE IN THE MISSISSIPPI RIVER ADJACENT TO ILLINOIS
We used isotopic analytical techniques in an attempt to identify the sources of NO3 in the Mississippi River. Water samples were collected seasonally from the river at the intakes of 3 municipal water companies adjacent to Illinois and analyzed for NO3- and its isotopes (ä15N - ä18O). Additional samples were collected seasonally from drainage tiles located beneath cropped land treated with artificial fertilizer in central Illinois. All of the data lie within a narrow band along a denitrification vector on a ä15N - ä18O scatter plot that originates from the domains of both artificial fertilizer and soil organic matter. River and tile water samples overlapped each other on the plot, suggesting that agricultural-N from cultivated fields may be the dominant source of NO3- in the Mississippi River during all seasons. The highest and lowest concentrations of NO3 occurred in the spring and summer, respectively. The isotopic values of NO3 were lowest in the spring (i.e., least denitrified). There is a significant negative correlation between NO3 concentrations and ä15N values due to denitrification. Most of the denitrification probably occurs in the soil zone and smaller tributaries prior to dicharging into the Mississippi River.