SOURCES OF NITRATE IN THE MISSISSIPPI RIVER ADJACENT TO ILLINOIS

Nitrate (NO₃) contamination in surface water and groundwater, especially as it relates to land use, is a critical issue in the Midwestern United States. NO₃ and other nutrients that enter surface streams via springs, seeps, and runoff in the Midwestern US, discharge into the Mississippi River. Rabalais et al. (1996) have shown that in the vicinity of the Mississippi Delta, nutrient-rich water from the Mississippi River may contribute to a 15,500 km2 zone of hypoxia in the Gulf of Mexico, resulting in the death of marine life. NO₃ concentrations in the Mississippi River have been increasing since the 1960s, and many investigators have suggested that the dominant source of this additional NO₃ is from application of artificial fertilizers in the corn belt states of the Midwest. However, other investigators have suggested other sources of NO₃, such as natural soil organic matter, sewage, livestock manure, and atmospheric deposition, may also significantly contribute to the NO₃ load.

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.