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TECHNICAL REPORTS

Surface Water Quality

Isotopic Evidence of Nitrate Sources and Denitrification in the Mississippi River, Illinois

^a Illinois State Geological Survey, Natural Resources Building, 615 E. Peabody Street, Champaign, IL 61820
^b Illinois State Water Survey, 2204 Griffith Drive, Champaign, IL 61820-7495

^* Corresponding author (panno{at}isgs.uiuc.edu)

Received for publication January 14, 2005. Anthropogenic nitrate (NO₃^–) within the Mississippi–Atchafalaya River basin and discharge to the Gulf of Mexico has been linked to serious environmental problems. The sources of this NO₃^– have been estimated by others using mass balance methods; however, there is considerable uncertainty in these estimates. Part of the uncertainty is the degree of denitrification that the NO₃^– has undergone. The isotopic composition of NO₃^– in the Mississippi River adjacent to Illinois and tile drain (subsurface drain) discharge in agricultural areas of east-central Illinois was examined using N and O isotopes to help identify the major sources of NO₃^– and assess the degree of denitrification in the samples. The isotopic evidence suggests that most of the NO₃^– in the river is primarily derived from synthetic fertilizers and soil organic N, which is consistent with published estimates of N inputs to the Mississippi River. The 1:2 relationship between ¹⁸O and ¹⁵N also indicate that, depending on sample location and season, NO₃^– in the river and tile drains has undergone significant denitrification, ranging from about 0 to 55%. The majority of the denitrification appears to have occurred before discharge into the Mississippi River.

Abbreviations: ¹⁵N, isotopic composition of nitrogen • ¹⁸O, isotopic composition of oxygen • , isotope enrichment factor • MARB, Mississippi–Atchafalaya River Basin • OM, organic matter

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