Isotopic and chemical composition of inorganic and organic water-quality samples from the Mississippi River Basin, 1997-98

Nitrate (NO₃) and other nutrients discharged by the Mississippi River combined with seasonal stratification of the water column are known to cause a zone of depleted dissolved oxygen (hypoxic zone) in the Gulf of Mexico each summer. About 120 water and suspended sediment samples collected in 1997 and 1998 from 24 locations in the Mississippi River Basin were analyzed for the isotope ratios δ¹⁵N and δ¹⁸O of dissolved NO₃, and δ¹⁵N and δ¹³C of suspended particulate organic material (POM). Sampling stations include both large rivers (drainage areas more than 30,000 square kilometers) that integrate the effects of many land uses, and smaller streams (drainage areas less than 2,500 square kilometers) that have relatively uniform land use within their drainage areas. The data are used to determine sources and transformations of NO₃ in the Mississippi River.

Results of this study demonstrate that much of the NO₃ in the Mississippi River originates in the agriculturally dominated basins of the upper midwestern United States and is transported without significant transformation or other loss to the Gulf of Mexico. Results from major tributaries that drain into the Mississippi River suggest that NO₃ is not significantly altered by denitrification in its journey, ultimately, to the Gulf of Mexico. The spatial variability of isotope ratios among the smaller streams appears to be related to the dominant nitrogen source in the basins. There are some distinct isotope differences among land-use types. For example, for both NO₃ and POM, the majority of δ¹⁵N isotope ratio values from basins dominated by urban and undeveloped land are less than +5 per mil, whereas the majority of values from basins dominated by row crops and row crops and/or livestock production are greater than +5 per mil. Also, the median δ¹⁸O of NO₃ isotope ratio value (+14.0 per mil) from undeveloped basins is more than 6 per mil higher than the median value (+7.3 per mil) from the row crop dominated basins and 5 per mil higher than the median value (+9.0 per mil) from the row crop and/or livestock production dominated basins. The median δ¹⁸O of NO₃ isotope ratio value (+21.5 per mil) from urban basins is 6.5 per mil higher than the median value (+14.0 per mil) from the undeveloped basins. The majority of NO₃ concentrations are greater than 3 milligrams per liter (mg/L) in basins dominated by row crops and row crops and/or livestock production, whereas all NO₃ concentrations are less than 2 mg/L in basins dominated by urban and undeveloped land.