HR: 14:35h
AN: H52C-04    [PDF]
TI: What can $\Delta^{15}$N and $\Delta^{18}$O isotopes tell us about sources, transport, and fate of nitrate in the Mississippi River Basin?
AU: * Battaglin, W A
EM: wbattagl@usgs.gov
AF: U.S. Geological Survey, Box 25046 MS 415 Denver Federal Center, Denver, CO 80225
AB: Water and nutrients, primarily nitrate (NO$_{3}$) in Mississippi River discharge, affect the size and severity of the Gulf of Mexico hypoxic (depleted dissolved oxygen) zone. Approximately 120 water samples were collected from 16 sites on small streams and 6 sites on large rivers within the Mississippi River Basin in 1997-98 to see if NO$_{3}$ sources and transformations can be identified using the stable isotopic ratios $\Delta^{15}$N and $\Delta^{18}$O. Results from Lagrangian sampling at the large river sites indicate that nitrate mass decreases slightly, while $\Delta^{15}$N and $\Delta^{18}$O isotope ratios are unchanged in the 1500 river kilometers between the Upper Mississippi-Ohio River confluence and the Gulf of Mexico. Results also show that $\Delta^{15}$N and $\Delta^{18}$O values from small streams draining lands dominated by row crops or livestock tended to be distinct from those dominated by urban or undeveloped land. Mean $\Delta^{15}$N values at the 16 sites on small streams were most strongly correlated (Pearson's r) with manure production rate (0.64), percent residential land use (-0.45), and urea use rate (0.43). The best multiple linear regression (MLR) model for mean $\Delta^{15}$N values (r2=0.69) used manure production rate and ammonium nitrate use rate as explanatory variables. Mean $\Delta^{18}$O values were most strongly correlated with percent wetlands (0.72), mean NO$_{3}$ concentration (-0.71), and percent residential land use (0.58). The best MLR model for mean $\Delta^{18}$O values (r$^{2}$=0.85) used percent residential land use, percent wetlands, and ammonium nitrate use rate as explanatory variables. Mean NO$_{3}$ concentrations were most strongly correlated with percent row-crops land use (0.84), nitrogen-fertilizer use rate (0.74), and hog-manure production rate (0.66). The best MLR model for mean NO$_{3}$ concentration (r$^{2}$=0.85) used percent row-crops land use and percent grain-crops land use as explanatory variables. MLR equations developed from the 16 smaller streams were used to predict mean $\Delta^{15}$N and $\Delta^{18}$O values and NO$_{3}$ concentrations at the 6 large river sites. The predicted mean $\Delta^{15}$N and $\Delta^{18}$O values tended to be less (lighter) than the measured values, and the predicted mean NO$_{3}$ concentrations were greater than the measured values. These results indicate that a significant portion of the nitrate that leaves relatively small Mississippi Basin watersheds is denitrified or assimilated during transport to the Mississippi River and its major tributaries.
DE: 1803 Anthropogenic effects
DE: 1871 Surface water quality
SC: Hydrology [H]
MN: 2003 Fall Meeting