HR: 11:00h
AN: H52B-03    [Abstracts]
TI: Tracing the Importance of Atmospheric Nitrate Deposition in Watersheds with Triple Oxygen Isotopes
AU: * Showers, W J
EM: w_showers@ncsu.edu
AF: N Carolina State University, Dept MEAS Box 8208, Raleigh, NC 27695 United States
AB: Measuring the flux of atmospherically deposited nitrate in watersheds is important because increasing nitrogen loads in many rivers are degrading aquatic ecosystems. Atmospheric nitrate is enriched in $^{17}$O and $^{18}$O. These isotopes can be used to quantify the flux of atmospheric N through watersheds. In the Neuse River Basin, NC it is estimated that up to 50% of the externally supplied "new" nitrogen flux that enters the estuary is from atmospheric sources. The $^{15}$N / $^{18}$O relationship of nitrate in urban and agricultural watersheds indicates that atmospheric nitrogen may be an important part of the river nitrogen flux in these areas. This system is light limited by sediment turbidity, so phytoplankton uptake in the main stem is not important. The $^{15}$N / $^{18}$O relationship of nitrate in the river main stem indicates that denitrification also is not important. Yet the concentration of riverine nitrate deceases down basin. Dissolved phosphate and HD / $^{18}$O values of river and groundwater indicate that decreased nitrate concentrations in the lower basin result from significant deep groundwater inputs. $^{17}$O of rainfall varies from 10 to 25 per mil over the year, with the lightest values occurring in the winter months. Over a 7 year period, the flux of nitrate N is slightly higher than ammonium N, and organic N deposition rates are half the DIN deposition rates in wet deposition. Dry N deposition is one third the wet N deposition on an annual basis. $^{17}$O of nitrate in surface and ground waters varies between different watersheds with different land use. The heaviest values are found in forested watersheds indicating the importance of atmospheric deposition in these environments. Urban creeks have the greatest $^{17}$O nitrate variation, but the average $^{17}$O nitrate of urban, agricultural and main stem samples are below 1 per mil indicating that atmospheric nitrogen is not important in river nitrogen flux. Two hydrographic events were sampled on an hourly basis in an urban watershed over 3-5 day periods to see if discharge variations biased the discrete sample $^{17}$O results. Large changes were observed in the $^{15}$N, $^{18}$O, and $^{17}$O of nitrate as well as nitrate, ammonium and chloride concentrations. $^{17}$O of nitrate varied from 0.3 to 11 per mil in a 12 hour period. The flux of atmospheric N can be calculated using the rates of deposition of DIN, DON, wet and dry N deposition, and the $^{17}$O composition of rainfall nitrate. The greatest flux of atmospheric nitrogen occurs in this urban watershed at the end of the falling discharge hydrograph. On an event basis atmospheric N can account for over 20 % of the DIN flux. Discharge variations are important when calculating atmospheric N fluxes from $^{17}$O data, and the importance of atmospheric N deposition can be underestimated if these variations are not taken into account.
DE: 1860 Runoff and streamflow
DE: 1871 Surface water quality
DE: 1040 Isotopic composition/chemistry
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting