Sources of nitrate in rivers draining sixteen watersheds in the northeastern US: Isotopic constraints

The feasibility of using nitrogen and oxygen isotope ratios of nitrate (NO3-) for elucidating sources and transformations of riverine nitrate was evaluated in a comparative study of 16 watersheds in the northeastern U.S.A. Stream water was sampled repeatedly at the outlets of the watersheds between January and December 1999 for determining concentrations, delta(15)N values, and delta(18)O values of riverine nitrate. In conjunction with information about land use and nitrogen fluxes, delta(15)N(nitrate) and delta(18)O(nitrate) values provided mainly information about sources of riverine nitrate. In predominantly forested watersheds, riverine nitrate had mean concentrations of less than 0.4 mg NO3- -N L-1, delta(15)N(nitrate) values of less than +5parts per thousand, and delta(18)O(nitrate) values between +12 and +19parts per thousand. This indicates that riverine nitrate was almost exclusively derived from soil nitrification processes with potentially minor nitrate contributions from atmospheric deposition in some catchments. In watersheds with significant agricultural and urban land use. concentrations of riverine nitrate were as high as 2.6 mg NO3--N L-1 with delta(15)N(nitrate) values between +5 and +8parts per thousand 3 and delta(18)O(nitrate) values generally below +15parts per thousand. Correlations between nitrate concentrations, delta(15)N(nitrate) values, and N fluxes suggest that nitrate in waste water constituted a major, and nitrate in manure a minor additional source of riverine nitrate. Atmospheric nitrate deposition or nitrate-containing fertilizers were not a significant source of riverine nitrate in watersheds with significant agricultural and urban land use. Although complementary studies indicate that in-stream denitrification was significant in all rivers, the isotopic composition of riverine nitrate sampled at the Outlet of the 16 watersheds did not provide evidence for denitrification in the form of elevated delta(15)N(nitrate) and delta(18)O(nitrate) values. Relatively low isotopic enrichment factors for nitrogen and oxygen during in-stream denitrification and continuous admixture of nitrate from the above-described sources are thought to be responsible for this finding