, Volume 57-58, Issue 1, pp 171-197

Sources of nitrate in rivers draining sixteen watersheds in the northeastern U.S.: Isotopic constraints

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The feasibility of using nitrogen and oxygenisotope ratios of nitrate (NO3 ) forelucidating sources and transformations ofriverine nitrate was evaluated in a comparativestudy of 16 watersheds in the northeastern U.S.A. Stream water was sampled repeatedly at theoutlets of the watersheds between January andDecember 1999 for determining concentrations,δ15N values, and δ18Ovalues of riverine nitrate.

In conjunction with information about land useand nitrogen fluxes,δ15Nnitrate andδ18Onitrate values providedmainly information about sources of riverinenitrate. In predominantly forested watersheds,riverine nitrate had mean concentrations ofless than 0.4 mg NO3 -N L−115Nnitrate values of lessthan +5‰, and δ18Onitratevalues between +12 and +19‰. This indicatesthat riverine nitrate was almost exclusivelyderived from soil nitrification processes withpotentially minor nitrate contributions fromatmospheric deposition in some catchments. Inwatersheds with significant agricultural andurban land use, concentrations of riverinenitrate were as high as 2.6 mg NO3 -NL−1 with δ15Nnitratevalues between +5 and +8‰ andδ18Onitrate values generallybelow +15‰. Correlations between nitrateconcentrations, δ15Nnitratevalues, and N fluxes suggest that nitrate inwaste water constituted a major, and nitrate inmanure a minor additional source of riverinenitrate. Atmospheric nitrate deposition ornitrate-containing fertilizers were not asignificant source of riverine nitrate inwatersheds with significant agricultural andurban land use. Although complementary studiesindicate that in-stream denitrification wassignificant in all rivers, the isotopiccomposition of riverine nitrate sampled at theoutlet of the 16 watersheds did not provideevidence for denitrification in the form ofelevated δ15Nnitrate andδ18Onitrate values. Relativelylow isotopic enrichment factors for nitrogenand oxygen during in-stream denitrification andcontinuous admixture of nitrate from theabove-described sources are thought to beresponsible for this finding.