Whole-Stream Response to Nitrate Loading in Three Streams Draining Agricultural Landscapes

Authors

• John H. Duff, USGSFollow
• Anthony J. Tesoriero, USGS
• William B. Richardson, USGS
• Eric A. Strauss, University of Wisconsin–La Crosse
• Mark Munn, USGS

Document Type

Article

Date of this Version

2008

Comments

Published in J. Environ. Qual. 37:1133–1144 (2008).

Abstract

Physical, chemical, hydrologic, and biologic factors affecting nitrate (NO₃^-) removal were evaluated in three agricultural streams draining orchard/dairy and row crop settings. Using 3-d “snapshots” during biotically active periods, we estimated reachlevel NO₃^- sources, NO₃^- mass balance, in-stream processing (nitrification, denitrification, and NO₃^- uptake), and NO₃^- retention potential associated with surface water transport and ground water discharge. Ground water contributed 5 to 11% to stream discharge along the study reaches and 8 to 42% of gross NO₃^- input. Streambed processes potentially reduced 45 to 75% of ground water NO₃^- before discharge to surface water. In all streams, transient storage was of little importance for surface water NO₃^- retention. Estimated nitrification (1.6–4.4 mg N m⁻² h⁻¹) and unamended denitrification rates (2.0–16.3 mg N m⁻² h⁻¹) in sediment slurries were high relative to pristine streams. Denitrification of NO₃^- was largely independent of nitrification because both stream and ground water were sources of NO₃^-. Unamended denitrification rates extrapolated to the reach-scale accounted for <5% of NO₃^- exported from the reaches minimally reducing downstream loads. Nitrate retention as a percentage of gross NO₃^- inputs was >30% in an organic-poor, autotrophic stream with the lowest denitrification potentials and highest benthic chlorophyll a, photosynthesis/respiration ratio, pH, dissolved oxygen, and diurnal NO₃^- variation. Biotic processing potentially removed 75% of ground water NO₃^- at this site, suggesting an important role for photosynthetic assimilation of ground water NO₃^- relative to subsurface denitrification as water passed directly through benthic diatom beds.