HR: 14:30h
AN: NB23E-05    [Abstracts]
TI: Comparison of Whole-stream and Hyporheic-zone Estimates of Denitrification Determined Simultaneously During an Isotope Tracer Injection in a Nitrate-Rich Stream
AU: * Harvey, J W
EM: jwharvey@usgs.gov
AF: U.S. Geological Survey, 430 National Center, Reston, VA 20192 United States
AU: Bohlke, J K
EM: jkbohlke@usgs.gov
AF: U.S. Geological Survey, 430 National Center, Reston, VA 20192 United States
AU: Voytek, M A
EM: mavoytek@usgs.gov
AF: U.S. Geological Survey, 430 National Center, Reston, VA 20192 United States
AB: ¹⁵N labeled nitrate is increasingly being used as a reactive tracer in stream tracer tests to estimate whole-stream denitrification averaged at a spatial scale large enough to allow comparisons across disparate stream ecosystems. No matter how valuable, these whole-stream estimates are not very informative about controlling processes and will have limited transfer value unless processes controlling denitrification are investigated simultaneously at finer scales. Insights about the processes that influence the whole-stream rates could be especially informative if simultaneous rate measurements are made representing variable hydrologic and biogeochemical conditions near reactive surfaces in the stream and in the streambed. Our approach was to investigate factors that control denitrification by simultaneously measuring denitrification in-situ in a variety of streambed environments by sampling evolution of the (¹⁵NO₃^-) tracer during transport through shallow hyporheic flow paths. Here we report results from two tracer studies conducted in Sugar Creek, western Indiana, in a basin dominated by corn and soybean agriculture. The two tracer experiments were conducted in September 2001 and September 2003, when streamflows (40 and 20 L s^-1) and stream NO₃^- concentrations (70 and 175 μmoles L^-1) in the two reaches were near their annual minimum values. The experiments involved co-injection of conservative (Br), reactive (¹⁵NO₃^-), and dissolved gas (SF6) tracers into streamflow allowing quantification of advection, dispersion, gas evasion, hydrologic retention in "storage" zones, and also allowing in-situ estimation of denitrification within selected hyporheic flow paths. The experiments resulted in estimates of both whole-stream and hyporheic-zone rates of denitrification and related nitrogen reactions. The streambed of Sugar Creek is covered in most areas with a relatively thin layer (ranging from <1 to 3 cm) of fine granular and organic sediment and periphyton, overlying a thicker layer (30 to 100 cm) of relatively coarse granular sediment with a median grain size of 2 mm. In-situ sampling in the streambed was conducted simultaneously with the tracer injection using the USGS MINIPOINT sampler to withdraw small-volume water samples from depths ranging between 1.5 and 15 cm in the hyporheic zone at 6 locations along the two stream reaches. A total of 22 in-situ estimates of denitrification were derived from those samples. Zero-order denitrification rates in the hyporheic zone ranged broadly across two orders of magnitude (2 to 230 μmoles L^-1 h^-1) with a trend of decreasing rates with increasing depth below the streambed. The hyporheic-zone dentrification rates (54 to 430 μmoles m^-2 h^-1) bracketed the reach-scale rates (120 to 305 μmoles m^-2 h^-1), demonstrating that denitrification in hyporheic flow paths significantly contributed to whole-stream denitrification in this nitrate-rich Midwestern stream.
UR: http://water.usgs.gov/nrp/jharvey/site/index.html
DE: 0400 Biogeosciences
DE: 1871 Surface water quality
DE: 4845 Nutrients and nutrient cycling
SC: North American Benthological Society [NB]
MN: 2005 Joint Assembly