Collaborative Research: Controls on hyporheic nitrate retention - discriminating among transport, reaction-rate, and substrate limitation

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0409591GooseffThe transport and fate of nitrogen in stream networks is critical to understanding watershed exportsof nitrogen. Nitrate (NO3 - ) retention, and denitrification in particular, is known to be dependent upona variety of factors: geochemical conditions (chiefly RedOx potential), substrate (largely availabilityof NO3 - and dissolved organic carbon (DOC)), and transport of nitrate and DOC to favorablelocations for denitrification. We will examine the controls of nitrate retention in the hyporheic zonesof streams in Oregon and Wyoming: 4 forested stream reaches, 4 agricultural streams, and 4 urbanstreams. Each reach will be the subject of 15 NO3 - injections in partnership with the Lotic InterbiomeNitrogen eXperiment II (LINX II) project which is using nitrogen-15 ( 15 N) techniques to studyuptake and retention. Collaborating with LINX II allows synergistic and cost-effective examinationof stream and hyporheic nitrogen cycling. The objectives of our research are to examine the factorscontrolling nitrate retention and denitrification in hyporheic zones of small streams and to quantifythe fraction of nitrate retention in these streams due to hyporheic exchange. We will test thefollowing hypotheses: (1) hyporheic denitrification in headwater, forested streams will be lowbecause of substrate and rate limitations, yet biotic assimilation will be high, relative to agriculturaland urban streams, because of inorganic N-limitation; (2) hyporheic denitrification will be greatestin mid-network locations where surrounding land use is predominantly agricultural, however, totalloss of NO3 - will be transport-limited and biotic assimilation will be reduced because nitrogen is lesslimiting; and (3) potential rate of denitrification in the hyporheic zone will be high in the urbanstream reaches, but total nitrate retention in the hyporheic zone will be low because bothdenitrification and biotic assimilation will be severely transport-limited.Elevation surveys of the longitudinal profiles of the stream reaches will be used to quantify channelmorphology and provide a base for groundwater flow and reactive transport simulations. We havesuccessfully used this technique to model groundwater flow and calculate hyporheic residence timedistributions [e.g., Kasahara and Wondzell, 2003; Wondzell et al., 2003]. We will use tracer testsbased on new understanding of their use for measuring residence time distributions in the hyporheiczone [Gooseff et al., 2003; Haggerty et al., 2000; Haggerty et al., 2002].We will couple our work to the LINX II NSF project by installing a sampling well andpiezometer network in the hyporheic zone of 12 LINX II experimental reaches in Oregon andWyoming. The sampling wells will provide access to hyporheic water, from which we will obtainsamples for measurement of 15 NO3 - , 15 N2(g), and 15 N2O(g) as well as DO, DOC and other physical andgeochemical parameters, which will be used to quantify biotic assimilation, denitrification, andgroundwater flow in the hyporheic zone. We will have access to LINX II stream data on these andother (e.g., 15 NH4 + ) N-species. The work will build upon, and enhance the current LINX II project.The LINX II project will benefit from quantification of nitrogen cycling in the hyporheic zone of 12of its sites and will gain a method to refine models of nitrogen dynamics.Three of the experimental reaches are on OSU campus, which allows us to use Oak Creek as a hands-on, active-learning laboratory in 3 courses with a combined annual enrollment of more than 300students. Through these courses, undergraduate and graduate research projects, and the participationof undergraduates in our field work, a large number of students will gain field experience inhydrology at experience-appropriate levels. The USU research team will continue to interact withmiddle school and high school students through the Teton Science School, demonstrating fieldactivities and facilitating hands-on field experience for these students. Together, the OSU and USUresearch teams will develop a web site with our data and project-related science activities anddissemination of results for K-12 students and teachers, and will promote learning about surfacewater hydrology and water quality in local schools through the NSF-sponsored GLOBE program.