Water flux and nitrogen cycling in the hyporheic zones of a semi-arid watershed: Hydrologic and geomorphic driving forces in a transitional climate

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0450317SiegelHyporheic exchange, the temporary storage of surface water in stream bank sediments, affects the transport of solutes, including nutrients, through watersheds. Water diverted into the hyporheic zone has a longer residence time and more interaction with biogeochemically active sediments than water in other flow paths. Hyporheic flux rates and geochemistry have not been studied in semi-arid intermountain watersheds, transitional in climate between alpine catchments and desert lowlands. Hyporheic exchange in watersheds in humid regions is enhanced by stream meanders, variable flow rates, and sediment hydraulic conductivity. In water poor regions other geomorphic characteristics, particularly beaver dams, may equally influence hyporheic exchange. We propose to complete an intensive study of hyporheic interaction and nitrogen uptake potential in an intermountain semi-arid watershed with significant beaver activity. The study will include field experiments and numerical hydrologic models designed to 1.) identify and quantify hyporheic pathways and fluxes of water and dissolved solutes across the surface-groundwater interface and 2.) identify the causes of hyporheic flux variability. We will test the hypothesis that in-stream flow obstructions, particularly small beaver dams, particularly enhance the extent of the hyporheic zone along semi-arid streams compared to the effects of other geomorphic and hydrologic controls. We will test our hypothesis by inversely modeling a series of in-stream tracer tests to characterize hyporheic storage parameters along various reaches of Red Canyon Creek, Wyoming. We will use Darcy flux calculations and observations of tracers at near-stream wells to independently test the results of the in-stream tracer tests. We will test the hypothesis that reaches with a greater degree of hyporheic interaction have a greater potential for uptake of nitrate. Dissolved nitrate will be paired with the conservative tracers during in-stream tracer tests. Using the rate of decline of nitrate concentrations downstream, measurements of ammonia and other nitrogen species, and an exponential nutrient uptake model, we will quantify nitrogen uptake lengths along Red Canyon Creek and compare these results with hydrologic measures of hyporheic exchange. Furthermore, we will do microbial investigations incorporating molecular techniques and analysis of the geochemistry of near-stream waters to independently evaluate the degree to which denitrification and nitrogen uptake drives nitrogen processing in the hyporheic zone. Results of in-stream tracer tests are empirical in nature, and therefore difficult to generalize to other sites. For this reason, we will use three-dimensional numerical computer models of near-stream flow paths to evaluate the physical processes controlling the aerial extent and flux of water through hyporheic zones. The results of these models will be combined with detailed measurements of stream geomorphological and sedimentological characteristics in a series of multivariate geostatistical approaches to explore what stream characteristics most affect hyporheic exchange in the study setting and in similar order streams elsewhere. This project will build on the ongoing collaborative relationships between Syracuse University, the University of Missouri's Branson geology field camp, and the Wyoming Nature Conservancy. Previous collaborations between these institutions have resulted in hydrologic instrumentation of part of the Red Canyon Creek watershed for student research and education in fields from glacial history to ecology. We will build on this previous work, enhancing the site with instrumentation that will provide research grade hydrologic data at a watershed scale. Through this project, students from across the Nation studying at the Branson geology field camp will have unprecedented access to a broader range of hydrogeological field experiments at a wider variety of sites within the Red Canyon Creek watershed. In addition, students from Central Wyoming College, a 2-year community college with a large Native American enrollment, will be recruited to participate directly as summer research assistants for the project. These collaborations, undergraduate through graduate in level, will potentially have broad educational impact at both the local and larger scale.