HR: 14:05h
AN: H52C-02    [PDF]
TI: Fate and Transport of Nitrate in an Alpine Catchment Based on Dual-Isotopic and End-Member Mixing Analysis
AU: * Williams, M W
EM: markw@snobear.colorado.edu
AF: Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309 United States
AU: * Williams, M W
EM: markw@snobear.colorado.edu
AF: Department of Geography, University of Colorado, Boulder, CO 80309 United States
AU: Liu, F
EM: fengjing@snobear.colorado.edu
AF: Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO 80309 United States
AU: Liu, F
EM: fengjing@snobear.colorado.edu
AF: Department of Geography, University of Colorado, Boulder, CO 80309 United States
AU: Kendall, C
EM: ckendall@usgs.gov
AF: US Geological Survey, 345 Middlefield Road MS434, Menlo Park, CA 94025 United States
AB: Attempts to understand nitrate loss from small watersheds have been the focus of much research. In general, these results have shown that understanding how sources and flowpaths of nitrate-rich waters change with time necessitates a combination of isotopic, chemical, and hydrometric data. New and evolving tools to understand the fate and transport of nitrate include dual-isotopic analysis of both the nitrogen and oxygen atoms in the nitrate molecule, and end-member mixing analysis (EMMA). However, to-date no one has combined these two approaches. Here we present results from both dual-isotope analysis and EMMA to understand the fate and transport of nitrate in the high-elevation Green Lakes watershed of the Colorado Front Range. Thirty-five samples were collected and analyzed for the dual isotopes of nitrate, and showed that values of $\delta^{18}$O in atmospheric deposition had considerable overlap with values in stream waters. There was a strong counterclockwise pattern of hysteresis when $\delta^{18}$O is plotted versus stream discharge, consistent with a mixture of atmospheric and microbial nitrate sources on the rising limb of the hydrograph, then mostly a microbial source on the recession limb of the hydrograph. A chemograph of nitrate sources developed using EMMA shows atmospheric deposition the predominant source at the initiation of snowmelt runoff, with soil water and talus becoming the dominant source at peak runoff, and talus-water and baseflow both important on the recession limb of the hydrograph. We were able to qualitatively evaluate the EMMA results by developing a 2-component nitrate source model using the dual-isotope values. The dual-isotope mixing model agreed well with the atmospheric and subsurface sources of nitrate identified using EMMA.
DE: 1806 Chemistry of fresh water
SC: Hydrology [H]
MN: 2003 Fall Meeting