USE OF NITROGEN STABLE ISOTOPES TO UNDERSTAND HYDROLOGIC AND GEOCHEMICAL PROCESSES IN SEMIARID VADOSE ZONES

We have been using nitrogen stable isotopes (d¹⁵N) to better understand nitrogen cycling and transport within the thick vadose zone of the Pajarito Plateau in north-central New Mexico.  The Plateau vadose zone has been impacted by releases of nitrate and other nitrogen compounds (e.g., high explosives) from Los Alamos National Laboratory.  We are using nitrogen isotopes too address three common vadose zone problems; 1) as a method for source attribution, 2) as a way of monitoring nitrate degradation, and 3) as a way of understanding flow and transport in a complex canyon vadose zone.  Our presentation discusses examples related to the problems listed above and these examples help illustrate the dynamics of nitrogen cycling in the vadose zone and nitrogen transport.  With regard to source attribution, we have been using a simple leaching procedure for recovering nitrate from unsaturated core samples, which then can be analyzed for nitrogen isotopic composition.  Our preliminary results suggest that this method is sensitive enough that it can be used to differentiate between natural vadose zone nitrate and historical sanitary waste sources.  We have also examined nitrogen isotopes in waters and plants from a nitrate and high explosives contaminated spring, stream, and alluvial aquifer system to look for evidence of degradation.  Our data to date show a great deal of temporal and spatial variability in nitrogen isotope composition consistent with multiple nitrogen sources and the variable redox conditions in the stream/alluvial environment.  Relatively high isotopic compositions from grasses suggest that some nitrate and/or high explosives degradation may be occurring.  Finally, we have utilized the unsaturated core leaching technique to understand flow processes in a canyon system contaminated with nitrate, perchlorate, and radionuclides.  During the 1980’s, a nitrate source was released into the canyon that had an extremely negative isotopic composition.  For example, some of our core samples have d¹⁵N values less than –130 permil.  By analyzing nitrate isotopes in boreholes up and down the canyon, we have been able to use the unique isotopic composition of the 1980’s release as a tracer to better define the importance of alluvial aquifer leakage and the residence time of water in the canyon vadose zone.