SUBSURFACE FATE OF NITRATE AS A FUNCTION OF DEPTH AND LANDSCAPE POSITION IN MISSOURI FLAT CREEK WATERSHED, USA

Increased nitrate concentrations in groundwater associated with the application of nitrogen fertilizers have led to inquiries concerning the fate of nitrate beneath agricultural fields. This study was conducted to identify the processes affecting the distribution of nitrate in the unsaturated and saturated zones beneath an agricultural field and to assess how each process is influenced by factors associated with slope position. Nested piezometers were installed at two slope positions at the study site in southeastern Washington, U.S.A. Unsaturated- and saturated-zone sediment cores were analyzed for water content, pH, total and soluble organic carbon, ammonium, nitrate, and denitrification potential. Waters from the piezometers showed decreasing nitrate concentrations with depth below the water table. Trends in measured parameters indicated depth intervals where the distribution of nitrate could be attributed either solely to transport or to a combination of transport and biological denitrification. Denitrification explained the distribution of nitrate in the root zone while transport explained the interval between the root zone and the water table. There was a higher potential for denitrification below the water table at the bottom slope than at the top slope. Factors associated with slope position, such as a shallow water table and impeding stratigraphic layers, may explain this higher potential. Regardless of slope position, comparing nitrous oxide and carbon dioxide production from nitrate- and carbon-amended or -unamended samples indicated that denitrifier populations present in high-potential zones arc nitrate-limited. Results from spherical microsite modelling suggest that anoxic conditions are possible in the bulk sediment despite the presence of oxygenated groundwaters beneath both slope positions. Advective-dispersive transport will continue to transport nitrate through the unsaturated and saturated zones. The data from this study suggest that there is greater potential for nitrate attenuation by denitrification beneath the bottom slope than the top slope. The data also show that large masses of nitrate reside in deep subsoil vadose zones. These regions must therefore be monitored to detect threats to future groundwater quality.