NITROGEN TRANSPORT AND CYCLING IN THE INTERFLOW ZONE OF A MANTLED KARST WATERSHED IN NORTHWEST ARKANSAS

Karst hydrogeologic systems typically are categorized as having minimal capacity for processing of nutrients because of the predominance of thin soils, rapid infiltration, and focused-type flow. Whereas nutrient behaviors in karst soils and conduits are reasonably well understood, a flow-path element that has been overlooked in terms of nutrient processing is the interflow zone. Dissolved organic carbon concentrations and bioavailability, concentrations of reactive (nitrate) versus conservative (chloride and bromide) constituents, and nitrate isotopic composition data were collected in specific hydrogeological zones in the mantled karst of the Ozark Highlands at the University of Arkansas Savoy Experimental Watershed, with particular focus on the interflow zone. The interflow zone is an intermediate zone between the focused-flow and diffuse-flow soil zones where a strong horizontal component of flow occurs over permeability contrasts such as at the epikarst. The interflow zone, with increased ground-water retention time and water-matrix interaction, may be important to nutrient processing in comparison to the lack of processing in the focused-flow and thin, karst soil zones. Data indicate considerable short-circuiting or bypass of dissolved species past the soil zone, but that a majority of flow spends some residence time in the interflow zone. Data also indicate that nearly 40 percent of nitrate moving through the interflow zone may have been microbially processed. The level of processing was highly variable and dependent upon flow-path and hydrologic conditions. Bioavailability of dissolved organic carbon in the interflow zone was elevated relative to the focused-flow zone under high-flow conditions, providing a needed substrate for nitrate processing in this zone. Results suggest the interflow zone appears to be a potentially important zone for nitrate attenuation in karst settings.