BIOGEOCHEMICAL/HYDROLOGICAL INVESTIGATION OF THE FATE OF NITRATE IN THE INTERFLOW ZONE OF MANTLED KARST

Groundwater nitrate (NO^₃-) contamination associated with animal production and land application of animal manures is a common problem in karst terrains. These terrains offer limited nutrient bioremediation potential due to thin soils and predominance of conduit flow. Sustainable manure management in these terrains requires an understanding of in situ (NO^₃-) dynamics and bioremediation processes, which is presently lacking. Dissolved organic carbon concentrations and bioavailability, concentrations of reactive (NO^₃-) versus conservative (chloride and bromide) constituents, and (NO^₃-) stable isotope data were collected across a hydrologic gradient in a mantled karst watershed in the Ozark Highlands, with particular focus on the interflow zone situated between the focused-flow and diffuse-flow soil zones, where increased ground-water retention time and water-matrix interaction may favor (NO^₃-) bioremediation processes such as denitrification. Mass balance calculations indicated that although mixing was the primary process removing (NO^₃-) along the interflow zone flow-paths, up to 33 percent of (NO^₃-) moving through the interflow zone may have been microbially processed. The magnitude of this processing was highly variable, depending upon the 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 (NO^₃-) processing in this zone. Results suggest the interflow zone may be a potentially important zone for (NO^₃-) attenuation in karst.