NITRIFICATION AND DENITRIFICATION IN A GROUNDWATER CONTAMINANT PLUME ON CAPE COD—SORTING THROUGH PHYSICAL, GEOCHEMICAL AND BIOLOGICAL CONTROLS ON PROCESS FUNCTION

Nitrification and denitrification are key contributors to the cycling and flux of nitrogen in the subsurface. Both processes involve multiple-step redox reactions that control the valence, mobility and fate of inorganic nitrogen. The function of these processes is controlled by a combination of the physical, geochemical and biological factors present within an aquifer and must be assessed within the in situ context of that environment. An overview will be presented that describes varied approaches used to characterize and quantify nitrification and denitrification in an unconfined sand and gravel aquifer that had been contaminated by the long-term discharge of dilute, treated wastewater. The resulting large contaminant plume (>6 km) is characterized by steep vertical geochemical gradients that included nitrate and ammonium, as well as all intermediates in the nitrification and denitrification pathways and longitudinal gradients characterized by differing zones of available electron supply. Techniques used in this study include interpretation of stable isotope profiles, incubations with cores, in situ natural-gradient tracer tests, injection tests to assess process response to altered geochemistry, turnover of trace-level intermediate compounds, and molecular characterization of microbial communities. Results suggest that vertical dispersion greatly limits both processes in the aquifer. In many zones denitrification is electron-donor limited, but capable of utilizing a variety of potential electron sources (H₂, CH₄, Fe⁺²). Nitrification is additionally limited by cation exchange, to the point that even though nitrifying microorganisms are present and active when ammonium and oxygen are both co-injected, there is little evidence that the process has any substantial effect on plume geochemistry or transport of ammonium within the aquifer, with the exception of a very narrow vertical zone (5-10 cm) along the upper plume boundary.