Origin and fate of industrial ammonium in anoxic ground water -: 15N evidence for anaerobic oxidation (anammox)

Elevated ammonium concentrations in pump-and-treat effluent from a volatile organic compounds-contaminated municipal aquifer originate from two industrial sources: infiltration of drainage from the blending operations of a fertilizer company (FC) located in the recharge area (NH4+ of 500 to 700 parts per million [ppm] N and NO3- of 150 to 300 ppm N) and leakage from waste water treatment ponds maintained by an adjacent chemical company (CC) (NH4+ of 50 to 70 ppm N, with no NO3-). Geochemical and isotope data are used to trace the mechanisms for the strong attenuation observed between the source areas and the municipal ground water treatment wells (NH4+ < 10 ppm N). Conservative mixing calculations demonstrate a loss of NH4+ and NO3- along the flowpath relative to K+ and Cl-. Reactive loss of NH4+ in these anoxic ground water is attributed to anaerobic oxidation by anammox bacteria. Lines of evidence leading to this conclusion include (1) loss of both NH4+ and NO3- under anoxic conditions along the flowpath; (2) a progressive enrichment of delta N-15(NH4) and delta N-15(NO3), indicating reactive loss of ammonium and nitrate; (3) delta N-15(NO3) values greater than coexisting delta N-15(NH4), which precludes NH4+ loss by nitrification to NO3-; and (4) significant N-2 overpressuring with increasing delta N-15(NH4) values. Anaerobic ammonium oxidation by anammox bacteria uses nitrate as the electron donor: 3NO(3)(-) + 5NH(4)(+) -> 4N(2) + 9H(2)O + 2H(+). The recently discovered anammox reaction is more energetically favorable than denitrification and is now considered to play a major role in the global nitrogen cycle. It has been observed in waste water bioreactors and sea water but not previously in ground water.