Origin and Fate of Industrial Ammonium in Anoxic Ground Water— 15 N Evidence for Anaerobic Oxidation (Anammox) by Ian Clark, Robert Timlin, Annie Bourbonnais, Katie Jones, Dennis Lafleur, and Kevin Wickens Abstract Elevated ammonium concentrations in pump-and-treat effluent from a volatile organic compounds–contaminated muni- cipal aquifer originate from two industrial sources: infiltration of drainage from the blending operations of a fertilizer com- pany (FC) located in the recharge area (NH þ 4 of 500 to 700 parts per million [ppm] N and NO  3 of 150 to 300 ppm N) and leakage from waste water treatment ponds maintained by an adjacent chemical company (CC) (NH þ 4 of 50 to 70 ppm N, with no NO  3 ). 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 (NH þ 4 < 10 ppm N). Conservative mixing calculations demon- strate a loss of NH þ 4 and NO  3 along the flowpath relative to K + and Cl  . Reactive loss of NH þ 4 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 NH þ 4 and NO  3 under anoxic conditions along the flowpath; (2) a progressive enrichment of d 15 N NH 4 and d 15 N NO 3 , indi- cating reactive loss of ammonium and nitrate; (3) d 15 N NO 3 values greater than coexisting d 15 N NH 4 , which precludes NH þ 4 loss by nitrification to NO  3 ; and (4) significant N 2 overpressuring with increasing d 15 N NH 4 values. Anaerobic ammonium oxida- tion by anammox bacteria uses nitrate as the electron donor: 3NO  3 þ 5NH þ 4 ! 4N 2 þ 9H 2 O þ 2H þ . The recently discov- ered 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. Introduction Un-ionized ammonia in surface water poses considerable risk to fresh water and marine organisms (e.g., Thurston et al. 1981; Hecnar 1995). Further, oxidation of ammonia to nitrate in ground water and surface water poses a threat to drinking water quality. Accordingly, the origin and trans- formation of ammonia in ground water can be important considerations in water quality assessments. In most agricultural settings, the degradation of organi- cally bound nitrogen, the mineralization of urea, and the dissociation of ammonium nitrate fertilizer are the principal sources contributing ammonium and nitrate to fresh water (Wassenaar 1995, Goss et al. 1998). Inputs from localized activities such as landfill leachates (Lo 1996; North et al. 2004) and septic effluent (Aravena and Robertson 1998) are important point sources of these contaminants in sur- face water and ground water. Attenuation of nitrate and/or ammonia in impacted water occurs by dilution through dis- persion and mixing and by well-known reaction pathways. In oxidizing environments, ammonia can be lost through volatilization or through bacterially mediated O 2 oxidation to nitrate (Burrell et al. 2001). In reducing ground water, ammonia is considered to be stable, while reductive loss of nitrate by denitrifying bacteria is common (Korom 1992). Recently, an anaerobic ammonia oxidation reaction has been identified (Mulder et al. 1995). The anammox reac- tion is mediated by Candidatus ‘‘Brocadia’’ anammoxidans and two other closely related bacteria, Candidatus ‘‘Sca- lindua brodae’’ and Candidatus ‘‘Scalindua wagneri’’ (Jetten et al. 2001; Schmid et al. 2003), and involves nitrate and nitrite as electron acceptors for the oxidation of ammonia to N 2 . It has now been identified in a variety of settings, including anoxic ocean water (Dalsgaard et al. 2003; Devol 2003; Kuypers et al. 2003) and soils (Jetten 1999) as well as waste water streams (Strous et al. 1999). The variety of potential sources and transformations of ammonium in ground water often makes tracing its origin a difficult task. This is particularly so in rural settings where various diffuse and point sources can all contribute. Even with an understanding of the aquifer architecture, recharge and flowpaths, subsurface dispersion, mixing, and reaction complicate efforts to determine sources of Copyright ª 2008 The Author(s) Journal compilation ª 2008 National Ground Water Association. Ground Water Monitoring & Remediation 28, no. 3/ Summer 2008/pages 73–82 73