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Anammox is increasingly shown to play a major role in the aquatic nitrogen cycle and can outcompete heterotrophic denitrification in environments low in organic carbon. Given that aquifers are characteristically oligotrophic, anammox may represent a major route for the removal of fixed nitrogen in these environments, including agricultural nitrogen, a common groundwater contaminant.

Groundwater dating using anthropogenic and natural tracer substances is a powerful tool for understanding groundwater dynamics for improved management of groundwater resources. Due to limitations in individual dating methods, often multiple tracers are used to reduce ambiguities. It is commonly accepted that there is a need for further complementary age tracers, in addition to current ones (e.g., tritium, SF 6 , and CFCs). We propose a potential new groundwater age tracer, Halon-1301 (CF 3 Br), which can easily be determined using gas chromatography with an attached electron capture detector (GC/ECD) developed by Busenberg and Plummer (2008). Its peak was noted by Busenberg and Plummer (2008), but they believed it to be CFC-13 (CF 3 Cl) at that time. We performed rigorous tests on gases containing or excluding Halon-1301 and CFC-13 and modern water samples and concluded that the two compounds have extremely similar retention times. Additionally, we found that the ECD response of CFC-13 is far too low to be detected in groundwater or air using standard volumes and sampling techniques. However, the peak areas and concentrations Busenberg and Plummer (2008) reported are in line with what would be expected for Halon-1301. Thus, we are confident that the peak formerly identified as CFC-13 is actually Halon-1301. Busenberg agrees with our findings. We further suggest that Halon-1301 has potential as a (complementary) age tracer, due to its established atmospheric history, and could hypothetically be used to date groundwater recharged in the 1970s or onward. We discuss known relevant properties, such as solubility and stability of Halon-1301 in the context of how these effect its potential application as a groundwater age tracer. Some open questions remain concerning how conservative Halon-1301 is-is it subject to degradation, retardation, and/or local contamination in groundwater. We are confident that Halon-1301 possesses important tracer relevant properties, but further work is required to fully assess its applicability and reliability as a groundwater age tracer in different groundwater environments.

Abstract. Groundwater dating is an important tool to assess groundwater resources in regards to their dynamics, i.e. direction and timescale of groundwater flow and recharge, contamination risks and manage remediation. To infer groundwater age information, a combination of different environmental tracers, such as tritium and SF6, are commonly used. However, ambiguous age interpretations are often faced, due to a limited set of available tracers and their individual restricted application ranges. For more robust groundwater dating multiple tracers need to be applied complementarily (or other characterisation methods need to be used to complement tracer information). It is important that additional, groundwater age tracers are found to ensure robust groundwater dating in future. We have recently suggested that Halon-1301, a water soluble and entirely anthropogenic gaseous substance, may be a promising candidate, but its behaviour in water and suitability as a groundwater age tracer had not yet been assessed in detail. In this study, we determined Halon-1301 and inferred age information in 17 New Zealand groundwater samples and various modern (river) water samples. The samples were simultaneously analysed for Halon-1301 and SF6, which allowed for identification of issues such as contamination of the water with modern air during sampling. All analysed groundwater sites had also been previously dated with tritium, CFC-12, CFC-11 and SF6, and exhibited mean residence times ranging from modern (close to 0 years) to over 100 years. The investigated groundwater samples ranged from oxic to highly anoxic. All samples with available CFC data were degraded and/or contaminated in one or both of CFC-11 and CFC-12. This allowed us to make a first attempt of assessing the conservativeness of Halon-1301 in water, in terms of presence of local sources and its sensitivity towards degradation, which could affect the suitability of Halon-1301 as groundwater age tracer. Overall we found Halon-1301 reliably inferred the mean residence time of groundwater recharged between 1980 and 2014. Where direct age comparison could be made 71% of mean age estimates for the studied groundwater sites were in agreement with ages inferred from tritium and SF6 (within an uncertainty of 1 standard deviation). The remaining (anoxic) sites showed reduced concentrations of Halon-1301 along with even further reduced concentrations of CFCs. The reason(s) for this need to be further assessed, but are likely to be caused by sorption or degradation of the compounds. Despite some groundwater samples showing evidence of contamination from industrial or agricultural sources (inferred by elevated CFC concentrations), no sample showed a significantly elevated concentration of Halon-1301, which suggests no local anthropogenic or geologic sources of Halon-1301 contamination.