Potential groundwater age tracer found: Halon-1301 (CF3Br), as previously identified as CFC-13 (CF3Cl)

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, SF6, and CFCs). We propose a potential new groundwater age tracer, Halon‐1301 (CF3Br), 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 (CF3Cl) 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. Key Points Halon‐1301 is easily determinable in 1 L water samples Halon‐1301 possesses important key properties for application as tracer We advise further assessment of its applicability and reliability as an age tracer