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

Abstract

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.

Assessment of Halon-1301 as a groundwater age tracer

“…We have previously and unexpectedly identified the presence of Halon-1301 (CBrF 3 ) in modern water samples. Our paper immediately following this discovery (Beyer et al, 2014b) has detailed this identification, has discussed known Halon-1301 properties and has suggested this compound might have potential as a new, complementary groundwater age tracer (for water recharged < 100 years ago) to join the limited set of established compounds commonly used for this purpose. We have not inferred ages from Halon-1301 concentrations in that paper.…”

Supplementary material to &quot;Medusa-Aqua system: simultaneous measurement and evaluation of novel potential halogenated transient tracers HCFCs, HFCs and PFCs in the ocean&quot;

“…However, several HCFCs and HFCs (i.e. HCFC-22, HFC-134a and HFC-125) have low responses and large uncertainties when they are measured by the ECD (Lobert et al, 1995;Beyer et al, 2014). Besides, the HCFCs tend to be more soluble, making it more difficult to quantitatively purge all of the tracers from a water sample.…”

Halon-1301 – further evidence of its performance as an age tracer in New Zealand groundwater

“…The gas samples were subsequently purged and analysed on a gas chromatograph with attached electron capture detector (GC/ECD). Simultaneous analysis of Halon-1301 and SF 6 is detailed in Beyer et al (2014Beyer et al ( , 2015. The analytical setup for determination of Halon-1301 and SF 6 also allowed the simultaneous determination of CFC-12 (Busenberg and Plummer, 2008;Beyer et al, 2014;Bartyzel et al, 2016).…”