Assessment of Halon-1301 as a groundwater age tracer

Abstract

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.

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).…”

“…Actual solubility measurements of Halon-1301 are not available in the literature (according to our searches and further backed up by personal communication with Daniel Deeds, 06/03/2015). In our previous study (Beyer et al, 2015), we used modern (equilibrated tap and river) water to estimate solubility and to validate the solubility estimates. In this study, we confirmed our previous estimate by using solubility estimated from four additional modern (river) water samples.…”

“…We previously assessed Halon-1301's performance as an age tracer against two relatively reliable established tracers, SF 6 and tritium, at 18 sites (Beyer et al, 2015), and found that in the majority of assessed groundwater samples ages inferred Hydrol. Earth Syst.…”

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

“…2). To more accurately determine the solubility of Halon-1301 and reduce uncertainty in its determination, further study is needed (also pointed out in Beyer et al, 2015). Accurate measurement of the solubility of Halon-1301 is beyond the scope of this study, as specialized equipment is required due to its extremely low solubility.…”

“…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 and Rozanksi, 2016).…”