The practicalities of using CFCs and SF6 for groundwater dating and tracing

Abstract

Knowledge of groundwater residence time is important in understanding key issues in the evolution of water quality, whether this occurs due to water-rock interaction or simply by mixing or contamination. The build-up in the atmosphere of the trace gases chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF 6 ) from the middle of the last century offers a convenient way of dating waters up to ~60 yrs old. The gases are well-mixed in the atmosphere so their input functions are not area-specific as is the case with tritium. While any one of these trace gases can in principle provide a groundwater age, when two or more are measured on water samples the potential exists to distinguish between different modes of flow including piston flow, exponential flow and simple end-member mixing. As with all groundwater dating methods, caveats apply. Factors such as recharge temperature and elevation must be reasonably well-constrained. Primarily for SF 6 , the phenomenon of 'excess air' also requires consideration. Primarily for the CFCs, local sources of contamination need to be considered, as do redox conditions. For both SF 6 and the CFCs, the nature and thickness of the unsaturated zone need to be factored into residence time calculations. However, as an inexpensive dating method, the trace gases can be applied to a wide range of groundwater problems where traditional age indicators might once have been used more sparingly. Examples include tracing flowlines, detecting small modern inputs in 'old' waters, and pollution risk assessment. In the future, with the main CFCs are already declining in the atmosphere, new anthropogenic trace gases are likely to take their place.

Isotope signatures and hydrochemistry as tools in assessing groundwater occurrence and dynamics in a coastal arid aquifer

“…The principle of age dating with CFCs is based on comparing their concentrations in groundwater with the known worldwide atmospheric concentrations of these anthropogenic gases over the past 60 years to estimate the year of recharge at each site (Darling et al 2012). However, several complications may arise.…”

“…It is well recognized that major and minor chemical elements may give an insight into water-rock interactions as well as anthropogenic influences affecting the groundwater system as demonstrated in numerous studies (Hem 1989;Mahlknecht et al 2004;Ledesma-Ruiz et al 2015;Cruz-Fuentes et al 2014;Hernández-Antonio et al 2015;Russak et al 2015), while water isotopes may define groundwater recharge areas, mixing and evaporation sources (Edmunds et al 2003;Horst et al 2007;Jiménez-Martínez and Custodio 2008;Mohammed et al 2014;Gonçalvès et al 2015), and radiocarbon isotopes residence times of old groundwater and its distribution (Fontes and Garnier 1979;Clark and Fritz 1997;Edmunds and Smedley 2000;Mahlknecht et al 2006a). More recently, it has been demonstrated that chlorofluorocarbons are useful in evaluating residence times of modern groundwater, mixing patterns and aquifer's vulnerability to contamination (Busenberg and Plummer 1992;Cook et al 1995;Goody et al 2006;Mahlknecht et al 2006b;Horst et al 2008;Darling et al 2012).…”

Using 18 O/ 2 H, 3 H/ 3 He, 85 Kr and CFCs to determine mean residence times and water origin in the Grazer and Leibnitzer Feld groundwater bodies (Austria)

“…Most wells in the Leibnitzer Feld show CFC-11 values lower than expected for recent groundwaters. Variable reducing and oxidizing condition in parts of the Leibnitzer Feld could be the reason for the low CFC-11 values (Darling et al, 2012). Höhener et al (2002) reported that in 12 of 16 aquifers groundwater was locally contaminated with CFCs in concentrations exceeding equilibrium with respect to modern air.…”

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

“…The currently used age tracers have limited application ranges and reliability (Darling et al, 2012). For example, SF 6 has natural sources (see for example Bunsenberg andPlummer, 2000, 2008;Stewart and Morgenstern, 2001;Koh et al, 2007), which can interfere with the interpretation of age from its concentration.…”