Assessing the applicability of global CFC and SF6 input functions to groundwater dating in the UK

Abstract

Chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF 6 ) are increasingly being used to date recent groundwater components. While these trace gases are generally well-mixed in the atmosphere, there is evidence that local atmospheric excesses (LAEs) exist in some areas of the world, primarily associated with urbanisation and thereby affecting the interpretation of data derived from groundwater studies. Since the soil acts as a low-pass filter for atmospheric trace gas fluctuations, the possible existence of LAEs in the UK has been investigated by measuring the mixing ratios of CFC-11, CFC-12 and SF 6 in soil gases from sites within the UK's two largest cities (London and Birmingham) and a smaller urban area, Bristol. While there was some evidence of excesses, most of the measured mixing ratios for CFC-12 and SF 6 were less than 10% above the current northern hemisphere atmospheric mixing ratio (NH-AMR) values. CFC-11 was more variable, but usually less than 20% above the NH-AMR value. Surface waters were also investigated as possible short-term archives of trace-gas information but were much less consistent in performance.While the lack of significant current LAEs for SF 6 can justifiably be extrapolated to past decades, different global emission patterns mean that this is much harder to justify for the CFCs. Nevertheless, in the absence of further evidence it is concluded that the use of CFC and SF 6 input functions based on the NH-AMR curves is generally justified for the UK, with the proviso that urban groundwater investigations should not rely on the CFCs as age tracers.

Interaction between groundwater, the hyporheic zone and a Chalk stream: a case study from the River Lambourn, UK

“…Each of the methods have advantages and drawbacks which can often be site specific, so choosing the best method(s) for a particular site is not straightforward. For example using CFCs in UK lowland catchments in urban settings can be problematic due to point source contamination of the aquifer system with CFCs (Darling and Gooddy, 2007). In most situations a combination of several techniques is required to understand the complex hydrological processes (Cook et al, 2003, Gooddy et al, 2006.…”

Factors controlling terrigenic SF6 in young groundwater of the Odenwald region (Germany)

“…Santella et al (2008) estimate that significant areas of the US, Europe and Asia potentially show elevated SF 6 levels of 10% or more over clean air background. However, studies by Gooddy et al (2006) and Darling and Gooddy (2007) indicate that SF 6 levels close to industrialised urban areas are not always elevated.…”

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)

“…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. Pathways of CFC input to groundwater such as local atmospheric pollution (Darling and Gooddy, 2007), landfills (Darling et al 2010), and industrial solvent spills (Morris et al, 2006) are widely reported throughout the literature.…”