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

Chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF6) 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, associated with urbanisation and thereby complicating the interpretation of data derived from groundwater studies. Previous research has shown that the soil acts as a low-pass filter for short-term atmospheric trace gas fluctuations, so that soil gases should yield integrated evidence for LAEs if present. To investigate the possible existence of LAEs in the UK, mixing ratios of CFC-11, CFC-12 and SF6 have been measured in soil gases from sites within the UK’s two largest cities (London and Birmingham) and a third urban area, Bristol. While there was some evidence of excesses, most of the measured mixing ratios for CFC-12 and SF6 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 trace-gas archives but were much less consistent in performance. Overall, given the errors associated with trace-gas groundwater dating (e.g. precision of measurement, assumptions about recharge temperature, excess air, contamination or degradation in the subsurface), there is little justification for factoring in the minor LAEs found in this study to urban or peri-urban groundwater investigations, and accordingly the use of CFC and SF6 input functions based on the NH-AMR curves appears to be justified for the UK.