Inter-laboratory comparison of the analyses of sulphur hexafluoride (SF6) and three chlorofluorocarbonate (CFC-11, -12 and -113) in groundwater and an air standard

International audienceAn inter-laboratory comparison exercise involving the environmental tracers used for groundwater datingwas organized in 2012 in France. This paper focuses on SF6 and CFC tracers. Sampling and analyticalprotocols were compared in three different exercises using: (1) groundwater from a homogeneous aquifer,(2) groundwater from a fractured heterogeneous aquifer and (3) an air standard. The results showgood agreement between laboratories (except some outliers) for the aquifers and air standard. Variationin the SF6 results on the air standard was low (RSD = 2%) compared to CFCs (RSD 3–7%), even though itsconcentration was two orders of magnitude lower. Results obtained on recent groundwater (rechargedpost 1980) show that the inter-laboratory uncertainty for groundwater dating with SF6 is 3–4 years. Thislarge uncertainty is mainly due to sampling and/or analytical problems. For CFCs, the uncertaintiesobtained from all laboratories were less than 2 years for groundwater recharged between 1965 and1996. A higher RSD was observed for CFCs with the air standard analysis, as compared to groundwatermeasurement, which could indicate an additional uncertainty due to inadequate standardization. Differentsampling protocols were used by the laboratories (glass bottles, steel cylinders, etc.) but no systematiceffect on measurement uncertainty could be attributed to any of these protocols for either CFCs orSF6. The better precisions obtained through these exercises indicate that, except for CFC-11 and SF6,the corresponding uncertainty in recharge date determination on recent groundwater (1993 – present)is higher than the commonly assumed 1–2 years (i.e., 7 years for CFC-12 and 4 years for CFC-113). Theseresults confirm the need for regular inter-laboratory comparison exercises to improve the analytical andsampling procedures used in groundwater dating