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

- We present the results of three inter-laboratory exercises on CFCs and SF6 in groundwater and air.
- The dispersion of the results is analyzed by parameter and by laboratory.
- Influence of the sampling protocols is evaluated.
- The effect of the dispersion of the results is analyzed on the “apparent age” of groundwater.

An inter-laboratory comparison exercise involving the environmental tracers used for groundwater dating was organized in 2012 in France. This paper focuses on SF6 and CFC tracers. Sampling and analytical protocols 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 show good agreement between laboratories (except some outliers) for the aquifers and air standard. Variation in the SF6 results on the air standard was low (RSD = 2%) compared to CFCs (RSD 3-7%), even though its concentration was two orders of magnitude lower. Results obtained on recent groundwater (recharged post 1980) show that the inter-laboratory uncertainty for groundwater dating with SF6 is 3-4 years. This large uncertainty is mainly due to sampling and/or analytical problems. For CFCs, the uncertainties obtained from all laboratories were less than 2 years for groundwater recharged between 1965 and 1996. A higher RSD was observed for CFCs with the air standard analysis, as compared to groundwater measurement, which could indicate an additional uncertainty due to inadequate standardization. Different sampling protocols were used by the laboratories (glass bottles, steel cylinders, etc.) but no systematic effect on measurement uncertainty could be attributed to any of these protocols for either CFCs or SF6. 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). These results confirm the need for regular inter-laboratory comparison exercises to improve the analytical and sampling procedures used in groundwater dating.