Accuracy of CFC groundwater dating in a crystalline bedrock aquifer: Data from a site in southern Sweden

The concentrations of chlorofluorocarbons (CFC-11, CFC-12, and CFC-113) and tritium were determined in groundwater in fractured crystalline bedrock at Finnsjön, Sweden. The specific goal was to investigate the accuracy of CFC dating in such an environment, taking potential degradation and mixing of water into consideration. The water was sampled to a depth of 42 m in three boreholes along an 800-m transect, from a recharge area to a local discharge area. The CFC-113 concentration was at the detection limit in most samples. The apparent recharge date obtained from CFC-11 was earlier than from CFC-12 for all samples, with a difference of over 20 years for some samples. The difference was probably caused by degradation of CFC-11. The CFC-12 dating of the samples ranged from before 1945 to 1975, with the exception of a sample from the water table, which had a present-day concentration. Conclusions about flow paths or groundwater velocity could not be drawn from the CFCs. The comparison between CFC-12 and tritium concentrations showed that most samples could be unmixed or mixtures of waters with different ages, and the binary mixtures that matched the measured concentrations were determined. The mixing model approach can be extended with additional tracers.