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CFCs are useful tracers for age dating young water. Two critical assumptions are typically invoked: (1) the initial concentration needs to be known, and (2) the tracer must be stable. A series of 8000 CFC air data from four sites on the west European continent (from high‐alpine clean air down to urban sites) show site‐specific CFC excesses relative to the global background ranging from 125% at an urban site (Heidelberg) to only 30% at a rural site (Wachenheim). In both cases we find regular diurnal, weekly, and annual variations but also episodic variations due to changing air mass and sporadic local emission as well as a decrease of the CFC excess due to decreasing emissions in Europe. However, in soil air above the groundwater table the CFC excess variation is low‐pass filtered, allowing reconstruction of the site‐specific CFC input anomaly in relation to the global background. Thus local CFC soil air measurements can provide a valuable local correction factor for CFC dating and extend the applicability of CFC studies in specific environments. To investigate the chemical stability assumption, five field sites and two laboratory experiments show CFC 11 and CFC 12 degradation under anoxic conditions with degradation rates ranging from 0.05 up to 3×105 pmol L−1 yr−1. However, the CFC 12/CFC 11 degradation ratio is found to be constant over six orders of magnitude, with CFC 12 always being less reactive by a factor of about 10.

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