GROUNDWATER AGE DATING WITH CHLOROFLUOROCARBONS

CFCs are useful tracers for age dating young water. Two critical assum ptions are typically invoked: (1) the initial concentration needs to b e known, and (2) the tracer must be stable. A series of 8000 CFC air d ata from four sites on the west European continent (from high-alpine c lean air down to urban sites) show site-specific CFC excesses relative to the global background ranging from 125% at an urban site (Heidelbe rg) to only 30% at a rural site (Wachenheim). In both cases we find re gular diurnal, weekly, and annual variations but also episodic variati ons due to changing air mass and sporadic local emission as well as a decrease of the CFC excess due to decreasing emissions in Europe. Howe ver, 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 so il air measurements can provide a valuable local correction factor for CFC dating and extend the applicability of CFC studies in specific en vironments. To investigate the chemical stability assumption, five fie ld sites and two laboratory experiments show CFC 11 and CFC 12 degrada tion under anoxic conditions with degradation rates ranging from 0.05 up to 3 x 10(5) pmol L(-1) yr(-1). However, the CFC 12/CFC 11 degradat ion ratio is found to be constant over six orders of magnitude, with C FC 12 always being less reactive by a factor of about 10.