Chlorofluorocarbons as Tracers of Groundwater Transport Processes in a Shallow, Silty Sand Aquifer

Detailed depth profiles of Chlorofluorocarbons CFC‐11(CFCl 3 (, CFC‐12 (CF 2 Cl 2 ) and CFC‐113 (C 2 F 3 Cl 3 ) have been obtained from a well‐characterized field site in central Ontario. Aquifer materials comprise predominantly silty sands, with a mean organic carbon content of 0.03%. Nearly one‐dimensional flow exists at this site, and the vertical migration of a well‐defined 3 H peak has been tracked through time. Detailed vertical sampling has allowed CFC tracer velocities to be estimated to within 10%. Comparison with 3 H profiles enables estimation of chlorofluorocarbon transport parameters. CFC‐12 appears to be the most conservative of the CFCs measured. Sorption at this site is low ( K d < 0.03), and degradation does not appear to be important. CFC‐ 113 is retarded both with respect to CFC‐12 and with respect to 3 H ( K d = 0.09−0.14). CFC‐11 appears to be degraded both in the highly organic unsaturated zone and below 3.5 m depth in the aquifer, where dissolved oxygen concentrations decrease to below 0.5 mg L −1 . The half‐life for CFC‐11 degradation below 3.5 m depth is less than 2 years. While apparent CFC‐12 ages match hydraulic ages to within 20% (up to 30 years), apparent CFC‐11 and CFC‐113 ages significantly overestimate hydraulic ages at our field site.