CHLOROFLUOROCARBONS AS TRACERS OF GROUNDWATER TRANSPORT PROCESSES IN A SHALLOW, SILTY SAND AQUIFER

Detailed depth profiles of chlorofluorocarbons CFC-11 (CFCl3), CFC-12 (CF2Cl2) and CFC-113 (C2F3Cl3) 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 H-3 peak has been tracked through time. Detailed vertical sampling has allowed CFC tracer velocities to be estimated to within 10%. Comparison with H-3 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 H-3 (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.