The Reston Chlorofluorocarbon Laboratory

Guidelines for Assignment of Apparent CFC Age

The most reliable results are obtained if the apparent ages from all three CFC compounds agree.

When the CFC-11 and CFC-12 ages agree and the CFC-113 age is younger, mixing of old and young water has likely occurred.

If the CFC-11 age is old relative to the CFC-12 and CFC-113 ages, CFC-11 may be degraded and the most likely age is closer to the CFC-12 (and/or CFC-113) age.

If all the CFCs give different apparent ages and the youngest is from CFC-12, the next older is from CFC-113, and the oldest is from CFC-11, the water has most likely been affected by degradation. If CFC-11 and CFC-113 are still present in the sample, CFC-12 has probably not been significantly affected by degradation. If CFC-11 and CFC-113 are completely removed, CFC-12 has likely been at least partially degraded as well.

If degradation has occurred, the chromatogram may have extra peaks, many of which are broad and poorly defined, and the baseline may not be straight.

Use of other dissolved gases: If the DO is high, no CFC degradation has likely occurred unless the water is a mixture. If the DO is low, caution must be used in utilizing CFC ages, particularly those obtained from CFC-11. If N2O is present (seen on chromatogram; indicates nitrate reduction), CFC-12 and CFC-113 are sometimes in close agreement, but CFC-11 can be low. If H2S is present, CFC-11 can be significantly degraded, the CFC-113 age may be a few years older than that based on CFC-12, and CFC-12 is likely not affected by degradation. If CH4 is present, all three CFCs can be degraded. The CFC dating technique probably cannot be utilized in methanic environments except to say that an age is younger than the apparent CFC-12 age.

In methanic environments, CFC-12 is not significantly degraded in samples with high DOC, but is significantly altered when high particulate organic matter (i.e. plant debris) is available to act as a substrate for degradation. In bottles that contain suspended sediment, degradation is likely (see Plummer and others 1998a, 1998b in bibliography section).

It is best to collect 4 to 5 bottles at each site. A single apparent age may not be obtained at a given site if the discharge is not at hydraulic steady state.

No age can be assigned if all CFC concentrations are below the detection limit. In this case, the age is denoted as “older than” a given year depending on the tracer. No age can be assigned if all CFC concentrations are greater than that of “modern.”

The apparent age is not necessarily the age since confinement as there may be effects from matrix diffusion, hydrodynamic dispersion, complex mixing in the aquifer or in the well bore during sampling, etc.

In order to make the best assessment of age, it is necessary to utilize other geochemical and hydrologic data. Other geochemical data could include. 3H, 3H / 3He, dissolved gases, redox indicators, etc. Other hydrologic data could include information on pump type, method of sample collection, environmental setting, hydrologic setting, etc.