CFCs (Chlorofluorocarbons)

Background

CFCs are anthropogenic compounds with virtually no natural background. Their industrial production started in the 1930s and 1940s. Between this time and the 1990s, their atmospheric concentration increased, first quasi-exponentially, then quasi-linearly (Fig.). In the late 1980s, the Montreal protocol led to a drastically reduction in CFC production. Consequently, the atmospheric concentrations of CFCs leveled off. Presently, the atmospheric concentrations of CFC11 and CFC 113 are decreasing significantly. That of CFC12 is in a plateau and will drop at a lower rate.

CFC production and use

CFCs or chlorofluorocarbons are compounds that are essentially inert in the troposphere. The main compounds used as tracers in natural systems are CFC 11 (CCl₃F), CFC 12 (CCl₂F₂) and CFC 113 CCl₂F-CClF₂). All compounds have no known natural sources. In the early literature, CFC 11 and CFC 12 were also called chlorofluoromethanes due to their methane-type structure. The trade name of CFCs (DuPond) is Freon (F 11, F 12, F 113). The main use of CFCs is summarized in Table 1.

Atmospheric CFC sink

CFCs are degraded in the stratosphere by photo dissociation. The resulting chlorine radicals contribute to the destruction of the ozone layer and led to the ban of CFC production (Montreal Protocol). The individual CFCs have different atmospheric life times. These were determined during the so-called ALE experiment (Atmospheric Lifetime Experiment; see Cunnold et al.). The life times of the individual CFCs is summarized in Table 1.

Production and mean life time:

	Boiling Point [^oC]	Use	Delay in release to the atmosphere	Mean life time in the atmosphere
CFC 11	23.8	Aerosol spray cans Solvent Foam blowing agent	» 6 months » 6 months 6 months to 20 ys	57 - 105 ys mean: 74 ys
CFC 12	-29.8	Refrigerators Air conditioners Foam blowing agent	4 – 12 years 4 – 12 years 6 months to 20 ys	67 – 333 ys mean: 111 ys
CFC 113	47.6	Cleaning solvent for manufacturing processes and electronic components		136-195 ys
CCl4	76.8	Cleaning agent

CFC life cycle (Fig):

Production
release to troposphere; lag times
mixing laterally and vertically
decomposition in stratosphere
dissolution in natural waters

Atmospheric concentrations (Fig)

Determined by

Production rate (Fig.)
Decomposition
Hemispheric and inter-hemispheric mixing (Fig.)
Vertical mixing (Fig.)
Local sources (Fig.)

Determined by measurements and historic industrial production records

Typically well known in clean air
Small hemispheric gradients
Inter-hemispheric gradients are of the order of several percent
Local sources are highly variable and lead to concentrations in excess of clean-air conc. Of the order of several percent to several 10 percent (Oster et al; Ho et al.). Fig.

Dating with CFCs

There are two basic principles for determining the mean residence time of ‘age’ of a groundwater parcel using CFCs:

pCFC method: If the CFC concentration of a water sample is measured and the solubility for the temperature and salinity (typically zero) of the water at the time of formation at the groundwater table are known, the CFC concentration in the soil air above the water table can be calculated. The CFC solubilities have been measured by Warner and Weiss and Bu and Warner. The recharge temperature can be determined through N₂/Ar temperatures. Comparing the CFC concentration in soil air determined in this way, the age can be estimated by matching this value with the atmospheric CFC concentration curve and reading the age off the graph (time at which the atmospheric concentration and the soil air concentrations determined from the groundwater measurement match (Fig).
The ratio of certain CFCs (e.g., CFC 11 and CFC 12) are time dependent, at least over certain time intervals (Fig.). This time dependence can be used to estimate the age of a water parcel. Dating with CFC ratios has been used mainly in oceanography. In hydrology, the ratio dating is more problematic due to the fact that CFC 11 is frequently degraded, especially in anoxic environments.

CFCs in aquifers

In clean environments, CFCs frequently are an efficient tool for age dating. There are virtually no signs of pollution or degradation and the pCFC ages are in good agreement with other tracer ages (Fig.) or with flow modeling results (Fig.)
However, CFCs can have both sources and sinks in groundwater.

For example, sources are related to sewage and land fills (Fig.).
Sinks are typically related to anoxic environments (Fig.). So far, degradation rates have been determined for ocean waters (Fig.) and some artificial environments (Fig.). Whereas a study in a Norwegian Fjord (Fig.) shoed little evidence for CFC 12 degradation, other experiments (Oster et al., 1996) claim a ratio of CFC 11 to CFC 12 degradation of about 10.

The fact that CFCs are not conservative in many aquifers requires cross checks with other tracers. This eliminates some of the advantages of CFC measurements (relatively easy technique; low costs) compared to e.g. tritium/³He measurements.

Resources

USGS CFC laboratory