Chloroflourocarbons (CFCs) are anthropogenic organic compounds that
have been produced since the 1930s for a number of industrial and domestic
purposes ranging from aerosol propellants to refrigerants. There is a short
lag time between production and release to the atmosphere, where concentrations
have been increasing steadily over the past 60 years. However as a result
of various environmental regulations limiting the use of CFCs, current
production estimates are less than half of the peak values of the late
1980s. CFC-11 (CFCl3), CFC-12 (CF2Cl2)
and CFC-113 (C2F3Cl3) have relatively
long residence times in the atmosphere (44, 180 and 85 years, respectively;
Cunnold et al., 1994; Ko and Jackman, 1994), where they undergo equilibration
with surface waters as a function of temperature. As a consequence, atmospheric
concentrations show little spatial variation, with only 10% variation observed
between average concentrations in Ireland, Oregon, Barbados, Samoa and
Tasmania (Cunnold et al., 1994).
Two CFCs that have gained recent attention as potential tracers and
age-dating tools are trichlorofluoromethane (CCl3F) and dichlorodifluoromethane
(CCL2F2). CFCs and tritium can be used in a similar
manner for tracing modern water. CFCs have certain advantages over tritium
because CFCs are detectable in lower concentrations than tritium, and are,
therefore, more sensitive indicators of modern water where modern and old
water mix. In addition to acting as tracers of modern water, CFCs can yield
actual recharge ages when mixing and environmental contamination are significant
(Hinkle and Snyder, 1997). Concentrations of CFCs in ocean basins have
been used to study mixing processes, and the movement of deep ocean currents
(Trumbore et al., 1991; Wallace et al., 1992). CFC concentrations in groundwater
have been used as tracers and to estimate groundwater age (Thompson et
al., 1974; Randall and Schultz, 1976; Schultz et al., 1976; Thompson and
Hayes, 1979; Busenberg and Plummer, 1992; Dunkle et al., 1993; Plummer
et al., 1993; Ekwurzel et al., 1994; Reilly et al., 1994; Hinkle and Snyder,
By measuring CFC concentrations in groundwater and determining or estimating
the recharge temperature of the groundwater, a CFC-model age can be assigned
to the sample. Apparent CFC ages are obtained by converting measured CFC
concentrations in groundwater to equivalent air concentrations using known
solubility relationships (Warner and Weiss, 1985; Bu and Warner, 1995)
and the recharge temperature. Corrections for excess air are made if appropriate
(Busenberg and Plummer, 1992). These concentrations are compared with the
atmospheric concentration curve to obtain an apparent CFC age. Groundwater
containing any amount of CCl3F and CCL2F2
must have a component of modern recharge water no older than approximately
1948 or 1944 for CCl3F and CCL2F2, respectively.
The sensitivity of the CFC dating method depends on the rate of change
of the atmospheric CFC concentration with time, and thus the ability to
date very young water will diminish with time. However, the ability to
date groundwater that entered the saturated zone prior to the Year 2000
will not change for several decades.
Concentrations of chlorofluorocarbons in groundwater samples are measured
by gas chromatography (Bullister and Weiss, 1988) with an analytical precision
that is approximately ± 3% for concentrations above 50 pg kg–1.
This corresponds to an error in apparent CFC ages of less than 1 year for
groundwaters recharged since the mid 1960s (Dunkle et al., 1993). The sensitivity
of apparent CFC age to excess air is less than 0.1 year per cm3-air
kg–1-water at 0 C and less than 1 year per cm3 kg–1
at 20 degrees C for CFC-12 and CFC-113. Sensitivity of apparent CFC age
to recharge temperature is less than 2 years per degree C for CFC-12 and
CFC-113 (for recharge temperatures below 20 degrees C) and less than 1
year per degree C for CFC-11 (Cook and Solomon, 1996). Hence, allowing
for analytical precision and uncertainty in atmospheric concentrations,
the accuracy of apparent CFC ages in a purely advective flow system is
better than ± 4 years, provided that excess air can be estimated
to within 1 cm3 kg–1, and recharge temperature to
within 1 degree C.
One of the assumptions of groundwater dating with CFCs is that concentrations
in the soil gas immediately above the water table are in equilibrium with
the atmosphere. However, this is not always the case, particularly if the
unsaturated zone is thick (Weeks et al., 1982; Severinghaus et al., 1994;
Cook and Solomon, 1995). A time lag associated with gas diffusion through
the unsaturated zone is strongly dependent on the soil water content and
CFC solubility, and to a lesser extent on the recharge rate. This time
lag is negligible if the unsaturated zone thickness is less than 5 m, and
varies between 0.5-3 years for a unsaturated zone thickness of 10 m and
between 5-20 years for a 25 to 30 m thickness (Johnston, 1994; Cook and
Solomon, 1995). The effect of dispersion on apparent CFC concentrations
and ages has been discussed by Busenberg and Plummer (1992), Plummer et
al. (1993), Ekwurzel et al. (1994) and Reilly et al. (1994). Because the
atmospheric concentration curve is approximately linear with time, dispersion
has a minimal effect on concentration profiles. Modeling results show that
only waters older than 20 years are significantly affected. For dispersivities
of less than 0.5 m, the age error will be less than 3 years for groundwaters
recharged since 1955 (Plummer et al., 1993; Ekwurzel et al., 1994). Sorption
of CFCs to aquifer materials may cause CFC velocities to be lower than
water velocities in some aquifers, resulting in apparent CFC ages which
are older than groundwater ages (Cook et al., 1995; Busenberg and Plummer,
1993). However, sorption of CFCs does not appear to be important processes
in low organic carbon aquifers.
Contamination of groundwater with chlorofluorocarbons appears to be
the greatest limitation to CFC dating. Other organic contaminants may contain
small quantities of CFCs, sufficient to cause serious contamination at
the parts per trillion level. Groundwater samples from residential and
industrial sites often contain concentrations of chlorofluorocarbons above
modern atmospheric levels, in some instances by several orders of magnitude
(Thompson and Hayes, 1979; Jackson et al., 1992; Busenberg and Plummer,
1992). Elevated atmospheric concentrations may occur close to industrial
centers (Lovelock, 1972; Cook et al., 1996; Johnston, 1994).
Source of text: This review was assembled by Eric Caldwell, primarily
from Solomon et al. (1998) and Hinkle and Snyder (1997).
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