Periodic Table--Chlorofluorocarbons

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, 1997).

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).