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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 kg1. 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 kg1-water at 0 C and less than 1 year per cm3 kg1 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 kg1, 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).

References
Bu, X. and Warner, M.J. (1995). Solubility of chlorofluorocarbon 113 in water and seawater. Deep Sea Res., 42: 1151-1161.
Bullister, J.L. and Weiss, R.F. (1988). Determination of CCl3F and CCl2F2 in seawater and air. Deep Sea Res., 35: 839-853.
Busenberg, E. and Plummer, L.N. (1992). Use of Chlorofluorocarbons (CCl3F and CCl2F2) as hydrologic tracers and age-dating tools: The alluvium and terrace system of central Oklahoma. Water Resour. Res., 28: 2257-2283.
Busenberg, E. and Plummer, L.N. (1993). Use of trichlorofluorocarbon-113 (CFC-113) as a hydrologic tracer and age-dating tool of young ground water. In: GSA Annual Meeting, Boston, p. 365.
Cook, P.G. and Solomon, D.K. (1995). The transport of atmospheric trace gases to the water table: implications for groundwater dating with chlorofluorocarbons and krypton 85. Water Resour. Res., 31: 263-270.
Cook, P.G., Solomon, D.K., Plummer, L.N., Busenberg, E. and Schiff, S.L. (1995). Chlorofluorocarbons as tracers of groundwater transport processes in a shallow, silty sand aquifer. Water Resour. Res., 31: 425-434.
Cook, P.G., Solomon, D.K., Sanford, W.E., Busenberg, E., Plummer, L.N. and Poreda, R.J. (1996). Inferring shallow groundwater flow in saprolite and fractured rock using environmental tracers. Water Resour. Res., 32: 1501-1509.
Dunkle, S.A., Plummer, L.N., Busenberg, E., Phillips, P.J. Denver, J.M., Hamilton, P.A., Michel, R.L. and Coplen, T.B. (1993). Chlorofluorocarbons (CCl3F and CCl2F2) as dating tools and hydrologic tracers in shallow groundwater of the Delmarva Peninsula, Atlantic Coastal Plain, United States. Water Resour. Res., 29: 3837-3860.
Ekwurzel, B. Schlosser, P., Smetthie, W.M., Jr., Plummer, L.N., Busenberg, E., Michel, R.L., Weppernig, R. and Stute, M. (1994). Dating of shallow groundwater-Comparison of the transient tracers 3H/3He, chlorofluorocarbons, and 85Kr. Water Resour. Res., 30: 1693-1708.
Elkins, J.W., Thompson, T.M., Swanson, T.H., Butler, J.H., Halls, B.D., Cummings, S.O., Fisher, D.A. and Raffo, A.G. (1993). Decrease in growth rates of atmospheric chlorofluorocarbons 11 and 12. Nature, 364: 780-783.
Hinkle, S.R. and Snyder, D.T. (1997). Comparison of Chlorofluorocarbon-Age Dating with Paticle-Tracking Results of a Regional Ground-water Flow Model of the Portland Basin, Oregon and Washington. U.S. Geological Survey Water-Supply Paper 2483, 47 p.
Jackson, R.E., Lesage, S. and Priddle, M.W. (1992). Estimating the fate and mobility of CFC-113 in groundwater: results from the Gloucester Landfill Project. In: R.E. Jackson and S. Lesage (Editors), Groundwater Contamination and Analysis at Hazardous Waste Sites, Marcel Dekker, New York, pp. 511-526.
Khalil, M.A.K. and Rasmussen, R.A. (1989). The potential of soils as a sink of chlorofluorocarbons and other man-made chlorocarbons. Geophys. Res. Lett., 16: 679-682.
Lovelock, J.E. (1972). Atmospheric turbidity and CCl3F concentrations in rural southern England and southern Ireland. Atmos. Environ., 6: 917-925.
Lovley, D.R. and Woodward, J.C. (1992). Consumption of freons CFC-11 and CFC-12 by anaerobic sediments and soils. Environ. Sci. Tech., 26: 925-929.
Plummer, L.N., Michel, R.L., Thurman, E.M. and Glynn, P.D. (1993). Environmental tracers for age dating young ground water. In: Regional ground-water quality. Van Nostrand Reinhold, New York, pp. 255-294.
Randall, J.H. and Schultz, T.R. (1976). Chlorofluorocarbons as hydrologic tracers: A new technology. Hydrology and Water Resources in Arizona and the Southwest, 6: 189-195.
Reilly, T.E., Plummer, L.N., Phillips, P.J. and Busenberg, E. (1994). The use of simulation and multiple environmental tracers to quantify groundwater flow in a shallow aquifer. Water Resour. Res., 30: 421-433.
Russell, A.D. and Thompson, G.M. (1983). Mechanisms leading to enrichment of the atmospheric fluorocarbons CCl3F and CCl2F2 in groundwater. Water Resour. Res., 19: 57-60.
Schultz, T.R., Randall, J.H., Wilson, L.G. and Davis, S.N. (1976). Tracing sewage effluent recharge - Tucson, Arizona. Groundwater, 14: 463-470.
Solomon, D.K., Cook, P.G. and Sanford, W.E. (1998). Dissolved Gases in Subsurface Hydrology. In: C. Kendall and J.J. McDonnell (Eds.), Isotope Tracers in Catchment Hydrology. Elsevier, pp. 291-318.
Thompson, G.M. and Hayes, J.M. (1979). Trichlorofluoromethane in groundwater - a possible tracer and indicator of groundwater age. Water Resour. Res., 15: 546-554.
Thompson, G.M. and Hayes, J.M. and Davis, S.N. (1974). Fluorocarbon tracers in hydrology. Geophys. Res. Lett., 1: 177-180.
Trumbore, S.E., Jacobs, S.S. and Smethie, W.M. (1991). Chlorofluorocarbon evidence for rapid ventilation of the Ross Sea, Deep Sea Res., 38: 845-870.
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