CFC-Derived Recharge Age Calculation
Once the sample has been analyzed and the concentrations of CFC-11, CFC-12 and CFC-113 in the water sample have been determined the recharged age is calculated as follows:
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Using the temperature dependent solubility function and the measured CFC concentration in the water, an "equivalent atmospheric concentration" for each CFC is calculated using the following equation:
CEA = CW/F
Where CW = measured CFC concentration in the water sample, F is the temperature dependent solubility constant, and CEA is the equivalent atmospheric concentration.
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The equivalent atmospheric concentration for each compound is then compared to a plot of atmospheric CFC concentration versus time (see figure 1) to determine the year in which the sample was recharged.
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The derived recharged age for each compound is then calculated by subtracting the year of recharge from the sampling date. The recharge ages derived from each compound are then compared to each other. If no problems are detected (see section D below) the ages derived from each compound are averaged to determine the CFC-derived recharge age.
Recharge Temperature and Other Parameters
Because the solubility of the CFCs is dependent on temperature, an estimate of the recharge temperature (temperature of the water as it enters the aquifer) must be provided. Final CFC eqilibration usually occurs as rainwater is percolating through the unsaturated zone. The temperature of the unsaturated zone, for unsaturated zones less than 30m thick, is usually very close to the mean annual air temperature of the local area. Therefore the mean annual air temperature is usually a good estimate of the recharge temperature, if recharge is occurring locally. If the water sample is taken from a surfacial aquifer the measured water temperature is a good approximation of the recharge temperature. Recharge temperature may also be determined by measuring dissolved helium and neon in the groundwater (Noble Gas Laboratory ) and calculating the temperature based on the helium and neon solubility functions. Please provide an estimate of the recharge temperature when sending samples. Because CFC concentrations in the water sample are reported in picomoles of CFC per kg of water, the salinity (or conductivity) of the water is also needed if sending samples by syringe. Please also provide dissolved oxygen and dissolved hydrogen sulfide concentrations, if these data are available.
Solvent Contaminated Aquifers
If the aquifer being sampled is known to be contaminated with solvents then the CFC concentrations will almost always be supersaturated with respect to the highest atmospheric CFC concentration, or there will be many interfering peaks in the chromatogram which make accurate CFC quantification difficult. Therefore, in most solvent contaminated aquifers, CFCs will not give valid recharge ages. Keep in mind that the maximum CFC-11, CFC-12 or CFC-113 concentration expected, if the atmosphere is the only source of CFCs to the aquifer, is ~ 10,000 times lower than the drinking water standards for these compounds. Therefore, even if drinking water standards are met, the CFCs may still be supersaturated with respect to the highest atmospheric CFC concentration.
Another way to think about this is that the edges of a solvent-contaminated plume of groundwater extend beyond the boundaries set by drinking water standards. If you expect that the groundwater you are sampling is contaminated with solvents, then the 3H/3He method [Noble Gas Laboratory] hyperlink of deriving recharge age may be more appropriate. You will be charged for the analysis even if the CFC concentrations are greater than can be expected from equilibration with the atmosphere.
Potential Problems
CFC-11 is known to be degraded under anoxic conditions, and anoxic groundwaters will frequently have CFC-11 derived recharge ages that are older the CFC-12 derived recharge ages. CFC-113 was commonly used as a solvent in the electronics industry and can be found as a contaminant in some groundwaters. The atmospheric concentrations of CFC-11 and CFC-113 have leveled out or started to decrease in the recent past, which makes their input functions non-unique (i.e. for some CFC-11 and CFC-113 concentrations there will be two possible ages). CFC-12 does not suffer from any of these problems. It is not known to degrade under anoxic conditions. Although the rate of increase in the atmospheric concentration of CFC-12 has decreased in recent years, its concentration it still slowly increasing. Its low boiling point (~ -50oC) precludes its use as a solvent, although CFC-12 contamination of an aquifer is possible. Some possible sources of CFC-12 contamination include, buried refrigerators, buried air conditioners, or heat-pumps which may leak CFC-12 into the groundwater that is commonly used as a heat sink in these devices.
References
Warner, M.J. and R.F. Weiss, Solubilities of chlorofluorocarbons 11 and 12 in water and seawater, Deep-Sea Res., 32, 1485-1497, 1985.
Bu, X. and M.J. Warner, Solubility of chlorofluorocarbon 113 in water and seawater, Deep-Sea Res. I, 42, 1151-1161, 1995.
