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Chlorofluorocarbon (CFC) Analysis

The technique of using CFC's as transient tracers to derive estimates of seawater ventilation ages is well established and the use of these tracers to derive estimates of groundwater recharge age is becoming increasingly common. CFCs are potentially useful tracers of groundwater flow because they are non-reactive and their input history is well known. CFC-12 (CCl₂F₂) and CFC-11 (CCl₃F) were introduced into the atmosphere in the late 1940's while CFC-113 (CCl₂FCClF₂) was first released to the atmosphere in the 1960’s. The atmospheric concentrations of these CFCs have been increasing ever since, although CFC-11 and CFC-113 concentrations have leveled off or decreased over the last 5 years (Figure 1).

When water is in contact with the atmosphere (i.e. during recharge) it picks up a CFC signature based on the atmospheric concentration and the temperature dependent solubility coefficient of each compound. When isolated from the atmosphere the groundwater retains its characteristic CFC concentration because the compounds are non-reactive (conservative) under aerobic conditions, and relatively little mixing occurs in ground water systems (compared to the ocean). The CFC tracers offer several advantages to the more commonly used tritium/helium-3 tracer pair. CFC-11 and CFC-12 can be used to date groundwater up to 50 years old, while the tritium/helium-3 tracer pair is limited to less than 40 years. CFCs also offer the advantage of essentially real-time data. After receipt of a sample, the typical analysis time is 1 day, while samples for tritium/helium-3 analysis can take from 10 days to several months for complete analysis. Offsetting some of these advantages are the stringent sampling requirements needed to obtain valid CFC-derived recharge ages. For addition information on sampling groundwater for CFC tracer analysis go to: Advice on CFC Sampling.

Water samples are analyzed for CFCs using a custom built purge-and-trap gas chromatograph with electron capture detection. Briefly, water samples are purged with inert gas to remove dissolved gases.

Purge-and-trap gas chromatograph

The CFCs are adsorbed from the purging gas stream onto trap held at -10^o C. Once the CFCs are purged and trapped, the trap is heated to release the CFCs onto a small volume-focusing trap held at -15^o C. When CFC transfer is complete, the focusing trap is heated releasing the CFC's into the gas chromatograph Separation of the CFCs is achieved on a capillary column and the compounds are detected using an electron capture detector. This method is extremely sensitive and the limit of detection for CFC-11, CFC-12 and CFC-113 is 0.007, 0.01, and 0.01 x10-12 moles per kilogram of water (pmol kg-1), respectively. Precision values for all three CFCs are 2 % or less. The accuracy of CFC-derived recharge ages from these measurements is 3 years or less.