CFC and SF6 Analysis

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 increased until 1993 for CFC-113, 1994 for CFC-11, and 2004 for CFC-12 and decreased since the maximum concentrations were reached. The recent decreasing CFC concentrations have complicated their use as transient tracers because in some cases two ages are possible. SF₆ was first introduced into the atmosphere in the 1950’s and has been increasing in concentration to the present. Therefore by measuring the CFCs and SF₆ a distinct recharge age for groundwater can be determined for samples ranging from 0 to 70 years old.

When water is in contact with the atmosphere (i.e. during recharge) it picks up a CFC or SF₆ 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 and SF₆ 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 and SF₆ tracers offer several advantages to the more commonly used tritium/helium-3 tracer pair. A CFC/SF₆ analysis is less costly than the tritium/helium-3 analysis. CFCs also offer the advantage of essentially real-time data. The typical turnaround time is 1 week, while samples for tritium/helium-3 analysis can take from several weeks to several months for complete analysis. Offsetting some of these advantages are the stringent sampling requirements needed to obtain valid CFC and SF₆-derived recharge ages. For addition information on sampling groundwater for CFC/SF₆ tracer analysis go to: Advice on CFC/SF₆ Sampling.

Water samples are analyzed for CFCs and SF₆ 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.

The CFCs and SF₆ are adsorbed from the purging gas stream onto trap held at -20^o C. Once the CFCs and SF₆ are purged and trapped, the trap is heated to release the CFCs and SF₆ onto a small volume-focusing trap held at -20^o C. When CFC and SF₆ transfer is complete, the focusing trap is heated releasing the CFCs and SF₆ into the gas chromatograph Separation of the CFCs and SF₆ 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 the CFCs are 0.005 x10^-12 moles per kilogram of water (pmol kg^-1). The limit of detection for SF₆ is even more sensitive at 0.05 x 10^-15 moles per kilogram of water (fmol kg^-1). Precision values for all three CFCs are 2 % or less. SF₆ precision is 5% or less. The accuracy of CFC and SF₆-derived recharge ages from these measurements is 2 years or less.