6.6 Discussion of Tracer-Specific Issues
The CFC dating method is conceptually straightforward but depends on assumptions such as conservative geochemical behavior in the subsurface and the atmosphere being the only source for CFCs in groundwater. In addition, the reduction of CFC atmospheric concentration since the late 1990s results in a non‑uniqueness in the age‑concentrations relationship. These issues are briefly discussed in this section.
To function as age‑dating tracers, CFC concentrations in groundwater cannot be altered by biogeochemical processes in the subsurface. Minimal sorption of CFCs has been demonstrated in column experiments consisting of ground Ottawa sand (Ciccioli et al., 1980) and CFCs may be considered stable with respect to microbial degradation under aerobic conditions (Lovely & Woodward, 1992). However, for soils and sediments containing modest fractions of organic carbon (e.g., foc > 0.03 percent) and under reducing conditions, both sorption and microbial degradation can strongly affect CFC concentrations.
In general, CFC‑12 is the least affected by these processes and comparisons of tracer ages derived from the different CFCs have been used to identify biogeochemical processes affecting CFC‑11 and CFC‑113 (Cook et al., 1995). In laboratory studies, for example, Archbold and others (2012) showed that both CFC‑11 and CFC‑113 degraded in the presence of zero‑valent iron with half‑lives of less than one day. In a field study, Cook and others (1995) showed stability of CFC‑12 but almost complete degradation of CFC‑11 and modest sorption of CFC‑113 in a sandy aquifer near Sturgeon Falls, Ontario, Canada. As a general guide, CFC‑11 tends to be most affected by microbial degradation under anerobic conditions whereas CFC‑113 is most affected by sorption onto organic matter in soils and sediments.
Contamination of groundwater samples by sources other than the atmosphere can alter CFC concentrations in groundwater. Although the absolute concentration from other sources is often small, it can result in values that exceed atmospheric solubility and make dating impossible. Contamination sources include seepage from septic tanks, landfills, releases from polyurethane foam waste, leaky sewer lines, leakage from underground storage tanks, and recharge from rivers that are contaminated with CFCs, often from sewage effluents (Cook, Plummer, et al., 2006). Samples for CFC analyses can also become contaminated during collection, often when plastics are used in the collection process. While sampling equipment such as plastic discharge lines may not inherently contain CFCs, their exposure to the modern atmosphere along with slow desorption into groundwater samples can lead to contamination. When measured concentrations exceed atmospheric solubility, it is easy to recognize such contamination but identifying trace amounts of CFCs that simply elevate groundwater concentrations is difficult. Eliminating the possibility of contamination during sampling—using proper materials and abundant flushing—is extremely important.
As shown in Figure 24, atmospheric concentrations of CFCs started declining in the late 1990s to early 2000s, resulting in a non‑uniqueness in the relationship between age and concentration. Although dating waters that recharged since 2000 is problematic with CFCs, it is still possible to date waters that recharged between about 1950 and 2000. In the future, it may be possible to date young waters using the declining limb of the atmospheric curve along with some other tracer and/or hydrogeologic information that identifies the water as being recharged after about 2000.