Use and application of CFC-11, CFC-12, CFC-113 and SF₆ as environmental tracers of groundwater residence time: A review

Groundwater residence time is a fundamental property of groundwater to understand important hydrogeological issues, such as deriving sustainable abstraction volumes, or, the evolution of groundwater quality. The anthropogenic trace gases chlorofluorocarbons (CFC-11, CFC-12 and CFC-113) and sulphur hexafluoride (SF₆) are ideal in this regard because they have been released globally at known rates and become dissolved in groundwater following Henry's Law, integrating over large spatial (global) and temporal (decades) scales. The CFCs and SF₆ are able to date groundwater up to ~100 years old with the caveat of certain simplifying assumptions. However, the inversion of environmental tracer concentrations (CFCs and SF₆) to derive groundwater age rests on the accurate determination of groundwater recharge parameters, namely temperature, elevation, salinity and excess air, in addition to resolving the potential for contamination, degradation and unsaturated zone effects. This review explores the fundamentals of CFC-11, CFC-12, CFC-113 and SF₆ as environmental tracers of groundwater age and recommends complementary techniques throughout. Once this relatively simple and inexpensive technique has been used to determine initial concentrations at the recharge zone, setting the groundwater dating ‘clock’ to zero, this review then explores the meaning of groundwater ‘age’ in relation to measured environmental tracer concentrations. It is shown that the CFCs and SF₆ may be applied to a wide-range of hydrogeological problems and suggests that environmental tracers are particularly powerful tools when integrated with numerical flow and transport models.