If the input concentration of an environmental tracer has been variable, but is well known, concentrations measured in groundwater may be compared directly with the known input concentrations to provide information on the groundwater age. 3H, 36Cl, 85Kr, CFC and SF6 concentrations are commonly interpreted in this way. Of course, this method is effective only over the period during which input concentrations have changed and in which they have been measured or can be modelled reliably. It is therefore generally limited to groundwater that entered the subsurface over the past 100 years. Concentrations of 3H, 36Cl and 14C increased in the atmosphere during the 1950s and 1960s as a result of above-ground nuclear bomb testing and have decreased gradually toward natural levels since then. 85Kr, CFCs and SF6 have been released to the atmosphere from various industrial sources since the middle of the 20th century (Figure 6). CFCs were most useful as a groundwater dating tool until the 1990s, as until that time atmospheric concentrations of these compounds were increasing monotonically, and so a measured concentration in groundwater could be uniquely assigned a groundwater age. However, international agreements to reduce anthropogenic emissions of these compounds have reduced atmospheric concentrations, so that measured concentrations can no longer be uniquely related to a recharge date. For SF6 and 85Kr, however, atmospheric concentrations continue to increase, so these tracers still have good resolution for young water samples. Other atmospheric contaminants are also being explored as potential event markers (Busenberg and Plummer, 2008; Beyer et al., 2017).
One of the advantages of CFCs and SF6 is that their atmospheric concentrations are relatively uniform globally, and so the input concentration is usually known with a high precision. 85Kr shows greater latitudinal variation. 3H and 36Cl show the greatest amount of spatial variation of input concentrations, although the time of peak concentrations was essentially the same at all locations.