The age of groundwater refers to the time that has elapsed since a water particle entered the saturated zone as groundwater recharge. This is also called the residence time of the water particle in the aquifer. The advective travel time is the time required for a particle of water to move from one location to another. Residence time and advective travel time are equal when the starting point for the advective travel time is the point of recharge. The mean age or mean residence time refers to a group of water particles. This might be the mean age of a water sample pumped from a well (all the individual water particles need not have the same age) or the mean residence time of water in the entire aquifer. This is perhaps best understood by analogy to human age, where we can talk about the age of an individual or the mean age of a group of individuals. The mean residence time of water in the aquifer (average age at discharge) is then analogous to the life expectancy of the population (average age at death).
It needs to be recognized that while groundwater age may be estimated using environmental tracers, it is only an estimate because measured tracer concentrations are also affected by other processes. Therefore, some authors prefer to use terms such as apparent groundwater age or piston-flow groundwater age so that this distinction is clear, although these terms are somewhat cumbersome. However, it is useful to refer to tracer ages in general, or for example to CFC-12 ages or 14C ages in particular, to differentiate between the concept of age and the estimate obtained with a specific tracer.
Environmental tracers that can be used for estimating groundwater age can be divided into radioactive isotopes, event markers and radiogenic tracers. The most commonly used of these tracers, and the approximate age ranges that they can be used to estimate are illustrated in Figure 2.