Mean residence times (ages) in subsurface water

The transit time (age) of groundwater can be obtained e.g., by applying so called lumped-parameter models to the time records of environmental tracers measured in the input and in the output of the groundwater system. Those models are based on an assumption that the transit time distribution function (exit age distribution function) of the tracer particles in the investigated system is known. The transit time distribution function describes the whole spectrum of the transit times of a single tracer particles transported between the entrance (recharge area) and the exit (discharge area: a spring, a stream, pumping wells) of the system. The type of the function and the values of its parameter define the model of a system. The main parameter is the mean transit time of tracer through the system, which is commonly assumed to be equal to the mean transit of water molecules. The parameters of the model are found by fitting the model to the experimental records of tracer concentrations measured at the outlet. The found value of the mean transit time serves in combination with some other data for determining other useful parameters, e.g. the recharge rate or the volume of water in the system. The paper presents the transit time distribution functions of some lumped-parameter models, which can be used in practice.