On some methodological problems in the use of environmental tracers to estimate hydrogeologic parameters and to calibrate flow and transport models

Environmental tracers are used qualitatively for a better formulation of conceptual models and quantitatively for assessing groundwater ages with the aid of box models or for calibrating numerical transport models. Unfortunately, tracers often yield different ages that do not represent uniquely the water ages. Difficulties result also from different definitions of age, e.g. water age, advective age, tracer age, or radiometric tracer age, that are measured differently and depend on aquifer parameters and characteristics of particular tracers. Even the movement of an ideal tracer can be delayed with respect to the advective movement of water due to diffusion exchange between mobile and immobile water zones, which for fissured rocks or thin aquifers, may lead to significant differences between advective and tracer ages, i.e. also between advective and tracer velocities. The advective velocity is of importance in water resources considerations as being related to Darcy velocity, whereas the tracer velocity is a more useful term for the prediction of pollutant transport. When a groundwater system changes from one hydrodynamic steady state to another, environmental tracers need much more time to reach a new steady state. Several tracer studies are recalled as examples of tracer-specific effects on the estimations of groundwater age.