Modern groundwater that has recharged aquifers within the past 50 a shows the influence of humans globally, either by the presence of small concentrations of environmental tracers or in some cases by severe pollution. This study describes important environmental tracers (e.g.
85Kr, chlorofluorocarbons, SF
6) and contaminants (e.g. NO
3-, pesticides, chlorinated solvents) for modern groundwater dating and recognition of human impacts. Some applications of the described tracers in aquifers investigated in the PALAEAUX study are presented in order to illustrate the advance of modern waters in European aquifer systems. The study shows that the location of the modern water interface varies within a range of between c. 10 and c. 100 m in the investigated aquifers due to variations in hydrogeological setting, climate and exploitation of the groundwater resource. The subsurface distribution of the modern water indicators and contaminants demonstrate that the advance of modern groundwaters and the fate of harmful substances in them have important implications for protection and development of the water resources. Contaminants that do not degrade or degrade only very slowly will advance further into the aquifers and may eventually contaminate even deep groundwater systems.