AbstractThe Mean Residence Time (MRT) of groundwater is an important factor for the development of hydrogeological concepts influencing the origin, recharge and exchange with the aquifer rocks. In addition, contamination and risk assessment depend strongly on the estimated residence of groundwater.Combinations of different techniques have to be applied to estimate MRTs of groundwater depending on their time span. Recent recharge can be evaluated by the seasonal variation of 18O/2H, 35S and actively introduced tracers. Modern water with a residence time between 1 to 60 years can be measured by 3H/3He, 85Kr, CFCs and SF6. Old waters in the range of hundreds and thousands of years are frequently investigated by 39Ar and 14C. Very old groundwater up to 1.5 Mio. years can be investigated by 14C, 40Ar, 36Cl, 4He and 81Kr.Mean Residence Times of groundwater in different depths of the Vienna Basin is estimated by combinations of 18O/2H, 3H/3He, 85Kr, CFCs, SF6 and 14C evaluating previous hydrogeological concepts and risk assessments.