Radioactive decay and mixing leads to tritium activity decrease in groundwater. In south-eastern Lithuania, the unconfined layer and precipitation water tritium activity decrease rate is dominated by radioactive decay for the period of 1980–2013. Confined aquifers are vastly affected by mixing; therefore, it is hard to predicted tritium activity values. To achieve it, the monitoring type of sampling must be applied in order to calculate tritium activity gradient value. Moreover, gradient values in different sampling points along the water pathway must be similar. It is a necessary condition when comparing theoretical decay curves with actually calculated. Only the shallowest confined aquifer (ag II–III md-gr) in this study met these conditions. An additional method for modelled data interpretation was tritium peak analysis. According to the modelled tritium peak, a few possible dating water age alternatives exist. The data analysis suggests that no certain water age could be determined. The tritium peak analysis could be a helpful method to gain more comprehensive and valuable hydrogeological information.
