Tritium is an isotope of hydrogen, produced in the upper level of the atmosphere. It enters the water cycle after its oxidation and through the precipitation. The measurement of tritium concentration in rainwater is of great importance because it can be used for hydrology investigations such as the recharge mode or the vulnerability of aquifers.

For this purpose, rainwater samples were collected for a period of 16 months from October 2002 to January 2004 from a sampling station at Ioannina, northwestern Greece. Each sample was filtered and measured without any further treatment for tritium activity, using a super low-level background, liquid scintillation analyzer. Five mL of the sample were mixed with 15 mL of scintillation cocktail, specially designed for low-level tritium measurements and its beta activity was measured for 500 min.

Results show that during the sampling period, the tritium concentration varied up to two fold, increasing during the spring and summer months. The measured tritium concentrations in rainwater, which ranged from 9 ± 4 to 23 ± 5 TU, can be used for local hydrology studies.