Characterization and analysis of groundwater recharge through tritium measurements

The August, 2 2017 Decree, which standardized the health protection requirements from the presence of radioactive substances in the waters intended for human consumption, provides for the obligation to verify the value of two parameters relating to the radioactivity content in drinking water: the concentration of tritium, which must be less than 850 UT (unit of tritium), and the total indicative dose, related to ingestion, which must be less than 0.1 mSv / year.

Tritium atoms produced in the atmosphere, by combination with oxygen in the air, tritiated water (HTO) which, having greater molecular mass than the H2O molecule, has a shorter residence time in the atmosphere from which it tends to be removed with precipitation. The short periods of residence in the atmosphere and the short period of decay mean that the concentrations of tritium in the rains are low and almost constant, as there is a balance between the speed of formation, the removal by the rains and the total quantity of natural tritium in the environment which is ∼ 70 × 106 Ci (US Department of Energy, 2002). This means that in groundwater with a long residence time in subsoil, infiltrated before 1950 (nuclear test period), the concentration of tritium is below the analytical detection limit.

In 1960, rainwater had an abnormal concentration (due to the emission into the atmosphere following nuclear tests) corresponding to an average value of 1000 UT. In the subsoil the decay of tritium produces its continuous loss which, in the absence of rainwater recharge and without the compensation of new atmospheric inputs, causes a decrease. To date, in the absence of any infiltration, this water would contain 35 UT.

Given that waters with over 50 years generally have dilution factors from 10 to 20 times with tritium-free fossil water, today we expect, due to mixing, detectable UT values but lower than 4. Aquifer values greater than 9 UT would therefore be related to recent anthropogenic recharge or surface percolation factors.

In the case of recharge with rain water rich in tritium, the concentration reflects the balance between the loss due to decay and the supply of rain water enriched with tritium. Based on the abundance of tritium, in the absence of sources of anthropogenic contamination it is possible to establish the average age of groundwater under the age of 50. This data is very important, because by analyzing the concentrations of tritium present in groundwater it is possible to trace the age of the aquifer and / or define if the aquifer is polluted by anthropogenic activities.

ENEA's Environmental Traceability and Radiometry Laboratory is at the forefront of the analysis of radioisotopes in the environment, including low-concentration of tritium analyzes. In this regard, an activity with ARPAV of Treviso was started for the determination and evaluation of the concentrations of tritium found in groundwater samples of the area around the city of Treviso.