Characterization and analysis of groundwater recharge through tritium measurements
Telloli C., Salvi S., Rizzo A., Ubaldini A.
Poster Conferenza Internazionale con referaggio
The August, 2 2017 Decree, which standardized, on the national territory, the health protection requirements from the presence of radioactive substances in the waters intended for human consumption defined by Legislative Decree 28/2016 and updating the Legislative Decree 31/2001, 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 (3H) is an unstable isotope of the hydrogen element which by emission of ß-particles decays into a helium atom (3He). The corresponding half-life is approximately 4500 days (12.3 years). It is an element mainly of cosmogenic origin as it is generated naturally in the atmosphere by the interaction between cosmic ray and nitrogen present in the stratosphere and upper troposphere. However, it also has an anthropogenic origin: from 1952 to 1963, tritium was released into the atmosphere in large quantities following atmospheric thermonuclear tests which introduced non-cosmogenic tritium into the atmosphere. These events changed the geochemical footprint of rainfall with significant increases that stopped after 1963, when an international agreement (CTBT) banned all nuclear tests. Thanks to this, in the last 25 years the equilibrium conditions that control the natural values of tritium was observed. Tritium atoms produced in the atmosphere, by combination with the oxygen present in the air, tritiated water in the form of 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, Annual Report). This means that in groundwater with a long residence time in the subsoil, infiltrated before 1950, 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 recharge of rainwater and therefore 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 (e.g. landfills) it is possible to establish the average age of groundwater under the age of 50 (Clark and Fritz, 1997). 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. The analysis of these phenomena requires a good capacity of tritium analyses in water in concentrations below the limits of concentration of tritium in the waters established by the August, 2 2017 Decree and this is possible only through a tritium enrichment procedure and analysis with low-level liquid scintillation spectrometers. ENEA's Environmental Traceability and Radiometry Laboratory (FSN-SICNUC-TNMT) is at the forefront of the analysis of radioisotopes in the environment, including low-concentration of tritium analyzes. In this regard, following collaborations with the Department of Physics and Earth Sciences of Ferrara, 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. The study of the tritium concentration in groundwater is therefore a very useful tool for the determination of any recent exchanges with surface water or the presence of recent anthropogenic contributions.
REMTECH EXPO: Emerging pollutants and diffuse pollution. Ferrara (Italy)