Electrolytic enrichment method for the determination of tritium in aquifers: tritium as an indicator of anthropogenic pollution

Abstract

Environmental Tritium is a powerful tracer of groundwater age and it is a key tracer of groundwater movement. Consequently, monitoring of Tritium concentration in groundwater can be a very useful tool for the determination of recent exchanges with surface water and for the presence and the traceability of anthropogenic contributions. Tritium (3H) is a radioactive isotope of hydrogen with a half-life of 12.3 years, which emits low- energy beta particles. It is a naturally occurring radionuclide through the interaction of high-energy cosmic rays with oxygen and nitrogen atoms in the upper atmosphere. The environmental levels of Tritium increased after nuclear weapon tests between 1945 and 1963, and after that it was mainly released from nuclear facilities, especially the heavy water reactor (HWR).However, Tritium concentrations in precipitation have exponentially declined over the past decades as the anthropogenic bomb tritium peak has all but disappeared. At the present, due to the natural decay of tritium and the prohibition of nuclear activities, the activity of tritium in water decreases year by year, and environmental tritium concentrations are close to natural levels of cosmic production. To detect low concentration of tritium in water, a certain degree of enrichment is essential to obtain adequate tritium count rates, through electrolytic system. The principle of electrolytic concentration of tritium is to use the isotope fractionation effect of hydrogen isotopes in the gas and liquid phases. Water samples contain mainly HHO, HDO and some HTO molecules, so by passing an electric current through a conductive water solution, the bonds of the water molecules are broken. Discharge of hydrogen from the cathode is highly isotope-selective for protium, thus, tritium and deuterium are concentrated, leaving most of the tritium in the residual water after reduction of the water sample. One method of 3H electrolytic enrichment was developed and implement in Italy by the ENEA’s Traceability Laboratory (FSN-SICNUC-TNMT – Brasimone research center). The ENEA electrolytic enrichment procedure, which precedes the counting of tritium by liquid scintillation counting (LSC), is performed by an electrolysis system consisting of 20 steel cells, a cooling system, a temperature control unit and three multiple distillation batches for the tritium samples. At present, given the decline in tritium activity in the atmosphere and hydrosphere, low tritium concentrations in groundwater can no longer be neglected, as they can be important indicators of anthropogenic pollutants. A good contribution to the assessment of the vulnerability of aquifers from anthropogenic impact could be offered by the determination of the low tritium in groundwater via the electrolytic enrichment methodology.