New methodologies for uranium, radium and tritium groundwater analysis

Abstract

Aim of this thesis was to ascertain whether the uranium behavior, studied at LNGS with respect to the seismic activity in the region, could be related to the Gran Sasso aquifer local dynamics. In order to discern these dynamics and to relate them with the uranium behavior over a time span including intense seismic activity (L’Aquila 2009) and periods preceding and following it, a new approach has been developed of studying together and over time 3H and 226Ra concentrations and 234U/238U isotopic ratio, their specific characteristics enabling different groundwater dynamics features to be investigated. Tritium was chosen because it defines groundwater masses movements and mixing without being affected by water-rock interactions. It was preconcentrated by means of electrolytic enrichment and detected using Liquid Scintillation Spectrometry. 234U/238U and 226Ra instead allow to investigate water-rock interactions and disequilibrium along the 238U decay-chain. They were detected by means of Inductively Coupled Plasma Mass Spectrometry and, in 226Ra case, a specific method for radium separation and preconcentration by means of sequential use of selective resins was developed and optimized. Data analysis of two sampling sites, located on the two sides of the main overthrust fault, highlighted different mean contents and also different patterns over time, thus excluding water mixing between the sides, therefore locally configuring the aquifer as two separate sectors divided by an impermeable septum such as the overthrust fault. Data showed a common flat behavior during pre-seismic, co-seismic and post-seismic periods which means an inhibited mixing with young superficial water. After these periods a resumption of water table movement along with changes occurring in water-rock interaction pattern were observed. These results highly support the hypothesis of groundwater uranium concentration spike-like anomalies, observed before L’Aquila earthquakes in all sites, being related to geodynamical processes and not to the input from the vadose zone through variations of percolation processes linked to meteoric events.