GNGTS 2013 - Atti del 32° Convegno Nazionale

The gases dissolved in groundwaters provide information on the gas-water interactions (GWI) occurring in the subsurface hydrologic systems, in order to distinguish a dissolved atmospheric component (air saturated water, ASW) and gas phases released from deeper sources. The waters from the Monfalcone site, and in particular Grado-1 and Lignano wells, lie far from the typical ASW composition, showing enrichments in both He and CO 2 . Since the 4 He abundance produced by the α decay of the parent uranium and thorium nuclides which are present in rocks and sediments increases with the rock-water-gas contact time, high 4 He concentrations are common in groundwaters characterized by long residence times. The 3 He/ 4 He helium isotope-ratio normalized to the atmosphere (R/Ra) is in the range 0.08-0.27 at Monfalcone, and has values of 0.04 and 0.02 in the waters from the Grado-1 and Lignano wells, respectively. 3 He values below the atmospheric 3 He/ 4 He ratio, with 4 He resulting from mixing of deep-derived and atmospheric-derived helium to various extents support the hypothesis of a significant input of gas from crustal sources, as highlighted by the correlation in the He/Ne vs. R/Ra diagram (Fig. 2), which indicate the mixing between a crustal end- member and a water component equilibrated with the atmosphere. This is mostly evident in the Lignano well waters. The dissolved gas data also indicate that samples are characterized by the interaction with a CO 2 -rich gas phase, in particular at the Grado-1 well, The temperature of the end-member seawater in the reservoir, modeling the evolutionary trends of seawater-carbonate interaction using PHREEQC and the observed correlations, is estimated between 65-70 °C. The various amount of mixing increasing from the Lignano end-member saline water, to the Grado and Monfalcone reservoirs is also consistent with the inverse trend of decreasing temperatures from Lignano-Cesarolo (58-65 °C), to Grado well (42-44 °C) and Monfalcone springs (32-41 °C). Conclusions. The thermal waters hosted in the Mesozoic carbonatic platform of the Friuli Venezia Giulia coastal area have the nature of ancient, diagenetically modified seawater. Sr- isotopes indicate that the deep saline reservoir might represent remnants of seawater entrapped in the Cretaceous carbonate strata during the late Oligocene - Miocene sea transgression which followed the uplift of some portions of the carbonatic units during upper Eocene. The carbonatic aquifer has a complex geometry, is confined and interested by important fault systems which allow the development of hydrothermal cells and some local mixing with the shallower freshwater aquifers. The data indicate that the waters outflowing at Monfalcone and sampled by the deep wells of Grado-1 and Lignano likely fed by the same regional saline reservoir, which underwent interactions with more superficial aquifers at different extent in the different sites. Acknowledgements. Società Imprese Lignano-SpA is acknowledged for allowing waters of the Lignano deep well to be sampled. Fig. 2 – He/Ne vs. R/Ra ratios. Samples lie on a mixing line between ASW and a radiogenic-type end-member of crustal origin (solid line). 272 GNGTS 2013 S essione 1.3