GNGTS 2013 - Atti del 32° Convegno Nazionale

represented by geothermal waters from the Lignano well, and a low-chloride term, which is consistent with karst-type waters. Waters from the Grado-1 well are in intermediate position, suggesting that also in this case a dilution process likely occurs at depth. However, some elements deviate from these correlations. In particular, the spring and well waters collected at Monfalcone are characterized by sulfate excess with respect to the mixing trend. The sulfate surplus might represent the record of processes occurring at depth. In particular, high sulfate values with increasing temperature are found in thermal waters hosted in carbonate aquifers and are attributed to the reaction of sulfuric acid produced by oxidation of hydrogen sulfide generated by the microbial reduction of sulfates with the calcite of the hosting carbonates, providing that sufficient oxygen becomes available, according to reactions such as: H 2 S+2O 2 =H 2 SO 4 and H 2 SO 4 +2CaCO 3 =2Ca 2+ +SO 4 2- +2HCO 3 - . These reactions also contribute to the creation of secondary porosity. In this scenario, a deeper mixing process of the saline end-member with oxygenated waters would occur, at least in this sector eastern of the carbonatic platform. In water samples from the Roman thermal baths, characterized by an increasing in dilution, the sulfate excess is not observed, possibly reflecting the additional admixing of shallow freshwaters from soils. On the contrary, waters from the Grado-1 well are displaced towards lower sulfate content with respect to the mixing trend, likely reflecting the depletion of SO 4 2- due to reduction processes and sulfide production. Sulfate depletion is even larger in the Lignano well waters, suggesting that in this western sector of the aquifer reductive reactions were enhanced. The Ca ion content in waters from the Monfalcone springs and wells, Grado-1 well and Lignano well also shows an excess in concentration with respect to a binary mixing, still assuming chloride as conservative. The Ca surplus is the highest in Lignano well waters. This is interpretable as the interaction of the waters with Cretaceous limestones of the carbonatic platform which provide a surplus of Ca, hence reflecting diagenetic reactions of the thermal waters with hosting lithologies. The oxygen and hydrogen isotopic data also support the hypothesis of a thermal marine component that progressively mixes with low-salinity waters of meteoric origin; the effects of mixing are negligible or small for the Lignano well waters. This is consistent with the observation that these waters are the closest to the saline end-member. The Sr isotopic composition ( 87 Sr/ 86 Sr ratio) is significantly lower with respect either to the 87 Sr/ 86 Sr ratio of modern seawater standard (MSS, 87 Sr/ 86 Sr = 0.70918) or to the present-day seawater sampled offshore the Friuli coastline ( 87 Sr/ 86 Sr ratio of 0.70917±0.00002). These data indicate that the saline end-member is likely represented by connate fluids. In addition, the Sr-isotope ratios are higher compared with those reported for the Upper Cretaceous limestone (average 87 Sr/ 86 Sr=0.70760). It has also to be noted that in the Monfalcone dilution series, the Sr isotopic composition does not correlate with electrical conductivity, chloride content and, in general, with major and trace ion content. This suggests that the 87 Sr/ 86 Sr ratio is not significantly affected by the mixing of the saline thermal component with freshwaters, as expected on the basis of mass-balance considerations. Thus, the Sr-isotope ratio can be used to infer the pristine isotopic composition of the marine end-member. This has been obtained by mass balance calculations; the obtained results yield an 87 Sr/ 86 Sr ratio ranging between 0.7083 and 0.7088, corresponding to a Miocene “age”. It is hence suggested that the deep saline reservoir in Mesozoic carbonates from the Friuli platform represents the remnant of paleo-seawater, entrapped in the deeper formations during the late Oligocene - Miocene sea transgression, connected with the northward tilting of the ramp anticlines. 271 GNGTS 2013 S essione 1.3