Water and gas geochemistry of the Euganean and Berician thermal district (Italy)

Between 1987 and 1995 more than 100 chemical and isotopic analyses were car ried out on the thermal fluids discharged at surface from wells and springs of the Euganean and Berician thermal district. Results for delta D and del ta(18)O in waters, delta(13)C in CO2 and in C-1-C-4 n-alkanes, delta D in C H4, He-3/He-4 and Ar-40/Ar-36 ratios in natural gases were coupled with che mical analyses in an attempt to determine the main characteristics and evol utionary trends of thermal fluids emerging in the region. The isotopic and chemical composition of thermal waters has led to the postulation of a mete oric origin of discharged thermal fluids and of a "maturation" trend as wat er moves from the peripheral manifestations of the Berici Hills towards tho se of the Battaglia, Montegrotto and Abano springs in the inner part of the geothermal field. Numerical simulation suggested that the observed evoluti onary path is consistent with differentiation due to processes of water-roc k interaction. The results of bulk analyses have shown that the gases are made up mainly o f N-2 (65-95 vol%), CO2 (0.5-20.5 vol%) and CH4 (up to 10 vol%), with relat ively high Ar and He contents (up to 1.5 vol% and 0.16 vol%, respectively) and detectable amounts of C-2-C-6 saturated hydrocarbons. The chemical and isotopic composition of the gases suggests that both the meteoric and crust al contributions to the natural discharges are significant, while any signi ficant magmatic contribution, possibly related to vestiges of the volcanic activity that occurred in the Abano area during the Tertiary age, can be ru led out. (C) 2000 Elsevier Science Ltd. All rights reserved.