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

Between 1987 and 1995 more than 100 chemical and isotopic analyses were carried out on the thermal fluids discharged at surface from wells and springs of the Euganean and Berician thermal district. Results for δD and δ18O in waters, δ13C in CO2 and in C1–C4 n-alkanes, δD in CH4, 3He/4He and 40Ar/36Ar ratios in natural gases were coupled with chemical analyses in an attempt to determine the main characteristics and evolutionary trends of thermal fluids emerging in the region. The isotopic and chemical composition of thermal waters has led to the postulation of a meteoric origin of discharged thermal fluids and of a “maturation” trend as water moves from the peripheral manifestations of the Berici Hills towards those of the Battaglia, Montegrotto and Abano springs in the inner part of the geothermal field. Numerical simulation suggested that the observed evolutionary path is consistent with differentiation due to processes of water–rock interaction. The results of bulk analyses have shown that the gases are made up mainly of N2 (65–95 vol%), CO2 (0.5–20.5 vol%) and CH4 (up to 10 vol%), with relatively high Ar and He contents (up to 1.5 vol% and 0.16 vol%, respectively) and detectable amounts of C2–C6 saturated hydrocarbons. The chemical and isotopic composition of the gases suggests that both the meteoric and crustal contributions to the natural discharges are significant, while any significant magmatic contribution, possibly related to vestiges of the volcanic activity that occurred in the Abano area during the Tertiary age, can be ruled out.