A multi-tracer study of groundwater origin and transit-time in the aquifers of the Venice region (Italy)

Noble gas excesses in respect to atmospheric equilibrium occur in all samples, particularly in the deeper aquifers of the Piave river, but also in phreatic water of the undifferentiated aquifers. 3He-3H ages in the phreatic aquifer and in the shallower level of the multi-aquifer system indicate recharge times in the years 1970-2008. The progression of 3H-3He ages with the distance from the recharge areas together with initial tritium concentration (3H + 3Hetrit) imply an infiltration rate of about 1 km/y and the absence of older components in these GW. SF6 and 85Kr data corroborate these conclusions. 3H − 3He ages in the deeper artesian aquifers suggest a dilution process with older, tritium free waters. 14C Fontes-Garnier model ages of the old GW components range from 1 to 12 ka, yielding an apparent GW velocity of about 1-10 m/y. Increase of radiogenic 4He follows the progression of 14C ages. 39Ar, radiogenic 4He and 14C tracers yield model-dependent age-ranges in overall good agreement once diffusion of 14C from aquitards, GW dispersion, lithogenic 39Ar production, and 4He production-rate heterogeneities are taken into account. The rate of radiogenic 4He increase with time, deduced by comparison with 14C model ages, is however very low compared to other studies. Comparison with 14C and 13C data obtained 40 years ago on the same aquifer system shows that exploitation of GW caused a significant loss of the old groundwater reservoir during this time.