Samples from 19 wells located South and 1 well located North of the spring belt in the Friuli alluvial plain were collected and the radiocarbon isotope content was evaluated. Nineteen wells exploit eight confined aquifers of the low plain and the remaining well exploits the unconfined aquifer of the high plain. Unless three wells located inside the Grado lagoon, age versus depth of the remaining wells exhibits an increase of the age with the depth of the aquifer. The increasing time of residence of the water in the hydrologic circuit may derive from a decreasing permeability or from an increasing of the length of the hydrologic circuit.