Does the recharge area of a spring vary from year to year? Information from the water isotopes

Abstract

This study presents the characterization of the recharge zone and the water renewal mechanisms of the Gorgovivo spring, which feeds the aqueduct of the Ancona province and represents one of the most important water resources in central Italy. Measurements of chemical, isotopic and tritium contents were taken to get a detailed understanding of the hydrogeological setting and water circulation features. The isotopic investigation carried out in this study was performed thanks to the presence of different gauging stations for measuring rainfall heights and isotopic contents in precipitations, which allowed us to draw the altitude gradient that correlates the oxygen-18 content in groundwater with the isotopic height of infiltration. This value changed over time (from -0.23‰ to -0.2‰/100 m) indicating the intake altitude for the Gorgovivo spring at a height ranging from 1030 to about 1200 m a.s.l. Tritium contents (about 8-13 TU for shallow springs present in the area, 6-8 TU for the Gorgovivo spring) and the different isotopic compositions confirm the supposed presence of a double pathway in the groundwater: the first one being shallow and very ast - the small springs in the area studied belong to this pathway- while the second is slower and deeper, reaching the bottom part of the basal aquifer. The groundwater recharging the Gorgovivo spring is therefore a combination of these two pathways. Isotopic composition and tritium contents have also allowed the recharge area and recharge mechanisms of this spring to be investigated, by comparing the results with surface water and shallow spring waters. As a result, the present study introduces the likelihood of the recharge area of the Gorgovivo spring extending to more distant places in the Umbria-Marche ridge, thus confirming the suitability of isotopic techniques as a remarkable tool for recharge area evaluation.