Harmonic analysis and isotopic investigation for recharge area characterization of the Promise Spring (Aosta Valley, NW Italy)

The response of alpine aquifer systems to changing climatic conditions is dependent on the specific geographical and geo-logical context. Consequently, understanding fluctuations in hydrogeological balances at the scale of spring catchments has become increasingly crucial for anticipating future scenarios related to water availability. This study focuses on Promise Spring in the Aosta Valley region (NW Italy) between October 2011 and July 2024 and employs innovative methodologies, including Fourier transform analysis, to characterize spring hydrograph signals and their relationships with atmospheric temperature, snow depth, and rainfall data. In addition, isotopic analyses of water samples were conducted to gain a bet-ter understanding of the origin of the water that feeds the spring. The analysis of the hydrodynamic behavior of the spring revealed a clear correlation between the environmental variables and their temporal variations. The main discharge peaks were associated with the completion of the snowmelt process between April and May, indicating that snowmelt was one of the primary water sources that fed the spring. Recent increases in discharge and the temporal shift of the recession curve minima toward the autumn and winter months are attributable to altered meteorological conditions that have modified snow accumulation regimes at higher altitudes in the catchment area. Isotopic analyses revealed that the Promise Spring exhibited an isotopic composition that is indicative of the absence of glacial paleowater contributions and implies a primarily meteoric origin derived from snowmelt and precipitation.