Using GIS for modelling the impact of current climate trend on the recharge area of the S. Susanna spring (Central Apennines, Italy)

Though the S. Susanna spring system is one of the biggest water sources in the central Apennines, its hydrogeological dynamics have been scarcely investigated. This study tried to clarify some of the factors controlling the recharge/discharge processes of this spring by modelling the available climate series, water balance equations and new isotopic and quantitative data, using statistical and raster overlay functions embedded in a Geographic Information System (GIS). Oxygen and hydrogen isotopic data were recorded monthly over a 2-year period at the spring itself and in eight rain gauges in Reatini Mountains. The effective infiltration rate was calculated using the Kennessey coefficients and the Turc equation. Finally, the recharge area was identified with the help of an expert evaluation procedure. Local d18O and dD versus altitude regression curves were used to validate the digital recharge model by comparing their expected values with the values actually measured. Recharge process was framed within the perspective of the ongoing local climate trends. The current discharge rate of 4.1 m3/s1 is significantly lower than the average value of 5.5 m3/s1 measured up to the 1980s, confirming the fall in the recharge/discharge rate. The hydrogeological system shows a delayed response, due to an average groundwater residence time in the aquifer, which is estimated to be about 15/20 years on the basis of the offset between calculated and observed isotope data at the main spring. For this reason the system is presently not equilibrated and is gradually changing towards a final equilibrium discharge estimated in about 3.4 m3/s.