Vertical variation in the amplitude of the seasonal isotopic content of rainfall as a tool to jointly estimate the groundwater recharge zone and transit times in the Ordesa and Monte Perdido National Park aquifer system, north-eastern Spain

The time series of stable water isotope composition relative to meteorological stations and springs located in the high mountainous zone of the Ordesa and Monte Perdido National Park are analyzed in order to study how the seasonal isotopic content of precipitation propagates through the hydrogeological system in terms of the aquifer recharge zone elevation and transit time. The amplitude of the seasonal isotopic composition of precipitation and the mean isotopic content in rainfall vary along a vertical transect, with altitudinal slopes for δ18O of 0.9‰/km for seasonal amplitude and −2.2‰/km for isotopic content. The main recharge zone elevation for the sampled springs is between 1950 and 2600m·a.s.l. The water transit time for the sampled springs ranges from 1.1 to 4.5yr, with an average value of 1.85yr and a standard deviation of 0.8yr. The hydrological system tends to behave as a mixing reservoir. [Display omitted] •Amplitude of seasonal isotopic composition of rainfall depends on elevation.•Moisture generating rainfall at the PNOMP comes from a main source.•The flow system tends to a better mixing the longer the system transit time is.•The aquifer recharge zones located at elevations between 2600 and 1950ma.s.l.•Short transit times in agreement with the karstic nature of the aquifer system