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

Abstract

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 d18O 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 2600 m·a.s.l. The water transit time for the sampled springs ranges from 1.1 to 4.5 yr, with an average value of 1.85 yr and a standard deviation of 0.8 yr. The hydrological system tends to behave as a mixing reservoir.Peer ReviewedPostprint (author's final draft

Global Isotope Hydrogeology―Review

“…Though most groundwaters are widely recognized to be mixtures of waters recharged at different elevations, isotope hydrologists often implicitly assume recharge takes place at a single elevation. Yet groundwaters recharged at high elevations likely mix with groundwaters recharged at lower‐elevations prior to sampling (Dafny et al, ; Jódar et al, ; Matter et al, ). Should such mixing take place, no single recharge elevation can characterize all of the groundwater in a sample.…”

Hydrogeological research in the Ordesa and Monte Perdido National Park (Huesca, Spain)

“…The method used to estimate t and ZR takes profit of the observed variation of both, the amplitude of the seasonal isotopic composition of precipitation Ain(Z) and the mean isotopic content in rainfall along a vertical transect δin(Z). Once and Ain(Z) are characterized, it is possible to estimate the spring altitude recharge ZR, then the corresponding amplitude of the seasonal isotopic content in rainfall at that altitude Ain(Z), and finally the mean transit time t as a function of the amplitude dampening f, which is defined as the ratio between the amplitude of the seasonal isotopic content in the sampled groundwater Aout(Zspring) and Ain(ZR) (Jódar et al, 2016b).…”

“…Estos valores representan aproximadamente el 10%, 20% y 70% de la precipitación media anual. Estos resultados son coherentes con los valores esperables para un acuífero (1) desarrollado en calizas con un importante proceso de karstificación, tanto en superficie como en profundidad, (2) donde las mayores precipitaciones se registran entre los meses de otoño y primavera en forma de nieve porosa y (3) donde la nieve cubre durante esos meses la superficie del PNOMP y controla el funcionamiento hidrológico del sistema, como lo muestra el contenido isotópico del agua en los manantiales de la zona (Lambán et al, 2015;Jódar et al, 2016b).…”

“…Este efecto también se ha observado en las estaciones meteorológicas localizadas a lo largo de un transecto altitudinal a través de los Alpes (Jódar et al, 2016a). En el caso del agua en los manantiales, las diferentes amplitudes están relacionadas con el tiempo de tránsito (Jódar et al, 2016b).…”

“…Las funciones estacionales de entrada y salida de un trazador se pueden describir mediante el uso de una función sinusoidal (DeWalle et al, 1997;Kabeya et al, 2007;Jódar et al, 2016aJódar et al, , 2016b (2) Figura 13. Contenido en δ 18 O observado (círculos) y ajustado (línea) para las muestras de agua procedentes de los pluviómetros (MS#) y manantiales (Site #) muestreados periódicamente en el PNOMP.…”