Isotopic content in high mountain karst aquifers as a proxy for climate change impact in Mediterranean zones: The Port del Comte karst aquifer (SE Pyrenees, Catalonia, Spain).

The objective of this work is to characterize the impact of climate change in the karst aquifer of the Port del Comte Massif (PCM). Six regional climate models (RCMs) from CLYM'PY Project are used to analyse the magnitude and trends of changes on precipitation and temperature (RCP4.5 and RCP8.5 scenarios) and how these changes propagate through the hydrogeological system as groundwater resources availability and the associated water isotopic content. The study uses the RCMs climate change forcings as input data to a combination of (1) a semi-distributed hydrological model for simulating the hydrodynamical response of the aquifer, and (2) a lumped parameter model for simulating the isotopic content in groundwater at the outlet of the aquifer. A mean decrease of 2.6% and 1.9% in yearly precipitation and a mean increase of 1.9 and 3.1 °C in average temperature is expected in PCM at the end of the 21st century in the RCP4.5 and RCP8.5 scenarios, respectively. This climate signal entering the hydrogeological system results in a mean decrease in recharge of 3.9% and 0.5% from rainfall and of 59.3% and 76.1% from snowmelt, and a decrease of 7.6% and 4.5% in total system discharge, but also generates an isotopic enrichment in groundwater discharge (δ18OGW) of 0.50‰ and 0.84‰, respectively. Moreover, from a long-term (2010-2100) perspective, the mean trend in δ18OGW is 0.7‰/100 yr and 1.2‰/100 yr for RCP4.5 and RCP8.5, respectively, resulting in easily measurable annual lapse rates with the current analytical methods.

Evaluation of natural background levels of high mountain karst aquifers in complex hydrogeological settings. A Gaussian mixture model approach in the Port del Comte (SE, Pyrenees) case study.

The hydrogeological processes driving the hydrochemical composition of groundwater in the alpine pristine aquifer system of the Port del Comte Massif (PCM) are characterized through the multivariate statistical techniques Principal Component Analysis (PCA) and Gaussian Mixture Models (GMM) in the framework of Compositional Data (CoDa) analysis. Also, the groundwater Natural Background Levels (NBLs) for NO3 and SO4 and Cl are evaluated, which are specially important for indicating the occurrence of groundwater contamination derived from the anthropic activities conducted in the PCM. The different hydrogeochemical facies found in the aquifer system of the PCM comprises low mineralized Ca-HCO3 water for the main Eocene karst aquifer, and Ca-SO4 and highly mineralized NaCl water types in the minor aquifers discharging from the PCM. The NBL values of SO4, Cl and NO3 obtained for the main karst aquifer are 14.33, 4.06 and 6.55 mg/L, respectively. These values are 35, 3 and 1.2 times lower than the respective official NBLs values that were determined by the water administration to be compared with in the case of conducting a pollution assessment characterization in the main karst aquifer. Official overestimation of NBLs can put important groundwater resources in the PCM at risk.

Contribution of isotopic research techniques to characterize high-mountain-Mediterranean karst aquifers: The Port del Comte (Eastern Pyrenees) aquifer.

Water resources in high mountain karst aquifers are usually characterized by high rainfall, recharge and discharge that lead to the sustainability of the downstream ecosystems. Nevertheless, these hydrological systems are vulnerable to the global change impact. The mean transit time (MTT) is a key parameter to describe the behavior of these hydrologic systems and also to assess their vulnerability. This work is focused on estimating MTT by using environmental tracers in the framework of high-mountain karst systems with a very thick unsaturated zone (USZ). To this end, it is adapted to alpine zones a methodology that combines a semi-distributed rainfall-runoff model to estimate recharge time series, and a lumped-parameter model to obtain ΜΤΤ. The methodology has been applied to the Port del Comte Massif (PCM) hydrological system (Southeastern Pyrenees, NE Spain), a karst aquifer system with an overlying 1000 m thick USZ. Six catchment areas corresponding to most important springs of the system are considered. The obtained results show that hydrologically the behavior of the system can be described by an exponential flow model (EM), with MTT ranging between 1.9 and 2.9 years. These ΜΤΤ values are shorter than those obtained by considering a constant recharge rate along time, which is the easiest and most applied aquifer recharge hypothesis when estimating ΜΤΤ through lumped-parameter models.