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