The unsaturated zone of karst aquifers influences the dynamics and the chemistry of water. Because of a lack of direct access, other than via caves, flows in the aquifer matrix and the smallest conduits remain poorly characterized. The few artificial underground structures in the unsaturated karst provide a rare opportunity to study the variety of flow processes. At the low noise underground research laboratory (Laboratoire Souterrain à Bas Bruit, LSBB) in Rustrel (France), 12 variables (temperature, pH, electrical conductivity, alkalinity, major anions and cations, total organic carbon) have been monitored on 12 perennial or temporary flows and leakages over a 10-year period covering contrasting climatic periods. This unique dataset of 1,135 samples has been used to discriminate, identify, and rank the processes associated with the hydrochemical variability of these different types of flows. A principal component analysis and a hierarchical cluster analysis, using mean values and standard deviation of the flow along the principal components, were performed. The results indicate that seasonal variability, mean water residence time, and the depth of acquisition of the chemical characteristics are the main factors of the variability of chemistry at the monitored flow points. Distinguished clusters highlight the great diversity of flows and processes occurring in the fine pathways that may be neighboring the large and structured fractures and conduits. Long-term monitoring with various climatic conditions appears to be a useful tool for assessing this diversity.
The hydrodynamical response at 45 flow points in the gallery of the Low Noise Underground Laboratory-carbonate aquifer (Rustrel, southern France) was monitored in order to determine the relationship between the hydrodynamical functioning of each flow component-slow, intermediate and quick, the depth and the fracturing density. Analysis of the relationship between the distribution of each flow component in function of depth and fracturing density in the karst vadose zone revealed the importance of (1) the variation of flow activation conditions with the depth, (2) the evolution of the flow component distribution with the depth and the fracturing density, and (3) the variability of the vadose zone role in supplying baseflow discharge, in function of carbonate thickness and fracturing state of the study area.
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