Comparison of Flow Processes in Drains and Low Permeability Volumes of a Karst System in the French Jura Mountains: High-Resolution Hydrochemical Characterization During a Flood Event

Abstract

The Fertans karst system is a field site of the hydrogeological observatory “Jurassic Karst” in the French Jura Mountains. The site is located within karstified upper Oxfordian limestones. Two monitoring stations were installed for simultaneous monitoring of a spring, draining a fracture, and a borehole within the microfractured limestone giving access to the low permeability volume (LPV). A flood event was followed in June 2012, with high-frequency physicochemical (pressure, T°C, E.C) and hydrochemical measurements (major and trace elements, total organic carbon) at both stations. Infiltration (TOC, Fe, Al) and long-time residence markers (Mg) were studied in order (1) to analyze the response of the drain (spring SA) and the LPV (borehole C3) to the rainfall event, (2) to identify the origin of the flood waters, and (3) to characterize water exchange between the 2 compartments. Based on statistical analysis (PCA, DFA), it was possible to characterize different mixing end-members and to identify exchanges between drains and LPV. The hydrochemical signal of the flood was observed rapidly at the spring (3 h); in contrast, the borehole showed a much higher inertia and an arrival of infiltration water in two steps: a first peak attributed to drain contribution, and a second peak attributed to diffusively infiltrated rain water, spreading out slowly through the LPV. The infiltration water signature was typically enriched in organic carbon, Fe, and Al derived from interaction with the clay–humus complex of the surface soil. Nitrates which are often used as indicator for direct surface infiltration seemed to have accumulated over longer time periods prior to the flood in the LPV, which behaved like a diffusive nitrate source.