Hydrochemical behavior of a multi-lithological karst hydrosystem during flood events in the French Pyrenees Mountains
Résumé
Hydrological changes play a pivotal role in hydrochemical dynamic of karst critical zone.
In this regard, the streamwater hydrochemical composition was monitored at the outlet of the karstic Baget Catchment (BC) at high-resolution during storm events throughout two hydrological years. Using this high-frequency survey, the objective was to identify stream water chemistry responses based on hydrological conditions, streamflow component contributions (quick-response/karst, subsurface/epikarst and baseflow) and biogeochemical processes. The structural and morphological karst characteristics of BC led to an impulsive response to flood events, but was limited by the rainfall intensity and the preceding hydrological conditions. Variations in stream water chemistry were linked to the lithology and to the hydrological conditions over the year. The concentration-discharge (C-Q) relationships for Ca2+ and HCO3-, showed an overall chemostatic behavior throughout the hydrological year. This response reflected the counterbalance exerted by the dissolution of carbonates by biogenic CO2-rich water to the stream water dilution by rainwater. Meanwhile, SO42- concentration exhibited the widest variation and followed a dilution pattern as discharge increases. During typical flood events (Qmax < 4.4 m3.s-1), a general accretion of concentrations or a chemostatic response to discharge increase
was identified, unlike extreme flood episodes (Qmax > 8.3 m3.s-1) with prolonged rainy events that exhibited an overall dilution behavior. For C-Q relationships, two simple indicators were used to characterize the hysteresis patterns and to establish the typology of floods according to the hydrological antecedent conditions and flood characteristics (intensity and shape). The hysteresis of almost all dissolved elements showed clockwise loops or more complex responses. Consequently, the most concentrated waters mainly came from the quick-response flow (karst and surface runoff fraction). For instance, turbidity and dissolved organic carbon (DOC) increased during the hydrograph rising limb and were controlled by surface runoff and through flow processes. Conversely, the epikarst flushing (subsurface flow) under dry conditions resulted in counterclockwise hysteresis loops of Ca2+ and HCO3-. High frequency monitoring combined to multi-geochemical tracers proved to be useful tools to improve the understanding of the hydrochemical responses of karstic catchments to flood events.