What can we learn about the hydrochemical dynamics of streamwater during flood events in a forested karstic catchment from the Pyrenees Mountains (Southwestern France)?

Hydrological variations hold a significant influence over the water chemistry in the karst systems within the critical zone. In this context, the Baget catchment (BC) was monitored at high-resolution over two hydrological years. The high-frequency survey at the outlet of BC displayed multiple hydrochemical patterns in response to hydrological variations, mixing water sources, and biogeochemical processes. Among the major elements, sulfate (205 ± 105 µeq/L, N=205) exhibited the widest relative variation during flooding and showed a significant dilution, whereas calcium (3.0 ± 0.2 meq/L) and bicarbonate (3.1 ± 0.3 meq/L) revealed a chemostatic behavior as a result of carbonate dissolution in the karst. Nitrate (29 ± 7 µeq/L) and chloride (46 ± 8 µeq/L) concentrations increased only during the rising limb of the hydrograph, suggesting a biogenic and rainfall origin, respectively, and a contribution for nitrates from scarce cow breeding in the upper catchment. Hysteretic analysis evidenced the control of different hydrological reservoirs compartments and antecedent moisture conditions over the stream transport processes of dissolved elements. The concentrations of suspended solids and dissolved organic carbon increased also during the hydrograph rising limb and were controlled by surface runoff and throughflow contributions. The relationships between Ca²⁺ and HCO₃^- concentrations and stream-discharge exhibited hysteresis patterns with counterclockwise loops, unlike all other elements, due to the carbonate weathering by biogenic CO₂-rich water. A slight increase of the dissolved element concentrations during the hydrograph rising limb was observed in the context of successive flood events, because of the epikarst and infiltrated-water contributions. Finally, high-frequency sampling during storm events improved the understanding of the factors controlling the hydrochemical dynamic of the Baget stream water. The relative contributions of the karst and epikarst zones, of rainwater, as well as the role of different biogeochemical processes and the hydrological conditions (past and successive floods), were highlighted.