Abstract
Hydrological variations hold a significant influence over the water chemistry in the karstic critical zone. In this context, the karstic Baget Catchment (BC) has been monitored at a high resolution over two years at the outlet in order to set up a typology of the flood events. The objective was to assess the multiple streamwater physico-chemical patterns in response to hydrological variations, streamflow component (quick-response, subsurface, and baseflow) and lithological contributions, and biogeochemical processes. The karstic catchment exhibited an impulsive response to flood events in relation to the typical structural and morphological characteristics of the karst. In addition, this response was constrained by the magnitude of the rainfall and the preceding hydroclimatic conditions. The variability of the dissolved load in streamflow was closely associated with the characteristics of the weathered rocks and the hydrological conditions throughout the year. Two simple indicators allow to characterize the concentration–discharge relationships with different hysteresis patterns on a set of floods with various intensities and shapes of the hydrograph and under different hydrological conditions before the flood. Almost all elements exhibited either clockwise loops or more complex behaviors, suggesting a higher overall concentration when the major water contribution comes from the quick-response flow (karst and surface runoff fraction). Besides, the epikarst flushing under dry conditions led counterclockwise hysteresis patterns for calcium (Ca2+) and bicarbonate (HCO3−) which revealed an overall chemostatic behavior as a result of carbonate dissolution in the karst. On the contrary, sulfate (SO42−) exhibited the widest relative variation during flooding and showed a significant sensitivity to the dilution process with increasing discharge. For medium flood episodes (Qmax < 4.4 m3·s−1), an overall concentration increase or chemostatic behavior could be observed during the rising limb of the hydrograph. On the opposite, under extreme flood episodes (Qmax > 8.3 m3·s−1) occurring after several rain events, a dilution pattern was noted for all elements originating from rock weathering. Finally, high-frequency sampling during storm events improved the understanding of the factors controlling the hydrochemical dynamic in karstic catchments.
Highlights
The ‘critical zone’ is the heterogeneous support zone of the Earth’s surface, in which occur complex interactions between rocks, soils, water, air, and living organisms
The main objective of this study is to assess in a typical multi-lithological karstic critical zone (K-CZ) catchment, the Baget Catchment (BC), the multiple physico-chemical patterns of streamwater in response to hydrological variation, streamflow components and lithological contributions, and biogeochemical processes
All parameters reacted to different extents to the hydrological variations, as indicated by their coefficient of variation (CV = σ/X)
Summary
The ‘critical zone’ is the heterogeneous support zone of the Earth’s surface, in which occur complex interactions between rocks, soils, water, air, and living organisms. It is located between the lower atmosphere and the deepest layer of aquifers, regulating the natural habitat and determining the availability of resources necessary to sustain life on earth [1,2,3]. Based on the water saturation of the hydrological system, dissolved matter can be stored in the epikarst and/or aquifer and/or transported to the outlet through the piston effect [16,17]. High frequency temporal data series are crucial to better understand and constrain the hydrogeological compartment contributions and biogeochemical processes influencing the variation of the hydrodynamic and hydrochemical responses within the K-CZ
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