Stream Hydrochemical Response to Flood Events in a Multi-Lithological Karstic Catchment from the Pyrenees Mountains (SW France)

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 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

Role of Pond Sediments for Trapping Pesticides in an Agricultural Catchment (Auradé, SW France): Distribution and Controlling Factors

In agricultural areas, ponds are suitable wetland environments to dissipate and reduce the occurrence of pesticides in aquatic environments. However, their impact at a catchment scale is still poorly understood. This study aims to determine how these organic contaminants were trapped in a pond located in an agricultural critical zone from SW France (Auradé catchment). The spatial distribution of pesticide concentrations and their different controlling factors were investigated in waters and sediments collected during two distinct seasons. The results highlighted (i) the link between the presence of the molecules and the agricultural practices upstream, (ii) the influence of hydrological/seasonal conditions, especially on hydrophobic molecule accumulation such as tebuconazole, (iii) the key role of clay content in sediments on the control of moderately hydrophilic pesticides (metolachlor and boscalid), but also the unexpected role of coarse particles for boscalid; and (iv) the influence of sediment depth on pesticide storage. Nevertheless, other physico-chemical parameters, such as mineralogical composition of sediment, needed to be considered to explain the pesticide patterns. This study brings a new hypothesis to be investigated in the future about pesticide behaviour in such pond environments

A Forty-Year Karstic Critical Zone Survey (Baget Catchment, Pyrenees-France): Lithologic and Hydroclimatic Controls on Seasonal and Inter-Annual Variations of StreamWater Chemical Composition, pCO2, and Carbonate Equilibrium

The long-term trends and seasonal patterns of stream water chemical composition in a small remote forested karst catchment, were investigated from 1978 to 2018. Calcium, magnesium, and bicarbonates, the dominant ions, increased over the period together with temperature, while sulfates decreased. Carbonate and sulfate mineral dissolution was the main source of these elements. These trends and the seasonal opposite patterns of discharge vs. temperature, calcite saturation index vs. pCO2 and bicarbonate vs. sulfates, suggested the influence of discharge, of reduced long-range atmospheric pollution, and of increasing air temperature on biological activity and carbonate dissolution. Furthermore, the hydrological regime controlled the seasonal stream water chemical composition and fluxes by: (i) a dilution during the high discharge period, (ii) a change in the contribution rate of the waters draining different lithological areas in the catchment, e.g., the increased sulfates to bicarbonates ratio during summer low flows, with a maximum alkalinity decrease of 24%, and (iii) a “piston” and a “flushing” effects of dissolved elements stored in soils and epikarst with the first autumn heavy rains. Long-term stream water hydrochemical surveys of karst system have proved to be powerful indicators of biogeochemical processes, water sources and pathways under variable natural and anthropogenic environmental pressure conditions

SNO KARST: a French network of observatories for the multidisciplinary study of critical zone processes in karst watersheds and aquifers

Karst aquifers and watersheds represent a major source of drinking water around the world. They are also known as complex and often highly vulnerable hydrosystems due to strong surface groundwater interactions. Improving the understanding of karst functioning is thus a major issue for an efficient management of karst groundwater resources. A comprehensive understanding of the various processes can be achieved only by studying karst systems over a wide range of spatio-temporal scales under different geological, geomorphological, climatic and soil cover settings. The objective of the French Karst National Observatory Service (SNO Karst) is to supply the international scientific community with appropriate data and tools, with the ambition of i) facilitating the collection of long-term observations of hydro-geo-chemical variables in karst, and ii) promoting knowledge-sharing and developing cross-disciplinary research on karst. The present paper provides an overview of the monitoring sites and of collective achievements such as the KarstMod modular modelling platform and the PaPRIKa toolbox. It also presents the research questions addressed within the framework of SNO Karst, along with major research results regarding i) the hydrological response of karst to climate and anthropogenic changes, ii) the influence of karst on geochemical balance of watersheds in the critical zone, and iii) the relationships between the structure and hydrological functioning of karst aquifers and watersheds

Forty years’ fluctuations of streamwater chemistry in the Baget karstic catchment (Ariege, Pyrennees mountains, SW France): impact of atmospheric deposition on carbonate dissolution.

International audienc

Baget catchment: a Critical Zone Observatory of a karstic area in the Pyrenees mountains, South West of France.

International audienc

Stream organic matter exports from a montane peatland (2015-01-01 to 2018-01-01)

High frequency (30 min) data of discharge, fluorescence of the organic matter and turbidity used to calculate respectively the exports of dissolved and particulate organic carbon from a mountainous watershed containing a peatland. Measurements were done both at the inlet and at the outlet of the peatland for three years from 2015, 1st January to 2018, 1st of January. This depository integrates low frequency organic carbon concentration data which were used for optical sensors calibrations and tracking the origin of the organic carbon in the watershed