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 the efficient management of karst groundwater resources. A comprehensive understanding of the various processes can be achieved only by studying karst systems across a wide range of spatiotemporal 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 hydrogeochemical variables in karst, and (ii) promoting knowledge sharing and developing cross-disciplinary research on karst. This paper provides an overview of the monitoring sites and collective achievements, such as the KarstMod modular modeling platform and the PaPRIKa toolbox, of SNO KARST. It also presents the research questions addressed within the framework of this network, 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.Abbreviations: CADI, cellular automata-based deterministic inversion; Ex/Em, excitation/emission; NOM, natural organic matter; SLP, sea level pressure; SNO Karst, the French Karst National Observatory Service.Karstified carbonate formations contain 25% of the world's water resources. They cover a very large extent of the continental surface: 10% of the global continental surface, 30 to 70% of the Mediterranean area, 22% of the land in Europe, and 50% in France (Chen et al., 2017). In carbonate karst hydrosystems, the presence of fractures, conduits, and surface solution features leads to strong surface-subsurface interactions that result in significant water, mass, energy, and contaminant transport within the critical zone. Such heterogeneous systems are highly dynamic, with complex hydrologic, geochemical, and biological processes occurring across a wide range of spatiotemporal scales. As a result, they Core Ideas • SNO KARST is dedicated to the study of karst functioning. • Hydrodynamics and geochemistry are measured at springs and in karst compartments. • Process sampling was set up at nine sites in various climatic contexts.
International audiencePurposeThe accumulation of carbon in peatlands originates from the slow rate of Sphagnum litter decomposition. Vegetation shifts can alter the Sphagnum decomposition rate through a litter mixture effect. This is rarely studied in peatlands. In a site colonised by vascular plants, we examined the effect of mixing litters of Sphagnum species with those of Molinia caerulea and Betula spp. on litter carbon (C) dynamics. We tested that water content and pH may explain the potential non-additive litter mixing effect.Materials and methodsLitter bags with Sphagnum cuspidatum or Sphagnum rubellum and M. caerulea or Betula spp. were placed in situ, in a Sphagnum decomposing environment and retrieved after 1 year of incubation. In the laboratory, the specific interaction between S. rubellum and M. caerulea was investigated. Solid, soluble and gaseous forms of C were studied in addition to the fluorescence of the dissolved organic matter (FDOM).Results and discussionLaboratory and field experiments showed that there is a non-additive effect of mixing S. rubellum and M. caerulea litter on C dynamics. The analyses of FDOM suggested a relatively higher living biomass in the laboratory-measured mixture than in the expected one. The in situ S. rubellum moist environment could stimulate the decomposition of M. caerulea that experiences much drier conditions in its native environment. In the laboratory experiment, M. caerulea were kept moist, and no significant difference in water content between the measured and expected mixture was found. Also, pH decreased in the measured mixture, ruling out any direct effect on microbial activity.ConclusionsThe non-additive mixture effect observed in the laboratory may be triggered by an increase of the microbial biomass. This increase was not explained by direct moisture or a pH effect. The suggested hypothesis that a lower pH could affect the availability of labile organic substrate through increased organic matter (OM) hydrolysis and thus stimulate microbial growth has to be further studied
Massive underground storage of hydrogen could be a way that excess energy is produced in the future, provided that the risks of leakage of this highly flammable gas are managed. The ROSTOCK-H research project plans to simulate a sudden hydrogen leak into an aquifer and to design suitable monitoring, by injecting dissolved hydrogen in the saturated zone of an experimental site. Prior to this, an injection test of tracers and helium-saturated water was carried out to validate the future protocol related to hydrogen. Helium exhibits a comparable physical behavior but is a non-flammable gas which is preferable for a protocol optimization test. The main questions covered the gas saturation conditions of the water, the injection protocol of 5 m3 of gas saturated water, and the monitoring protocol. Due to the low solubility of both helium and hydrogen, it appears that plume dilution will be more important further than 20 m downstream of the injection well and that monitoring must be done close to the well. In the piezometer located 5 m downstream the injection well, the plume peak is intended to arrive about 1 h after injection with a concentration around 1.5 mg·L−1. Taking these results into account should make it possible to complete the next injection of hydrogen.
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