Hydrochemistry of Avène thermal waters (Avène-les-Bains, France).

Avène Thermal Spring Water (Avène TSW) has been used since 1743 in the treatment of various pathologies. The hydrothermal system is delimited by a reservoir mainly composed by dolomite and quartz in which meteoric waters infiltrate allowing rock-water interactions that control the water's chemical composition. Water analysis and geochemical calculations highlight that water composition is stable and low mineral salt content are controlled by natural geochemical equilibrium and the solubility of the minerals calcite, dolomite and quartz according to temperature and CO2 pressure within the reservoir. In addition, the annual monitoring of surface waters sampled on the impluvium surface area confirms the absence of anthropic impact on water quality. The forest and the low human activity of the watershed contribute to preserve the natural groundwater quality. This confirms that nowadays, the recharge water is still of great quality for the future generation.

In-depth prospection of Avène Thermal Spring Water reveals an uncommon and stable microbial community.

BACKGROUND: Avène Thermal Spring Water (TSW) exhibits therapeutic properties in the treatment of skin pathologies. Arising from a dolomitic aquifer system, its physico-chemical properties are well-established and its bacteriological quality regularly monitored. The microbiota of this aquifer have been characterized. OBJECTIVES: We aimed to describe the structure of the bacterial community inhabiting the deep aquifer and to examine its dynamics over time. METHODS: The Avène TSW was collected at the catchment point and filtered through 0.1 µm pore size filters. The sampling was carried out every 3 months to generate a 4-year time series. The DNA extracted from filters was analysed using high-throughput 16S rRNA gene amplicon sequencing, and the microorganisms and their contribution were characterized by the taxonomic assignment of sequence variants generated from each sample. RESULTS: Bacteria were distributed into 39 phyla. Nitrospirae and Proteobacteria were the most prevalent, accounting for 38% and 23% of the total community on average, respectively. A stable pattern was observed throughout the study. A few bacterial species were always detected, forming a core community of likely chemolithoautotrophic organisms which might use energy sources and nutrients produced from water-bedrock interactions. Most of the species were distantly related to organisms described to date. CONCLUSIONS: Avène TSW provided by the deep aquifer system harbours a unique microbial community, shaped by the physico-chemical characteristics of the deep environment. Its remarkable stability over time has revealed a high level of confinement of the water resource.

On the controllability of distributed systems.

To "control" a system is to make it behave (hopefully) according to our "wishes," in a way compatible with safety and ethics, at the least possible cost. The systems considered here are distributed-i.e., governed (modeled) by partial differential equations (PDEs) of evolution. Our "wish" is to drive the system in a given time, by an adequate choice of the controls, from a given initial state to a final given state, which is the target. If this can be achieved (respectively, if we can reach any "neighborhood" of the target) the system, with the controls at our disposal, is exactly (respectively, approximately) controllable. A very general (and fuzzy) idea is that the more a system is "unstable" (chaotic, turbulent) the "simplest," or the "cheapest," it is to achieve exact or approximate controllability. When the PDEs are the Navier-Stokes equations, it leads to conjectures, which are presented and explained. Recent results, reported in this expository paper, essentially prove the conjectures in two space dimensions. In three space dimensions, a large number of new questions arise, some new results support (without proving) the conjectures, such as generic controllability and cases of decrease of cost of control when the instability increases. Short comments are made on models arising in climatology, thermoelasticity, non-Newtonian fluids, and molecular chemistry. The Introduction of the paper and the first part of all sections are not technical. Many open questions are mentioned in the text.