Scientific context

Scientific questions

The SNO KARST aims to understand and model the processes that govern the quantity and quality of the water resource in karst environment. The fast circulation of water in the karst Critical Zone in response to extreme meteorological forcing increases the vulnerability of aquifers that are often large and close to cities, densely populated areas, or exceptional historital site. The network is well poised to help understand these responses in the context of global climate change, increasing urban pressure and varying agricultural pressure. The main scientific issues are:

• The charaterization at various scales (local and régional) of the underground hydrodynamics of karst aquifers;
• The understanding of the interactions between surface water and groundwater, especially during extreme floods in the Mediterannean area;
• The determination of the vulnerability of groundwater resource.

Sites and measured variables

The network includes 9 instrumented sites allowing the collection of data on flow, piezometer levels, water quality (major elements, water isotopes, carbon, suspended matter and micro-organisms) and hydro-meteorological parameters. Acquisition frequencies are variable and adapted to the dynamics of the studied systems. High frequency measurements are made possible thanks to the in situ deployment of multiparameter probes. Approach is often spatialized: from borehole to regional aquifer.

The 9 components of the SNO Karst network and their major characteristics are:

Medycyss Observatory

The Multi-scale Observatory of Flood Dynamics and Subterranean Hydrodynamics in Karst Environments consists of a large karstic hydrosystem (Lez-Mosson-Coulazou basin, 1,200 km²) located between the Hérault river, the Vidourle river and the Mediterranean sea (Montpellier region). Hydro-meteorological (15 stations) and hydrodynamic (pressure, temperature, CE, 30 stations) monitoring has been set up since 2008 on boreholes/piezometers, perennial and temporary springs, avens/losing streams, from the regional scale to the local scale of the experimental site (22 boreholes over 0.5 km²). Hydro-geochemical samples are taken occasionally or regularly (bimonthly, flood monitoring): major elements and trace, dissolved organic matter (COT and natural fluorescence), various isotopes (SNO RENOIR), noble and anthropogenic gases, and bacterial community. The Lez aquifer exploited for the water supply of the Montpellier agglomeration (340,000 inhabitants) is particularly monitored with the instrumentation of the Lez spring (continuous monitoring of electrical conductivity, temperature, pH, dissolved O₂, COT, Cl, NO₃, turbidity, natural fluorescence) and the deep borehole of the Triadou (333 m, with continuous pressure and temperature monitoring and sampling at 5 different depths) highlighting the dynamics of the interactions between the aquifer and a temporary watercourse nearby.

Fontaine de VaucluseObservatory

Located about 30 kilometers from the city of Avignon, the Fontaine de Vaucluse is the unique outlet of an exceptional karstic system due to its surface (its catchment area reaches 1,160 km²) and the average thickness of its vadoze zone (an average of 800 m of elevation is necessary for the infiltrated water to reach the drowned zone). As a consequence of its extent, the average interannual flow of the spring reaches 20 m³/s, making it one of the most important in Europe. The water quality (major isotopes and natural fluorescence at the Fontaine de Vaucluse spring) as well as the flow rates and quality of several springs located in the catchment area are observed. This site is linked to the LSBB underground laboratory (SNO H+).

Fontaine de Nîmes Observatory

The catchment of the Fontaine de Nîmes, with an area of about 55 km² ,is located in the Garrigues of Nîmes with a part of the upstream area in an urban context. It includes 8 hydrological and 8 hydro-geological stations and 9 pluviometers. Dissolved and particulate carbon is monitored continuously.

Port Miou Observatory

Port-Miou site integrates a vast regional carbonate aquifer, with a watershed of about 400 km² composed of agricultural areas and scrubland near the city of Marseille and whose outlets are submarine springs. It is a unique observation site for the study of coastal karsts. Flow, salinity and temperature of the main brackish spring are measured, in an in situ underground laboratory located at 500 m from the sea shore, directly in the main karstic pipe accessible by a tunnel. Continuous natural fluorescence and water isotopes are also measured (SNO RENOIR).

Baget Observatory

The Baget watershed is located near the city of Saint-Girons in the Ariège. It is a mid altitude mountain ecosystem (about 1,000 m) drained by a karstic network that feeds the Lachein stream (Lez tributary, Garonne basin). This karstic basin has been the subject of an exceptional monitoring since 1968 at the outlet for hydro-climatic parameters (air temperature, rainfall and stream flow) and since 1978 for physico-chemical parameters (pH, conductivity, alcalinity, major cations and anions, water and carbon isotopes). A probe (since 2014) and an automatic sampler (since 2018) have made it possible to continuously monitor a variety of physico-chemical parameters and to sample floods. Historically, in addition to the main weir, the basin has been subject to hydrological monitoring of 3 full stations, 3 piezometers, including one in the abyss and the other one in the underground loss. At its periphery, the basin is equipped with a meteorological station and 2 collectors (which are also part of the SNO RENOIR) for chemical (major elements) and isotopic (water isotopes) measurements of precipitation.

Aquitaine Karst Observatory

The Aquitaine Karst Observatory gathers different karstic sites located at the edge of the Aquitaine sedimentary basin. The surface area of these watersheds ranges between 0.01 km² (Lascaux cave) and 1 km² (Cussac cave), up to 100 km² (Toulon springs). This region is strategic because of its exceptional archeological heritage. The valleys of the Vézère (Vallée de l’Homme) and the Dordogne have a large number of karstic caves classified as World Heritage of Humanity for long-term preservation. Measurements of CO₂ pressure are performed in 7 boreholes and in the Cussac and Lascaux caves. The analysis of the transfer of dissolved organic and inorganic carbon in the infiltration water is carried out by continuous fluorescence measurements and hydro-chemical and isotopic monitoring.

Jurassic Karst Observatory

The topographic context of the Jurassic Karst Observatory, specific to the Jura Arc, enables the study of karst aquifers at increasing altitudes along a climate and vegetation gradient. The Jurassic Karst Observatory monitors 3 sites: Fourbanne-Fontenotte (330 m), Lods-Epenoy (380 m) and Doubs spring (950 m). Physico-chemical parameters (water levels, conductivity, temperature) are continuously monitored at all sites and are complemented at Fourbanne and Lods by continuous monitoring of organic carbon, nitrates and turbidity (spectro UV-vis probes), as well as by analysis of major elements on two weekly samples by automatic samplers.

Val d’Orléans Observatory

The Val d’Orléans is a wide depression in the main course of the Loire river, 37 km long and 4 to 7 km large. It is a saturated karstic aquifer, within the highly porous Beauce limestone, covered by the quaternary alluvium of the Loire river. The Loire river feeds more than 80% of the water contained in the carbonated karstic aquifer, which flows towards several resurgences of the Loiret river through a saturated karstic network. The observatory monitors the Bouillon spring with high frequency multi-parameter probes and by analysing of major elements in the laboratory in order to follow the evolution of these underground waters in which organic matter degrades, dissolving the carbonates and consuming oxygen until it reachs periods of anoxia in summer.

Craie Observatory

It consists in the observation of the Chalk water table which supplies many cities, including Le Havre. The watersheds studied have a surface area ranging from 10 to 200 km². Several sites allow an almost exhaustive characterization of the different hydrological processes at work in the karstified aquifer of the chalk of the Paris Basin: natural karstic resurgences in Norville and Radicatel; borehole catchments in Norville and Yport; vadose zone in the chalky environment with the underground quarry of Saint Martin le Nœud; recharge zone with flow in a perched aquifer in the cover formations at Bouville. In addition, the Radicatel and Yport sites supply drinking water to almost the entire Le Havre area.

Partners and further information

SNO KARST is a national observation service accredited by the CNRS. It relies on several OSU, BRGM, ONEMA/AFB, urban communities and national parks. It collaborates with several Zones Ateliers (Arc Jurassien, PYGAR Seine), companies and water agencies.

The observations of SNO KARST are coupled with those from other networks that provide complementary measurements and approaches: SNO Tourbières, SNO RENOIR, SNO H+ and SNO HYBAM.

In addition to the academic partners who carry out the monitoring of the sites, the SNO Karst receives the support of the Parc Naturel Régional du Quercy, the Parc Naturel des Grands Causses and the Parc National des Cévennes.