Karsts Aquitains

Périgord et Pyrénées atlantiques

This region has been of particular interest for decades because of its strategic importance in terms of water resources and because of its exceptional historical background. The valleys of the Vézère (Valley of Man) and the Dordogne host a large number of karst cavities classified as World Heritage Sites to be preserved in perpetuity.

The uniqueness of these sites resides in the possibility of observing various karst compartments; epikarst (Montignac hill, Lascaux); perched karst (site of the Cussac cave), drowned karst (Toulon springs at Perigueux).

In the subsurface, the sites of decorated caves make it possible to:

  • expand our knowledge of the infiltration processes and recharge of underlying aquifers as related to the impact of climate variations,
  • study the mechanisms of CO2 production, processes of storage and transfer to the atmosphere and the vadose zone of karst systems (monitoring of PCO2 in 7 borings and in the caves of Cussac and Lascaux), a) Study CO2 production mechanisms, b) processes of storage and transfer to the atmosphere, and c) processes of storage and transfer to the vadose zones of karst systems
  • study the transfer of organic and inorganic carbon in dissolved form into infiltrating water (measurement of continuous fluorescence, hydrochemical and isotopic monitoring),
  • conduct modeling of transfers into the epikarstic compartment and into perched karst.

The size of the drainage basins studied is on the order of 1 km2, the annual average precipitation is on the order of 800 mm, and discharges range from several m3 per day (Lascaux) to several tens of m3 per day (Cussac). Monitoring of the Cussac basin began in 2000 during preparation of a I2M dissertation in partnership with support from the FEDER fund. Lascaux hill (cave discovered in 1940) has been the subject of particular attention by the DRAC Aquitaine for several decades. Hydrologic measurements have been collected since 1996, were upgraded in 2005, and again in 2014 when the I2M laboratory conducted two thesis projects.

The Toulon springs system in Périgueux is the outlet of a drainage basin whose size is on the order of 80 km2. It is a major concern for the city of Périgueux because it is the city’s only water resource. The operator Suez Environnement has monitored flows at this site since 2005. In 2016 within the framework of a I2M dissertation, equipment was upgraded to continuous monitoring of multiple physico-chemical and hydrochemical parameters (Conductivity, T°C, Turbidity, pH, O2, COT, COD, NO3). These observations are geared toward:

  • studying the vulnerability of the karst resource to inputs including: nitrates, organic carbon, turbidity, and bacteriology,
  • understanding the functioning of the multi-layer karst system in which polyphase karstification has occurred,
  • modeling transfers of pressure and mass in a multilayer karst system.

The observations conducted at Toulon result from a partnership of the City of Périgueux, Suez Environnement, the Water Agency AEAG, the Departmental Council of Dordogne through a project funded by the Région Nouvelle Aquitaine (New Aquitaine Region). These are registered by the will of the partners, including the BRGM, to make it a pilot site of aquifers on the margin of the Aquitaine basin.