The Chalk Karst observatory groups different karst sites on the Cretaceous Chalk located at the Paris Basin (Norville, Radicatel, Yport, Saint-Martin-Le-Nœud). These karst watersheds range from 10 to 200 km2 and the land use consists of agriculture and grazing under oceanic climate. There are characterized by chalk plateaus covered with clay-with-flints owing to chalk weathering constituting a fairly impervious layer and with quaternary silts. These surficial formations range from 3 to 20 meters depth and are highly susceptible to crusting, compaction, and erosion, particularly during autumn and winter. A numerous swallow holes locally penetrates the chalk through the above-mentioned impervious layer, resulting in a strong connection of the surface with the aquifer inducing infiltration of turbidity releases at spring and well used to drinking water (up to 500 NTU). These Chalk karst sites are one the sites of the French SO-KARST labellised by INSU-CNRS and are a part of the French RBV-Network and ZA Seine.