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The Marais Poitevin is a 970 km² marshland in Western France. Two thirds of the marsh (the dry marsh) is used for farming and breeding. One third of the marsh (the wet marsh) is a maze of islets criss-crossed by canals nicknamed The Green Venice. This wet marsh is an important place for biodiversity (birds, fish, plants..). Although the area was declared a Regional Natural Park in 1979, it lost that status in 1997 as intensive agricultural development around the Marsh endangered its natural biodiversity. Cereal and corn cultures need big quantities of water during dry season and the intensive water pumping was lowering down the water level of the marsh and endangering its existence. In order to preserve the agricultural economy of the region and its unique natural environment, it was decided in 2006 to built 10 artificial irrigation reservoirs upstream of the marshland in order to store 3,2 million m³ of water during the wet season when there is an excess of water and use it during the dry season to irrigate the cereals and corn. In parallel, a collective and equitable management of water pumping was established. This 16 million € (Euro) project took place between 2006 and 2010. Nine of those 10 water reservoirs have been made watertight using an EPDM (Ethylene Propylene Diene Terpolymer) geomembrane 1,1 mm (460.000 m²). EPDM geomembrane was chosen for its long durability and fast installation process. The project is too recent to already draw any conclusion but first results are very encouraging as a positive impact on the water table level during the dry season has been measured. This case study presents the use of geosynthetics for biodiversity and agricultural protection and the step by step construction of a water reservoir made watertight with a synthetic liner.