The basin project is one of the important steps being taken by CERN to improve its responsible management of water release, notably for the protection of the Lion river.

Following discussions with internal and external stakeholders during tripartite meetings between France, Switzerland and CERN, in the framework of an environmental Memorandum of Cooperation signed in 2007, a hydrological study was carried out on the operating conditions of the rainwater networks and the related surface runoffs on the Prévessin site. This study highlighted the need to build a retention basin in order to prevent potential accidental pollution.

As such, a basin with a volume of approximately 6000 m3 (the equivalent of over two Olympic-size swimming pools), which covers a surface area of around 6000 m2, has been built outside the fenced part of the CERN site, in the commune of Saint-Genis-Pouilly (France) close to Building 869 (BA2). The basin will be entirely enclosed by a fence to prevent access.

The basin will retain, and also under certain conditions regulate, water from heavy rainfall flowing off the CERN Prévessin site into the Lion river. In particular, it will allow the retention of any accidental pollutants, such as hydrocarbons or fire extinguishing foam.