What may be the consequences of a CO 2 leakage? Insights from soil gas measurements in an urban area – Clermont-Ferrand, French Massif Central

Abstract : This project is aimed to better constrain the geographical distribution of CO 2 geological emanations in an urban context and to suggest recommendations for risk management. Natural emanations may have some similarities with emissions from an anthropogenic deep storage site that experiences unexpected gas releases. Such gas emanations may generate hazards to which local authorities have to face, especially those concerning human exposure. In 2015, the Regional Bureau of Environment, Physical Planning and Housing from the Auvergne District (DREAL) and the Clermont-Ferrand Urban Community (CLERMONT-CO) have co-financed site investigations to better characterize gas emanations over selected territories (7 cities) and to establish guidelines for the management of associated risks. Work has been done under the coordination of the Regional Prefecture. Located in the Northern part of the French Massif Central, the Clermont-Ferrand city is a populated city (c.a. 140 000 inhabitants) that is located mostly over quaternary volcanic rocks belonging to a former maar structure. To the East, volcanic rocks are progressively replaced by continental formations belonging to the Tertiary graben of Limagne. The limit between sedimentary domain and basement/volcanic domain is underlined by deep-rooted normal faults along which several well-known mineral springs are emplaced.