France is committed to reducing its greenhouse gas emissions, which in particular includes the transposition of the European Directive on the geological storage of CO2. Indeed, groundwater protection is not explicitly taken into account in the European Directive on CO2 storage, although it is indirectly covered by the global philosophy of the text summarized in Article 1.2: “prevent and, where this is not possible, eliminate as far as possible negative effects and any risks to the environment and human health”. The main actions mentioned in the text in relation to groundwater protection are data collection and monitoring. In this context, the CIPRES project focuses on the characterization of potential impacts of CO2 leakage on groundwater quality. The first objective is to characterize the biogeochemical mechanisms that may impair the quality of groundwater resources, especially in deep aquifers, such as the Albian in the Paris Basin in France, as yet little explored. The microbial composition of such deep aquifers and their role in controlling water quality is unknown. Another aspect already highlighted by previous studies is the role of sorption-desorption processes on the mobility of trace elements. The presence of glauconite in the Albian sands may play a role in the mobility of trace elements in the Albian aquifer. The second objective, in view of the fact that future groundwater monitoring will be confronted with several issues, is to validate a monitoring methodology. The thresholds values of the parameters to be monitored will be validated in a natural context during an experimental CO2 leakage and our methods will be tested by equipping a monitoring observation well in a deep aquifer, the Albian aquifer (0 to 1000 m deep). The major parameters to be monitored in the case of CO2 leakage are pH, alkalinity and dissolved CO2. In deep conditions, pH sensors are not stable, CO2 sensors are not validated and measuring alkalinity requires particular sampling conditions to avoid degasing before analysis. To reach the abovementioned objectives, this project proposes three complementary study contexts: (i) the laboratory for the characterization of biochemical and geochemical (sorption / desorption) processes that may impact water quality, and experiments carried out on Albian samples (ii) an experimental site to validate the monitoring methodology by experimental CO2 leakage in groundwater and to characterize the in situ mechanisms having an impact on water quality and (iii) a deep well to apply deep monitoring methodology for deep aquifers. The characterization of mechanisms in the laboratory and in situ will be based partly on the acquisition of experimental data and partly on the calibration of numerical models that take into account the diffusion of CO2 into the environment (unsaturated zone and aquifer) and reactive transport. This numerical calibration is designed to reinforce the numerical modelling work carried out for predictive purposes during the site characterisation, impact studies and design of monitoring networks. One outcome of the project will be to integrate the results in order to propose recommendations for (i) taking into account the impacts on groundwater in the future characterization of geological storage sites, (ii) defining the mechanisms to be considered in the studies to qualify and quantify the impacts on groundwater quality, and (iii) setting up groundwater monitoring networks.