Assessing the potential consequences of CO2 leakage to freshwater resources: A batch-reaction experiment towards an isotopic tracing tool

Abstract The assessment of the environmental impacts of CO 2 geological storage requires the investigation of potential CO 2 leakages into fresh groundwater, particularly with respect to protected groundwater resources. The geochemical processes and perturbations associated with a CO 2 leak into fresh groundwater could alter groundwater quality: indeed, some of the reacting minerals may contain hazardous constituents, which might be released into groundwater. Since the geochemical reactions may occult direct evidence of intruding CO 2 , it is necessary to characterize these processes and identify possible indirect indicators for monitoring CO 2 intrusion. The present study focuses on open questions: Can changes in water quality provide evidence of CO 2 leakage? Which parameters can be used to assess impact on freshwater aquifers? What is the time scale of water chemistry degradation in the presence of CO 2 ? The results of an experimental approach allow selecting pertinent isotope tracers as possible indirect indicators of CO 2 presence, opening the way to devise an isotopic tracing tool. The study area is located in the Paris Basin (France), which contains deep saline formations identified as targets by French national programs for CO 2 geological storage. The study focuses on the multi-layered Albian fresh water aquifer, confined in the central part of the Paris Basin a major strategic potable groundwater overlying the potential CO 2 storage formations. An experimental approach (batch reactors) was carried out in order to better understand the rock–water–CO 2 interactions with two main objectives. The first was to assess the evolution of the formation water chemistry and mineralogy of the solid phase over time during the interaction. The second concerned the design of an isotopic monitoring program for freshwater resources potentially affected by CO 2 leakage. The main focus was to select suitable environmental isotope tracers to track water rock interaction associated with small quantities of CO 2 leaking into freshwater aquifers. In order to improve knowledge on the Albian aquifer, and to provide representative samples for the experiments, solid and fluid sampling campaigns were performed throughout the Paris Basin. Albian groundwater is anoxic with high concentrations of Fe, a pH around 7 and a mineral content of 0.3 g L −1 . Macroscopic and microscopic solid analyses showed a quartz-rich sand with the presence of illite/smectite, microcline, apatite and glauconite. A water–mineral–CO 2 interaction batch experiment was used to investigate the geochemical evolution of the groundwater and the potential release of hazardous trace elements. It was complemented by a multi-isotope approach including δ 13 C DIC and 87 Sr/ 86 Sr. Here the evolution of the concentrations of major and trace elements and isotopic ratios over batch durations from 1 day to 1 month are discussed. Three types of ion behavior are observed: Type I features Ca, SiO 2 , HCO 3 , F, PO 4 , Na, Al, B, Co, K, Li, Mg, Mn, Ni, Pb, Sr, Zn which increased after initial CO 2 influx. Type II comprises Be and Fe declining at the start of CO 2 injection. Then, type III groups element with no variation during the experiments like Cl and SO 4 . The results of the multi-isotope approach show significant changes in isotopic ratios with time. The contribution of isotope and chemical data helps in understanding geochemical processes involved in the system. The isotopic systems used in this study are potential indirect indicators of CO 2 –water–rock interaction and could serve as monitoring tools of CO 2 leakage into an aquifer overlying deep saline formations used for C sequestration and storage.