CO2 intrusion in freshwater aquifers: Review of geochemical tracers and monitoring tools, classical uses and innovative approaches

•Review of applied geochemical methods for monitoring CO2 leak in fresh groundwater.•Application of multi-isotope approach for a powerful monitoring strategy.•Deciphering of chemical and isotopic signal associated with CO2 presence.•New perspectives on isotopic systematic not yet applied in the context of CCS.

This review of applied geochemical methods for monitoring CO2 leakage focuses on shallow freshwater aquifers overlying CO2 storage areas. Geochemical tracer tools form a set of geochemical techniques, each of which is examined in this review and classified according to its sensitivity in CO2 detection. The purpose of any monitoring programme is to be able to provide sufficient information to enable site remediation in the case of unforeseen events and also to enable a satisfactory site closure strategy. Therefore, CO2 monitoring tools must be able to detect a precursor signal or an early warning signal of leakage associated with potentially minute geochemical modifications (i.e. associated with small amounts of CO2).We have classified the monitoring/tracing tools into two groups: (A) direct indicators of the CO2 itself, and (B) indirect indicators (i.e. reaction products) of the presence of CO2 that take into account displacement of the chemical equilibria under the conditions imposed by CO2 dissolution. Included in these tools are isotopic monitoring tools that are very sensitive to physico-chemical changes and can therefore provide early CO2 detection. These tools include carbon and oxygen isotope systematics that are conventionally used with respect to CO2 in the Carbon Capture and Storage (CCS) context. Finally, the review offers new perspectives on sensitive indirect detection methods using isotopes that are ‘non-traditional’ in the sense that they have not yet been applied to the field of CO2 geological storage. The complementarity of these geochemical methods provides a powerful monitoring strategy.