Monitoring of Leakage from CO2 Stores in Shallow Groundwater Aquifers – Lessons Learned from Natural Analogues

In the case of leakage from a deep aquifer storage, ascending saline formation waters and CO2 can mix with fresh waters and react with shallow aquifer rocks. These processes reduce concentrations of chemical species in contaminant plumes and cause overlapping concentration ranges of chemical elements with back ground waters. Some examples are presented from areas where natural CO2 and carbonic water discharge together with fresh groundwater. The main reasons of uncertainty in leakage quantification are: natural variability of background waters, chances of plume detection in thick alluvial aquifers with high groundwater flow rates and the calculation of mass balances from geochemical point data obtained from limited numbers of observation wells. These inherent uncertainties have to be taken into account in the preparation of risk management and monitoring plans for groundwater protection, e.g. the choice of indicative chemical species and the definition of threshold levels for actions to be taken.