Detection of simulated leaks from geologically stored CO 2 with 13 C monitoring 1 2

13 14 Precise methods for the detection of geologically-stored CO2 within and above soil surfaces are 15 an important component of the development of carbon capture and storage (CCS) under 16 terrestrial environments. Although CO2 leaks are not expected in well-chosen and operated 17 storage sites, monitoring is required by legislation and any leakage needs to be quantified under 18 the EU Emissions Trading Directive. The objective of the present research was to test if C 19 stable isotope motoring of soil and canopy atmosphere CO2 increases our detection sensitivity 20 for CCS-CO2 as compared with concentration monitoring only. A CO2 injection experiment 21 was designed to create a horizontal CO2 gradient across 6×3-m plots, which were sown with 22 oats in 2011 and 2012. Injected CO2 was methane derived and had an isotopic signature of 23 46.2‰. The CO2 concentrations were measured within the soil profile with passive samplers 24 and at several heights within the crop canopies. The CO2 fluxes and their C signatures were 25 also measured across the experimental plots. In situ monitoring and gas samples measurements 26 were conducted with a cavity ring down spectrometer (CRDS). The plots displayed hot spots 27 of injected-CO2 leakage clearly detectable by either concentration or isotopic signature 28 measurements. In addition, the C signature measurements allow us to detect injected CO2 in 29 plot regions where its presence could not be unequivocally ascertained based on concentration 30 measurement alone. 31 32