Baseline soil gas measurements as part of a monitoring concept above a projected CO2 injection formation-A case study from Northern Germany

Applying a simplified non-steady state 1D-diffusion leakage model to the data set reveals that CO2 concentration is quite sensitive to additional leakage in this stable zone, both in terms of concentration increase and response time. Monitoring in deeper parts of the soil column is much more sensitive to additional seepage than surface emission measurements. For the study site, we have developed criteria for rating the effectiveness of the monitoring locations based on absolute and variability of the carbon dioxide concentration, the local water level fluctuations and estimated seepage rates. The evaluation of δ13C isotopic composition of background CO2 at deeper measuring intervals shows that δ13C can discriminate leakage even when isotopic composition of injected CO2 is similar to soil CO2.