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

Abstract Detection and quantification of different gas species in the vadose zone of soils and/or in the atmospheric boundary layer is a key method in many fields of environmental research and also considered as one element of monitoring related to carbon storage operations. We have developed a robust and economic continuous soil gas monitoring system, which requires little maintenance and includes an automatic data transfer. We report on one of the largest continuous baseline data sets ever recorded. They were acquired during a long-term (up to 4 years) continuous soil gas monitoring program completed during a case study in the Altmark area (Germany). Permanent monitoring stations were operated at 12 well sites and one reference location. We detected considerable and site specific variations of CO 2 concentration at shallow depths (1–3 m), which show that soil gas baseline concentrations, including quantification of variations caused by natural influences, can only be performed by means of a long-term continuous monitoring with reasonably short measuring intervals. The major result from the analysis of the data set is that much of the complexity of soil gas monitoring can be avoided if the correct sites and depths are chosen. In particular, we can demonstrate that CO 2 concentrations are very stable over long time periods (more than one year) when measured below the biological active soil zone (here at 0.5 m depth in average) and above the water table, where they are marginally touched by atmospheric influences. Seasonal variations trend to level out providing a constant signal over longer time periods (weeks/months). Applying a simplified non-steady state 1D-diffusion leakage model to the data set reveals that CO 2 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 δ 13 C isotopic composition of background CO 2 at deeper measuring intervals shows that δ 13 C can discriminate leakage even when isotopic composition of injected CO 2 is similar to soil CO 2 .