Since the Kyoto Protocol came into effect in February 2005, international community of states have for the first time been obliged to establish and develop action goals and implement methods to help global climate protection. As a result of this, obligations have arisen to create, monitor and verify emission inventories. This includes, alongside the development of strategies to achieve emission reduction goals, actual conception of effective monitoring methods that allow extensive observation and control of greenhouse gases.

A potential bridging technology for the reduction of anthropogenic CO2 emissions is Carbon Capture and Storage (CCS) where separated CO2 is stored into deep underground formations. The stored CO2 can potentially originate from fossil fuel supply systems, industrial supplies or from the use of biomass as a waste-to-energy converted resource. The subterranean CO2 storage can only make an effective contribution in the fight against climate change when the stored CO2 are continually and fully maintained in these reservoirs. This requirement is a main aspect in the Carbon Dioxide Storage Law (Kohlendioxidspeichergesetz) in Germany. Effective and rapid monitoring methods for comprehensive observation of the near-surface environment before, during and after the operational phase is essential to guarantee the safety of the process and to avoid any problems with storage integrity,

The UFZ- Helmholtz Centre for Environmental Research coordinated the joint research project MONACO (MONitoring-Approach for geological CO2 storage sites using a hierarchical observation concept) between 2011 and 2014. The aim of this project was the development of an integrated, hierarchical monitoring concept for the reliable identification of CO2 emissions from natural and anthropogenic storage formations in near-surface areas (e.g. aquifer systems, the unsaturated soil zone) and into the atmosphere. Successful implementation of this project encompassed the efficient combination of individual geophysical, geochemical and geotechnical methods, which were used at several different reference sites - with each subject to its own variable site-specific influencing factors.    

This hierarchical approach, based on a modular monitoring concept, allows monitoring of large-scale areas (in km2-scale) with adequate spatial and temporal resolution to enable reliable detection of CO2 migration pathways and seepage into the atmosphere. Given that evidence of apparent atmospheric gas leakages is especially crucial when establishing an early warning system, ground-based optical remote sensing systems were applied and tested, so as to monitor near-surface atmospheric CO2 concentrations.

Alongside the investigation of natural spatial variability, targeted attempts were made with respect to monitoring the temporal variations at natural COsources using ground-based Infrared Spectroscopy and Eddy Covariance methods. As such, development of a new Infrared Spectrometer prototype with specific analysis software was undertaken in the course of the project. This device operates with a high degree of accuracy and makes it possible to map large areas over long periods of time, for specific CCS areas of interest.

Subsequently applied meso- and point-scale monitoring techniques focus on the identification of structural settings in the subsurface. With the help of geophysical data, characteristic soil COconcentration patterns as well as COground emission rate behavior models potential areas of biologically- and geologically-influenced CO2 emissions at the test sites within the project could be identified. This therefore shows that a site-specific combination of periodically-repeated measurement campaigns utilizing geophysical methods in combination with analytical soil gas measurements and permanent in-situ installations is essential to directly obtain timely, high resolution measurements of petrophysical and soil analytical parameters. As such, the site-specific spatial and temporal dynamics of the various petrophysical parameters are crucial for our understanding of transport processes and the resultant geophysical response functions, as well as for the design of our monitoring concept. For the spatial and temporal investigation of COmigration behavior in the subsurface, a prototype device was successfully tested and developed. . With the determination of soil gas isotope ratios, it was possible to conclusively identify CO2 origins – respectively, the proportions originating from different CO2 sources and CO2 migration behavior.

The results of the MONACO Project demonstrate that a successful monitoring concept for the investigation of near-surface areas must be based on a modular, hierarchical approach, so as to successfully identify COmigration at different temporal and spatial scales with varying levels of resolution. These techniques in turn represent a dependable and reliable means of identifying potential COmigration pathways and even possible leakage areas.

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