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Selected European studies have illustrated the impacts of elevated CO 2 concentrations in shallow soils on pasture. For the first time, general unified conclusions can be made, providing CO 2 thresholds where effects on plants and soil microbiology are observed and making recommendations on how this information can be used when planning projects for CO 2 storage. The sites include those where CO 2 is being naturally released to the atmosphere from deep geological formations; and a non-adapted site, with no previous history of CO 2 seepage, where CO 2 has been injected into the unsaturated soil horizon. Whilst soil gas concentrations will be influenced by flux rates and other factors, the results suggest that a concentration of between 10% and 15% CO 2 soil gas at 20 cm depth, which is within the root zone, is an important threshold level for observing changes in plant coverage. Site-specific plant ‘indicators’ are also observed for CO 2 concentrations at ≥35%. Microbiological changes are seen where CO 2 soil gas concentrations are between 15% and 40%. As part of site characterisation, an evaluation of the risks of leakage and their potential environmental impacts should be undertaken. [ABSTRACT FROM AUTHOR]

Abstract: Two airborne remote-sensing flights were conducted above a geothermal field in central Italy (the Latera caldera) where deep, naturally produced CO2 is migrating to surface along faults and leaking to the atmosphere at spatially restricted gas vents. The goal of these surveys was to understand if it is possible to locate CO2 leaking from a CO2 geological storage site through the application of indirect remote-sensing methods that primarily measure plant stress and subsequent ground-based verification using near-surface gas geochemistry techniques. The overall success rate obtained by integrating six different datasets was 39%, although some individual techniques, such as one NDVI survey, achieved a 47% success rate. While the work did discover some vents that were previously unknown, it also failed to locate five vents that are known to exist and, perhaps, other unknown vents. Future work will focus on understanding the various causes of false positives, automation of preliminary data interpretation, and the direct hyperspectral measurement of atmospheric CO2 produced by these natural seeps. [Copyright &y& Elsevier]

Abstract: A clear and detailed understanding of gas migration mechanisms from depth to ground surface is fundamental to choose the best locations for CO2 geological storage sites, to engineer them so that they do not leak, and to select the most appropriate monitoring strategy and tools to guarantee public safety. Natural test sites (or “natural analogues”) provide the best opportunity to study migration mechanisms, as they incorporate such issues as scale, long-time system evolution, and interacting variables that cannot be adequately addressed with laboratory studies or computer models. To this end the present work examines the migration to surface of deep, naturally produced CO2 along various buried and exposed faults in the Latera caldera (central Italy) by integrating structural geology and near-surface gas geochemistry surveys. Results show how gas migration is channelled along discrete, high-permeability pathways within the faults, with release typically occurring from spatially restricted gas vents. Size, distribution, and strength of these vents appear to be controlled by the evolution and deformation style of the fault, which is in turn linked to the rheology of the lithological units cut by the fault. As such gas migration can change drastically along strike. Gas migration in the vadose zone around these vents is also discussed, focussing on how the physical–chemical characteristics of various species (CO2, CH4, and He) control their spatial distribution and eventual release to the atmosphere. [Copyright &y& Elsevier]