Continuous monitoring of near surface gases at a natural CO2 emission site near Rome – lessons for low-level CO2 leakage detection

Regulations for geological storage of CO2 (such as the EU Directive on Geological Storage of CO2 and that covering the Emissions Trading Scheme) require monitoring for leakage detection and quantification of any emissions to the atmosphere or marine water column. Whilst early indications of migration within the storage reservoir and storage complex, and leakage from the storage complex, may be provided by deep-focussed monitoring, the ultimate detection of surface emissions and their quantification will need to be made by near-surface measurements. There have been no significant leaks from CO2 storage sites operated to date at pilot, demonstration or larger scales that range up to more than 1 Mt of CO2 injected per year and totals in excess of 20 Mt. In the absence of such leaks, assessment of leakage detection technologies has been undertaken at controlled injection and release sites, such as the ZERT site in Montana or the CO2 Field Lab in Norway, or at sites of natural CO2 emission. At many of such sites the gas emissions occur at clear seepage points, restricted in areal extent (a few metres to tens of metres across) and with CO2 concentrations in the soil, fluxes across the soil-to-atmosphere interface and concentrations in the near surface atmospheric boundary layer that are readily distinguishable from background values. However, there is also some evidence for more subtle emissions, from isotopic data or gas ratios that can fall within normal baseline ranges and are therefore more difficult to detect. Also there exists the possibility that such low-level emissions could lead to significant loss of stored CO2 if they occurred over large areas. We report here investigations carried out at a natural CO2 emission site near the main Rome airport at Fiumicino.