A. Annunziatellis

Short- and long-term gas hazard: the release of toxic gases in the Alban Hills volcanic area (central Italy)

Abstract In the Alban Hills area, strong areally diffuse and localised spot degassing processes occur (Tivoli, Cava dei Selci, Solforata, Tor Caldara). The gas comprises a large proportion of CO 2 , with minor CH 4 , H 2 S and Rn. These advective features are generated by fluid leakage from buried reservoirs hosted in the structural highs of the Mesozoic carbonate basement. Gas migration towards the surface is controlled by fault and fracture systems bordering the structural highs of the carbonate formations (e.g. Ciampino high). His release is triggered when the total pressure of the fluid phase exceeds the hydrostatic pressure, thus forming a free gas phase. Furthermore, both the sudden and catastrophic, and slow and continuous gas release at surface, of naturally occurring toxic species (CO 2 , H 2 S and Rn) poses a serious health risk to people living in this geologically active area. This paper presents data obtained from soil gas and gas flux surveys, as well as gas isotopes analyses, which suggest the presence of a deep origin gas flux enriched in carbon dioxide and minor species (CH 4 and H 2 S), as well as a channelled migration of geogas mixtures having a Rn component which is not produced in situ. In regards to the health risk to local inhabitants, it was found that some anomalous areas had been zoned as parkland while others had been heavily developed for residential purposes. For example, many new houses were found to have been built on ground which has soil gas CO 2 concentrations of over 70% and a CO 2 flux of about 0.7 kg m −2 day −1 , as well as radon values of more than 250 kBq/m 3 . In addition, an indoor radon survey has been conducted in selected houses in the town of Cava dei Selci to search for a possible correlation between the local geology and the radon concentration in indoor air. Preliminary results indicate high indoor values at ground floor levels (up to 1000 Bq/m 3 ) and very high values in the cellars (up to 250.000 Bq/m 3 ). It is recommended that land-use planners incorporate soil gas and/or gas flux measurements in the environmental assessment of areas of possible risk (i.e. volcanic or structurally active areas).

Characterization of a CO2 gas vent using various geophysical and geochemical methods

An understanding of gas migration along faults is important in many geologic research fields, such as geothermal exploration, risk assessment, and, more recently, the geologic storage of man-made carbon dioxide (C O2 ) . If these gases reach the surface, they typically are discharged to the atmosphere from small areas known as gas vents. In a study of an individual gas vent located in the extinct Latera caldera, central Italy, near-surface geochemical and geophysical surveys were conducted to define the spatial distribution of gas-induced effects in the first few meters of the soil and, by inference, the 3D structure and geometry of the associated gas-permeable fault. Grid surveys and detailed profiles were performed across this vent using time-domain reflectometry (TDR), ground-penetrating radar (GPR), frequency-domain electromagnetics (FDEM), electrical resistivity tomography (ERT), and gas geochemistry measurements. Detailed profilesurveys indicate that the leaking C O2 has changed the physical, chemic...

NEAR-SURFACE GAS GEOCHEMISTRY TECHNIQUES TO ASSESS AND MONITOR CO2 GEOLOGICAL SEQUESTRATION SITES

As is well known the long-term effects and stability of a man-made CO2 geological storage facility is very difficult to predict with laboratory or modeling experiments due to the size and long time scales involved. Instead attractive additional sources of information are natural sites where CO2 produced at great depths is either trapped in porous reservoirs or leaks to the surface. These sites can be considered as “natural analogues” of what may occur over geological time spans within an engineered CO2 geological storage site. The study of these sites can be subdivided into three broad fields: i) understanding why some reservoirs leak while others don’t; ii) understanding the possible effects of CO2 should it leak into the near-surface environment; and iii) using the leaking sites to develop, test and optimise various monitoring technologies. The present article summaries many of the near-surface gas geochemistry results obtained in central Italy during the EC-funded NASCENT project (Natural Analogues for the Storage of CO2 in the Geological Environment). These include a comparison of leaking (Latera) and a non-leaking (Sesta) CO2 reservoirs, detailed soil gas surveys to outline migration pathways, the development of a geochemical continuous-monitoring station to study temporal changes in CO2 concentrations, and field experiments involving the injection of a gas mixture in the shallow subsurface to outline migration pathways and to understand the behaviour of various gas species based on their different chemical-physical-biological characteristics. Put together this data provides useful information for site selection, risk assessment and monitoring techniques, which is needed if CO2 geological storage is to become an accepted and widely-applied technology.

Allaying public concern regarding CO2 geological sequestration through the development of automated stations for the continuous geochemical monitoring of gases in the near surface environment

Publisher Summary The principle concern for both regulators and the public at large regarding the large-scale application of on-shore geological carbon dioxide (CO2) sequestration is related to possible health risks due to the leakage of CO2 at surface. Development and “proof of concept” of the automated geochemical monitoring station during research is seen as an important step forward in providing tools that help in assuring the public at large about the safety of geological CO2 sequestration. The advances in microchip technology, portable analytical instrumentation, and decreasing prices means that, once fully developed, a significant number of these relatively low cost stations might be deployed above an injection site to monitor CO2 leaks. These might be particularly useful if placed around any deep wells that penetrate the reservoir or deep aquifer in which CO2 is being injected, as gas injection tests have shown how breakthrough of monitored gases is often first observed in the vicinity of the injection borehole, implying that fracturing during drilling provided preferential flow pathways. Preliminary data provided by the development and installation of a prototype automated geochemical-geophysical monitoring station in the San Vittorino area has shown that this technology exists and that it can provide, together with other techniques, a useful tool for the safety assessment and monitoring of geological sequestration sites.