Lombardi, Salvatore

The Valentano area is located in the volcanic district of Vulsini Mounts, in the northern part of the Latium region (Central Italy). Geometry and shallow distribution of gas anomalies together with geophysical data on deep structural elements and anomalous geothermal gradient, confirm the presence of gas fluxes from a geothermal reservoir. The Valentano area is characterized by widespread natural emission phenomena, hazardous to human and animal health (many animals deaths have been recorded). A soil gas survey has been carried out in this area with the aim to understand the spatial distribution of the gaseous species and so the location of the main pathways (faults and fractures) causing the gas uprising. A detailed survey has been carried out at NW of Valentano town, where gas vent phenomena are characterised by high CO2 and CH4 concentrations. In this area, 75 soil gas samples (60 samples/km) have been collected. Data have been treated statistically (by normal probability plots), and by contour maps. CO2 and CH4 concentrations show maximum values up to 100% and 1000ppm respectively, detected over the highest emission points. CO2 and CH4 contour maps show high anomalous concentrations in the central and in the southern sector of the studied area and distributions have a spot geometry. Highest Rn concentrations (> 100Bq/L) show a good spatial continuity in N-S direction overlapping the CO2 and CH4 highest values only in the central part of the anomaly. He concentrations show anomalous values only in the southern sector. The good correlation of the gas concentrations allow us to infer that the geochemical anomalous areas and the deep geothermic reservoir are strictly related. Besides, the good association between Rn and CO2 distributions confirms the CO2 carrier gas role for trace elements. Since this study has been partially funded by local authorities for the gas risk assessment, radon hazard map has been elaborated in order to highlight the risk areas for the human and animal health.

Continuous monitoring has been carried out at a fluvial flood-plain site near Rome for over a year. There is a mix of biogenic CO2 and deep geogenic CO2 at the site at relatively low concentrations and fluxes compared with other natural CO2 seepage sites studied previously. Factors such as temperature and soil moisture clearly affect the CO2 concentration and flux and seasonal and diurnal influences are apparent. Statistical approaches are being used to try to define these relationships and separate out the two gas components, which would be necessary in any quantification of leakage from CO2 storage.

The monitoring of the integrity of onshore geological carbon capture and storage projects will require an approach that integrates various methods with different spatial and temporal resolutions. One method proven to be quite effective for site assessment, leakage monitoring, and leakage verification is near-surface gas geochemistry, which includes soil gas concentration and gas flux measurements. Anomalous concentrations or fluxes, relative to the natural background values, can indicate the potential occurrence of a leak. However, the natural background can be quite variable, especially for CO2, due to biological production and accumulation in the soil that changes as a function of soil type, land use, geology, temperature, water content, and various other parameters. To better understand how these parameters influence natural, near-surface background values, and to examine the potential of different sampling strategies as a function of the survey goals, this paper reports results from two highly different case studies, one from northern Europe (Volund, Denmark) and one from southern Europe (Sulcis, Sardinia, Italy). The small Voulund site, with its homogeneous soil, climate, and topography, was surveyed twice (in fall and in spring) within the EU-funded SiteChar project to examine the effects of different land-use practices and seasons on baseline values. Forested land was found to have lower CO2 concentrations during both campaigns compared to cultivated and heathland, and higher CH4 values during the spring sampling campaign. Continuous monitoring probes showed much more detail, highlighting seasonal changes in soil gas CO2 concentrations linked primarily to temperature variations. The much larger Sulcis site, studied within an ENEA-funded project on potential CO2-ECBM (Enhanced Coal Bed Methane) deployment, was surveyed at the regional scale and on detailed grids and transects for site assessment purposes. Despite the completely different soil and climate conditions, the statistical distribution of the Sulcis data was similar to that of Volund. Much higher soil gas CO2 anomalies were found at this site, however, due to the less permeable sediments (i.e., better water retention and greater gas accumulation) and the warmer temperatures. Detailed surveys at this site highlighted various significant anomalies, some of which can be explained by near-surface biological processes, whereas others, especially helium anomalies, were more difficult to explain. These results show the utility of baseline surveys and highlight the need for follow-up studies to clarify any unexplained anomalies before any CO2 storage.

The EC funded Geochemical Seismic Zonation program (EEC GSZ Project 1996-98) chose Sardinia as a low-seismicity site, in which the relationships between fluid geochemistry and seismo-tectonics had to be investigated and results compared with outcomes from other selected high-seismicity sites. A first paper, examining the role of fault segmentation and seismic quiescence on the geochemical composition of groundwaters and gases, has already been presented (Angelone et al. 2005). This paper deals with environmental isotopes which, together with selected hydrochemical data, give hints on tectonically-related fluid circulations. Four water-dominated hydrothermal systems were considered, all located along regional fault systems and discharging groundwaters belonging to the Na-HCO3 and Na-Cl facies. In the considered systems, groundwater circulation takes place, principally, in the Palaeozoic Crystalline Basement (PCB), with the exception of the Logudoro system, where hydrological circuits develop in the Mesozoic Carbonate Platform (MCP). The high CO2 contents, the non-attainment of fluid-rock equilibrium and the large lithological variability prevent the construction of a unique hydrogeological-geochemical conceptual model. In this case, stable isotopes provide a useful tool to describe the origin of fluids and their subterranean movements. Stable isotopes of water, integrated with hydrochemical data, indicate that fluids are derived from three main endmembers. The dominant component is a relatively recent local meteoric water, the second one is marine water, and the third one is a fossil freshwater, depleted in heavy isotopes with respect to modern rains. The latter endmember entered the aquifer system in the past, when climatic conditions were greatly different from today. At least two circulation systems can be recognised, namely a shallow cold system and a deep hydrothermal system, as well as two distinct hydrological processes: (1) gravity-controlled descent of cold water towards greater depths and (2) convection linked to a thermal gradient, causing deep fluids to rise up from the hydrothermal reservoir towards the surface. The highly variable 13CTDIC values suggest the presence of two distinct CO2 sources, namely a biogenic one and a thermogenic one. The relation between the isotopic compositions of CO2 and He indicates an increased mantle signature in uprising CO2-rich fluids.

National Italian funding has recently been allocated for the construction of a 350 MWe coal-fired power plant / CCS demonstration plant in the Sulcis area of SW Sardinia, Italy. In addition, the recently approved EC-funded ENOS project (ENabling Onshore CO2 Storage in Europe) will use the Sulcis site as one of its main field research laboratories. Site characterization is already ongoing, and work has begun to design gas injection experiments at 100-200 m depth in a fault. This article gives an overview of results to date and plans for the future from the Sapienza University of Rome research group.

The physical properties of clay allow argillaceous formations to be considered geological barriers to radionuclide migration in high-level radioactive-waste isolation systems. As laboratory simulations are short term and numerical models always involve assumptions and simplifications of the natural system, natural analogues are extremely attractive surrogates for the study of long-term isolation. The clays of the Orciatico area (Tuscany, Central Italy), which were thermally altered via the intrusion of an alkali-trachyte laccolith, represent an interesting natural model of a heat source which acted on argillaceous materials. The study of this natural analogue was performed through detailed geoelectrical and soil–gas surveys to define both the geometry of the intrusive body and the gas permeability of a clay unit characterized by different degrees of thermal alteration. The results of this study show that gas permeability is increased in the clay sequences subjected to greater heat input from the emplacement of the Orciatico intrusion, despite the lack of apparent mineral and geotechnical variations. These results, which take into consideration long time periods in a natural, large-scale geological system, may have important implications for the long-term safety of underground storage of nuclear waste in clay formations.

Gas-geochemistry has been proven to be a reliable and simple technique to apply, at different scales, in many geological scenarios. The study of spatial distribution of soil-gas anomalies, at the surface, can give important and interesting information on the origin and processes involving deep and superficial gas species. This information can be applied and studied in different frameworks, for example: I) seismic zonation, examining, at the surface, anomalous concentrations of deep gas species that upraise throughout preferential pathways (faults and/or fractures); II) environmental protection, monitoring naturally occurring toxic gases and highlighting zones with high health risks for humans; III) geomorphological and structural research, detecting the aggressive fluid piping that causes carsic phenomena; IV) radionuclide migration, both in the pollution assessment from abandoned uranium mines and in the study of high-level radioactive-waste isolation systems. Soil-gas distribution could be affected by surface features such as pedological, biogenic and meteorological factors: these are supposed to have only a subordinate effect on gas leakage. However, it is possible to properly interpret soil-gas anomalies and recognize, and avoid, influences of surface features studying the association of different gases (with different origin and physical/chemical behavior), collecting a large number of samples during periods of stable meteorological and soil moisture conditions (e.g., during dry season) and using appropriate statistical treatment of data (i.e., experimental variograms to investigate the spatial dependency of gas concentrations). We will try, in this paper, to give hints for a better comprehension of the main mechanisms ruling soil-gas features both displaying and discussing some data obtained in either prospecting or monitoring case studies. Soil-gas geochemistry involves the study of many gaseous species (i.e., radiogenic, trace and diagenetic gases) each of them can give specific information on the conditions that allow their formation, accumulation and/or migration. In this study, we outline the results from two analyzed soil-gases: radon (222Rn), a radiogenic trace gas, and carbon dioxide (CO2) which generally acts as carrier for trace gases.

The physical properties of clay allow to consider argillaceous formations as geological barriers to radionuclide migration in high-level radioactive-waste isolation systems. As laboratory simulations are short term and numerical models always involve assumptions and simplifications of the natural system, natural analogues are extremely attractive surrogates for the study of long-term isolation. The thermally altered clays of the Orciatico area (Tuscany, Central Italy) represent an interesting natural model of a heat source which acted on argillaceous materials. The study of this natural analogue was performed through detailed geoelectrical and soil-gas surveys in order to define both the geometry of the intrusive body and the gas permeability of a clay unit characterized by different thermal alteration degrees. In particular, soil-gas radon and carbon dioxide distributions highlighted that the clay sequences, in spite of their thickness and plasticity, if fractured and metamorphosed, form a lesser impermeable barrier for naturally migrating gas.

The Sulcis Basin is an area situated in SW Sardinia (Italy) and is a potential site for the development of CCS in Italy. This paper illustrates the preliminary results of geological characterization of fractured carbonate reservoir (Miliolitico Fm.) and the sealing sequence, composed by clay, marl, and volcanic rocks, with a total thickness of more than 900 m. To characterize the reservoir-caprock system an extensive structural-geological survey at the outcrop was conducted. It was also performed a study of the geochemical monitoring, to define the baseline conditions, measuring CO2 concentrations and flux in the study site.

CCS communication has proven a tough challenge, particularly for the difficulty in raising interest for the technology, which is still unknown to the majority of the population, and for the complexity of conveying information about its potential for reducing emissions. In this paper, we present a research based effort for bringing CCS nearer to people, through visual material developed taking into account emotional needs related to the technology. The production of a short introductory film on CCS is illustrated and its testing with a sample of 700 high school students.